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Aarts, H.J.M.

Heppener, M.

Simons, D.G.

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The driftless gas scintillation proportional counter

Leiden University Scholary Publications

Adv. SpaceRes.Vol. 10, No. 2, pp. (2)317—(2)321, 1990 0273—1177/90$0.00+50Printedin GreatBritain. All rights reserved. Copyright© 1989 COSPARTHE DRIFTLESS GAS SCINTILLATIONPROPORTIONAL COUNTERM. Heppener,H. J. M. Aarts and D. G. SimonsLaboratory for SpaceResearchLeiden, Niels Bohrweg2, 2333 CA Leiden,The NetherlandsABSTRACTIn this article we present the performance characteristics of a so-called driftless Gas Scintillation Propor-tionalCounter which operates in the energy range 0.1-15 key. Typically, an energy resolution of 30.8 %Full Width at Half Maximum (FWHM) is obtained at 0.28 keV (C-K radiation) whereas a rejection effi-ciency of 99.6 % for Co-60 induced background events is achieved in the energy range 0.4-10 keV. Bymaking use of a position sensitive microchannelplate read-out a typical position resolution of 0.95 mmFWHM at 7.5 key (Ni-K radiation) is obtained.INTRODUCTIONThe driftless Gas Scintillation Proportional Counter (GSPC) differs from a conventional one in the ab-sence of a separate absorption region. In stead, the X-ray photons are absorbed directly in the scintilla-tion region, in which a high electric field is present. The advantages of the driftless concept are an ex-pected better low energy performance due to the absence of electron loss to the detector entrance win-dow and an improved background rejection capability as a result of the smaller electron cloud size. Bymaking use of an Csl-coated microchannelplate (MCP) with a wedge and strip anode as detector for thescintillation light the GSPC can be made position sensitive. In this way an instrument is obtained that isvery well suited for a variety of imaging X-ray applications including the use as focal plane instrument forgrazing incidence telescopes such as employed in X-ray astronomy.EXPERIMENTAL ARRANGEMENTDescription of the detector and principle of operationThe driftless GSPC consisted of a cylindrical Macor body, 30 mm deep and with an internal diameter of60 mm. The 25 mm diameter entrance window was made of 3.3 pm thick polypropylene coated withcolloidal carbon to provide the electrical conductivity needed to apply the high voltage. In order to with-stand the 1 atm. differential pressure across the thin window a support grid has been included. The exitwindow of the gascell was made of 4 mm thick MgF2 with an aperture of 40 mm. A grounded grid of 80% optical transmission was mounted just above the exit window in order to define the electric field. Asscintillation gas pure xenon at a pressure of 1 atm. was used. The maximum scintillation v~ltageduringthe experiments was 11.7 kV. This corresponds to a reduced electric field of ca. 5 V cm- Torr~,wellbelow the onset of the gas amplification process (Ca. 6 V crn1 Torr1)/1/. A SAES getter at an operat-ing temperature of 400 C was used to continuously purify the noble gas.After absorption of an X-ray photon in the xenon gas a cloud of primary electrons is produced which isaccelerated instantaneously by the electric field. During acceleration the electrons acquire sufficient en-ergy to create xenon excimers which deexcite under the emission of UV photons in the waveband 150-190 nm/2/. The number of these photons is proportional to the number of primary electrons and henceto the energy of the incident X-ray photon. In a driftless GSPC the light output is also dependent on theabsorption depth of the incoming X-ray photon. Therefore the duration of scintillation (hereafter calledburstlength) of each event has to be measured and the pulse height corrected accordingly. Detection ofthe UV photons was normally accomplished by means of a 3.5 EMI photomultiplier tube (PMT) with aquartz window and a bialkali photocathode (type D 319 QB) with an effective quantumefficiency (QE) ofapproximately 20 %.For position sensitive measurements a 39 mm diameter microchannelplate chevron, coated with 1400nm CsI, was used as read-out device. The ~,uantumefficiencyof the CsI photocathode has been mea-sured to be 9 % at an incident angle of 20 . Details on the fabrication and calibration as well as thedependence of the quantumefficiency on wavelength and angle of incidence (in