Nuclear Instruments and Methods in Physics Research A 344 (1994) 384-393North-Holland

Ultra-high pressure proportional countersPart II. XenonR.K. Sood a°*, Z. Ye a, R.K. Manchanda bDepartment of Physics, University College, Univ. of NSW, ADFA, Canberra ACT 2600, Australia

b Tata Institute of Fundamental Research, Bombay, 400 005, India

(Received 14 october 1993)

Proportional counters filled with xenon-methane mixtures at ultra-high pressures have been tested since the publication of thefirst part of this paper. These tests have clearly demonstrated the feasibility of using large area high pressure xenon detectors forapplication in the hard X-ray and low energy gamma ray regions . Our measurements also indicate that the mobility of positive ionschanges with gas pressure thereby affecting the gas amplification process and the anode pulse shape in proportional counters . Thespatial and energy resolution of counters at high pressures depends not only on the increased fluctuations in the number of primaryelectrons produced and the statistics of the charge multiplication process, but also on the gas pressure, concentration of quenchinggas and the presence of electro-negative impurities .

1 Introduction

Proportional counters filled with argon or xenon atpressures up to - 3 atm (- 300 kPa) have been usedsuccessfully to detect X-rays of energies up to - 150keV in the areas of nuclear physics, nuclear medicineand X-ray astronomy [1-6]. In order to investigate thedesirable increase in detection efficiency for higherenergy photons, we have constructed several proto-types of a proportional counter which can be filled toultra-high pressures up to 50 atm. In a previous paper(called paper I hereafter) we presented for the firsttime, results obtained from such counters filled withargon-methane mixture up to - 30 atm [7], where weobtained excellent stability over a long-period of timeand excellent energy and spatial resolution . The work-able energy range of such detectors is limited to pho-ton energies up to - 150 keV.

The initial tests of xenon-methane filled prototypecounters have revealed a different behaviour of suchcounters at high pressures compared to argon-methanefilled ones . The spatial and energy resolution of thesedetectors deteriorates rapidly with increasing gas pres-sure [8] . Similar conclusions of resolution deteriorationat pressures up to 10 atm have been reported recentlyby Sakurai et al . [9] .

In order to optimize the use of xenon filled ultra-high pressure counters for future large area and high

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NUCLEARINSTRUMENTS& METHODSIN PHYSICSRESEARCH

Section A

resolution detectors for hard X-ray astronomy beyond100 keV, we have continued further tests on the char-acteristics of such detectors at pressures up to 17 atm.

This paper describes detailed investigations of vari-ous parameters which affect the performance of largearea, sealed xenon-methane filled detectors and dis-cusses the basic limiting processes. The data analysisshows that the purity level of the gas mixture is crucialfor enhancing the lifetime of the electrons produced inthe initial photon interaction . This increased lifetime isessential, since the number of inelastic collisions whichdo not lead to ionization is enhanced at high pressures.The increased number of collisions of the primaryelectron cloud not only affects the gas gain but alsocontributes to a large variance in the amount of ioniza-tion thereby leading to a worsening of the detectorresolution at higher pressure . We find a substantialchange in the mobility of the positive ions at increasedpressures, a factor which determines the properties ofthe anode pulse. Pressure dependence of the basicelectron drift and multiplication processes further lim-its the energy and spatial resolution of the high pres-sure counter and the geometrical dimensions of eachcell in a large area detector . Present results demon-strate that it is feasible to build a large area spectrome-ter with a detection sensitivity of 10-5 photons cm -2s -1 keV-1 at 300 keV. The observed resolution of12% at 120 keV for the high pressure counter thoughnot ideal, is comparable with that of scintillation detec-tors which tend to suffer from detector generatedbackground .

2. Experimental arrangement

The ultra-high pressure proportional counters usedfor the tests with xenon gas consisted of long tubes ofcylindrical geometry . The counters were similar to theone used for the argon measurement (paper 1) withsome modification on the end faces. These modifica-tions yielded improved signal handling and made possi-ble negative high voltage operation for absolute gainmeasurement using the current measurement method .A schematic diagram of the apparatus is shown in Fig.1 . The aluminum coaxial proportional counter tubeshad a diameter of 30 mm, wall thickness 2 mm andlengths 1100 mm and 300 mm respectively . The anodeswere gold plated tungsten wire of diameter 25 wm . Thecounters were thoroughly cleaned before assembly andbaked under vacuum for a few weeks and were sealedafter filling . The fill gases were cleaned using an ox-isorb cartridge purifier to remove O,, HO and otherelectro-negative impurities, which are fatal to counteroperation at high pressures.

