Sylvain Chaty
email: chaty@cea.fr

Multi-wavelength observations of the microquasar XTE J1720-318: a transition from high-soft to low-hard state

To gain a better understanding of high-energy Galactic sources, we observed the Galactic Xray binary and black hole candidate XTE J1720-318 in the optical and near-infrared, just after the onset of its X-ray outburst in January, 2003. These observations were obtained with the ESO/NTT as a Target of Opportunity, in February and April 2003. We performed an accurate astrometry and analysed photometrical observations. We then produced a colour-magnitude diagram, looked at the overall evolution of the multi-wavelength light curve, and analysed the spectral energy distribution. We discovered the optical counterpart in the R-band (R ∼ 21.5) and confirmed the near-infrared counterpart. We show that, for an absorption between 6 and 8 magnitudes, XTE J1720-318 is likely to be an intermediate mass X-ray binary located at a distance between 3 and 10 kpc, hosting a main sequence star of spectral type between late B and early G. Our second set of observations took place simultaneously with the third secondary outburst present in X-ray and near-infrared light curves. The evolution of its spectral energy distribution shows that XTE J1720-318 entered a transition from a high-soft to a low-hard state in-between the two observations. We finally discuss the different phases of the outburst of this source in the general scheme of outbursts from microquasars.

Discovery and characteristics of XTE J1720-318

X-ray binaries are constituted of a compact object and a companion star, the former attracting matter from the later, either through an accretion disc or the wind. They are usually divided in 2 sub-classes: high mass X-ray binaries and low mass X-ray binaries, hosting early-type and latetype stars, respectively. Since accretion and ejection phenomena usually occur in these objects, they are ideal laboratories for studying relativistic phenomena and the formation and evolution of compact objects in binaries. However, to study them, we first have to derive the important parameters related to the nature of these systems, i.e. the distance, nature of the compact object, spectral type of the companion star, type of accretion, orbital parameters, etc. Because of the way they are formed, most of the observed Galactic X-ray binaries are located in the Galactic plane or even towards the Galactic centre, and therefore associated with very high absorption (up to A v ∼ 50 magnitudes) because of the presence of gas and dust in this region. In this case, nearinfrared (NIR) observations prove to be particularly useful, since the radiation is less absorbed at NIR wavelengths than at optical ones (see, e.g., [START_REF] Chaty | Near-infrared observations of Galactic black hole candidates[END_REF]). Furthermore, X-ray binaries have to be studied in a multi-wavelength context to disentangle all parts of the system emitting at various wavelengths: the accretion disc from high-energies to NIR, the companion star from ultra-violet to NIR, the jets from radio to X-rays, etc. (see, e.g., [START_REF] Chaty | The role of microquasars in astroparticle physics in proceedings of Frontier Objects in Astrophysics and Particle Physics[END_REF]).

On January 9, 2003, the All Sky Monitor (ASM) of the Rossi-XTE satellite discovered a new source in the X-ray sky: XTE J1720-318, in the direction of the Galactic bulge, at 6 • from the Galactic Centre. The 2-12 keV flux was initially ∼ 130 mCrab and reached ∼ 430 mCrab in one day, on January 10, 2003 (25). Spectroscopic observations with XMM-Newton were carried out on February 20, 2003, allowing (19) to estimate the column density of hydrogen on the line of sight: N H = 1.33 × 10 22 cm -2 . An iron line was detected at 6.2 keV with 95 eV equivalent width, and no low or high frequency oscillation was detected [START_REF] Markwardt | Refined position and spectrum for XTE J1720-318[END_REF]. The 2-10 keV flux was estimated to be 1.6 × 10 -9 erg cm -2 s -1 (15). The source was not detected during INTEGRAL/IBIS observations on February 28, 2003, but became visible at the end of the outburst during IBIS surveys of the Galactic centre from the end of March 2003 [START_REF] Goldoni | INTEGRAL detection of Hard X-ray emission from XTEJ1720-318[END_REF]. The source reached ∼ 25 mCrab in the 15-40 keV band and became detected in the 40-100 keV energy band at ∼ 30 mCrab on April 6 and 7, 2003, indicating that the source had undergone a change of state, as suggested by [START_REF] Goldoni | INTEGRAL detection of Hard X-ray emission from XTEJ1720-318[END_REF]. The high-energy observations suggest that the compact object is a black hole, first because of its early spectral evolution that was very similar to black hole X-ray transients (25), and also because of Xray spectral parameters [START_REF] Bel | High-Energy observations of the state of the X-ray nova and black hole candidate XTE J1720-318[END_REF] and the presence of an iron line [START_REF] Markwardt | Refined position and spectrum for XTE J1720-318[END_REF]. A radio counterpart was discovered with the VLA on January 15, 2003, and confirmed with ATCA on January 16, 2003: only one radio source included in the Rossi-XTE error box significantly varied from 0.32 ± 0.04 to 4.9 ± 0.1 mJy at 4.9 GHz (27). Radio observations took place from January to August 2003, allowing [START_REF] Brocksopp | The 2003 outburst of a new X-ray transient : XTE J1720-318[END_REF] to study the correlation between radio and X-ray fluxes. A NIR counterpart was discovered then by (17) on January 18, 2003. Thirteen observations in J, H, and K s , until May 21, 2003, allowed (21) to measure the exponential decay following the outburst, which was equal to 60 days.

In this paper, we will first describe our optical and NIR observations and data reduction in Sect. 2.1, then report on our astrometry and photometry results in Sect. 2.2. We will then focus on constraining the companion star spectral type in Sect. 3, and finally we will analyse the evolution of the XTE J1720-318 light curve and spectral energy distribution (SED) in Sect. 4. Readers who want to have more details might refer to [START_REF] Chaty | Optical/near-infrared observations of the black hole candidate XTE J1720-318 : from high-soft to low-hard state[END_REF].

ESO Optical/NIR observations 2.1 Observations and data analysis

Our observations were carried out as part of the Target of Opportunity (ToO) programme 070.D-0340 (PI: S. Chaty) dedicated to the study of new Galactic high-energy sources and jet sources. They were triggered so as to be conducted at the same time as INTEGRAL ToO observations, and we asked for two periods of observations. The first set of observations took place on February 28, 2003, and the second one on April 24, 2003. On February 28, 2003, we obtained NIR photometry in J-, H-, and K s -bands with the spectra-imager SofI, and optical photometry in B-, V -, R-, and I-bands with EMMI, both installed on the NTT (La Silla Observatory, Chile, European Southern Observatory). We used the large field imaging of SofI's detector, giving an image scale of 0.288" pixel -1 and a field of view of 4.94 ′ × 4.94 ′ , and the EMMI detector with an image scale of 0.32" pixel -1 and a binning 2 × 2, giving a field of view of 9.9 ′ × 9.1 ′ . Concerning the NIR observations, we repeated one set of observations for each filter with 9 different 30" offset positions, including XTE J1720-318, with an integration time of 90 seconds for each exposure, following the standard jitter procedure that allows us to cleanly subtract the blank NIR sky emission. We observed two photometrical standard stars of the faint NIR standard star catalogue of ( 23): sj9157 on February 28, 2003, and sj9172 on April 24, 2003. Concerning the optical observations, we acquired 300 s exposures in each filter, except for the B-band (200 s), using a 2x2 binning to increase the sensitivity. We observed the standard star RU152 in R-and I-bands. Since we did not have any standard star observations in the B and V filters, we used mean zero-points taken from the EMMI website 1 . We used the Image Reduction and Analysis Facility (IRAF) suite to perform data reduction, carrying out standard procedures of optical and NIR image reduction, including flat-fielding and NIR sky subtraction. As we had only one standard star observation available for each night, we used characteristic extinction coefficients at la Silla: ext B = 0.214, ext V = 0.125, ext R = 0.091, ext I = 0.051, ext J = 0.08, ext H = 0.03, and ext Ks = 0.05, to transform instrumental magnitudes into apparent magnitudes. The observations were performed through an airmass between 1 and 1.4.

