IEEE Transactions on Nuclear Science, Vol. NS-34, No. 1, February 1987

A GAS SCINTILLATION PROPORTIONAL COUNTER FOR THE X-RAY ASTRONOMY SATELLITE SAX

A. Smith1, A. Peacock' and T.Z. Kowalskil21 Space Science Department of the European Space Agency, ESTEC, postbus 229, Noordwijk 2200 AG, The Netherlands.2 University of Mining and Metallurgy, Inst. of Physics and Nuclear Techniques, Krakow, Poland.

Sunnary

A driftless gas scintillation proportional counter isdescribed. The instrument has several advantages overconventional GSPCs especially for applications at lowx-ray energies (-0.l - 30 keV) and small dimensionssuch as imaging focal plane detectors for x-raytelescopes or concentrators. The x-ray astronomysatellite project SAX has such a requirement and animaging GSPC of this configuration is presented in thiscontext.

Introduction

In a driftless GSPC [1] the x-rays are absorbed in thescintillation region rather than in a forward diffusionregion. This has the disadvantage that the observedlight amplitude is dependant upon the depth ofpenetration of the observed x-ray, however this can becompensated for and the instrument has a number ofadvantages including greater simplicity of design andlimited electron diffusion.

loss of electrons to an intermediate grid. If thepenetration depth effect can be overcome then theinstruments should show a better performance than aconventional GSPC [1]. The driftless GSPC is mostsuited to low x-ray energies where penetration issmall anyway, and to small instruments such as focalplane detectors.

A GSPC for SAX

The performance requirements of the low energy gasscintillation proportional counter (LEGSPC) for SAXare defined by various mission constaints especiallythe design of the concentrator optics. Table 1 liststhese parameters together with their implications foran ideal LEGSPC.

Table 1.

SAX Concentrator/LEGSPC characteristics

The Italian national satellite project SAX will be anastronomical observatory in low earth orbit devoted tox-ray astronomy. The scientific payload will consistof four experiments that will make both wide and narrowfield x-ray observations. One of the narrow fieldinstruments will be a set of four x-ray concentratorseach with a geometrical area of 80 cm [2]. The focalplane detectors will be imaging GSPC, three of whichwill be sensitive in the range 1.0 - 10 keV while onewill have an extended range of 0.1 - 10 keV. It is thislast instrument that is the subject of this work. Thewider energy range is obtained by the use of plasticrather than beryllium for the entrance window.

This work describes a proposed solution for the LEGSPCbased on labaratory work with a representative gas celland readout system, and permeability tests of plasticfoils.

A Driftless GSPC

In a conventional GSPC (see e.g. [1] ,[3]) x-rays areabsorbed in a foward drift region where an electroncloud is produced that drifts under a relatively weakelectric field through a grid into a higher fieldregion where the electrons obtain enough energy betweencollisions to excite and scintillate but not ionise thegas. The light produced is measured with aphotomultiplier and the amount of light is proportionalthe energy of the incident photon. GSPCs of this typehave a major advantage over proportional counters inthat the loss of energy resolution caused by anavalanche is avoided. Since the photons are detected inthis forward buffer the amount of light produced isindependant of their actual penetration into thedetector. Photons absorbed in the scintillation regionare a nuisance but easily identified by virtue of theirrelatively short burst lengths.

A GSPC can be configured such that there is no driftregion ( hence driftless) but in which the photons areabsorbed directly in the scintillation region. In sucha situation the burstlength can be used to determinethe depth of penetration of the photons and hence torecover the high energy resolution. Such aconfiguration has the advantage of simplicity ofdesign, electron clouds are kept small and there is no

Parameter

Mission:-AltitudeInclinationAbsolute pointing accuracyAbsolute measurement accuracyDuration

