text:
User talk:TaoPhoenix

Anyone reading this? Feel free to email me.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: User talk:TaoPhoenix


text:
OpenOffice.org/Introduction

OpenOffice.org is a free, open source alternative to the costly Microsoft Office. It possesses many of the same day-to-day capabilities as Microsoft Office, except that it is free to use and it is much more cross-platform than Microsoft Office. This makes it ideal for students or home users and people that have non-Windows Platform (Linux, MacOS, Unix, etc).

About OpenOffice.org
OpenOffice.org is a free, full-fledged Office Suite based on Sun's StarOffice Suite. OpenOffice.org is composed of six tools: Writer, Calc, Impress, Draw, Math, and Base. Writer is a word-processor, it can be used to write text documents from a simple letter to novels. Calc is a spreadsheet program, it can be used to store, process, and present data and formats them in a table layout. Impress is a presentation software, it can be used to create various types of presentations with various animations (effects). Draw is a vector-based drawing program, the images are based on lines and shapes which creates an image with virtually unlimited resolution. Math is a formula editor, it can be used to write from your high-school algebra to a complex rocket science mathematical formulas. Base, the newest tool of the suite, is a database program, it can be used to design, create, and manipulate a database. OpenOffice also supports macro scripting through its OpenOffice Basic scripting language, which obviously is based on BASIC programming language. OpenOffice is also very extensible, OpenOffice has a plugin system which allows third-parties to add functionalities into the suite, available plugins include a somewhat tongue-in-the-cheek lorem ipsum generator to a professional database report generator to remote control add-on for Impress.

OpenOffice.org's project started on 13 October 2000, with Sun's announcement of the project. Today, OpenOffice is available on all major platforms including Windows, Linux, Mac OS X, UNIX, and of course Sun's Solaris, and it is available in over 80 languages and localizations. OpenOffice also pioneered the OASIS OpenDocument Standard and it has been its native format since OpenOffice 2.0. A number of other Office Suites has also adopted OpenDocument Standard to some degree, but unfortunately Microsoft Office, another Office Suite which has the de facto largest user base, declined to follow the standard and developed their  own XML standard instead.

Due to its open source nature, there are many other office suites that has their codes in partial or in whole derived from OpenOffice.org. Sun's StarOffice, in particular, bases most of its code on OpenOffice's codes, but added some additional proprietary components. The free and open source nature (and its quality) also makes OpenOffice.org a favorite choice among Linux distros makers to include OpenOffice.org in their distros as the default Office software. OpenOffice.org is also favored among some governments due to its open nature, they claim that a closed, proprietary document format like Microsoft's binary .doc format is detrimental in the long-term, when the company owning the format it have gone out-of-business, the cost of conversion in the future would be much higher with a closed, proprietary format. In the short term, the cost deploying thousands of computers with an expensive, proprietary suite may also be higher than deploying free, open suite.

How to Get It?
OK, now I know what OpenOffice is, so how do I get it?

There are three official ways of distribution of OpenOffice, all of which are free:
 * Direct Download from OpenOffice's download page.
 * Bittorrent download (Tracker files)
 * CD-ROM distribution, List of OpenOffice's CD distributor

title: OpenOffice.org/Introduction


text: Literary Criticism/Shakespeare Statistics


text:
Basic Physics of Nuclear Medicine/Gas-Filled Radiation Detectors

We have learned in the last two chapters about how radiation interacts with matter and we are now in a position to apply our understanding to the detection of radiation.

One of the major outcomes of the interaction of radiation with matter is the creation of ions as we saw in Chapter 5. This outcome is exploited in gas-filled detectors as you will see in this chapter. The detector in this case is essentially a gas, in that it is the atoms of a gas which are ionised by the radiation. We will see in the next chapter that solids can also be used as radiation detectors but for now we will deal with gases and be introduced to detectors such as the Ionization Chamber and the Geiger Counter.

Before considering these specific types of gas-filled detectors we will first of all consider the situation from a very general perspective.

Gas-Filled Detectors
As we noted above the radiation interacts with gas atoms in this form of detector and causes ions to be produced. On the basis of what we covered in Chapter 5 it is easy to appreciate that it is the Photoelectric and Compton Effects that cause the ionisations when the radiation consists of gamma-rays with energies useful for diagnostic purposes.

There are actually two particles generated when an ion is produced - the positive ion itself and an electron. These two particles are collectively called an ion pair. The detection of the production of ion pairs in the gas is the basis upon which gas detectors operate. The manner in which this is done is by using an electric field to sweep the electrons away to a positively charged electrode and the ions to a negatively charged electrode.

Let us consider a very simple arrangement as shown in the following figure:

Here we have two electrodes with the gas between them. Something like a capacitor with a gas dielectric.

The gas which is used is typically an inert gas, for example argon or xenon. The reason for using an inert gas is so that chemical reactions will not occur within the gas following the ionisations which could change the characteristics of our detector.

A dc voltage is placed between the two electrodes. As a result when the radiation interacts with a gas atom the electron will move towards the positive electrode and the ion will move towards the negative electrode. But will these charges reach their respective electrodes? The answer is obviously dependent on the magnitude of the dc voltage. For example if at one extreme we had a dc voltage of a microvolt (that is, one millionth of a volt) the resultant electric field may be insufficient to move the ion pair very far and the two particles may recombine to reform the gas atom. At the other extreme suppose we applied a million volts between the two electrodes. In this case we are likely to get sparks flying between the two electrodes - a lightning bolt if you like - and our detector might act something like a neon sign. Somewhere in between these two extremes though we should be able to provide a sufficient attractive force for the ion and electron to move to their respective electrodes without recombination or sparking occurring.

We will look at this subject in more detail below. Before we do let us see how the concept of the simple detector illustrated above is applied in practice. The gas-filled chamber is generally cylindrical in shape in real detectors. This shape has been found to be more efficient than the parallel electrode arrangement shown above.

A cross-sectional view through this cylinder is shown in the following figure:

The positive electrode consists of a thin wire running through the centre of the cylinder and the negative electrode consists of the wall of the cylinder. In principle we could make such a detector by getting a section of a metal pipe, mounting a wire through its centre, filling it with an inert gas and sealing the ends of the pipe. Actual detectors are a little bit more complex however but let us not get side-tracked at this stage.

We apply a dc voltage via a battery or via a dc voltage supply and connect it as shown in the figure using a resistor, R. Now, assume that a gamma-ray enters the detector. Ion pairs will be produced in the gas - the ions heading towards the outer wall and the electrons heading towards the centre wire. Let us think about the electrons for a moment. When they hit the centre wire we can simply think of them as entering the wire and flowing through the resistor to get to the positive terminal of the dc voltage supply. These electrons flowing through the resistor constitute an electric current and as a result of Ohm's Law a voltage is generated across the resistor. This voltage is amplified by an amplifier and some type of device is used to register the amplified voltage. A loud-speaker is a fairly simple device to use for this purpose and the generation of a voltage pulse is manifest by a click from the loud-speaker. Other display devices include a ratemeter which displays the number of voltage pulses generated per unit time - something like a speedometer in a car - and a pulse counter (or scaler) which counts the number of voltage pulses generated in a set period of time. A voltage pulse is frequently referred to in practice as a count and the number of voltage pulses generated per unit time is frequently called the count rate.

DC Voltage Dependence
If we were to build a detector and electronic circuit as shown in the figure above we could conduct an experiment that would allow us to explore the effect of the dc voltage on the magnitude of the voltage pulses produced across the resistor, R. Note that the term pulse height is frequently used in this field to refer to the magnitude of voltage pulses.

Ideally, we could generate a result similar to that illustrated in the following figure:

The graph illustrates the dependence of the pulse height on the dc voltage. Note that the vertical axis representing the pulse height is on a logarithmic scale for the sake of compressing a large linear scale onto a reasonably-sized graph.

The experimental results can be divided into five regions as shown. We will now consider each region in turn.

 * Region A    Here Vdc is relatively low so that recombination of positive ions and electrons occurs.  As a result not all ion pairs are collected and the voltage pulse height is relatively low. It does increase as the dc voltage increases however as the amount of recombination reduces.

 * Region B    Vdc is sufficiently high in this region so that only a negligible amount of recombination occurs.  This is the region where a type of detector called the Ionization Chamber operates.

 * Region C    Vdc is sufficiently high in this region so that electrons approaching the centre wire attain sufficient energy between collisions with the electrons of gas atoms to produce new ion pairs.  Thus the number of electrons is increased so that the electric charge passing through the resistor, R, may be up to a thousand times greater than the charge produced initially by the radiation interaction.  This is the region where a type of detector called the Proportional Counter operates.

 * Region D    Vdc is so high that even a minimally-ionizing particle will produce a very large voltage pulse.  The initial ionization produced by the radiation triggers a complete gas breakdown as an avalanche of electrons heads towards and spreads along the centre wire.  This region is called the Geiger-Müller Region, and is exploited in the Geiger Counter.