Astrometry and photometry results

We used the K s image of the XTE J1720-318 field taken on January 21, 2003 (22) to identify XTE J1720-318 in our NTT images. We then determined the position of the XTE J1720-318 NIR counterpart by deriving the astrometric solution, using ∼ 12 stars taken from the GSC2 catalogue: the position we measured was: α = 17 h 19 m 58. s 988 ± 0. s 008; δ = -31 • 45 ′ 01. ′′ 21 ± 0. ′′ 15 (equinox J2000). This position is consistent with other determinations (Table 1). We discovered the optical counterpart in the R-and I-bands at α = 17 h 19 m 58. s 994 ± 0. s 007, δ = -31 • 45 ′ 01. ′′ 46 ± 0. ′′ 15 (equinox J2000), a position that is consistent with the NIR counterpart. We present BV RI magnitudes in Table 2. The R and I magnitudes are consistent with detection limits given by ( 21): R > 18 and I > 16.5. We give a lower limit for the B-and V-bands, as we did not detect any counterpart in these bands. Since XTE J1720-318 is located close to the Galactic centre, we had to perform crowded field photometry to obtain precise NIR magnitudes, using the noao.daophot package. This procedure, described in [START_REF] Massey | A user's guide to stellar ccd photometry with iraf[END_REF], consists of creating an empirical point-spread function with isolated bright stars, applying this model to the whole field, cancelling the contributions of neighbour stars, measuring the flux of the object itself, and then applying aperture correction (due to the use of a smaller aperture for measuring XTE J1720-318 magnitude than for standard stars). This procedure allows us to get photometry with better than 1% accuracy. We present the apparent J, H, and K s magnitudes measured in February and April 2003 in Table 2. Uncertainties were determined from CCD readout and signal noise. We point out that we do not include the H-band observation of February 28, 2003, here because of bad sky subtraction.

Our NIR photometrical observations are reported in Fig. 1, where we also included results from [START_REF] Nagata | The Infrared Counterpart of the X-Ray Nova XTE J1720-318[END_REF]. Our magnitudes, indicated by ' * ', are consistent with measures from [START_REF] Nagata | The Infrared Counterpart of the X-Ray Nova XTE J1720-318[END_REF]. We notice the similar behaviour of X-ray and NIR light curves, particularly during the first maximum when the coverage is more complete. Although similar, the NIR flux decayed less rapidly than the X-ray flux, as indicated by the red lines in Figure 1. This is a consequence of the accretion-disc instability model, where the optical and NIR emission emanates from the outer part of the accretion disc, which is kept hot enough long until the cooling front from inside reaches the outer part [START_REF] Lasota | The disc instability model of dwarf novae and low-mass X-ray binary transients[END_REF]. We point out that we observed an increase in NIR on April 24, 2003 (MJD 52753), observed by chance at exactly the same time as an X-ray increase seen on the light curve. These ESO observations are therefore simultaneous to the 2 outbursts, as indicated by the red ellipses in Figure 1.

The nature of the binary system and companion star

We will now try to constrain the nature of the companion star with a colour-magnitude diagram (CMD). XTE J1720-318 optical and NIR magnitudes allow us to constrain the nature of the binary system by comparing its absolute magnitudes with those of well-determined spectral type stars (see, e.g., [START_REF] Chaty | Near-infrared observations of Galactic black hole candidates[END_REF]). For this purpose, we use template absolute magnitudes related to spectral types (taken from [START_REF] Ruelas-Mayorga | distribution and studies of the infrared stellar population in the galaxy. i. the model[END_REF]). The conversion of apparent magnitudes m to absolute magnitudes M depends on both distance d and interstellar absorption A v , via: M = m + 5 -5logd(pc) -A v . Concerning the interstellar absorption, we have three different estimates: First, XMM-Newton spectroscopy obtained in February 2003 gave N H = 1.24 ± 0.02 × 10 22 cm -2 (5). This column density corresponds to an absorption of A v = 6.9 magnitudes using the relation A v = 5.59 × 10 -22 N H [START_REF] Predehl | X-raying the interstellar medium: Rosat observations of dust scattering halos[END_REF]. Second, ( 21) obtained A v = 8 by assuming a high temperature blackbody emission just after the X-ray outburst. They also noted that extinction derived from the 2MASS survey is A v ∼ 6. Here, we will use only one observing epoch, on April 24, 2003, when the source is fainter, to minimise the accretion disc contribution in the observed NIR flux. Even if the object was still far from quiescence, in this way, we determine a lower limit for the companion star spectral type by assuming that the accretion disc emission reddens the NIR flux. To derive the possible spectral types, we computed the absolute magnitudes of XTE J1720-318, taking various distances d and absorption in the visible A v . The results are reported with ' * ' in the (J -Ks, Ks) colour-magnitude diagram (CMD) presented in Fig. 2. The distance was computed between 1 and 10 kpc (from bottom to top, respectively) and the absorption between 6 and 8 magnitudes (from right to left, respectively).

From this CMD, we first derive that, in any case, the companion star must belong to the main sequence, the favoured region being indicated by the red ellipse. Furthermore, if the interstellar absorption is high, A v ∼ 8 magnitudes, the spectral type would be between late B and early A, and the source far away, between 6 and 10 kpc. With an intermediate value of the interstellar absorption, A v ∼ 7 magnitudes, the spectral type would be between late A and early F, and the distance between 5 and 7 kpc. Finally, with a small interstellar absorption, A v ∼ 6 magnitudes, the spectral type would be between late F to early G, and the distance between 3 and 6 kpc. Therefore, we can conclude that, for an absorption between 6 and 8 magnitudes, the XTE J1720-318 companion star is a main sequence star of spectral type between late B and early G, located at a distance between 3 and 10 kpc. This estimate of distance makes the source closer than suggested by [START_REF] Nagata | The Infrared Counterpart of the X-Ray Nova XTE J1720-318[END_REF]: it is therefore possible that the source is not located in the Galactic bulge. We point out that from this analysis, XTE J1720-318 can be added to the list of intermediate mass X-ray binaries, like, e.g., V4641 Sgr (8).

Transition from high-soft to low-hard state

In Fig. 3, we show the overall outburst light curve of XTE J1720-318 from mid-January to the end of August 2003. First we can see that the light curve grossly has the form of a Fast-Rise Exponential-Decay light curve (so-called FRED) with a timescale of 60 days, but superimposed on this FRED, the source exhibits a complex behaviour: after the main outburst on January 16 (MJD 52656), we can see both in X rays and in NIR a secondary outburst on January 29 (MJD 52669), then a second one on February 22 (MJD 52693), and finally a third one on April 24, 2003 (MJD 52753), exactly at the time of our second epoch NIR observations. The main outburst and the last event are also associated with radio outbursts, and therefore with ejection events. We indicate the time of these events in Fig. 3 by O, 1, 2, and 3 in the top, respectively, and by the red lines. There are other sources that exhibit clear secondary maxima in their X-ray light curves, such as A0620-00, GS 1124-68, GRO J0422+32 (11), 4U 1543-47 (3), XTE J1550-564 ( 16) and XTE J1859+226 (2). However, XTE J1720-318 seems to be the second source after A0620-00 to exhibit clear multi-secondary maxima in the optical/NIR, correlated with the X rays, as seen in Fig. 3. This complex lightcurve and presence of multi-secondary outbursts might be explained by the accretion-disc instability model (such as an "outside-in"-type outburst).