Concentrator:-Geometric areaFocal lengthField of viewAngular resolution

LEGSPC:-Image scaleAperture diameterEnergy rangeEnergy resolutionLife-time

Value

500-600 km12 degrees3 arc min1 arc min> 2 years

80 cm2180 cm30 arc min1 arc min

0.54 mm/arc min1,6 cm.1 - 10 kev< 8 % at 6 keV> 2 years

Fig. 1 is a schematic drawing of the proposedinstrument. The entrance window is 2 micron Lexan(polycarbonate) foil with a diameter of 5mm. The foilis supported by a copper mesh of 0.6 mm square holeswith a transparency of 70%. The cell depth is 50 mmwith a 3 mm MgF window. The scintillation light isdetected with a 5 anode Hamamatsu PM tube. The fillinggas is pure xenon at 1 atmosphere. The light yield ofvarious combinations of window, gas depth, gas typeand PM tube have been discussed in [4]. The choice ofcell depth and window material are a consequence ofthe work leading to [4]. The purity of the gas ismaintained by 2 passive getters and the loss of gas bydiffusion through the plastic window is compensated bya gas reservoir. Differential diffusion prevents theuse of a second filling gas.

The relatively small window is required to reduce theloss of xenon by diffusion through the Lexan, the sizeof which is determined by the concentrator optics andspace-craft pointing stability. A second window willpermit simultaneous measurement of the sky background.This design causes no loss of performance on a pointx-ray source but will require multiple pointings on anextended object.

0018-9499/87/0200-0057$01.00 © 1987 IEEE

57

Forward WindowPump l

Getter _l_ =_ l_l Getter + Gas

RA

HVCELL (X

\/ RAWindows Ceramic

e)

PM

Fig 1. A schematic of the proposed LEGSPC.

In order to prevent the diffusion of atmosphericconstituents into the cell through the Lexan foil thereis an evacuated forward chamber, the window of which isruptured after launch.

In-flight calibration is performed using Fe55 radio-active sources in otherwise unused parts of the focalplane.

100.0

LEXAN

cj:~

I-f10.0 BERYLLIUMI

1.0

0.1 1.0 10.0

ENERGY KEV

Fig 2. X-ray transmission of 2 micron Lexan and 40micron beryllium foils.respectively. Note, the Cdl09 x-rays lie outside ourenergy range and indicates a worst case situationsince x-rays penetrate right through the cell. Theobtained energy resolution after correction is poorerthan that observed for a very narrow burstlength range

(7.0% vis 6.4 % at Fe55 and 8.4% vis 5.6% for Cdl09).The slight wing to the peak in fig 4c is caused by theMn k beta line at 6.4 kev. Below

-

5 keV the

difference between reconstructed energy resolution andthat obtained for a narrow range of burstlengths isnegligible since there is little penetration and theintrinsic resolution is lower.

Various aspects of the design are discussed below.

Quantum efficiency.

5 atm.cm of xenon gas is highly absorbant over theenergy range 0.1 - 10 keV and so the QE is determinedlargely by the entrance window although there is someloss. due to the particle background rejectiontechnique, (see below). Fig 2 gives the QE for a 2micron Lexan window compared to a 40 micron berylliumwindow (appropriate to the other three focal planeGSPCs on board SAX). The use of thin Lexan foil fordetector windows has been discussed elsewhere (5]. Themanufacture of the foils involves a multi-layerprocedure in which very fine films (0.05 microns) arestretched onto a water surface. These films are builtup to give a foil of the required thickness. The multi-layer technique has the advantage that pin holes areunlikely however the foil is intrinsically quite weak.

Energy resolution

Although a detector is yet to be assembled inaccordance'with fig 1, representative tests .have beenmade on a similar cell with a 4 cm scintillationregion. The applied voltage was 12 KV, the PM tube wasa Hamamatsu R1652 with a quartz window and the cellwindow was 5mm MgF2. Fig 3 is a plot of burstlength vspulse height fot Cdl09 22.4 kev x-rays. The bright bandshows the more or less linear dependace of one upon theother. The Ag k-al ha and kT ta lines are clearlyseparated. By allowihg for this dependance a correctioncan be applied to each event and so the high resolutionof the GSPC may be recovered. Fig 4cd shows the resultof such a process on the spectra of Cd109 and Fe55 (5.9Kev) x-rays. Fig 4a,b are the uncorrected spectra

_

Fig 3. Burstlength (Y) vs109 (22.4 keV) x-rays.