The gas amplification factor was measured forxenon-methane mixtures at pressures between 1 and17 atm. At ultra high pressures, the percentage ofmethane in the gas mixture affects the energy resolu-tion of the detector [7] . The measurements were there-fore made with varying concentrations of methane. Ahighly stabilized 0-8 kV power supply was used for themeasurements . The signal from the proportional coun-ters was amplified and investigated, using in turn, oneof two different charge sensitive pre-amplifiers, (a) theORTEC/EG&G model 142PC unit and (b) an in-house unit built around the LeCroy TRA1000 chip .For absolute gas gain measurements the current mea-suring method was adopted . The gas gain obtained bythe pulse matching method was calibrated against the

Safety Valve

R.K. Sood et al. /Nucl. Instr. and Meth. i n Phys. Res . A 344 (1994) 384-393

Iligh Pressure Gas

Valve

30-110 cm

Cylindrical Wall (Cathode)

Iligh Vacuum System

Fig. 1 . Schematic of the new proportional counter .

current measurement mode . A detailed discussion ofthe gas amplification process at high pressures is givenelsewhere [10] .

In order to measure the energy resolution, energylinearity, and gas gain factors, pulse height spectrawere obtained using Z°tAm which emits photons at 16 .5and 60 keV, 57Co at 122 keV and t33Ba at ^ 350 keV.The measurements were made using both uncollimatedsources and collimated sources of a few mm aperture .Pulse height spectra were obtained at each pressurefor a series of anode voltages in order to study varia-tions in gas gain and energy resolution .

3. Results and discussions

3 .1 . Gas gain and general characteristics

Following procedures similar to our earlier mea-surements with argon, the xenon filled detectors weretested at various pressures between 1 and 17 atm withmethane concentration of 1%, 2%, 5% and 10%. Fig.2. shows the gas gain as a function of applied voltagefor the 98 : 2 mixture, which was found to be the mostsuitable . Representative curves for 95 :5 (dashed line)and 90 :10 (dash-dot line) mixtures at 1, 3 .5 and 7 atmpressure are also plotted in the figure . The gas gainvalues shown on the Y-axis correspond to those ob-tained using the current measurement method . Thedata indicate that at the higher pressures, the outputpulse is quenched by 50% when the methane concen-tration changes from 2% to 5% and from 5% to 10%.The corresponding reduction is only about 10% atlower pressures. At 15 atm, the gas gain curves revealsome unexpected behaviour, as seen in Fig. 3. Unlikethe trend in Fig. 2, the gain is lower for a lower

a

Anode Wire

CeramicValve

385

cô0NaCD

386

Anode Voltage

Fig. 2. Gas gam vs anode voltage for xenon-methane (98 :2) atvarious pressures. Representative curves for 95 :5 (dashedline) and 90 :10 (dash-dot line) mixtures are shown for 3.5

and 7 atm pressure .

concentration of methane. It is seen from Fig. 2, thatindividually, each of the curves shows exponential be-haviour (straight line on a log-linear plot) . This isexpected from simplified ionization theory where thegas gain M is given by [11]

M=exp(frb

adr l = ko ew.r o

R K Sood et al. /Nucl. Instr. and Meth . i n Phys. Res. A 344 (1994) 384-393

Here, V is the anode voltage, and C is the capacitanceof the counter given by C = 24rEOE r(p, T)/ln(rb/r a)where the dielectric constant Er for the gas is a func-tion of pressure p(atm) and temperature T('C), and rband ra are the cathode and anode radii respectively .The first Townsend coefficient of ionization a whichdetermines the degree of ionization is given by

a = krp exp[ -k2É 1>

(2)

where E is the electric field and kl and k2 areconstants. We also notice in Fig. 2, that as in ourearlier measurements, the exponent of the gas gaincurves shows a marked pressure dependence . The vari-ation of this exponent with pressure is shown in Fig. 4.Data from our earlier tests on high pressure argoncounters are plotted for comparison .

