Nuclear Instruments and Methods in Physics Research A281 (1989) 283-290North-Holland, Amsterdam

MULTIWIRE PROPORTIONAL CHAMBER FOR THE IUCF COOLER RING

K. SOLBERG, A. EADS, J. GOODWIN, P. PANCELLA, H.O . MEYER, T. RINCKEL and A . ROSSIndiana University Cyclotron Facility and Department of Physics, Indiana University, Bloomington, IN 47405, USA

Received 18 April 1989

The design and construction of a multiwire proportional chamber for experiments with the Indiana Cooler is described. The novelfeatures of this device are an opening in the center of the sensitive area and fast high-voltage switching, both required byexperimental demands. Data are presented on the performance of this chamber which is currently in use for the first nuclear physicsexperiment with an internal target in a stored, cooled beam .

1. Introduction

The Indiana Cooler [1] is one of the first ion storagerings with electron cooling which are currently beingbuilt for internal target physics. Its design goal is tooffer stored, cooled light-ion beams with magnetic rigid-ities of up to 3.6 Tm. The wire chambers which aredescribed in this report are part of an experimentalsetup which has been installed in a straight section ofthe ring . This setup serves a number of the early mea-surements planned with the new accelerator but hasbeen constructed specifically for an investigation ofneutral pion production in proton-proton collisionsclose to threshold [2] . In this reaction the momenta andthe energies of the two outgoing protons will be mea-sured. Since the observed particles fall within a narrow

Fig. 1. Front and side view of the multiwire chamber assemblyconsisting of two x- y modules at 45' with respect to each

other.

0168-9002/89/$03 .50 © Elsevier Science Publishers B.V .(North-Holland Physics Publishing Division)

283

forward cone, it is possible to cover most of the availa-ble phase space with a simple detector system. It istherefore desirable for the wire chamber to completelyintercept all angles within a forward cone . With thepresent chambers this is the case, except for a centralhole which allows the storage ring beam pipe to passthrough. This hole was made as small as practical .

The experimental setup consists of three planes ofconventional plastic scintillators and four multiwirechamber planes in a stack of two x-y modules, tilted by45 ° with respect to each other, as shown in fig. 1 . Morethan one wire chamber is needed in order to unambigu-ously determine the particle angles in a multiprongevent. The wire spacing is determined by the desiredangular resolution .

The construction of our wire chambers is standardexcept for the central hole . The rim structure for thishole is suspended solely by the sense wires and thinpolyester foils of the detector, requiring no additionalstruts or other supports, hence optimizing the sensitiveregion .

2. Description of the wire chamber

Each wire chamber unit has a sensitive area of 71 .1cm by 71 .1 cm . It consists of a stack of square framesmade from FR-4 [3] and aluminum which support thefoil and wire planes . The sequence of the stack is shownin fig . 2. Two sense wire planes, each with 112 gold-plated tungsten wires, 20 pm in diameter, separated by6.35 mm (1/4 in .), run perpendicular to each other (thetwo planes sense orthogonal positions) . On either sideof the wire planes there are high-voltage cathode planesmade from 0.025 mm graphite-coated polyester . Thecathode to anode (wires) spacing is 9.53 mm (3/8 in .) .This relatively large spacing was chosen to allow a long

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separation is also less sensitive to small positioning

errors of the wires which would adversely affect the gain

of the detector. On either side the stack terminates with

a double layer of 0.025 mm aluminized polyester

windows to contain the chamber gas and to provide a

path for the gas flow . In the center of the chamber there

is a hole, large enough to clear the beam pipe . The

configuration of this hole ("hub") is a cylindrical stack

of rings in which the sequence of the outer frames is

repeated (see fig . 2). In each wire plane, ten of the sense

wires are interrupted by the hub . The two halves of

these wires are electrically connected to opposite sides

of the chamber . The inside diameter of the completely

assembled hub structure is 57 mm. The outside diame-

ter is 72 mm; this latter number was made as small as

possible in order to minimize the insensitive area, as

well as to minimize the amount of material to be

suspended from wires and polyester planes .

In a conventional chamber the gas flow is from one

side to the opposite side . In this design, however, stag-

nant flow regions might occur in the wake of the hub .

In order to prevent this, the gas is brought to the center,

using the space between the outer and an inner window,

which was added for this purpose. The gas then flows

radially outward from the center of the hub . The gas

flow is indicated in fig . 2 .

