Performance of high pressure Xe/TMA in GEMs for neutron and X-ray detection

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1 Performance of high pressure Xe/TMA in GEMs for neutron and X-ray detection R. Kreuger, C. W. E. van Eijk, Member, IEEE, F. A. F. Fraga, M. M. Fraga, S. T. G. Fetal, R. W. Hollander, Member, IEEE, L. M. S. Margato, T. L. van Vuure Abstract In the framework of development of a single-gem detector for thermal neutron detection, with a position resolution of 1 mm, the charge multiplication in the GEM has been studied for different stopping gases at pressures up to 8 bar. The effect on the gain of addition of several bars of He to these gases has been determined. It has been found that a Penning mixture of Xe/TMA has the highest gain. At a pressure of 7 bar Xe + 2.5% TMA a gain of 10 could still be obtained for a 50 µm thick GEM with 80 µm holes. A gain of is possible in a mixture of 4 bar Xe, 2.5% TMA and 6 bar He, which makes the above mentioned position resolution possible. In addition it has been found that the Xe/TMA mixture scintillates around 300 nm in the multiplication process in the GEM. The light yield per secondary electron depends only slightly on the Xe pressure or the TMA concentration. Index Terms Gas detectors, gas electron multiplier, high-pressure Xenon detector, Penning mixture, scintillation, neutron detector. I. INTRODUCTION The Gas Electron Multiplier (GEM) [1], recently introduced by F. Sauli at CERN is a good device for thermal neutron detection in neutron scattering instruments to be installed at new spallation sources like the SNS and ESS. The GEM offers several advantages over present position sensitive detectors. At these new neutron sources the neutron count rate in detectors can be as high as 10 6 mm -2 s -1. GEMs can handle such a rate, contrary to multi-wire proportional chambers (MWPCs). Furthermore the read-out electronics is less sensitive to discharges, because the gas multiplication process takes place in the GEM holes and no strong electric field is applied to the read-out anode. Another advantage is the fact that the GEM can be used as a universal building block for detectors with different position resolution requirements. It's just a matter of adapting the pixel or strip layout of the pickup electrode. The best possible This work is supported by the European Union research program. R. Kreuger, C. W. E. van Eijk, R. W. Hollander and T. L. van Vuure are with the Delft Univ. of Techn., Mekelweg 15, 2629 JB Delft, Netherlands ( Rob.Kreuger@iri.tudelft.nl). F. A. F. Fraga, M. M. Fraga, S. T. G. Fetal and L. M. S Margato are with LIP-Coimbra, , Coimbra, Portugal ( francisco@lipc.fis.uc.pt). resolution is determined by the size of the primary charge cloud. Some of the planned neutron scattering instruments need a resolution of 1x1 mm 2. Others require a lower resolution (cm) but huge detector area (10 m 2 ). Then the GEM is a cheaper solution than MWPCs. As described in [2] our goal is to develop a GEM detector which complies with most of these detector demands: millimeter position resolution, a few microseconds time resolution, high efficiency and handling of high count rates. This position resolution can only be achieved by sufficiently reducing the range of the 3 He neutron capture reaction products by adding a so-called stopping gas. We studied different stopping gases and only a Xe/TMA (TriMethylAmine) gas mixture provided sufficient gain in a single GEM detector at the required pressure. With 3 He such a detector serves as a thermal neutron detector, however, without 3 He it can serve as an efficient position sensitive X-ray detector. This detector can also be operated with an optical read-out method, instead of the charge collection on the read-out electrode, as we will show that the amount of scintillation light emitted by the avalanches in the GEM holes at a wavelength of around 300 nm is high. By installing an optical window, instead of an anode, this light can be detected with a photo-multiplier, APDs or a CCD. A CCD is a very simple position sensitive read-out method compared to a charge read-out. II. DETECTOR REQUIREMENTS A. Choice of gas for thermal neutron detection 3 He and 10 BF 3 are common choices for counting gases in gas filled neutron detectors. We discard BF 3 since it is known to be strongly electronegative at higher pressures that would reduce the charge gain of the detector by electron attachment. We are left with the 3 He neutron capture reaction for neutron detection: 3 He + n p (573keV) + T (192keV) The cross section of 3 He is 5333 barn at a neutron wavelength of 1.8 Å. The cross section is larger for longer wavelengths. The detection efficiency depends on the gas pressure and the thickness of the detection volume. The latter is the drift volume between the entrance window and the GEM foil. This cannot be chosen at will, if one wants to /02/$ IEEE 457