Pulse Height (X)x

plot for Cd

Imaging

Although the instrument descibed has a relativelysmall aperture, imaging is required both to determinethe location of the celestial x-ray source, (theaperture is larger than the concentrator image), forthe rejection of the particle background, and to allowsome estimation of the sky background.

Imaging is accomplished by use of a nine anode PMtube. The amplitudes from the nine anodes arecombined in a Anger Camera algorithm to provide x-yco-ordinates [6]. The anode signals are summedtogether to give pulse height information. Fig. 5shows the image of four 0.6 mm holes illuminated by aFe55 x-ray source (5.9 kev), the cell configurationwas the same as described above. The apparentdistortion in the image is merely the inaccuracies ofdrilling the holes in the mask. The outer boundaryrepresents a 5 mm diameter window. The spatialresolution derived from fig 5, after allowing for thefinite hole size is 0.38 mm rms o0.89mm FWHM. The

spatial resolution scales with E-'5.

58

59

LUJ

CCLU

Lli'

co.L

LU]

0.0

,

zI-I

CO

,-

0 .0,

FC

-JLuzzCC

LLJ

COC)

0 .0

-3LLzza:C)LUCrI

C)

0n

O .0

PULSE HEIGHT

Fig 4 a: Uncorrected response to Fe 55 (5.9 kev)c: Fe55 corrected for burstlength dependance.

Background rejection

Background rejection is accomplished by virtue of themeasured burstlength and pulse height. Not all regionsof the burstlength to pulse height plane are

appropriate to x-ray events. The boundry for 95 %acceptance of x-rays is shown in fig 6a. Thediscontinuity in the boundary at about 4.8 keVcorresponds to the xenon-l edge and the consequentlysmaller penetration above this energy. Fig 6b is theresponse of the detector to Co 60 gamma-rays (Cd 109has been shown in fig 3). The effective backgroundrejection can be determined by summinmg the counts infig 6b over the boundary in fig 6a and comparing to thetotal counts in 6b, including those above the thresholdof electronics. The background rejection to Co60 foran energy range of 0.1 - 10 kev and for 95% acceptanceof x-rays throughout the range is 92.2%. The responseto Fe 55 within the 95% boundry is shown in fig 6c.

The imaging capability is used to reject particlebackground events since any observed photon must have a

determined position that is consistant with theentrance window(s). The effective loss of background isthen just the positional resolution divided by theimaged area and is approximately 10000 at 6 kev anddecreases linearly with energy.

d

0 .00PULSE HEIGHT 256 .00

b: Uncorrected response to Cdl09 with a burstlengthcutoff corresponding to 2 cm penetration.d: Cdl09 corrected for burstlength dependanceNote figs a,c have a different energy scale to b,d.

Further rejection is possible if the burstlengthprofile is examined and annomalous profiles arerejected. This is currently being investigated using ahigh speed digitiser connected to a micro computer toselect bad profiles that would otherwise not havebeen rejected. Similarly the distribution of lightover the nine anodes of the PM tube should beconsistant with a unique location of the x-rayconversion or otherwise the event can be rejected. Theimprovements involved in these areas is presentlybeing investigated and will be the subject of a futurepublication.

Window considerations

The use of a thin platic window introduces problems ofgas purity, gas diffusion and mechanical reliability.

The diffusion of gas into the detector is avoided witha forward chamber that is either evacuated orpressurised with xenon. After launch the front windowof this chambet will be allowed to rupture.

60

500

-JLuzM

L)I-cn

Fig 5. An image of 4 0.6 mm diameter holes at 5.9 kev.The width of the circle corresponds to 5 mm.

I>1

50

-JLii

cc(J a

U)

I

0Fig 7a(top) LEGSPC Ib) LEGSPC response Idensity in 10000 s.

