Viviane Bouyer

Isabelle Ranc-Darbord

Philippe Hervé

Gérard Baudin

Christian Le Gallic

François Clément

Guy Chavent

Isabelle Darbord
email: isabelle.darbord@u-paris10.fr

Shock-to-detonation transition of nitromethane: Time-resolved emission spectroscopy measurements

Keywords: Nitromethane, Shock-to-detonation transition, Emission spectroscopy, Temperature profile, Absorption coefficient models, Inverse problems, Equation of radiative transfer

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Introduction

This work deals with the study of the shock initiation of a liquid high explosive, nitromethane (NM). Many experimental investigations as well as theoretical researches have been performed on NM for about 40 years but the detonation mechanisms are not still completely explained. Their understanding rests mainly on pressure and velocity measurements. However, to improve knowledge of the initiation and detonation of high explosives, temperature is an essential factor. It is of great interest for the understanding of the involved chemical kinetics, for the accuracy of hydrodynamic codes related to condensed explosives, and for the characterization of the thermal effects of high explosives.

For many years a nonintrusive technique, optical pyrometry, has been used to get the temperature during the shock-to-detonation transition (SDT) of NM and also during the detonation of very nonideal high explosives [START_REF] Léal-Crouzet | Application de la pyrométrie optique à la mesure de température des produits de réaction d'explosifs condensés en régime d'amorçage et de détonation[END_REF][START_REF] Kato | Contribution à l'étude des détonations des mélanges hétérogènes de nitrométhane et d'aluminium[END_REF]: it has a response time of only a few nanoseconds, compatible with the swiftness of the phenomena. However, the temperature determination, based on thermal radiation emitted by the explosive, requires knowing the emissivity of the gaseous or solid species involved during the SDT. This parameter is not measurable under such conditions of pressure (several GPa) and temperature (several thousands of K). These thermodynamic states peculiar to detonation are not easily reachable and most of the techniques used to determine the emissivity in static states [START_REF] Hervé | Techniques de l'ingénieur[END_REF][START_REF] Sacadura | Colloque SFT[END_REF] are unable to get sufficient temporal resolution. Moreover, the modeling of emission spectra from databases is restricted to lower pressures and temperatures corresponding to combustion [START_REF] Rothman | [END_REF][6][7]. That is why temperature determination by pyrometry is based on an assumption on the monochromatic emissivity ε λ . The simplest solutions consist of black (ε λ = 1) or gray (ε λ1 = ε λ2 ) body hypotheses [START_REF] Von Holle | 7th Symposium (International) on Detonation[END_REF][START_REF] Xianchu | 8th Symposium (International) on Detonation[END_REF]: if the wavelengths of the pyrometer are remote, the determination of the temperature is inaccurate. The development of multichromatic pyrometers [START_REF] Gardner | High Temperatures High Pressures[END_REF], such as the six-wavelength pyrometer of the CEG [START_REF] Gallic | 51st ARA Meeting[END_REF], leads to different solutions: the emissivity is stated as a polynomial of the wavelength λ [START_REF] Léal-Crouzet | 11th Symposium (International) on Detonation[END_REF][START_REF] Coates | [END_REF]. In the case of NM, the reaction and the detonation products are probably semi-transparent and the previous assumptions may then be unsuitable.

Therefore, to get still more information on the optical properties of the initiation and detonation products, a time-resolved emission spectroscopy technique has been developed and used to analyze plane shock impacts on NM targets [START_REF] Bouyer | 52nd ARA Meeting[END_REF]. These measurements give the radiation emitted by the explosive as spectral radiance. It is possible to determine the temperature profiles (spatial and temporal distributions) from the radiance values. These two data are linked by the equation of radiative transfer. In this paper, we determine the temperature using a method based on the mathematical inversion of the equation of radiative transfer of a semitransparent medium.

Experimental configuration

NM is a liquid explosive that has the property of semitransparency at ambient pressure and temperature. We measured its transmission spectrum for a thickness of 25 mm (Fig. 1): NM is transparent between 0.4 and 0.85 µm.

The detonation experiments consist of plane shock impacts on explosive targets at 8.6 GPa, under conditions of one-dimensional strain. These experiments have been performed previously for pyrometry measurements [START_REF] Léal-Crouzet | [END_REF]. A single-stage powder gun propels the projectile at the target at a velocity of 1940 m/s to initiate the detonation. NM is confined in a polyethylene chamber of depth 25 mm, closed on one side by a copper transfer plate and on the other side by a lithium fluoride window (LiF) (Fig. 2). Several measurement techniques are used: a polarization electrode records the time of shock entrance and the time of formation of the superdetonation and the detonation, piezo-electric pins signals lead to the measurement of the shock and detonation velocities.

An optical probe using achromatic doublets collects the radiation emitted during the detonation and transmits it to the emission spectroscopy device (Fig. 3) with an optical fiber. The optical probe has been designed to collect thermal radiation be- The spectroscopy system has to be calibrated to get a conversion between the radiance (thermal radiation) and the voltage output (or current for the PMT). Because of the relatively high spectral resolution, the 16 channels of the PMT may be considered as monochromatic. A MIKRON M390 black body source is used to perform the calibration (900-3300 K).

Experimental results

We have performed several experiments of plate impacts at ∼8.6 GPa on 25-mm-thick NM targets at ambient pressure. Table 1 shows the characteristics of these shots: V is the projectile velocity, p is pressure, t 1 is the time of formation of the superdetonation, and t 2 is the time of formation of the strong detonation according to the classical model of the shock-to-detonation transition (SDT) proposed by Chaiken [16]. The emission spectroscopy measurements are reproducible in terms of radiance signals versus wavelength.

Here we analyze one of our experiments. Fig. 4 represents the 3D radiance profiles obtained during shot 1020 and a typical temporal profile at a given wavelength. Emission spectroscopy measurements with the piezoelectric pins and electrode signals clearly show the different stages of the SDT as described by Chaiken. A first jump at 1.65 µs characterizes the formation of a detonation wave propagating in shocked NM resulting of a thermal explosion. This wave is called superdetonation and overtakes the initial shock wave at a second jump at 2.15 µs. A strong detonation is then formed and gradually decays into a steady-state detonation that propagates until the interaction with the LiF window at 4.5 µs.

Radiance temperature (T radiance ) profiles (also called brightness temperature profiles) have been calculated with the Planck law, assuming the emissivity ε λ is 1, i.e., emissivity of a black body:

(1)

T radiance = C 2 λ ln C 1 λ -5 L λ mes + 1
with λ for the wavelength, L λ mes for the measured radiance, C 1 = 3.741 × 10 -16 W m 2 , and C 2 = 1.4388 × 10 -2 m K are constants of the Planck law.

The true temperature will be at least equal to the maximum radiance temperature.

Because of the noise of the system, minimum temperature corresponding to the measured radiance is about 1500 K. The main result is that the radiance temperature is not constant during the propagation of the superdetonation and the detonation wave (Fig. 5). The reaction products, which consist of gaseous species and solid carbon, are probably semi- transparent. We obtain the minimum temperature for each step of the SDT that are shown in Table 2.

Experimental uncertainties have been studied in [START_REF] Bouyer | Etude de la transition choc-détonation du nitrométhane par spectrométrie d'émission[END_REF]. For low wavelengths (<0.5 µm) and low temperatures (<1500 K-noise limit), the error on radiance can reach 60%. But this error decreases when the temperature increases and is lower than 10% for temperatures higher than 1800 K and for all wavelengths. On the contrary, the error on radiance temperature is negligible (lower than 0.6% in all cases).

Changes in radiance depending on wavelength have been studied for various typical stages of the SDT (Fig. 6). From the formation of the superdetonation, a hollow appears between 0.65 and 0.75 µm. It remains until the end of the propagation of the detonation wave. This hollow characterizes semitransparent optical properties of the gaseous species involved.

Discussion

We compared our measurements to those obtained by the pyrometry technique, in particular results from Leal-Crouzet et al. [START_REF] Léal-Crouzet | Application de la pyrométrie optique à la mesure de température des produits de réaction d'explosifs condensés en régime d'amorçage et de détonation[END_REF] and Urtiew [START_REF] Urtiew | [END_REF] (Fig. 7). Urtiew emission results put in evidence the wavelength dependence on radiance temperature. It is thus not likely that the detonation wave behaves as a black body as proposed by Leal-Crouzet and other authors [START_REF] Sellam | 8th Symposium (International) on Detonation[END_REF][START_REF] Dremin | [END_REF], who obtained constant radiant temperature. Their results can be explained by the larger spectral resolution they are using (two or three times larger than ours); the wavelength dependence on the emissivity is then integrated. The choice of the pyrometer wavelengths can thus affect the estimation of the optical characteristic of the studied medium [START_REF] Bouyer | Shock Compression of Condensed Matter-2001[END_REF].

