a low-pressure gas detector for heavy-ion tracking and particle identification

Nuclear Instruments and Methods in Physics Research A 495 (2002) 216–231

A low-pressure gas detector for heavy-ion tracking and particleidentification

A. Cunsoloa,b, F. Cappuzzelloa, A. Fotib,c, P. Gangnantd, A. Lazzaroa,b,J.F. Libind, A.L. Melitaa,b, W. Mittigd, C. Nociforoa,b, S. Orrigoa,b,

P. Roussel-Chomazd, C. Spitaelsd, J.S. Winfielda,*a INFN-Laboratori Nazionali del Sud, Via S. Sofia 44, 95123 Catania, Italy

bDipartimento di Fisica e Astronomia, Universit "a di Catania, Via S. Sofia 64, 95123 Catania, Italyc INFN-Sezione di Catania, Corso Italia 57, 95129 Catania, Italy

dGANIL (CEA/DSM—CNRS/IN2P3), BP 5027, 14076 Caen cedex 5, France

Received 12 April 2002; received in revised form 6 August 2002; accepted 20 August 2002

Abstract

A low-pressure gas tracker for heavy ions is described. It is based on a drift chamber with four series of multiplying

wires for energy loss measurement and underlying strips for position and angle determination. Results of tests with an

a-source and with low-energy heavy-ion beams are presented. They indicate the suitability of such a detector in a wide

range of experiments with heavy ions where a low energy threshold, a large solid angle, together with a precise

measurement of ion trajectories and identification is needed. In particular, the application as a prototype of the focal

plane detector for the large-acceptance spectrometer MAGNEX is discussed.

r 2002 Elsevier Science B.V. All rights reserved.

PACS: 29.40.Gx; 29.40.Cs

Keywords: Low-pressure drift chamber; Strip readout; Low energy threshold

1. Introduction

Low-pressure gas-filled detectors are establishedtools for nuclear physics. In particular, Low-Pressure Multi-Wire Proportional Counters(LPMWPC) or Drift Chambers working withheavy hydrocarbon gases at pressures around

10mbar are commonly used to track fissionfragments [1] and heavy-ions [2]. They can coverlarge areas, give good time and position resolution(around 0.5mm and better), support high countingrates (often 100 kHz), are inexpensive, robust andresistant to radiation damage. On the other hand,they have rather poor energy resolution, and asfocal plane detectors for magnetic spectrometersthey are usually followed by long ionisationchambers for energy loss measurement and plasticscintillators for residual energy measurement, as in

*Corresponding author. Tel.: +39-95-542-383; fax: +39-

0975-7141815.

E-mail address: [email protected] (J.S. Winfield).

0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 1 6 1 0 - 8

Ref. [3]. Such a modular design results in addi-tional intermediate foils, increasing the detectionthreshold for slow heavy-ions.

A strong impetus in the development of new gastrackers for low-energy heavy ions comes from thespecific requirements of focal plane detectors oflarge acceptance magnetic spectrometers. In parti-cular, we have been involved in the MAGNEXproject at LNS, Catania [4,5]. For such spectro-meters, the interplay of large acceptance and goodmomentum resolution leads to focal planes ofabout 1m length and 20 cm height. Moreover,software techniques to reconstruct ion trajectoriesback through the spectrometer usually require aprecision in the horizontal and vertical position atthe focal plane of better than a millimetre and inthe angle of the order of 10mrad [6,7]. TheMAGNEX spectrometer is designed for heavy-ions with magnetic rigidities from 0.2 to 1.8 Tm.This covers an energy range from the Bragg peakregion (about 0.3 to 0.7MeV/A, depending on themass) up to intermediate energies (B40MeV/A).A particular motivation for MAGNEX is to studyreactions induced by radioactive beams near theCoulomb barrier [8,9]. These beams would bedelivered by the EXCYT project [10], which has aspost-accelerator a 15MV Tandem accelerator.Many cyclotron beams can also be accepted. Forions with such energies, particle identification canbe achieved through the usual DE � E method (foratomic number Z) and time-of-flight coupled withmagnetic rigidity (for mass). The spectrometercould also be used at rigidities as low as 0.2 Tm,where many interesting experiments may beperformed in the field of nuclear astrophysics. Asan example, experiments in the region of CNOcycle require the detection of very low-energy ions(e.g., 12MeV 12C) with good angular and energyresolution [11]. Thus, a low energy detectionthreshold and the minimisation of straggling arealso important for the design of the focal planedetector. For these very low-energy ions, where theionisation is close to the Bragg peak, the DE � E

method alone is no longer able to distinguishbetween ions with neighbouring Z: However, insuch experiments the particle identification re-quirements are usually met by the coincidentdetection of light particles in an auxiliary detector

[11]. In summary, to be generally useful, the focalplane detector must adapt to heavy-ions with abroad range of energies.