Since the functional dependence of a on pressureand the electric field is highly non-linear, a variety ofempirical linearized functional forms based on simpli-fied assumptions have been suggested in the literature[11] . For example, following Rose and Korff [12] theequation can be re-written in terms of the gas gain andthe reduced electric field S = E/p, and is given by

In M

k4

praSa- ks

where ra and Sa are the radius and the reduced

Fig. 3. Gas gain at 15 aim for 1% and 2% methane concentra-tion .

electric field at the anode surface respectively . Fig. 5shows a plot of the observed data in terms of thetheoretical functional form in Eq . (3) . The non-linearbehaviour of the gas gain and thereby a, is quiteevident from the figure . Similarly, other empirical formsderived from data taken at low pressures do not fit ourhigh pressure data [10] .

The discrepancy between theory and experimentcannot be related to the limitations of the detectionsystem . The data suggest that at the low values of thereduced field strengths typical for proportional opera-tion at high pressures, the charge production andcharge collection processes are radically modified .Compared to operation at low pressures, even thoughthe reduced field in a high pressure proportionalcounter is small, the radial distance at which a specific

30

20

10

04

cô

NO

1000

100

10

Xenon + methane15 aim

" 98 2

a 99 I

Anode Voltage (kV)

0

Argon (98 2 ) -

0 Xenon (98 2) -

5 10 15 20 25

Gas Pressure (atrn)

60

30

Fig. 4. Variation of the gradient of the gas gain curve withpressure . A curve for argon is shown for comparison .

0.05

0.04

p0.03

0.02w

c 0.01

R.K. Sood et al. INuel. Instr. and Meth. in Phys. Res. A 344 (1994) 384-393

0.000.02 0.03 0.04 0.05 0.06 0.07

Sae [Vlcm kPa ]

Fig. 5. Linearized gas gain curve for xenon-methane (98 :2),following Rose and Korff [101 .

value of the electric field occurs is farther from theanode by a factor of - 4. This enlarges the regions ofscattering and ionization around the anode. The basicfeatures which will therefore determine proportionalbehaviour at high pressures are :- increased inelastic scattering due to the increase in

the gas density;- variation of the Fano factor at higher pressure (if

any) ;- electron drift velocity and the lifetime of electrons

before they combine with electro-negative impuri-ties ;

- mobility of the positive ions .The electric field is highly non-linear for a cylindrical

geometry and it falls off as E(r) a V/r where r is the

radial distance from the anode. Very close to the

anode wire the field strength is high enough for the

development of an avalanche and the developed charge

is proportional to the number of primary electrons. Far

away from the anode wire, the field is just sufficient fordrifting the ions . The number of electrons in the pri-mary cloud in the drift region is determined by theattachment of these electrons to any electro-negativesubstances present. The charge development cantherefore be written as

N(t) = Nt( 1 -e(4)Te

Here N is the average number of electrons producedper incident photon of energy E, and Te is the lifetimeof the electrons, defined as -re = 1/k s , where ks is theattachment coefficient which is a function of the impu-rity concentration, mainly Oz . Low levels of contami-nation up to 16 ppm do not appreciably affect-theproportional characteristics in a situation where thedrift dimension is limited to - 1 cm and the pressureis less than 10 atm. However, we believe that in the

larger drift dimension of 30 mm used in our case, theattachment effect may be appreciable since at pres-sures above > 5 atm the observed detector characteris-tics are significantly altered.

Between the proportional and the drift regions onecan define a cylindrical shell in which the detectorcharacteristics resemble those of an ionization cham-ber rather than a proportional counter. The electricfield is such that, between two collisions, an electronattains enough energy to excite rather than to ionizethe xenon atoms. The increased number of inelasticcollisions of the primary electrons thus not only givesrise to an enhanced variance in the electron statistics,but also leads to a lower value of the gas gain . Theincrease in the gas pressure results in a reduced meanfree path I for the electrons. Only those electronswhich have a mean free path x(> 1) sufficiently long toacquire the energy necessary to ionize the gas, willcontribute to the secondary ionization .

The number N of electrons which have path lengths> 1 is given by

N=N e

The mean free path 1, and the minimum path lengthrequired for ionization x can be written as

and

387

where w is the ionization potential of the gas, Ng is the

number density of the gas molecules and o- is thescattering cross-section . The number density of elec-trons giving rise to ionization can thus be written in theform

N( r) =N e -k ,p~ v

(

and the total collected charge q a (1/p) e-krpI v, whichclearly represents the exponential behaviour seen inthe gradient of the gas gain curve at different pres-sures, as plotted in Fig. 2. The pressure dependence ofthe electron numbers which give rise to ionization alsoexplains the low value of the gas gain seen in ourmeasurements .