Then we tried to find out the optical properties of shocked NM and of the reaction products behind the superdetonation wave and behind the detonation wave.

The evolution of the signal emitted after the shock entrance leads us to conclude that the shock front is transparent. Indeed, if the shock front were opaque, the emitted radiance would be constant until the formation of the strong detonation. Moreover, the radiance is about the same as the radiance emitted by the copper impactor at 500 K [START_REF] Bouyer | Etude de la transition choc-détonation du nitrométhane par spectrométrie d'émission[END_REF]. This means that shocked NM is transparent and we will consider that its absorption coefficient is the same as that for neat NM. Besides, Yoo et al. [START_REF] Yoo | Symposium on Decomposition, Combustion and Detonation Chemistry of Energetic Materials[END_REF] pointed out that shocked NM remains transparent in the spectral range 0.35-0.75 µm. The radiant temperature after shock entrance, prior to the detonation transition, reaches 2500 K. Such a temperature is not in agreement with Chaiken model, which predicts a shock temperature of about 1000 K. It is also clearly greater than those predicted by Lysne and Hardesty [START_REF] Lysne | [END_REF] (1100-1200 K) or Winey and Gupta [24] (1000 K). Yet a similar high temperature has been also recorded by pyrometry technique in the visible range in the same single sustained shock experiments [START_REF] Léal-Crouzet | [END_REF] and in multiple-shock experiments performed at the CEG [START_REF] Baudin | Symposium HDP V, Hautes pressions dynamiques[END_REF][START_REF] Serradeill | Symposium HDP V, Hautes pressions dynamiques[END_REF]. The reverberating shock tests described in [START_REF] Serradeill | Symposium HDP V, Hautes pressions dynamiques[END_REF] consist in pyrometry measurements (eight wavelengths from visible, 0.5 µm, to IR, 3.6 µm, with 5 ns time resolution) and material velocity measurements using VISAR. In this work, it appears that the temperature profile in the NM layer after the fourth shock wave (13.2 GPa) is not uniform (see Fig. 8). Indeed, for visible and near IR wavelengths of the pyrometer where NM is transparent (same wavelength range as our emission spectroscopy device), shocked NM reaches temperatures greater than 2200 K. On the contrary, measurements in IR range (2.2 and 3.6 µm) lead to radiance temperatures between 500 and 1000 K which is more in agreement with Chaiken model. At these wavelengths, NM is opaque and these low temperatures correspond to shocked NM at the interface with the pyrometry mea-surement window. Thus, the higher temperatures are necessarily located close to the aluminum transfer plate and can be explained by the presence of local decomposition reactions inside the shocked liquid NM, leading to hot spots. The authors also noticed that the intensity of the laser in the VISAR signals recorded at the transfer plate/NM interface do not decrease, which implies that there is not a bulk decomposition of NM. These results support the presence of these local reactions, created after the passage of shock waves on bubbles and heterogeneities inside NM. These hot spots contribute to increase the radiance measured in the visible range without changing the optical properties of neat NM. The mechanism of emission is shown in Fig. 9.

The chemical composition of the reaction and detonation products has been calculated with the CHEETAH code [START_REF] Fried | CHEETAH 2.0 User's Manual[END_REF] using two classical equations of state and several data bases. Table 3 shows the major species (with a molar fraction greater than 1%), gas and carbon clusters which form a semitransparent medium [START_REF] Bouyer | Etude de la transition choc-détonation du nitrométhane par spectrométrie d'émission[END_REF][START_REF] Bouyer | Shock Compression of Condensed Matter-2001[END_REF]. The radiation emitted by this medium is given by the complete equation of radiative transfer (ERT),

dL λ ( i) dx = K λ L 0 λ (T ) -β λ L λ i (2) + σ λ 4π 4π L λ i Φ λ i → i dΩ i ,
with β λ the extinction coefficient such as β λ = K λ + σ λ ; K λ (λ, T ) and σ λ are absorption and scattering coefficients; L λ ( i) is the spectral radiance in the direction i, dΩ( i) is the solid angle centered in the direction i, and Φ λ is the spectral scattering phase function [28]. The scattering phase function represents the probability of the scattering of the radiation coming from the direction i toward direction i.

Carbon cluster size has been estimated as 50 Å by Winter and Ree [START_REF] Winter | 11th Symposium (International) on Detonation[END_REF]. Their particle size parameter x (x = πd/λ, where d is the particle diameter or size of the cluster) is between 0.02 and 0.04 for λ between 0.4 and 0.85 µm. The scattering regime of carbon is therefore Rayleigh scattering. Thus, the ERT is simplified:

(3)

dL λ dx = K λ L 0 λ (T ) -K λ L λ (x).
In a first stage, we have studied the optical properties of the detonation products during the propagation of the steady-state detonation. According to the Chapman-Jouguet theory, the temperature profile behind the detonation front is constant. Thus, K λ only depends on λ and during this stage, the measured radiation L λ ( ) can be written with the equation of radiative transfer, (4)

L λ ( ) = L 0 λ (T) 1 -exp -K λ (x D -x 0 )
, where is the cell depth, T is the detonation products temperature, K λ is the absorption coefficient of the detonation products, and x D and x 0 are the detonation front and impactor/explosive interface positions.

According to Eq. ( 4), the radiance should increase when the detonation wave moves forward as (x Dx 0 ) increases, but we can notice in Fig. 4 that the emitted radiation before the interaction with the LiF window is constant. It could be explained by surface area emissivity of the detonation front. Yet, the radiance temperature values obtained during the detonation propagation show a deviation of about 500 K depending on the wavelength (Fig. 5). This confirms that the detonation products are semitransparent. Moreover, we can see in Fig. 10 that the shape of the radiant spectrum is the same before and after the interaction with the window. That would not be possible in the case of an opaque detonation front.

The constant signal is more likely due to the fact that the radiation comes from a small thickness behind the detonation wave, remaining constant during the propagation (see Fig. 11). The detonation products can be considered optically thick. The emission spectroscopy system measures the radiation emitted by detonation products that have just formed. Indeed, the same shape obtained in Fig. 10 for the radiance curves before and after the interaction with the LiF window means that the same chemical species are emitting.

The reaction products behind the superdetonation wave are also semitransparent. Indeed, Doppler laser interferometry measurements [START_REF] Baudin | Symposium HDP V, Hautes pressions dynamiques[END_REF] performed at the CEG show that the laser beam at 0.6 µm crosses the reaction products thickness. By focusing on the signal emitted during the propagation of the superdetonation, it seems that for wavelengths higher than 0.6 µm, the signal is constant, whereas under 0.6 µm, it in- creases. The limit 0.6 µm is not accurate because of the temporal and spectral resolutions of the recording.

In order to precise it, it would be necessary to perform measurements with a higher time sampling implying that acquisition is stopped before the end of the propagation. Moreover, to increase spectral resolution, the grating or the detectors have to be changed.

However, we can already analyze the differences between the two spectral regions. Fig. 12 represents this emission mechanism. In the region 0.6-0.85 µm (radiation 2 in figure), reaction products behind the superdetonation behave as detonation products; i.e., the emitted radiation comes from a layer of products behind the superdetonation front. The reaction products are said to be optically thick. In contrast, in the region 0.3-0.6 µm (radiation 1 in figure), reaction products are transparent.

The results obtained on the optical properties of shocked NM and reaction and detonation products lead us to try to find out which species cause the hollow in the radiance spectra. The knowledge of the optical parameters as emissivity or absorption factor is still rather weak. Therefore, only a mathematical inversion of the equation of radiative transfer allows determining both temperature profiles and absorption coefficient.

Determination of temperature profiles and absorption coefficient

The inversion consists in determining the temperature profile and the absorption coefficient for each wavelength from the experimental radiance spectra, as functions of time and position. This work is based on the automated determination of physical properties of detonation (temperature, celerity) and absorption coefficient of the medium in the successive stages of the shock to detonation transition by trying to reproduce at best the experimental measurements. The identification is achieved by a least square algorithm, applied to the equation of radiative transfer for a semitransparent medium in the Rayleigh approximation (Eq. ( 3)) [START_REF] Bouyer | Shock Compression of Condensed Matter-2001[END_REF].