We also report the SED of XTE J1720-318 in Figures 4 and5 for the two observing epochs ('+' for February 28, 2003, and ' * ' for April 24, 2003, respectively), where we put together quasisimultaneous ESO/NTT optical/NIR observations from this paper, INTEGRAL/IBIS high-energy observations ( 5)), and ATCA/VLA radio data (1). We will now describe the light curve and both SEDs and analyse XTE J1720-318's evolution between these two observing epochs.

The February 2003 epoch: high-soft state

Observations obtained with XMM-Newton, INTEGRAL/IBIS, and Rossi-XTE in February 2003 showed that the source spectrum was very soft (power law photon index of Γ = 2.7): while the 2-12 keV flux was ∼ 100 mCrab, IBIS detected a source at only ∼ 2.1 mCrab in the 20-120 keV band [START_REF] Bel | High-Energy observations of the state of the X-ray nova and black hole candidate XTE J1720-318[END_REF]. XTE J1720-318 was not detected in radio (≤ 0.27 mJy at 4.8 GHz, (1)), which is consistent with typical high-soft state. This is corroborated by the analysis of XTE J1720-318 SED from February 28, to March 2, 2003 (reported with '+' in Fig. 4), where we notice the characteristic shape of the accretion disc emission in X-rays (strong flux and soft spectrum), and the absence of radio emission. We overplot in green the multi-colour black-body disc emission and the soft power law in X-rays, and in red the emission in optical and NIR.

The April 2003 epoch: low-hard state

On the other hand, after March 25, 2003 (MJD 52723), XTE J1720-318 hard X-ray flux as seen by INTEGRAL/IBIS increased by a factor of 100 with respect to the high-soft state [START_REF] Bel | High-Energy observations of the state of the X-ray nova and black hole candidate XTE J1720-318[END_REF], while the soft X-ray flux (RXTE/ASM) remained constant, as seen on the light curve of Fig. 3. Besides, we also observed a radio outburst (1), simultaneous with an X-ray outburst and an increase in the NIR flux. Therefore, XTE J1720-318 seems to have entered a transition towards a low-hard state in-between these 2 observing epochs, as suggested by [START_REF] Goldoni | INTEGRAL detection of Hard X-ray emission from XTEJ1720-318[END_REF]. This is confirmed by the analysis of the SED (data of this second epoch are reported with ' * ' in Fig. 5, where we overplot in green the hard power law in X rays, in red the emission in optical and NIR and in blue the power law in radio): the source shows all the usual signs of the low-hard state (see, e.g., [START_REF] Chaty | Multiwavelength observations revealing the evolution of the outburst of the black hole XTE J1118+480[END_REF]). Firstly, we immediately notice that the source hardened in the high-energy domain with a high-energy power law photon index of Γ = 1.8. Secondly, the radio emission is usually interpreted in this state as synchrotron emission emanating from a jet. From the SED, we can derive the power law index α (in S ν ∝ ν α ): ∼ -0.3 in the radio and ∼ 1.6 in the optical/NIR. Therefore, the extrapolation of the radio flux towards the optical/NIR domain is significantly fainter than the observed optical/NIR flux. This strongly suggests that the synchrotron emission from the jet is contributing only for a small part, if any, in the NIR emission. It seems also likely that the optical/NIR emission comes from the thermal accretion disc and the irradiated companion star, since the slope in the NIR and optical remarkably remains the same in both observing epochs, while the X-ray decreased at the same time. Therefore, the NIR emission is dominated by the contribution of the companion star, which is consistent with XTE J1720-318 being an intermediate mass X-ray binary.

Trying to put everything together

From the analysis of the outburst lightcurve and the SED of XTE J1720-318 we therefore see that the source exhibited the first part of its outburst in the high-soft state, then transited to the lowhard state. The source probably began its outburst in the low-hard state, as tentatively suggested by spectral analysis of X-ray data [START_REF] Brocksopp | The 2003 outburst of a new X-ray transient : XTE J1720-318[END_REF]. It might even have passed through the intermediate state while it was crossing the jet line, just before the transition to the low-hard state (see e.g. ( 12)).

We can also point out that XTE J1720-318 does not follow the radio-X-ray luminosity correlation of black hole candidates in the low-hard state (see e.g. ( 13)). Indeed, it lies below this correlation even if put at a distance of 1 kpc. But XTE J1720-318 is not the first source to deviate from this not-so-universal correlation (see for instance SWIFT J1753.5-0127 in (4)).

Conclusions

We have reported ESO/NTT optical and NIR observations of the microquasar XTE J1720-318, taken as Target of Opportunity observations following the January 2003 outburst of this source. By performing accurate astrometry, we discovered the optical counterpart in the R-band (R ∼ 21.5) and confirmed the near-infrared counterpart. From photometric observations, analysis of a colourmagnitude diagram, and a basic modelling of its SED, we found that, for an absorption between 6 and 8 magnitudes, XTE J1720-318 is likely to be an intermediate mass X-ray binary, hosting a black hole and a main sequence star of spectral type between late B and early G, located at a distance between 3 and 10 kpc. We also analysed the XTE J1720-318 X-ray and near-infrared light curves: superimposed to a main FRED lightcurve, it exhibited three secondary outbursts, and our second set of observations took place simultaneously with the third one. The study of such outburst lightcurves is of prime importance to test accretion disc models. Comparing the SEDs during and after its outburst, we confirm the change of state of this source, from high-soft to low-hard state.

To further understand this source, it would be useful i) to get spectroscopic observations of XTE J1720-318 in quiescence to better characterise the companion star when the photospheric flux of the star dominates and ii) to observe the radial velocity of the binary system to derive the mass function and orbital parameters. With these parameters we will be able to further analyse its SED (as in, e.g., [START_REF] Chaty | Multiwavelength observations revealing the evolution of the outburst of the black hole XTE J1118+480[END_REF] with the source XTE J1118+480). And obviously, we need some more continuous multi-wavelength coverage of such outbursts to understand how they work.

Figure 1 :

 1 Figure 1: Multi-wavelength XTE J1720-318 light curve. From bottom to top: Rossi-XTE X-ray light curve (indicated by diamonds); NIR J, H, and K s light curves, respectively. NIR data taken from (21) are reported with '+', and data from this paper with ' * ' (MJD = JD -2400000.5). The red lines indicate that the NIR flux decayed less rapidly than the X-ray flux, in agreement with the Disc Instability Model. The red ellipses indicate the time of our observations, allowing to reveal two simultaneous X-ray and NIR outbursts.

Figure 2 :

 2 Figure 2: Colour-magnitude diagram showing characteristic absolute magnitudes of various spectral types and XTE J1720-318 absolute magnitudes computed for interstellar absorption A v between 6 and 8 magnitudes (' * ' from right to left) and distance between 1 and 10 kpc (' * ' from bottom to top). We used the April 24, 2003, observations, when the object was fainter, to reduce the accretion disc contribution in the NIR flux. From this CMD we can conclude that XTE J1720-318 is an intermediate mass X-ray binary located at a distance between 3 and 10 kpc, and that the companion star is a main sequence star of spectral type between late B and early G (see text for more details). The ellipse indicates the favoured region.

