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A multiple sampling proportional counter for particle identificationof relativistic heavy ionsK. Kimura, Y. Akiba 1), Y. Miake 2) and S. Nagamiya 3)

Department of Physics, Kyushu University, Fukuoka 812, Japan1 ) Institute for Nuclear Study, University of Tokyo, Tokyo 188, Japan`1 Brookhaven National Laboratory, Upton, NY 11973, USA31 Department of Physics, Columbia University, New York, NY 10027, USA

Received 18 April 1990

A multiple sampling dE/dx counter using a multiwire proportional chamber equipped with cathode pads was constructed for themultiple detection of dE/dx values along a particle trajectory . For low-energy particles this counter was proved to be useful as aBragg-curve detector . At relativistic energies around E =14.6 GeV/nucleon good particle identification was obtained by cathode padsignals as well as anode signals for the range of projectile fragments from Z =1 (minimum ionization) up to a beam charge of Z =14.

1 . Introduction

A multiple sampling proportional counter (MSPC)has been constructed to measure the change of thed E/dx value along the path of an incident particlecaused by the change of its nuclear charge due to, forexample, its spontaneous decay. By such measurementsone can deter.-nine vertex points of decays and therebydecay lengths or relatively long-living particles, such ashyperons or hypemuclei [1], which can be produced in

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Nuclear Instniments and Methods in Physics Research A297 (1990) 190-198North-Holland

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the projectile frame of relativistic heavy ion collisions[2] . Because of Lorentz boost they can have decaylengths of the order of a meter.A multiple sampling ionization chamber, called

MUSIC, was constructed at LBL [3] and was proved tobe powerful for particle identification of high-energyheavy ions having large atomic numbers (around Z =50) . For our purposes, however, it is necessary to re-solve low-Z particles (below Z =10) . A gaseous ioniza-tion chamber is not suitable to measure such low-Z

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Fig . 1 . Schematic picture of the multiple sampling proportional counter (MSPC) .

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particles of relativistic energies because of their smallspecific ionizations . Thus a proportional counter modewas adopted .

2 . Construction of the MSPC

Fig. 1 shows a schematic view of the MSPC, whichconsists of a ;geld cage and a multiwire proportionalchamber (MWPC). These two parts are separated by agrid plane . Incident particles pass through the field cage(1 m long, 7 cm high and 6 cm wide) parallel to itslongest side as shown in fig. 1 . Electrons liberated fromgas molecules along particle tracks are forced to drift uptowards the grid by the electrostatic field. After passingthrough the grid they are amplified by the MWPC. Thegrid plane is made of 1 mm spaced Cu-Be wires (100Rm diameter), which are parallel to the direction of theincident particles .

In the MWPC part anode wires and cathode wiresare alternately placed with a pitch of 5 mm. They aredisplaced from the grid plane by 5 mm and are parallelto the grid wires . The effective length of the anode wiresis 1 m. A cross sectional view of the MSPC is shown infig. 2 . Six anode wires (30 tLm diameter Au-plated W)and five cathode wires (100 Rm diameter Cu-Be) makeup six counter cells, each of which has a cross section of1 cm x 1 cm . Here the two anode wires at both sides aremade thicker (100 Rm diameter) than others to preventgas multiplication at these wires. They simply serve asfield shaping wires for the inner cells, i .e . to maintain

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K. Kimura et al. /A multiple sampling proportional counter

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uniformity of gas gains for the four inner anode wires.The effective width of the MWPC is thus 4 cm .

Multiple sampling of the dE/d x value is done bycathode pads arranged 5 mm above the anode plane atthe opposite side of the grid plane. Cathode pads weremade by photoetching of a Ni-plated G-10 board . Thelength of one pad along the anode wires is 3.8 cm and 6cm perpendicular to this covering the whole width ofthe MWPC. In total 24 pads are arranged, each sep-arated by 2 mm. Since the two pads at both ends sufferfrom edge effects, 22 pads are actually used. In order tobe able to evacuate the counter before filling it with gas,the assembled structure of the field cage and the MWPCwas housed in an aluminum chamber of about 15 mmthick. For the entrance and exit windows of the counter(5 cm diameter hole), mylar foils (50 Rm thick) wereused.