 * Region E    Here Vdc  is high enough for the gas to completely breakdown and it cannot be used to detect radiation.

We will now consider features of the Ionisation Chamber and the Geiger Counter in more detail.

Ionisation Chamber
The ionisation chamber consists of a gas-filled detector energised by a relatively low dc voltage. We will first of all make an estimate of the voltage pulse height generated by this type of detector. We will then consider some applications of ionisation chambers.

When a beta-particle interacts with the gas the energy required to produce one ion pair is about 30 eV. Therefore when a beta-particle of energy 1 MeV is completely absorbed in the gas the number of ion pairs produced is:

$$n = \frac{1\ \text{MeV}}{30\ \text{eV}} = \frac{1 \cdot 10^6}{30} \approx 3 \cdot 10^4\ \text{ion pairs}$$

The electric charge produced in the gas is therefore

$$Q = n \cdot e$$

$$\therefore (3 \cdot 10^4\ \text{ion pairs}) \cdot (1.6 \cdot 10^{-19}\ \text{C})$$

$$\therefore Q = 5 \cdot 10^{-15}\ \text{C}$$

If the capacitance of the ionisation chamber (remember that we compared a gas-filled detector to a capacitor above) is 100 pF then the amplitude of the voltage pulse generated is:

$$V = \frac{Q}{C} = \frac{5 \cdot 10^{-15}\ \text{C}}{100 \cdot 10^{-12}\ \text{F}} = 5 \cdot 10^{-5}\ \text{V}$$

$$\therefore V = 50\ \mu\text{V}$$

Because such a small voltage is generated it is necessary to use a very sensitive amplifier in the electronic circuitry connected to the chamber.

We will now learn about two applications of ionisation chambers. The first one is for the measurement of radiation exposures. You will remember from Chapter 4 that the unit of radiation exposure (be it the SI or the traditional unit) is defined in terms of the amount of electric charge produced in a unit mass of a air. An ionization chamber filled with air is the natural instrument to use for such measurements.

The second application is the measurement of radioactivity. The ionisation chamber used here is configured in what is called a re-entrant arrangement (see figure below) so that the sample of radioactive material can be placed within the detector using a holder and hence most of the emitted radiation can be detected. The instrument is widely referred to as an Isotope Calibrator and the trickle of electric current generated by such a detector is calibrated so that a reading in units of radioactivity (for example MBq or mCi) can be obtained. Most well-run Nuclear Medicine Departments will have at least one of these devices so that doses of radioactivity can be checked prior to administration to patients.

Here are some photographs of ionisation chambers designed for various applications:

Geiger Counter
We saw earlier that the Geiger Counter operates at relatively high dc voltages (for example 400-900 volts) and that an avalanche of electrons is generated following the absorption of radiation in the gas. The voltage pulses produced by this detector are relatively large since the gas effectively acts as an amplifier of the electric charge produced.

There are four features of this detector which we will discuss. The first is that a sensitive amplifier (as was the case with the Ionization Chamber) is not required for this detector because of the gas amplification noted above.

The second feature results from the fact that the generation of the electron avalanche must be stopped in order to reform the detector. In other words when a radiation particle/photon is absorbed by the gas a complete gas breakdown occurs which implies that the gas is incapable of detecting the next particle/photon which enters the detector. So in the extreme case one minute we have a radiation detector and the following moment we do not.

A means of stopping the electron avalanche is therefore required - a process called Quenching. One means of doing this is by electronically lowering the dc voltage following an avalanche. A more widely used method of quenching is to add a small amount of a quenching gas to the inert gas. For example the gas could be argon with ethyl alcohol added. The ethyl alcohol is in vapour form and since it consists of relatively large molecules energy which would in their absence give rise to sustaining the electron avalanche is absorbed by these molecules. The large molecules act like a brake in effect.

Irrespective of the type of quenching used the detector is insensitive for a small period of time following absorption of a radiation particle/photon. This period of time is called the Dead Time and this is the third feature of this detector which we will consider. Dead times are relatively short but nevertheless significant - being typically of the order of 200-400 µs. As a result the reading obtained with this detector is less than it should be. The true count rate, T, can be obtained using the following equation:

$$T = \frac{A}{1 - \tau A}$$

where A is the (actual) reading and τ is the dead time. Some instruments perform this calculation automatically.

The fourth feature to note about this detector is the dependence of its performance on the dc voltage. The Geiger-Müller Region of our figure above is shown in more detail below:

Notice that it contains a plateau where the count rate obtained is independent of the dc voltage. The centre of this plateau is where most detectors are operated. It is clear that the count rate from the detector is not affected if the dc voltage fluctuates about the operating voltage. This implies that a relatively straight-forward dc voltage supply can be used. This feature coupled with the fact that a sensitive amplifier is not needed translates in practice to a relatively inexpensive radiation detector.


text:
User:JCarriker~enwikibooks

"But I don't want to go among mad people," Alice remarked.

"Oh you can't help that," said the Cat, "We're all mad here. I'm mad.  You're mad."

''"How do you know I'm mad?" said Alice.''

"You must be," said the Cat, "or you wouldn't have come here."

 * --An excerpt from Alice in Wonderland

title: User:JCarriker~enwikibooks


text:
User talk:JCarriker~enwikibooks

Please post at the bottom of the page

Ancient History
I'd be delighted if you would contribute to this project. I think I got to be editor by being the only active contributor for the few days while they were setting up the infoboxes.

I think it would be best if this book was an Ancient History textbook and not an Ancient Egypt or Ancient Greece textbook. By this, I mean that your favorite dynasties and my favorite classical events rate a paragraph or so, and not several pages. We should stick to broad themes and overall forces, rather than nitty-gritty details. However, because this is an open-source project, I'm trying to keep an open mind about it.

I also think that we should have a discussion going on the main page about this, so that future contributors can have a sense of the overall intent of the book and its driving philosophies. Does this make sense to you? -Andrew Watt 14:23, 6 June 2006 (UTC)

P.S., would you please read and edit the Egypt page which gives the overview paragraphs, and add whatever you think appropriate, and then do the same at Egyptian Geography? I'd be tremendously grateful. Andrew Watt 19:52, 6 June 2006 (UTC)

The additions to Egypt, including Predynastic Egypt, look good. I made a few editorial changes, including spelling and a few comma movements; I also shifted the voice of some sentences to active, because the passive voice in history textbooks drives me up the wall. I hope you approve of my changes. Glad to work with you! -Andrew Watt 19:50, 7 June 2006 (UTC)

Paleolith & Neolith
J -- I've got substantial sections of the Paleolithic and Neolithic pages filled out. I'm not really clear on my hominids so I want to go back in and research some of that before I try writing those sections. Secretly I'm hoping that someone who knows more than I do will come along and volunteer to do those. I'm planning to leave Egypt to you for the moment, and get to work on Mesopotamia next, so that each section has at least some text that can be filled in around and/or edited as other people join the project. Any thoughts on this? -Andrew Watt 15:15, 9 June 2006 (UTC)

Talk:Wikijunior Ancient Civilizations
I'd like to point you in the direction of the discussion on date notation on this page. Since you are a regular contributor to our history textbooks, your input would be particularly valuable.

First, however, I'd like to note how we go about deciding things on Wikibooks, in case you are unfamiliar with this. Essentially we discuss them together, taking account of all concerns raised, and find a way of resolving the issue that everyone is happy to go along with. Very rarely, once all issues raised have been discussed, we may go to a confirmatory vote, or request a dissenting editor to step aside from his argument - but on the whole it does not come to that. So if there are things you are concerned about, please feel free to raise them.

Turning to Talk:Wikijunior Ancient Civilizations itself, you will note that in the discussion there, whilst some users have stated their own personal preference in date style notation, those who have been arguing that the text should use one notation over another have been considering what is most appropriate for the book's target audience. Personally, I very much think that this should be the determining factor, and that it is therefore right that this is where the discussion has been heading. You will note that, for this particular book, which has as its audience children aged 8 to 12 living anywhere in the English-speaking world, most contributors are supportive of using the BC/AD notation rather than the BCE/CE notation. As you are a contributor to our history books (and I very much hope remain a contributor) to that book, it would be useful if you could visit the discussion and either raise further points that you believe should be considered, or alternatively indicate whether you personally would foresee any difficulties in using BC/AD notation for that book.

