Design and characteristics of soft X-ray sealed gas electroluminescence detector

NUCLEAR

INSTRUMENTS

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DESIGN AND C H A R A C T E R I S T I C S OF SOFT X-RAY SEALED GAS E L E C T R O L U M I N E S C E N C E D E T E C T O R D. A. G O G A N O V , N. 1. K O M Y A K , V. B. ELKIND, A. A. SCHULTZ

Special Design Bureau ~?f X-ray Apparatus, I Stakhanovtsev street, Leningrad 195 112, U.S.S.R. Received 7 November 1977 and in revised form 7 February 1978 The design of a sealed type for a gas electroluminescence (proportional scintillation) detector having a plane condenser field is considered. The results of the main detector characteristics study are given. Energy resolution determined with a SSFe source (5.9 keV) amounted to 9.7%, a peak-to-valley ratio being 200.

Gas electroluminescence detectors (called scintillation proportional counters or proportional counters with discharge-glow registration) have offered much promise in the spectrometry of soft Xrays I 3,5,6). The analysis of various designs for detectors of a similar type j-3) as well as our experience in the field of X-ray gas electroluminescence detectors 4) have revealed the prospects for application of detectors with a plane condenser electric field, which made the grounds for designing the detector described in the present paper. The electroluminescence detector (fig. 1) comprises a vacuum-tight cylinder body employing metal-ceramic units with end positioning of entrance and exit windows, two metal grids serve as electrodes going parallel to these windows. The detector working volume is divided by means of these electrodes into two functional parts: 1) ionizing radiation absorption zone limited by the detector entrance window serving as the cathode and by the first grid with a positive potential in respect of the entrance window; 2) electroluminescence zone limited by the first and second grids, the second one being used as the detector anode. The grids transmittance was about 90% (1 cell/ram). Be-entrance window was of 100/xm thickness and 30 m m diameter. MgF 2-crystal, 30 m m diameter served as an exit window of the counter. For the mounting of the windows there was used a vacuum-tight composition material on the basis of polyorganosiloxaneT). The electroluminescence detector was evacuated at 150 °C within 5-6 h to 10 6 torr, then washed with xenon and pumped out. Xenon filling was effected up to 700 tort after which the detector was sealed. Xenon purity a m o u n t e d to 99.987% ( K r 0 . 0 0 7 % , 02

0.001%, N 2 0.005%, aqueous vapour 0.004 g / m 3, H2, CO2 and hydrocarbons 0.001%). The problem which springs up in the process of designing the sealed version of a gas electroluminescence detector lies in the fact that the detector exit window fails to transmit entirely the total spectral composition of electroluminescence arising in xenon and does not conform to a spectral sensitivity of a photomultiplier, which reduces the amplitude of the detector light yield and consequently makes worse the spectrometer energy resolution as a whole. In a great n u m b e r of gas electroluminescence detectors for converting the uv portion of xenon radiation spectrum into a portion of longer wavelengths there are applied light converters (spectrum shifters). The short-coming of the known light converters lies in the fact that ¢

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they c o n t a m i n a t e a gas v o l u m e of a counter chamberS,9). This probably puts a limitation on the d e v e l o p m e n t of sealed off gas electroluminescence counters with long-term stabilityl°'l~). To eliminate the detector v o l u m e contamination resulted from gassing of the light converter, one as a rule applies various methods of purification, e.g. a calcium purifier periodically or permanently connected to the detector, which is not the case with the sealed type. In addition the use of a light converter brings about the possibility of extra noises to be raised due to electron interaction with light converter materiaF). Our design features in the application as the detector exit window a magnesium fluoride crystal with a short wavelength boundary of optical transparence a m o u n t i n g to about 120 n m ; the crystal possesses the properties of v a c u u m material and this permits to r e m o v e the material of a light converter from the interior v o l u m e of the detector. The electroluminescence from the detector was registered by a FEU-39 photomuhiplier with a quartz entrance window having an optical contact with the detector exit window• A signal from the photomultiplier via a shaper arrived at a V A - V -

100 linear amplifier-analyzer and then was applied to an AI-256 pulse analyzer and a VA-G-120 scaler. The best energy resolution from a 5SFe source along the MnK~ line (5.9 keV) with the FEU-39 o