the ranges 25-200 nmand 0~900,respectively) of this device can be found in ref./3/. The spatial distribution of the electroncloud leaving the MCP stack is determined by means of a wedge and strip anode/4/. This device hasthe advantage of yielding a good linearity while needing only three signal connections. The incoming(2)318 M. Heppeneret a!.position of the X-ray photon is then calculated as the centre of gravity of this distribution. In order toprevent degradation of the photocathode the whole assembly has been stored under vacuumcontinuously. The LiF entrance window of the MCP housing served at the same time as exit window forthe GSPC.Signal processingSeveral parameters have to be extracted from the output of the read-out device. These are: pulse height,pulse shape and, optionally, position. Pulse shape information is needed for two reasons. First, as al-ready mentioned above, the burstlength has to be measured in order to correct for pulse height varia-tions arising from the variation in scintillation pathlengths. Second, a very powerful background rejectiontechnique is based on the different pulse shapes obtained from (point-like) X-ray events when comparedto those from extended tracks originating from the passage of high-energy particles through the detectorvolume. It will be shown that especially the pulse decay time is an extremely sensitive parameter in thisrespect.A detailed description of the analogue and digital signal processing equipment can be found in refs./5/and /6/. Briefly, the pulse height signal was obtained by shaping the output of the charge sensitivepreamplifier coupled to the read-out system. Burstlength and decay time information was derived fromthe differentiated preamplifier signal (i.e. the scintillation time profile, see fig.8) by means of constantfraction timing. Finally, in the case of the MCP read-out three preamplifiers were used, each for onesegment of the wedge and strip anode, and shaped independently. Burstlength and pulse height weredetermined in the same way as for the EMI PMT.Storage of the data as well as subsequent data analysis was performed by the AT running programswritten in the scientific computer language ASYST. X and Y positions were calculated as the fractions fand ~ of the total charge falling on the s and w anodes, respectively, normalized to the total amount o~charge leaving the MCP, according to:(la)(1.b)with O~,0~and Q the charge signals on the s, w and z anodes, respectively. The total pulse height wasin this case caIcula~tedas the sum of the s-, w- and z-signals. The analysis programs accounted also forburstlength correction of the raw pulse height spectra, the generation of histograms and application ofGauss fits to the pulse height and position spectra to obtain energy and position resolutions.X-ray generationX-ray photons with an energy up to 2 key (i.e. B-K, C-K and Al-K radiation) were produced in an X-raygenerator comprising an electron emitting filament and a cathode coated with the relevant material. Ifnecessary a filter was used to suppress the bremsstrahlung continuum. For the higher energies (Ti-K,Cr-K and Fe-K) fluorescent X-ray photons were produced from a suitable target, excited with character-istic radiation and bremsstrahlung from a Ni anode. Care has been taken that, whenever appropriate, thebeam spot size was at least a factor two smaller than the FWHM position resolution of the GSPC.RESULTSEnergy resolution1000 300C500 -2400 . .200 .100 fl—K ~50 1600FEK I I40 00 20 lID 200 0 40 60 120 lOt 200— DURSTLENGTS CHANHEI. — 8009TLENGTK CHANNELFig.1. Measured burstlength distributions for Ti-K and Fe- Fig.2. The measured burstlength/pulse height responseK The solid lines represent the exponential fits to the data. for Ag-Kr and Ag-KU X-rays.Driftless GSPC (2)319Since the QE of the EMI photomultiplier was approximately twice as high as that of the CsI-coated MCPwe will present in this section only results obtained with the former read-out device.Fig.1 shows the measured burstlength distributions for Ti-K and Fe-K X-rays (4.5 and 6.4 key, respec-tively) obtained from the differentiated preamplifier signal. From the exponential fits to the data one ob-tains mean absorption depths of 6.1 and 3.5 mm, respectively, which is in good agreement with theattenuation cross sections for Xe as reported by Veigele/7/. As a result of these non-negligible absorp-tion