I also believe that the arguments in that history book readily transfer for any other history book with a general audience, which includes some of the history books that you have been contributing too on Wikibooks. If you have comments regarding that, again, I should be grateful if you would note them on Talk:Wikijunior Ancient Civilizations. If you believe there is anyone else who, in particular, ought to be invited to comment, please let them know about the discussion, so that any decision we come to is one that everyone can accept. All the best, Jguk 21:53, 18 June 2006 (UTC)

Saylor.org's Ancient Civilizations of the World
Hey there, I noticed that you have been a sginificant contributor to the textbook on ancient civilizations. I am currently working with the Saylor Foundation to create a similar textbook that we can host and use on our website for our history course on ancient civilizations (which we offer for free to anybody on the internet). We would love for you to check out Saylor.org's Ancient Civilizations of the World and give us some feedback or make some contributions! --Azin (discuss • contribs) 21:17, 6 June 2012 (UTC)

Your account will be renamed
Hello,

The developer team at Wikimedia is making some changes to how accounts work, as part of our on-going efforts to provide new and better tools for our users like cross-wiki notifications. These changes will mean you have the same account name everywhere. This will let us give you new features that will help you edit and discuss better, and allow more flexible user permissions for tools. One of the side-effects of this is that user accounts will now have to be unique across all 900 Wikimedia wikis. See the announcement for more information.

Unfortunately, your account clashes with another account also called JCarriker. To make sure that both of you can use all Wikimedia projects in future, we have reserved the name JCarriker~enwikibooks that only you will have. If you like it, you don't have to do anything. If you do not like it, you can pick out a different name.

Your account will still work as before, and you will be credited for all your edits made so far, but you will have to use the new account name when you log in.

Sorry for the inconvenience.

Yours, Keegan Peterzell Community Liaison, Wikimedia Foundation 23:18, 17 March 2015 (UTC)

Renamed
 This account has been renamed as part of single-user login finalisation. If you own this account you can |log in using your previous username and password for more information. If you do not like this account's new name, you can choose your own using this form after logging in: . -- Keegan (WMF) (talk) 05:03, 19 April 2015 (UTC)

title: User talk:JCarriker~enwikibooks


text:
Umbraco/Samples and Articles/Creating umbraco pages programmatically

Creating umbraco pages programmatically
This is based on an original article at http://www.umbraco.org/frontpage/documentation/implementingnetcontrols/creatingumbracopagesprogrammatically.aspx

Getting Started
It's easy to create new pages programmatically using the umbraco api (businesslogic). Just make sure you reference the umbraco.dll (or if running 2.1: cms.dll and businesslogic.dll).

Here's a few lines of sample code that shows how to do it.

using umbraco.cms.businesslogic.web; // The documenttype that should be used, replace 10 with the id of your documenttype

DocumentType dt = new DocumentType(10); // The umbraco user that should create the document, // 0 is the umbraco system user, and always exists

umbraco.BusinessLogic.User u = new umbraco.BusinessLogic.User(0); // The id of the parent document int parent = 100; // Create the document Document d = Document.MakeNew("My weblog post", dt, u, parent); // Add values to the generic properties of the document // (where bodyText is the alias of your property)

d.getProperty("bodyText").Value = "Lorem Ipsum"; // Set the publish status of the document and there by create a new version

d.Publish(u);

// Tell the runtime environment to publish this document

umbraco.library.PublishSingleNode(d.Id);

In umbraco version 2.1, you can use the DocumentType.GetByAlias(string Alias), so you're not dependent on identifiers. This is best practice and very important in order to make your code re-usable. If you're running 2.0, make sure that the id of the documenttype can be changed from umbraco by making a public property.


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Basic Physics of Nuclear Medicine/Scintillation Detectors

The second type of radiation detector we will discuss is called the scintillation detector. Scintillations are minute flashes of light which are produced by certain materials when they absorb radiation. These materials are variously called fluorescent materials, fluors, scintillators or phosphors.

If we had a radioactive source and a scintillator in the lab we could darken the room, move the scintillator close to the source and see the scintillations. These small flashes of light might be green or blue or some other colour depending on the scintillator. We could also count the number of flashes produced to gain an estimate of the radioactivity of the source, that is the more flashes of light seen the more radiation present.

The scintillation detector was possibly the first radiation detector discovered. You might have heard the story of the discovery of X-rays by Wilhelm Roentgen in 1895. He was working one evening in his laboratory in Wurzburg, Germany with a device which fired a beam of electrons at a target inside an evacuated glass tube. While working with this device he noticed that some platino-barium cyanide crystals, which he just happened to have close by, began to glow – and that they stopped glowing when he switched the device off. Roentgen had accidentally discovered a new form of radiation. He had also accidentally discovered a scintillator detector.

Although scintillations can be seen we have a more sophisticated way of counting and measuring them today by using some form of photodetector.

We will learn about the construction and mode of operation of this type of detector in this chapter. In addition, we will see how it can be used not just for detecting the presence of ionizing radiation but also for measuring the energy of that radiation.

Before we do however it is useful to note that scintillators are very widely used in the medical radiations field. For example the X-ray cassette used in radiography contains a scintillator (called an intensifying screen) in close contact with a photographic film. A second example is the X-ray Image Intensifier used in fluoroscopy which contains scintillators called phosphors. Scintillators are also used in some CT Scanners and as we will see in the next chapter, in the Gamma Camera and PET Scanner. Their application is not limited to the medical radiations field in that scintillators are also used as screens in television sets and computer monitors and for generating light in fluorescent tubes – to mention just two common applications. What other applications can you think of?

So scintillators are a lot more common than you might initially think and you will therefore find the information presented here useful to you not just for your studies of nuclear medicine.

Fluorescent Materials
Some fluorescent materials are listed in the following table. Thallium-activated sodium iodide, NaI(Tl) is a crystalline material which is widely used for the detection of gamma-rays in scintillation detectors. We will be looking at this in more detail below.

Another crystalline material sodium-activated caesium iodide, CsI(Na) is widely used for X-ray detection in devices such as the X-ray image intensifier. Another one called calcium tungstate, CaWO4 has been widely used in X-ray cassettes although this substance has been replaced by other scintillators such as lanthanum oxybromide in many modern cassettes.

Notice that some scintillation materials are activated with certain elements. What this means is that the base material has a small amount of the activation element present. The term doped is sometimes used instead of activated. This activating element is used to influence the wavelength (colour) of the light produced by the scintillator.

Silver-activated zinc sulphide is a scintillator in powder form and p-terphenyl in toluene is a liquid scintillator. The advantage of such forms of scintillators is that the radioactive material can be placed in close contact with the scintillating material. For example if a radioactive sample happened to be in liquid form we could mix it with a liquid scintillator so as to optimise the chances of detection of the emitted radiation and hence have a very sensitive detector.

A final example is p-terphenyl in polystyrene which is a scintillator in the form of a plastic. This form can be easily made into different shapes like most plastics and is therefore useful when detectors of particular shapes are required.

Photomultiplier Tube
A scintillation crystal coupled to a photomultiplier tube (PMT) is illustrated in the following figure. The overall device is typically cylindrical in shape and the figure shows a cross-section through this cylinder:

The scintillation crystal, NaI(Tl) is very delicate and this is one of the reasons it is housed in an aluminium casing. The inside wall of the casing is designed so that any light which strikes it is reflected downwards towards the PMT.

The PMT itself consists of a photocathode, a focussing grid, an array of dynodes and an anode housed in an evacuated glass tube. The function of the photocathode is to convert the light flashes produced by radiation attenuation in the scintillation crystal into electrons. The grid focuses these electrons onto the first dynode and the dynode array is used for electron multiplication. We will consider this process in more detail below. Finally the anode collects the electrons produced by the array of dynodes.

The electrical circuitry which is typically attached to a PMT is shown in the next figure:

It consists of a high voltage supply, a resistor divider chain and a load resistor, RL. The high voltage supply generates a dc voltage, Vdc which can be up to 1,000 volts. It is applied to the resistor divider chain which consists of an array of resistors, each of which has the same resistance, R. The function of this chain of resistors is to divide up Vdc into equal voltages which are supplied to the dynodes. As a result voltages which increase in equal steps are applied to the array of dynodes. The load resistor is used so that an output voltage, Vout can be generated.

Finally the operation of the device is illustrated in the figure below:

The ionizing radiation produces flashes of light in the scintillation crystal. This light strikes the photocathode and is converted into electrons. The electrons are directed by the grid onto the first dynode.

Dynodes are made from certain alloys which emit electrons when their surface is struck by electrons with the advantage that more electrons are emitted than are absorbed. A dynode used in a PMT typically emits between two and five electrons for each electron which strikes it.

So when an electron from the photocathode strikes the first dynode between two and five electrons are emitted and are directed towards the second dynode in the array (three are illustrated in the figure). This electron multiplication process is repeated at the second dynode so that we end up with nine electrons for example heading towards the third dynode. An electron avalanche therefore develops so that a sizeable number of electrons eventually hits the anode at the bottom of the dynode chain.

These electrons flow through the load resistor, RL and constitute an electric current which according to Ohm's Law generates a voltage, Vout which is measured by electronic circuitry (which we will describe later).

A number of photographs of devices based on scintillation detection are shown below:

The important feature of the scintillation detector is that this output voltage, Vout is directly proportional to the energy deposited by the radiation in the crystal. We will see what a useful feature this is below. Before we do so we will briefly analyze the operation of this device.