3 . Test experiment with low-energy particles

3.1 . Experimental setup

The MSPC detector was first tested by low energyprotons and deuterons obtained from the tandem accel-erator at Kyushu University. The MSPC was filed withP-10 gas (Ar 90% + CH4 10%) of 1 atm after completelyevacuating the chamber by a rotary pump. Because nogas flow system was used we repeated evacuation andgas filling several times to degas moistures from thecounter wall . After this treatment no deterioration ofcounter resolution was observed within a one-day mea-surement .

The MSPC was located 30 cm downstream of a thinexit window (havar foil) of a scattering chamber . Byintroducing scattered beams from this window at about25' into the MSPC through a collimator (2 mm diame-ter) the dE/dx curves for 10 MeV protons and deu-terons were measured . The anode voltage was set to+ 1500 V. The field voltage was selected to be -1500V, to ensure that the drift velocity of the electrons in theP-10 was high enough to reach the maximum value(about 5 cm/Its) . The grid wires and the cathode padswere both grounded. Since cathode pad signals have apositive polarity, they were inverted by inverting trans-formers after preamplification and then fed to an ADC(LeCroy 2249W) .

3.2. Bragg curves

The results obtained in this measurement are sum-marized in fig. 3 . Experimental points show peak chan-nels of the ADC spectra obtained from the cathodepads . They are proportional to the energy loss withinthe width of a single cathode pad (4 cm) . Filled circlesare the data from proton beam and filled squares are

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those from deuteron beam. The distance in the horizon-tal axis was measured between the exit window of thescattering chamber and the center of each cathode pad .The length of the vertical bars indicate the FWHM ofthe ADC spectra . Typical resolutions are 15%, whichseems to be mainly determined by energy loss strag-gling. The contribution from electronic noise is about1% . Nonuniformity of anode wire gains contributes lessthan 1%. This was checked by comparing peak channelsof the 59.5 keV -y-ray spectra (241Am source) measuredby the individual anode wires. From the summedanode-wire spectrum, energy resolution of 5% was ob-tained for this -y-ray.

As seen in fig. 3, the measured energy losses for bothprotons and deuterons show a rapid increase towardsthe end of their tracks and abruptly become zero, acharacteristic aspect of the so-called Bragg curve . TheBragg-curve for protons is compared with the calculatedone, shown by a solid curve, using the tabulated stop-ping powers by Northcliff and Schilling [4] . The verticalscale is suitably normalized . The overall shape agreesvery well with the experiment . Thus smearing of Braggcurves due to induction of charges on the cathode padsfrom distant charges appears to be small .

4. Test experiment with relativistic heavy ions

4.1 . Setup of the test experiment

A high-energy test experiment was carried out at theBrookhaven National Laboratory using Si beams ofenergy 14.6 GeV/nucleon supplied by the tandem AGSaccelerator complex. The MSPC detector was located

K Kimura et al. / A multiple sampling proportional counter

on the beam line of the MPS course about 10 mdownstream from the MPS magnet . In front of theMSPC, a trigger scintillator T1 (2 cm x 2 cm x 5 mm` )(superscript t : thickness) and a large area veto scintilla-tor TA, having a 2 cm diameter hole at the position ofthe T1 scintillator, were placed. At the back of theMSPC, another trigger scintillator T2 (4 cm x 4 cm x 5mmt) was placed . The distance between the two triggercounters was 115 cm . This trigger system was devised totrigger single track events and reject multiparticle eventsas far as possible . Since thick materials other thantrigger counters and air existed in front of the MSPC,for example aluminum frames, we expected to observevarious projectile fragments from Z =1 to Z =14 at atime with almost comparable strength to direct beam.

After evacuating the MSPC by a rotary pump it wasfilled with P-10 gas of 1 atm . The field voltage was setto be the same as before, while the anode potential wasset to be slightly lower (+ 1400 V) because the maxi-mum energy loss was larger than the low-energy experi-ment .

Signals from the four anode wires and the 22 cathodepads together with those from the trigger scintillatorswere fed to ADCs (LeCroy 2249W) and were readthrough a CAMAC-PC system under the trigger condi-tion T1 * T2 * TA. Trigger rates were typically 10 countsper spill . The gate width for the gas counter ADC wasadjusted to be 2 lis to cover the maximum drift time ofelectrons in the field cage (7 cm deep) . A gate width of1 .5 [Ls was not enough to get the best resolution .