depths the pulse height distributions show an appreciable tail towards the lower energies. In fig.2 thecorrelation between pulse height and burstlength is shown. Restored pulse height spectra are obtainedby applying a burstlength correction algorithm based on a second order polynomial fit to the data in fig.2.In fig.3 the results before and after correction are shown for Ti-K X-rays. The energy resolution derivedfrom a Gauss fit to the data (taking into account the presence of the ~ç8line at 4.9 key) is 8.5 %. In thesame way an energy resolution of 7.3 % is obtained at Fe-KG (6.4 key). By irradiating the GSPC with aradioactive 109Cd source the energy resolution at 22.1 keV (Ag-KG) has been measured to be 4.2 %(fig.4)600 2000- TI—K S—RAYS a TI—K S—HATS b640 - 1600i~~ 1.1110 00 120 ISO 240 300 0 00 120 100 240 300PULSE HEIGHT CHANNEL ———-~ CORRECTEDPULSE HEIGHT CHANNELFig.3. The raw (a) and burstlength corrected (b) pulse height distribution measured for Ti-K X-rays.For the lower energies the influence of differences in scintillation pathlengths is negligible as a result ofthe small absorption depths in xenon. The measured energy resolutions at Al-K (1.48 keV) and C-K (0.28keV) are 14.7 and 30.8 % FWHM, respectively. In fig.5 the observed pulse height distribution obtainedwith an anode coated with a carbon-boron mixture is shown. Fitting the data with two gaussians, the pa-rameters of one of which correspond to those obtained for the pure carbon spectrum, an energy resolu-tion of 36.4 % at 0.18 key is obtained.200 005I2~ J ~ ~ 11,00 ISO 240 320 450 G AG OG 120 lOG 200CORRECTEDPULSE HEIGHT CHANNEL PGLSE HEIGHT CHANNELFig.4. Corrected pulse height distribution for Ag-K X-rays. Fig.5. The observed pulse height distribution obtainedfrom a mixed boron-carbon target. The solid lines resultfrom a gaussian fit to the data performed to obtain thewidth of the boron peak.(2)320 M. Heppeneret al.Position resolutionPosition resolution measurements were necessarily only performed with the MCP read-out since the EMIPMT does not give spatial information.Fig.6 shows the theoretical and experimental values of the FWHM position resolution as measured atvarious X-ray energies. In this figure also the contribution of lateral diffusion of the primary electron cloudhas been included, both for a standard and a driftless configuration. As can be seen the position resolu-tion is completely determined by the QE of the MCP. The advantage of the driftless concept will howeverbecome appreciable when using a read-out system with a QE comparable to that of a conventional PMT.A particularly interesting feature is the dependence of the measured position on the absorption depth foroff-axis X-rays. This phenomenon, which is due to the finite size of the read-out system, can be seenmost clearly in fig.7 where the measured mean positions at different absorption depths for Ti-K X-raysare depicted. It can be shown that the measured radial position Rm, relative to the centre of the detectorobeys the following simple relationship:Rm(L)=R0/(1 +c.L) (2)with L the scintillation pathlength, R the incoming position and c a constant equal to 0.020 mm-1. Forvery larg read-out systems c shou?d approach zero. Eq.2 provides a simple correction procedure forthe reconstructionof the incoming position for off-axis photons.I IlJ~IIusionlimitModel ~rittlessl Fig.6. Measured and calculated FWHM position resolu-5 — — iuvon lent tions as a function of X-ray energy. Also Indicated are the— Total diffusion limits to the position resolution for a driftless and— a standard GSPC.0’ I’ :::~•: :1X-ray energy IkeVI= 22.,I•Fig.7. The measured mean positions for Ti-K X-rays mci- ~ 20 £ ‘adent on the detector at different positions at three absorp- •tion depths: 0 mm (circles), 5 mm (triangles) and 10 mm(squares). IS I I I I I I I I I10 21 23 25 27 20MEAN MEASUREDU POSITION I..IBackground rejectionBackground events were simulated by placing a 60Co source at the side of the detector. The issuinggamma ray photons in the MeV range liberate energetic electrons from the detector walls and in thecounter gas via the Compton effect. These electrons create extended tracks of ionization in the gas. TrueX-ray events, on the other hand, have a very limited extent which is determined by the diffusion of theprimary electron cloud only. This difference can be exploited as background rejection