Mathematical Model
A simple mathematical model will be presented below which will help us get a better handle on the performance of a scintillation detector. We will do this by quantifying the performance of the scintillator, the photocathode and the dynodes.

Let's use the following symbols to characterize each stage of the detection process:

 * m: number of light photons produced in crystal
 * k: optical efficiency of the crystal, that is the efficiency with which the crystal transmits light
 * l: quantum efficiency of the photocathode, that is the efficiency with which the photocathode converts light photons to electrons
 * n: number of dynodes
 * R: dynode multiplication factor, that is the number of secondary electrons emitted by a dynode per primary electron absorbed.

Therefore the charge collected at the anode is given by the following equation:

$$Q = m k l R^n e\,\!$$

where e: the electronic charge.

For example supposing a 100 keV gamma-ray is absorbed in the crystal. The number of light photons produced, m, might be about 1,000 for a typical scintillation crystal. A typical crystal might have an optical efficiency, k, of 0.5 – in other words 50% of the light produced reaches the photocathode which might have a quantum efficiency of 0.15. A typical PMT has ten dynodes and let us assume that the dynode multiplication factor is 4.5.

Therefore

$$Q = 1000(0.5)(0.15)(4.5^{10})(1.6 \cdot 10^{-19})\ \text{C}$$

$$\therefore Q = 41 \cdot 10^{-12}\ \text{C}$$

$$\therefore Q \approx 40\ \text{pC}$$

This amount of charge is very small. Even though we have used a sophisticated photodetector like a PMT we still end up with quite a small electrical signal.

A very sensitive amplifier is therefore needed to amplify this signal. This type of amplifier is generally called a pre-amplifier and we will refer to it again later.

Output Voltage
We noted above that the voltage measured across the resistor, RL, is proportional to the energy deposited in the scintillation crystal by the radiation. Let us consider how the radiation might deposit its energy in the crystal.

Let us consider a situation where gamma-rays are detected by the crystal. We learnt in Chapter 5 that there were two interaction mechanisms involved in gamma-ray attenuation – the Photoelectric Effect and the Compton Effect. You will remember that the Photoelectric Effect involves the total absorption of the energy of a gamma-ray, while the Compton Effect involves just partial absorption of this energy. Since the output voltage of a scintillation detector is proportional to the energy deposited by the gamma-rays it is reasonable to expect that Photoelectric Effects in the crystal will generate distinct and relatively large output voltages and that Compton Effects will result in lower output voltages.

The usual way of presenting this information is by plotting a graph of the count rate versus the output voltage pulse height as shown in the following figure:

This plot illustrates what is obtained for a monoenergetic gamma-emitting radioisotope, for example 99mTc – which, as we have noted before emits a single gamma-ray with an energy of 140 keV.

Before we look at it in detail remember that we noted above that the output voltage from this detector is proportional to the energy deposited by the radiation in the crystal. The horizontal axis can therefore be used to represent the output voltage or the gamma-ray energy. Both of these quantities are shown in the figure to help with this discussion. In addition note that this plot is often called a Gamma-Ray Energy Spectrum.

The figure above contains two regions. One called the Photopeak and the other called the Compton Smear. The Photopeak results because of Photoelectric absorption of the gamma-rays from the radioactive source – remember that we are dealing with a monoenergetic emitter in this example. It consists of a peak representing the gamma-ray energy (140 keV in our example). If our radioisotope emitted gamma-rays of two energies we would have two photopeaks in our spectrum and so on.

Notice that the peak has a statistical spread. This has to do with how good our detector is and we will not get into any detail about it here other than to note that the extent of this spread is a measure of the quality of our detector. A high quality (and more expensive!) detector will have a narrower statistical spread in the photopeaks which it measures.

The other component of our spectrum is the Compton Smear. It represents a range of output voltages which are lower than that for the Photopeak. It is therefore indicative of the partial absorption of the energy of gamma-rays in the crystal. In some Compton Effects a substantial scattering with a valence electron can occur which gives rise to relatively large voltage pulses. In other Compton Effects the gamma-ray just grazes off a valence electron with minimal energy transfer and hence a relatively small voltage pulse is generated. In between these two extremes are a range of scattering events involving a range of energy transfers and hence a range of voltage pulse heights. A 'smear' therefore manifests itself on the gamma-ray energy spectrum.

It is important to note that the spectrum illustrated in the figure is simplified for the sake of this introductory discussion and that actual spectra are a little more complex – see figure below for an example:

You will find though that your understanding of actual spectra can easily develop on the basis of the simple picture we have painted here.

It is also important to appreciate the additional information which this type of radiation detector provides relative to a gas-filled detector. In essence gas-filled detectors can be used to tell us if any radiation is present as well as the amount of that radiation. Scintillation detectors also give us this information but they tell us about the energy of this radiation as well. This additional information can be used for many diverse applications such as the identification of unknown radioisotopes and the production of nuclear medicine images. Let us stay a little bit longer though with the fundamental features of how scintillation detectors work.

The photopeak of the Gamma-Ray Energy Spectrum is generally of interest in nuclear medicine. This peak is the main signature of the radioisotope being used and its isolation from the Compton Smear is normally achieved using a technique called Pulse Height Analysis.

Pulse Height Analysis
This is an electronic technique which allows a spectrum to be acquired using two types of circuitry. One circuit is called a Lower Level Discriminator which only allows voltages pulses through it which are higher than its setting. The other is called an Upper Level Discriminator which only allows voltage pulses though which are lower than its setting.

The result of using both these circuits in combination is a variable-width window which can be placed anywhere along a spectrum. For example if we wished to obtain information from the photopeak only of our simplified spectrum we would place the discrimination controls as shown in the following figure:

A final point to note here is that since the scintillation detector is widely used to obtain information about the energies of the radiation emitted from a radioactive source it is frequently referred to as a Scintillation Spectrometer.

Scintillation Spectrometer
Types of scintillation spectrometer fall into two basic categories – the relatively straight-forward Single Channel Analyser and the more sophisticated Multi-Channel Analyser.

The Single Channel Analyser is the type of instrument we have been describing so far in this discussion. A block diagram of the instrument is shown below:

It consists of a scintillation crystal coupled to a photomultiplier tube which is powered by a high voltage circuit (H.V.). The output voltages are initially amplified by a sensitive pre-amplifier (Pre-Amp) as we noted above before being amplified further and conditioned by the amplifier (Amp).

The voltage pulses are then in a suitable form for the pulse height analyser (P.H.A.) – the output pulses from which can be fed to a Scaler and a Ratemeter for display of the information about the portion of the spectrum we have allowed to pass through the PHA. The Ratemeter is a display device just like the speedometer in a car and indicates the number of pulses generated per unit time. The Scaler on the other hand usually consists of a digital display which shows the number of voltage pulses produced in a specified period of time.

We can illustrate the operation of this circuitry by considering how it might be used to generate a Gamma-Ray Energy Spectrum. What we would do is set up the LLD and ULD so as to define a narrow window and place this to pass the lowest voltage pulses produced by the detector through to the Scaler and Ratemeter. In other words we would place a narrow window at the extreme left of the spectrum and acquire information about the lowest energy gamma-ray interactions in the crystal. We would then adjust the LLD and ULD settings to acquire information about the interactions of the next highest energy. We would proceed in this fashion to scan the whole spectrum.

A more sophisticated detector circuit is illustrated in the following figure:

It is quite similar to that in the previous figure with the exception that the PHA, Scaler and Ratemeter are replaced by a Multi-Channel Analyser and a computer. The Multi-Channel Analyser (MCA) is a circuit which is capable of setting up a large number of individual windows to look at a complete spectrum in one go. The MCA might consist of 1024 individual windows for example and the computer might consist of a personal computer which can acquire information simultaneously from each window and display it as an energy spectrum. The computer generally contains software which allows us to manipulate the resultant information in a variety of ways. Indeed the 137Cs spectrum shown above was generated using this approach.


text:
Talk:Cherokee

Untitled
I think it's a great idea to create a Cherokee Wikibook. I have always been interested in learning a Native American language. Are you going to create lessons? If so, please read the article Bite-sized language lessons for some ideas of how to go about it. Junesun 09:19, 6 June 2006 (UTC)

Tones
As Cherokee is a tonal language, the tones should be explained and marked in the transliterations of vocabulary words.

title: Talk:Cherokee


text:
Basic Physics of Nuclear Medicine/Nuclear Medicine Imaging Systems

Topics we have covered in this wikibook have included radioactivity, the interaction of gamma-rays with matter and radiation detection. The main reason for following this pathway was to bring us to the subject of this chapter: nuclear medicine imaging systems. These are devices which produce pictures of the distribution of radioactive material following administration to a patient.

The radioactivity is generally administered to the patient in the form of a radiopharmaceutical – the term radiotracer is also used. This follows some physiological pathway to accumulate for a short period of time in some part of the body. A good example is 99mTc-tin colloid which following intravenous injection accumulates mainly in the patient's liver. The substance emits gamma-rays while it is in the patient's liver and we can produce an image of its distribution using a nuclear medicine imaging system. This image can tell us whether the function of the liver is normal or abnormal or if sections of it are damaged from some form of disease.