In contrast to argon mixtures at high pressure, xenonmixtures show considerable asymmetry in the shape ofthe energy peak at higher energies, specially for inter-actions farther away from the anode. We believe thattwo factors contribute to this behaviour (a) signal lossdue to electron attachment and (b) multiple Comptonscattering of the primary photon before it is absorbedin the gas mixture. Thus, the electrons released duringan interaction, though large in number have a largevariance in number and in energy. The Compton scat-tering process leads to an initiation of the proportional

388

process from more than one site, thus enhancing theasymmetry in the shape of the observed energy peaks.

3.2 . Detection efficiency

Gas detectors operating at low pressures have alower quantum efficiency at 100 keV compared toconventional scintillation counters and solid state de-tectors . At high pressures the detection efficiency in-creases to quite comparable levels . Also, the usefulenergy range is extended to beyond 300 keV, a regionwhich has remained unexplored so far in X-ray astron-omy. In addition, due to simplicity of construction, it iseasy to make large area detectors of - 1 m2 using amodular approach . The total detection efficiency canbe further increased by using a pile detector of multilayer construction . One can define a quality factor fora detector given by

Q(E) = Area x detection effi-

_ôTca>.U

WC0.U

0

ôNNIYTiNCW

Fig. 6. Detector efficiency for a 17 atm xenon filled propor-tional counter and a comparison of the quality factor of a 32element UHPC array and a 5 mm thick scintillation counter .

25

15

10

5

0

R.K. Sood et al. /Nucl Instr. and Meth. in Phys. Res. A 344

Gas Gain

104

103

210

c0

ôô

Fig. 7 Energy resolution versus gas gain at 1 and 13 .5 atm.

(1994) 384-393

ciency/resolving power. For a multi-layer xenon detec-tor with an effective area of 1 m2 and a volume of - 1m3, this parameter is an order of magnitude higherthan crystal scintillator and germanium detectors ofavailable sizes, since the background counting rateintrinsic to proportional counters is negligible, and theuse of electronic background reduction in such detec-tors further reduces the total background countingrate . The estimated sensitivity for our test detector,which consists of 32 tubes filled with xenon at 17 atmand is arranged in four layers with an effective volumeof 2000 cm2 x 10 cm, is 10-5 photons cm -2 s- ' keV- 'at 300 keV. The detection efficiency and the qualityfactor Q(E) as a function of energy for this detectorare shown in Fig. 6. The quality factor of a Nal(TI)scintillation counter of 400 cm2 area and 5 mm thick-ness is plotted for comparison .

4. Energy resolution measurements

In order to optimize the energy response at eachpressure we measured the energy resolution as a func-tion of gas gain . A representative plot for the resolu-tion curve measured at 1 and 13 .5 atm pressure for 60keV photons is shown in Fig. 7. It is seen from the Fig.that at the lower pressure there is a very broad mini-mum. At the higher pressure the energy resolutionshows fast degradation for gas gains above 10 3 . This isprobably due to the change in the operating regionfrom limited proportional to Geiger, set off by thelarge number of UV photons released due to thede-excitation of the xenon molecules. This mechanismappears to be the dominant mode of energy loss for theelectrons in the gas volume. Therefore, for all oursubsequent measurements at different pressures, we

6

4

Gas Pressure (atm)Fig . 8. Optimum anode voltage versus pressure for xenon-

methane and argon-methane mixtures .

N

C

0U

G

U

0

R.K Sood et al. /Nucl. Instr and Meth .

Energy (keV)

Fig . 9 . Typical spectra for 241Am at 1, 3 .5 and 15 atm.

operated the counter at a gas gain of 10 3 . A plot of theoptimum operating voltage at various pressures, for agas gain of 10 3 , is shown is Fig. 8. High pressure argondata are plotted for comparison .

In Figs . 9 and 10 we have plotted representativespectra taken over a range of pressures. The datacorrespond to X-ray emission at 60 keV from 241Am

and 122 keV emission line from 57Co. At low pressuresthe spectra appear quite good . Unlike the spectra fromargon counters, the spectra here are quite complexbecause of the appearance of the escape peak at about

140

Energy (keV)

Fig . 10 . Typical spectra for 24'Am and S7Co at 7 arm.