One is naturally led to a low-pressure gasdetector combining good position and energymeasurements without intermediate foils. Onepossibility is the use of gaseous microstripchambers [12,13], which have been adapted tothe detection of intermediate-energy heavy-ions[14]. These can achieve extremely high positionresolution: of the order of 10 mm for lightlyionising particles. However, such a precision isredundant for the detection of low-energy heavy-ions, where straggling in the gas and window foilwould surely exceed this value. For a detectorwhere a rather large angular spread is expected,several hundreds of microstrips would need to beread and analysed per event, even with zero-suppression of the data. A more suitable approachfor the present purposes is that of multi-wire stripdetectors [15,16] in which the avalanche ofelectrons produced near the wires induces a chargeon a few underlying strips. In the past, the stripswere connected to taps of a delay line and theposition derived from the time difference of thesignals from either end of the line. The positionresolution from such detectors is limited to about0.4mm [17], principally by the quality of thecomponents of the delay line. The delay line can beavoided by individually reading the strip signalsand constructing the centre of gravity of theinduced charge distributions. There is no loss ofsignal by transmission along the delay line whichsignificantly reduces the gas pressure and highvoltage on the wire needed to provide sufficientsignal for the position measurement. The electro-nic noise is reduced by taking the signals fromgrounded strips. The advent of multiplexedmicroelectronic read-out systems such as Gassi-plex [18] makes such a solution feasible for largedetectors. An energy-loss signal can be obtainedfrom the summed output of the proportionalwires.

In this paper we describe a new hybrid gasdetector, working as a Heavy Ion Tracker (HIT),which is designed to both measure position tosub-millimetre accuracy and give a good energysignal while avoiding intermediate foils. The

A. Cunsolo et al. / Nuclear Instruments and Methods in Physics Research A 495 (2002) 216–231 217

disadvantage compared to the use of LPMWPCs isthe loss in modularity and the limited countingrate (several kHz). The latter is because of the needto wait for the signal to arrive at the anode afterthe trigger (indeed, this is the principle of thevertical position measurement). For the case ofreactions induced by radioactive beams, which areweak in intensity, the limited counting rate abilitywould usually not be a problem. Indeed, manysuch experiments would not be possible without alarge solid angle detector.

The paper is organised as follows. Section 2describes the design and construction of thedetector. The experimental details and results ofthe tests are discussed in Sections 3 and 4. Thesuitability of HIT as a small prototype of the finalfocal plane detector for the MAGNEX spectro-meter is discussed in Section 5.

2. Design and construction

The detector is composed of an ionisation driftchamber, four independent position-sensitive pro-portional counters, one after the other, andstopping silicon detectors. It is emphasised thatno intermediate foils separate the sections. A

schematic drawing of the detector is shown inFig. 1.

The drift chamber is 120mm wide, 114mm highand 210mm deep, with a cathode plate above anda Frisch grid below. The gas used in the testsdescribed here was 99.95% purity isobutane atvarious pressures from 7 to 30mbar. The gas-containment window of the external chamber was1.5 mm thick Mylar, supported by a grid of fivewires. The Frisch grid is made of 81 gold-platedtungsten wires, 50 mm in diameter and separatedby 2.5mm centre to centre. The geometricaltransparency is thus about 98%, while the shield-ing efficiency with respect to the anode wires 1 cmbelow the grid is about 89%. There are lateralfield-shaping strips on the sidewalls of the driftchamber and corresponding horizontal wiresacross the entrance and exit openings. These areconnected to each other by 220 kO resistors toform a chain with the cathode.

The proportional counter section is mountedindependently from the drift chamber, and consistsof four sets of 16 anode strips, orientated as shownin Fig. 2. Each strip is 50mm long and 5mm wideand separated by 0.8mm from its neighbour. Thusthe detector covers a horizontal span of about92mm. To reduce a possible non-linearity in the

Fig. 1. Schematic side view of the HIT detector. The gas containment chamber continues beyond the right margin of the picture.

A. Cunsolo et al. / Nuclear Instruments and Methods in Physics Research A 495 (2002) 216–231218

position measurement, the second and fourth setof strips are shifted by half a strip width from thepreceding one. One can then in principle averagetwo position measurements.1 There are five am-plifying wires above each set of strips, in a senseperpendicular to the long dimension of the strips.They are gold-plated tungsten, 20 mm diameter,and are located 10mm below the Frisch grid and5mm above the strips. The wires in each set areconnected in common and are spaced 10mmapart.