Different radiopharmaceuticals are used to produce images from almost every region of the body:

Note that the form of information obtained using this imaging method is mainly related to the physiological functioning of an organ as opposed to the mainly anatomical information which is obtained using X-ray imaging systems. Nuclear medicine therefore provides a different perspective on a disease condition and generates additional information to that obtained from X-ray images. Our purpose here is to concentrate on the imaging systems used to produce the images.

Early forms of imaging system used in this field consisted of a radiation detector (a scintillation detector for example) which was scanned slowly over a region of the patient in order to measure the radiation intensity emitted from individual points within the region. One such device was called the Rectilinear Scanner. Such imaging systems have been replaced since the 1970s by more sophisticated devices which produce images much more rapidly. The most common of these modern devices is called the Gamma Camera and we will consider its construction and mode of operation below. A review of recent developments in this technology for cardiac applications can be found in Slomka et al (2009).

Gamma Camera
The basic design of the most common type of gamma camera used today was developed by an American physicist, Hal Anger and is therefore sometimes called the Anger Camera. It consists of a large diameter NaI(Tl) scintillation crystal which is viewed by a large number of photomultiplier tubes.

A block diagram of the basic components of a gamma camera is shown below:

The crystal and PM Tubes are housed in a cylindrical shaped housing commonly called the camera head and a cross-sectional view of this is shown in the figure. The crystal can be between about 25 cm and 40 cm in diameter and about 1 cm thick. The diameter is dependent on the application of the device. For example a 25 cm diameter crystal might be used for a camera designed for cardiac applications while a larger 40 cm crystal would be used for producing images of the lungs. The thickness of the crystal is chosen so that it provides good detection for the 140 keV gamma-rays emitted from 99mTc – which is the most common radioisotope used today.

Scintillations produced in the crystal are detected by a large number of PM tubes which are arranged in a two-dimensional array. There are typically between 37 and 91 PM tubes in modern gamma cameras. The output voltages generated by these PM tubes are fed to a position circuit which produces four output signals called ±X and ±Y. These position signals contain information about where the scintillations were produced within the crystal. In the most basic gamma camera design they are fed to a cathode ray oscilloscope (CRO). We will describe the operation of the CRO in more detail below.

Before we do so we should note that the position signals also contain information about the intensity of each scintillation. This intensity information can be derived from the position signals by feeding them to a summation circuit (marked ∑ in the figure) which adds up the four position signals to generate a voltage pulse which represents the intensity of a scintillation. This voltage pulse is commonly called the Z-pulse which, following pulse height analysis, (PHA) is fed as the unblank pulse to the CRO.

So we end up with four position signals and an unblank pulse sent to the CRO. Let us briefly review the operation of a CRO before we continue. The core of a CRO consists of an evacuated tube with an electron gun at one end and a phosphor-coated screen at the other end. The electron gun generates an electron beam which is directed at the screen and the screen emits light at those points struck by the electron beam. The position of the electron beam can be controlled by vertical and horizontal deflection plates and with the appropriate voltages fed to these plates the electron beam can be positioned at any point on the screen. The normal mode of operation of an oscilloscope is for the electron beam to remain switched on. In the case of the gamma camera the electron beam of the CRO is normally switched off – it is said to be blanked.

When an unblank pulse is generated by the PHA circuit the electron beam of the CRO is switched on for a brief period of time so as to display a flash of light on the screen. In other words the voltage pulse from the PHA circuit is used to unblank the electron beam of the CRO.

So where does this flash of light occur on the screen of the CRO? The position of the flash of light is dictated by the ±X and ±Y signals generated by the position circuit. These signals as you might have guessed are fed to the deflection plates of the CRO so as to cause the unblanked electron beam to strike the screen at a point related to where the scintillation was originally produced in the NaI(Tl) crystal. Simple!

The gamma camera can therefore be considered to be a sophisticated arrangement of electronic circuits used to translate the position of a flash of light in a scintillation crystal to a flash of light at a related point on the screen of an oscilloscope. In addition the use of a pulse height analyser in the circuitry allows us to translate the scintillations related only to photoelectric events in the crystal by rejecting all voltage pulses except those occurring within the photopeak of the gamma-ray energy spectrum.

Let us summarise where we have got to before we proceed. A radiopharmaceutical is administered to the patient and it accumulates in the organ of interest. Gamma-rays are emitted in all directions from the organ and those heading in the direction of the gamma camera enter the crystal and produce scintillations (note that there is a device in front of the crystal called a collimator which we will discuss later). The scintillations are detected by an array of PM tubes whose outputs are fed to a position circuit which generates four voltage pulses related to the position of a scintillation within the crystal. These voltage pulses are fed to the deflection circuitry of the CRO. They are also fed to a summation circuit whose output (the Z-pulse) is fed to the PHA and the output of the PHA is used to switch on (that is, unblank) the electron beam of the CRO. A flash of light appears on the screen of the CRO at a point related to where the scintillation occurred within the NaI(Tl) crystal. An image of the distribution of the radiopharmaceutical within the organ is therefore formed on the screen of the CRO when the gamma-rays emitted from the organ are detected by the crystal.

What we have described above is the operation of a fairly traditional gamma camera. Modern designs are a good deal more complex but the basic design has remained much the same as has been described. One area where major design improvements have occurred is the area of image formation and display. The most basic approach to image formation is to photograph the screen of the CRO over a period of time to allow integration of the light flashes to form an image on photographic film. A stage up from this is to use a storage oscilloscope which allows each flash of light to remain on the screen for a reasonable period of time.

The most modern approach is to feed the position and energy signals into the memory circuitry of a computer for storage. The memory contents can therefore be displayed on a computer monitor and can also be manipulated (that is processed) in many ways. For example various colours can be used to represent different concentrations of a radiopharmaceutical within an organ.

The use of digital image processing is now widespread in nuclear medicine in that it can be used to rapidly and conveniently control image acquisition and display as well as to analyse an image or sequences of images, to annotate images with the patient's name and examination details, to store the images for subsequent retrieval and to communicate the image data to other computers over a network.

The essential elements of a modern gamma camera are shown in the next figure. Gamma rays emitted by the patient pass through the collimator and are detected within the camera head, which generates data related to the location of scintillations in the crystal as well as to the energy of the gamma rays. This data is then processed on-the-fly by electronic hardware which corrects for technical factors such as spatial linearity, PM tube drift and energy response so as to produce an imaging system with a spatially-uniform sensitivity and distortion-free performance.

A multichannel analyzer (MCA) is used to display the energy spectrum of gamma rays which interact inside the crystal. Since these gamma rays originate from within the patient, some of them will have an energy lower than the photopeak as a result of being scattered as they travel through the patient's tissues – and by other components such as the patient table and structures of the imaging system. Some of these scattering events may involve just glancing interactions with free electrons, so that the gamma rays lose only a small amount of energy. These gamma rays may have an energy just below that of the photopeak so that their spectrum merges with the photopeak. The photopeak for a gamma camera imaging a patient therefore contains information from spatially-correlated, unattenuated gamma rays (which is the information we want) and from spatially-uncorrelated, scattered gamma rays. The scattered gamma rays act like a variable background within the true photopeak data and the effect is that of a background haze in gamma camera images.

While scatter may not be a significant problem in planar scintigraphy, it has a strong bearing on the fidelity of quantitative information derived from gamma camera images and is a vital consideration for accurate image reconstruction in emission tomography. It is the unattenuated gamma rays (also called the primary radiation) that contain the desired information, because of their direct dependence on radioactivity.

The scatter situation is illustrated in more detail in the figure below, which shows estimates of the primary and scatter spectra for 99mTc in patient imaging conditions. Such spectral estimates can be generated using Monte Carlo methods. It is seen in the figure that the energy of the scattered radiation forms a broad band, similar to the Compton Smear described previously, which merges into and contributes substantially to the detected photopeak. The detected photopeak is therefore an overestimate of the primary radiation. The extent of this overestimate is likely to be dependent on the specific imaging situation because of the different thicknesses of tissues involved. It is clear however that the scatter contribution within the detected photopeak needs to be accounted for if an accurate measure of radioactivity is required.

One method of compensating for the scatter contribution is illustrated in the figure below and involves using data from a lower energy window as an estimate for subtraction from the photopeak, i.e.

Primary Counts =Photopeak Window Counts- k (Scatter Window Counts)

where k is a scaling factor to account for the extent of the scatter contribution. This approach to scatter compensation is referred to as the Dual-Energy Window (DEW) method. It can be implemented in practice by acquiring two images, one for each energy window, and subtracting a fraction (k) of the scatter image from the photopeak image.

