Cathode readout multiwire proportional chamber for ISTRA setup

242

Nuclear Instruments and Methods m Phy,,ics Research 227 (1984) 242 248 North-Holland Am,,terdam

CATHODE READOUT MULTIWIRE PROPORTIONAL V N. BOLOTOV,

S.N. G N I N E N K O ,

Yu.M. KLUBAKOV,

R.M. JILKIBAEV,

V.D. LAPTEV, O.M. RADCHENKO

CHAMBER FOR ISTRA SETUP V.V

I S A K O V , O.V. K A R A V A S H E V ,

and A.N. TOROPIN

Instttute for Nuclear Research, Academy of Scten¢ e.~ of the USSR, 60th O~ toher A nnwersarv Prospec t. 7a. Moscow 117312, USSR

Received 13 March 1984

The main characteristics of cathode readout multlw, re proportional chambers w~th 760 × 760 mm 2 achve area are presented for particle loads up to 103 p a r t / m m 2 s The following chamber characteristics were obtained cffioency, ~ = 99%, space resolution, 6 = 0 3 mm, rms of charge ratio Q x / Q r for charges on the X- and Y-plane of one chamber, 8Q~/¢?, = 7% A descnphon as g, ven of chamber des,gn and operauon, cahbratlon and detector check-up procedures, and reg,stered charged parUcle coordinate reconstruction methods Calculated dependenoes for the probabdlty of obtaining the true mult~track event coordinate on the 8O ~/Q~ value are presented

1. Introduction Recently, a m o n g the charged part,cle track detectors, c a t h o d e readout multlwtre p r o p o r t i o n a l c h a m b e r s ( M W P C ) have been a p p h e d on an ever m c r e a s m g scale [1-3] The following reasons for this can be g,ven firstly, the n u m b e r of measuring c h a n n e l s is cons~derably reduced as c o m p a r e d with the conventional proportlonal c h a m b e r s ; secondly, mult,track events can be registered without any auxiliary c h a m b e r , thirdly, standardized multlchannel systems with h n e a r analogto-digital conversion are apphcable. The latter are extens,vely used in registering signals from hodoscope systems wxth full absorption. The M W P C p e r f o r m a n c e is based on the fact that the motion of a charged particle through the c h a m b e r forms a 10-12 C charge, localized within a zone of 10 3 cm, which reduces c o r r e s p o n d i n g charges on the cathode planes (X, Y). Measuring the spacial distribution of the reduced charges on the cathode planes, the charged particle coordinates can be determined Thts p a p e r presents a discussion of M W P C characterlstlcs for the I S T R A setup of I N R desxgned for rare charged part,cle decay studies that are carried out o n the I H E P accelerator [4]

2. Chamber description T h e proportional c h a m b e r (PC) schemattc desagn Is shown m fig. 1 The c h a m b e r consists of two cathode a n d one a n o d e wire plane There is a 8 m m gap between the a n o d e a n d c a t h o d e planes. The actwe area of the c h a m b e r as 760 x 760 m m 2 The c h a m b e r design m0 1 6 8 - 9 0 0 2 / 8 4 / $ 0 3 . 0 0 © Elsevier Science P u b h s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b h s h m g Dlws~on)

cludes two s u p p o r t m g flanges and four rings to which fibre-glass electrodes are glued The c h a m b e r gas volume is insulated a n d screened w~th 50 ~tm aluminized mylar film The c h a m b e r weight is 100 kg The cathode planes are wound w~th a bronze w,re of 0.1 m m dmmeter, w,th 2 m m spacing and 200 g tens,on, m one plane ( X ) the wires are perpendicular to the a n o d e ones, whde ,n the other ( Y ) they are parallel to them. Each four adjacent w,res m a cathode plane are connected together m a " s t r i p " from whxch the o b t a i n e d s,gnals are transmitted to an amphfier The high voltage a n o d e plane is w o u n d w,th a 20 ~ m dmmeter VR-20 gold-plated tungsten w~re, w,th 2 m m spacing and 100 g tens,on All a n o d e wires are connected together and, via a 220 p F high voltage d,vldmg c a p a o t o r a n d a relay, can be coupled either with an a m p h f i e r or with a pulse generator The strip width has been selected equal to the a n o d e / c a t h o d e gap This provides good space resolution [1-3] at reasonable n u m b e r (190) of c h a m b e r amp h t u d e channels

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//F~g 1 Chamber schematic design

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V N Bolotov et al / Cathode readout M W P C