2 preserve the time resolution of the neutron pulses of the source. In general the absorption region should not be thicker than the moderator in the source. We choose the drift gap to be 1.5 cm. Neutrons of 1.8 Å have a velocity of 2200 m/s. They cross the drift gap in 7 µs. This is also the upper limit of the time resolution of the detector and much less than the neutron pulse width of >15 µs at spallation sources. The detection efficiency of the GEM detector has to be at least as good as the present 3 He-tubes: 70%. For a 1.5 cm thick detection volume this implies a 6 bar 3 He gas pressure for the detector. B. Choice of the stopping gas The point of interaction of the neutron in the gas does not coincide with the center of gravity of the ionization along the proton and triton tracks. This is caused by the fact that the proton track is three times longer than the triton track with the highest ionization density at the end of the tracks. If all possible centers of gravity for a fixed neutron interaction point are projected onto the read-out plane one obtains a center of gravity distribution around that interaction point. The width of this distribution is the intrinsic resolution of the detector. The FWHM of this distribution is approximately 70% of the proton range [3]. Unfortunately the range of the proton from the 3 He neutron capture reaction is rather long: 1.1 cm in 6 bar 3 He. For a 1 mm (FWHM) resolution this would imply a pressure of 46 bars of 3 He. This is not convenient for the detector construction. Adding a so-called stopping gas with heavy atoms or molecules can reduce the proton range. In Table I the proton stopping range for some stopping gas candidates as calculated by the SRIM package [4] is shown. The pressure mentioned for 1 mm resolution is the partial gas pressure to be added to the 6 bar 3 He already present in the detector. TABLE I 573 KEV PROTON RANGE IN SOME STOPPING GASES. Gas Proton range at (mm) Pressure for 1 mm position resolution (bar) He Xe C 3 H CF C 3 F The GEM has to provide enough charge multiplication in these high-pressure gas mixtures to make sure that electronics connected to the read-out plane can detect at least the energy deposited by half a triton track, i.e. kev. This corresponds to about 3000 electrons in the primary charge cloud. For simplicity we would like to use only one single GEM in our detector. Considering our amplifiers, we need a gain of the GEM of at least 10, while the maximum voltage across the GEM is limited to about 800 V. III. SETUP In Fig. 1 the setup is shown that we have used to measure the gain in different gas mixtures. It consists of a pressure vessel that has been tested up to 20 bar. Inside, a GEM foil and an anode foil have been mounted on frames. The GEM foil is mounted 3 mm behind the Be entrance window. The window has been soldered into a 2 cm thick steel plate. A 1 kv/cm strong electric drift field is applied between the front plate and the GEM foil. The window has been made of Be in order to be able to test the detector with X-rays instead of neutrons. The GEM to anode distance is 3 mm. In this volume the collection field is 4 kv/cm. The GEMs are made of 50 µm thick Kapton foils, the holes having a diameter of 60 µm or 80 µm at a 140 µm pitch. The charge induced on the anode is measured with a PAC-LP87 preamplifier. The transfer function of this amplifier has been calibrated with a test capacitor and a pulser. Fig.1. Schematic overview of the test detector. IV. MEASUREMENTS A. Carbon containing stopping gases The detector has been filled with different carbon containing gases from Table I. We measured the gain of the GEM for different gas pressures and different voltages across the GEM. There was no He present in the detector during these measurements. The detector was irradiated with either a 55 Fe source or a Cr X-ray tube. First we tried CF 4. The results are shown in Fig.2. 3 bar cm Be Fig. 2. The gain of the GEM as a function of the GEM voltage for CF 4 gas at different pressures. The maximum gain of the curves is the one at which discharges start to occur. It is clear that the gain at a given voltage across the GEM gets smaller as the pressure 458