ENERGY 10 KEVresponse to SNR Cas A in 10000 s.:o 1% Cas A at zero hydrogen column

0'o -* x

Fig 6 a(top) 95% x-ray acceptance region for theburstlength(Y) pulse height (X) plane. b(middle) burst-length/pulse height response for Co6O. c(bottom)burstlength/pulse height response for Fe55.

The loss of gas from the detector by virtue of thepermiability of the plasic is a more serious problem. Anumber of samples of Lexan foil were examined in a highvaccuum test facility in which one side of the foilcould be pressurised with a particular gas while onthe other side the leakage was measured. These testsindicated an equivalent leak rate to xenon of 5.2*10-6torr.litrs/sec for a 2 micron Lexan window of 5 mmdiameter. This implies a loss of xenon of 0.82atm.litres in a 2 year mission. Although this amount isnot large in itself it would constitute an unacceptabledrop in gas pressure and so a gas resovoir will berequired.

Presently under investigation is a polypropelene/Lexanwindow. Polypropelene stretched foils are stronger thanLexan but more subject to pin holes. Such aLexan/polypropelene combination has the best propertiesof each.

The window is coated on the inside with a thin layerof aluminium in order to provide a conductive surfaceand keep out optical light. Graphite is not favouredin this respect because of its relatively pooroutgassing properties. Outgassing of the foil, itssupport 0-ring and other components will be handled bytwo passive getters.

Discussion

The proposed instrument for the LEGSPC role on SAXwill provide good energy resolution and highbackground rejection over the range 0.1 - 10 keV. Therequirement of a low energy response forces a thinplastic window which in turn means a small aperture inorder to limit the loss of gas by permeation. Bymaking the window large enough such that attitudeerrors and concentrator point spread function areaccomodated, and by using a second window to determinethe sky background, the performance of the instrumentto a point x-ray source is unaffected. For an extendedsource multiple pointings will be required. SThespatial resolution can be expected to vary as E- -which implies a FWHM of 7 mm at Ol keV. Because ofthis there is an advantage in an effective aperturestop for extended (>10 arc min) sources in order toisolate small source regions (c.f. the slit in aspectrometer)

61

In order to provide an astrophysical setting for thisenergy resolution and wide band-width, the response ofthe instrument to the supernova remnant SNR Cas A issimulated in fig 7a. Fig 7b. is the response to a SNRlike Cas A but of 1% its brightness and at a low valueof the hydrogen column density. In this f igure noallowance has been made for the losses due to the x-rayoptics and an exposure of 10000 seconds is assumed.

The measured values given in this work have not beenwith an optimised detector configuration. From [4] wecan expect an improvement in light yield by a factor of2.5 by careful choice of PM window and cell depthleading to a further improvement in performanceespecially in spatial resolution.

References

[1] D.G Simon, P.A.J. de Korte, A. Peacock and J.A.M.Bleeker, Energy resolution limitations in a gasscintillation proportional counter, Proceedingsof the SPIE symposium, 1985,

[2] G.F. Spada, SAX Scientific Instrumentation, inProceedings of the Workshop on Non-thermal andVery High Temperature Phenomena in X-rayAstronomy, Rome, 1983, pp217 - 234.

[3] J. Davelaar, A. Peacock, and B.G. Taylor, A gasscintillation camera for x-ray astronomy, IEEETrans. Nucl. Sci., NS-29,1982, p142.

[4] T.Z. Kowalski, A. Peacock, A. Smith and B.G.Taylor, Light Yield Improvements in GasScintillation Proportional Counters, submitted toN.I.M., 1986.

[5] H. Huizenga J.A.M. Bleeker, W.H. Diemer and A.P.Huben, Submicron Entrance Windows for anUltrasoft X-ray Camera, Rev. Sci. Instrum. 1981,52(5), pp 673-677.

[6] H.O. Anger, Rev. Sci Instr., vol 29, 1958, pl.

Acknowledgements

The authers are pleased to acknowlede the assistance ofN. Striker, R. Englehardt and j Valero. T.Z.Kacknowledges the receipt of an IAEA fellowship.