Based on its long history of successful use, the gas

mixture chosen is 33% isobutane and 67% argon, bub-

bled through an isopropyl alcohol bath which is kept at

10 ° C . Since a large amount of polyester is present in

the detector, isopropyl alcohol was substituted for the

customary methylal [5], as it is known that the latter

attacks polyester .

Each sense wire is equipped with its own amplifier

and discriminator . Commercially available, integrated

electronics (LeCroy 2735b cards [6]) are used . The dis-

criminator level is set remotely to 2 .5 V (corresponding

to a nominal 5 WA) . The ECL outputs are fed into

LeCroy 4448 48-bit coincidence registers and read out

through CAMAC for every valid trigger .

3 . Details of construction

Even though a sense wire is fragile (the breaking

force is about 0 .1 N), several wires together can hold a

light object such as the narrow ring that forms part of

the hub structure . Realizing this fact, we developed a

technique analogous to a method commonly employed

in wire chamber construction [7] . In this technique

every plane represents a unit by itself ; the chamber

assembly is achieved by stacking planes in the required

order . The assembled chamber then contains a stack of

outer frames that provide mechanical rigidity and a

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stack of carefully designed inner rings that make up thehub structure .

The procedure for the construction of the wire cham-bers is as follows: First, a wire plane is wound on awinding table onto a printed-circuit FR-4 frame. Thewires are then glued to the frame and soldered to thepads of the printed circuit . Next, an inner ring isfabricated from copper-clad FR-4 with pads etched out(fig. 3) . An alignment jig is used to position the ringprecisely in the center of, and underneath, the wireplane. The sense wires are then soldered to the FR-4ring. After this, the wires inside the ring are cut (see fig.3), wrapped around the ring, and secured with glue .This method works as long as the center ring is lightenough to be supported by the sense wires, and strongenough not to be distorted by the tension of the wires.In this detector, the center ring is made from 3.2 mmthick FR-4 with inner and outer diameters of 61 .9 mmand 61 .9 mm, respectively. Since the wire spacing is 6.35mm, only ten wires are connected to the inner ring oneither side . No distortion problems have yet been en-countered. For applications with different geometry, itis likely that problems of distortion could be overcomeby slightly altering the technique of construction .

The high-voltage cathode planes are produced in asimilar manner . First, a 0.025 mm thick polyester sheetis spray-painted with graphite [8] . A mask is placed onthe polyester to match the active area of the detector .The polyester plane is glued to the FR-4 frame usingEccobond [9] no . 45 (clear) with catalyst no. 15 (clear)in a mixing ratio giving a "rigid" bond . Next, a jig isused to place a machined nylon ring on the plane (fig.4a). The ring is epoxied in place using Epoxy-Patch [10]no . 907 (green) . After the epoxy has cured, a gasketpunch is used to open a hole in the center of thepolyester. The polyester remaining inside the ring isnow cut radially every 3 mm (see fig . 4b). A thin layer

K. Solberg et al. / MWPCfor the IUCFCooler ring

of Epoxy-Patch is placed on the inside of the ring and aballoon is inflated to press the polyester against theepoxy-covered inside of the nylon ring (fig . 4c). Thepolyester-epoxy forms a boundary which is effective inpreventing sparking to the center of the hub which is atground potential .

The two polyester planes that form the doublewindow are made by the same method . The outerwindow is made from 0.025 mm thick aluminized poly-ester . The aluminum acts as an rf shield and decreasesthe rate of diffusion of air into the chamber gas. Thealuminum coating on the polyester also carries theelectrical ground to the center cylinder of the hubshown in fig . 2. The inner window is made of clearpolyester 0.025 mm thick. Aluminized polyester cannotbe used here, because electrostatic attraction wouldcause the inner window to stick to the nearest high-volt-age plane. The outer and inner windows are attached toaluminum and nylon rings, respectively, in the samemanner as the cathode high-voltage planes.

The assembly of the center hub and the outer framesis shown in fig. 2. The foot of the aluminum cylinderforms the sealing surface for O-ring no . 2. The outersurface of the aluminum cylinder provides the sealingsurface for O-ring no . 1 . A pushing ring (shown in fig . 2

a

NYLONRING

025mm MYLAR

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Fig. 4 . Procedure to mount the center nylon ring on thepolyester foil planes : (a) shows a side view, (b) describes thecutting of the hole and (c) illustrates the method used to glue

the polyester foil overlap .