The 64 preamplifiers for the anode strips aremounted on a liquid-cooled motherboard directlybeneath the anode plate. The gain of the pre-amplifiers is 200mV/MeV (silicon-equivalent).Similar preamplifiers are used externally for theproportional wires. Besides the need for goodposition resolution, the possibility of using thewire signals for energy-loss measurement was alsoexplored. The advantage compared to the use ofan anode plate is that the small DE signal isamplified, which reduces the amount of noise pick-up and eliminates the problem of cross-talk withthe large proportional wire signals [19]. Secondly,in the context of application as a focal planedetector, the use of the position-measurementsection as a supplemental DE section reduces the

total depth of the detector, which leads to a lowerenergy detection threshold.

For the a-source tests, two 50mm� 50mm�180 mm silicon detectors (‘‘A’’ and ‘‘B’’) weremounted side-by-side at the exit of the driftchamber and within the gas volume. For the beamtests, these were supplemented by an additional515 mm thick silicon detector ‘‘C’’ also of 50mm� 50 mm active area. To cover the fullacceptance of the HIT, four such silicon detectorswould be needed.

2.1. Principle of operation

The ionisation produced by the passage ofcharged particles through the gas leaves atrack of primary electrons and positive ions. Auniform reduced electric field E=p of about2V cm�1/Torr makes the electrons drift towardthe Frisch grid. After passing through the grid, theelectrons are accelerated in an increasing electricfield. Near the wires the field reaches valuessufficient for a multiplication by an average factorof around 200 for each primary electron. Theavalanche both induces charge on the neareststrips underneath and produces a direct energy-loss signal that can be read from the wires. Large-area silicon detectors at the back of the ionisationchamber measure the residual energy and providea timing signal.

beam

window

proportional wire

anode strip

silicondetectors

Fig. 2. Schematic plan view of detector. The detector is rotated to 121 and intercepts the direct beam.

1This was not necessary for the tests described in this paper.


3. Experimental details

The detector was first tested on the bench withan a-source, and subsequently in a scatteringchamber with heavy-ion beams at the LNS,Catania. The goals of these tests were to measure:(i) horizontal position and angular resolution andvertical (drift time) resolution, and (ii) DE resolu-tion for different ions, while investigating theoptimum pressure/voltage conditions.

3.1. Bench test

For the bench test with an a-source, an 241Amsource (EaE5:5MeV, activity 3.2� 104 Bq) wasused to illuminate the detector with a continuousrange of angles. A diaphragm with three apertureswas placed about a centimetre in front of thesource. The central slit was 0.5mm wide; it wasseparated horizontally by 3.0mm from the outeropenings, which were sets of 1 and 2mm diameterholes (see sketch in Fig. 3). The detector was filledwith isobutane gas at 20mbar pressure, which wasthen flowed at approximately 0.1 lmin�1 (STP).Because of the low-energy of the a-particles, thesource was placed within the gas volume, 76mmfrom the plane of the middle of the first row ofstrips. The voltages applied to the cathode and theanode wires were–700V and +500V, respectively,

with the Frisch grid connected to ground. Thereduced field E=p was then 2.3V cm�1/Torr, givinga drift velocity of about 2 cm/ms [20].

3.2. Beam tests and beam preparation

The beam tests were performed with 80MeV16O, 46MeV 6Li, and 152MeV 40Ca from the LNS15 MV Tandem. The detector was mounted on amoveable arm in a 2m diameter scatteringchamber. It was offset 3.8 cm to the right of thecentre of the arm to allow a greater range of anglesto be explored when the well-collimated beam wassent directly into the detector (see Fig. 2). Theavailable devices for the beam preparation were:(i) an ‘‘advanced’’ target location (approximately4m from the centre of the scattering chamber),having three tantalum plates with small holes, twoof which were covered with foils of 5.2 mm goldand 6 mm Mylar, the third being open, (ii) a set ofmoveable horizontal and vertical slits about 50 cmfrom the chamber, and (iii) a target ladder at thecentre of the chamber in which was mounted aselection of scattering foils, a plate with a 0.36mmdiameter hole, and a plate with five 0.8mmdiameter holes. The configuration used for eachparticular measurement will be described in theappropriate section below. The technique toprepare a well-collimated beam of sufficiently

Fig. 3. Histogram of the reconstructed image (right) of the three ‘‘slits’’ (left) in front of the a-source (this is for demonstration

purposes only, without elimination of important straggling effects, and was not used to estimate the detector resolution). The reason

for the different heights of the peaks is explained in Section 4.1.


reduced intensity so that it could be sent directlyinto the detector is then as follows. The thickadvanced target disperses the beam so that it iseasy to pass a small fraction of the central partthrough the subsequent narrow slit opening(typically 1mm� 2mm) and the fine collimatorin the target location.

