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Progress in Cherenkov ring imaging
Nuclear Instruments and Methods 216 (1983) 79-91 North-Holland Publishing Company
79
P R O G R E S S IN CHERENKOV RING IMAGING Part 2: Identification of charged hadrons at 200 G e V / c Ph. M A N G E O T ,
G. C O U T R A K O N
*, J . R . H U B B A R D ,
J. M U L L I I ~ , J. T I C H I T a n d A. Z A D R A
DPhPE, CEN Saclay, Gif -sur- Yvette, France R. B O U C L I E R ,
G. CHARPAK,
J. M I L L I O N ,
A. P E I S E R T , J.C. S A N T I A R D
a n d F. S A U L I
CERN, Geneva, Switzerland
C.N. B R O W N FNAL, Batavia, Illinois, USA D. FINLEY,
H. G L A S S , J. K I R Z a n d R . L . M c C A R T H Y
SUNY, Stony Brook, New York, USA Received 11 April 1983
We have used a ring-imaging Cherenkov detector to separate Tr's, K's, and antiprotons in a 200 GeV/c beam at Fermilab. This device was built as a prototype for a large-aperture counter now in operation in Fermilab experiment E605. The radiator consisted of 8 m of atmospheric-pressure helium gas. The photon detector was a multistep proportional chamber. Cherenkov photons near 8 eV were detected by photoionization of triethylamine (TEA) vapor in the chamber. An average of 2.5 to 2.7 Cherenkov photons were observed per event, corresponding to a figure of merit NO= 45 per cm. A single-photon radius uncertainty of 0.47 mm was obtained with a helium/TEA/CH 4 gas mixture in the photon detector. The rms uncertainty in the determination of the Cherenkov angle was AOc/O ..... = 0.006, corresponding to one-standard-deviation ~r/K separation at 500 GeV/c. At 200 GeV/c, the particle identification efficiency in a beam containing 95.2% ~r-, 4.3% K-, and 0.5% antiprotons was 92% for the ~r's, 83% for the K's, and 90% for the antiprotons.
1. Introduction A ring-imaging Cherenkov detector has been used to separate ~r , K - , and antiprotons in a 200 G e V / c beam at Fermilab. The ring-image was formed in the entrance-plane of the radiator by an 8 m focal-length spherical mirror. The p h o t o n detector was a 20 × 20 cm 2 multistep proportional chamber containing triethylamine (TEA) vapor as the photo-sensitive component. Some details of the construction, operation, and perform a n c e of the multistep chamber as a detector of photons in the vacuum ultraviolet (VUV) region were presented in Part 1 of this paper [1].
2. Cherenkov ring-imaging technique Cherenkov ring-imaging was first p r o p o s e d by Roberts [2] as a technique for accurate measurement of particle velocities and directions. When a charged par* Also at SUNY. Stony Brook, USA. 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
ticle passes through a medium of index of refraction n at a velocity (v = fic) greater than the velocity ( c / n ) of light in the same medium, p h o t o n s are emitted at an angle O c whose cosine is the ratio of the velocity of the light to that of the charged particle: cos 0 C = l / f i n .
(1)
Photons emitted at various points along a particle's straight-line trajectory, when reflected from a spherical mirror of radius R, are focused onto a circle of radius r =ftan
~c
(2)
in the focal plane, a distance f = R / 2 from the mirror. If the particle is inclined at an angle O D to the optical axis, the center of the circle will be displaced a distance Aq = f t a n @D
(3)
from the axis. Thus, measurement of the center of the Cherenkov circle determines the particle direction. Inversely, external measurement of the particle direction determines the center of the Cherenkov circle, so that each detected p h o t o n gives a measure of the Cherenkov
80
Ph. Mangeot et a L / Progress in Cherenkov ring imaging, 11
ring radius, and therefore of the particle velocity. In our application the particle momentum and direction are both determined externally, so a single detected photon yields a measurement of the particle mass.
3. Design considerations The work reported here was undertaken to develop a prototype for a detector to identify ~r's, K's, and protons of momentum 100 to 400 G e V / c for Fermilab experiment E605. This is a high-luminosity experiment designed to study high-P t single particles and particle pairs. The Cherenkov counter is downstream of two momentum-analyzing magnets. It has a 2 × 2 m 2 entrance plane and accepts particles with wide angular dispersion (120 mrad × 60 mrad). The detector must have large acceptance, good velocity resolution (~'ma~ --- 800), and reasonably good time resolution (_< 1/~s), but it is only expected to identify at most two particles per event. 3.1. Choice of the photo-detection technique Cherenkov ring-imaging was first demonstrated experimentally using image-intensifiers operating at visible wavelengths [3-8]. This approach is unsuitable for our purposes because of the limited acceptance and high cost of these devices. The detector surface can be reduced by optical subtraction, a s in the spot-focusing Cherenkov counter [9,10], but this technique is limited to beams with small divergence. In order to obtain a large detection surface compatible with our high rates, we use a proportional wire chamber containing photo-sensitive vapor, as suggested by S6guinot and Ypsilantis [11]. Difficulties were encountered in the development of this technique because the known compounds with reasonable vapor pressures are sensitive only in the vacuum ultraviolet (VUV) region. This fact sharply limits the choice of radiators and windows, since most materials absorb in the VUV region. Furthermore, chromatic dispersion increases for these higher photon energies. These problems have been brought under control by the development of detectors using vapors with very low emission potentials. The further problem of efficient detection and localization of the single photoelectrons produced has been resolved by use of a multistep structure for the proportional chamber, as described in ref. 1. 3.2. Choice of the photo-sensitive vapor The early work on Cherenkov ring-imaging with gaseous detectors [12-14] used acetone (ionization potential 9.69 eV) or benzene (9.24 eV) as the photosensitive vapor. Lithium fluoride crystals, which are highly
hygroscopic and difficult to handle, were required for the windows. The introduction of triethylamine (TEA) vapor, with a photo-ionization threshold at 7.5 eV, provided considerable simplification. Crystals of CaF 2 or MgF2, transparent to about 10 eV and more stable than LiF, can be used with TEA. The vapor pressure of T E A is sufficiently high (40 Torr at 15°C) that photons are completely absorbed in a few mm of gas. Considerable effort has been invested in the development of photon detectors using TEA vapor [15-18]. Noble gases are normally used as radiators in order to minimize the chromatic dispersion. We chose TEA vapor for our detector, even though several compounds with much lower ionization potentials are now available [19-22]. These new compounds can be used with quartz windows (transparency cut-off around 7.5 eV for UV grade), instead of the fragile fluoride crystals required for TEA, but they have the disadvantage of very low vapor pressures and correspondingly long absorption lengths. Tetrakis dimethylamino-ethylene (TMAE), for example, has a photoionization threshold of 5.4 eV, but a vapor pressure of only 0.35 Tort and an absorption length [23] of about 20 mm at 20°C. Such a long absorption length implies a timing jitter of a /~s or more and significant loss in spatial resolution due to parallax. With TEA vapor at 50% saturation at 15°C, the absorption length is only 1 mm [16], and both timing jitter and parallax are acceptably small. 3.3. Choice of the detector gas mixture Once the photosensitive vapor has been selected, we must choose an appropriate carrier gas. Usually a noble gas is used to avoid photon capture by the carrier gas. We have used helium as the principal component of our gas mixture, rather than the more common argon, for two reasons: first, because charged particles ionize much less in helium, so we avoid large pulses which can mask the smaller photon pulses and which can lead to chamber instabilities. Second, because the excitation and ionization levels are much higher in helium than in argon, and we can obtain higher gains before breakdown occurs. We use TEA at 50% saturation to avoid condensation in the photon chamber. The TEA temperature was 15°C. Thus, the first gas mixture used was He(97.5%) + TEA(2.5%). A second gas mixture was formed by adding methane (CH4) to the above mixture, yielding He(90%) + TEA(2.5%) + CH4 (7.5%). The methane was added to improve resolution by the absorption of high-energy photons, as discussed in section 6.2. (The transmission of 7.5% methane in our gas
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, II 100
I
100
--.. CH 4
"
\ x
Helium (n2-1)
75
75
Ca g 2 - ' " - . . . "
81
T E A [16,22], are shown in fig. 1. Fig. 2 shows the C h e r e n k o v angle for various particle types as a function of m o m e n t u m . The C h e r e n k o v threshold for protons is a r o u n d 100 G e V / c , a n d we can hope to separate ¢r's a n d K's out to 400 G e V / c .
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4. Experimental set-up
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Fig. 1. The helium index of refraction [26] at STP, the quantum efficiency of TEA [16,22], the transmission of CaF 2, and the transmission of the methane in our H e / T E A / C H 4 gas mixture [24,25], as a function of photon energy.
Our prototype particle identifier is shown schematically in fig. 3. The radiator vessel was a stainless-steel tube with a spherical mirror m o u n t e d at one end and the p h o t o n detector at the other end. The spherical mirror, at the d o w n s t r e a m end, was accessible through a bolted cover in order to allow the proper alignment. A C a F 2 window and the multistep p h o t o n detector were m o u n t e d with conventional O-ring seals at the upstream end. A control tube m o u n t e d above the radiator tube could be used to measure the transmission of the radiator gas. Drift c h a m b e r s measured the particle trajectory, a n d scintillation counters provided the trigger. 4.1. Radiator
mixture, calculated from measured absorption coefficients [24,25], is shown in fig. 1.) 3.4. Choice o f radiator gas
Helium was chosen for the radiator gas in order to limit chromatic dispersion a n d allow particle identification at high energy. Atmospheric-pressure helium at 0 ° C (STP) has an index of refraction of 1.0000386 for p h o t o n s of 8.6 eV (near the peak of the T E A q u a n t u m efficiency). The energy d e p e n d e n c e of the index of refraction [26], as well as the q u a n t u m efficiency of
The radiator tube was 8 m long a n d 30 cm in diameter. G a s purity was an i m p o r t a n t consideration in the design, because the p h o t o n absorption cross sections of oxygen a n d water are large in our p h o t o n energy range. The stainless-steel vessel was cleaned, then v a c u u m - b a k e d at 100°C before the initial use at C E R N . The ~adiator gas was o b t a i n e d as boil-off from a liquid helium dewar. G a s flow was m a i n t a i n e d at one volume change per hour during most data runs. The gas flowed out through the control tube to the recovery system. 4.2. Control tube
9
lr
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E v LU
8
ZT/, 7
> 0 x," W
kU I
6 100
I 200
[ 300
400
PARTICLE MOMENTUM(GeV)
Fig. 2. Cherenkov angle for different momenta and particle types. The refractive index of helium used here was 1.0000386 at STP for 8.6 eV photons [26].