Figure 3 :

 3 Figure 3: Multi-wavelength light curve showing the outburst of XTE J1720-318 and its transition from highsoft to low-hard state. Top panel: NIR observations ('+': (21), ' * ': this paper); Middle panel: high-energy observations ('⋄': RXTE/ASM, '△': INTEGRAL/IBIS, (5)); Bottom panel: radio VLA/ATCA observations ('x': (1)). We indicated at the top the times when the main outburst (O) and the three secondary outbursts (1, 2, 3 respectively) occurred, indicated with red lines. MJD = JD -2400000.5

Figure 4 :

 4 Figure 4: Spectral Energy Distribution of XTE J1720-318 during the two observing epochs: the February 28, 2003, data are represented by '+', and the April 24, 2003, by ' * '. The observations were taken nearly simultaneously with the VLA in the radio (1), the NTT/EMMI and SOFI in the optical and NIR (this paper), and INTEGRAL/IBIS in the high-energy (5). For the first epoch, the VLA data were taken on February 26, 2003, NTT/SOFI data on February 28, 2003, and INTEGRAL/IBIS data on February 28 -March 2, 2003. For the second epoch, VLA data were taken on April 26, 2003, NTT/SOFI data on April 24, 2003, NTT/EMMI on April 27, 2003, and INTEGRAL/IBIS data on February 28 -March 2, 2003. The February 28, 2003, observations correspond to the high-soft state: high and soft X-ray flux and no radio emission. On the contrary, the April 24, 2003, observations correspond to the low-hard state: low and hard X-ray flux and detection of radio emission. Optical and NIR fluxes were de-reddened, assuming an interstellar absorption in the visible of A v = 7 magnitudes. It is remarkable that they remain the same in both states. We overplot the High Soft state in this Figure: in green the multi-colour black-body disc emission and the soft power law in X-rays, and in red the emission in optical and NIR.

Figure 5 :

 5 Figure 5: Same Figure as above, with the Low Hard State overplot: in green the hard power law in X-rays, in red the emission in optical and NIR and in blue the power law in radio.

Table 1 :

 1 17 h 19 m 58. s 988 ± 0. s 008 17 h 19 m 59. s 000 ± 0. s 014 17 h 19 m 58. s 994 ± 0. s 004 δ -31 • 45 ′ 01. ′′ 21 ± 0. ′′ 15 -31 • 45 ′ 01. ′′ 2 ± 0. ′′ 2 -31 • 45 ′ 01. ′′ 25 ± 0.05 Summary of astrometry results (equinox J2000) for the XTE J1720-318 NIR counterpart.

		This paper	(21)		(22)
	Date	MJD	R	I	J	H	K s
	2003 02 28 52698		17.47 ± 0.05	-	16.00 ± 0.06
	2003 04 24 52753 21.5 ± 0.3 20.6 ± 0.1 17.66 ± 0.05 16.99 ± 0.07 16.34 ± 0.05

α

Table 2 :

 2 Apparent R, I, J, H, and K s magnitudes of XTE J1720-318 (MJD = JD -2400000.5). XTE J1720-318 is not detected in B (> 23.2 ± 0.4) or V (> 23.1 ± 0.4).

www.ls.eso.org/lasilla/sciops/ntt/emmi/


G Rodeghiero

Y Zhong

M Lambert

D Lesselier

X Chen

An efficient interpolation for calculation of the response of composite layered material and its implementation in MUSIC imaging

To build

• Accurate computational models of complex anisotropic multi-layer composite panels • Robust, fast, end-user's friendly imaging procedures

NdT-E from eddy-currents to microwaves in aeronautics and automotive industries

Electromagnetic modeling and preliminary numerical results

Undamaged structure

x y z N N -1 N -2 1 2 3
J (r)
• Each layer: homogeneous anisotropic (different e.m. properties from layer to next)

• Uniaxial dielectric (glass-based) or conductive (graphite-based) effective media large-scale hypothesis

• Diagonal tensor in eigenframes along and orthogonal to fibers' axes • Dyadic Green function in need as well as response to known distributed electric source anywhere in the structure ε p r q " diag " ε n;11 ; ε n;22 p r q ; ε n;22 p r q ‰ in local coordinate system Damaged structure

• E.M. parameters differ from background stratified panel within layers or at interfaces

• 3-D volumetric defects (voids, fluid-filled cavities, localized damaged zones, etc.), or delaminations (thin, air-type slabs)

• Method of Moments upon vector contrast-source integral formulations, or change of dyads via supplementary reflection/transmission

Constructing the spectral and spatial response of the laminate (forward modeling)

New recurrence relations based on the propagator matrix method [8] • To efficiently calculate the spectral response of the laminate • Capable of stably dealing with distributed source along z • More efficient compared to the traditional Green's function method

• To numerically solve the state equation d dz ψ pzq " Ān ¨ψ pzq `f pzq with ψ pzq containing the tangential components of the fields and f pzq being the source term Fast algorithm to calculate the spatial response of the laminate [7] • To efficiently calculate the spatial response on the rectilinear mesh • Goal is to compute the I-FT of fast oscillating spectrum in the k x ´ky plane G px, y q " 1 4π2 8 ij ´8 G0 pk x , k y qe p ik x x`ik y y q dk x dk y

• Interpolation of the non-oscillating part at the Padua points with Chebyshev's polynomial interpolant

L n G0 `kx , k y ˘" n ÿ k "0 k ÿ j"0 c j, k ´j Tj pk x q Tk´j `ky ˘´1 2 c n,0 Tn pk x q T 0 `ky ˘(2)
with weights c j, k ´j computed using [START_REF] Caliari | [END_REF] • Fourier transform of Chebyshev polynomials given by ż 1

´1 Tn pk x q expp´ik x xq dk x

(3) are managed using [6] among other good options.

MUSIC images of anisotropic layered media affected by two defects • A cross type antenna array with 5 antennas on each arm.

• Each antenna as transceivers with x and y polarizations.

• Two small inclusions with dimension of 0.1λ 0 ˆ0.1λ 0 ˆ0.1λ 0 .

ǭb1 " diag r4.5 `i0.2, 6 `i0.05, 6 `i0.05s; θ b1 " 45 ǫb2 " diag r2 `i0.3, 3 `i0.1, 3 `i0.1s θ b2 " 60 ǫS 1 " Īǫ 0 θ S 1 " 0 ˝θS 2 " 120 ǫS

•

  DIGITEO : PhD Scholarship jointly involving L2S and CEA-LIST (Département Imagerie et Simulation pour le Contrôle) • Univ Paris-Sud : invited assistant-professor • Internal ressources (DRÉ & NUS)

Figure :

 : Figure :Reproduction of the results found in[2]

• 1 -Figure :

 1: Figure :The Padua points with their generating curve for n " 12 (left, 91 points) and n " 13 (right, 105 points), also as union of two Chebyshev-Lobatto sub-grids (red and blue bullets). Image taken from[START_REF] Caliari | 5th European Congress of Mathematics[END_REF].

2 (x s 1

 21 , y s 1 , z s 1 ) (x s 2 , y s 2 , z s 2 ) θ S 2

" diag r4.5 `i0.2, 6 `i0.05, 6 `i0.05s l 1 " 0.5λ 0 `xs 2 , y s 2 , z s 2 ˘" p´0.2λ 0 , ´0.1λ 0 , ´0.55λ 0 q l 2 " 0.2λ 0 `xs 1 , y s 1 , z s 1 ˘" p0.15λ 0 , 0.2λ 0 , ´0.35λ 0 q l

" 0.2λ 0 • Standard MUSIC imaging method[1]

v "1 ˇˇū j ¨Ḡ v p r q ˇˇ2

• Enhanced MUSIC imaging method [5] φp r q " 1 1 ´řσ j ąσ L ˇˇū j ¨Ḡ p r q ¨ā test ˇˇ2

Conclusions & perspectives

• Numerical integration method based on Padua points is proposed to avoid directly interpolating on the fast oscillating function.

• The approach is validated by comparison with configurations found in the literature • Green's function constructed by the proposed method is applied in MUSIC imaging.