In order to dramatically reduce the number of unknowns, we introduce a parameterization of temperature and absorption coefficient. We first take advantage of the fact that the different species encountered at the different stages of the shock to detonation transition always correspond to different values of the temperature. Hence, the absorption coefficient for each wavelength can be seen as a function of the sole temperature. As described in [START_REF] Bouyer | 12th Symposium (International) on Detonation[END_REF][START_REF] Clément | Détermination des profils de température pendant la détonation d'un explosif liquide, le nitrométhane [Determination of Temperature Profiles during the Detonation of a Liquid Explosive: Nitromethane[END_REF], the shape of the absorption coefficient is estimated from the analysis of the radiance spectra when the steady state detonation is reached. Then, we notice that the temperature profile presents shocks (reactive or nonreactive) that move at various speeds. This is the typical shape of the solution of a scalar conservative law. So we take the temperature as the solution to a convectiondiffusion problem of the form ( 5)

∂T ∂t + ∂f (T ) ∂x = g T (x, t) ,
where f (T ) is an advection term and g(T ) a reaction term. The parameters to be determined have been chosen according to a sensitivity analysis taking into account the noise level of the experiment. The number of parameters was further reduced with additional information on absorption coefficient and by searching the temperature profile among a limited family that is physically admissible, through specific choices for the advection term f and the reaction term g.

The least-squares approach consists in minimizing the normalized quadratic criterion

J (L cal ) = k,j L mes,k j -L cal,k j σ k j 2 (6) k,j L mes,k j σ k j 2 ,
where L mes is the measured radiance, L cal is calculated with ERT from the selected temperature and absorption coefficient, and σ stands for the noise and relative errors of measurements. The indices j and k respectively stand for wavelengths and time steps. This definition of the normalized criterion makes it possible to compute the percentage of retrieved radiance profile through 1 -J .

The difficulties and the details of the inversion method are described in [START_REF] Bouyer | Etude de la transition choc-détonation du nitrométhane par spectrométrie d'émission[END_REF][START_REF] Bouyer | 12th Symposium (International) on Detonation[END_REF][START_REF] Clément | Détermination des profils de température pendant la détonation d'un explosif liquide, le nitrométhane [Determination of Temperature Profiles during the Detonation of a Liquid Explosive: Nitromethane[END_REF]. The inversion algorithm has been validated for simple cases of detonation as steady-state detonation and detonation with reaction products relaxation [START_REF] Bouyer | 12th Symposium (International) on Detonation[END_REF][START_REF] Clément | Détermination des profils de température pendant la détonation d'un explosif liquide, le nitrométhane [Determination of Temperature Profiles during the Detonation of a Liquid Explosive: Nitromethane[END_REF].

We present here the results of the inversion method applied to the experimental radiance profiles we measured during shot 1020. The computation is divided in two steps: the first one consists in retrieving the signal between the shock entrance and the overtaking of the shock wave by the superdetonation wave (identified parameters are the temperature just behind the shock front, the temperature just behind the superdetonation front, and the celerity of the superdetonation front); the second one consists in retrieving the signal during the propagation wave until the interaction with the LiF window (identified parameter is the detonation temperature). The recorded radiance profiles between 2.12 and 2.9 µs correspond to the propagation of the strong detonation with a temperature profile which is difficult to model. This part of the signal was not processed.

SDT phase between 0 and 2.12 µs

The sensitivity analysis showed a low sensitivity of the measured radiance with respect to the celerity of the superdetonation front. A boundary on celerity has been introduced to limit its value to a physically range. Fig. 13 shows computed and measured radiance profiles for several wavelengths; 83% of the radiance profiles are retrieved. The oscillations visible on the measured radiance profiles do not penalize the criterion J , which is due to the use of the L 2 norm.

Detonation-propagation phase between 2.9 and 4.5 µs

In this case, 89% of the radiance profiles are retrieved (Fig. 14). The search for the temperature profile has been reduced to the search for a constant temperature profile. Indeed, difficulties still remain in the inversion of the release of the reaction products part of the profile. Fig. 15 shows the temperature profiles versus position in the explosive cell for both stages, SDT and detonation propagation. The inversion gives one profile every 10 ns but fewer profiles are drawn on the figure for more clarity. The shock temperature value T S thus determined is 2810 K, the superdetonation temperature value once the wave is sustained, T SD , is 3166 K, and the detonation temperature T D is 3253 K. At the entrance of the shock, the result shows high temperatures, which confirms the existence of hot spots, due to local chemical reactions, as explained earlier.

Taking into account the noise level of the radiance measurements, the proportion of retrieved radiance profiles is rather high. However, apart from the shock temperature, the superdetonation and the detonation temperature are a bit lower than the minimum temperature given by the brightness temperature we determined from the emission signals (Table 2). The relative error in superdetonation temperature compared to this minimum is 1.1% and the relative error in detonation temperature is 9.6%. The inversion method gives relevant results for the SDT phase but for the detonation-propagation phase, the temperature is rather weak because the calculation was based on the search of a constant temperature profile, whereas it is possible that the steady-state detonation was not already reached.

Absorption coefficient profiles

Fig. 16 shows the computed absorption coefficient during SDT and steady-state detonation (for wavelengths lower than 0.4 µm, NM is opaque). Among the major species listed in Table 3, only two species in the reaction products emit in the wavelength measurement range: carbon clusters [START_REF] Modest | Radiative Heat Transfer[END_REF] and water vapor (main lines are given in Table 4 at 0.72 and 0.82 µm). We cannot define a more accurate equation of state because the detonation velocities calculated with CHEETAH agree well with experimental data. Therefore concentrations are only indicative.

The calculated volume fraction of carbon varies from 0 up to 9% depending on the equation of state. So, in a first approximation, we choose to represent the Rayleigh scattering regime of the particles by an independent regime [START_REF] Bouyer | 12th Symposium (International) on Detonation[END_REF][START_REF] Tien | Annual Review of Numerical Fluid Mechanics and Heat Transfer[END_REF]. The absorption coefficient of particles for different volume fractions f v is shown on Fig. 17 with a refractive complex index 1.8 -0.9i [START_REF] Modest | Radiative Heat Transfer[END_REF]. Carbon particles produce a continuous background. Comparing these predicted values with the computed absorption coefficient (Fig. 16), we can deduce that the volume fraction of carbon is very small (<10 -3 %). This result is more in agreement with the data base of the Sandia National Laboratory for the gaseous BKW and JCZ3 equations of state.

Regarding the contribution of water vapor, a model is required to represent the absorption coefficient depending on wavelength. The model proposed for the emission of water vapor in the reaction products is simple in view of the lack of knowledge on the emission of gases at the pressure and temperature encountered. It is based on the absorption lines of H 2 O in the visible range and near-infrared range at ambient pressure and temperature (Table 4) and on a representation of the line width with the exponential wideband model of Edwards [START_REF] Modest | Radiative Heat Transfer[END_REF] that qualitatively considers line broadening with temperature and pressure. Fig. 18 represents the absorption coefficient calculated with the same bandwidth for all lines, as a first approximation. According to such a model, the absorption coefficient spectrum would be the superposition of a continuous background and broad lines of water vapor. The spectral resolution of our spectrometry measurement can exhibit such an absorption coefficient spectrum. But the inversion method does not compute such a spectrum (Fig. 16). We obtain a continuous spectrum without the right spectral lines. However, the contribution of water vapor is conceivable because of the effects of the pressure. The thermodynamic states attained in the NM reaction products (local constant volume explosions of shocked NM at 2810 K and 14-17 GPa, superdetonation at 3166 K and 25.1-31.6 GPa, detonation at 3253 K and 11.4-13.2 GPa, according to the measurements and the thermochemical calculations) are unusual and out of the validity range of the known data of gas and particle emission spectra. In such conditions of temperature and pressure, the main cause of lines broadening can be the pressure effect (Doppler lines broadening from high temperature is negligible). But other effects are conceivable. The convolution of the molecules collisions effect and autoabsorption is a possible explanation of continuous spectral absorption coefficient. The number of molecules per unit volume is high: the molecular density is about 10 29 molecules/m 3 at 3500 K and 15 GPa. In such a thermodynamic state, the mean distance between molecules (∼10 -10 m) is close to their mean diameter collision, so that no model exists at present. In dense plasma, highly nonhomogeneous and optically thick (as the detonation products), autoabsorption of lines can lead to an excessive broadening and a total reversal of the lines. The idea of nonthermal emissivity emerges in the work of Gruzdkov and Gupta about shock-induced decomposition of NM [START_REF] Gruzdkov | [END_REF]. They measured the emission spectrum between 0.4 and 0.75 µm of NM shocked at 16.7 GPa under a stepwise loading process and a peak appeared at 0.65 µm. They explained it as luminescence from reaction products, possibly NO 2 . The fluorescence of NO 2 can indeed last 44 µs [START_REF] Herzberg | Electronic Spectra and Electronic Structure of Polyatomic Molecules[END_REF] so our emission measurements during the SDT could record the chemiluminescence of NO 2 after its formation during the NM decomposition. However, as already said, comparison of the radiance spectra during the steady-state detonation and after the interaction with the LiF window shows the same shape (Fig. 10). Indeed the decomposition of NM is finished and the chemiluminescence of NO 2 cannot occur. Moreover, the hot spot temperature is slightly lower than the calculated constant volume adiabatic explosion temperature of shocked NM (Table 3). This temperature is thus compatible with a purely thermal emission, unlike the conclusions of Gruzdkov and Gupta [START_REF] Gruzdkov | [END_REF]. Therefore, although the absorption coefficient profiles measured and retrieved can hardly be explained in view of today's knowledge at high pressure and temperature, the temperature profiles obtained with the inversion method are compatible with a purely thermal emission mechanism (retrieved temperature values are not slightly different from the temperature calculated with CHEETAH, shown in Table 3).