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Fig. 5 . The pushing ring which locks the hub structure into aunit and seals the unit by compressing O-ring no. 2 in fig. 2 .

and, in detail, in fig . 5) was designed to compress therings together after assembly and to compress O-ringno . 2. The set screws shown in fig . 5 are inserted just farenough to hold in clearance holes provided in thealuminum cylinder . Then the "compression" screws aretightened sufficiently to compress O-ring no . 2 (fig. 2) .

When the chamber was designed, it was anticipatedthat the hub would be held in place transversely by the

K. Solberg et al. / MWPC for the IUCFCooler ring

polyester planes . Since those planes exert a relativelysmall restoring force on any movement perpendicular tothe plane, we thought it would be necessary to use thebeam pipe to hold the hub in the precise alignmentrequired for the proper operation of a multiwire propor-tional chamber. When the wire chamber was filled withgas, however, we were pleasantly surprised to find thatthe small pressure from the filling gas pushed the outerwindows out and held the hub rigidly in place in muchthe same way as a bicycle tire is given shape by the gaspressure it contains . The pressure difference betweenthe inside of the chamber and the outside atmosphere is0.5 Torr _+ 0 .1 Torr . The window is 73 .7 cm x 73 .7 cm .This means that the total force exerted by the gas oneach side of the hub structure is approximately 35 N .Hence this chamber can be safely operated in a horizon-tal or a vertical position with no additional externalsupport required for the center hub.

One problem encountered in this design is that ad-jacent cathode planes will attract each other if they areat a different potential. Hence, if x and y planes arepowered separately and a spark occurs in, say, the xplane causing the x high voltage to trip off, the yhigh-voltage plane nearest the x detector will attract theclosest x high-voltage plane and they will actually sticktogether . Usually, the only way to get them apart againis to disassemble the chamber. To avoid this problem,

Fig. 6. Experimental setup used to investigate the efficiency close to the hub.

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WIRE NUMBERFig. 7. Distribution of test events in the vicinity of the central

hole . The axes are labeled by wire number .

the high-voltage planes of both chambers are connectedto the same supply . The high voltage is operated at thevalue required by the higher of the two plateau voltages .

4. Operation and performance tests

4.1 . Investigation of the efficiency near the hub

Although it is possible to map out local efficiencieson the bench using a radioactive beta source, the needfor tight collimation causes problems with scatteredradiation and low counting statistics . It is thereforemuch better to use a well localized source such as oneobtains from small-angle Rutherford scattering of pro-tons from a heavy target . The experimental setup usedfor this purpose is shown in fig . 6. A 45 MeV protonbeam from the Indiana University Cyclotron Facilitywas directed at a 400 wg/cmz 56 Fe target . The protonsused to test the new wire chamber are defined by acoincidence between two scintillators and two x-y posi-tion-sensitive delay line wire chambers . The delay linechambers have 4 mm spacing between sense wires. Thedistribution of events as measured by the chamber to betested is shown in fig . 7. Since the particles responsiblefor firing the new chamber are tagged independently, itis possible to determine efficiency as function of posi-tion . Shown in fig. 8 are the x and y projections (in thereference frame of the new chamber) of trigger events(dashed line) and all such events for which there wasactually a response in the new chamber (solid lines) .The two minima in the dashed curves reflect the factthat protons which hit the material in the hub fail toreach the second scintillator and thus do not lead to atrigger. The accumulated data indicate that particles

K. Solberg et al. / MWPCfor the IUCF Cooler ring

passing 4.5 cm or greater from the center of the hub aredetected with the full efficiency (solid and dashed linesidentical) . Beyond the outer radius of the hub structure(3 .5 cm) there is thus an insensitive annular zone of 10mm width due to field effects close to the groundpotential of the hub. This width is, as expected, ap-proximately the distance from anode to cathode (9 .5mm).

4.2. Operation of the chamber in the Cooler; efficiency

The chambers described here are part of a setupintended to measure with the Cooler the reaction

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near threshold [2] . The experimental arrangement, shownin fig . 9, is designed to measure the energies andmomenta of the two outgoing protons. Apart from the

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Fig. 8. (a) x position data and (b) y position data from thefirst delay line chamber of fig. 6, for slices which pass throughthe center of the CE-01 chamber being tested . The dashedhistogram is all scintillator triggers, the solid line is all triggerswhich also produced a signal in the appropriate coordinate ofthe CE-01 chamber. Where the dashed and solid line overlaponly the solid line can be seen. The increase in intensity in thex direction reflects the variation of the Rutherford cross sec-tion for scattering from the 56 Fe target . The vertical axis is thenumber of counts and the horizontal axis is the wire number in

the delay line chamber .

