c pion beams

Nuclear Instruments and Method,, m Physics Research 227 (1984) 237-241 North-Holland, Amsterdam

A LONG LIQUID C H E R E N K O V C O U N T E R FOR 300 TO 460 M e V / c D

ZAVRTANIK,

237

PION B E A M S

F S E V E R , M. P L E S K O , M. MUS;I(~ a n d G . K E R N E L

J Stefan ln~'tltute and Department of Phv~l~,~, E Karde0 Umversttv, Llubljana, Yugo~lat,ta N W. T A N N E R

Department of NuJear Phy~t~~, Oxford UntuersttV, England E.G. MICHAELIS

* a n d A. S T A N O V N I K

**

CERN, Geneea, Swttzerland Received 24 Aprd 1984

A long hquld Cherenkov counter has been used to measure the proportion of muons in positive and negative plon beams m the momentum range between 300 and 460 MeV/c A nine-parameter function fits all the spectra well The data show a smooth dependence on incident momenta and agree with calculations of plon and muon pulse heights

1. Introduction Secondary beams obtained by lrradtatmg a production target are m general c o m p o s e d of several types of particles even after having undergone m o m e n t u m analysis in a b e a m - t r a n s p o r t system. In any experiment aiming at absolute cross sections it ~s imperative to know the proportion of particles which lmtmte the reaction of interest. Often only a small fraction of the beam particles ~s used in the scattermg target As a consequence the composition of the beam before h~ttlng the target and the " s p e n t " beam, are almost identical Therefore a destructive analysis of the beam composition can be made on the spent beam. We describe an experiment performed at the C E R N synchrocyclotron on beams emerging from a production target b o m b a r d e d by 600 MeV protons A long liquid Cherenkov counter (LLC) was used in a similar way as reported by Rachard-Serre et al [1] but the measurements were extended over both ~r+ and 7r beams w~th m o m e n t a from 300 M e V / ~ up to 460 M e V / ~ . At higher m o m e n t a where the rr//z distributions are less well resolved one may introduce into the least-squares fitting procedure mitial estimates of peak positions as calculated (section 3) and of their relahve strengths and

* Present address Department of Nuclear Physics, Oxford University, England ** Present address J Stefan Institute, E KardeIj University, Ljubljana, Yugoslavia 0168-9002/84/$03.00 © Elsevier Science Pubhshers B V ( N o r t h - H o l l a n d Physics Publishing Division)

widths as obtained by extrapolation from low momentum data Thus we show that by systematic measurements over a broad m o m e n t u m range and with the aid of calculations it is posstble to extract meaningful results even at m o m e n t a up to 460 M e V / c The attractions of this method for beam composition d e t e r m m a t i o n are simple experimental set-up and analysis

2. Experimental set-up The measurement of beam composahon was related to an experiment which alms at a determmatlon of 7rp ~ ¢r~rN cross sections near threshold [2,3]. Fig 1 shows the layout of beam transport elements as used in the ~r+p part of the experiment An almost identical set-up was used for the Tr-p channel The main problem in determining the fraction of muons in plon beams is the small difference of ~r and velocities. In long Cherenkov counters m which plons and muons are slowed down to threshold for Cherenkov hght emission, however, the relative difference in the total amount of hght emitted ~s larger than in thin transmission counters Th~s is due to the fact that muons not only radmte more hght per unit distance but also radiate on a longer path (higher initial kinetic energy, smaller energy loss and lower energy threshold) For the Cherenkov radiator we have chosen FC75 (C8F160), which besides bemg liquid at room temperature and thus easy to handle, also has a high density

238

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Long hqutd Cherenkot, counter