0 20 40 60 80 100 120 140

Energy (keV)

Fig . 11 . Energy resolution vs energy at 1, 3.5 and 13 .5 atm.

30 keV and the difference energy peak . For photonsof energy above the K-edge of xenon, 87% of theinteractions lead to the emission of K-photons whichmay escape from the gas volume thus leaving behindan effective energy deposit of E - EK . In the spectraplotted in Fig. 10, we also detect the fluorescent linefrom our lead collimator and its corresponding escapepeak. It is also seen that at high pressures, because ofthe aforementioned asymmetry in the shape of theenergy peak, the apparent energy resolution is consid-erably worse.

Fig. 11 shows the variation in energy resolution withgas pressure for xenon-methane (98 : 2) measured at agas gain of 10 3. It is seen that at pressures up to

4 atm, the energy resolution is extremely good withvalues of - 6% at 30 keV and - 4% at 122 keV. Athigher pressures, the observed resolution worsens by afactor of about - 3 because of various factors, i.e . gasimpurity, effect of the quenching gas, change in themobility of the positive ions and pressure effects asdiscussed earlier . Tests on a (99 :1) xenon-methanemixture showed only a slight improvement in the reso-lution at higher energies . The spectra at higher pres-sures, (Figs. 9 and 10), show an asymmetry in eachenergy peak, the reasons for which have been dis-cussed earlier . The observed resolution at pressures upto 3 .5 atm can be fitted with a functional form .9d =0.46E -1 /2 ; similar to that obtained for argon counters .

Classically, _the energy resolution M(_

DE/E --_8P/P where P is the mean pulse height at energy E)of a single wire proportional counter depends on thevariance of the primary charge cloud and the fluctua-tions in the gas gain. However, the long track length ofthe primary charge distribution due to multiple Comp-ton scattering of a high energy photon can give rise tosystematic effects because of the limitation of the mea-

n Phys. Res A 344 (1994) 384-393 389

20xenon + methane(98 .2)-

+ 1 aim.s~.- ° 3.5oim

15x 13 .5 arm

100NN

o+ 5

WCtil

0

39 0 R.K. Sood et al.INucl. Instr. and Meth. in Phys . Res. A 344 (1994) 384-393

suring system as manifested in the pulse asymmetry. Asa first approximation, we can therefore define :

P ) 2= ( N l z+ N( M )2+ NI T)

Z,

(P

)2=( N

)

z+ N ( 77(p,E)M I z.

where N is the mean number of primary ion pairsproduced by a photon of energy E and mean tracklength T, M is the mean amplification factor and 8P,8N, 8M, and ST are the standard deviations of themean quantities . The third term in Eq . (9) in effect isthe variation in the mean amplification due to thefinite shaping time constant of the measuring system,and may therefore be combined with the second termby introducing an energy dependent variable r1 in thedenominator in the second term . Thus,

(10)

The classical equation which describes the resolutionof a proportional counter [12] may then be modifiedusing Eq . (10) to yield

f.W=236 F+

(11)71(p , E) E

where F is the Fano factor and f = (8M/M)z . Usingour measurements of the energy resolution at variousenergies and at different pressures, we can qualita-tively describe the nature of the functional dependenceof the Fano factor on pressure, and the dependence ofthe variable 77 on pressure and energy .

The observed deviation of energy resolution withenergy, from the ideal functional form .M a E-1 /2 (forv7 =1 in Eq. (11)), and the apparent constant resolu-tion at all energies above 100 keV as seen in the Fig.11, clearly point to 77(P, E) < 1 . The limiting func-tional dependence will thus approximate to v1(p,E) -E'/ z since the second term in Eq . (11) is mainlyresponsible for the poor resolution . Similarly, the vari-ation of energy resolution with pressure at differentenergies is ascribable to both the variation of the Fanofactor and the variation of n with pressure . Our dataare consistent with a weak exponential increase of bothparameters with gas pressure . Optimal fitting of thedata suggests about 40% increase in the value of F at17 atm compared to its value at 1 atm.

An increase in the Fano factor F = (bJ/J)z at higherpressures, where J is the mean number of electronsproduced per photon, is expected naturally due to adecrease in the mean free path of the 8-rays resultingin an enhanced probability of re-combination and anincreased ratio of the number of excited to ionizednuclei Nex/N� compared to the canonical value of-0.4 .