• Preliminary numerical results show the efficiency of the proposed method in a fully complex anisotropic configuration


B E Mironov
email: bemironov@gmail.com

H M Freeman
email: helenfreeman88@gmail.com

A P Brown

F S Hage

A J Scott

A V K Westwood

Jean-Pierre Da Costa

R.M.D P Weisbecker

R. M. D Brydson

J. -P Da Costa

Electron irradiation of nuclear graphite studied

come

Introduction

Over 80% of the UK's current nuclear reactors are graphite-moderated Advanced Gas Cooled Reactors (AGR) or Magnox reactors [START_REF]Table of past and present nuclear reactors in the UK[END_REF]. In addition to moderating the energies of neutrons in the fission process, the graphite core provides structural support, contains the fuel and control rods and allows for coolant flow. The graphite blocks are subject to high levels of neutron irradiation resulting in chemical and physical property changes, which in turn affect neighbouring reactor components. The lifetime of such reactors is therefore primarily limited by the performance of the irreplaceable graphite within the working reactor, so an accurate measure of its condition is essential for economic success and plant safety. Nuclear graphite is a synthetic material produced from pitch and petroleum coke particles, with a high degree of crystallinity following thermal treatment at high temperatures (graphitization) [START_REF] Nightingale | Nuclear Graphite[END_REF]. When the graphitization process is complete, two main features can be distinguished: the majority filler particles and a minority binder phase, both of which are formed by domains of aligned individual crystallites and appear as a single colour in a polarised light micrograph. Both features have potentially inter-and intra-structural porosity ranging from Mrozowski cracks between crystallites (50 nm -10 m) to micro-and macropores around domains and particles (Figure 1) [START_REF] Wen | Microcracks in nuclear graphite and highly oriented pyrolytic graphite (HOPG)[END_REF].

For over 70 years, a considerable body of evidence has been assembled to understand the behaviour of irradiated graphite [START_REF] Nightingale | Nuclear Graphite[END_REF][START_REF] Hennig | Vacancies and dislocation loops in graphite[END_REF][START_REF] Thrower | Microstructural changes in neutron-irradiated graphite[END_REF][START_REF] Bollmann | Electron microscope observations of irradiated graphite single crystals[END_REF]. The bulk properties of damage features have been thoroughly investigated and theoretical models of induced structural changes derived [START_REF] Heggie | ruck and tuck: A proposed new model for the response of graphite to neutron irradiation[END_REF].

Although this has allowed property changes in the irradiated bulk to be partly understood and accounted for in current and future graphite based reactor designs, the mechanisms of such processes at the nanoscale still remain uncertain. This work investigates the effect of electron irradiation on nuclear grade graphites within a transmission electron microscope (TEM) in an attempt to understand the fundamental processes involved in radiation damage.

Irradiation of nuclear graphite

Throughout this paper we will compare the effects of neutron and electron irradiation therefore it is important to understand the key differences between the two. The atomic displacement rate of the carbon atoms in the graphite is measured in displacements per atom (dpa) and is dependent on the kinetic energy of the incident particle [START_REF] Nightingale | Nuclear Graphite[END_REF]. According to calculations by Thrower and Mayer [START_REF] Thrower | Point defects and self-diffusion in graphite[END_REF] a 1 MeV electron and neutron produce an average of 1.6 and 500 atomic displacements respectively. It is generally understood that cascades of atomic displacements are the most common route for large scale structural disturbances and models have been developed to calculate the number of atoms involved in cascade events resulting from different incident energies [START_REF] Yazyev | Early stages of radiation damage in graphite and carbon nanostructures: A first-principles molecular dynamics study[END_REF].

When mimicking the effects of neutron irradiation damage with electron irradiation in the TEM, it is important to account for both the higher dose rate of electrons compared to neutrons in a nuclear reactor (greater by about 10 4 ) and the reduced displacements per atom from electrons due to the lower mass [START_REF] El-Barbary | Electron spectroscopy of carbon materials: experiment and theory[END_REF]. Electron irradiation causes point defect damage whereas the higher mass and lower dose rate of neutrons causes cascade damage. However, the relatively wide spacing of graphite's basal planes results in a low density of the cascade events, and the low neutron dose rates (10 -7 dpa s -1 ) and high temperatures (~450 °C) in the nuclear reactor allow damage to partially anneal out between cascade events [START_REF] Simmons | Radiation damage in graphite[END_REF][START_REF] Egerton | Electron Energy-Loss Spectroscopy in the Electron Microscope[END_REF][START_REF] Kane | Microstructural characterization and pore structure analysis of nuclear graphite[END_REF].

Interstitial and vacancy defects created during irradiation can behave independently or coalesce into clusters and gradually deform the crystal lattice ultimately resulting in both chemical and physical changes. Damage accumulation at temperatures below 200 °C increases the Wigner energy and heat content of the graphite, due to a lack of atomic diffusion [START_REF] Baker | Graphite as a neutron moderator and reflector material[END_REF]. It is widely agreed that single vacancies become mobile at 100 -200 °C whereas interstitial atoms become mobile at temperatures of 500 °C [START_REF] Trevethan | Extended Interplanar Linking in Graphite Formed from Vacancy Aggregates[END_REF]. Thus only at higher temperatures such as those in the Gen IV graphite moderated Very High Temperature Gas Reactors (> 300 °C) does stored energy dissipation occur by diffusion driven atomic reordering and the problem is addressed in the short term. Longer term exposure to a high temperature environment (> 400 °C) however, gives rise to creep and dimensional change [START_REF] Goggin | Dimensional changes in the irradiated graphite lattice[END_REF][START_REF] Henson | Lattice parameter and dimensional changes in graphite irradiated between 300 and 1350[END_REF][START_REF] Brocklehurst | Analysis of the dimensional changes and structural changes in polycrystalline graphite under fast neutron irradiation[END_REF].

The key observed changes in nuclear grade graphite as a result of neutron irradiation are micro-crack closure resulting from expansion in the c-direction and dimensional change from irradiation induced creep, both of which depend on the overall level of initial crystallinity [START_REF] Baker | Graphite as a neutron moderator and reflector material[END_REF][START_REF] Karthik | In situ transmission electron microscopy of electron-beam induced damage process in nuclear grade graphite[END_REF]. Dimensional change is determined in a number of ways, such as directly measuring specimens before and after irradiation, using X-ray diffraction to assess crystallite behaviour, and measuring changes in cracks and porosity with electron and light microscopy and smallangle neutron scattering [START_REF] Goggin | Dimensional changes in the irradiated graphite lattice[END_REF][START_REF] Henson | Lattice parameter and dimensional changes in graphite irradiated between 300 and 1350[END_REF][START_REF] Mileeva | A study of the porosity of nuclear graphite using small-angle neutron scattering[END_REF]. The fundamental dimensional changes are known to involve crystallographic expansion in the c-direction and contraction in the a-direction [START_REF] Wen | Microcracks in nuclear graphite and highly oriented pyrolytic graphite (HOPG)[END_REF].

Initially, the expansion is largely accommodated for in cracks and pores created during the manufacturing process; Mrozowski cracks arise from the anisotropy in graphite thermal expansion coefficients and lie perpendicular to the c-direction hence the initial accommodation of expansion, so that the initial macroscopic response is a net shrinkage in the a-direction [START_REF] Mrozowski | Mechanical strength, thermal expansion and structure of cokes and carbons[END_REF]. Upon further irradiation and once the cracks and pores are fully closed, irreversible net macroscopic expansion occurs. The transition between contraction and expansion is referred to as 'turnaround' [START_REF] Hall | The microstructural modelling of nuclear grade graphite[END_REF][START_REF] Windes | Graphite Technology Development Plan[END_REF].

Transmission electron microscopy is an established tool for characterising both electron and neutron irradiated graphite [START_REF] Kane | Microstructural characterization and pore structure analysis of nuclear graphite[END_REF][START_REF] Banhart | Irradiation effects in carbon nanostructures[END_REF]. There are however, very few detailed TEM-EELS studies on nuclear graphites but a significant volume of work on graphitizing and nongraphitizing carbons [START_REF] Daniels | Investigating carbonization and graphitization using electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM)[END_REF][START_REF] Zhang | Investigating the structure of non-graphitising carbons using electron energy loss spectroscopy in the transmission electron microscope[END_REF]. In this work we will focus on quantitative analysis of atomic lattice imaging and EEL spectroscopy to elucidate the nanoscale changes that occur in irradiated graphite.