(Tl 2+T 22 ) 1/2Fig . 4 . Correlation of the ADC vaiue, from the two triggerscintillators (TI and T2) obtained in the high-energy testexperiment . The horizontal axis is taken to be the root meansquares of the ADC values of the two scintillators and thevertical axis is taken to be the ratios of them . The event cutapplied in the analysis is indicated by arrows . See the text for

details.

4.2 . Results of the experiment

4.2.1 . Total energy loss spectraFig. 4 shows a correlation of the ADC values from

the two trigger scintillators T1 and T2 in a typical run .The horizontal axis and the vertical axis are taken to bethe root mean squares and ratios, respectively, of thetwo ADC values . The thick concentration of eventsaround a horizontal line are thus due to the particleshaving nearly the same energy loss in the two triggerscintillators, i .e. the particles passed through the MSPCwithout changing their nuclear charges . A software cutwas set on these events as indicated in the figure. Thiscut does not guarantee at she same time that all thegated events are single track events. Closely spacedmultiparticles can pass through the two trigger countersand deposit equal energies on them.An energy loss spectrum obtained by summing the

signals from all four anode wires is shown in fig . 5a .Pulse summing was done by using a sum amplifier. A

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K Kimura et al. / A multiple sampling proportional counter

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Fig. 5 . Energy loss spectra of relativistic particles aroundE =14.6 GeV/nucleon obtained from the MSPC detector by(a) summing anode wire ADC values and (b) averaging cathode

pad ADC values.

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clear separation of the spectrum into 14 distinct peakscan be seen . Each of them represents the energy loss ofthe detected particles for each 14 different nuclearcharges from Z = 1 to the maximum charge 14 . Theintensity of Z = 1 particles is suppressed to about 1 /10of the actual intensity by adjusting a lower level dis-criminator of the trigger circuit .

Almost the same energy loss spectrum was obtainedby averaging the ADC values from all 22 cathode padsas shown in fig. 5b . Peak positions of these two spectra,i .e . energy losses, as a function of atomic number Z aregiven in figs . 6a and 6b. We tried to fit them by thefunction DE = a + bZ", where a, b and n are adjusta-ble parameters. Fitted curves are drawn by solid curves .In the case of the anode spectrum (fig . 6a) a best fit wasobtained at n = 1.90, whereas for the cathode spectrum(fig. 6b), it was obtained at n = 2.00. These results arequite consistent with the predicted Z2-dependence ofthe energy loss of relativistic particles from the Bethe-Bloch formula .

The peak shapes of the two spectra look like Gaus-sians rather than the Landau-Vavilov distribution ex-pected for energy-loss strugglings in a thin material.This is probably because the energy losses we measuredare not actual energy losses but are energy depositswithin the sensitive volume of the filed cage as dis-cussed in ref. [5] . In the present case it is also probablethat the energy spread of the detected partcles andmultiparticle events which survived our event cut areadditionally contributing to the peak shapes, especiallyin the low-Z region .FWHM values of the energy-loss spectra are given in

fig . 7 after normalizing the gain of the cathode spectrumto that of the anode spectrum. It is evident that the twospectra have almost the same Z-resolution . The FWHMvalues increase almost linearly with Z and behave dif-ferent than the Z2-dependence expected for the simpleLandau distribution [6] of energy loss, shown by a solidcurve. Since the difference of energy losses for twoneighbouring Z-values increases linearly with Z, the Zresolutions (AZ = 0.25) are kept nearly constant for allobserved Z. It should be nosed here that no improve-ment of the Z-resolution was observed by making thepad averaged ADC spectrum excluding five successiveADC values from the largest one . This would be anatural consequence for the Gaussian shape of the padspectra.

In figs . 5a and 5b,

Z= 14 events are seen fairlyweakly . This is not because the incident Si beam hadbeen lost due to interactions with thick materials infront of MSPC but because the counter was locatedslightly off the beam line and thus only projectile frag-ments had been detected . Incident angles and positionsof the particle tracks projected on the horizontal plane,i .e . anode plane, can be known from the relative magni-tudes of signals from different anode wires . Figs. Sa and

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Fig . 6. Atomic number Z dependence of energy loss of relativistic particles obtained from (a) the anode spectrum and (b) the cathodespectrum shown in fig . 5 . Fitted curves by the function AE = a + bZ" are drawn by solid curves.