Strip

ALC-8

Oetaq

ALE-32

QDC

P O P- 11//-,0

Ftg 2 Electromcs block dmgram

As a gas fill,ng an Ar + 15% CO 2 + 0 3% 13B1 Freon gas mixture has been apphed, hawng a number of advantages over other m~xtures In the first place, ~t contains no organic compounds capable of polymerization under the charged particle flux and setthng on the wires. In the second place, this mixture is fireproof and can be released directly into the experimental hall In the third place, an mdustrml supply of the purified mixture is available The chamber was fed from high voltage sources [5] w~th fast current overload protection operated e~ther under computer control or m the usual, non-automatic regime. The protecuon triggering threshold was set at 200/~A Fig 2 shows the PC electromcs block dmgram. The cathode strips are connected with the Inputs of fast low noise LCA-8 preamphfiers [6] mounted on the PC. The preamphfier output signals are transmitted wa 45 m RK-50-2 cable towards the electronic bay to fast LCA-32 amphfiers [6] and then passed to the charge sensitive analog-to-&gltal converter (QDC) [7] inputs. The signal was registered in a Q D C only under the condition of simultaneous receiving of a stroboscopic trigger pulse generated by a logic electronic clrcmt. When measurmg the charge from a PC strip within a sufficiently narrow stroboscopic pulse of - 60 ns, ~t ~s ~mportant to know the relative spread of different electromc channel t~me delays to choose an optimum over-

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all delay for the stroboscopic pulse. The relative spread of the electromc channel time delays was measured by means of a computer controlled delay block Varying the stroboscopic pulse delay versus the GIS-2 generator pulse w~thm 1-255 ns, an optimum delay for each chamber channel was determined The distribution of time delays over all the PC channels is presented m fig 3. It is seen that the delay spread does not exceed 9 ns

3. Chamber calibration and operation checkout

Callbrauon and checkout operations were carried out over the whole measuring tract including the hnear amphfiers and QDCs. The cahbratlon was to get nd of the m&vzdual channel amphflcatlon factor spread and to equahze the measuring tracts to an accuracy of several percent A 10-digit computer controlled GIS pulse generator [8] has been used for the cahbratlon procedure The generator pulse had a leading edge of 15 ns and a back exponential edge w~th a characteristic damping time of 10 #s The s~gnal from the generator was transmitted vm the 220 pF high voltage &vldmg capacttor to the anode plane and &fferentaated on the strip-anode capacity and the input resistor of the LCA-8 preamphfier At the LCA-32 amphfier output the shape of the generator s~gnal was similar to that of a charged particle in the PC, with a leading edge of - 15 ns and a rear edge of - 70 ns. Both the LCA-8 500 D input resistors and 50 pF strip-anode capaclt~es were equal to each other within an accuracy of several percent, thus the generator amplitudes from the cathode strips were equal with the same accuracy. To obtain cahbratlon coeffic]ents the dependence of the generator amphtude q. on the Q D C readout u',, was measured over twenty points (q. = 16-640). At every gwen q. amphtude the generator was started-up 100

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Fig 3 Distribution of tlme delays over all electromc channels of the PC

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Fig 4 Conversion coefficient drift for one PC channel

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V N Bolotoo et al / Cathode readout MWPC I

times to determine the u~, dispersion. The expertmental results were approximated by a square polynome (1)

,~ = A'q 2 + B'q, + C',

where t is the number of the measuring channel, A', B', C' are cahbratlon coefficients Typical coefficient values were. A' = (2-8) × 10 -s, B' = 0 5-0.8, C' = 1-5. During the I S T R A setup operation the calibration of the PC electronics was carried out once every two days and did not take more than 15 mm The electronacs of the PC measurmg tract was checked out every 7 s by means of the G1S-2 generator. The conversion coefficient drift for one PC channel is shown m fig 4, tt did not exceed 1,5% per day after a day's heating, The PC operatton was checked out with a code pack Technical codes checked out the pedestals of the charge-dtgltal converters, notse level and stablhty of calibratmn coefficients for each PC channel. The phystcal checkout codes analyzed the registered events by means of stmphfled algorithms and determined the current efficiency of PC operation

4. P C c h a r a c t e r i s t i c s

The chamber's spatial resolution depends on the signal/noise ratio. Thus, when employing a MWPC, the noise reduction ts of great Importance The electrofiics intrinsic noise level was about 0 5 (conv. un.) When coupled with a PC strip under the chamber's working condmons, the noise level Increased up to 2 (conv. un). Ftg. 5 shows the mean particle mduced charge and noise averaged over the chamber versus the PC working voltage.