Experimental Details

Sample preparation

Virgin Pile Grade A (PGA) graphite sourced from the University of Manchester was chosen for inspection. PGA is a medium to coarse grain anisotropic nuclear graphite of typical density 1.74 g cm -3 . The anisotropy of this particular graphite comes from the tendency of the needle-like grain particles in the filler to align in the extrusion direction during the manufacturing process. Samples were crushed using an agate pestle and mortar and mixed with acetone before being dispersed onto a holey carbon-coated copper TEM grid (3mm diameter; Agar Scientific Ltd). The analysed areas had a thickness less than 0.3 times the mean free path for inelastic scattering (Λ).

Microscope conditions

TEM investigations were performed on an FEI CM200 field emission TEM operated at 197 kV with a tip extraction bias of 3.21 kV routinely providing an electron flux of approximately 4.24 x 10 18 electrons cm -2 s -1 and an EELS energy resolution of 0.7-0.8 eV, measured as the full width at half-maximum (FWHM) of the ZLP. For EELS, the microscope was operated in diffraction mode with the smallest selected area aperture inserted, giving a circular projection of approximate diameter 150 nm, a collection semi-angle of 1.6 mrad and a convergence semi-angle of approximately 0.8 mrad, (corresponding to the magic or orientation independent angle [START_REF] Daniels | Experimental and theoretical evidence for the magic angle in transmission electron energy loss spectroscopy[END_REF]). Digital images and energy loss spectra were captured using a Gatan Imaging Filter (GIF) 200 with a 1 megapixel slow scan CCD array. Data from the array (i.e. images and spectra) were processed using Gatan's Digital Micrograph software.

Unless stated otherwise, all experiments were performed at room temperature where the localised heating effect from the electron beam was considered to be negligible due to the high thermal conductivity of graphite [START_REF] Williams | Transmission Electron Microscopy A textbook for Material Science[END_REF]. In-situ electron irradiation damage at higher temperatures was investigated using a Gatan TEM heating holder.

Determination of dose

Throughout this paper we will refer to electron and neutron irradiation in three ways: fluence refers to the number of electrons or neutrons that intersect a unit area; flux is the fluence rate; and dose is the energy transferred at a given fluence. The dose, D (in dpa), is calculated using Equation 2.1 where J is the electron fluence and σ d is the displacement cross section.

d J D    Equation 2.1
The displacement cross section varies with electron energy and displacement threshold energy. Referring to figure 18 from [START_REF] Oen | Cross sections for atomic displacements in solids by fast electrons[END_REF] [START_REF] Oen | Cross sections for atomic displacements in solids by fast electrons[END_REF] we measure a displacement cross section value of 16.25 barns for an electron energy of 200 keV and a displacement threshold energy of 20 eV [START_REF] Krasheninnikov | Ion and electron irradiation-induced effects in nanostructured materials[END_REF]. The value of displacement threshold energy has not yet been agreed upon within the literature, with values ranging from 15 -30 eV, resulting in a significant variation in σ d and the resultant dose estimates [START_REF] Egerton | Electron Energy-Loss Spectroscopy in the Electron Microscope[END_REF][START_REF] Karthik | In situ transmission electron microscopy of electron-beam induced damage process in nuclear grade graphite[END_REF][START_REF] Banhart | Irradiation effects in carbon nanostructures[END_REF][START_REF] Krasheninnikov | Ion and electron irradiation-induced effects in nanostructured materials[END_REF].

During EELS experiments, the intensity of the beam was varied considerably for the acquisition of low loss and core loss spectra resulting in an inconsistent electron flux. For the acquisition of low loss spectra the beam was spread over a large area to give a low intensity (so as to avoid saturating the ccd) resulting in a near-negligible flux; when collecting the core loss however, the beam was focused over a smaller area giving a much higher intensity, comparable to the intensity during imaging. The change in fluence was accounted for by measuring the electron flux at the two acquisition intensities and recording the time spent at each; the cumulative fluence was then determined and converted to dpa.

Results and Discussion

TEM

Four areas of thin (<50 nm) PGA graphite were subjected to an average electron fluence of 4.2 x 10 18 electrons cm -2 s -1 (2.4 x10 -4 dpa s -1 ). Images of the basal planes and electron energy loss spectra were recorded periodically throughout. The micrographs and their corresponding selected area electron diffraction (SAED) patterns shown in Figure 2 are typical of the damage produced by a 200 kV electron beam. In particular, the tortuosity (or curvature) of the (002) planes can be seen to increase, the spacing increases, and the lattice fringe length decreases. These results suggest the breakup of the graphitic structure into nanocrystalline regions with increasing dose, which in the high dose limit produce an amorphized structure.

These micrographs are comparable to those obtained by Karthik and Kane [START_REF] Karthik | In situ transmission electron microscopy of electron-beam induced damage process in nuclear grade graphite[END_REF] and Muto [START_REF] Takeuchi | Structural change in graphite under electron irradiation at low temperatures[END_REF] who also investigated the effects of electron irradiation in nuclear graphite.

For comparison, Figure 2 (e) shows the effect of performing the same procedure at 400 °C.

Interestingly, significantly less alteration of the atomic structure is observed, suggesting that damage is continuously annealed out at these temperatures. Further work is in progress to investigate this phenomenon in more detail. It must be noted that while these descriptions provide qualitative analysis, understanding the exact mechanisms of defect creation remains an area for investigation. A contrast change in a phase contrast TEM micrograph may not necessarily relate to a change in atomic position but may instead be due to a change in thickness or defocus. While every effort was made to ensure that images were acquired at Scherzer defocus, awareness of the issues associated with contrast reversal is highly important when analysing these disordered structures [START_REF] Scherzer | The theoretical resolution limit of the electron microscope[END_REF]. Diffraction patterns were also recorded at regular intervals during electron beam exposure and the spacing of (002) spots were measured using intensity profiles in Digital Micrograph.

Figure 3 shows an increase in plane spacing of 10% following electron irradiation of 0.31 dpa, as measured from the diffraction patterns. The arcing of the diffraction spots was also measured to assess the misalignment of layers induced during electron irradiation. To extract this information the SAED patterns were intensity normalised and the full width at half maximum (FWHM) of each diffraction arc was measured using a Graphite Anisotropy Analysis Program (GAAP) provided by A. A. Campbell at the University of Michigan [START_REF] Campbell | In situ proton irradiation-induced creep at very high temperature[END_REF].

