Gas differential and threshold Cherenkov counters for high energy particle identification

136

Nuclear Instruments and Methods in Physics Research A248 (1986) 136-142 North-Holland, Amsterdam

GAS DIFFERENTIAL AND THRESHOLD CHERENKOV COUNTERS FOR HIGH ENERGY PARTICLE IDENTIFICATION S.P . DENISOV

Institute for High Energy Physics, Serpukhoo, USSR

The design and characteristics of differential and threshold Cherenkov counters are considered. Possibilities for using them for particle identification at future accelerators are discussed. An application of Cherenkov counters in accelerator experiments is presented.

1. Differential counters In this section we deal with differential Cherenkov counters without chromatic corrections used for ir, K and p-separation in high energy beams. Fig. 1 presents a schematic cross section of the counters used at IHEP [1]. The Cherenkov light is focused by a spherical mirror (1) into a ring in the plane of an annular diaphragm (4). The light passing through the diaphragm slit is detected by 12 PMs with quartz windows. The signals from 2, 3, 4 and 6 PMs are summed to form 6-, 4-, 3- and 2-fold coincidences . The counters were filled with helium. The advantages of helium are its low dispersion and a good transparency in the far ultraviolet. The lengths of the counters are 5 and 10 m. The 5 m counter detects Cherenkov light emitted at 23 mrad with respect to the counter axis, for the 10 m counter this angle is 12 mrad . Fig. 2 shows the pressure curve for the 5 m counter placed in the 40 GeV/c negative beam. With a slit

width of 1 .6 mm the velocity resolution is equal to 6 x 10 -6 for 6-fold coincidences. Fig. 3 illustrates 45 GeV/c particle identification with the 10 m counter. For 6-fold coincidences Af3 = 2 x 10 -6 . The counter inefficiency is mainly determined by the angular spread of the beam . The background level for 6-fold coincidences is 10 -5-10 -6 . It is independent of the slit width and is thus mainly due to accidentals and PM noise but not to helium scintillation and Cherenkov light of 8electrons. Differential counters [2] used in the experiments at FNAL are shown in fig. 4. The counter lengths are 10 and 13 m, the radiating gas is helium or nitrogen. The Cherenkov angle is 7-10 mrad . The Cherenkov light is focused by a spherical mirror (1). The light passes through an annulus (2) and is collected by a mirror (3) onto one (10 m counter) or two (13 m counter) PMs (4), whose signals are in coincidence with the beam monitor. The Cherenkov light from unwanted particles is de-

Fig. 1. Schematic cross-section of the differential counter [11 : (1) spherical mirror, with radius P. =10286 mm for 0 = 23 mrad and R = 20140 mm for 0 =12 mrad, (2) .steel tube, (3) diaphragm, (4) annular diaphragm, (5) quartz window, (6) collecting mirror, (7) XP1023 . 0168-9002/86/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

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S. P. Denisou / Cherenkov counters for high energy particle identification

8

9

10

2 .0 PHe , atm

2.5

3.0

3.5

40 PHe' at m

Fig. 2. Efficiency dependence of a 5 m counter on helium pressure for 3-, 4- and 6-fold coincidences . The annular diaphragm slit width is 1.6 mm . Particle momentum is 40 GeV/c. Background level for 6-fold coincidences is less than 10 -5 .

Fig. 3. Pressure curves for 4- and 6- fold coincidences of 10 m counter placed m a 45 GeV/c beam . Background level for 6-fold coincidences is 3 X 10 -7 . Diaphragm slit width is 3 mm .

tected by an "anticomcidence" PM (5). The PMs used are of RCA C31000 type with quartz windows and high efficiency first dynodes. The average number of photoelectrons is Ne = No L0 2, where No = 150 cm -1 and L is the counter length . Fig. 5 gives the counting rate dependence for 100 GeV/c particles on helium pressure in the 13 m counter (B = 10 mrad). A low background to these counters is achieved by the efficient detection of the unwanted particles. The first differential counters with anticoincidence channels are described in ref. [3]. Differential Cherenkov counters are widely used for particle identification in hadron beams at existing accelerators. Can they be used to tag particles at future accelerators? As has been shown in ref. [4], the velocity resolution of differential counters may be very high if the Cherenkov radiation angle is chosen small enough . In order to obtain sufficient light intensity I it is necessary to increase the counter length (at small angles I - L0 2). Let us calculate, for instance, the parameters