mechanism bymeasuring the decay time of the light pulse for each event. Typical X-ray and background pulse shapescan be seen in fig.8. With the exception of ionization tracks that are exactly parallel to the exit windowone expects the decay time of background events to be always longer than for X-ray event~.This can beseen in fig.9 where the sharp decay time distribution obtained with 5.9 key X-rays from an ~5Fesource iscompared to the distribution obtained with the 60Co source. Since the lower 1 cm of the detector doesnot contribute significantly to the sensitivity of the instrument in the desired energy band the distributionfor background events compatible with X-ray absorption in the upper 2 cm of the detector is also in-cluded. By applying a suitable decay time window simultaneously with the burstlength window needed toreject events from the lower 1 cm of the detector a rejection efficiency of 98.9 % is obtained at an X-rayacceptance level of 85 %. Off-line application of an absorption depth-dependent burstlength window canDriftlessGSPC (2)321improve the rejection efficiency even further to 99.6 % in the energy range 0.4 - 10 keV/6/. Additionalimprovements are expected from the use of the spatial information provided by the read-out system.F 55 F ~ Fig.8. Typical pulse shapes observed with a drthless GSPCfor true X-rays (Fe-K) and background (00-60) events.__________ ___________ :::_________________ _________________ ::~ ::: UPPER 2T~n,e ~ _.T~ne ~pSJ ~ 150Fig.9. The pulse decay time distributions for true X-raysfrom a ~Fe source compared to those obtained with a _______ I I I ICo source used to simulate background events. Also 00 120 100 240 500given is the distribution for 6°Coevents compatible with X -______ — PULSE DECAY TIME CHANNELray absorptIon in the upper 2 cmof the detector.CONCLUSIONSDriftless gas scintillation proportional counters provide superior performance in terms of energy resolu-tion and background rejection when compared to GSPC’s with a separate drift region. Most striking inthis respect are the energy resolutions obtained at sub-key X-ray energies such as B-K or C-K radiation.From these results it is clear that the problem of primary electron loss to the detector entrance win-dow/B! has been solved. The background rejection efficiencies reported here surpass those obtainedwith gas proportional counters in a comparable energy range/9/. The position resolutions of our in-strumentare completely determined by the QE of the MCP read-out. It appears to be difficult to coat theMCP with other materials exhibiting higher quantumefficiencies at 170 nm such as Cs2Te. Moreover, thehigh resistivity of the MCP material gives rise to gain degradation at higher countrates/1 0/. Further theexpected hfetime of channeiplates is not compatible with the use in a Ion~duration satellite mission.Therefore the solution of this problem is sought in the use of a position sensitive PMT that has been de-veloped recently/il/. Results of this investigation will be published shortly.ACKNOWLEDGEMENTSThe authors would like to thank A. van der Heijden, L de Jong, M.G.A. Kengen and LWeijts for continu-ous support during the measurements. The Laboratory for Space Research Leiden is supported finan-cially by the Netherlands Foundation for Scientific Research (NWO).REFERENCES1. T.H.V.T. Dias, A.D. Stauffer and C.A.N. Conde, IEEE Trans. Nuc!. Sc!. NS-34 (1983) 389.2. A. Gedanken, J. Jortner, B. Raz and S. Szöke, J. Chem. Phys. 57 (1972) 3456.3. D.G. Simons, G.W. Fraser, P.A.J. de Korte, J.F. Pearson and L de Jong, Nuci. Instr. and Meth. A261(1987) 579.4. O.H.W. Siegmund, R.F. Malina, K. Coburn and D. Werthimer, IEEE Trans. Nuci. Sc!. NS-31 (1984) 776.5. D.G. Simons, Thesis, Leiden University, 1988.6. D.G. Simons and P.A.J. do Korte, accepted for publication in Nuci. Instr. and Meth. A.7. Wm.J. Veigele, Atomic data tables 5(1973)51.8. H. lnoue, K. Koyama, M. Matsuoka, V. Tanaka and H.Tsunemi, NucI. Instr. and Meth. A157 (1978)295.9. T.A. Bailey, A. Smith and M.J.L. Turner, Nuci. Instr. and Meth. 155 (1978) 177.10. D.G. Simons, P.A.J. de Korte and M. Heppener, Nuc!. Instr. and Moth. A273 (1988) 512.ii. H. Kume, S. Muramatsu and M.Iida, IEEE Trans. Nuci. Sc!. NS-33 (1988) 359.

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The driftless gas scintillation proportional counter

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