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Fig 6 Registration effmoency vs PC voltage

One can see from the figure that, with respect to the signal/noise ratio, the optimum working voltage is 4 4-4.5 kV At this voltage four strips operate on average (see fig 7), which is qmte sufficmnt to obtam charged particle coordinates with an accuracy better than 0 7 ram. When determmmg the registration efficiency, only those events were accepted for which the amplitudes exceedmg the threshold level (5 readouts) appeared on three or more adjacent strips on each cathode plane ( X and Y). In fig 6 the chamber registration effictency ts shown versus the voltage A rather high efflcmncy (99%) is observed in the working point (4.4-4 5 kV). Fig 8 demonstrates the dependence of the chamber reglstra-

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Fig 5 Mean partmle reduced charge and averaged no=se, 8, vs PC voltage

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STR I P Fig 7 Distribution of operated strip number for a charged partmle passmg perpendicular to the PC electrodes

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V N Bolotov et al / Cathode readout M W P C I

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Fag 8 Registration efficiency vs particle coordinate m the PC

tton efficiency on the Y-coordinate. The efficiency is lower at the edges by several percent The c h a m b e r time resolution can be estimated according to the curve of the overall cathode plane s i g n a l / s t r o b o s c o p i c pulse delayed comcldence. On the right b r a n c h of the curve the time resolution ts not worse than 40 ns. For defining the c h a m b e r ' s spatial resolution several algorithms for coordinate calculation can be employed. The best p o s m o n m g accuracy can be reached with a description of the cathode plane amplitudes by an induced charge distribution curve o b t a i n e d from special cahbrat~on m e a s u r e m e n t s But such a procedure requires a large c o m p u t i n g c a p a b t h t y and Is not convenient for real-t~me operation To obtain the coordinates of charged particles traversing the c h a m b e r in real time of an experiment, a simple method of the centre of gravity calculation for the cathode plane charge distribution has been used. xc=EQ,x,/EQ

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where x, is the coordinate of the l t h strip centre, and s u m m i n g Is done over all operated strips Q, values were calculated according to u, a m p h t u d e s registered m the QDC, wlth subtraction of a corresponding pedestal and a bias level ~/B, z - 4 A , ( C , - u , ) - B,

Q' =

ZA,

(3)

The spatial resolution defined by this method is illustrated in fig 10, where the distribution of coordinate differences for two identical adjacent c h a m b e r s is shown The spatial resolution obtained for one c h a m b e r from this distribution is ~ = 0 38 m m In ref. [9] the x, value was shown to be a good but somewhat shifted estimate of the true x c o o r d m a t e of a passing particle However, in the case of equal strip width and PC gap, for the calculation of the true x coordinate it is sufficient to take into account only h n e a r correction factors to x~ X = X~ +(X~ -- X ma × )K 1 + a / 2 K 2,

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(4)

where x,,,x is the centre c o o r d m a t e of a strip with m a x i m u m amplitude, and a is the strip width. The K 1 a n d K 2 coefficient values were determined u n d e r the condition of c h a m b e r resolution minimization They depend essentially on the n u m b e r of operated strips E.g. for three strips K i = 0.19. K 2 = 0: for four strips K i = 0 12, K 2 = 4-0.12 d e p e n d i n g on where

V N Bolotov et al / Cathode readout MWPC

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more strips operated, from the left or the r,ght side respechvely; for five stl'tps K 1 = 0 06, K 2 = 0, for six or seven strips K 1 = K 2 = 0 The linear corrections to the calculated particle coordinate give a better resolution, 8, = 0 27 m m (fig 11) The coordinate distribution of passing particles registered on the Y-plane of a single PC consists of separate peaks divided by a &stance equal to the anode wire spacing. The PC spatial resolution can also be estimated from the Y-plane peak widths, this resolution having no systematic error connected with the method of particle coordinate reconstruction. In fig 12 a partlcle coordinate d l s t r l b u t m n is shown taken from a narrow interval on the Y-plane of a PC The spatial resolutmn is 8~ = 0 25 m m So it IS seen that the linear correction m e t h o d (5) of particle coordinate reconstruction e h m m a t e s the systematm error of the centre-ofgravity method W h e n a particle traverses a PC the overall cathode p l a n e induced charges Q x and Qy are strictly correlated, though each charge value fluctuates considerably (fig 13). The Q x / Q Y charge ratm has a mean value equal to unity with 80,/Q ' resolution The 80,/Q ' value d e p e n d s on b o t h the electromcs noise level and the fluctuanons of the avalanche ion cloud shape ,n the PC T h e & s t r t b u t l o n of the Q x / Q ~ ratio is presented in fig 14. The mean Q x / Q r value v a n a t i o n and the resolution 8o ~/o, were studied d e p e n d i n g on the Y-coordinate of a b e a m passing through the PC The 8 o , / o ' value v a n e d from 6% to 8% d e p e n d i n g on the Y-coordinate, the m e a n 80x/O ~ value being 7%. Fig 15 shows the dependence of the m e a n Q x / Q Y ratm o n the Y-coordinate. Thus, the m e a n Q x / Q Y spread does not exceed + 2 5%