The control tube was 6 m long a n d 10 cm in diameter. It was used to m o n i t o r continuously the outflowing gas purity by measuring its transmission for V U V photons. A V U V p h o t o n source was m o u n t e d at one end of this tube, a n d a solar-blind photomultiplier with a M g F 2 window at the other end. The p h o t o n source, described in detail in ref. l, used secondary emission in k r y p t o n [27], which matches closely the T E A q u a n t u m efficiency response. The photomultiplier response was measured while the helium flowed through the tube. T h e n the control tube was isolated from the radiator and evacuated, a n d the p h o t o m u l t i p l i e r response was measured again. The ratio of the response with a n d without gas in the tube gave a reliable m e a s u r e m e n t of the transmision of the radiator gas: 87% during the run without methane, 81% during the r u n with m e t h a n e in the p h o t o n chamber.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, I1
82 UV S o u r c e
PM
[
,,
Beam
Solar-blind
C o n t r o l Tube
}
'I OaF2
If" I
Helium
r
Drift Chambers
R a d i a t o r Tube (L=8m)
I I I I I I I I I I
I I [ I I
Drift Chambers
Mirror (f=8m)
Photon Detector 20x2Ocm 2 Multi-step Proportional Chamber
I I I I I
Fig. 3. Experimental set-up.
4.3. Spherical mirror T h e mirror was m a n u f a c t u r e d by v a c u u m depositon over a pyrex substrate of a reflecting layer of a l u m i n u m 822 A thick, coated with 165 A of MgF~ to e n h a n c e reflectivity and to avoid oxidation. Its reflectivity (fig. 4) was measured to be (70_+ 4)% over the region of high T E A q u a n t u m efficiency. The focal length was (794 _+ 1.2) cm, so the mirror was positioned 794 cm from the conversion gap of the p h o t o n detector.
the energy range 7.5 to 10 eV. For the H e / T E A / C H 4 run, the average transmission was also weighted by the effective transmission of the methane, yielding 65% for this run. The crystals were m o u n t e d on a brass frame to m a t c h the Caf 2 thermal expansion (for details, see ref. 1). The 1 cm wide arms formed a dead-space for photons (an additional loss of 9%), b u t not for b e a m tracks. T h e effective window transparency was thus 0.64 × 0.91 = 58% for the H e / T E A run, and 0.65 × 0.91 = 59% for the H e / T E A / C H 4 run.
4.4. Calcium fluoride window
4.5. Multistep proportional chamber
A square array of four 4 m m thick C a F z crystals separated the radiator from the detector. The transmission of the CaF2, measured with a m o n o c h r o m a t o r , is shown in fig. 1. The average transmission for the H e / T E A run was 64%, obtained by weighting the measured transmission by the q u a n t u m response of T E A in
The 20 x 20 cm 2 multistep proportional c h a m b e r is s h o w n schematically in fig. 5. Cherenkov p h o t o n s pass through the C a F 2 window a n d ionize T E A molecules in the conversion gap (C), producing single photoelectrons. T h e photoelectrons are drifted into the preamplification gap (PA), where each photoelectron initiates an
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Fig. 4. Reflectivity versus photon energy. Measurements were made with an ultraviolet monochromator. The circles correspond to source intensity measurements made before inserting the mirror and crosses correspond to source intensity measurements made after the mirror was withdrawn, indicating some intensity fluctuations.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, I1 PWC
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gas mixtures. With H e / T E A the PA gap field was below 600 V / r a m , and the PWC a n o d e - c a t h o d e voltage difference was 1100 V. With H e / T E A / C H 4 the PA gap required 1000 V / r a m , and the PWC operated at 2000 V. We experienced some difficulties with chamber stability, and the higher voltages necessary with H e / T E A / C H 4 increased these difficulties. Further details of the construction and performance of the multistep proportional chamber used as a photon detector are given in ref. 1.
x
Fig. 5. Photon detector structure.
avalanche in the strong electric field• A fraction of the electrons in each avalanche, typically about 103 , emerge into the drift space and are transferred to the proportional wire chamber (PWC). A second amplification takes place around the anode wires, bringing the total charge to about 107 , which permits easy electronic detection. The conversion and amplification gaps were formed of three stainless-steel meshes of 50/~m diameter wires, with a 500 t~m pitch• The first mesh was in contact with the CaF 2 window. The optical transparency of this mesh was 81%. Some photoelectrons produced in the conversion gap are lost because of electric field distortions near the 100 # m thick meshes. Conversions which take place in the first 50 btm or so of the detector can be lost due to field distortions in that region. The mean free path of photons in 2.5% T E A is 1 ram, implying a 5% loss from these first 50/~m. Furthermore, some photoelectrons fail to enter the preamplification gap, and are lost, collected on the second mesh. We estimate a loss of 8% of the photoelectrons at the second mesh for our operating conditions. The conversion gap was 6 mm thick (6 mean free paths), so there was no loss due to photon escape. The effective transmission of the conversion gap was therefore 0.18 × 0.95 × 0.92 = 71%. The PWC half-gap width was 3.2 mm. This narrow gap was chosen to reduce the width of the cathode pulses, and thereby improve the multiple-hit resolution. The cathode planes, made with 50 ~ m diameter, ½ram pitch wires, were perpendicular to each other and parallel to the detector frames, defining two orthogonal coordinates X and Y. The anode plane was implemented with 20 /~m wires, 2 mm apart, mounted at a 45 ° angle with respect to the other electrodes, and provided an inclined coordinate U. All three planes were read out every 2 mm into analog-to-digital converters (ADCs). There were 96 channels per plane which covered most of the active surface of the detector. The multistep proportional chamber was operated with two gas mixtures, H e / T E A and H e / T E A / C H 4 , as discussed in section 3.3. The drift fields in the conversion and transfer gaps were 100 V / m m for both
4.6. Electronics
Signals from the 96 anode wires and 2 × 96 cathode strips were amplified on the chamber by discretecomponent amplifiers with a gain of 125 m V / p C , a time constant of 300 ns, and a noise level equivalent to less than 5 × 104 electrons. The pulses were transmitted in differential mode to the control room (a distance of 30 m), where they were referenced to ground and further amplified ( × 6 ) by M E C L 10115 receivers. These receivers had trimmers which allowed a channel-bychannel adjustment of the gain. The signals were fed to charge-sensitive A D C s (LeCroy model 2249) with input impedance of 50 $2. Standard N I M logic units were used to trigger the system and provide the gates for the ADCs. A 75 ns gate was used, delayed by 800 ns to compensate the drift time of the electrons. The digitizing time of the A D C s was 60/~s. 4. 7. Particle beams
The complete prototype particle identifier was first tested in a secondary electron beam (about 8 G e V / c momentum) at C E R N . This run provided information on the detection efficiency and multihit resolution, but the radius resolution was poor because of multiple Coulomb scattering in the windows and in the gas. A second test run, with a hadron beam, was necessary to demonstrate the particle-identification possibilities of the device. This run was performed in the Meson Laboratory at Fermilab with an unseparated 200 G e V / c negative beam. Results from this second run are presented below. 4.8. Drift chambers
Two sets of high-accuracy drift chambers, before and after the radiator, were used to measure the trajectory of the incident particles. A third set of drift chambers was installed to determine drift-chamber resolution. Each of the six planes of drift chambers contained four wires, one wire every 5 cm, with one T D C per wire. The drift chambers were positioned so that the beam was measured on a single wire of each chamber•
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, 11
84
4.9. Data acquisition A coincidence of four scintillation counters, two before and two after the C h e r e n k o v counter, defined a particle event and provided the trigger for readout of the drift chambers and the multistep proportional chamber. All data were read from standard C A M A C units by a Hewlett-Packard mini-computer. The raw data recorded on magnetic tape included 24 T D C values from the drift c h a m b e r s and 288 A D C values from the p h o t o n chamber. The data acquisition rate was computer-limited to 40 events per second. The quality of the data was controlled by an on-line program that included a simple reconstruction of individual events.
planes for the two runs are shown in fig. 7. The value of the pulse height for each entry in the histogram is the sum of the A D C contents of each channel in a cluster of cathode strips above threshold (20 counts) multiplied by a conversion factor 2.2 × l0 -4 pC per A D C count. The pulse-height threshold for the cluster was 200 counts, corresponding to a m i n i m u m detectable signal of 3 x 105 electrons. Fig. 8 shows the correlation in cathode X a n d Y pulse heights for single photo-electrons. The cathode X pulse heights were 20% larger than the cathode Y pulse heights,
~--T--~ 400
a
~]j~
He/TEA CATHODE
b He/TEA CATHODE Y
5. Data analysis
30O
5.1. Pulse-height distributions w
The A D C pulse-height information from a typical event is shown infig. 6. The cathodes defined the orthogonal coordinates X and Y, and the anodes provided the inclined coordinate U, with U = ( X + Y ) / ~ / 2 . The preamplified charge from a single photoelectron usually fell on one or two adjacent wires (see ref. l), while the induced charge on each cathode plane was spread over six or seven 2 m m channels. The pulse-height spectra on the cathode X and Y
~ 200 E
100
.4
.4
.8
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Ax(PC) Fig. 6. An example of photon reconstruction for an event with four photons and a beam track. The beam track is not at the center of the Cherenkov ring, which is determined by the particle direction. Two ghost points appear, but they are easily eliminated because all three coordinates are required for other, real points.
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Fig. 7. Cathode X and Y pulse height distributions for He/TEA (a) and (b) and H e / T E A / C H 4 (c) and (d). The amplitude is the sum of the ADC values for a cluster of channels above threshold. The conversion from ADC value to picocoulombs is discussed in the text. Only clean photon candidates were included in these plots.
85
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, I1 i
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Fig. 8. Correlation between the cathode X and Y amplitudes. (a) Low statistics correlation plot A x versus A v for unambiguous photon hits. (b) High statistics distribution showing the cathode X amplitudes to be 20% larger than the cathode Y amplitudes.
because the preamptified charge arrived at the a n o d e wire closer to c a t h o d e X than to cathode Y.
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Fig. 9. Pulse height distribution of individual cathode strips. Here ax~ is the amplitude of the pulse from the strip with coordinate x i, am. . is the maximum pulse height in the group of strips, and X is the coordinate calculated for the group of strips by the center of gravity method.