of a counter to select kaons in a 1 .5 TeV/c beam . The velocity difference of 7T and K is 5 X 10 -8 for this momentum. The counter resolution is equal to Aß = tan 0 - dB + An . It will mainly be determined by the dispersion do of the radiator . Let do=0 .5 4ßK,r and let us choose helium as the radiating gas. If the counter is sensitive in the wavelength range of 200-600 nm, then An = 0.05(n - 1) and n - 1 = 5 X 10 -7 . For such a value for n - 1 the helium pressure is equal to 1 .4 X 10 -2 and the Cherenkov angle is 0 = [2(n - 1) - (m K/p)2]i/2 = 1 mrad . With a light collection efficiency of 0.75 and a PM quantum yield of 0.3, the number of photoelectrons is equal to Ne = 3.1 X 10-2 L[m], i.e . for L = 200 m the counter efficiency will be high enough . The amount of helium in the counter is 5 X 10 -2 g/cm2 and absorption and multiple scattering will be negligible . It is convenient to make the counter from several independent sections . For the diffraction not to influence the counter resolution, the length LS of each section should III . ELEMENTARY PARTICLE PHYSICS

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S.P. Denisoo / Cherenkov counters for high energy particle identification

Fig. 4. Conceptual design for the differential counter [2]: (1) focusing spherical mirror, (2) mirror diaphragm, (3) light collection mirror, (4) coincidence PM, (5) veto PM . satisfy the following condition :_ /Ls << 5 X 10 -8 . At )` = 400 nm, LS >> 8 m. The counter may consist of four units, each 50 m long . The mean radius of the Cherenkov ring image in the focal plane of the spherical mirror will be LSO = 5 cm . The efficiency of each section is 0.8 assuming 100% detection probability for a single photoelectron . The counter angular acceptance is 25 grad (p i - 40 MeV/c) . To reduce the background it will be useful to detect unwanted particles as was done in ref. [2]. In conclusion to this section the two following remarks should be made concerning the counter for the TeV energy range. (1) Widening the spectrum range of the counter will not allow a considerable reduction of its length, since in this case 4 n increases and the Cherenkov angle must be decreased. (2) A 200 m counter may seem very large. However, one should bear in mind that the 1.5 TeV/c beams will be more than 1 km long [5]. In such a beam the 200 m

Fig. 5. Identification of 100 GeV/c particles with 13 m differential counter [2]. Cherenkov angle is 10 mrad.

Fig. 6. Schematic cross section of the differential counter for particle identification in a 1 .5 TeV/c beam : (1) spherical mirror, (2) flat mirror, (3) optics for chromatic correction, (4) annular mirror diaphragm, (5) coincidence PM, (6) veto PM . counter would not seem so huge . Still, in some particular cases (hyperon beam, etc.) shorter counters will be needed . This may be achieved by using conical optics for chromatic correction [6] (fig . 6) *. In this case the angle O may almost be doubled and the counter length reduced down to 50 m without decreasing its angular acceptance and efficiency . Note that a counter with chromatic correction may be made even shorter, if its spectrum range be extended below 200 nm. 2. Threshold Cherenkov counters Threshold counters detect particles with velocities above the threshold value ßi = 1/n . Conventionally an inclined spherical mirror is used to collect the light onto the PM (fig . 7) . Sometimes quartz lenses [8] or mirror light guides (conical or Winston type [9]) are used for light collection . The main advantage of inclined mirrors is their minimum light loss . In threshold counters either quartz-face PMs or glass PMs with a thin layer of wavelength shifter on the photocathode are used [10] . P-terphenyl is often used as a shifter. Recently it has been proposed to use photoionization detectors [11,12] instead of PMs. They have a high photon detection efficiency in the far ultraviolet and may work in strong magnetic fields . The threshold curve e(n) i.e . the dependence of counter efficiency on the refractive index (or pressure) of the radiating gas, is the main characteristic of a threshold counter (fig . 8) . The velocity difference (or equivalent difference in n), which corresponds to a change of e(n) from 0 to 0.63, determines the counter * Counters with chromatic correction are considered in another report at this seminar.

S.P. Denisov / Cherenkov counters for high energy particle identification

13 9

Fig. 7. The design of a threshold counter [7]: (1) steel tube, (2) diaphragm, (3) spherical mirror, (4) mylar window, (5) 56 UVP, (6) quartz window . resolution . The background level (probability to detect particles with velocity ß < /3,) and efficiency on the plateau are also important characteristics of the counter. For the counter shown in fig. 7 the resolution is equal to 6 .5 x 10 -6 , the background level is 3 x 10 -° , the efficiency on the plateau is 0.9999994. A low background level and a high efficiency make it possible to use only one counter in anticoincidence with the beam monitor, to identify not only light but heavy particles as well, even if their contamination is only 10 -3 (fig . 9) . The characteristics of the counter may be improved by pulse height analysis [13] . The quality of threshold counters may also be characterized by the value of No , which depends on the counter spectrum range, the light absorption in the radiator and optical system and the photon detection

efficiency . Good counters with glass PMs have No = 50 cm -1 . The same value holds for counters with photoionization detectors. For quartz-face or glass PMs with wavelength shifter No is about 150 cm -1 . The record value for No is 260 cm -1 [14] . Even this record figure is not a limit. It may be increased by a factor of 2 or 3 if a method for efficient detection of Cherenkov light in the wavelength range from - 100 to - 600 rim is found. To separate particles with P = 1.5 TeV/c a threshold counter of hundreds of meters is needed. For instance, for pion identification with 90% efficiency (Ne = 2.3) the counter with No = 260 cm -t will be L = N,/NO0 2 = 1 km long (02 = 24ßK ) . When filling the counter with helium the pressure will be 1.6 x 10 -3 aim. A counter for K and p separation in this beam may be three times shorter.