For the spectrum shown above, it can be seen that the scaling factor, k, is about 0.5, but it should be appreciated that its exact value is dependent on the scattering conditions. Gamma cameras which use the DEW method therefore generally provide the capability of adjusting k for different imaging situations. Some systems use a narrower scatter window than that illustrated, e.g. 114-126 keV, with a consequent increase in k to about 1.0, for instance.

A host of other methods of scatter compensation have also been developed. These include more complex forms of energy analysis such as the Dual-Photopeak and the Triple-Energy Window techniques, as well as approaches based on deconvolution and models of photon attenuation. An excellent review of these developments is provided in Zaidi & Koral (2004).

Some photographs of gamma cameras and related devices are shown below:

We will continue with our description of the gamma camera by considering the construction and purpose of the collimator.

Collimation
The collimator is a device which is attached to the front of the gamma camera head. It functions something like a lens used in a photographic camera but this analogy is not quite correct because it is rather difficult to focus gamma-rays. Nevertheless in its simplest form it is used to block out all gamma rays which are heading towards the crystal except those which are travelling at right angles to the plane of the crystal:

The figure illustrates a magnified view of a parallel-hole collimator attached to a crystal. The collimator simply consists of a large number of small holes drilled in a lead plate. Notice that gamma-rays entering at an angle to the crystal get absorbed by the lead and that only those entering along the direction of the holes get through to cause scintillations in the crystal. If the collimator was not in place these obliquely incident gamma-rays would blur the images produced by the gamma camera. In other words the images would not be very clear.

Most gamma cameras have a number of collimators which can be fitted depending on the examination. The basic design of these collimators is the same except that they vary in terms of the diameter of each hole, the depth of each hole and the thickness of lead between each hole (commonly called the septum thickness). The choice of a specific collimator is dependent on the amount of radiation absorption that occurs (which influences the sensitivity of the gamma camera), and the clarity of images (that is the spatial resolution) it produces. Unfortunately these two factors are inversely related in that the use of a collimator which produces images of good spatial resolution generally implies that the instrument is not very sensitive to radiation.

Other collimator designs beside the parallel hole type are also in use. For example a diverging hole collimator produces a minified image and converging hole and pin-hole collimators produce a magnified image. The pin-hole collimator is illustrated in the following figure:

It is typically a cone-shaped device with its walls made from lead. A cross-section through this cone is shown in the figure. It operates in a similar fashion to a pin-hole photographic camera and produces an inverted image of an object – an arrow is used in the figure to illustrate this inversion. This type of collimator has been found useful for imaging small objects such as the thyroid gland.

Example Images
A representative selection of nuclear medicine images is shown below:

Emission Tomography
The form of imaging which we have been describing is called Planar Imaging. It produces a two-dimensional image of a three-dimensional object. As a result images contain no depth information and some details can be superimposed on top of each other and obscured or partially obscured as a result. Note that this is also a feature of conventional X-ray imaging.

The usual way of trying to overcome this limitation is to take at least two views of the patient, one from the front and one from the side for example. So in chest radiography a posterio-anterior (PA) and a lateral view can be taken. And in a nuclear medicine liver scan an antero-posterior (AP) and lateral scan are acquired.

This limitation of planar X-ray imaging was overcome by the development of the CAT Scanner about 1970 or thereabouts. CAT stands for Computerized Axial Tomography or Computer Assisted Tomography and today the term is often shortened to Computed Tomography or CT scanning (the term tomography comes from the Greek word tomos meaning slice). Irrespective of its exact name the technique allows images of slices through the body to be produced using a computer. It does this in essence by taking X-ray images at a number of angles around the patient. These slice images show the third dimension which is missing from planar images and thus eliminate the problem of superimposed details. Furthermore images of a number of successive slices through a region of the patient can be stacked on top of each other using the computer to produce a three-dimensional image. Clearly CT scanning is a very powerful imaging technique relative to planar imaging.

The equivalent nuclear medicine imaging technique is called Emission Computed Tomography. We will consider two implementations of this technique below.

Single Photon Emission Computed Tomography (SPECT)

 * This SPECT technique uses a gamma camera to record images at a series of angles around the patient. These images are then subjected to a form of digital image processing called Image Reconstruction in order to compute images of slices through the patient.

 * The Back Projection reconstruction process is illustrated below. Let us assume for simplicity that the slice through the patient actually consists of a 2x2 voxel array with the radioactivity in each voxel given by A1...A4:

 * The first projection, P1, is imaged from the right and the second projection, P2, from the right oblique and so on. The back projection process involves firstly adding the projections to each other as shown below:

 * and then normalising the summed (or superimposed) projections to generate an estimate of the radioactivity in each voxel. Since this process can generate streaking artefacts in reconstructed images, the projections are generally filtered prior to back projection, as described in a later chapter, with the overall process referred to as Filtered Back Projection (FBP):

 * An alternative image reconstruction technique is called Iterative Reconstruction, a successive approximation technique. The Maximum-Likelihood Expectation-Maximisation (ML-EM) algorithm is widely applied where a division process is used to compare the actual and estimated projections, as shown below:

 * One cycle of data through this processing chain is referred to as one iteration. Sixteen or more iterations can be required in order to generate an adequate reconstruction and, as a result, computation times can be rather long.  The Ordered-Subsets Expectation-Maximisation (OS-EM) algorithm can be used to substantially reduce the computation time by utilising a limited number of projections (called subsets) in a sequential fashion within the iterative process.  Noise generated during the reconstruction process can be reduced, for example, using a Gaussian filter built into the reconstruction calculations or applied as a post-filter:

 * Images generated using different iterations, subsets and filtration settings can be found in an online book.

 * A comparison of these image reconstruction techniques is shown below for a slice through a ventilation scan of a patient's lungs:

 * The gamma camera is typiclly rotated around the patient in order to acquire the images. Modern gamma cameras which are designed specifically for SPECT scanning can consist of two camera heads mounted parallel to each other with the patient in between.  The time required to produce images is therefore reduced by a factor of about two.  In addition some SPECT gamma cameras designed for brain scanning have three camera heads mounted in a triangular arrangement.

 * A wide variety of strategies can be used for the acquisition and processing of SPECT images.

Positron Emission Tomography (PET)

 * You will remember from chapter 2 that positrons can be emitted from radioactive nuclei which have too many neutrons for stability. You will also remember that positrons do not last for very long in matter since they will quickly encounter an electron and a process called annihilation results.  In the process the positron and electron vanish and their energy is converted into two gamma-rays which are emitted at roughly 180o degrees to each other.  The emission is often referred to as two back-to-back gamma-rays and they each have a discrete energy of 0.51 MeV.

 * So if we administer a positron-emitting radiopharmaceutical to a patient an emitted positrons can annihilate with a nearby electron and two gamma-rays will be emitted in opposite directions. These gamma-rays can be detected using a ring of radiation detectors encircling the patient and tomographic images can be generated using a computer system.  The detectors are typically specialised scintillation devices which are optimised for detection of the 0.51 MeV gamma-rays.  This ring of detectors, associated apparatus and computer system are called a PET Scanner:

 * The locations of positron decays within the patient are highlighted by the solid circles in the above diagram. In addition only a few detectors are shown in the diagram for reasons of clarity.  Each detector around the ring is operated in coincidence with a bank of opposing detectors and the annihilation gamma-rays thus detected are used to build up a single profile.

 * It has also been found that gamma cameras fitted with thick crystals and special collimators can be used for PET scanning.

 * The radioisotopes used for PET scanning include 11C, 13N, 15O and 18F. These isotopes are usually produced using an instrument called a cyclotron.  In addition these isotopes have relatively short half lives.  PET scanning therefore needs a cyclotron and associated radiopharmaceutical production facilities located close by.  We will consider cyclotrons in the  next chapter of this wikibook.

 * Standardized Uptake Value (SUV) is a semi-quantitative index used in PET to express the uptake of a radiopharmaceutical in a region of interest of a patient's scan. Its typically calculated as the ratio of the radioactivity in the region to the injected dose, corrected for body weight.  It should be noted that the SUV is influenced by several major sources of variability and it therefore should not be used as a quantitative measure.

 * A number of photographs of a PET scanner are shown below:


text:
Modern Greek/Appendix I

= Summary of Declensions and Conjugations =

Group 1
Σκύλος, dog, is a masculine noun. Note the different forms of the definite article.

Group 1
The noun Η ώρα (the hour, the time) is shown below:

Group 1
Το παιδί (the child)

The verbs έχω, to have, and είμαι, to be
Two important verbs are έχω, to have, and  είμαι, to be.  The first-conjugation verb  έχω is regular in the present tense, so it has the same endings as γράφω.

To be in English is expressed in the active voice, but the Greek είμαι is passive, and doesn't have an active form. The ending -μαι is a typical, regular ending for passive verbs.

Pronouns
Pronouns have cases, number and gender just as regular nouns do. To make the tables appear simpler, the pronouns have been separated into persons.