5.2. C o o r d i n a t e calculation
The pulse-height i n f o r m a t i o n was used to calculate coordinates by the center-of-gravity m e t h o d for each group of wires hit. For the i t h group of wires in the X plane
x,=Exka~,/Ea~,, k
k
Axi = E axk, k
where axk is the A D C pulse height on the wire or strip at position x k, a n d X i a n d Xx~ are the center-of-gravity coordinate a n d its amplitude. The distribution of c a t h o d e pulse heights a b o u t the calculated coordinate position is shown in fig. 9. The m i n i m u m separation for which two coordinates of equal pulse height can be resolved is 6 mm. F o r the anodes, the m i n i m u m separation is 4 mm. W h e n multiple peaks were a p p a r e n t in a given group of wires, multiple coordinates were calculated by a r o u g h partition of the measured pulse heights. Some coordinates were missed or poorly determined at this stage, because of overlapping pulse-height distributions. 5.3. P o i n t reconstruction
A n initial list of point candidates was o b t a i n e d by selecting all coordinate triplets (Xi, ~ , Uk) with good spatial correlation, unless their amplitudes differed by a very large factor. Explicitly, we selected triplets satisfy-
ing the following three requirements: (a) correlated in space to better than 2 mm, (b) cathode amplitudes equal within a factor 4, a n d (c) cathode a n d anode amplitudes equal within a factor 9. Some points did not satisfy the above criteria because of unresolved coordinates. In order to recover these points, a second class of candidates was defined as follows: F o r each coordinate pair ( X i, Yj), (X,, ~ ) , or ( ~ , Uj), the expected value of the third coordinate U~, Y~, or X k was calculated; if the contents of the A D C s near the calculated coordinate were large enough to m a t c h the amplitudes of the original pair, then a new p o i n t candidate was created. A final list of points was selected by removing poorer candidates from this c o m b i n e d list until a " b e s t ' ' fit to the overall event was found. C a n d i d a t e points were most likely to be eliminated if all three coordinates were used elsewhere in the candidate list, a n d if the amplitudes of the three coordinates were very different. W h e n the final list was established, final coordinates were determined as follows: (1) A coordinate was considered unambiguous, and its measured value was retained, if it was used in a single point in the final list. (2) If a point h a d only one ambiguous coordinate, that coordinate was recalculated from the two u n a m b i g u o u s coordinates. (3) If more than one coordinate was ambiguous, we a t t e m p t e d to recalculate these ambiguous coordinates by removing from the center-of-gravity calculation the c o n t r i b u t i o n from previously resolved points.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, 11
86
5. 4. Recalculation of ambiguous coordinates
5.5. ,~r'stem resolution
This last technique can be illustrated by considering the specific example in fig. 10. The X and Y coordinates of p h o t o n 2 of this event are b o t h ambiguous, so the position of that p h o t o n is poorly determined. But, the ambiguous coordinate X t and its amplitude A ~ can be expressed as the sum of the contributions from p h o t o n s 1 a n d 2:
The drift c h a m b e r measurements of the angles of the particle trajectory were used to determine the center of the p h o t o n ring. For each point candidate, a ring radius was calculated (the distance from the ring center). Single-photon radii in the region of the ~r peak are plotted in fig. 11 for the two data runs. The distribution
X]-
X(I)Ax(1
) + X(2)A~(2)
A,,=Ax(1)+A~(2
150
).
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,,>,
U(1)
-
r(l)
=/2U,
Ax(l)--Av(1)=Avi, Therefore, the X amplitude due to p h o t o n 2, A ~(2), can be calculated from the coordinates amplitudes:
_
Calculated Chromatic Dispersion He/TEA
"6
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90-
60-
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N o w these values can be used to calculate t h e X coordinate of p h o t o n 2: x(2) -
0 66
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A., (2)
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We calculate Y(2) in a similar way. Then we verify that the triplet [X(2), Y(2), U2] is well-correlated in space before accepting this point for further calculations.
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i
g
Y,.
-
Furthermore, the X and Y amplitudes due to this photon are closely correlated, so
X,A.,,
I
M e a s u r e d Distribution
The X coordinate of p h o t o n 1, X(1), can be calculated from the u n a m b i g u o u s coordinate pair (Y~, U~)
=/2
I
- - !
A~(1) +A~(2)
Z
He/TE A//C H4
t
I
90
,
E
//
/
,/
I
[
i
t
l
//
./
\ ',;, I I Xl
/
Z
i \ GHOST /
ii i l
/
60
//
®,,/,.,"
30 -
0 _ _ INDIVIDUAL AMPLITUDES
SUMMED AMPLITUDES
Fig. 10. Example of two pulse overlap problem. The circles mark the position of the photon hits. The coordinates were calculated by the center of gravity method and are labeled on the sides. Photon 2 in this figure has an ambiguous coordinate in X and Y.
66
//
I
L
]
67
68
69
-
I 70
b
71
RADIUS (mml
Fig. 11. The measured one photon radius distribution and the expected chromatic dispersion (a) with He/TEA in the chamber, and (b) with H e / T E A / C H 4. The chromatic dispersion was calculated from eq. (10), using the helium index of refraction in fig. 1 and the TEA quantum response in fig. 15.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, II of single-photon radii for all 200 G e V / c 7r's should give the system resolution function. The fwhm of this distrib u t i o n is 1.5 m m for the H e / T E A data a n d 1.1 m m for H e / T E A / C H 4. These distributions are not Gaussian. They are convolutions of the detector resolution with the chromatic dispersion of the helium radiator (see section 6.2). The equivalent rms widths are 0.64 m m a n d 0.47 mm, respectively. 5.6. Photon selection The final point list c o n t a i n e d b e a m tracks as well as p h o t o n candidates. Indeed, the incident particles traversed the multistep c h a m b e r near its center, leaving a trail of ionization. This b e a m track was often detected along with the C h e r e n k o v photons, even though the p h o t o n s reach the c h a m b e r with a 53 ns delay. The b e a m track was located easily by interpolation from its d r i f t - c h a m b e r coordinates. With the b e a m track removed, the remaining points were the p h o t o n candidates. Background points were them removed by a clustering technique. F o r a m u l t i - p h o t o n event, all p h o t o n radii were required to lie within 3 m m of each other. W h e n this was not true, the cluster with the most p h o t o n s satisfying this criterion was used. W h e n two ore more clusters h a d the same n u m b e r of photons, the cluster with the smallest range of radii was used. Backg r o u n d points were removed efficiently by the cluster cuts except in the case of z e r o - p h o t o n events, where any b a c k g r o u n d point was interpreted as a single p h o t o n (1% of all events). The distributions of the n u m b e r of p h o t o n s selected by the cluster technique are shown in fig. 12. F o r the H e / T E A run, an average of 2.64 p h o t o n s were found
I
i
I
I
I
I
I
I
3_
N=2.64
per event; for the H E / T E A / C H 4 run, the average was 2.48 p h o t o n s per event. These distributions are not perfectly Poissonian, however, even after shifting b a c k g r o u n d events down to the zero-photon bin. Limited two-photon resolution a n d other program inefficiencies shift m a n y higher-multiplicity events down to lower bins, with a net loss of a b o u t 8% of all photons. The lowest multiplicity bins, which determine the n u m b e r of unidentified events, correspond to the higher averages, 2.9 p h o t o n s per event for H e / T E A a n d 2.7 p h o t o n s p e r e v e n t for H E / T E A / C H 45. 7. Particle identification The distribution of average p h o t o n radii is shown in fig. 13. The a n t i p r o t o n peak is well separated from the c o m b i n e d ~r-K peak. This latter region is shown in more detail in fig. 14. The superior resolving power of the H e / T E A / C H 4 gas mixture is evident here. The rms width of the 7r peak was 0.53 m m for the H e / T E A run, a n d 0.42 m m for the H e / T E A / C H 4 run, for a ~r-K radius difference of 2.64 mm. These widths are smaller t h a n the widths of the single-photon distributions, but larger than expected for a N I/2 improvement. F o r the H e / T E A data run, 1% of the ~r's and 25% of the K's are a m b i g u o u s between the ~ a n d K hypotheses.
f
[
i
104
I
u-lr-
'
I
103
K-
g
I
He/T EA/CH4
He/r E A 25%
i
87
> U.I
=2.48
~ 10 2
g
E
20%
Z
zLU 1 5 %
10
LIJ 10%
5%
4
6
8
110 Number
0
of
-L•
6 Photons /Event
i
k
8
10
Fig. 12. Histogram of number of events versus number of photons per event, with and without C H 4.
50
60
70
80
RADIUS (mm)
Fig. 13. Radius distribution with 1 mm bin size, showing the ~v ~ K and/~ peaks. The radius shown here is the average of all photons in the event.
88
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, lI i
I
I
I
i
I
(including b a c k g r o u n d events which appear in the onep h o t o n bin) account for 6% of the ~r's, 7% of the K's, a n d 9% of the antiprotons. The particle identification efficiency is thus 93% for the ~"s, 68% for the K's, and 91% for the antiprotons. For the H e / T E A / C H 4 run, 7r K ambiguities are only half as frequent. Less than 1% of the ~r's and 9% of the K's are ambiguous between the two hypothesis. Assigning these events to the most p r o b a b l e hypothesis, 4% of the K's and 0.2% of the 7r's are misidentified (0.4% of all events). Z e r o - p h o t o n events account for 7% of the ~r's, 8% of the K's, and 10% of the antiprotons. The identification efficiency is thus 92% for the ~r's, 83% for the K's, and 90% for the antiprotons.
71"
He/TEA ,
103
03
E
K-
LIJ 10 2
o ..el E
5.8. Beam composition 10
I
I
64
I
66
I
I
,
I
~
68 RADIUS (ram) T
I
i
I
a
70
72
[
71-l
He/TEA/CH4
The measured Cherenkov ring radii were 68.3, 65.7, a n d 57.5 m m for ~r-. K - , and antiprotons, respectively. The average index of refraction of the radiator gas was thus 1.0000372, reflecting the atmospheric conditions d u r i n g these data runs. The 200 G e V / c b e a m composition was determined to be 95.2% 7r , 4.3% K , and 0.5% antiprotons.
6. Cherenkov detector system evaluation 6.1. Figure of merit
10 3
Cherenkov systems are often characterized by their figure-of-merit, No, defined such that the n u m b e r of p h o t o n s of C h e r e n k o v angle O c detected from a radiator of length L is
o3
~3
N = NoL sin2Oc.
; ~°2I
Z
,o [
I64
I 66
,
1 68
i
I 70
b 72
R A D I U S (mm)
Fig. 14. Radius distributions with 200 #m bin size, (a) for the He/TEA gas mixture, and (b) for H e / T E A / C H 4. If we assign these events to the most p r o b a b l e hypothesis, then 11% of the K's a n d only 0.4% of the ~r's are misidentified (0.8% of all events). Z e r o - p h o t o n events
T h e experimental figure-of-merit can be calculated directly from the ring-image radius a n d the average number of detected photons. For the H e / T E A run, N = 2.64 detected p h o t o n s per event and the ring radius is 68.3 mm, so N 0 = 45 per cm. For the H e / T E A / C H 4 run, N = 2.48 and r = 68.3 mm, so N o = 42 per cm. The figure-of-merit is supposed to represent the system characteristics i n d e p e n d e n t of the radiator used in a particular experiment. Correcting the above values for the transmission of the radiator gas in each run, we find N O = 4 5 / 0 . 8 7 = 52 per cm for the H e / T E A run, and N o = 42/0.81 = 52 per cm for the H e / T E A / C H 4 run. We expect a larger N 0 value for the H e / T E A data, since the methane in the H e / T E A / C H 4 gas mixture removes some of the photons. The equality of the corrected values is u n d o u b t e d l y due to the lower pulse heights in the H e / T E A run (see fig. 7). In fact, the Y-cathode pulse-height distribution from the H e / T E A / C H 4 run seems to indicate that the c h a m b e r was not completely efficient even for these data.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, H The experimental figure-of-merit can be c o m p a r e d to the value expected on the basis of the physical properties of the detector system, o b t a i n e d by integrating the detector response over the energy spectrum of the C h e r e n k o v emission. Since C h e r e n k o v p h o t o n s are emitted in a flat energy spectrum, the n u m b e r of photons produced in an energy interval d E is given by
d N = KL sin2OcdE,
(4)
where K = a / h c = 370 p h o t o n s per cm per eV. The n u m b e r of p h o t o n s detected is
dN
= KL
sin2e~,
( E)dE,
(5)
where ~ ( E ) is the detection efficiency for p h o t o n s of energy E. If we neglect a small energy d e p e n d e n c e of the C h e r e n k o v angle (see section 6.2), we can express the total n u m b e r of detected p h o t o n s as
N
= KL
sinZO¢fe(E)dE
=
NoL sinZOc.