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8b show the correlations of the ADC values betweentwo neighbouring anode wires . Four active anode wiresare numbered from 2 to 5 . The concentration of eventson discrete lines, inclined by -45", can be clearlydistinguished. Each of these lines corresponds to therespective nuclear charge from Z =1-14 of the de-tected particles . These distinct correlations of the anodewire signals indicate that all the particle tracks pro-jected on the anode plane are nearly parallel with eachother but not parallel to the anode wires. The crossingangles of the projected tracks with the anode wires arenot so large that they intersect more than c:;o anodewire sections because the sum of the ADC values fromonly two anode wires already gives a good particleidentification spectrum. They are about 2/100 rad . It isalso suggested that the spatial spread of 8-rays is mainlyconcentrated within 2 cm.

For single track events dip angles with respect to theanode plane can also be measured by taking TDCvalues from all the cathode pads. Thus a three dimen-sional tracking is possible through anode wire ADCvalues and cathode pad TDC values . if we use flash-ADCs, multiparticle separation only in the vertical di-rection can be made just like the TPC.

4.2.2. Cathode pad ADC valuesHow good the particle identification by a single

cathode pad is, is an essential point of the MSPC. Atfirst particle identification spectra from a single cathodepad looked very bad because of the high-energy tailespecially for high-Z particles. But soon it was realized

that this tail is completely correlated with the positionof particle tracks and occurs for the particles whosetracks projected on the anode plane cross with onecathode wire just above the reference pad [7] . At thiscrossing point drifting electrons are divided into twoMWPC cells and are collected onto two anode wires

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Fig . 8 . Correlations of the anode wire ADC values of the MSPC for two different combinations of neighbouring wires in ahigh-energy heavy-ion experiment.

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K. Kimura et al. / A multiple samplingproportional counter

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separately . Thus after gas multiplication the spacecharge drops to zero on one anode wire and rises on theother wire . If the counter operation is approaching gainsaturation, this decreasing space charge density causesthe anode wire gains to increase locally, especially forhigh-Z particles .

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of the cathode pad ADC values of the MSPC obtained in the high-energy heavy-ion experiment. (a) Correlationof neighbouring two pad ADC values and (b) correlation of three pad averaged ADC values. Degree of concentration of events into

discrete groups indicates goodness of separation of Z-values of the detected particles.

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We have corrected this local gain variation usingtrack position information obtained from the anodewire signals . After this correction, particle separation bysingle pad ADC values slightly improved but for thepad averaged ADC values no improvement was ob-served because most of the detected particles crossedonly one anode wire and thus the correction was equiv-alent to modifying the overall gain slightly .

The quality of particle identification by the cathodepad ADC values can be seen from fig . 9a, a correlationof the ADC values between two neighbouring cathodepads close to the exit of the MSPC. From the ADCvalues of individual cathode pads Z values cannot beuniquely identified. But as seen from fig. 9a, a slightconcentration of events into discrete groups on a diago-nal line tells us that the correlation of the ADC valuesfor two neighbouring pads barely allows us to identifyZ values . The most straightforward way to improve thepad resolution is to operate the MSPC with increasedgas pressure (2-3 atm) . But this is very cumbersomeand also causes an increase of unwanted beam-gasinteractions . An alternative way is to take averaged

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K Kimura et al. / A multiple sampling proportional counter

ADC values on several neighbouring pads. In this caseposition resolution has to be sacrificed to some extent .A correlation of the three pad averaged ADC values attwo different places is shown in fig . 9b . Evidentlyparticle separation improve: and it becomes possible toresolve the Z values .

The fluctuation of energy loss along a particle trajec-tory is expected to be slightly different from that seenby a single cathode pad described above because in theformer case the fluctuation of the average energy lossdue to the energy spread of the detected particles ormultiparticle events do not explicitly contribute ; insteadgain variations along the anode wires additionally comeinto play. Typical fluctuations of energy loss alongparticle tracks are shown in fig. 10a for various differentparticles from Z =1 to Z = 14. Large staggerings ofdata can be seen . They are almost as wide as the energyloss distribution seen by a single cathode pad in fig. 9a.Thus it is rather difficult to identify the position wherea sudden change of Z value by one unit occurs but fortwo units of change it seems to be identifiable. Thesefluctuations are again reduced by taking pad averages .