The front of the entrance window of thedetector was approximately 17 cm from the targetladder in the chamber.

3.3. Amplifier gain matching

For all the tests the gains of the amplifiers forthe strips were matched by applying a voltagesignal from a pulse generator directly to the anodewires and reading the induced charge from allstrips in a given set. A slight disadvantage with thismethod is that the strips next to the ends appear tosee more charge than the others. However, eventsare rejected in the position-reconstruction algo-rithm if these end strips have the peak charge.

4. Results and observations

With a-particles, the amplitude of the signalsfrom the wires was approximately 1.4V after thepreamplifier. Since the preamplifier gain scaled forisobutane is 31.5mV/MeV and the energy loss in5 cm of gas is about 270 keV, this indicates a gasgain of B180. The signals from the strips wereabout four times smaller. For 95MeV 40Ca ions in7mbar of isobutane, the gas gain was reduced toB42. The cross-talk between strips, measured byturning off the voltage to one set of wires andobserving the residual induced signals, was about10% of the peak signal. This is well below thethreshold set in the software for determining thethree strips with highest charge and is of minorimportance.

Every strip is read out through separateelectronic channels into the acquisition system.The centroid X of the induced charge distributionon the strips position is estimated in the dataacquisition program. In our case, because there area limited total number of strips, we used only threestrips (the one with the maximum charge and its

immediate neighbours). One of the simplestmethods for the centroid determination is bycentre-of-gravity (COG), for which

X ¼ XC þ aXn

i¼1xiQi=

Xn

i¼1Qi

where XC is the location of the strip with themaximum charge, and xi is the location and Qi isthe charge on the ith strip. The COG methodsuffers from a discontinuous behaviour as thecentre of the electron avalanche passes fromone strip to another [21]. The parameter a can beempirically adjusted to try to smooth out theeffects of the discreteness of the strips, but since a

is strictly dependent on the width of the chargedistribution, it should also take into account, forexample, the vertical position of the origin of theionisation. Hence any attempted correction is non-trivial in practice.

A second method is derived from fitting theshape of the charge distribution Qi by a hyper-bolic-secant-squared function (SECHS) with threevariables:

Qi ¼ a1=cosh2ðpðxi � a2Þ=a3Þ:

The merits of both these and other methods, suchas Gaussian and Lorentzian shapes, are discussedby Lau and Pyrlik [21] with regard to cosmic rayand 190GeV muon data. They found that not onlydid the SECHS function give the best approxima-tion to their model charge distribution, but it alsohad the smallest systematic error. In Fig. 4, weshow that for heavy ions there is little differencebetween the Gaussian and SECHS models forthe induced charge distribution. We have used theSECHS model for all the results discussed below.

4.1. Reconstruction of image by ray tracing

The reconstruction of the image of the three-‘‘slit’’ mask in front of the a-source is shown inFig. 3. It is made by extrapolating event-by-eventthe best straight-line fit to the position measure-ments from the strips. No conditions are put onthe analysis except that all four positions wererecorded in each event. As mentioned before, thesource was located about 1 cm before the mask, sothat a significant fraction of the particles passingthrough the outer holes would have too great an


angle to reach the last sets of strips. This accountsfor the fact that relatively more counts arerecorded from the central slit than would other-wise be expected compared to those from the outerholes.

4.2. Horizontal position resolution

For the bench test, the angular spread of thea-particles was essentially unrestricted. Therefore,to measure the intrinsic position resolution narrowgates (0.5mm wide) were set on the positionparameters (centroid determinations) of the 3rdand 4th sets of strips. The effects of straggling inthe gas may then be removed by analysingcorrelation plots of one position measurementagainst another. An example of such a correlationplot between the centroids measured from the firstand second sets of strips is shown in Fig. 5. Theintrinsic resolution of the detector is representedby the width of the line. The full-width at half-maximum (FWHM) is 0.2670.02mm.

For the tests with 16O and 6Li, the advancedtarget used to produce the dispersed beam was a5.2 mm gold foil glued over a 0.2mm hole in atantalum plate. For the 40Ca beam, this wassubstituted by a 6 mm Mylar foil glued over a0.2mm hole. The slit settings were 70.5mm

horizontal and 71.0mm vertical. A plate with a0.36mm diameter hole was used to define the finalobject size at the centre of the target chamber. For6Li and 16O, this configuration allowed a beam ofless than 100 particles per second to enter thedetector. For 40Ca, despite the use of the less-dispersive Mylar advanced target, a count rate ofseveral kHz was obtained because scatteredparticles went above and below the frame of theplate in the chamber. This was not a problem forthe position measurements, since it could be easilyrejected in the software analysis.