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OU DELAY LINE

CHAMBER GATEDBY CE-01

100-

0- 1 1 ,

0 50 ,100

288 K Solberg et al. / MWPCfor the IUCF Cooler ring

Fig . 9 . The IUCF CE-01 experimental setup .

wire chambers, it contains an internal hydrogen gas jettarget in a vacuum chamber and three planes of plasticscintillators . The scintillators are divided into eight seg-ments. When a trigger signal, which is based entirely onthe fast scintillators, is received, full information on theevent is read out . This includes the contents of coinci-dence registers that are connected to the discriminatoroutputs of the individual wire amplifiers of each of the448 wires in the four wire chamber planes . The bitpattern on the registers is translated in software to oneor several wire numbers for further use, e.g., in incre-menting position spectra. An example of an x-y distri-bution is shown in fig . 10. To make the display moreinteresting, only triggers from every other of the eightscintillator segments have been used .An aspect of Cooler experiments which has a pro-

found impact on data acquisition arises from the factthat the Cooler operation is cyclic . A typical cycle starswith the filling of the storage ring. The current injectionmethod of dissociating an incident HZ beam causes avery large radiation background during the few millisec-onds required to inject the beam. During this time theanode current in the wire chambers would be excessive .This current would severely limit the lifetime of thechamber . It also makes it difficult to protect the cham-ber against sparking by setting a current limit on thehigh-voltage supply . In order to avoid such problems,the wire chamber high voltage is lowered, prior toinjection, to a quiescent level of Uq� = 1300 V . Afterfilling, the chamber voltage is raised to the operatinglevel of U.P = 2950 V, and data acquisition is enabled .

The components are labelled in the figure and discussed in the text . The beam istravelling from left to right .

The circuit used to supply the chamber high voltage isshown in fig. 11 . A start signal, synchronized with theCooler cycle, triggers a square wave generator whichdetermines the "on" time . The two voltage levels andthe duration of the on time are set independently . Thefast rise and fall of this signal is then smoothed by avariable RC filter (RC = 115 ms) . This low-voltage sig-nal is processed by an amplifier [111 with a voltage gainof 10 3 and delivered to all of the high-voltage planes . Infig . 12 the chamber voltage is shown as a function of

Fig . 10. Position distribution of 45 MeV protons scatteredfrom a 1°N gas jet. Only triggers from every other of the eight

scintillator segments are enabled .

time, together with the efficiency of one plane coordi-nate. It is obvious that maximum efficiency is achievedas soon as the plateau voltage is applied to the chamber .Since in fig. 12 the operating voltage (U.P) is consider-ably higher than the plateau (2200 V) full efficiencyoccurs well before the full operating voltage is reached .So far, after a few hundred hours of chamber operation,no problems have been encountered which could betraced to this fast cycling of the high voltage.

The chambers were disassembled and inspected aftera few hundred hours of operation . No evidence ofsparking could be found in the chambers . However, thehigh-voltage planes had wrinkled . The wrinkles did notadversely affect the efficiency of the chambers and didnot cause sparking. We believe the wrinkling was causedby either the isopropyl alcohol, or manufacturing de-fects in the polyester which was used . We are currentlyconducting tests to find the cause of the wrinkling andwill alter the isopropyl content or change the polyesterin the high-voltage planes to cure the problem.We have investigated the efficiency of the wire

chambers under the actual running conditions of aCooler experiment. The best data to date came fromcooled 108 MeV protons scattering from a thick carbon

v500 800 900 1100Time After Cycle Start (ms)

Fig. 11 . Block diagram of the fast cycling high-voltage supply. The amplifier is the TREK [10] model 609 .

Fig. 12 . Wire chamber bias voltage (upper curve) and effi-ciency of a single wire plane (lower curve) vs time near thestart of data taking cycle . The events are caused by 45 MeVcooled protons scattered from an aluminum target . Efficiencyhere is defined as total wire chamber counts divided by scintillator double coincidences (triggers). This includes nonconsecu-

tive events, unlike the definition in the text.