Links
| A page with more on pronouns

title: Modern Greek/Appendix I


text:
Talk:Modern Greek/Restructuring

Untitled
This is in response to: ''The question is whether it is better to have the Dialog in the second lesson but be unable to fully explain it. Or to have it later and be able to explain everything.--IKnowNothing 20:49, 5 June 2006 (UTC)''

I believe that every lesson should contain a text or dialogue, because many people are unable to understand grammar and apply it if they don't see it in use. Also, seeing how the stuff taught in one lesson helps one get by in a real-life situation is a very motivating factor for the learner.

I am sure that it would be possible to start every lesson (except the one teaching the alphabet) with a small dialogue, without introducing too much grammar or vocabulary at once. German is similar in grammar complexity as Greek and see how it was accomplished there: BLL German/A1. Also see my comment to the discussion at the main Modern Greek page.

Mind you, it's even easier to have a meaningful text or dialogue in each lesson if you're ready to introduce several conjugated forms or several cases at once. I still believe that splitting is the key to learner motivation though, particularly for those who haven't studied Latin yet. To provide a general overview of every difficulty a language has to offer on a particular area of grammar is the task of a grammar book, not a language course. You could link to the complete tables for those who'd rather try to memorise everything at once.

Junesun 13:30, 6 June 2006 (UTC)

title: Talk:Modern Greek/Restructuring


text:
Managing Groups and Teams/How Do You Build High-performing Virtual Teams?

Introduction
“Virtual teams are groups of geographically and/or temporally dispersed individuals brought together via information and telecommunication technologies.” (Piccoli and Ives, 2003, p365) Virtual teams are increasingly becoming a key feature of projects in modern organizations, while the landscape of communication tools continues to change dramatically. The benefits of virtualization include diversity of perspectives, large member selection pool, extended hours of productivity, and reduced transportation costs.

However, many new challenges arise, such as difficulty in performance monitoring, disappearance of social cues, member isolation and anonymity, and technology issues. In addition, virtual teams must deal with cultural, temporal, and geographic barriers. Using a framework based on Larson and LaFasto’s Teamwork, conventional team strategies have been adapted for the virtual context to help virtual team leaders and members overcome these challenges.

A Clear and Elevating Goal
Larson and LaFasto (1989) stress the importance of a clear and elevating goal in the performance of an effective team. They define goal clarity as “a specific performance objective, phrased in such concrete language that it is possible to tell, unequivocally, whether or not that performance objective has been attained” (p28) and elevating as “personally or collectively” challenging and that it “makes a difference.” (p31) While Larson and LaFasto consider this to possibly be the most important component of an effectively functioning team, it appears to be even more critical for a virtual team.

To stress this point, Kirkman and Rosen, et al. (2002) quote a team member from Sabre, Inc. as saying “virtual teams need to understand much more so than co-located teams what goal they are working towards because you are working in such different areas, and in our case, in different countries. It plays a much stronger role if you know what your ultimate target is going to be. Everyone is working towards the same thing.” Further work on the performance of virtual teams by Kirkman and Rosen, et al. (2004) demonstrates a positive correlation between empowerment and virtual team performance. They define empowerment as having four dimensions, two of which, potency and impact significantly overlap the concepts of goal clarity and elevation.

This research therefore supports and elevates the arguments made by Larson and LaFasto about the importance of a clear and elevating goal for virtual teams. Virtual team managers should make sure to have a clear and elevating goal for the group and should make sure to prominently display the team mission statement on the virtual work space, team web-site, and other electronic communications to the group.

Results-Driven Structure
Team structure is a key factor which differentiates successful teams from unsuccessful ones. Team structure encompasses many aspects such as tools, processes, communications systems, facilities, and organization of the team. However, the best structure depends on the objectives and composition of the team.

Virtual teams tend to be very effective in project development tasks. However, the lack of face to face contact can be a challenge in building consensus. For decision-making teams, having the right team composition can help to overcome this. Collectivistic teams have higher levels of collaborative conflict management than individualistic teams, and are motivated more by outcomes which benefit the entire team. (Paul, et al, 2005)

Research has also shown that highly diverse or heterogeneous teams outperform homogenous teams in many performance measures, though they take longer to reach consensus. Virtuality can be beneficial for highly diverse teams since it obfuscates potentially divisive demographic differences. (Kirkman et al, 2002)

Larson and LaFasto outlined four necessary features of effective team structure, which apply equally to virtual teams, but become more challenging.

First, team members must have clear roles and accountabilities. Lack of visibility may cause virtual team members to feel less accountable for results, therefore explicit facilitation of teamwork takes on heightened importance for virtual teams. Temporal coordination mechanisms such as scheduling deadlines and coordinating the pace of effort are recommended to increase vigilance and accountability. (Massey, Montoya-Weiss, and Hung, 2003)

Second, teams require effective communications systems. This is especially important for virtual teams because geographic and temporal differences may rule out the most common communication channels. “Many in our study found e-mail a poor way for teams as a whole to collaborate.” (Majchzak et al, 2004) Individual email conversations cause others to feel left out, but copying everyone causes email overload. Sharing documents over email often leads to conflicting versions. Many successful teams prefer to use virtual work spaces, online forums, instant messaging, and file repositories. Regardless of the communication system, team norms and rewards structures should support use of the systems.

Third, effective teams should monitor individual performance and provide feedback. Assessment and development of virtual team members is very limited in the virtual environment. Performance management is an enormous challenge when employees are out of sight. Managers should monitor group communication to assess subjective factors such as idea generation, leadership, and problem-solving skills. (Kirkman et al, 2002) Utilizing peer and customer feedback helps assess contributions to team effectiveness. Managers should also consider using "richer" communication media (such as video conferencing) to more effectively deliver evaluation feedback.

Finally, teams must rely on fact-based decision making, for which information and communication tools are vital. Teams can adapt decision-making software to facilitate fact-based problem solving and decision-making. (Kirkman et al, 2002) They might also assign one person to ensure accurate information is clearly communicated (Piccoli and Ives, 2003). Rocketdyne, for example, used collaborative technology to manage knowledge. They allowed all communications to be recorded and all information to be visible to the team, and even prohibited face to face discussions. (Malhotra, Majchzak, Carmen, Lott)

Members
Larson and LaFasto recognize three common features of competent team members: technical skills and abilities, desire to contribute, and capability of collaborating effectively. Virtual teams amplify the importance of using appropriate criteria when selecting people for the team. A critical element in high-performance teams is creating functional diversity among team members, while productively managing resulting task conflict. Work group studies suggest that “such conflict evidently fosters a deeper understanding of task issues and an exchange of information that facilitates problem solving, decision making, and the generation of ideas.” (Pelled, et al, 1999, p22)  A best-practice study of successful virtual teams concludes that diversity among disciplines, working styles, and problem-solving approaches can be exploited to produce “solutions instead of acrimony.”  (Majchrzak, et al, 2004, p133)  Conflict researchers have found that task conflicts can improve team performance if managed collaboratively. (Weingart and Jehn, 2000)

However, regardless of other qualifications, not everyone can handle the social isolation of a virtual team. Maintaining this challenging environment requires selection of team members with interpersonal skills, self-regulatory skills, a high level of knowledge, and comfort with technology. (Gibson and Cohen, 2003) This critical balance between technical and interpersonal skills must include the ability to work with others to identify, address, and resolve issues.

Managers should consider using behavioral interviewing techniques and simulations to select team members with unique areas of competence that will contribute to a high-quality solution. Ideally some members should have team process backgrounds. Managers should also provide potential team members with a realistic assessment of virtual team challenges, and the choice to opt out.

Unified Commitment
Larson and LaFasto suggest that lack of unified commitment is often the most important feature of ineffective teams. They identify two key elements: dedication to the endeavor (commitment) and dedication to the team (unity).

High-performance teams are distinguished by passionate dedication to goals, identification and emotional bonding among team members, and a balance between unity and respect for individual differences. Virtual teams face the challenge of developing and sustaining unified commitment in the absence of face-to-face contact. In particular, they must identify and deal with the most serious threat, competition between individual and team goals.

Kerber and Buono recommend appointing a strong team leader, willing to maintain frequent contact with team members, take full advantage of collaborative technologies, demonstrate a high level of personal commitment, and deal quickly with self-serving and non-contributing team members. Larson and LaFasto suggest that commitment can be enhanced by involving team members in project planning and in defining team identity, goals, and processes. Virtual team leaders should identify commonalities among members early on, while focusing the team on achieving key performance objectives and providing a clear context for recognizing team success.

Finally, interdependence of goals, tasks, and outcomes among members of virtual teams can overcome motivational challenges, particularly early on. (Hertel, et al, 2004) Virtual team managers can create the experience of connectedness by delegating goal setting, assigning interdependent tasks, and rewarding both personal contributions and team performance.