PH atm e Fig. 8. Pressure curves of a threshold counter [7] for 50 GeV/c peons. Figures near the curves indicate the PM high voltage. III. ELEMENTARY PARTICLE PHYSICS

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S P. Denisoo / Cherenkov counters for high energy particle identification

TL-

w 1

to-, K-

10 3

10

4 0

05

10

1,5

20

25

Pc~atrn

Fig. 9. Particle selection m a 20 GeV/c beam using a threshold counter [7]. e=MC/M, where M is the counting rate of the beam monitor. Background level above the antiproton threshold is 10 -6 .

Fig. 10 . Conceptual design of the multicell Cherenkov counter [15] .

We have considered threshold counters for particle identification to monochromatic beams. Wide aperture Cherenkov counters [15,16] are widely used for secondary particle selection. Fig. 10 illustrates the design of a counter of this type . The optical system of the counter consists of eight spherical mirrors, each of them focuses the Cherenkov light onto its own PM . In front of the PM there is a mirror light guide. In the case of particles going out of the small region (target, colliding beams intersection) elliptical mirrors may be used instead of spherical ones. One of the problems in the construction of such counters is the manufacturing of a large number of thin mirrors [17] . The performance of multicell counters is close to that described above. 3. Application of Cherenkov counters in accelerator experiments Cherenkov counters are used in experiments to solve the following main problems : 1) Particle identification in primary beams. 2) Search for new stable particles and antiparticles with single-arm spectrometers . 3) Particle identification in double-arm spectrometers . 4) Particle identification in wide-aperture spectrometers, including colliding beam facilities . Below, some examples of using Cherenkov counters in real experiments are listed . Counters [1,7] have been used at IHEP to measure the total cross sections . In these experiments an increase of the total cross sections has been observed for the first time [18] . Classical examples of using Cherenkov counters in the search for new particles and antiparticles are the experiments in which the antiproton [19], the anttdeuteron [20], antihelium-3 [21] and antitritium [221 have been discovered . The layout of the single-arm spectrometer used at CERN to study particle production and search for new particles is shown in fig. 11 . In this spectrometer two differential Cherenkov counters of the DISC type with chromatic correction and three threshold counters were used . The quality of particle identification is shown in fig. 11 . An important role was played by Cherenkov counters to the discovery of CP-violation [24], of J/4, particles and T resonances . All these experiments were carried out with double-arm spectrometers. Many of the wide aperture spectrometers (SIGMA (IHEP), tagged photon spectrometer (FNAL), MPS II (BNL), Omega (CERN), EMC (CERN), LASS (SLAG)) and colliding beam facilities (MD-I (Novosibirsk), DELCO (SLAG), HRS (SLAG), TASSO (DESY), PLUTO (DESY)) are equipped with Cherenkov counter hodoscopes . An unusual application of Cherenkov counters has

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E

10 0

rp

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+30 GeV/c

d

3 e

TT 2500

3000

i1 1 Tll l itil

3500

4500 n

Fig. 11 . Layout of a single-arm spectrometer [23] and the counting rate versus index of refraction of the radiating gas (SF6) in differential Cherenkov counters DISC-1 and DISC-2 . Beam momentum : 30 GeV/c (S,A - scintillation counters, C threshold Cherenkov counters, Q - quadrupoles, B - bending magnets) . been demonstrated in an experiment on the search for monopoles at the IHEP accelerator [27] . This experi-

ment exploited the fact that the Cherenkov light polarization for monopoles differs from the polarization of particles with an electrical charge . The list of experiments and spectrometers using Cherenkov counters may be continued. However, the examples given above are sufficient to show the importance of Cherenkov counters in high energy physics.

4. Conclusion Gas Cherenkov counters are one of the main devices in high energy physics. They are widely used in experiments both at fixed target accelerators and on colliding beams. At present experimenalists have no other tool to

identify ultrarelativistic particles which can match their good time resolution, high velocity resolution, high ef-

ficiency and low background level . No doubt Cherenkov counters will remain one of the basic tools even for considerably higher energies than those available at modern accelerators .

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