Conclusions

We developed a time-resolved emission spectroscopy device in the visible range 0.3-0.85 µm. Sixteen radiance profiles have been obtained for each experiment of plane shock impact on liquid explosive target at about 8.6 GPa. The combination of other metrological tools and the spectrometer makes it possible to display the different stages of the SDT as described by Chaiken. The calculation of the radiance temperature gives the minimum temperature reached during each stage. For shocked NM, this value reaches 2500 K and during the superdetonation and detonation propagations, respectively 3200 and 3600 K.

The spectral analysis of the emitted intensity related to the temporal analysis shows a particularity in the emission spectrum that appears from the formation of the superdetonation wave. This result could not be recorded by the former pyrometer measurements. The following conclusions on the optical properties during the different steps of the initiation as well as the propagation of the detonation have been proposed:

-Our results correlated to multiple shock experiments conducted at the CEG [START_REF] Baudin | Symposium HDP V, Hautes pressions dynamiques[END_REF][START_REF] Serradeill | Symposium HDP V, Hautes pressions dynamiques[END_REF] show that NM remains transparent under shock and confirm that the 2500 K high value of shock temperature are consecutive to local chemical reactions. -The chemical composition of reaction products changes with the formation of the detonation wave. They are still semitransparent but become optically thick in the range 0.4-0.85 µm. The emission comes from a small-thickness layer just behind the detonation front. Therefore, the common black or gray body assumption cannot be used. -Reaction products appearing with the superdetonation wave are semitransparent. It seems that they are transparent in the range 0.4-0.6 µm but they absorb light in the range 0.6-0.85 µm, where they are called optically thick.

We carried out a mathematical inversion of the equation of radiative transfer in order to determine the temperature profiles and the absorption coefficient from the radiance measurements. The inversion method is of great interest to obtain temperature profiles during the SDT. With this approach, 80% of the measured spectra can be retrieved. We discussed the measured emission spectrum and the computed absorption coefficient obtained through classical emission models of solid particle and gases. It appears that without precise data at high pressure and high temperature, it is difficult to predict the effect of pressure on emission spectrum. Nevertheless, the inversion method based on thermal emissivity enables to help interpreting the measurements and progressing in the modeling of the SDT.

Fig. 1 .

 1 Fig. 1. Transmission spectrum of NM for a thickness of 25 mm measured by a FT-IR Fourier spectrometer.

Fig. 2 .Fig. 3 .

 23 Fig. 2. Detonation target.

Fig. 4 .

 4 Fig. 4. 3D radiance profiles of shot 1020 and one typical temporal profile.

Fig. 5 .

 5 Fig. 5. Radiance temperature profiles for the 16 channels of shot 1020.

Fig. 6 .

 6 Fig. 6. Radiance spectra at different times for shot 1020: (a) after shock entrance (0.3 µs), (b) before the superdetonation formation (1.2 µs), (c) superdetonation formation (1.65 µs), (d) 1.9 µs, (e) catch-up of the shock wave (2.15 µs), (f) strong detonation (2.45 µs), (g) overdriven steady state detonation (3.8 to 4.4 µs). Radiance values were averaged around the given time value.

Fig. 7 .

 7 Fig. 7. Radiance temperature of detonation wave of nitromethane versus wavelength compared to other works.

Fig. 8 .

 8 Fig. 8. Radiation emitted during multiple reverberating shocks experiments performed in CEG [25].

Fig. 9 .

 9 Fig. 9. Radiation emitted during the propagation of the shock wave.

Fig. 10 .

 10 Fig. 10. Comparison of the radiance spectra before and after the interaction with the LiF window.

Fig. 11 .

 11 Fig. 11. Radiation emitted during the propagation of the steady-state detonation.

Fig. 12 .

 12 Fig. 12. Radiation emitted during the propagation of the superdetonation wave.

Fig. 13 .

 13 Fig. 13. Comparison of the computed (retrieved) radiance and the measured radiance profiles during SDT.

Fig. 14 .

 14 Fig. 14. Comparison of the measured radiance and the computed radiance profiles during the propagation of the detonation.

Fig. 15 .

 15 Fig. 15. Temperature profiles versus position obtained by the inversion method.

Fig. 16 .

 16 Fig. 16. Computed absorption coefficient at shock (2810 K), superdetonation (3166 K), and detonation (3253 K) temperatures.

Fig. 18 .

 18 Fig. 18. Absorption coefficient of water vapor calculated with Eq. (4).

Table 1

 1 Characteristics of plate impact shots

		Shot No.
		76	1020	1045	2019
	V (m/s)	1926	1937	1936	1946
	p (GPa)	8.57	8.64	8.64	8.7
	t 1 (µs)	1.7	1.65	1.38	1.6
	t 2 (µs)	2.36	2.15	1.84	2.15
	tween 0.3 and 1.6 µm and the focus area has a
	2-mm diameter. The optical fiber is a 1-mm core
	silica fiber transmitting in the same spectral range.
	A JOBIN YVON TRIAX 180 grating spectrome-
	ter disperses the light beam. The grating, blazed at
	0.5 µm, has a groove density of 150 g/mm. The dis-
	persion is 32.8 nm/mm at the wavelength 565 nm.

A HAMAMATSU R5900U-01-L16 multianode photomultiplier tube (PMT) is used as a 16-detectors array to detect the signal in the spectral range 0.3-0.85 µm with a response time of 0.6 ns. The detectors are 0.8 mm large and 16 mm high and separated by a 0.2-mm space. Four TEKTRONIX TDS684 digitizers are used for data acquisition. During impact experiments, time sampling is 1 GHz. The time and spectral resolutions obtained are respectively 1 ns and 32 nm, each channel of the PMT detecting light flux on a 26-nm-wide spectral range.

Table 2

 2 Minimum temperature of the different stages of the SDT

	Stage of the SDT	Temperature (K)
	After shock entrance	2500
	Before the superdetonation formation	2600
	Superdetonation	3200
	Detonation	3600

Table 3

 3 Chemical composition (in mol/mol of explosive) of the reaction products from a steady-state detonation of NM calculated with CHEETAH using two classical equations of state (BKW and JCZ3) and several data bases (Lawrence Livermore National Laboratory BKWC, Sandia National Laboratory BKWS and JCZS)

		EOS BKW data base BKWC LLNL	EOS BKW data base BKWC SNL	EOS JCZ3 small data base JCZS SNL
		Det.	V cste expl.	Super det.	Det.	V cste expl.	Super det.	Det.	V cste expl.	Super det.
	P (GPa)	11.4	14.0	25.1	13.2	17.0	31.6	11.7	14.9	27.4
	T (K)	3666	3094	3509	3626	2914	3317	3442	2935	3123
	H 2 O	1.397	1.492	1.499	0.646	0.665	0.608	0.857	0.974	0.827
	N 2	0.499	0.500	0.500	0.558	0.449	0.429	0.458	0.460	0.479
	CO	0.358	0.134	0.061	0.070	-	-	0.538	0.212	0.119
	CO 2	0.120	0.186	0.219	0.558	0.593	0.534	0.254	0.362	0.412
	H 2	-	-	-	0.112	-	-	0.230	0.062	0.036
	CH 4	0.032	-	-	0.165	0.159	0.081	-	-	-
	H 3 N	-	-	-	0.092	0.101	0.140	0.083	0.079	0.042
	C 2 H 6				0.052	0.082	0.108	0.073	0.098	0.152
	CH 2 O 2	-	-	-	0.075	0.066	0.154	0.041	0.044	0.115
	C(s)	0.466	0.674	0.719	0.000	0.000	0.000	0.000	0.182	0.049

Notes. Det.: detonation. V cste expl.: constant volume explosion at 8.6 GPa, 1.842 g/cm 3 (shocked NM). Super det.: detonation wave propagating in compressed NM, i.e., superdetonation wave.

Table 4

 4 Position and intensity of absorption lines of water vapor[START_REF]Atmospheric Propagation of Radiation[END_REF]

	Wavelength (µm)	Intensity (a.u.)
	0.698	0.2
	0.724	1.08
	0.823	0.93
	0.906	2.4
	0.942	10
	0.977	2.1

Fig. 17. Absorption coefficient of carbon clusters with varying volume fraction according to Rayleigh scattering.