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(p = 1.75 g / c m 3) so the particles are slowed down to threshold (n = 1.277) in a m a n a g e a b l e length The h q m d radiator was c o n t a i n e d mn a 10 cm dmmeter, 45 cm long perspex vessel viewed by a Phdlps 58 A V P photomultmp h e r at the d o w n s t r e a m end The set-up for b e a m compos~tlon measurements (fig 2) was located a b o u t 3 m d o w n s t r e a m from the spectrometer where the b e a m profile dimensions were of the order of the & a m e t e r of the LLC counter. Part=cles going sideways through the r a & a t o r and those scattered out of It give smaller signals producing tails and b r o a d enxng of the peaks In order to select only particles near the C h e r e n k o v counter axis ( r < 1 cm, 9~ < 1 °) a scintillation counter telescope was used m the trigger ( S I ' 1.2×1.2×1.2cm 3,$2' O=2cm, d=04cm, S3: 1 0 × 10 × 0.4 cm 3 w~th 2 5 cm diameter hole). The whole assembly of telescope a n d Cherenkov counters was m o u n t e d on a support which was adjusted so that their axis coincided with the b e a m axis. As different particles

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Fig 3 LLC pulse height dlstnbuuons of the 355 MeV/c negative beam with the gas Cherenkov counter triggering (a) and inhibiting (b) the ADC gate

D Zavrtamk et al / Long hquM Cherenkou counter m the beam - defined by the main spectrometer " b e a m trmgger" - have different spatial d~stnbut~ons horizontal a n d vertical scans were m a d e to d e t e r m i n e variations in composition over the b e a m profile For the m e a s u r e m e n t of negatwe beams a 0 6 M P a C O 2 gas C h e r e n k o v counter was also required Fig. 3 shows the LLC spectra of the 355 M e V / ~ negauve b e a m taken with the gas counter triggering (fig 3a) and inhibiting (fig 3b) the A D C gate It is seen that the electron part of the spectrum (fig. 3a) is very b r o a d It even reaches and overlaps with the ~-/~ part of the spectrum (fig 3b) In order to allow for a better determination of m u o n s m negatwe beams - where the electron fraction ts high - the LLC signals were measured with the gas counter m ant~comcldence while electrons were just counted The LLC counter is also c o n v e n i e n t for d e t e r m i n i n g the effic,ency of the gas c o u n t e r a n d wce-versa The measured effic,encies for electrons were 95% for the gas counter and 99% for the LLC The 7r/# part of the spectrum cons,sis of the plan a n d m u o n peaks corresponding to part,cles emitting all the " a v a d a b l e " C h e r e n k o v light as they are gradually slowed down from incident m o m e n t u m to threshold. a n d of a low-pulse-height c o n t i n u u m mainly due to p l a n s undergoing nuclear inelastic colhslons in which they are abruptly slowed down below threshold This c o n t m u u m extends from zero pulse height up to the p l a n peak

239

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Fig 4 Examples of measured LLC pulse height distributions (histograms) compared with the fitted function F (smooth line)

3. A n a l y s i s and r e s u l t s

M e a s u r e m e n t s of LLC pulse height distrlbuUons were m a d e both for positive a n d negative b e a m s of the following m c l d e n t m o m e n t a . 295, 315, 335, 355, 375, 415 a n d 455 M e V / c . Some of these spectra are shown m fig. 4. Measured spectra have been fitted by a least-squares m e t h o d using the p r o g r a m H F I T from the C E R N comp u t e r program library [4] The trial function h a d the following form F(z)=

A f - +°~ ~ i(z',a,b)

e x p [[-[ z ~- z~' ]-2 l /

I dz'

taken into account by the convolution The second term ts the pton " t o t a l h g h t " peak with the same width p a r a m e t e r as the first term. The third term represents the m u o n peak. The function F ( z ) has n m e parameters which were allowed to vary wlthm certain hmtts. These h m i t s were determined either by estimates from measured spectra (for low incident m o m e n t a ) a n d / o r by extrapolation from low m o m e n t u m data (for tugh incident momenta). F r o m fig. 4 where the fits to the data

ith 20 sm2.~.dx 0 15

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T h e first term represents the c o n t r i b u t i o n from plons undergoing nuclear interactions m the radmtor. F o r the functton l(z; a, b), we have chosen a stmple form which approximately reproduces the shape of the spectrum below the plon peak. Finite experimental resolution is

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D Za[:rtamk et al / Long hqutd Cherenkov ¢ounter