(6)
T h e efficiency c ( E ) is the product of the efficiencies (at p h o t o n energy E ) of all the c o m p o n e n t s of the system reflectivity of the mirror, transmission of the radiator gas, the C a F 2 window, a n d the conversion gap of the multistep chamber, q u a n t u m efficiency of the TEA, single photoelectron efficiency of the chamber, a n d pattern-recognition efficiency of the analysis program:
e( E )
=
RmTrTwTcQteaCpe,
pr .
89
Table 1 System efficiencies
Radiator gas transmission Mirror reflectivity CaF z crystal transmission CaF 2 window frame First wire mesh transparency Conversions in first 50/~m of detector Detector carrier gas transmission Second wire mesh transfer efficiency Multistep chamber photoelectron efficiency Analysis program efficiency TEA quantum efficiency
fQt¢~(E)dE
He/TEA
He/TEA/CH 4
0.87
0.81
0.70
0.70
0,64
0.65
0.91
0.91
0.81
0.81
0.95
0.95
1.00
0.83
0.92
0.92
0.78
0.93
0.92
0.92
0.68 eV
0.68 eV
0.121 eV
0.114 eV
System efficiency
fc(E)dE
(7)
The main energy d e p e n d e n c e comes from Qtea, the T E A q u a n t u m efficiency. The form of the T E A quantum response [16] is shown in fig. 1. The value at the m a x i m u m has been measured to be Q m a x = 0.65 [22]. We have used the functional dependences in fig. 1 to p e r f o r m the numerical integration indicated by eq. (6). The results are shown in table 1. The average mirror reflectivity ( R m) was 70%. The effective window transmission (Tw) was 58% for the H e / T E A run a n d 59% for the H e / T E A / C H 4 run, as discussed in section 4. The effective transmission of the conversion gap To) was 71% for H e / T E A . T h e average m e t h a n e transmission in the H e / T E A / C H 4 gas mixture was 83% (see fig. 15). This reduced T~ to 59% for the H e / T E A / C H 4 run. The r e m a i n i n g terms - transmission of the radiator gas, photoelectron detection efficiency, a n d pattern-recognition efficiency - are rather d e p e n d e n t on the particular experimental conditions. Ignoring them for the m o m e n t , we find maximum expected values of N O of 73 per cm for H e / T E A a n d 61 per cm for H e / T E A / C H 4. In order to calculate N O for this particular experiment, we must include the measured radiator transmission ( T r = 87% for the H e / T E A r u n a n d 81% for the H e / T E A / C H 4 run), a n d the analysis efficiency (tpr = 92%). Then, if we assume the photoelectron detection efficiency ( t p e ) to be 100%, we find expected values of N o = 58 per cm for the H e / T E A run, a n d N O = 45 per
cm for the H e / T E A / C H 4 run. These calculated figures-of-merit are still larger than the experimental values, especially for the H e / T E A run, indicating that the multistep c h a m b e r was not completely efficient, as already suggested above. W e can use the expected and experimental values of N O to estimate the photoelectron efficiency in the two
I
I
i
I
I
50%
l
I
I
I
He/TEA He/TEA/CH4
4O%
~30% u.I E 20%
2c
(~ 10%
7.5
8.0
8.5
9.0
9.5
10.0
Photon Energy (eV)
Fig. 15. Quantum efficiency curves versus photon energy with and without CH 4 in the multistep chamber. These curves include the TEA quantum efficiency [16,22], the transmission of the CaF 2 crystals, and the absorption in the methane [24,25]. Further corrections can be found in table 1.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, 11
90
runs. For the H e / T E A run, we find ( p e = 4 5 / 5 8 = 78%. For the H e / T E A / C H 4 run, we find cp¢ = 4 2 / 4 5 = 93%. These values seem. compatible with the pulse-height distributions in fig. 7.
6.2. Photon detector spatial resolution The system resolution, determined from the singlep h o t o n radius distributions (see section 5.5), depends in part on the choice of radiator for this particular experiment. F r o m eqs. (1) and (2), the ring radius is given by
r = f ( f l Z n z - 1) '/2
(8)
But the index of refraction, and thus the ring radius, depends on the p h o t o n energy, as shown in fig. 1 for our helium radiator. The range of p h o t o n energies over which our detector is sensitive is given by the efficiency ((E), which depends principally on the T E A q u a n t u m efficiency, as discussed above. The expected dispersion is given by dN
dN
dr
dN
/ dr dn)
dr
(9)
Differentiating eq. (8) and using eq. (5), we obtain
dN dr
K~L sin3 Oc ~(dn/dE)"
(10)
where d n / d E , the chromatic dispersion of the helium index of refraction at p h o t o n energy E, can be read from the curve in fig.l. In eq. (10), ¢, O c, and d n / d E are all functions of the ring radius, r = r[n(E)], through their dependence on the p h o t o n energy E. The dispersion thus calculated is shown on the single-photon radius distributions in fig. 11. The m e t h a n e in the H e / T E A / C H 4 gas mixture cuts off high-energy photons, as shown in fig. 15, a n d eliminates any potentially long tails in the radius distribution. The rms chromatic dispersion (calculated in the region where the distribution exceeds 5% of the peak values) is found to be 0.34 m m for H e / T E A a n d 0.29 m m for H e / T E A / C H 4. The detector resolution is found from the deconvolution of the system resolution and the chromatic dispersion. We find o = 0.54 m m for the detector operating with H e / T E A , and o = 0.37 m m with H e / T E A / C H 4.
7. Conclusions We have identified particles in a 200 G e V / c b e a m containing 95.2% ~r-, 4.3% K - , a n d 0.5% a n t i p r o t o n s with an efficiency of 92% for the ~r's, 83% for the K's (68% with H e / T E A in the p h o t o n chamber), a n d 90% for the antiprotons. The multistep c h a m b e r detected p h o t o n s with a spatial resolution of 0.54 m m with H e / T E A a n d 0.37 m m with H e / T E A / C H 4. The sys-
tem resolution (including the effect of chromatic dispersion in the radiator gas) was 0.64 m m for the H e / T E A run and 0.47 m m for the H e / T E A / C H 4 run. The experimental figure-of-merit of the ring-imaging system was N O ---45 per cm. A ring-imaging systems based on this prototype is now in operation in Fermilab experim e n t E605 [28]. We would like to t h a n k T. Ypsilantis, H. J6stlein, A. Cattai, A. Breskin, and R. Praca for their help in various stages of this experiment. We would also like to acknowledge the support of the N a t i o n a l Science F o u n d a t i o n , the Commissariat h l'Energie Atomique, and CERN.
References [1] R. Bouclier, G. Charpak, A. Cattai, G. Million, A. Peisert. J.C. Santiard, F. Sauli, G. Coutrakon, J.R. Hubbard, Ph. Mangeot, J. Mulli6, J. Tichit, H. Glass, J. Kirz and R.L. McCarthy, Nul. Instr. and Meth. 205 (1983) 403. [2] A. Roberts, Nucl. Instr. and Meth. 9 (1960) 55. [3] M.M. Butslov, M.N. Medvedev, I.V. Chuvilo and M.V. Sheshunov, Nucl. Instr. and Meth. 20 (1963) 263. [4] G.T. Reynolds, J.R. Waters and S.K. Poultney, Nucl. Instr. and Meth. 20 (1963) 267. [5] D.M. Binnie, M.R. Jane, J.A. Newth, D.C. Potter and J. Waiters, Nucl. Instr. and Meth. 21 (1963) 81. [6] R. Iredale, G.W. Hinder, A.G. Parham and D.J. Ryden, IEEE Trans. Nucl. Sci. NS-13 (1966) 399. [7] R. Geise, O. Gildemeister, W. Paul and B. Schuster, Nucl. Instr. and Meth. 88 (1973) 83. [8] B. Robinson, Phys. Scripta 23 (1981) 716. [9] M. Benot, J.M. Howie, J. Litt and R. Meunier, Nucl. Instr. and Meth. 111 (1973) 397. [10] M. Benot, J.C. Bertrand, A. Maurer and R. Meunier, Nucl. Instr. and Meth. 165 (1979) 439. [1 I] J. S6guino and T. Ypsilantis, Nucl. Instr. and Meth. 142 (1977) 377. [12] R.S. Gilmore, J. Malos, D.T. Bardsley, F.A. Lovett, J.P. Melot, R.J. Tapper, D.I. Giddings, L. Lintern, J.A.G. Morris, P.H. Sharp and P.D. Wroath, Nucl. Instr. and Meth. 157 (1978) 507. [13] S. Durkin, A. Honma and D.W.G.S. Leith, Proc. 1978 Isabelle Summer Workshop BNL 50885 (1979) 120. [14] J. Chapman, D. Meyer and R. Thun, Nucl. Instr. and Meth. 158 (1979) 387. [15] G. Charpak, S. Majewski, G. Melchart, F. Sauli and T. Ypsilantis, Nucl. Instr. and Meth. 164 (1979) 405. [16] J. S6guinot, J. Tocqueville and T. Ypsilantis, Nucl. Instr. and Meth. 173 (1980) 283; D. Solomon and A.A. Scala, J. Chem. Phys. 62 (1975) 1469. [17] G. Comby, Ph. Mangeot, J. Tichit, H. de Ligni~res, J.F. Chalot and P. Monfray, Nucl. Instr. and Meth. 174 (1980) 77. [18] G. Comby, Ph. Mangeot, J.L. Augu6res, S. Claudet, J.F. Chalot, J. Tichit, H. de Ligni~res and A. Zadra, Nucl. Instr. and Meth. 174 (1980) 93. [19] F. Sauli, Phys. Scripta 23 (1981) 526.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, H [20] T. Ekelof, J. S+guinot, J. Toqueville and T. Ypsilantis, Phys. Scripta 23 (1981) 718. [21] G. Charpak, A. Peisert, F. Sauli, A. Cavestro, M. Vascon and G. Zanella, Nucl. Instr. and Meth. 180 (1981) 387. [22] E. Barrelet, T. Ekelof, B. Lund-Jensen, J. S6guinot, J. Tocqueville, M. Urban and T. Ypsilantis, Nucl. Instr. and Meth. 200 (1982) 219. [23] K. Fransson, private communication.