Acknowledgments

The work described here was carried out with financial support from DGA/SPNuc, for the interest of CEA. Each impact experience was performed at Physics of Explosive Laboratory at CEG with the assistance of ARES and Metrology staff. We are thankful to Regis Serradeill for his advice.


Martine Tamang

Mazaudon

Martine Mazaudon

I have been asked to give some advice on the development of a writing system for Tamang and tools to use in the classroom in order to teach reading and writing in Tamang. Further development of the use of Tamang (or for that matter other minority languages of Nepal) in education at a higher level will be outside the scope of my remarks.

Some valuable attempts have already been made to transcribe Tamang. I know of two systems presently and there may be more. I will say from the start that there are several possible solutions, and will outline some of the possibilities and difficulties with each. I will also say that the choice between possible solutions is not entirely technical but also largely political, and on that aspect I will refrain from expressing any opinion at all.

From the technical point of view, we will have to distinguish between what would be a perfect transcription from the point of view of the linguist, and what will be practical for the user. If we can achieve both all the better; if we cannot we will have to compromise. I will first discuss what would constitute completely correct transcriptions of the language, basing my analysis on Eastern Tamang, mainly the dialect of Risiangku, and see if the same system can be used for all dialects, in so far as I am informed on the other dialects.

Transcription and spelling

Ideally, a transcription of a language should represent each individual sound used in that language by a unique letter or symbol, or, less desirably, by a unique combination of letters or symbols. Such a system should be easy for speakers to learn, because in principle the speaker already "knows" the sounds of his language, although it may take him a while to get used to analysing words into individual sounds. All he has to learn is the equation between each individual sound (there are often about 40 of these) and the letter or combination of letters used to write it.

The spelling systems (or "orthographies") which are actually used for the world's languages are more or less close to this ideal. Nepali spelling is quite good as a transcription, especially if compared to English or French or Tibetan. In English, for example, there are many ways of writing the same sound, for example the sound [u] (as a linguist would write it), which occurs in the words to, too, two, through, blue, you, crew, and the same letter or combination may represent different sounds in different words. One reason for these anomalies is that the pronunciation of languages changes over time, so that the older a spelling system is, the farther it is likely to be from an ideal transcription of the modern pronunciation. Another reason is that foreign spellings may have been kept for borrowed words, like French words in English or Sanskrit words in Nepali. Whatever the reason, learning such a system in school takes up time that could be used to teach mathematics or geography. So why not change it? Here people will object that thousands of books have been published in the old spelling system and that changing it would cut off the younger generations from their traditional literature.

In a language which has not been written before, this question does not arise, and it is advisable to adopt a transcription principle such that each different sound is represented by a different letter, and only one. This is what is done by linguists when they use the International Phonetic Alphabet (IPA). This alphabet, which is based on the Roman alphabet, has been designed on purpose to have a very large number of symbols, or letters, and can provide enough different symbols to write down any language correctly. On the other hand it is not available in most printing presses, and I would not recommend using it for everyday transcription of Tamang, for practical reasons.

So the best thing to do would be to use one of the local alphabets, which are already familiar to a large number of people in Nepal, and which can be printed locally, and adapt it to fit the Tamang language. What I will discuss here is how much adaptation is required and what problems that adaptation may encounter.

What is Tamang linguistically?

Tamang is a language of the Tibeto-Burman family, belonging in the same branch as classical Tibetan, but it is not a descendant of classical Tibetan; it is not a Tibetan dialect. To use a family metaphor, Tamang is a grand-nephew of Classical Tibetan, not a grandchild. So one option is not available, that is writing Tamang etymologically by using Tibetan spelling. Let us take an example.

The word for 'eight' in Classical Tibetan is spelled brgyad. In Lhasa Tibetan it is gyeÚ, in Tamang ¢brat. These three forms are related, in technical terms they are 'cognates'. But while there are regular rules which allow the transformation of the Tibetan spelling into its Lhasa pronunciation, such rules cannot be made for Tamang.

In Lhasa Tibetan we can learn that in a word which begins with brgy-only gy in pronounced. This is the case in the word 'eight' pronounced gyeÚ , and also in other words, like '100', written brgya and pronounced gya . In comparing Tamang to Classical Tibetan, we could observe that in the word 'eight' the initial group brgy is pronounced br . But the word for '100' in Tamang is gyarca , with gy instead of br corresponding to Written Tibetan brgy. So while in Lhasa a single pronunciation rule can be learned, in Tamang we would need two different rules: 'brgy-is sounded gy', for the word '100', but 'brgy-is sounded br' for the word 'eight'. This will not make it easier for Tamang children to learn how to read and write! On the other hand, the Tibetan alphabet, simply as an alphabet, not as a spelling system, is one of the possible choices. In that option, 'eight' in Tamang would be written dC [-or something like that, as we will see later, and not d›X [-.

Tamang speech sounds and possible writing methods

Two different non-Roman alphabets are more or less familiar to people living in Nepal, and especially to Tamangs: Devanagari, which is used to write Nepali, and the Tibetan script, itself derived from an ancestor of Devanagari many centuries ago. We will discuss transcriptions based on these two alphabets. Our first task, however, will be to study the sounds actually used by Tamang speakers to make up words -in technical terms, the 'phonology'. There are some differences between dialects, which we will discuss later, but they are not too great.

Let us start with the most difficult part, so the rest will seem easy afterwards. Tamang has tones. Nepali and English do not. Tibetan has some tones too, but 1) they are different from Tamang tones and 2) they are not transcribed as such in the Tibetan writing system. So whatever alphabet is used, some addition will be necessary if we want the transcription of Tamang to be completely correct.

Tones

All dialects of Tamang distinguish 4 tones. Their phonetic description here is based on the Risiangku dialect, but, with some reflection, speakers of other dialects can discover them in their own speech. Since there are some words which are not used in one or the other dialect, the examples given here will not work for everybody. You have to try and understand the principle and look for examples in your own dialect. If you do not speak Tamang or do not speak it fluently you should ask a village person to help you find examples. An illiterate grandmother might be a good choice.

One pair of words which seems to be found in all dialects, at least in the Eastern ones, is 'to sell' and 'to catch or tie up an animal'. Both could be transcribed as cu≥-ba in the phonetic alphabet2 , in Roman letters, or as r' ª-aF in Devanagari, or as g$ r-din the Tibetan alphabet. The problem is that these two words are not really homonyms. If a Tamang speaker tells another:

≥a-i ra cu≥-ci 'I -goat -sell/tie up -past' the hearer will be able to understand whether the goat was sold or tied up. There is a difference in the way the syllables cu≥-ci are pronounced. That difference is tone. It consists mostly of differences in the pitch, or level of the voice, in the speed of utterance, in the direction of the pitch change during the pronunciation (starting higher and becoming lower or remaining flat, or rising).

Let us use a new sign to indicate this difference, for instance a number. We could write ¡cu≥-ba for 'to catch and tie up' and ™cu≥-ba for 'to sell'.

When a language has tone, it is not just a few words that use it to distinguish meaning; the whole vocabulary is affected. For instance the words for 'ear of corn or wheat' and 'rain', both nam if we do not write tone, should be distinguished. One should be marked with the ¡ tone and the other with the ™ tone. By listening to yourself saying these two words, and comparing their melody with that of ¡cu≥ and of ™cu≥ you should be able (after some practice and effort!) to decide which tone to write. The answer in this case is ¡nam 'ear of grain' and ™nam 'rain'. In the same way 'bile' is ¡kam and 'chin' is ™kam; 'to stop a river' is ¡pa≥-ba and 'to speak' is ™pa≥-ba; 'to go away' as a respectful form is ¡see-ba, but ™see-ba means 'to know'. Words which do not share their consonants and vowels with another word, as in the examples above, still are pronounced with one of the tones of the language. For instance, the word mar meaning 'down' or 'downstream' is pronounced with the same tone as ¡cu≥ 'to sell', ¡nam 'ear of grain' and ¡kam 'bile', and should receive the tone mark ¡: ¡mar. I do not know of any word pronunced ™mar in Tamang, except the root of the verb ™mar-ba 'to extract oil by pressing' (in some dialects only). Even though no confusion in meaning would occur if 'downstream' were written ™mar or even simply mar, the correct complete transcription is ¡mar. The fact that neighbouring languages do not distinguish tone, so that there is no known way to write it, makes it seem as if it were not important. But actually it is just as important to pronounce (and write) tone ¡ and tone ™ differently as it is to distinguish between the vowels a and o. Once you become conscious of them, tones will be as natural to write as consonants and vowels.