The first step in the analysis procedure usedabove for a-particles of setting narrow gates on the3rd and 4th sets of strips was omitted. Slitscattering and other background were removedby applying a loose software gate on the 1stposition parameter and wire energy-loss signal. Asabove, correlation plots between two positionmeasurements were used to remove the effects ofsmall-angle multiple scattering, this time from theMylar entrance window as well as from the gas.The FWHMs obtained in this way for the threebeams are shown as solid symbols in Fig. 6.

There is a clear deterioration of the positionresolution as the angle of inclination of thedetector to the beam is increased. This presumablyreflects the broadening of the distribution of

0

500

1000

1500

2000

2500

3000

3500

2 4 6 8 10 12 14 16

40CaGaussian

SECHS

6Li

16O

strip no.

Qst

rip (

arbi

trar

y un

its)

Fig. 4. Observed charge distribution from the strips for sample events (histograms) compared with SECHS and Gaussian model

shapes.


induced charge on the strips as the angle increases,making the centroid determination less accurate.Nevertheless, even at 121 inclination the resolutionis no more than 0.6mm. While the results for 16Oand 40Ca are quite similar and the resolutionmeasured with the a-source is good, that for the6Li beam is apparently worse. However, part ofthe reason for the poor resolution appears to be

that not all the multiple scattering was removed bythe correlation plot technique alone. As a test, forcases where good statistics permitted, additionaltight gates were set on the last two wires (as donefor the analysis of the a-particles). The resolutionfor 6Li improved considerably, while that for 16Oappeared to be already at its limit (open symbolsin Fig. 6).

8 9 10 11 12 13 147

8

9

10

11

12

13

Posn 1 (mm)

Pos

n 2

(mm

)

Fig. 5. Correlation plot of position from the second set of strips against that of the first, measured with the a-source. The arrows

indicate the intrinsic resolution of the detector.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8

Angle (deg)

X R

esol

utio

n (m

m)

10 12

Fig. 6. Horizontal position resolution as a function of angle. Filled symbols: projection of correlation plot; open symbols: triple gate

methods. Error bars are statistical only.


4.3. Vertical position resolution

The vertical position is obtained from the drifttime of the electrons from the point of creation tothe anode wires. In practice, since the total drifttime is several microseconds, four Time-to-Ampli-tude Converters, each started by one of the silicondetectors and stopped by a given wire, are used.The cathode voltage was �600V and the gaspressure was 20mbar, leading to a reduced field of2V cm�1/Torr. From the results of Ref. [20], thedrift velocity vdrift is then 1.7 cm/ms. This reducedfield is not in the constant region of vdrift (which isabove 10V cm�1/Torr), and leads to a variation inthe drift time, and hence the vertical position, ifthe voltage or pressure should change. The great-est long-term observed fluctuations in the gaspressure were 70.05mbar in 20mbar. This by farexceeds the relative contribution of the 2mV ripplein the cathode HV power supply. The latter cantherefore be neglected. From the data of vdriftagainst E=p of Ref. [20], we estimate an uncer-tainty in vdrift of about 70.045mm/ms. The effecton the drift time and vertical position depends onthe height of the ionising particle from the Frischgrid. Taking as a mean value the half-height of ourionisation chamber, 57mm, we find a variation of70.15mm in the vertical position, which is smallbut could become significant for larger chambers.Thus, a close attention must be paid to the stabilityof the gas pressure.

To measure the vertical position resolution, aspecial mask with five 0.8mm holes shown inFig. 7 was used in the target position. Otherwise,the 16O beam made with the same advanced targetand slit settings as in Section 4.2 was used. Ananalysis similar to that for the horizontal positionintrinsic resolution using correlation plots betweensets of vertical signals was carried out. The verticalposition resolution measured by wires 1 and 2 is0.4570.04mm, that measured by wires 3 and 4 is0.5370.04mm. The weighted average of thesegives the intrinsic resolution for a single set of wireas 0.4870.03mm. Note that the effect of variationin gas pressure mentioned above would becancelled out by the correlation analysis.

The average of the FWHMs of the peaks in theraw Y-position spectrum (Fig. 8) of wire 1 timedagainst silicon C is 1.0970.09mm. The corre-sponding value for wire 3 is 1.4670.12mm. Thedifference of these and the intrinsic resolutionestimated above is not because of straggling in thewindow and gas, the effect of which we calculateby SRIM-2000 [22] as only 0.2mm, but because ofthe geometrical broadening of the beam passingthrough collimators.