91
[24] L.G. Christophorou, Atomic and Molecular Radiation Physics (Wiley, New York, 1971) p. 165. [25] P.G. Wilkinson and H.L. Johnston, J. Chem. Phys. 18 (1950) 190. [26] M.C.E. Huber and G. Tondello, J. Opt. Soc. Am. 64 (1974) 390. [27] A. Gedanken et al., J. Chem. Phys. 57 (1972) 3456. [28] H. Glass et al., submitted to IEEE Trans. Nucl. Sci.
79
P R O G R E S S IN CHERENKOV RING IMAGING Part 2: Identification of charged hadrons at 200 G e V / c Ph. M A N G E O T ,
G. C O U T R A K O N
*, J . R . H U B B A R D ,
J. M U L L I I ~ , J. T I C H I T a n d A. Z A D R A
DPhPE, CEN Saclay, Gif -sur- Yvette, France R. B O U C L I E R ,
G. CHARPAK,
J. M I L L I O N ,
A. P E I S E R T , J.C. S A N T I A R D
a n d F. S A U L I
CERN, Geneva, Switzerland
C.N. B R O W N FNAL, Batavia, Illinois, USA D. FINLEY,
H. G L A S S , J. K I R Z a n d R . L . M c C A R T H Y
SUNY, Stony Brook, New York, USA Received 11 April 1983
We have used a ring-imaging Cherenkov detector to separate Tr's, K's, and antiprotons in a 200 GeV/c beam at Fermilab. This device was built as a prototype for a large-aperture counter now in operation in Fermilab experiment E605. The radiator consisted of 8 m of atmospheric-pressure helium gas. The photon detector was a multistep proportional chamber. Cherenkov photons near 8 eV were detected by photoionization of triethylamine (TEA) vapor in the chamber. An average of 2.5 to 2.7 Cherenkov photons were observed per event, corresponding to a figure of merit NO= 45 per cm. A single-photon radius uncertainty of 0.47 mm was obtained with a helium/TEA/CH 4 gas mixture in the photon detector. The rms uncertainty in the determination of the Cherenkov angle was AOc/O ..... = 0.006, corresponding to one-standard-deviation ~r/K separation at 500 GeV/c. At 200 GeV/c, the particle identification efficiency in a beam containing 95.2% ~r-, 4.3% K-, and 0.5% antiprotons was 92% for the ~r's, 83% for the K's, and 90% for the antiprotons.
1. Introduction A ring-imaging Cherenkov detector has been used to separate ~r , K - , and antiprotons in a 200 G e V / c beam at Fermilab. The ring-image was formed in the entrance-plane of the radiator by an 8 m focal-length spherical mirror. The p h o t o n detector was a 20 × 20 cm 2 multistep proportional chamber containing triethylamine (TEA) vapor as the photo-sensitive component. Some details of the construction, operation, and perform a n c e of the multistep chamber as a detector of photons in the vacuum ultraviolet (VUV) region were presented in Part 1 of this paper [1].
2. Cherenkov ring-imaging technique Cherenkov ring-imaging was first p r o p o s e d by Roberts [2] as a technique for accurate measurement of particle velocities and directions. When a charged par* Also at SUNY. Stony Brook, USA. 0 1 6 7 - 5 0 8 7 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
ticle passes through a medium of index of refraction n at a velocity (v = fic) greater than the velocity ( c / n ) of light in the same medium, p h o t o n s are emitted at an angle O c whose cosine is the ratio of the velocity of the light to that of the charged particle: cos 0 C = l / f i n .
(1)
Photons emitted at various points along a particle's straight-line trajectory, when reflected from a spherical mirror of radius R, are focused onto a circle of radius r =ftan
~c
(2)
in the focal plane, a distance f = R / 2 from the mirror. If the particle is inclined at an angle O D to the optical axis, the center of the circle will be displaced a distance Aq = f t a n @D
(3)
from the axis. Thus, measurement of the center of the Cherenkov circle determines the particle direction. Inversely, external measurement of the particle direction determines the center of the Cherenkov circle, so that each detected p h o t o n gives a measure of the Cherenkov
80
Ph. Mangeot et a L / Progress in Cherenkov ring imaging, 11
ring radius, and therefore of the particle velocity. In our application the particle momentum and direction are both determined externally, so a single detected photon yields a measurement of the particle mass.
3. Design considerations The work reported here was undertaken to develop a prototype for a detector to identify ~r's, K's, and protons of momentum 100 to 400 G e V / c for Fermilab experiment E605. This is a high-luminosity experiment designed to study high-P t single particles and particle pairs. The Cherenkov counter is downstream of two momentum-analyzing magnets. It has a 2 × 2 m 2 entrance plane and accepts particles with wide angular dispersion (120 mrad × 60 mrad). The detector must have large acceptance, good velocity resolution (~'ma~ --- 800), and reasonably good time resolution (_< 1/~s), but it is only expected to identify at most two particles per event. 3.1. Choice of the photo-detection technique Cherenkov ring-imaging was first demonstrated experimentally using image-intensifiers operating at visible wavelengths [3-8]. This approach is unsuitable for our purposes because of the limited acceptance and high cost of these devices. The detector surface can be reduced by optical subtraction, a s in the spot-focusing Cherenkov counter [9,10], but this technique is limited to beams with small divergence. In order to obtain a large detection surface compatible with our high rates, we use a proportional wire chamber containing photo-sensitive vapor, as suggested by S6guinot and Ypsilantis [11]. Difficulties were encountered in the development of this technique because the known compounds with reasonable vapor pressures are sensitive only in the vacuum ultraviolet (VUV) region. This fact sharply limits the choice of radiators and windows, since most materials absorb in the VUV region. Furthermore, chromatic dispersion increases for these higher photon energies. These problems have been brought under control by the development of detectors using vapors with very low emission potentials. The further problem of efficient detection and localization of the single photoelectrons produced has been resolved by use of a multistep structure for the proportional chamber, as described in ref. 1. 3.2. Choice of the photo-sensitive vapor The early work on Cherenkov ring-imaging with gaseous detectors [12-14] used acetone (ionization potential 9.69 eV) or benzene (9.24 eV) as the photosensitive vapor. Lithium fluoride crystals, which are highly
hygroscopic and difficult to handle, were required for the windows. The introduction of triethylamine (TEA) vapor, with a photo-ionization threshold at 7.5 eV, provided considerable simplification. Crystals of CaF 2 or MgF2, transparent to about 10 eV and more stable than LiF, can be used with TEA. The vapor pressure of T E A is sufficiently high (40 Torr at 15°C) that photons are completely absorbed in a few mm of gas. Considerable effort has been invested in the development of photon detectors using TEA vapor [15-18]. Noble gases are normally used as radiators in order to minimize the chromatic dispersion. We chose TEA vapor for our detector, even though several compounds with much lower ionization potentials are now available [19-22]. These new compounds can be used with quartz windows (transparency cut-off around 7.5 eV for UV grade), instead of the fragile fluoride crystals required for TEA, but they have the disadvantage of very low vapor pressures and correspondingly long absorption lengths. Tetrakis dimethylamino-ethylene (TMAE), for example, has a photoionization threshold of 5.4 eV, but a vapor pressure of only 0.35 Tort and an absorption length [23] of about 20 mm at 20°C. Such a long absorption length implies a timing jitter of a /~s or more and significant loss in spatial resolution due to parallax. With TEA vapor at 50% saturation at 15°C, the absorption length is only 1 mm [16], and both timing jitter and parallax are acceptably small. 3.3. Choice of the detector gas mixture Once the photosensitive vapor has been selected, we must choose an appropriate carrier gas. Usually a noble gas is used to avoid photon capture by the carrier gas. We have used helium as the principal component of our gas mixture, rather than the more common argon, for two reasons: first, because charged particles ionize much less in helium, so we avoid large pulses which can mask the smaller photon pulses and which can lead to chamber instabilities. Second, because the excitation and ionization levels are much higher in helium than in argon, and we can obtain higher gains before breakdown occurs. We use TEA at 50% saturation to avoid condensation in the photon chamber. The TEA temperature was 15°C. Thus, the first gas mixture used was He(97.5%) + TEA(2.5%). A second gas mixture was formed by adding methane (CH4) to the above mixture, yielding He(90%) + TEA(2.5%) + CH4 (7.5%). The methane was added to improve resolution by the absorption of high-energy photons, as discussed in section 6.2. (The transmission of 7.5% methane in our gas
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, II 100
I
100
--.. CH 4
"
\ x
Helium (n2-1)
75
75
Ca g 2 - ' " - . . . "
81
T E A [16,22], are shown in fig. 1. Fig. 2 shows the C h e r e n k o v angle for various particle types as a function of m o m e n t u m . The C h e r e n k o v threshold for protons is a r o u n d 100 G e V / c , a n d we can hope to separate ¢r's a n d K's out to 400 G e V / c .
z 0
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4. Experimental set-up
/ TEA
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- -
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L / ~ II
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/
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/
I 8
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Fig. 1. The helium index of refraction [26] at STP, the quantum efficiency of TEA [16,22], the transmission of CaF 2, and the transmission of the methane in our H e / T E A / C H 4 gas mixture [24,25], as a function of photon energy.
Our prototype particle identifier is shown schematically in fig. 3. The radiator vessel was a stainless-steel tube with a spherical mirror m o u n t e d at one end and the p h o t o n detector at the other end. The spherical mirror, at the d o w n s t r e a m end, was accessible through a bolted cover in order to allow the proper alignment. A C a F 2 window and the multistep p h o t o n detector were m o u n t e d with conventional O-ring seals at the upstream end. A control tube m o u n t e d above the radiator tube could be used to measure the transmission of the radiator gas. Drift c h a m b e r s measured the particle trajectory, a n d scintillation counters provided the trigger. 4.1. Radiator
mixture, calculated from measured absorption coefficients [24,25], is shown in fig. 1.) 3.4. Choice o f radiator gas
Helium was chosen for the radiator gas in order to limit chromatic dispersion a n d allow particle identification at high energy. Atmospheric-pressure helium at 0 ° C (STP) has an index of refraction of 1.0000386 for p h o t o n s of 8.6 eV (near the peak of the T E A q u a n t u m efficiency). The energy d e p e n d e n c e of the index of refraction [26], as well as the q u a n t u m efficiency of
The radiator tube was 8 m long a n d 30 cm in diameter. G a s purity was an i m p o r t a n t consideration in the design, because the p h o t o n absorption cross sections of oxygen a n d water are large in our p h o t o n energy range. The stainless-steel vessel was cleaned, then v a c u u m - b a k e d at 100°C before the initial use at C E R N . The ~adiator gas was o b t a i n e d as boil-off from a liquid helium dewar. G a s flow was m a i n t a i n e d at one volume change per hour during most data runs. The gas flowed out through the control tube to the recovery system. 4.2. Control tube
9
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8
ZT/, 7
> 0 x," W
kU I
6 100
I 200
[ 300
400
PARTICLE MOMENTUM(GeV)
Fig. 2. Cherenkov angle for different momenta and particle types. The refractive index of helium used here was 1.0000386 at STP for 8.6 eV photons [26].