We are not quite done with tones yet. All the words quoted until now, whether transcribed with tone ¡ or tone ™ , are pronounced, in the great majority of Tamang dialects, with a high pitch, something that the Tibetan grammarians have called 'feminine' voice. We are now going to consider words which are pronounced with a low pitch (or 'masculine' voice). Take the words for 'gold' and 'butter'; here again consonants and vowels could be written as mar, just as for 'downstream'. It will be clear to all Tamang speakers, that 'gold' and 'butter' are rather pronounced with a 'masculine' voice or low pitch, while 'downstream ' is pronounced with a higher voice, more typical of 'feminine' voices. This is a tone difference again. The tone of 'butter' and 'gold' is different from that of 'downstream'; it is also different from the tone of the words that we have written with the mark ™, like ™nam 'rain'. In addition, the tone of 'butter' is different from the tone of 'gold'. So we need to introduce two more tone marks. In Roman characters, let us use the numbers £ and ¢. We will write £mar for 'butter' and ¢mar for 'gold'. Now all words which are pronounced with a 'masculine'

voice need to receive one of these two marks. In this way 'to bite' will be written as £cii-ba and 'to remember' will be ¢cii-ba. That is all: there are only 4 tones in Tamang.

In addition to their low pitch, tones £ and ¢ , in most dialects, have a redundant phonetic feature which is technically called 'breathy voice' or 'breathiness'. To simplify, it makes a word like 'one' ¢kik (or ¢ki depending on the dialect) sound somewhat as if it started with a Nepali 3 (or sometimes u). The vowel itself does not quite sound like the i in ™pit-pa 'to send' for example. The impression of 'blowing' or of heavy breathingthe breathiness -extends over the vowel too. In Tamang pronunciation the difference between voiced consonants (g, j, é d, d, b) and voiceless consonants (k, c, é t, t, p) is not used alone to differentiate words. It always comes along with a difference in tone. So that if we write tone we do not need to use the letters g, j, é d, d, b at all in transcribing

Tamang, although we can do it redundantly. (We will see later that conversely, if we write a difference in consonant voicing, we may not have to write some of the tone features.)

Tone numbers

How can we write the tones then? We need 4 symbols which do not exist either in Devanagari or in the Tibetan script. We have seen that in phonetic transcription we can use numbers. Can we do the same in Devanagari? We could incorporate into the Tamang alphabet the four symbols ! @ # $ to write the tones, placed on the line or slightly above the line , in the following way:

¡kan !sfg or ! sfg 'rice' ™nam @gfd @ gfd 'rain' £mi #dL # dL 'man' ¢me $d] $ d]
'cow' On two or more syllable words, only the beginning of the word needs to be marked: non-initial syllables and suffixes do not have their own tone. So :

£sa≥mo #;fªdf] or # ;fªdf] 'coppe r' ¡nana !gfgf ! gfgf 'sister' ¢meme $d] d] $ d] d] 'ancestor' ™yoma @of] df @ of] df 'young female'
With this transcription the sentence 'I ate rice' would look like this: !ªfL !sfg !rfrL

This system may look a little strange, but it has been used successfully for a major language like Thai, and for several minority languages in China. Its advantage, as we will see, is that it would be satisfactory for all dialects. Its disadvantage is that if people find it too strange they will simply omit the tone-number altogether and that will lead to confusions between words.

In the Tibetan script the same principle can be used. We can insert tone numbers at the beginning of words.

We will return to discussing alternative means of transcribing tone in Devanagari or in the Tibetan alphabet after we have introduced the consonants and the vowels. In the meantime I will use tone numbers, or leave the tones unmarked.

Vowels

Tamang vowels are simple. They require only 5 different symbols:

i, e, a, o, u and a mark for long or short. In the examples above I have written the vowel twice to show length, as in the word £cii-ba 'to bite'. The usual way in the International Phonetic Alphabet is to use a double dot, like £ciÚ-ba. This double dot exists also in Devanagari as the visarga. In Nepali, the visarga is not used to indicate that a vowel is long. But it is so used in Newari and could be used for that purpose in Tamang as in:

short Another possibility is to use Devanagari in a manner as close as possible to its use in transcribing Nepali. In this option, the columns for short vowels would remain the same, but the columns for long vowels would be modified for those letters for which Devanagari offers a possibility of distinguishing long from short, namely a, i, u.

c cf

or in linked form

s sF O O{ ls sL p π s¬ s" P PM s] s] M cF] cF] M sF] sF] M
There is no long e or long o in Devanagari. So for these two vowels there is no other choice than to use the visarga. This is the system used by Doreen Taylor, and in the primers presently in use in the Trisuli area. The advantages of this option would be that a written text in Tamang would look more like a text in Nepali, and I suspect that some people might think it more pleasing from an aesthetic point of view because it is more compact. From the pedagogical point of view, since the students are going to learn to read and write Nepali as well as Tamang, one might think that the less difference there is between the two systems, the easier it will be for the students. But the fact is that dirgha and hrasva are only orthographic headaches in modern Nepali, since they are pronounced exactly the same. So learning how to use them in Tamang on the basis of pronunciation would be no help when learning Nepali spelling.

The disadvantages of this option are as follows. First the system is not homogenous, since length is written differently for different vowels, and there is no typographical economy since visarga is needed anyway for e and o. Secondly, writing a vowel before a consonant as for Devanagari short i, may be pedagogically more difficult. Thirdly, the vowel a, as all Tamangs know, is different in Nepali and in Tamang. The difference in Nepali between c and cf is not only a difference in length, but also and mainly a difference in quality. In the International Phonetic Alphabet, Nepali short a would be written ˙] or [Ø], a central vowel, not as open as the Nepali long a, which would be transcribed as [aÚ]. The two a vowels of Tamang both have the quality of the Nepali long a, but one is short and the other is long. So it could be rather confusing to a Tamang student to see his own short a written with no symbol as in Nepali, and it could reinforce his usual mistake of pronouncing Nepali 'to die' [m˙rnu] with a Tamang short a, making it sound like Nepali 'to kill' [maÚrnu]. Lastly, if we decide to have one vowel unwritten, the inherent a, the use of halant will be necessary at the end of each syllable which ends in a consonant, and to indicate consonant clusters. Since the use of halant in Nepali printed books is very sporadic, it is likely that Tamangs would also omit it, leading to confusions. So whatever we do for long and short i and u, I would recommend always writing the vowel a. Hence sF for ka and sFM for kaÚ.

[

Vowels in the Tibetan script

If we want to use the Tibetan alphabet, q# q° q q( q% will transcribe ki, ke, ka, ko, ku correctly. But I do not know of a symbol equivalent to visarga which we could use for length. Some syllable-final letters of Classical Tibetan produce a long vowel in Lhasa Tibetan, -r for example. But since Tamang has a real final r, pronounced as r, we cannot use that symbol to show vowel length. Final -g in Tibetan also lengthens the vowel, as in dmag 'army', which is pronounced [maÚ]. But this -g has 'side effects' in Tibetan; it produces a falling pitch (or tone), and we have seen that Tamang needs to distinguish between level and falling tones. The best choice might be to reserve the Tibetan a-chung for marking length, as follows:

short long q# q# z q° q°z q qz q( q( z q% q% z

Nasal vowels

Tamang has nasal vowels in a small number of words. They can be written easily using anunasika in Devanagari, as in the word for 'worms' # uf] + tfO £kõtai in phonetic script. Since nasal vowels are always long it is not necessary to write the visarga with them, as length will be understood from anunasika.

If we use the Tibetan alphabet, I believe there exists also a very rarely used dot (only in a few loan words from Sanskrit) which is drawn over the line to mean about the same thing as anunasika.

Diphthongs

Diphthongs are found in all dialects of Tamang. I believe that the maximum number of vowel combinations or diphthongs found in any dialect is the following: ai au oi ui

In Devanagari such combinations can be written by using the linked form of the vowel for the first member, followed by the full form of the vowel for the second. Other systems would be possible, but since this is usual, it is best to keep it. So 'to fall' ¡tai-ba would be ! tfO-af, 'to belch' ¢kau-ba $ ufp-af, and 'to plough' ¡moi-ba ! df] O-af . In Risiangku, the diphthong oi can sometimes sound more like ue or we. This is a free variation.

The vowel sequence ui can also be considered as a diphthong, pO although it could also be written as a glide w followed by i, thus jL . For instance 'to carry', ¡pui-ba or ¡pwi-ba in phonetics, could be written either as ! k¬ O-aF or as ! kjL-aF (We will talk later about ligatures). If we write it as ! k¬ O-aF the form of the verb root will be less changed in the different forms of the conjugation, for example in the imperative ! k¬ of] ¡puyo. On the other hand, if most Tamang speakers feel that it sounds more like wi than like ui, there is no obstacle to writing ! kjL-aF . A decision on such points should only be made after experimentation, and variation in spelling can also be tolerated.