Dependence of the Fano factor on both energy andpressure, within a limited range of energy of a few

Fig . 12 . Linearity of the response of the counter as a functionof energy .

keV, has been reported by Kowalski et al . [13] . Theysuggested that the Fano factor may vary with energy tothe extent of (Fw)' /z = 0.0773 - 0.00446 E1 /z .

Notwithstanding the relatively poor energy resolu-tion seen at higher pressures, energy linearity of thepulse height was seen at all pressures. In Fig. 12 wehave plotted a sample pulse height output in the en-ergy range 30 to 350 keV. Linearity in detector re-sponse is essential for its stable operation over a widedynamic range of energy .

5. Spatial response

Energy (keV )

Factors which limit the spatial resolution of a pro-portional counters are (i) range of photoelectrons (ü)diffusion of the primary electron cloud (iii) fluorescentphotons emitted by the absorption of X-ray photonswith energy greater than the K-edge energy of thefilling gas (iv) geometrical size of the test beam (v)fluctuations in the position of the avalanche centroid(vi) re-combination during the drift of the electrons(vii) electronics noise

In contrast to argon, the K-fluorescent effect ishighly pronounced in xenon since for photons above 30keV, 87.5% of the interactions involve the K-shell.These interactions have 85% probability for fluores-cent photon emission . Secondly, the operation of ahigh pressure counter is limited to a maximum gas gainof about 103 as discussed above. The re-combinationprobability is therefore higher as there are an in-creased number of collisions which do not lead toionization . Spatial measurements are therefore neces-sary to determine operational constraints and to estab-lish the optimum geometrical parameters for ultra-highpressure counters . Pulse height spectra were measuredalong both the axial and radial direction by using a

R. K. Sood et al. /Nucl. Instr. and Meth . in

narrow X-ray beam at 60 keV obtained by collimatingan 241Am source in a lead collimator.

Fig. 13 shows sample energy spectra taken in theradial direction for xenon-methane (98 :2) at 3.5 atm,when the collimated 241Am was located at the centre,

5 mm from the edge and at the edge of the counterrespectively. The 60 keV peak is seen to move slightlytowards lower channels when the collimated X-raysource is moved towards the edge of the counter. Thechange in both pulse height and energy resolution is- 3% and is comparable to results obtained with ar-gon. However, when the collimated source is locatedclose to edge of the counter, the pulse is much more

asymmetric. Fig. 14 shows a set of24'Am

energy spec-

tra collected for xenon-methane (99 :1) at 15 atm. Atthese pressures, the observed variation of the pulseheight in xenon is much higher compared to argon.The peak-shift is more obvious compared to that ob-served at lower gas pressures. The two sets of energyspectra shown in Figs . 13 and 14 were collected using

the same counter filled with the same purity gas, theonly difference being gas pressure . In the axial direc-tion we did not find any noticeable change in the pulseheight to within 15 mm of the anode ends .We believe that the deterioration of the spatial

response in the radial direction is due to a number ofthe reasons outlined earlier. In addition, the increasedprobability of electron scattering, re-combination dueto the presence of electro-negative impurities, and theamount of quenching gas [5] become important param-eters when the first ionization takes place far awayfrom the anode. Present studies suggest a limit on the

N

0U

50 100 150 200 250 300 350

Pulse Height (Channel)

Fig. 13 . Spatial response of the counter m the radial directionat 1 atm, established with 241Am.

Phys. Res. A 344 (1994) 384-393

391

6. Quenching effect

Xenon+methane (99 1)

15 atm

0 100 200 300 400

Channel noFig. 14 . Spatial response of the counter in the radial direction

at 15 atm, established with 241Am

maximum size of the counter cell at high pressure, ofabout 25 X 25 mm.

In paper I we noted that the concentration ofquenching gas affects the detector resolution at higherpressures for argon-methane mixtures . Similar be-haviour is seen for xenon-methane mixtures . It is seenfrom the data that at low pressures, the amount ofmethane in the xenon-methane mixture does not showany significant effect on the detector response . Athigher pressures, once again the data show an expo-nential functional dependence similar to that observedin argon counters . It was noted that various otherparameters discussed above, which result in the wors-ening of detector resolution, do not vary with thechange of gas composition and hence the observedbehaviour must be intrinsic to the amount of quench-ing gas.