GAAP measures the intensity around a ring at the (002) radius the Gaussian curves are fitted to the intensity plots and the FWHM determined. The data are presented in Figure 4 and show a 40° increase in arcing of (002) diffraction spots following 0.86 dpa of electron irradiation. TEM micrographs were also analysed, using software provided by the 'PyroMaN' research group, to quantify the change in atomic arrangement following electron irradiation [START_REF] Raynal | Multi-scale quantitative analysis of carbon structure and texture: III. Lattice fringe imaging analysis[END_REF]. Based on analysis of (002) lattice fringes, the software provides information on fringe length, tortuosity and orientation using Fourier transform filtering and a level curve tracking algorithm [START_REF] Costa | Level curve tracking algorithm for tectural features extraction, Pattern Recognition[END_REF]. Although the algorithm detects fringes even if they exceed the image boundaries, they cannot be accounted for when performing measurements since the true length and tortuosity cannot be determined. Application of the software to ordered and (electron irradiation induced) disordered areas is displayed in Figure 5 with the extracted data presented in Figure 6, 9 and 10. The lack of fringe detection (outlined in white over a filtered micrograph) for ordered structures is emphasized in Figure 5 analysis of these orientation maps can provide additional insights into the nanostructure, including estimates of coherence lengths parallel and perpendicular to the fringes, together with the mean misorientation of the fringes at larger distances. In principle, the software is also capable of producing three dimensional models from a two dimensional electron micrograph by comparison and refinement with molecular dynamics simulations [START_REF] Farbos | Nanoscale structure and texture of highly anisotropic pyrocarbons revisited with transmission electron microscopy, image processing, neutron diffraction and atomistic modeling[END_REF]. The PyroMaN image analysis technique will be applied to other structures and collaborations with the PyroMaN research group will continue in order to realise the software's full potential. Data extracted from the fringe detection algorithm illustrated in Figure 5 (c) and (g) include fringe length (L 2 ) and tortuosity (τ) (the latter defined as the ratio of the total length of a fringe to its direct end-to-end length). Figure 6, shows that for low doses where the structure is still ordered, the tortuosity of each detected fringe is relatively low, with over 75% having a tortuosity of 1 -1.03 and a fringe length which is relatively high, extending to 8.9 nm. However when the structure becomes disordered following electron irradiation, the tortuosity increases significantly, with highs of nearly 1.14, and the (002) fringes break up into shorter lengths, 98% of which are below 1 nm in length. A second-order (i.e. pairwise) statistical analysis of the orientation maps provides information about the relative orientation of neighbouring fringes. Referring to the schematic in Figure 7, the angles of planes to the vertical axis (φ 1 ) were measured every 0.033 nm (pixel resolution) from the origin for a set of radii (r) up to a maximum of 4 nm. Data at each angle (θ) were then plotted on a graph of mean orientation difference (Δφ) versus distance to determine the average opening angle for the 4 nm domain as shown in Figure 7. The mean misorientation of the fringes corresponds to the plateau of the plots [START_REF] Costa | Investigating carbon materials nanostructure using image orientation statistics[END_REF]. For example, the plot taken at 0.7 dpa plateaus at Δφ max = 8.7° for both θ = 0° and 90° (the two extremes of θ).

The plateau is reached by the θ = 0° data before the θ = 90° data implying there is a quicker loss of orientation in the a-axis (θ = 0°) and domains are wider in the c-axis (θ = 90°). This automated procedure was performed on all micrographs acquired during electron beam exposure to produce a series of orientation maps and a plot of orientation angle against electron dose (Figure 8). As expected it suggests that the relative orientation of planes increases with electron irradiation, but appears to plateau at high doses, suggesting there is a misorientation limit, potentially associated with the unstable transition from strained sp 2 bonding to sp 3 interlayer bonding. Although both data sets plateau at different values of Δφ, they exhibit a similar trend suggesting there is a degree of short range homogeneity. Figure 7 Change in mean orientation difference with distance from origin (r) at θ = 0° and θ = 90° following an electron dose of 0.7 dpa ± 6.4 %. The coherence lengths can be defined as the distance at which 90% of the value of the plateau is reached for the diagrams at 0° and 90° [START_REF] Costa | Investigating carbon materials nanostructure using image orientation statistics[END_REF].

Figure 8 Change in mean orientation difference plateau, Δφ max , with electron irradiation for two irradiation data sets. Dose error = ± 6.4%

EELS

The bulk π+σ valence plasmon peak in the low loss region of the spectrum was analysed; the position of the peak being determined by taking the first derivative of the spectrum. The widely agreed value for the bulk plasmon peak position of highly oriented pyrolytic graphite (HOPG) in the literature is 27 eV [START_REF] Henrard | Electron Energy Loss study of plasmon excitation in curved carbon network[END_REF][START_REF] Laffont | Plasmon as a tool for in situ evaluation of physical properties for carbon materials[END_REF][START_REF] Gass | Free-standing graphene at atomic resolution[END_REF][START_REF] Egerton | Electron energy-loss spectroscopy in the TEM[END_REF] (for comparison, the corresponding value for amorphous carbon is 23 eV). As illustrated in Figure 9, the plasmon peak energy for PGA graphite is consistently lower than that of HOPG. This may be due to the misorientation of the specimen's c-axis relative to the normal of the incident beam [START_REF] Marinopoulos | Anisotropy and Interplane Interactions in the Dielectric Response of Graphite[END_REF], in combination with the choice of the spectrometer collection angle [START_REF] Egerton | Electron Energy-Loss Spectroscopy in the Electron Microscope[END_REF][START_REF] Brydson | Electron Energy Loss Spectroscopy[END_REF]. During the experiments the position of the plasmon peak appears unchanged, or perhaps slightly decreases, as a function of dose to within experimental error. A representative set of EELS C K-edge spectra as a function of electron irradiation is presented in Figure 10 (a)-(d). The π* peak maximum of all spectra was calibrated to 285 eV, and C K-edge spectra were acquired every 90-100 seconds along with the corresponding low loss peak which was used to deconvolute each K-edge spectra to remove plural scattering.

The data extracted from the spectrum acquired at t=0 were used to normalise subsequent sp 2 content data. Note that even after extensive electron irradiation damage, and even though the * component appears to undergo considerable change with a reduction of the graphitic order (Figures 345678), the presence of the π* peak indicates that the structure retains a distinct sp 2 character. To characterise the ratio of (planar) sp 2 bonded carbon to total carbon, an improved automated fitting routine for the C K-edge was achieved by inclusion of two additional Gaussian curves to the method proposed by Zhang [START_REF] Zhang | Investigating the structure of non-graphitising carbons using electron energy loss spectroscopy in the transmission electron microscope[END_REF] which employed three Gaussian peaks (Figure 11 While for fitting the extra two Gaussians (G4 and G5), the script fixed the FWHM and the centre of G1, G2 and G3 along with the area of G2 and G3, but allowed the area of G1 to vary in order to accommodate the new components, which were then fitted using the following constraints:

-G4 centred at ~286.5 eV (0.2 eV ≤ ≤ 1.5 eV) -G5 centred at ~288.5 eV (0.2 eV ≤ ≤ 1.5 eV).

The variation of planar sp 2 content was calculated by comparing the ratio of the π* intensity (G1) with the total C K-edge intensity (over a 20 eV window of onset 282.5 eV) which is proportional to the total number of carbon atoms present in the probed volume: where the intensity values from the initial C K-edge spectrum were used to calculate (I π* / I π*+σ* ) t=0 so as to ensure all spectral changes were relative to the initial structure. As can be seen in Figure 12 the sp 2 content changed approximately linearly with respect to electron dose. The sp 2 content dropped to 76% following an exposure of 0.32 dpa, a value consistent with the typical value of 75% sp 2 carbon derived from an amorphous carbon film using this fitting method. These changes provide evidence for the introduction of atomic disorder as a result of electron beam exposure. The errors shown in Figure 12 reflect an analytical error of ± 6% (determined by making slight changes to the energy windows to account for subjective discrepancy) and the variation in data between four separate experiments.

Figure 12 Change in sp 2 content with electron dose. Data were analysed using the 5 Gaussian fitting method and averaged over four regions of electron transparent (002) oriented PGA graphite. The error bars reflect the experimental variance and analytical error. Dose error = ± 6.4%

The signals from the G4 and G5 peaks were also analysed to provide information regarding the non-planar sp 2 carbon content of the specimen by measuring the ratio between the combined intensities under G4 and G5 (I R ) and the combined intensities under the G4, G5 and G1 peaks (I R +sp 2 ), as detailed in Equation 3.2. This ratio is proportional to the fraction of sp 2 -bonded carbon atoms which are bonded in a non-planar fashion and the increase in this quantity with increasing electron dose is plotted in Figure 13. Note the non-zero value of this quantity at zero-extrapolated dose is due to the inherent intensity in this spectral region (even in a pure planar sp 2 carbon based material) as well as any non-planar sp 2 carbon atoms located at crystallite grain boundaries within the analysed volume. Comparing the data in Figure 12 and Figure 13, it appears as though the loss of planar sp 2 bonding (very roughly a 20% decrease) is predominantly accounted for by an increase in non-planar sp 2 bonded carbon.