All of these diphthongs (including ui) can be preceded by a palatal vowel or glide (i or y) and by a labio-velar vowel or glide (u or w) in the following way: £kwai-ba y and w also occur in front of long vowels: yaÚ, yoÚ, yuÚ, waÚ, and (in one word only) ywaÚ. This last combination will be a little clumsy to write. It occurs in Risiangku in the word ™sywaÚ 'to sprinkle', which is pronounced as ™syaÚ in some other dialects. This, depending on the village, will thus be written @ ;ojfM-af or @ ;ofM-af

Diphthongs and triphthongs in the Tibetan script

Although Classical Tibetan does not have many groups of vowels, it has a way to write them by adding a vowel symbol to the a-chung used as support. We have proposed to use the a-chung as a mark of vowel length, but since Tamang diphthongs are always short there would be no confusion. The examples above could be transcribed as follows:

'to return' ¡yai-ba xz# -d- 'pond' £kyoi R( z# - 'water' ™kyui W& z# - 'ant' ¡syauri bz% -c# - 'to brew beer' £kwai-ba eK z# -d-

Consonants

Plain consonants

Simple consonants can be written easily either with Devanagari letters or with Tibetan letters. There is no need for illustration in initial position. In syllable-final position, it should be remembered that among stopped consonants only k, t, p (the voiceless stops) are found in Tamang, never the voiced stops (b, d, g). So when transcribing Tamang in Devanagari we should use s t k rather than u b a at the end of syllables. In using the Tibetan alphabet, however, it is traditional to write the voiced consonants in syllable-final position, and never the voiceless ones. This usage can be kept as it does not lead to any confusion. So Tamang ¢kik 'one' would be written in Devanagari as $ uLs gik and in Tibetan script as e# e gig.

Final consonants and halant

The use of halant with final consonants is not necessary if we decide to write all the vowels, including short a, and to have no inherent vowel. In this case, when reading dfg ¡man 'medicine' we will know that it cannot be mana (the measure word) because the latter would be written dfgf.

The same is true of two consonants which come to be in contact at the boundary of two syllables, as in £sa≥mo 'copper' # ;fªdf] . Since we write all the vowels we will know not to pronounce it sa≥amo.

But if we chose to use the Nepali system with the short a unwritten we have to write halant every time as in # ;fª\ df] or ! dfg\ .

Intervocalic consonants

Consonants which occur inside a word, especially between two vowels, sound voiced in Tamang. For instance the verbal suffix -pa, most of the time sounds like -ba . This is just the opposite of what happens at the end of syllables: inside a word we do not find k, t, p, but only g, d, b, or else people can vary from one to the other or pronounce a sound in between, without creating any confusion. After we talk about tones we will see that if we wish we can write Tamang perfectly without ever using g, j, é d, d, b. Consonants inside a word could always be written with a symbol of the voiceless series k, c, é t, t, p. For example ! rf-kf 'to eat', or, equally well, ! rf-af .

In Eastern Tamang, in particular in Risiangku, it is necessary to use both aspirated and non-aspirated consonants inside words. For example 'dog' needs a kh:

gfvL, while 'chicken' needs a k (or g) gfsf . In most other dialects, only k is required, and 'dog' is gfsL .

Initial consonants and tones

The Tibetan. We are coming to this problem now. You will remember that there is a connection between tone and initial consonant and we suggested that this could be used to propose alternative transcription systems. Until now we have been using numbers to transcribe the tones in our phonetic and Devanagari transcriptions. This is the most straightforward way to indicate the tones, though it is quite arbitrary: ¡ simply represents the category of words that have the same tone as ¡kan 'rice', whatever that is phonetically. Here again are some examples, with two different Devanagari equivalents: chest

¡ku ! s' ! s' ' chin ™kam @ sfd @ sfd back £ko # sf] # uf] thatch ¢ki $ sL $ uL
The first Devanagari column above uses s with all tones, and this is perfectly adequate.

We mentioned above, however, that with tones £ and ¢ this initial consonant usually sounds more like u or 3. So we have used u in the right-hand column above, although it is not necessary to do so. Similarly, with tones £ and ¢ we could write h 8 b a instead of r 6 t k. This does not make the numbers unnecessary, however: they are still needed to distinguish # u from $ u, and they are also needed to distinguish words beginning with other consonants such as m, y, r, l, s etc., for example in ¡nam 'ear of grain' ! gfd and ™nam 'rain' @ gfd ; or in ¡mar 'downstream' ! df/ , £mar 'butter' # df/ and ¢mar 'gold' $ df/ . Some more examples:

god ¡la ! nf eye ™miÚ @ dLM hill £ri # /L thief ¢joÚ $ of] M

Using consonants to write tone

The systems that have actually been used to write Tamang in Nepal so far usually ignore the difference between tones ¡ and ™, and between £ and ¢. That is, they put the two 'high' tones together and the two 'low' tones together. Instead of using numbers, they write the initial consonants differently for the high and the low tonesjust as we have shown above for s and u.

In her forthcoming dictionary of Western Tamang, however, Doreen Taylor intends to add an extra sign, the apostrophe, to her system, to mark words with tones ™ and ¢, making them different from words with tones ¡ and £, and thus making her transcription complete. We can add this sign to the Action Aid transcription as well.

Here, then, are the two systems, with the apostrophe added:

hill £ri # /L /≥ L /xL thief ¢joÚ $ of] M ' o≥ f] M 'Øxf] M or 'oxf] M chest ¡ku ! s' s' s' chin ™kam @ sfd 'sfd 'sfd back £ko # sf] uf] 4 uf] thatch ¢ki $ sL 'uL 'uL earth ¡sa ! ;f ;f ;f three ™som @ ;f] d ';f] d ';f] d to make £so-pa # ;f] -kf ;≥ f] -kf ;Ùf] -kf mushroom ¢syamo $ ;ofdf] ';≥ ofdf] ';Ùofdf] or ':ÙØfdf]
Both systems use u h 8 b a for tones £ and ¢. For other initial consonants, Doreen

Taylor writes an h after the consonant (because of the "breathiness") and Amrit Yoncan writes a dot under the consonant. Thus instead of marking the difference between high (¡, ™) and low (£, ¢) tones directly, by a tone symbol, they mark it indirectly, by using two sets of initial consonants. Neither of them marks the difference between ¡ and ™, or between £ and ¢, when writing ordinary Tamang text, although the apostrophe is available for this purpose when more accuracy is needed, as in a dictionary. Thus both of these systems make a compromise. This leads of course to some homographs (words which sound different but are written the same), but any language can tolerate a reasonable amount of homographs and homonyms.

Continuant initials

In the transcription of continuant initials on the low tones, the advantage of using the consonant h rather than the dot is that a dot, like a tone number is a small unusual sign that people may be tempted to omit when they write. The disadvantage of using h is that, although it is no problem with m, n, ≥, s, it interferes with the transcription of some clusters. For example, the word for 'hill', £ri in phonetics, has breathy voice like all tone £ words and would be written /ÙL5 . The word £la 'hillside' would be written nÙf or Nxf in that system. The word £wapa 'rooster, male' would be written jÙf-kf. In Eastern Tamang, there are a few words which begin with an aspirated r (or hr), aspirated l (or hl), aspirated w (or hw) and so on. We probably want to use the h symbol for those. Consider the word ¡hri 'the spirit of a dead child'. To keep it distinct from low tone £ri 'mountain', we could decide to write the h first: Ù/L (or XL ).

In some dialects, like Taglung in the North of the Kathmandu Valley, there are numerous words with initial aspirated w, or hw, which need to be distinguished from low tone breathy w. For example 'to cry' in that dialect is ¡hwa-pa (corresponding to ¡kra-pa in most other dialects). This would have to be written Ùjf-kf . If we use the dot for breathiness, the h remains free for aspiration. Otherwise we shall have to make a rule about the order of the h as compared to the initial.

Aspirated consonants

Aspirated consonants (kae, cae, é tae, tae, pae) occur only with the two high tones, that is without breathy voice. So they will never be written with the dot. Words which start with such initials only need to be differentiated for tone ¡ and tone ™. We can use the apostrophe for this, as follows (or use tone-numbers):

Dialectal variation and regular correspondences, the problem of standardisation

Standardisation

It is a well-established fact that any language spoken over a significant area has different dialects, and Tamang is no exception. It is also true that most written languages have a standard written form that is used by speakers of different dialects. For many Tamangs, these observations immediately raise the question of which dialect of Tamang should serve as the basis for written Tamang.