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V N Bolotoc et al / Cathode readout M WPC I

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reconstruction of several particle coordinates has been

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used 1) the charges were ranked according to their values, for X- and Y-p]anes, e g

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Q~ > Q2x > Q i >

Q~v > Q~ > Q3 >

2) the events were selected by the following criteria on the X- and Y-planes, e g. for two particles.

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3) the particle coordinates were mutually compared, e.g

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relative to the averaged value of 1 025 Based on the strict correlation between Qx and Q r charges, the true coordinates of multltrack events can be obtained w~thout any auxiliary c h a m b e r To illustrate th~s method we have carried out a simulation of the process of several particle registrations in a MWPC, taking into account the real induced charge spectrum (fig 13) and the real PC noise 6 = 2 (conv un ) For the simulation the following procedure of the

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(Q2x, Q 2 ) - , ( x , y ) 2

To define the fraction of true events depending on dispersion for two and three particles in one PC a M o n t e Carlo generation was used for the particle charges In figs 16 and 17 the dependencies of the true coordinate reconstruction efficiency F for several partlcles and the probabihty W of mixing up the coordinates on the dispersion the 6~2y/Q, for two and three particles in one PC at n = 1, 1 5 are given The efficiency F was defined as the ratio of events sufficing criteria (7) to the total number of events. The probability W was defined as the ratio of events with mixed-up coordinates after the selection (7) to the total number of accepted events It is seen that at n = 1 5 and 8O,/o ' = 7% the true coordinates can be reconstructed with 99 6% confidence

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Fig 15 value vs particle coordinate on the Y-plane

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Fig 16 Dependence of coordinate reconstruction effioency F and probabdlty /4: of mixing up the coordinates of two particles on the rms of the Qx/Qr ratio error Curves 1, 2 correspond to n = 1 and n = 1 5 respectively

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V N Bolotov et al / Cathode readout M W P C I

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W e should hke to express our a c k o w l e d g e m e n t to the directorate, a d m m i s i r a u o n a n d m a n a g e m e n t of the 1NR p r o d u c t i o n sections for suppol;t of this study T h e a u t h o r s are grateful to V M Lobashev, I V S h t r a m k h a n d V I V m o g r a d o v for incessant a t t e n t i o n , to V P Klrushkm, N.K K o z u b k o , L N . Latysheva, N , M S o b o l e v s k y a n d L I S t e p a n o v a for help m the preh m m a r y m e a s u r e m e n t s and also to S K Popov, V A Seregm, V E Postoyev a n d V P M a r m for valuable a s s i s t a n c e in d e t e c t o r m a n u f a c t u r i n g

1

References 2

~.

6

8

10

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Fig 17 Dependence of coordinate reconstruction effic=ency F and probability W of mixing up the coordinates of three parucles on the rms of the Q x / Q r rauo error Curves 1, 2 correspond to n = 1 and n = 1 5 respectively

level m 80% o f events w~th two p a m c l e s a n d m 40% of e v e n t s w~th three p a m c l e s .

[1] G C h a r p a k a n d F Sauh, Nucl lnstr a n d M e t h 113 (1973) 381 [2] F Sauh, Nucl lnstr and Meth 156 (1978) 147 [3] G Charpak, G Melchart, G Petersen and F Sauh, Nucl Instr and Meth 167 (1979) 455 [4] B A Arbuzov et al, Preprmt INR, P-0018 (1975) [5] A E Voronkov, R M Jilklbaev, A M Klabukov and I V Shtramkh, Preprmt INR, P-0221 (1982) [6] O M Gul'khandanyan, R M Jflklbaev, O M Radchenko and I V Shtramkh, Preprmt INR, P-0237 (1982) [7] Yu B Bushn,n et al, Pnb Tekh Exp 3 (1980) 80 [8] A E Voronkov, R M Jllklbaev, A M Klabukov and I V Shtramkh, Prepnm INR, P-0222 (1982) [9] 1 E n d o e t a l , N u c l Instr and Meth 188(1981)51