The control tube was 6 m long a n d 10 cm in diameter. It was used to m o n i t o r continuously the outflowing gas purity by measuring its transmission for V U V photons. A V U V p h o t o n source was m o u n t e d at one end of this tube, a n d a solar-blind photomultiplier with a M g F 2 window at the other end. The p h o t o n source, described in detail in ref. l, used secondary emission in k r y p t o n [27], which matches closely the T E A q u a n t u m efficiency response. The photomultiplier response was measured while the helium flowed through the tube. T h e n the control tube was isolated from the radiator and evacuated, a n d the p h o t o m u l t i p l i e r response was measured again. The ratio of the response with a n d without gas in the tube gave a reliable m e a s u r e m e n t of the transmision of the radiator gas: 87% during the run without methane, 81% during the r u n with m e t h a n e in the p h o t o n chamber.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, I1
82 UV S o u r c e
PM
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,,
Beam
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C o n t r o l Tube
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R a d i a t o r Tube (L=8m)
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Mirror (f=8m)
Photon Detector 20x2Ocm 2 Multi-step Proportional Chamber
I I I I I
Fig. 3. Experimental set-up.
4.3. Spherical mirror T h e mirror was m a n u f a c t u r e d by v a c u u m depositon over a pyrex substrate of a reflecting layer of a l u m i n u m 822 A thick, coated with 165 A of MgF~ to e n h a n c e reflectivity and to avoid oxidation. Its reflectivity (fig. 4) was measured to be (70_+ 4)% over the region of high T E A q u a n t u m efficiency. The focal length was (794 _+ 1.2) cm, so the mirror was positioned 794 cm from the conversion gap of the p h o t o n detector.
the energy range 7.5 to 10 eV. For the H e / T E A / C H 4 run, the average transmission was also weighted by the effective transmission of the methane, yielding 65% for this run. The crystals were m o u n t e d on a brass frame to m a t c h the Caf 2 thermal expansion (for details, see ref. 1). The 1 cm wide arms formed a dead-space for photons (an additional loss of 9%), b u t not for b e a m tracks. T h e effective window transparency was thus 0.64 × 0.91 = 58% for the H e / T E A run, and 0.65 × 0.91 = 59% for the H e / T E A / C H 4 run.
4.4. Calcium fluoride window
4.5. Multistep proportional chamber
A square array of four 4 m m thick C a F z crystals separated the radiator from the detector. The transmission of the CaF2, measured with a m o n o c h r o m a t o r , is shown in fig. 1. The average transmission for the H e / T E A run was 64%, obtained by weighting the measured transmission by the q u a n t u m response of T E A in
The 20 x 20 cm 2 multistep proportional c h a m b e r is s h o w n schematically in fig. 5. Cherenkov p h o t o n s pass through the C a F 2 window a n d ionize T E A molecules in the conversion gap (C), producing single photoelectrons. T h e photoelectrons are drifted into the preamplification gap (PA), where each photoelectron initiates an
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Fig. 4. Reflectivity versus photon energy. Measurements were made with an ultraviolet monochromator. The circles correspond to source intensity measurements made before inserting the mirror and crosses correspond to source intensity measurements made after the mirror was withdrawn, indicating some intensity fluctuations.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, I1 PWC
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gas mixtures. With H e / T E A the PA gap field was below 600 V / r a m , and the PWC a n o d e - c a t h o d e voltage difference was 1100 V. With H e / T E A / C H 4 the PA gap required 1000 V / r a m , and the PWC operated at 2000 V. We experienced some difficulties with chamber stability, and the higher voltages necessary with H e / T E A / C H 4 increased these difficulties. Further details of the construction and performance of the multistep proportional chamber used as a photon detector are given in ref. 1.
x
Fig. 5. Photon detector structure.
avalanche in the strong electric field• A fraction of the electrons in each avalanche, typically about 103 , emerge into the drift space and are transferred to the proportional wire chamber (PWC). A second amplification takes place around the anode wires, bringing the total charge to about 107 , which permits easy electronic detection. The conversion and amplification gaps were formed of three stainless-steel meshes of 50/~m diameter wires, with a 500 t~m pitch• The first mesh was in contact with the CaF 2 window. The optical transparency of this mesh was 81%. Some photoelectrons produced in the conversion gap are lost because of electric field distortions near the 100 # m thick meshes. Conversions which take place in the first 50 btm or so of the detector can be lost due to field distortions in that region. The mean free path of photons in 2.5% T E A is 1 ram, implying a 5% loss from these first 50/~m. Furthermore, some photoelectrons fail to enter the preamplification gap, and are lost, collected on the second mesh. We estimate a loss of 8% of the photoelectrons at the second mesh for our operating conditions. The conversion gap was 6 mm thick (6 mean free paths), so there was no loss due to photon escape. The effective transmission of the conversion gap was therefore 0.18 × 0.95 × 0.92 = 71%. The PWC half-gap width was 3.2 mm. This narrow gap was chosen to reduce the width of the cathode pulses, and thereby improve the multiple-hit resolution. The cathode planes, made with 50 ~ m diameter, ½ram pitch wires, were perpendicular to each other and parallel to the detector frames, defining two orthogonal coordinates X and Y. The anode plane was implemented with 20 /~m wires, 2 mm apart, mounted at a 45 ° angle with respect to the other electrodes, and provided an inclined coordinate U. All three planes were read out every 2 mm into analog-to-digital converters (ADCs). There were 96 channels per plane which covered most of the active surface of the detector. The multistep proportional chamber was operated with two gas mixtures, H e / T E A and H e / T E A / C H 4 , as discussed in section 3.3. The drift fields in the conversion and transfer gaps were 100 V / m m for both
4.6. Electronics
Signals from the 96 anode wires and 2 × 96 cathode strips were amplified on the chamber by discretecomponent amplifiers with a gain of 125 m V / p C , a time constant of 300 ns, and a noise level equivalent to less than 5 × 104 electrons. The pulses were transmitted in differential mode to the control room (a distance of 30 m), where they were referenced to ground and further amplified ( × 6 ) by M E C L 10115 receivers. These receivers had trimmers which allowed a channel-bychannel adjustment of the gain. The signals were fed to charge-sensitive A D C s (LeCroy model 2249) with input impedance of 50 $2. Standard N I M logic units were used to trigger the system and provide the gates for the ADCs. A 75 ns gate was used, delayed by 800 ns to compensate the drift time of the electrons. The digitizing time of the A D C s was 60/~s. 4. 7. Particle beams
The complete prototype particle identifier was first tested in a secondary electron beam (about 8 G e V / c momentum) at C E R N . This run provided information on the detection efficiency and multihit resolution, but the radius resolution was poor because of multiple Coulomb scattering in the windows and in the gas. A second test run, with a hadron beam, was necessary to demonstrate the particle-identification possibilities of the device. This run was performed in the Meson Laboratory at Fermilab with an unseparated 200 G e V / c negative beam. Results from this second run are presented below. 4.8. Drift chambers
Two sets of high-accuracy drift chambers, before and after the radiator, were used to measure the trajectory of the incident particles. A third set of drift chambers was installed to determine drift-chamber resolution. Each of the six planes of drift chambers contained four wires, one wire every 5 cm, with one T D C per wire. The drift chambers were positioned so that the beam was measured on a single wire of each chamber•
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, 11
84
4.9. Data acquisition A coincidence of four scintillation counters, two before and two after the C h e r e n k o v counter, defined a particle event and provided the trigger for readout of the drift chambers and the multistep proportional chamber. All data were read from standard C A M A C units by a Hewlett-Packard mini-computer. The raw data recorded on magnetic tape included 24 T D C values from the drift c h a m b e r s and 288 A D C values from the p h o t o n chamber. The data acquisition rate was computer-limited to 40 events per second. The quality of the data was controlled by an on-line program that included a simple reconstruction of individual events.
planes for the two runs are shown in fig. 7. The value of the pulse height for each entry in the histogram is the sum of the A D C contents of each channel in a cluster of cathode strips above threshold (20 counts) multiplied by a conversion factor 2.2 × l0 -4 pC per A D C count. The pulse-height threshold for the cluster was 200 counts, corresponding to a m i n i m u m detectable signal of 3 x 105 electrons. Fig. 8 shows the correlation in cathode X a n d Y pulse heights for single photo-electrons. The cathode X pulse heights were 20% larger than the cathode Y pulse heights,
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5. Data analysis
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5.1. Pulse-height distributions w
The A D C pulse-height information from a typical event is shown infig. 6. The cathodes defined the orthogonal coordinates X and Y, and the anodes provided the inclined coordinate U, with U = ( X + Y ) / ~ / 2 . The preamplified charge from a single photoelectron usually fell on one or two adjacent wires (see ref. l), while the induced charge on each cathode plane was spread over six or seven 2 m m channels. The pulse-height spectra on the cathode X and Y
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85
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, I1 i
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Fig. 8. Correlation between the cathode X and Y amplitudes. (a) Low statistics correlation plot A x versus A v for unambiguous photon hits. (b) High statistics distribution showing the cathode X amplitudes to be 20% larger than the cathode Y amplitudes.
because the preamptified charge arrived at the a n o d e wire closer to c a t h o d e X than to cathode Y.