3 increases. This is due to the increase of the mean free path for ionization at higher pressures. Obviously, CF 4 cannot be used as a stopping gas, because a gain of 10 is needed at 2.6 bar (Table I). We also tested C 3 F 8. We obtained a gain of 10 at a pressure of 300 mbar at a voltage across the GEM of 500 V, at higher voltages discharges over the GEM occurred. At higher pressures the maximum gain of the GEM will be smaller than 10 due to the increased mean free path for ionization. Therefore C 3 F 8 cannot be used as a stopping gas either. Finally we tried propane. We measured at most a gain of 30 at 900 mbar before discharges would occur. At the required pressure of 2.7 bar (Table I) the gain will also be lower than 10. Another stopping gas is clearly needed. VGEM (V) Xe/TMA 3bar Xe/TMA 5bar Volume percentage TMA (%) B. Xe/TMA stopping gas Finally we tried Xe as a stopping gas. The results are shown in Fig bar 4 bar 5 bar 6 bar 7 bar 8 bar pure Xe Fig. 3. The gain of the GEM as a function of the GEM voltage for Xe gas at different pressures with 200 mbar of TMA added. In this figure one sees a line for of pure Xe. The gain remains below. It is expected that the gain will drop below 10 at 4 bar of pure Xe. In order to decrease the operating voltage of the GEM, increasing the maximum gain, we tried a Penning mixture constituted by adding 200 mbar of TMA to the Xe gas. The first excited state of a Xe atom lies at 8.4 ev. That is just above the ionization energy of 8.3 ev of TMA. Energy transfer will occur from the excited Xe atoms to TMA molecules in the avalanche process in the holes. More electron-ion pairs will be created, yielding a higher gain at the same GEM voltage. This can be clearly seen in Fig. 3. The figure shows that at the required pressure of 4 bar a gain larger than can be obtained. We did measurements to optimize the TMA fraction. The results are shown in Fig. 4. Fig. 4. The voltage across the GEM that is needed to obtain a gain of in a detector filled with 3 or 5 bar of Xe as a function of the TMA fraction. Here one sees that the lowest voltage for a specified gain is obtained at a TMA fraction of about 2.5%, independent of the Xe pressure. Hence, the 4 bar Xe curve in Fig. 4 will show a slightly larger gain after reducing the TMA partial pressure from 200 mbar down to mbar. Finally we made sure that adding 6 bars of He to this Xe/TMA gas mixture does not reduce the charge gain considerably. Results are shown in Fig. 5. No He 3 bar 4 bar 5 bar 6 bar 7 bar Fig. 5. Gain of a GEM as a function of the voltage across the GEM for a gas mixture of 3 bar Xe/50 mbar TMA and different partial He pressures. Instead of 3 He we added 4 He to the Xe/TMA gas mixture because for gain measurements with X-rays 3 He is not needed and 4 He is much cheaper, while 4 He will have the same effect on the charge multiplication process. Comparing Fig. 5 and Fig. 3 one sees that the effect of adding He has much less effect on the gain than increasing the Xe pressure. This can be explained by the fact that the first excitation level of He lies at 19.6 ev. This is well above the ionization energy of Xe (12.1 ev) and TMA (8.3 ev). Therefore electrons will only interact elastically with the He atoms in the avalanche process. They will lose just a tiny bit of their energy in these interactions and the mean free path for ionization of Xe or TMA is hardly affected. The curves in Fig. 5 show the gain for a gas mixture with 3 bar Xe. We need 4 bar Xe to obtain a 1 mm position 459

4 resolution, but the curves for 4 bar show a similar behavior. 4 bar Xe/TMA together with 6 bar He will still give a gain above, which is well above our requirement of 10. C. Scintillation We have shown before [5] that using appropriate gaseous mixtures the GEM avalanches emit scintillation light. The scintillation properties of TMA have been reported in the past and we decided to perform some measurements using Xe/TMA at high pressure in a GEM detector operated in the scintillation mode. The detector is similar to the one shown in figure 1, but the readout electrode was removed and a quartz window was fitted on the side of the chamber opposite to the entrance window. All the electrons produced in the GEM avalanches will be collected on the back electrode of the GEM. Behind this window we installed an Applied Photophysics monochromator type 7300, read-out by an RCA C31034 photo-multiplier with a GaAs:CsO photocathode, covering the wavelength range between 185 and 930 nm. The photo-multiplier is used in single photon counting mode. During irradiation of the detector with an X-ray tube, both the primary current and the secondary current on the back electrode of the GEM were measured at constant X-ray flux. The GEM gain was defined as the ratio between this current and the primary current. A wavelength spectrum is measured by stepping the monochromator at 2 nm intervals through the wavelength