Non planar s𝑝 2 = 𝐼 𝑅 𝐼 𝑅 + 𝑠𝑝 2 Equation 3.2 relating to an increase in C-C bond length (Figure 14). The error associated with these data increases for higher doses where the MSR peak becomes wider leading to a degree of uncertainty in peak energy assignment; the error bars in Figure 14 represent experimental variation. Results obtained by Daniels [START_REF] Daniels | Investigating carbonization and graphitization using electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM)[END_REF] following a series of graphitization experiments are analogous, whereby the graphite began as a highly disordered material with an average bond length of 1.435 Å and through annealing became a near-perfect structure with a corresponding bond length of 1.420 Å. In Daniels' case, the decrease in bond length during graphitization was attributed to a decrease in sp 3 bonding (bond length = 1.54 Å) and the removal of heteroatoms and aliphatic molecules (bond length > 1.42 Å). In the present case however, no heteroatoms and aliphatic molecules are introduced to the system and the high energy barrier between graphite and diamond phases makes sp 2 to sp 3 transformation unlikely unless at very high temperature (~1000 K) [START_REF] Banhart | The role of lattice defects in the formation of new carbon structures under electron irradiation[END_REF]. However, we have established that the electron irradiation induces nanocrystallinity with atomic disorder at crystallite grain boundaries, (Figure 2 and7), and it is this disorder that reduces the packing efficiency of carbon atoms which is thought to increase the average bond length (e.g. for the most extreme case of amorphous carbon, the bond length increases to 1.44 Å [START_REF] Magnuson | Electronic structure and chemical bonding of amorphous chromium carbide thin films[END_REF]). It is thus suggested that introduction of dislocations and defects along with a bending of planes (the introduction of non-six-membered rings of carbon atoms [START_REF] Karthik | In situ transmission electron microscopy of electron-beam induced damage process in nuclear grade graphite[END_REF]) following electron irradiation increases the average C-C bond length. One might expect that an increase in bond length would lead to a reduction in valence electron density (and thus the possible slight reduction in plasmon energy with increasing dose as shown in Figure 9).

Final Discussion and Conclusions

We have presented a new methodology to quantitatively analyse TEM micrographs of irradiation damaged graphite. Following electron irradiation at 200 keV, a decrease in the graphite (002) fringe length and an increase in tortuosity and relative orientation was observed. Analysis of the low and core loss of several EEL spectral series indicates little or no change in valence electron density, a decrease in planar sp 2 content (to levels associated with amorphous carbon at the highest doses), an increase in non-planar sp 2 content (of inverse proportion to the planar sp 2 reduction and an increase in C-C bond length, all reflecting an increasing tortuosity of (002) layer planes and an increasing nanocrystallinity upon exposure to an increasing electron dose, Future work will involve the application of this analysis methodology for radiation damage to electron-irradiated samples at nuclear reactor temperatures (400°C) as well as neutron-irradiated samples from nuclear reactors and materials test reactors. The full potential of the image analysis software provided by the PyroMaN research group will also be investigated.

Figure 1 (

 1 Figure 1 (a) Polarised light micrograph of pile grade A (PGA) nuclear graphite showing the main constitutive elements, with the outlined area in red corresponding to (b), a schematic of short range features within the filler particle (outlined in green) and binder phase (h outlined in blue).

Figure 2 (

 2 Figure 2 (a) -(d) Electron micrographs of PGA graphite with their corresponding SAED patterns during electron beam exposure at 200 keV and room temperature, receiving 4.2 x 10 18 electrons cm -2 s -1 (2.4 x10 -4 dpa s -1 ±6.4%). (a) D = 0.01 dpa, (b) D = 0.1 dpa, (c) D = 0.2 dpa, (d) D = 0.3 dpa. (e) The same experiment was performed at 400 °C, the micrograph was recorded after a dose of 0.2 dpa.

Figure 3 Figure 4

 34 Figure 3 Change in plane spacing with respect to electron dose as measured from the spacing of (002) spot spacing in a series of SAED patterns. Error bars represent analytical error following the analysis of three damage series. Dose error = ± 6.4%

  (c) where only a third of the planes are detected. The orientation maps of these micrographs (Figure 5 (d) and (h)) show localised changes in the orientation of neighbouring planes where red indicates no relative change, yellow/green indicates a clockwise misorientation and purple/blue indicates an anticlockwise misorientation (as illustrated in the chart next to Figure 5 (h)). Statistical

Figure 5

 5 Figure 5 HRTEM images before (a-d) and after (e-h) 200 kV electron beam exposure for 5 minutes with electron flux 4.2 x 10 18 electrons cm -2 s -1 (2.4 x 10 -4 dpa s -1 ± 6.4%) (equating to 1 dpa). Images (a) and (e) show the raw HRTEM images, (b) and (f) the filtered HRTEM images, (c) and (g) illustrate the detection of (002) fringes within the HTREM, and (d) and (h) are orientation maps of the original HRTEM images.

Figure 6

 6 Figure 6 Comparison of fringe length and tortuosity data for the same region of interest having received low (red) and high (blue) electron irradiation doses.

Figure 9

 9 Figure 9 Change in plasmon peak position with dose. Data are averaged over two regions of electron transparent (002) oriented PGA graphite. The error bars reflect the experimental variance. Dose error = ± 6.4%

Figure 10

 10 Figure 10 Change in EEL spectra with dose (D): (a) D = 0 dpa; (b) D = 0.113 dpa; (c) D = 0.209 dpa; (d) D = 0.267 dpa ± 6.4%

  (a)): G1 centred at ~285 eV for the C=C π* component; G2 ~292 eV, the C-C σ* component and G3 ~300 eV, the C=C σ* component. The additional Gaussians were positioned under the "residual" peak (~286-288 eV) described in Zhang's method as of uncertain origin: either from the presence of additional heterospecies (e.g. O or H) or the presence of a non-planar sp 2 -bonded (fullerene-like) carbon component, as is the case here.

Figure 11 (

 11 Figure 11 (a) Circled residual peak signal from a 3 Gaussian fit performed in Gatan Digital Micrograph (b) Circled residual peak signal from a 5 Gaussian fit performed in Hyperspy. The residual peak from (a) was deconvoluted into two separate peaks in (b).

Figure 13

 13 Figure 13 Change in non-planar sp 2 content with dose over two regions of electron transparent (002) oriented PGA graphite. Error bars reflect experimental variance and analytical error. Dose error = ± 6.4%

Figure 14

 14 Figure 14 Change in MSR peak position with dose over two regions of electron transparent (002) oriented PGA graphite. Error bars reflect experimental variance and analytical error. Dose error = ± 6.4%

Acknowledgements

This paper was written by Brindusa Mironov and Helen Freeman. We acknowledge Abbie Jones of the University of Manchester, UK, for the provision of polarised light micrographs and Anne. A. Campbell at the University of Michigan, U.S.A, for the provision of the Graphite Anisotropy Analysis Program (GAAP). Fred S. Hage would like to acknowledge Magnus Kristofer Nord (Norwegian University of Science and Technology, Trondheim, Norway) for assistance and useful discussions when setting up the C-K edge fitting procedure in HyperSpy (freely available from http://hyperspy.org/). SuperSTEM is the UK Engineering and Physical Sciences Research Council (EPSRC) National Facility for aberration-corrected STEM. Funding was provided by the National Nuclear Laboratory and EPSRC (grants EP/J502042/1 and EP/I003312/1).