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5.2. C o o r d i n a t e calculation
The pulse-height i n f o r m a t i o n was used to calculate coordinates by the center-of-gravity m e t h o d for each group of wires hit. For the i t h group of wires in the X plane
x,=Exka~,/Ea~,, k
k
Axi = E axk, k
where axk is the A D C pulse height on the wire or strip at position x k, a n d X i a n d Xx~ are the center-of-gravity coordinate a n d its amplitude. The distribution of c a t h o d e pulse heights a b o u t the calculated coordinate position is shown in fig. 9. The m i n i m u m separation for which two coordinates of equal pulse height can be resolved is 6 mm. F o r the anodes, the m i n i m u m separation is 4 mm. W h e n multiple peaks were a p p a r e n t in a given group of wires, multiple coordinates were calculated by a r o u g h partition of the measured pulse heights. Some coordinates were missed or poorly determined at this stage, because of overlapping pulse-height distributions. 5.3. P o i n t reconstruction
A n initial list of point candidates was o b t a i n e d by selecting all coordinate triplets (Xi, ~ , Uk) with good spatial correlation, unless their amplitudes differed by a very large factor. Explicitly, we selected triplets satisfy-
ing the following three requirements: (a) correlated in space to better than 2 mm, (b) cathode amplitudes equal within a factor 4, a n d (c) cathode a n d anode amplitudes equal within a factor 9. Some points did not satisfy the above criteria because of unresolved coordinates. In order to recover these points, a second class of candidates was defined as follows: F o r each coordinate pair ( X i, Yj), (X,, ~ ) , or ( ~ , Uj), the expected value of the third coordinate U~, Y~, or X k was calculated; if the contents of the A D C s near the calculated coordinate were large enough to m a t c h the amplitudes of the original pair, then a new p o i n t candidate was created. A final list of points was selected by removing poorer candidates from this c o m b i n e d list until a " b e s t ' ' fit to the overall event was found. C a n d i d a t e points were most likely to be eliminated if all three coordinates were used elsewhere in the candidate list, a n d if the amplitudes of the three coordinates were very different. W h e n the final list was established, final coordinates were determined as follows: (1) A coordinate was considered unambiguous, and its measured value was retained, if it was used in a single point in the final list. (2) If a point h a d only one ambiguous coordinate, that coordinate was recalculated from the two u n a m b i g u o u s coordinates. (3) If more than one coordinate was ambiguous, we a t t e m p t e d to recalculate these ambiguous coordinates by removing from the center-of-gravity calculation the c o n t r i b u t i o n from previously resolved points.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, 11
86
5. 4. Recalculation of ambiguous coordinates
5.5. ,~r'stem resolution
This last technique can be illustrated by considering the specific example in fig. 10. The X and Y coordinates of p h o t o n 2 of this event are b o t h ambiguous, so the position of that p h o t o n is poorly determined. But, the ambiguous coordinate X t and its amplitude A ~ can be expressed as the sum of the contributions from p h o t o n s 1 a n d 2:
The drift c h a m b e r measurements of the angles of the particle trajectory were used to determine the center of the p h o t o n ring. For each point candidate, a ring radius was calculated (the distance from the ring center). Single-photon radii in the region of the ~r peak are plotted in fig. 11 for the two data runs. The distribution
X]-
X(I)Ax(1
) + X(2)A~(2)
A,,=Ax(1)+A~(2
150
).
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,,>,
U(1)
-
r(l)
=/2U,
Ax(l)--Av(1)=Avi, Therefore, the X amplitude due to p h o t o n 2, A ~(2), can be calculated from the coordinates amplitudes:
_
Calculated Chromatic Dispersion He/TEA
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N o w these values can be used to calculate t h e X coordinate of p h o t o n 2: x(2) -
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We calculate Y(2) in a similar way. Then we verify that the triplet [X(2), Y(2), U2] is well-correlated in space before accepting this point for further calculations.
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-
Furthermore, the X and Y amplitudes due to this photon are closely correlated, so
X,A.,,
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The X coordinate of p h o t o n 1, X(1), can be calculated from the u n a m b i g u o u s coordinate pair (Y~, U~)
=/2
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Fig. 10. Example of two pulse overlap problem. The circles mark the position of the photon hits. The coordinates were calculated by the center of gravity method and are labeled on the sides. Photon 2 in this figure has an ambiguous coordinate in X and Y.
66
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RADIUS (mml
Fig. 11. The measured one photon radius distribution and the expected chromatic dispersion (a) with He/TEA in the chamber, and (b) with H e / T E A / C H 4. The chromatic dispersion was calculated from eq. (10), using the helium index of refraction in fig. 1 and the TEA quantum response in fig. 15.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, II of single-photon radii for all 200 G e V / c 7r's should give the system resolution function. The fwhm of this distrib u t i o n is 1.5 m m for the H e / T E A data a n d 1.1 m m for H e / T E A / C H 4. These distributions are not Gaussian. They are convolutions of the detector resolution with the chromatic dispersion of the helium radiator (see section 6.2). The equivalent rms widths are 0.64 m m a n d 0.47 mm, respectively. 5.6. Photon selection The final point list c o n t a i n e d b e a m tracks as well as p h o t o n candidates. Indeed, the incident particles traversed the multistep c h a m b e r near its center, leaving a trail of ionization. This b e a m track was often detected along with the C h e r e n k o v photons, even though the p h o t o n s reach the c h a m b e r with a 53 ns delay. The b e a m track was located easily by interpolation from its d r i f t - c h a m b e r coordinates. With the b e a m track removed, the remaining points were the p h o t o n candidates. Background points were them removed by a clustering technique. F o r a m u l t i - p h o t o n event, all p h o t o n radii were required to lie within 3 m m of each other. W h e n this was not true, the cluster with the most p h o t o n s satisfying this criterion was used. W h e n two ore more clusters h a d the same n u m b e r of photons, the cluster with the smallest range of radii was used. Backg r o u n d points were removed efficiently by the cluster cuts except in the case of z e r o - p h o t o n events, where any b a c k g r o u n d point was interpreted as a single p h o t o n (1% of all events). The distributions of the n u m b e r of p h o t o n s selected by the cluster technique are shown in fig. 12. F o r the H e / T E A run, an average of 2.64 p h o t o n s were found
I
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3_
N=2.64
per event; for the H E / T E A / C H 4 run, the average was 2.48 p h o t o n s per event. These distributions are not perfectly Poissonian, however, even after shifting b a c k g r o u n d events down to the zero-photon bin. Limited two-photon resolution a n d other program inefficiencies shift m a n y higher-multiplicity events down to lower bins, with a net loss of a b o u t 8% of all photons. The lowest multiplicity bins, which determine the n u m b e r of unidentified events, correspond to the higher averages, 2.9 p h o t o n s per event for H e / T E A a n d 2.7 p h o t o n s p e r e v e n t for H E / T E A / C H 45. 7. Particle identification The distribution of average p h o t o n radii is shown in fig. 13. The a n t i p r o t o n peak is well separated from the c o m b i n e d ~r-K peak. This latter region is shown in more detail in fig. 14. The superior resolving power of the H e / T E A / C H 4 gas mixture is evident here. The rms width of the 7r peak was 0.53 m m for the H e / T E A run, a n d 0.42 m m for the H e / T E A / C H 4 run, for a ~r-K radius difference of 2.64 mm. These widths are smaller t h a n the widths of the single-photon distributions, but larger than expected for a N I/2 improvement. F o r the H e / T E A data run, 1% of the ~r's and 25% of the K's are a m b i g u o u s between the ~ a n d K hypotheses.
f
[
i
104
I
u-lr-
'
I
103
K-
g
I
He/T EA/CH4
He/r E A 25%
i
87
> U.I
=2.48
~ 10 2
g
E
20%
Z
zLU 1 5 %
10
LIJ 10%
5%
4
6
8
110 Number
0
of
-L•
6 Photons /Event
i
k
8
10
Fig. 12. Histogram of number of events versus number of photons per event, with and without C H 4.
50
60
70
80
RADIUS (mm)
Fig. 13. Radius distribution with 1 mm bin size, showing the ~v ~ K and/~ peaks. The radius shown here is the average of all photons in the event.
88
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, lI i
I
I
I
i
I
(including b a c k g r o u n d events which appear in the onep h o t o n bin) account for 6% of the ~r's, 7% of the K's, a n d 9% of the antiprotons. The particle identification efficiency is thus 93% for the ~"s, 68% for the K's, and 91% for the antiprotons. For the H e / T E A / C H 4 run, 7r K ambiguities are only half as frequent. Less than 1% of the ~r's and 9% of the K's are ambiguous between the two hypothesis. Assigning these events to the most p r o b a b l e hypothesis, 4% of the K's and 0.2% of the 7r's are misidentified (0.4% of all events). Z e r o - p h o t o n events account for 7% of the ~r's, 8% of the K's, and 10% of the antiprotons. The identification efficiency is thus 92% for the ~r's, 83% for the K's, and 90% for the antiprotons.
71"
He/TEA ,
103
03
E
K-
LIJ 10 2
o ..el E
5.8. Beam composition 10
I
I
64
I
66
I
I
,
I
~
68 RADIUS (ram) T
I
i
I
a
70
72
[
71-l
He/TEA/CH4
The measured Cherenkov ring radii were 68.3, 65.7, a n d 57.5 m m for ~r-. K - , and antiprotons, respectively. The average index of refraction of the radiator gas was thus 1.0000372, reflecting the atmospheric conditions d u r i n g these data runs. The 200 G e V / c b e a m composition was determined to be 95.2% 7r , 4.3% K , and 0.5% antiprotons.
6. Cherenkov detector system evaluation 6.1. Figure of merit
10 3
Cherenkov systems are often characterized by their figure-of-merit, No, defined such that the n u m b e r of p h o t o n s of C h e r e n k o v angle O c detected from a radiator of length L is
o3
~3
N = NoL sin2Oc.
; ~°2I
Z
,o [
I64
I 66
,
1 68
i
I 70
b 72
R A D I U S (mm)
Fig. 14. Radius distributions with 200 #m bin size, (a) for the He/TEA gas mixture, and (b) for H e / T E A / C H 4. If we assign these events to the most p r o b a b l e hypothesis, then 11% of the K's a n d only 0.4% of the ~r's are misidentified (0.8% of all events). Z e r o - p h o t o n events
T h e experimental figure-of-merit can be calculated directly from the ring-image radius a n d the average number of detected photons. For the H e / T E A run, N = 2.64 detected p h o t o n s per event and the ring radius is 68.3 mm, so N 0 = 45 per cm. For the H e / T E A / C H 4 run, N = 2.48 and r = 68.3 mm, so N o = 42 per cm. The figure-of-merit is supposed to represent the system characteristics i n d e p e n d e n t of the radiator used in a particular experiment. Correcting the above values for the transmission of the radiator gas in each run, we find N O = 4 5 / 0 . 8 7 = 52 per cm for the H e / T E A run, and N o = 42/0.81 = 52 per cm for the H e / T E A / C H 4 run. We expect a larger N 0 value for the H e / T E A data, since the methane in the H e / T E A / C H 4 gas mixture removes some of the photons. The equality of the corrected values is u n d o u b t e d l y due to the lower pulse heights in the H e / T E A run (see fig. 7). In fact, the Y-cathode pulse-height distribution from the H e / T E A / C H 4 run seems to indicate that the c h a m b e r was not completely efficient even for these data.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, H The experimental figure-of-merit can be c o m p a r e d to the value expected on the basis of the physical properties of the detector system, o b t a i n e d by integrating the detector response over the energy spectrum of the C h e r e n k o v emission. Since C h e r e n k o v p h o t o n s are emitted in a flat energy spectrum, the n u m b e r of photons produced in an energy interval d E is given by
d N = KL sin2OcdE,
(4)
where K = a / h c = 370 p h o t o n s per cm per eV. The n u m b e r of p h o t o n s detected is
dN
= KL
sin2e~,
( E)dE,
(5)
where ~ ( E ) is the detection efficiency for p h o t o n s of energy E. If we neglect a small energy d e p e n d e n c e of the C h e r e n k o v angle (see section 6.2), we can express the total n u m b e r of detected p h o t o n s as
N
= KL
sinZO¢fe(E)dE
=
NoL sinZOc.