In the case of high pressure argon counters, we hadargued in favour of a photon channel which gave riseto a fraction of the electrons during the multiplication

392 R.K Sood et al. /Nucl. Instr. and Meth. in Phys. Res. A 344 (1994) 384-393

process. We had envisaged the excitation of the gasmolecules to upper levels rather than ionization insome electron collisions, so that the subsequently emit-ted photons may have been re-absorbed by the gasmolecules to produce further ionization between thelower ionization state. Larger concentrations ofmethane would therefore have enhanced the quench-ing of the emitted photons thereby leading to largerfluctuations in the initial part of avalanche cascade,where excitation collisions dominate over ionization

collisions at high pressures. The dependence of the

detector resolution on the amount of quenching gas in

xenon at high pressures has led us to explore for an

additional fundamental process which can explain the

observed phenomena. The bulk of the emitted photons

in the xenon de-excitation process have energy ^- 7.9

eV and are below the methane resonance photon ab-

sorption peak at - 10 eV . Also, in the case of xenon,

charge exchange between the positive ions and the

methane molecules has a very low cross section be-

cause the ionization potential for xenon (w = 12 .1 eV)

is lower than that of methane (13 eV).

It is clear from Fig. 2 that an increase in the

concentration of the quenching gas leads to a lowering

of the gas gain at a given pressure and anode voltage.

This must arise partly because of resonant non-dis-

sociative and dissociative electron attachment pro-

cesses . A negative ion moving towards the anode can-

not contribute to the ionization due to a low drift

velocity . The dissociative electron attachment cross

section for methane shows a peak at 10 eV in the

electron energy range 7-12 eV [12] . At higher pres-

sures the scattering mean free path of an electron is

considerably lower than that at low pressures. There-

fore, the probability of an electron interacting with a

methane molecule before it has gained enough energy

for ionization increases proportionally with the

methane concentration. The increased attachment of

electrons to the methane molecules will result in larger

fluctuations in the total number of electrons and

thereby a deterioration in the detector resolution, as

observed experimentally .

7. Ion mobility and the pressure effects

The time development of the signal pulse in a

proportional counter is determined by the drift velocity

of the moving charge . The mobility of the ions will

therefore determine the rise time of the pulse. The

pulse height measured at the output of a charge sensi-

tive amplifier is a linear function of the input charge

which in turn depends upon the pulse rise time and the

value of the shaping time constant . This leads to a

functional interdependence between the gas gain and

the mobility of the ions.

T

L0

10

Xenon+methane (98 2)

1

1

1

1

1

1

1

12 4 6 6 10 12 14 16

Gas Pressure (atm)

Fig. 15 . Relative mobility of positive tons in xenon as afunction of pressure .

At values of the reduced electric field above those

normally encountered in proportional counters, the

average energy of ions is almost unmodified . There-

fore, the mobility of ions w+ moving in a gas is largely

constant and is specific to each ion. For a mixture of

gases the ion mobility can be obtained by using the

sum rule . At very high fields E/p > 20 V em -1 Torr-1

the ion mobility follows the relation w (x (E/p)'/2 [14] .

However, the reduced field E/p in a high pressure

co-axial proportional counter is fairly small and also

drops very significantly beyond a few mm from the

anode wire due to the cylindrical geometry . It is there-

fore reasonable to assume a constant mobility for the

xenon ions even at high pressures. However from our

study of the behaviour of xenon and argon at various

pressures, we conclude that ion mobility does vary with

pressure even at low values of the reduced electric

field. Such behaviour can be expected if the ion drift

velocity is mainly controlled by the scattering collisions

of the ions .Using our measurements of the absolute gas gain by

the current method, we have derived the mobility of

positive ions at various pressures for xenon [10] . A

value of /, = 0.53 em2 V-1 s -1 for xenon ions in their

own gas at atmospheric pressure was used for thecomputations . Relative changes in mobility with pres-

sure are plotted in Fig. 15 . Clear evidence of the effect

of pressure on ion mobility and thus on the detector

behaviour is seen in the Fig. 15 . The observed variation

represent an exponential slowing of the ions .

8. Conclusions

Proportional counters filled with high pressure

xenon-methane mixtures show proportionality, energy

R K Sood et al. /Nucl. Instr. and Meth. in Phys . Res A 344 (1994) 384-393

linearity and stability of operation similar to high pres-sure argon detectors as reported in paper 1 [7]. Degra-dation of energy resolution and non-linearity of spatialresponse appear to be the main constraints for the cellsize . Extremely low contamination levels are prere-quisite for large cell sizes.