Collaborative Climate
“Collaboration flourishes in a climate of trust.” (Larson and LaFasto, 1989, p. 87) Trust is based on social characteristics (familiarity, competence), immediate outcomes of interaction processes (reliability, integrity), and institutions (social norms, policies). Studies have shown that while trust has little impact on task performance, it can significantly reduce process losses. (Jarvenpaa, Shaw, Staples, 2004)

Trust affects how we interpret member behavior. “Trust is the lens through which these factors are interpreted.” (Jarvenpaa, Shaw, and Staples, 2004, p253) Therefore trust plays an important role in virtual teams where ambiguity is high. Unfortunately, building trust is an enormous challenge for virtual teams. "In virtual organizations, trust requires constant face-to-face interaction—the very activity the virtual form eliminates." (Kirkman et al, 2002) Structured opportunities for socialization are less satisfying in virtual environments, and slow development of relational ties.

When a team is formed, expectations about workloads, processes, and contributions lead to “psychological contracts” which can damage trust when broken. (Piccoli and Ives, 2003) Reneging and incongruence are heightened for virtual teams due to the limited ability to communicate. Studies show that in high-trust teams, structured behavior control mechanisms (rules, progress reports, explicit assignments) intensify the negative effects of reneging and incongruence because they increase salience of member behavior. (Piccoli and Ives, 2003) However, in low-trust teams strong structures actually mitigate the negative effects of trust by minimizing the role trust plays in interpreting member behavior.

Trust is not always dependent on social bonds. Instead, it can be founded on performance consistency. Task-based trust (vs. interpersonal trust) may be more achievable for virtual teams, and can be built by developing norms around communication patterns, ensuring reliable and rapid responses, and making team interaction timely and consistent. (Kirkman et al, 2002)

Standards of Excellence and External Support
Virtual teams are held to the same standards of excellence as conventional teams, but there are subtle differences. Virtual team members often function as the point of contact for their immediate physical group. They often have more autonomy than conventional team members as their teams may meet according to varying time zones which may not be understood by their local management. The presence of a true “invisible team” (Larson and LaFasto, 1989, p109) is also a unique component of a virtual team. The “invisible team” is the management team to which each of the members report. The invisible team sets the standards for each member.

Misunderstandings may arise if the “invisible team” does not align itself to the same set of expectations. A virtual team leader must understand the level and kind of support from each contributor. Larson and LaFasto observe that “loss of morale…decreased belief and commitment to the team’s goal” (p110) result when support is not visible to the team. The team leader should consider what expectations are reasonable to ask of members. Virtual teams that span various companies must create some form of “shared understanding” (Symons, 1997, p427) in order for members to develop a set of expectations. Failure to establish understanding may result in standards that are not achievable by all members. External support frequently determines how resources, such as incentives and capital expenses, are contributed by team members. During the Rocketdyne-Boeing Project, expectations of each member were established at the outset of the project. (Malhotra, Majchzak, Carmen, Lott) Resolving how each member would contribute time, resources, and expertise reduced misunderstandings as the project progressed. Standards of excellence and external support intersect on many levels. Time and energy is well spent at the outset of a virtual team to evaluate the level of excellence the team will achieve. This is especially true when financial contributions require resources outside of the control of the virtual team. Managers of virtual teams need to understand the feasibility of their requests given the context of their members’ management.

Leadership
Principled Leadership is the final ingredient identified by Larson and LaFasto (1989) for effective team performance. Pauleen (2004) tells us “leadership challenges are magnified in a virtual environment” and stresses the necessity of face-to-face meetings, stating “it is essential for them (leaders) to build personal relationships with team members before commencing a virtual working relationship. Strong relational links are associated with trust, creativity, motivation, morale, good decisions, and fewer process losses. However, Majchrzak, et al. (2004) argues that you can lead high-performing virtual teams without face-to-face meetings and provides several examples of successful teams whose members never met in person. This requires intensive communications to build a coherent identity and hold the team together, and their research found that the leaders of successful virtual teams “rarely let a day go by when members did not communicate with one another” and “frequent phone conversations between the team leader and individual members …were not unusual.” Research by Kirkman and Rosen, et al (2004) on the performance of virtual teams may provide a clue for leaders attempting to resolve this dilemma. They suggest that periodic face-to-face be held to focus on process improvement, but if this is not feasible “managers need to make extra efforts to empower virtual teams to deal directly and decisively with process improvement issues” (p. 188).

Gibson and Cohen (2003) suggest virtual team leaders need to engage the group in openly discussing cultural differences and similarities to help develop communication norms. Thompson’s (2000) work suggests the leader of a virtual team must also play a key role in assessing and balancing team performance levels across the four dimensions: team productivity; team satisfaction; individual growth; and organizational gains. Pauleen (2004) states that the leader of a virtual team must: assess team issues, boundaries, organizational policies, resources, and technology; target relationship levels necessary for performance; and develop effective strategies and select and utilize appropriate communication channels.

Conclusions
Virtual teams must deal with problems that befall face-to-face teams, along with some unique challenges. At the same time, they have the potential to realize additional process gains and deliver high-quality solutions by bringing together diverse individuals with complementary knowledge without the limitations of physical, organizational or cultural boundaries.

Our competitive environment places a premium on the quality and speed of solutions, and technology is providing increasingly richer collaboration tools – advancing from the telephone and the fax machine to video conferencing and virtual workspaces in a little over a decade. Organizations that learn to harness the power of virtual teams with these collaborative technologies will gain significant competitive advantage.


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WebObjects/Web Applications/Deployment/wotaskd

Troubleshooting
If WOtaskd did not start on your fresh install of WebObjects, check out the WOTaskd Didn't Start question in the troubleshooting section.


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User:KyrreKA

= Basic Info = Name: Kyrre Andersen

Age/sex: 26/male

Interests: Books (virtual and otherwise)

Academic Background: Mathematics and (a little) Philosophy

Current Interests: Topos Theory and Philosophy of Mathematics

title: User:KyrreKA


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Wikijunior talk:United States Charters of Freedom

Age target of this Wikibook
The way that this has been written seems to be targeted to a much older audience than the typical Wikijunior book. I would put the content and the questions to be typical American High School at the moment (ages 14-18). While this is a laudable goal, it would be good to try and determine exactly who this is aiming at. It needs to be toned down considerably in terms of word complexity and perhaps even the level of the questions if it is to be made for the more typical age range of Wikijunior books. --Rob Horning 16:32, 6 June 2006 (UTC)

 * Do you have any suggestions then? As I have attempted to tone it down already.

 * I would strongly suggest that you read through some of the comments with Wikijunior Solar System and the attempts to lower the target age of the content there. It is not an easy process, and it takes some effort to accomplish.  The real issue is that you have to reduce the complexity of the words that are used, avoiding the larger multi-syllabic words and instead use simplier words that are more commonly used by English speakers.  The specific formula or metric to determine what the grade level of content is can be argued, but there are several formulas and websites that cover this idea on how to determine what age a given set of prose is intended for. Also useful to try and cover is some specific curriculum guidelines by a school district or state education system (given the American emphasis on this book, American schools should especially be targets).  For some of these standards, look at Help:Textbook Standards for some specific standards that apply to many states (and other countries as well).  My experience is that this is indeed something that is covered in High School in the level that you are proposing here, particularly by including the full text of the original documents and providing annotated remarks (perfectly acceptable here on Wikibooks BTW, just unusual for Wikijunior).

Federalist Papers and Articles of Confederation
The Articles of Confederation, IMHO, is as significant to the people of the USA as is the Declaration of Independence. While it is true that it is viewed by historians as a failed document in the sense that it had no real power and had a number of flaws in its structure, it did establish guidelines for establishing a government for the United States of America, and was the legal basis for the U.S. government that existed prior to 1788. Even if all that is done is contrasting the weakness of the Articles against the Constitution, it is useful and important in terms of putting a context to the U.S. Constitution, and why the Constitution was written in the first place.

As far as the Federalist Papers are concerned, these are citable in arguments with the U.S. Supreme Court when trying to decide constitional issues, and were remarkably insightful in terms of what the founders of the U.S.A. thought the country would eventually become. This is a series of "letters to the editor" that were written by proponents of the Constition, and were instrumental in getting the Constition passed by state legislatures, notably New York and Pennsylvania. Imagine what the USA would be like without either state, and you see the magnitude of what was accomplished with these letters. They predicted the U.S. Civil War, and much of what was written in them has relevance today. Most recently, with the impeachment of Bill Clinton, the Federalist Papers were called into service to help interpret the meaning of what was an impeachable offense for a sitting U.S. President. Indeed the very procedure and process to perform the impeachment had to be developed by reading the Federalist Papers, particularly with the impeachment of Richard Nixon when such impeachments had never been previously even considered throughout most of U.S. History.

These are both significant documents in American history, and IMHO a part of the "cannon" of founding documents for the USA. Certainly any book aimed towards children that omits either document fails to effectively explain what the USA is really about and is missing a significant part of American history. If you want to claim that you understand what the U.S. government is about, these are simply must read documents as well. --Rob Horning 13:57, 8 June 2006 (UTC)