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amounted to 9.7%, the diameter of a collimated Xray beam being 3 r a m , counting rate 1000 pulses/sec, electric field intensity within the absorption zone E~ = 540 V / c m and that within the electroluminescence zone E2 = 4300 V/cm, integration and differentiation time constants for the VA-V-100 instrument 2 and 5/~sec, respectively. Fig. 2 shows an amplitude distribution of the spectrometer output pulses from the FEU-39. The detector parameters practically have not changed within several months of its operation. A peak-tovalley ratio (approximately 200) obtained was substantially better than that in the Palmer's design2). Fig. 3 shows a dependence of the energy resolution and spectrometer amplitude upon the field intensity within the electroluminescence zone at a constant field intensity within the absorption zone (540V/cm), The amplitude from the detector is well approximated to a linear function which indicates the lack of gas amplification in the electroluminescence zone within our range of electric field intensities. Shown in fig. 4 is a dependence of the energy resolution upon the working beam diameter for ~Fe radiation. The energy resolution obtained is not limiting for a given design. An increase in the detector light yield (e.g. owing to an increase of the detector exit window size and the diameter of the FEU photocathode or using MgF2 as the material for the photomultiplier entrance window as well as applying a light converter upon the exterior surface of the detector exit window) will make it possible (retaining all the advantages of the given detector design) to reach the spectrometer energy resolution obtained in refs. 1, 3, 5 and 6. It should

be noted that ref. l0 describes a detector design with an exit window made of a LiF crystal. The limiting resolution obtained in this paper for the MnK~,/~-line (5.9 keV) makes up about 14%.

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Due to the energy resolution already obtained one can successfully apply the developed detectors in X-ray analysis. Fig. 5 (a--c) (to the left) gives amplitude distribution curves for the main X-ray lines used in Xray structure analysis. In structure studies the interfering fl-line is filtered out with a loss up to 50% of the working K~-Iine. It is evident, that in case of Ag and Mo :z- and /,{-lines are resolved completely, 5.2% and 6.9%, respectively. As in what follows resolution values are given with no background correction applied and no fi-line taken into account. The working beam diameter is 3 ram. Significant background under the lines is explained by a marked absorption of the recorded radiation in the electroluminescence zone. For the CuK-line (8.79, resolution) the fl-line can be suppressed to the required ratio l~/l/s- 400 at a loss of approximately 25% of the K : l i n e . To take spectra, the radiation of a 1°gOd isotope was used

et al.

as well as Mo and Cu fluorescence radiation excited by means of the former. For comparison, shown in fig. 5 (a)-(c) (to the right) are spectra of the same lines measured by a Xe-CH4 proportional counter. Evidently, with designs distinguished for a high efficiency of AgK- and MoK-radiation detection, the electroluminescence detector can find a wide application in X-ray diffraction analysis instead of conventional gas proportional and scintillation detectors with a /J-filter being removed at the same time and a 1.5-2 times gain in the primary beam intensity obtained. Fig. 6(a,b) (the left-hand portion) shows T i K - M n K and V K - M n K spectra taken at 55Fe excitation (Ti was excited by transmission, a 25/~m foil was used; V was measured by reflection with an additional MnK radiation mixing). Fig. 6 (to the right) gives the same spectra taken by means of a proportional counter with a X e - C H 4 filling. Obviously, it is possible for the electroluminescence detector to resolve elements with atomic numbers Z, Z+2 in a system of elements, while the proportional counter separates elements Z, Z + 3 . E m p l o y m e n t of an electroluminescence detector instead of a gas proportional counter in the energy dispersive equipment will increase its efficiency substantially and widen its field of uses. References I) A. J. P. L. Policarpo, M. A. F. Alves, M. C. M. Dos Santos and M. J. T. Carvalho, Nucl. Instr. and Meth. 11t2 (1972) 337. 2) H. E. Palmer and L. A. Braby, Nucl. Instr. and Meth. 116 (1974) 587. ~) H. E. Palmer, IEEE Trans. Nucl. Sci. NS-22 (1975) 100. 4) D. A. Goganov, N. I. Komyak and V. B. Elkind, Apparatura i Metody Rentgenovskogo Analiza 14 (1974) 231, in Russian. ~) A. J. P. L. Policarpo, M. A. F. Alves, M. Salete S. C. P. Leite and M. C. M. Dos Santos, Nucl. Instr. and Meth. 118 (1974) 221. 61 M. Alice, F. Alves, A. J. P. L. Policarpo and M. Salete S. C. P. Leite, IEEE Trans. Nucl. Sci. NS-22(1975) 109. 7) N. P. Kharitonov, P. A. Veselov and A. S. Kuzinec, VakIIIIIHtlO/)/OII(VU kompoziciontoe I ~ I ( I I c I ' i ( I I V t l U O S t l O U t ) po/io/~t~anosiloksanov (Nauka, Leningrad, 1976)in Russian• ~) S. A. Baldin and V. V. Matveer, PTE 4 (1963) 5, in Russian. ')) 1. B. Birks, The t/wa G amt praclive (~/ scintillation countin:: (Pergamon Press, London, New York, 1964)• 10) R. D. Andresen, k Karlsson and B. G. Taylor, IEEE Trans. Nucl. Sci. NS-23 (1976) 1, 473. II) R. D. Andresen, E.-A. Leimann, A. Peacock, B. G. Taylor, G. Brownlie and P. Sanford, IEEE Trans. Nucl. Sci. NS-23 (1977) 1, 810.