4.4. Angular resolution

The angle and angular resolution of the detectorwere measured with the direct collimated beam fordifferent runs where the detector was rotated from

Fig. 7. Scatterplot (left) of the vertical against horizontal position measured with the 16O beam. A target with five holes, (pictured

right), was in front of the detector. The bands above and below the image of the five holes arise from the gaps between targets in the

ladder.


21 to 121. The same beam preparation was used asfor the horizontal position measurements inSection 4.2. A composite spectrum for the anglecalculated between the first and third set of strips isshown in Fig. 9. The average angular resolution is0.291 (5.0mrad) excluding the 121 point, for whichthe resolution is 0.381 (6.6mrad).

For the bench tests with the a-source a narrowgate of 0.7mm was put on the 4th positionparameter and events from the central slit selected.In this way a fixed direction was defined within4mrad. The angle was then calculated from theposition parameters from the remaining threewires. The angular spread observed was 7mrad

0

10

20

30

40

50

-6 -4 -2 0 2 4Y position (mm)

Cou

nts/

chan

nel

6

Fig. 8. Vertical position from the drift time measured by wire 1 timed against silicon detector C. A mask with three holes spaced

vertically apart by 3mm was in the target position. The width of the peaks is dominated by geometrical effects.

Fig. 9. Composite histogram of the calculated angle from separate runs when the detector was set from 21 to 121 in 21 steps. The beam

was 16O, collimated and sent directly into the detector.


FWHM, giving an intrinsic contribution of about6mrad.

The results for all the particles are collected inTable 1 together with estimates from SRIM-2000[22] of the small-angle scattering from the window(where appropriate) and the gas. Except for the a-particles, straggling is a small contribution to theobserved broadening.

A small non-linearity of the calculated angleis observed, especially at the largest angle. In orderto investigate this, the angle was calculated by alinear regression fit to the four position signals.The difference between the calculated angle and

the nominal rotation with respect to the beam isplotted in Fig. 10 (left). Up to 81 the residual angleis less than about 0.21, which is quite acceptable.The probable reason for the increasing magnitudeof the residuals beyond 81 is a non-uniformity ofthe electric field close to the sides of the ionisationchamber, despite the field shaping wires and strips(see plot of energy loss against position in rightpanel of Fig. 10). Supporting evidence for thiscomes from the observation that the anglecalculated by the set of strips 1 and 3 (avoidingthe last set 4) appears to be less affected than theregression method that uses set 4.

4.5. Energy-loss resolution

The energy-loss information is provided by thegas-amplified wire signals, which should be pro-portional to the amount of primary ionisation. Atest of signal amplitude versus cathode voltagewith the 40Ca beam showed that the ionisation

Table 1

Measured average angular resolution (excluding extreme

angles) and estimated contribution from straggling (SRIM)

a 6Li 16O 40Ca

dymeas (mrad) 6 5.9 5.0 5.3

dySRIM (mrad) 5.0 1.6 3.3 3.3

-20

-15

-10

-5

0

5

10

0 2.5 5 7.5 10 12.5

regression

tan-1((x1 - x3)/d13)

Θarm (deg)

Θca

lc -

Θar

m (

mr)

20 40 60 80 1000

100

200

300

400

Posn 4 (mm)

E W

ire (

keV

)

Fig. 10. Left: Residual angle (calculated–actual) as a function of angle. Results for two different methods used to calculate the angles

are shown. The beam was 16O, collimated and sent directly into the detector. Right: Energy loss signal from the 4th wire as a function

of position. Unconditioned illumination by a-particles.


chamber was working in the ‘‘plateau’’ region forelectron collection. Nevertheless, the inefficiencyof the Frisch Grid shielding and recombinationeffects in the gas may give some small dependencyof the signal on the vertical height of the ions.These latter effects should not, of course, bepresent for the tests with the collimated beams.

The results for 6Li and 16O are taken from thesame runs described in Section 4.2, with the directcollimated beam and the detector set at a nominal21. For the 16O beam, the energy loss in the gasabove each 5 cm length of strip is about 1.6MeV,and the energy loss resolution for individual wiresets is about 8%. This improves if the signal fromadjacent wires are summed together event-by-event because of the correlation of energy loss ingiven sections. For the 6Li beam, the energy-loss inthe gas is very small (B230 keV per section).Despite this, the observed DE resolution isrelatively good for a single set of wires (about20%). It only improves slightly as more signal isadded (only three wire sets were recorded for 6Li),presumably because the resolution is dominated bythe weak signal-to-noise ratio.