If we look at the history of well-known languages, however, we often find that the establishment of a standard came long after the languages began to be written. When Europeans started writing in their native languages instead of writing in Classical Latin, there was a long period of great variability in spelling and usage before standards were established, and these standards often did not correspond precisely to any one spoken dialect. Similarly, in the beginning, when Tamangs first write their language, it is natural that each will write in his own dialect, since that is the only one he can be sure of; he has not had the experience of reading dozens of books in a literary standard. Imposing an artificial standard at this point might well discourage Tamangs from writing at all, which would be most undesirable. Later, as literary culture develops, writers will naturally gravitate toward the systems that seem most satisfactory or that have the authority of the best writers behind them, and in the end it should be possible to compromise on a standard which has already been well-tested by usage.

Dialect variation

In a number of cases it may be possible to use transcriptions which will match several dialects in spite of some differences in the pronunciation. Whenever that is possible, these should be preferred, but it may not always be practical. A good example is tone.

The phonetic pronunciation of the tones is not exactly the same in all dialects. Even among Eastern dialects some misunderstandings can occur. For example, tone 3 words like mar 'butter' pronounced by a speaker from Ramechhap were always recognized by a speaker from Risiangku in a test. But tone ¢ words like ¢mar 'gold' were often a problem, because the Risiangku speaker expected them to be lower in pitch. In other dialects, like that of Taglung, the difference may be much more extreme.

Tone ¢ in Taglung is not at all low in pitch, it has no breathiness, and the initial stop consonant (e.g. p, t, k), if any, is not voiced (i.e. not pronounced b, d, g). The word 'one' sounds like ki (sL) not gi (uL or 3L). In these circumstances, if we write arbitrary tone numbers (as we have done) they work for all dialects because the numbers only indicate categories of words which have the same pitch, melody, and voice quality, without saying what these are precisely. The spelling $ sL is just as correct for Taglung, where the tone is high, as it is for Sahu, where the tone is low and where the kis more like g-. This is because it is true in all dialects that 'gold', 'one', 'cow', and 'to £ remember' are pronounced with the same tone, and differently from words which have tones 3, 2, or 1.

Vowels and consonants also vary between dialects. In many cases the variation is regular. For example, all words which end in -at in Risiangku end in -e in Taglung, Temal, or Sahu. Words which start with krin Risiangku, Sahu, and Temal, have hwin Taglung. Words in kwin Risiangku begin with win Sahu. And so on. We could try to invent an orthography such that words would be spelt the same for all dialects, and the pronunciation derived by regular rules. For instance, we could write ¢kyat for 'work', like the Risiangku pronunciation, and have Taglung speakers learn that, for them, -at is pronounced -e. I believe that this would be unnecessarily difficult and that it would be better, in the beginning at least, to write either ¢kyat or ¢ke depending on the dialect of the writer.

Classroom tools

It is always useful for a language to have a dictionary and a grammar for classroom use. There is a difference, however, between mother-tongue teaching and second language teaching. In the case of Tamang, most of the teaching will be mothertongue teaching. In this case, particularly in the case of adult education, the students already know the language, including the grammar and the vocabulary, and only need to learn to put it on paper and to read it.

The main incentive to reading is to have interesting reading material. Since Tamang is a language whose traditions are mostly oral, Tamang "texts" exist in the form of stories, songs, religious formulas of lamas or jhankris, etc., but they are not available in written form. These texts could be written down and constitute reading material for the classroom. To write down traditional texts in a readable form two principles should be borne in mind: 1) In as much as these texts constitute Tamang culture and tradition, you do not want to distort them, and 2) since you want to use them for teaching you want them to be grammatically correct. On the first point, the danger is to try to remake the texts, to rewrite them with a personal interpretation which may not be correct. On the second point, observe that when a child or an inexperienced adult starts writing his own language, he writes grammatically incorrect sentences, which, if you think about it, is amazing, because when he speaks, he does not make such mistakes. The time taken by the process of writing itself is enough to distort the language: the writer loses track of his sentence as he writes it down. For traditional texts, a good way to proceed is to go to people who know the texts, your grandfather or grandmother for instance, and ask them to tell the story, song etc. into a tape recorder. Some people will be better than others at this, and all will get better after they get used to the tape recorder. After that, someone who may not know the texts, but knows how to write, can transcribe them, possibly checking difficult points with the original speaker. If you think you know the texts and can transcribe them too, you should still try to distinguish the two phases, the telling and the transcription. Of course, Tamang literature will not be limited to traditional texts, and writers of short stories, novels, poems, essays, biography, history, and books on practical subjects, should be encouraged as well.

  yai ofO as in ! ofO-af'to return' or in IPA 3 ¡yai-ba yoi af 'to brew beer'

  observant reader must have felt uneasy with many of the examples given, because I keep writing k, c, p in phonetics, and I often use b, d, g in Devanagari or

  necessary to use the conjunct form of the letters in initial clusters. Example: kra-pa 'to cry' qç -d-. It would not be possible to write qc-dbecause that would read[karba].

  This is the system that has been proposed by the Tamang Language Council and used by Amrit Yoncan in the Adult Education Handbook (published by Action Aid) based on Eastern Tamang. It is also proposed by the Western Tamang journal Tamang. Like any system this has advantages and disadvantages. One advantage is that the same phonetic element, vowel length, is always transcribed in the same way, which is easier to learn.

		long		short long
	c	cM	or in linked form	sF	sFM
	O	OM	(shown with k)	sL	sLM
	p	pM		s¬	s¬ M
	P	PM		s]	s] M
	cF]	cF] M		sF]	sF] M

Published in Syomhendo, July-September 1993, 15-26 (Kathmandu, Nepal)

I use a hyphen in phonetics as well as in Devanagari to separate a root from its suffixes. In text orthography this can also be done or ignored.

I use y instead of the IPA symbol j for the palatal glide (the consonantal form of i) because this is more usual in Nepal and India.

The use of a dot to indicate low-breathy tone could be extended to stops consonants too, thus 'back' could be written s≥ f] and 'thatch' 's≥ L.

I do not think it advisable to use the conjunct X here as this means hr rather than rh, and as we will see presently we need to make that distinction.

Consonants and tones in the Tibetan alphabet

There are in Tibetan a large number of rarely used letters or symbols which are used mostly in loan words from Sanskrit, and with which I am not familiar enough to present them here. It is very likely that some such symbol could be found that could be used in the same way as we proposed to use the apostrophe in Devanagari. So I will leave this problem open for some further research. The interested reader can consult a learned lama on this matter. It should be remembered though that until such a symbol is added to the Tamang alphabet in Tibetan script, the transcription will miss half of the tonal features.

In order to transcribe the breathiness of low tones, we could use the series of voiced consonants for the stopped consonants, exactly as in devanagari. Ex. e(for £ko 'back' . The pronunciation of the series of voiced consonants in many dialects of Central Tibetan is very similar to what it is in Tamang, a devoiced consonant, with breathiness and low tone. So that would work very well. Tibetan also has a high-tone s n and a low-tone s ; , a high-tone sy b and a low-tone sy l which can be used to distinguish Tamang high tones from low tones with these initials.

For continuants on the other hand there is a difficulty. The Tibetan h is voiceless, and when associated with other consonants it leads to high tone. The Nepali h is voiced, which made it more suitable for use as a notation of Tamang breathiness.

Using the Tibetan h to indicate a low tone would be very counter intuitive for anyone knowing Tibetan. We would have to look for something else.

Consonant clusters

In syllable initial position, aside from the words in hr, hl, hy, hw which we have mentioned above, and which are not as numerous in all dialects, Tamang has frequent consonant clusters formed of a consonant followed by y, r, l, or w. There exist in Devanagari conjunct forms for all the clusters found in Tamang, and they can be used if we wish. On the other hand, most alphabets in the world keep the same form for the same letter in all contexts, and this solution is certainly pedagogically easier. As long as we always write the vowel a, there is no need to use conjunct letters, or to use halant.

Example: ¡kra-pa 'to cry' can be simply s/f-kf. If short a is left unwritten, then halant could be used on the first member of a cluster: s\ /F-kF.

When two consonants come into contact at the juncture of two syllables, the Nepali usage in Devanagari is to use conjuncts. This can be done in Tamang, but is not necessary. ™tar≥a 'fish' can be written 'tf/ªf ( or @tf/ªf ) and ¢palpa 'toad' 'afnkf ( or $kfnkf or 'k≥ fnkf according to the system chosen to write tones). The choice here is probably between aesthetic and pedagogical considerations. In a literacy program pedagogy may be more important. But here again experience in the classroom is the real teacher.

In the Tibetan script, syllables are separated by a dot, tseg, so there is no need for conjunct letters at the juncture of two syllables. Only in syllable initial position are clusters written in linked form. They are simpler than the Devanagari conjuncts in that the shape of the letters is less changed. Since we have used the a-chung to write length, we don't have another letter available to write short a. So in using the Tibetan alphabet, we will leave short a unwritten, as the inherent vowel. In this case it becomes