The majority of the photons above 30 keV absorbedin the gas result in the emission of escape photons.Because of the high probability of absorption of theescape photons within the counter volume, but at dif-ferent geometrical locations, the response of a highpressure xenon counter is very complicated and cannotbe fully understood quantitatively in the absence of anystandardized model of the behaviour of the detector.For example, in the case of high energy photon interac-tions at 100 keV, the mean free path for absorption inxenon at 17 atm is - 5 cm, the range of an Augerelectron at 100 keV is - 0.6 mm and the range of a Kaescape photon is - 10 mm. About 50% of the escapephoton materialize in the backward cone. The resul-tant pulse will thus correspond to two separate chargeclouds moving towards the anode at different dis-tances . The difficulty for any quantitative model is thatthe swarm of the electron from the farther chargecloud must pass through a sheath of positive ionsmoving towards the cathode. The coulomb interactionwith the ions at very small distances will dominate overthe scattering and thus result in the loss of electrons .This process may not only result in the reduction of gasgain but contribute to the large variance in the numberof electrons reaching the anode and the observedasymmetry in each energy peak .

The detailed results presented in this paper showthat the observed features of high pressure xenoncounters can be understood broadly, and that thecounter behaviour cannot be explained by simple ex-trapolation from low pressure data .

Our measurements further indicate that large areacounters with moderate energy resolution and highdetection efficiency for photon energies above 300keV, can be realized in practice for building large areadetector telescopes for hard X-ray and low energygamma ray astronomy. The observed change in ionmobility at the low values of the reduced field encoun-tered in high pressure counters must be carefully con-sidered from the point of view of the front end amplifi-

cation and the shaping time constant of the pre-ampli-fier .An array consisting of 32 individual cylindrical pro-

portional counters filled with xenon at high pressure iscurrently under fabrication for balloon borne measure-ments in the hard X-ray region for launch in 1994 .

Acknowledgement

It is pleasure to thank J. Panettieri for his valuabletechnical help and A. Willett for the fabrication ofvarious subsystems . Z. Ye was in receipt of a Univer-sity College Rector's Postgraduate Scholarship .

References

[1]

[2]

A.R . Rao, P.C . Agrawal and R.K. Manchanda, Proc .18th ICRC, Bangalore, India 8 (1983) 5.A. Bazzano, L. Boccaccini, C.D . La Padula, M. Mastropi-etro, R. Patriarca, V.F. Polcaro, P. Ubertini and R.K .Manchada, Nucl . Instr . and Meth . 214 (1983) 48 .D.P . Sharma, SW.B . Dieters, M.W . Emery, K.B . Fenton,A.B . Giles, J.G . Greenhill, A. Sprent, D.M . Warren, G.LSalmon, R.K. Sood and J.A . Thomas, Proc . 20th ICRC,Moscow, USSR 2 (1987) 285.

[4] R.K . Manchanda, Indian J. Radio Space Phys . 20 (1991)252.L. Kaufman, V. Perez-Mendez, D. Shames andG. Stoker,IEEE Trans. Nucl . Sci NS-19 (1972) 169.

[6] T. Ferbel, Experimental Techniques in high energyPhysics (Addison-Wesley, 1985).

[7] R.K. Manchanda, Z. Ye and R.K . Sood, Nucl . Instr . andMeth . A 292 (1990) 373.

[8] R.K. Sood, R.K . Manchanda and Z. Ye, Adv. Space Res.11 (8) (1992) 421.

[91 H. Sakurai, B.D . Ramsey and M. Weisskopf, Nucl . Instr.and Meth . A 307 (1991) 504.

[10] Z. Ye, R.K . Sood, D.P. Sharma R.K Manchanda andK.B . Fenton, Nucl . Instr . and Meth . A 329 (1992) 140.

[11] T. Aoyama, Nucl . Instr . and Meth . A 234 (1986) 125.[12] M.E . Rose and S.A . Korff, Phys . Rev. 59 (1941) 850.[13] T.Z . Kowalski, A. Smith andA. Peacock, Nucl . Instr and

Meth . A 279 (1989) 567.[14] L.G . Christophorou, Atomic and Molecular Radiation

Physics (Wiley-Interscience, 1970).[15] Peter Rice-Evans, Spark, Streamer, Proportional and

Drift Chambers (Richelieu, 1973).

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