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F~g 6 The relatmve difference of muon and p~on peak pulse heights as a functmon of momentum The data points are compared w~th the calculated curve

are shown, it can b e concluded that the function F ( z ) fits well all the data The m o m e n t u m dependence of the parameters obtained from the fitting procedure can be c o m p a r e d with calculated values. The relative plan and m u o n peak pulse heights have been calculated by a simple numerical integration of the light emitted per unit distance dL/dx=A sin20', where A is a c o n s t a n t a n d 0 is the C h e r e n k o v angle, cos 0 = 1/nB. Energy loss in the radiator is calculated from the B e t h e - B l o c h formula [5] T h e integration stops when the particle either reaches threshold or has traversed the whole length of the radiator. The d e p e n d e n c e of the pxon peak pulse heights o n incident m o m e n t a is shown m fig. 5. The experimental values - n o r m a h s e d to the calculated ones - are shghtly lower at low m o m e n t a a n d slightly higher at high m o m e n t a . A p a r t from this small systematic effect, which could be due to a m o m e n t u m d e p e n d e n t light collection efficiency, the agreement of the measured a n d calculated p l a n pulse heights is good. Experiment and

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calculation agree qmte well also for the separation of the pmon and m u o n peak positions (fig 6). At m o m e n t a above 400 M e V / c , where the fraction of m u o n s becomes very small a n d the separation of ~r and tz peaks ~s smaller than themr widths, the m u o n peak posmtmon m the fittmg procedure was fixed on the basins of calculated values M u o n s of incident m o m e n t a higher than 310 M e V / ¢ a n d pxons of incident m o m e n t a higher than 350 M e V / ~ radiate all the way through the 45 cm long radiator Thus m principle, a somewhat better 7r/t~ separation at higher m o m e n t a could be achieved at the price of a longer counter The p~on peak widths have been estmmated by assuming 500 sin20' emitted p h o t o n s per centmmeter, 100% transparency, 25% q u a n t u m efficmency of the p h o t o m u l tmpher, 1% fwhm m o m e n t u m spread for p o s t u r e and 4 5% for negative beams. F r o m fig 7 it is seen that the experimental wmdths are approximately 2.5 times larger t h a n those estimated This discrepancy indicates that a b o u t 80% of the C h e r e n k o v light ms lost before reaching the p h o t o c a t h o d e The proportion of m u o n s obtained from the least squares fitting of the measured C h e r e n k o v spectra is s h o w n m fig 8 as a function of b e a m m o m e n t u m The fractmon of plans undergoing nuclear mteractmons in the counter and thus c o n t r i b u t i n g to the c o n t i n u u m below the plan peak depends on the cross sections and will be treated m a subsequent p a p e r

4. Concluding remarks

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T h e h q m d FC75 seems to be a statable C h e r e n k o v radxator for destructive p~on b e a m composition measurements up to m o m e n t a of 460 M e V / c . A l t h o u g h the p l a n and m u o n peaks are not well separated at m o m e n t a above 400 M e V / c , we have d e m o n s t r a t e d that by systemauc, good-statistics m e a s u r e m e n t s over a larger range of incident m o m e n t a one m a y extract the m u o n fraction using a fitting m e t h o d The initial values for the p a r a m -

D Zavrtant~ et al / Long hqutd Cheren£ov ¢ounter

eters that have to be introduced into the procedure are estimated by extrapolation from low m o m e n t u m data a n d / o r by calculation We wish to members of the t~clpated in the C E R N SC staff erator

express our thanks to all Omlcron collaboration who present experiment, as well for excellent performance of

the other have paras to the the accel-

241

References [1] C Rlchard-Serre, M J M Saltmarsh and D F Measday, Nucl Instr and Meth 63 (1968) 173 [2] The Omlcron Collaboration, CERN/PSCC/P32 (September 1980) [3] G Kernel, P KnZan, M MlkuL A Stanovmk, D Zavrtanlk, G Engster, E G MLchaehs, A G Zephat, J Harvey and K O H Zlock, Nucl Instr and Meth 214 (1983) 273 [4] R Brun, |. Ivanchenko and P Palazzl, CERN Data Handling Division D D / 7 7 / 9 (revised 1979) [5] C Serre, yellow report CERN 67-5 (1967)