Rather poor DE resolution (B15%) was ob-tained with the 40Ca direct beam. The probablereason is the high counting rate (because of the slitscattering to large angles that went around thetarget frame), together with dense ionisation, bothof which could lead to space charge effects.Instead, elastic scattering from a nickel targetwas used to make estimates of the resolution for40Ca. For such scattering tests, the detector wasrotated to a central angle of 401 and the beam wasstopped in a Faraday cup. In order to obtainsufficient beam intensity (a few nA), the beam wasless-tightly collimated: a 2mm open hole was putin the ‘‘advanced target’’ position and the slitsbefore the scattering chamber opened to 73mm.A target of 154 mg/cm2 58Ni was used in the centreof the scattering chamber. The energy loss resolu-tion was investigated for several gas pressures, foreach of which the applied wire voltage wasadjusted to give the maximum induced signal onthe strips while avoiding saturation. The data wereconditioned by approximately 21 cuts in both thehorizontal and vertical directions in order toconstrain the path length differences and improve

the resolution. For e.g. DE plots are shown inFig. 11 both for a single wire set and for all fourwire sets summed together event-by-event. Notethat at least 5% DE resolution is required toresolve elements in the Z ¼ 20 region.

The results for all particle species are sum-marised in Table 2, in which the gas pressure andwire voltage settings are also given. Rows 6 and 8of this table give the energy loss stragglingcalculated by the SRIM-2000 program. For heavyions, the energy straggling would be dominated bythat coming from the 5.2 mm gold target. In orderto estimate the energy-loss straggling in the gas, wehave omitted the advanced target contributionfrom the calculation. The result divided bythe energy-loss is given as a percentage in thetable. The energy-loss straggling contribution tothe observed DE resolution is significant. Othercontributions to the resolution could includethe electronic noise and instabilities in the wirevoltage and gas pressure. Changes in the wire-electrode separation and the gas pressure producefluctuations in the gain, which is exponentiallydependant on these parameters [23]. These effects,as well as electronic noise, limit the energyresolution. For the configuration of the presentdrift chambers, the gain fluctuations are calculatedto contribute at the level of 4%, which is the bestresolution observed for 40Ca scattering with13mbar of isobutane, and indicates that we havereached the optimum conditions in that case.

Energy-loss against residual energy plots for thescattering of the 80MeV 16O beam from the540 mg/cm2 27Al and nickel targets are presented inFig. 12. These were taken under similar conditionsof gas pressure (20mbar) and wire voltage(+460V). Elastic scattering events are onlyobserved with the 58Ni target, the angle beingtoo large for any elastic scattering counts from27Al. Nevertheless, the data taken with the 27Altarget demonstrate a clean Z separation at least upto Z ¼ 10: These data are conditioned by a quitebroad cut on the horizontal angle: 31pyp61which restricts the range of path lengths forthe DE measurement and significantly improvedthe resolution. A further improvement might beexpected if a similar cut were applied to thevertical angles; this is not needed here.


0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

7 %

E wire (MeV)

Cou

nts/

chan

nel

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12 14

4 %

E wire sum (MeV)

Cou

nts/

chan

nel

Fig. 11. Energy loss resolution measured for the scattered 40Ca beam on the 58Ni target. Isobutane gas pressure 13mbar.

Table 2

Measured and calculated (SRIM) energy-loss resolutions (FWHM/mean). The measurements for 6Li and 16O are after the advanced

target of 5.2mm gold. The measurement for 40Ca is for elastic scattering on a 58Ni target and for two different gas pressures

a 6Li 16O 40Ca

Energy (MeV) 5.5 44 63 95

Gas pressure (mbar) 20 31 20 7 13

Wire voltage (V) 500 600 460 405 390

d(DE1)/DE1 meas (%) 12.0 20.4 7.9 9.2 7.0

d(DE1)/DE1 SRIM (%) 6.6 17.9 5.0 6.8 3.7

d(DES)/DES meas (%) 6.9 17.5a 4.3 6.0 4.0

d(DES)/DES SRIM (%) 4.5 10.7a 2.2 1.8 1.2

aSum of three wire sets. For 6Li, the 4th wire set was not recorded.