range. At each wavelength setting the number of pulses of the PM is counted during a fixed time. In Fig. 6 the spectra for some gas mixtures are shown, all with the detector operating at a charge gain of. Counts/I 2.0E-9 1.5E-9 1.0E-9 5.0E-10 He+40%CF4-1bar Ar+5%CF4-1bar Xe+5%TMA-1bar Step=2nm Slit aperture=1mm 0.0E Wavelength (nm) Fig.6. Light yield normalized to the secondary current I in the GEM as a function of its wavelength for different gas mixtures. The spectra are not corrected for the transmission of the monochromator or the quantum efficiency of the photo-multiplier. For comparison spectra of gas mixtures containing CF 4, taken in similar conditions using GEMs, are shown as well. The light yields at different wavelengths cannot be compared since the spectra have not been corrected yet for the transmission of the monochromator and the quantum efficiency of the PM. The light output has been normalized to the charge after multiplication. The number of counts/i is proportional to the number of photons per secondary electron. The latter has been measured once in absolute quantities for the Ar/CF 4 mixture and turned out to be 1 to 2 %. Considering that at higher pressures a much higher gain can be obtained in Xe/TMA than in Ar/CF 4 the resulting light output per primary electron might well be larger for the Xe gas mixture than for Ar/CF 4. We also checked the dependence of the light yield on the TMA fraction and the Xe pressure. The latter is shown in Fig. 7. One sees that the Xe pressure hardly plays a role. The normalized light output varies by 10% for TMA fractions between 0.6 and 5%. Of course the highest net light output is achieved for the highest charge gain in that case. Counts/I 2.0E-9 1.5E-9 1.0E-9 5.0E E+0 Xe+5%TMA - 1bar Xe+5%TMA - 3 bar Xe+5%TMA - 5 bar Step=2nm Slit aperture=1mm Gas Gain ~ Wavelength (nm) Fig. 7. Light yield normalized to the secondary current I in the GEM as a function of its wavelength for different Xe/TMA pressures. The spectra are not corrected for transmission of the monochromator or the quantum efficiency of the photo-multiplier. V. CONCLUSIONS We have shown that the GEM can be operated with sensible gain in Xe/TMA mixtures at high pressures. At 7 bar a gain of 10 can still be obtained with one GEM. The optimum fraction of TMA is 2.5%. By adding 6 bar of 3 He to this mixture thermal neutrons can be detected with 70% efficiency, and a position resolution of 1 mm can be reached. A neutron detector with Xe is in principle sensitive to background gamma radiation. However, we think that we can reject gamma's quite well. At spallation sources one expects gamma's of one MeV or higher. These will produce in the detector fast electrons, that will deposit an energy of about 3 kev/mm in a gas mixture of 4 bar Xe/TMA + 6 bar He. The corresponding signals can easily be discriminated from the signals produced after the absorption of neutrons, which deposit 770 kev over 2 mm. The Xe/TMA mixture alone can be used for the detection of X-rays. An 8 mm thick conversion volume with 5 bar 460

5 Xe/TMA will have a 50% detection efficiency for 17.5 kev X-rays, corresponding with the K a peak of the often-used Mo anode in an X-ray tube. The combination of a GEM with Xe/TMA at high pressure will allow for cheap large area X-ray detectors. Apart from the usual charge read-out we found that with Xe/TMA an optical read-out is also possible. With a CCD, and maybe some GEMs in cascade, a very simple position sensitive neutron or X-ray detector can be built as CCDs sensitive in the UV region are becoming available on the market. CCDs, however, have some disadvantages in combination with neutron detection. A CCD is an integrating device, which will not allow for gamma rejection on an eventby-event basis and will not give event time-information. Optical readouts using photon counting devices such as photo-multipliers or APDs can overcome this limitation. In the future we will investigate the effect of the GEM hole size on the gain and light emission in high-pressure Xe/TMA gas mixtures. We will build prototypes of a position sensitive thermal neutron detector with a 1 mm position resolution, as well as an X-ray detector for application in synchrotron radiation imaging. VI. REFERENCES [1] F. Sauli et al., "The gas electron multiplier," NIM A, vol. 386, p. 531, [2] T. L. van Vuure et al., "High pressure GEM operation aiming at thermal neutron detection," IEEE Trans. Nucl. Sc., vol 48, no. 4, pp , [3] F. D. van den Berg, "Gas-filled micro-patterned radiation detectors," thesis, section 6.4, Delft University Press, Delft, [4] [5] F.A.F. Fraga et al., "Optical readout of GEMs", NIM A, vol. 471, p. 125,