5. Conclusions

TO CHAMBERHV PLANE

289

target . The source of the scattered particles is well-local-ized, with low background, allowing us to test onechamber with particle tracks defined by the other cham-ber .A valid wire chamber event is defined as at least one

wire and at most four consecutive wires firing in agiving coordinate. For a valid event in one chamber,there are two types of inefficiencies possible in the otherchamber : "zero" inefficiency when no wires fire, andnonconsecutive inefficiency when multiple wires whichfired are separated by one or more wires that did notfire . This latter category is attributed to noise pickup,although it is possible that some small percentage ofthese occur when more than one particle is present inthe chamber. (The efficiency plotted in fig . 12 does notinclude this nonconsecutive inefficiency .) We confirmedthe earlier finding that the zero inefficiency is onlysignificant very close to the hub area . Away from thehub, the total inefficiency is due to nonconsecutive hitsincluding events where five or more wires fired . In allrespects the four coordinates in the two chambers dis-play the same efficiency characteristics . Nonconsecutivewire events are around 5% of all test events for everycoordinate, yielding an average efficiency of 95% .

The ratio of 2-wire events (consecutive) to singlewire events depends strongly on the angle of incidence,increasing for angles farther from perpendicular . Atnormal incidence, we observed 73.5% singles, 19.3%doubles, 1 .7% triples, 0.5% quadruples on the average.At angles --- 17% from perpendicular, this becomes 40%singles, 52% doubles, 2.2% triples, 0.8% quadruples.This indicates that 2-wire events are caused by trackswhich pass between wires . Therefore, the actual positionresolution of the chamber is better than the wire spac-ing (6.4 mm), and averages about 4.2 mm.

A new multiwire proportional chamber has beendesigned, built and tested for use in the new electron-cooled storage ring at IUCF .A new multiwire proportional chamber has been

designed, built and tested for use in the new electron-cooled storage ring at IUCF . It completely surrounds

K Solberg et al. / MWPCfor the IUCF Cooler ring

TRIGGER SIGNALFROM

COOLER CONTROL MONITOR1

ISQUARE WAVE _T-L TREK 609

PULSE X1000GENERATOR VARIABLE 1 HV AMPLIFIER

RC FILTER =

290

the vacuum pipe which contains the stored beam . Thecentral hole which admits the beam has been con-

structed to maximize the sensitive-detector area, bring-

ing the wires as close to the beam pipe as possible. The

presence of the hole does not detract from the perform-

ance of the chamber in any way outside of 1 cm fromthe physical boundary of the hub structure.

In addition, this chamber has operated successfullyin a pulsed high-voltage mode, where the bias voltage ischanged by 1650 V to the operating level in as little as100 ms, and turned off just as fast, as frequently as fivetimes per minute for extended periods of time. The gasmixture and other design parameters have proven togive great immunity to sparking problems, with low

noise, good efficiency and reliability . This is true de-

spite the lack of overcurrent protection in early test runsand the wrinkling of cathode planes . The performanceof this type of chamber through hundreds of hour ofoperation suggests that drift chambers could be con-structed and operated in the same manner .

Acknowledgement

The research presented here was supported by agrant from the National Science Foundation, NSF grantno . NSF Phy 87-144-06, 48-308-30.

K. Solberg et al. / MWPCfor the IUCF Cooler ring

References

H.O . Meyer, Cooler User Guide, 2nd ed ., IUCF internalreport (1981) .R.E. Pollock et al . . Measurement near Threshold ofNuclear and Few-Nucleon Pion Production in the Cooler,IUCF proposal, experiment CE-01 .FR-4 is an epoxy-glass laminate . FR-4 has, generally, thesame properties as the previously common G-10epoxy-glass laminate . The principal difference betweenFR-4 and G-10 is the fire retardant properties of FR-4 .M. Atac, IEEE Trans. Nucl . Sci. NS-31, no. 1 (1984).G. Charpak and F. Sauli, Nucl . Instr. and Meth . 162(1979) 405.

[6] LeCroy Corp ., 700 Chestnut Ridge Road, Chestnut Ridge,NY 01977-6499, USA.G. Charpak, G. Fischer, A. Minten, L. Naumann, F.Sauli, G. Flugge, Ch . Gottfried and P. Tirler, Nucl . Instr.and Meth . 97 (1971) 377.

[81 Acheson Industries, Inc., PO Box 8, Port Huron, MI48060, USA.

[9] Eccobond, manufactured by Emerson & Cuming Inc.,Canton, MA 02021, USA.

[10] Epoxi-Patch, manufactured by Hysol Division, DexterCorporation, Olean, NY 14760, USA.

[11] TREK Inc., 3932 Salt Works Road, Medina, NY 14103,USA.

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