(6)
T h e efficiency c ( E ) is the product of the efficiencies (at p h o t o n energy E ) of all the c o m p o n e n t s of the system reflectivity of the mirror, transmission of the radiator gas, the C a F 2 window, a n d the conversion gap of the multistep chamber, q u a n t u m efficiency of the TEA, single photoelectron efficiency of the chamber, a n d pattern-recognition efficiency of the analysis program:
e( E )
=
RmTrTwTcQteaCpe,
pr .
89
Table 1 System efficiencies
Radiator gas transmission Mirror reflectivity CaF z crystal transmission CaF 2 window frame First wire mesh transparency Conversions in first 50/~m of detector Detector carrier gas transmission Second wire mesh transfer efficiency Multistep chamber photoelectron efficiency Analysis program efficiency TEA quantum efficiency
fQt¢~(E)dE
He/TEA
He/TEA/CH 4
0.87
0.81
0.70
0.70
0,64
0.65
0.91
0.91
0.81
0.81
0.95
0.95
1.00
0.83
0.92
0.92
0.78
0.93
0.92
0.92
0.68 eV
0.68 eV
0.121 eV
0.114 eV
System efficiency
fc(E)dE
(7)
The main energy d e p e n d e n c e comes from Qtea, the T E A q u a n t u m efficiency. The form of the T E A quantum response [16] is shown in fig. 1. The value at the m a x i m u m has been measured to be Q m a x = 0.65 [22]. We have used the functional dependences in fig. 1 to p e r f o r m the numerical integration indicated by eq. (6). The results are shown in table 1. The average mirror reflectivity ( R m) was 70%. The effective window transmission (Tw) was 58% for the H e / T E A run a n d 59% for the H e / T E A / C H 4 run, as discussed in section 4. The effective transmission of the conversion gap To) was 71% for H e / T E A . T h e average m e t h a n e transmission in the H e / T E A / C H 4 gas mixture was 83% (see fig. 15). This reduced T~ to 59% for the H e / T E A / C H 4 run. The r e m a i n i n g terms - transmission of the radiator gas, photoelectron detection efficiency, a n d pattern-recognition efficiency - are rather d e p e n d e n t on the particular experimental conditions. Ignoring them for the m o m e n t , we find maximum expected values of N O of 73 per cm for H e / T E A a n d 61 per cm for H e / T E A / C H 4. In order to calculate N O for this particular experiment, we must include the measured radiator transmission ( T r = 87% for the H e / T E A r u n a n d 81% for the H e / T E A / C H 4 run), a n d the analysis efficiency (tpr = 92%). Then, if we assume the photoelectron detection efficiency ( t p e ) to be 100%, we find expected values of N o = 58 per cm for the H e / T E A run, a n d N O = 45 per
cm for the H e / T E A / C H 4 run. These calculated figures-of-merit are still larger than the experimental values, especially for the H e / T E A run, indicating that the multistep c h a m b e r was not completely efficient, as already suggested above. W e can use the expected and experimental values of N O to estimate the photoelectron efficiency in the two
I
I
i
I
I
50%
l
I
I
I
He/TEA He/TEA/CH4
4O%
~30% u.I E 20%
2c
(~ 10%
7.5
8.0
8.5
9.0
9.5
10.0
Photon Energy (eV)
Fig. 15. Quantum efficiency curves versus photon energy with and without CH 4 in the multistep chamber. These curves include the TEA quantum efficiency [16,22], the transmission of the CaF 2 crystals, and the absorption in the methane [24,25]. Further corrections can be found in table 1.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, 11
90
runs. For the H e / T E A run, we find ( p e = 4 5 / 5 8 = 78%. For the H e / T E A / C H 4 run, we find cp¢ = 4 2 / 4 5 = 93%. These values seem. compatible with the pulse-height distributions in fig. 7.
6.2. Photon detector spatial resolution The system resolution, determined from the singlep h o t o n radius distributions (see section 5.5), depends in part on the choice of radiator for this particular experiment. F r o m eqs. (1) and (2), the ring radius is given by
r = f ( f l Z n z - 1) '/2
(8)
But the index of refraction, and thus the ring radius, depends on the p h o t o n energy, as shown in fig. 1 for our helium radiator. The range of p h o t o n energies over which our detector is sensitive is given by the efficiency ((E), which depends principally on the T E A q u a n t u m efficiency, as discussed above. The expected dispersion is given by dN
dN
dr
dN
/ dr dn)
dr
(9)
Differentiating eq. (8) and using eq. (5), we obtain
dN dr
K~L sin3 Oc ~(dn/dE)"
(10)
where d n / d E , the chromatic dispersion of the helium index of refraction at p h o t o n energy E, can be read from the curve in fig.l. In eq. (10), ¢, O c, and d n / d E are all functions of the ring radius, r = r[n(E)], through their dependence on the p h o t o n energy E. The dispersion thus calculated is shown on the single-photon radius distributions in fig. 11. The m e t h a n e in the H e / T E A / C H 4 gas mixture cuts off high-energy photons, as shown in fig. 15, a n d eliminates any potentially long tails in the radius distribution. The rms chromatic dispersion (calculated in the region where the distribution exceeds 5% of the peak values) is found to be 0.34 m m for H e / T E A a n d 0.29 m m for H e / T E A / C H 4. The detector resolution is found from the deconvolution of the system resolution and the chromatic dispersion. We find o = 0.54 m m for the detector operating with H e / T E A , and o = 0.37 m m with H e / T E A / C H 4.
7. Conclusions We have identified particles in a 200 G e V / c b e a m containing 95.2% ~r-, 4.3% K - , a n d 0.5% a n t i p r o t o n s with an efficiency of 92% for the ~r's, 83% for the K's (68% with H e / T E A in the p h o t o n chamber), a n d 90% for the antiprotons. The multistep c h a m b e r detected p h o t o n s with a spatial resolution of 0.54 m m with H e / T E A a n d 0.37 m m with H e / T E A / C H 4. The sys-
tem resolution (including the effect of chromatic dispersion in the radiator gas) was 0.64 m m for the H e / T E A run and 0.47 m m for the H e / T E A / C H 4 run. The experimental figure-of-merit of the ring-imaging system was N O ---45 per cm. A ring-imaging systems based on this prototype is now in operation in Fermilab experim e n t E605 [28]. We would like to t h a n k T. Ypsilantis, H. J6stlein, A. Cattai, A. Breskin, and R. Praca for their help in various stages of this experiment. We would also like to acknowledge the support of the N a t i o n a l Science F o u n d a t i o n , the Commissariat h l'Energie Atomique, and CERN.
References [1] R. Bouclier, G. Charpak, A. Cattai, G. Million, A. Peisert. J.C. Santiard, F. Sauli, G. Coutrakon, J.R. Hubbard, Ph. Mangeot, J. Mulli6, J. Tichit, H. Glass, J. Kirz and R.L. McCarthy, Nul. Instr. and Meth. 205 (1983) 403. [2] A. Roberts, Nucl. Instr. and Meth. 9 (1960) 55. [3] M.M. Butslov, M.N. Medvedev, I.V. Chuvilo and M.V. Sheshunov, Nucl. Instr. and Meth. 20 (1963) 263. [4] G.T. Reynolds, J.R. Waters and S.K. Poultney, Nucl. Instr. and Meth. 20 (1963) 267. [5] D.M. Binnie, M.R. Jane, J.A. Newth, D.C. Potter and J. Waiters, Nucl. Instr. and Meth. 21 (1963) 81. [6] R. Iredale, G.W. Hinder, A.G. Parham and D.J. Ryden, IEEE Trans. Nucl. Sci. NS-13 (1966) 399. [7] R. Geise, O. Gildemeister, W. Paul and B. Schuster, Nucl. Instr. and Meth. 88 (1973) 83. [8] B. Robinson, Phys. Scripta 23 (1981) 716. [9] M. Benot, J.M. Howie, J. Litt and R. Meunier, Nucl. Instr. and Meth. 111 (1973) 397. [10] M. Benot, J.C. Bertrand, A. Maurer and R. Meunier, Nucl. Instr. and Meth. 165 (1979) 439. [1 I] J. S6guino and T. Ypsilantis, Nucl. Instr. and Meth. 142 (1977) 377. [12] R.S. Gilmore, J. Malos, D.T. Bardsley, F.A. Lovett, J.P. Melot, R.J. Tapper, D.I. Giddings, L. Lintern, J.A.G. Morris, P.H. Sharp and P.D. Wroath, Nucl. Instr. and Meth. 157 (1978) 507. [13] S. Durkin, A. Honma and D.W.G.S. Leith, Proc. 1978 Isabelle Summer Workshop BNL 50885 (1979) 120. [14] J. Chapman, D. Meyer and R. Thun, Nucl. Instr. and Meth. 158 (1979) 387. [15] G. Charpak, S. Majewski, G. Melchart, F. Sauli and T. Ypsilantis, Nucl. Instr. and Meth. 164 (1979) 405. [16] J. S6guinot, J. Tocqueville and T. Ypsilantis, Nucl. Instr. and Meth. 173 (1980) 283; D. Solomon and A.A. Scala, J. Chem. Phys. 62 (1975) 1469. [17] G. Comby, Ph. Mangeot, J. Tichit, H. de Ligni~res, J.F. Chalot and P. Monfray, Nucl. Instr. and Meth. 174 (1980) 77. [18] G. Comby, Ph. Mangeot, J.L. Augu6res, S. Claudet, J.F. Chalot, J. Tichit, H. de Ligni~res and A. Zadra, Nucl. Instr. and Meth. 174 (1980) 93. [19] F. Sauli, Phys. Scripta 23 (1981) 526.
Ph. Mangeot et al. / Progress in Cherenkov ring imaging, H [20] T. Ekelof, J. S+guinot, J. Toqueville and T. Ypsilantis, Phys. Scripta 23 (1981) 718. [21] G. Charpak, A. Peisert, F. Sauli, A. Cavestro, M. Vascon and G. Zanella, Nucl. Instr. and Meth. 180 (1981) 387. [22] E. Barrelet, T. Ekelof, B. Lund-Jensen, J. S6guinot, J. Tocqueville, M. Urban and T. Ypsilantis, Nucl. Instr. and Meth. 200 (1982) 219. [23] K. Fransson, private communication.
91
[24] L.G. Christophorou, Atomic and Molecular Radiation Physics (Wiley, New York, 1971) p. 165. [25] P.G. Wilkinson and H.L. Johnston, J. Chem. Phys. 18 (1950) 190. [26] M.C.E. Huber and G. Tondello, J. Opt. Soc. Am. 64 (1974) 390. [27] A. Gedanken et al., J. Chem. Phys. 57 (1972) 3456. [28] H. Glass et al., submitted to IEEE Trans. Nucl. Sci.