5. Relevance to the MAGNEX spectrometer

A large-acceptance (51msr) high-resolutionspectrometer MAGNEX [4] is under constructionat INFN-LNS Catania. It is intended as a multi-purpose device with high energy and mass resolu-tion (dE=EB1=1000; dM=MB1=150) for low andintermediate energy heavy ions (Brmax ¼ 1:8Tm)[5]. These properties make the spectrometer aunique device, particularly in connection with theTandem-accelerated Radioactive Ion Beams ofthe ISOL facility EXCYT [10] at the LNS. Theinnovative design of the spectrometer is based on avery detailed treatment of the magnetic fieldshaping, of the highly non-linear ion optics [6]and of the application of trajectory reconstructiontechnique. The latter strongly constrains thedesign of the associated focal plane detector.Design criteria also include the need to detectparticles with energies as low as 0.5MeV/nucleonand, at least for energies above the Bragg peak,identify ion species with Zp20: In Table 3 themain requirements for the focal plane detector inorder to achieve the optimum performance fromthe spectrometer are compared with the analogousparameters measured here with HIT. For the

horizontal position resolution, we give the aver-age of all the results from Fig. 6. The ‘‘worstcase’’ result, for 6Li at 111, is 0.6mm, but this

Fig. 12. Scatterplots of energy-loss from wire 2 against residual energy from silicon B for 16O scattering from 27Al (left panel) and 58Ni

(right panel) targets. The observed oxygen elastic group is circled for 58Ni; the predicted location for elastic scattering events is

indicated for 27Al.

Table 3

Comparison of the main requirements of the focal plane

detector to obtain the optimum performance from MAGNEX

[6] with the corresponding intrinsic values obtained with the

HIT detector

Required for

optimum

resolution

Achieved

with HIT

Horizontal position

resolution (mm)

0.5 0.3a

Horizontal angular

resolution (mrad)

o10b 7

Vertical position

resolution (mm)

1 0.5

Energy loss resolution for40Ca (%)

5 4

Detection energy

threshold (MeV/u)

0.5 1.3c

Horizontal size (mm) 920 120

Vertical size (mm) 200 114

Depth (mm) 160 210

aAverage of all results.bBetter angular resolution leads to better mass resolution [6].cNot tested below this value.


was probably affected by multiple scattering(Section 4.2).

The intrinsic resolution parameters of the HITdetector are all acceptable for MAGNEX andclose to the limit of straggling. Admittedly, theresolution tests have been performed with beamenergies per nucleon above the required limit forastrophysical experiments of 0.5MeV/u, so oneshould be cautious in extrapolating to this regime.However, we have shown that there is nodeterioration in the intrinsic position resolutiongoing from 7MeV/u (6Li) down to 2.4MeV/u(40Ca) or even to 1.3MeV/u (a-particles). We havealso demonstrated that it is possible to operate thecounter at gas pressures low enough (7mbar) toallow 0.5MeV/u ions to penetrate the silicondetectors. Another experimental challenge for suchlow energy heavy-ions is in particle identification,because the DE=E technique is compromised in theregion of the Bragg peak. This may limit the typesof experiments one could do below 1MeV/u. Onthe other hand, in a magnetic spectrometer theparticle identification is aided by both the selectionof momentum per ionic-charge as well as a longflight path for time-of-flight measurements.

The most obvious discrepancy between HITand the MAGNEX focal plane detector is in thesize of the detector. The horizontal dimension iseasily extendable due to the intrinsic modularityof the strip pattern and the weak dependenceof position resolution on the amplitude of thewire signals. We note that averaging thecharge collection on some tens of wires reducesthe dependence of energy-loss resolution on theuniformity of the wire diameter. Cathode platesand Frisch grids of 80 cm length have beenregularly constructed in the past (see, e.g.,[16,24,25]) and it should be possible to extend toa 1m long chamber without major difficulty. Therequired vertical size similarly should not be aproblem, having minor influence on the verticalresolution. Preliminary tests with a drift chamberwith larger vertical size show no gross difference inresolution if care is taken in the shaping of theelectric field. Reducing the strip length, andconsequently the distance between wires, can cutdown the depth of the detector. Tests show thatthe latter can be reduced to 12.5mm, so that a

factor of four compression on the position-sensitive part of the detector can be obtained.

6. Conclusions

A new hybrid detector for accurate heavy iontracking has been built and tested under a broadrange of experimental conditions. The excellentposition and angular resolutions measured, closeto the straggling limits, together with the lowenergy threshold and the good particle identifica-tion properties makes the HIT detector a multi-purpose device for applications where a maximumrate of a few thousand events per second areexpected. A particular interest for the HITdetector is for nuclear astrophysics experimentsin the region of the CNO cycle. We have alsodemonstrated the use of HIT as a prototype focalplane detector for trajectory reconstruction. This isnecessary for the software compensation of theaberrations of the modern large acceptancespectrometers such as MAGNEX at the LNS.

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a low-pressure gas detector for heavy-ion tracking and particle identification

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