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Fig . 10 . Typical fluctuations of energy loss of relativistic particles along their tracks observed by the cathode pads of the MSPC. (a)Fluctuation of single pad ADC values and (b) three pad averaged ADC values. The Z-numbers of the particles are explicitlyindicated .

Fig. lob shows the distributions of three pad averagedADC values . These pad averaged data allows us toidentify the change of Z-values by one unit .We observed many events showing various nuclear

interactions within the gas volume of the MSPC. Someof their event structures are depicted in figs. 11a andllb, all showing three pad averaged ADC values . Thetwo events in fig. lla seem to exhibit typical beam-gasinteractions. For example, the event connected with thesolid line can be interpreted as follows : a Z = 2 particleenters and scatters an atomic nucleus almost verticallyat pad no. 10. Then the scattered nucleus deposits muchenergy in the field cage.

The event connected with the dashed line is asfollows : a Z =10 particle interacts with an atomicnucleus and the resulting projectile fragments proceedforward, the target fragments going vertically . Projectilefragments deposit less energy than the incident particlebecause of their smaller charges while low-energy targetfragments deposit much energy . In this way most of thebeam-gas interactions are easily discriminated by in-specting event structures of the cathode pad ADC val-ues .

The event structures shown in fig . llb are veryinteresting because they seem to exhibit some chargechanging processes without accompanying any low-en-

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ergy particles having a large energy deposit. The upperthree events look like charge increasing processes whilethe lower three events look like charge decreasingprocesses . But whether they are actually due to sponta-neous decays of unstable particles or just beam-gasinteractions cannot be determined by these data only.Particle tracking by measuring TDC values of the padsignals or some other tracking devices in combinationwith a magnetic field will greatly help to visualize theexact images of these events.

5 . Summary

By using the multiwire proportional chamber it waspossible to resolve Z-values of relativistic particles fromZ =1 (minimum ionizing particles) to Z = 14, spanninga range of 200 : 1 in energy loss difference at a time. Theenergy loss of relativistic heavy ions in the MSPC (P-10gas) was found to follow an exact Z2-dependence asexpected from the Bethe-Bloch formula.

Under 1 atm gas pressure, statistical fluctuations ofenergy loss seen by single cathode pad (4 cm wide) didnot allow us to identify Z-values of relativistic heavyions uniquely at each cathode position . But by takingthree pa(, averages Z values could be resolved. If the

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Fig. 11 . Event structures of the three pad averaged ADC values showing sorne nuclear interactions or charge changing processes . (a)Typical beam-gas interaction events and (b) candidate events of spontaneous decay of unstable particles . Horizontal lines are drawn

to show the change of the average energy loss.

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MSPC detector is used as a TPC it might be possible to

do multiparticle tracking and thus it can be used as adecay counter.

Acknowledgements

One of the authors (K.K.) would like to expressthanks to Professors K. Nakai and S . Homma and Dr.T . Kobayashi for their supports and advice on theconstruction of the MSPC. He is most indebted to theBrookhaven National Laboratory and to the membersof the BNL E802 group, especially Professors S . Hayanoand C. Chasman, For doing the test experiment andalso to Mr . Y . Matsuyama for construction of thecounter. This work was supported by a Grant-in-Aidfor Scientific Research of the Japanese Ministry ofEducation, Science and Culture .

K. Kimura et al. / A multiple samplingproportional counter

References

[1] K. Nakai and S . Nagamiya, University 4 Tokyo ReportUTPN-193 (1982).

[2] M . Wakai, H . Bando and M. Sano, Phys . Rev . C38 (1988)748 .

[3] W.B . Christie et al ., Nucl. Instr. and Meth. A255 (1987)466.

[4] L.C. Northcliff and R.F . Schilling, Nucl . Data Tables A7(1970) 233 .

[5] K. Nagata, J. Kikuchi, T . Doke and C.R . Gruhn, Nucl .Instr . and Meth. 196 (1982) 41 .

[6] L. Landau, J. Phys . USSR 8 (1944) 201 ;H . Schmidt-Bflcking, Lecture Notes in Physics 83 (Springer,1978) p. 81 .

[7] K . Kimura, Y. Akiba, M. Miake and S. Nagamiya, Proc .Symp. on Radiation Detection, Fukuoka, 1989 .