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A compact Čerenkov counter for detecting electrons
N U C L E A R I N S T R U M E N T S A N D M E T H O D S 53
(I967) 261--265, ~) N O R T H - H O L L A N D P U B L I S H I N G
CO.
A COMPACT (~ERENKOV COUNTER FOR DETECTING ELECTRONS G. C U L L I G A N and T. W. Q U I R K
Department of Nuclear Physics, University of Oxford, Oxford, U.K. Received 3 April 1967 A compact gas (~erenkov counter for detecting electrons is described. The counter is 0.017 radiation lengths thick along the
beam direction with" an efficiency greater than 0.95 when subtending a solid angle at the electron source of 0.02 of 4n.
1. Introduction
a relativistic electron in a radiator of length 1, with refractive index (1 +r/) is given by the expression
Threshold ~erenkov counters are widely used in high energy physics for detecting eleetronsX-3). However, the thickness of the radiator required for efficient detection produces significant radiation losses and multiple scattering which makes measurements of momentum and direction difficult. This limitation has been overcome in some cases by measuring the direction and momentum of the electron before it passes into the Cerenkov counter 1' 2). However, if the solid angle subtended by the counter is required to be greater than one per cent of 4n, the physical dimensions of the counter make construction difficult and can result in a low efficiency with a high background counting rate. An alternative solution to the problem is described below. It is a transmission ~erenkov counter with an efficiency greater than 95% which presents less than 0.02 radiation lengths of material to particles traversing it. The counter was designed for use in an experiment in which electrons with momenta between 50 MeV/c and 250 MeV/c were produced in the presence of a background of muons and pions in the same momentum range.
n = 800 hi,
(r/,~ 1).
For an average quantum efficiency of 15% and a transport efficiency to the photocathode of 25%, 30 (~erenkov photons produce on average one photoelectron. Thus approximately 150 photons are required for efficient operation, i.e. for 5 photoelectrons. Three possible radiators are compared in table 1. The fraction of a radiation length required to produce 150 photons is given. It is seen from this table that liquid hydrogen is the most efficient radiator. However, its advantages are offset by the technical problems associated with liquid hydrogen. Also, it has a low velocity threshold and is therefore more susceptible to background radiation. Propane at six atmosphere pressure and room temperature is a more suitable choice. The corresponding properties of freon 12 at the same pressure are given for comparison. The refractive constant q for propane at 17°C is given as a function of pressure in fig. I. The refractive constant has been obtained from the density4) and refractive index of propane at NTP, using the ClausiusMosotti equationS).
2. Radiator
The number of (~erenkov photons n, with wavelength between 3500 A units and 5000 A units, produced by
3. ConsU'uetion The structure and apperance of the (~erenkov counter
TABLE I Radiator materials.
Material
Liquid hydrogen Propane :reon 12
Refractive constant r/ at NTP
97 × 10-a l . l O x 10-a 1.15 x 10-a
Density at NTP (rag/era3)
70 2.01 6.33
Radiation length (g/C4"fl2)
58.0 45.0 21.7
* At 17°C; + At 0°C.
261
Fraction of radiation length for 150 photons
Gas pressure (atm)
0.0026 0.008 0.044
6 6
Radiator thickness (cm)
2 29* 25 +
Velocity threshold
0.915 0.994* 0.993 +
262
G. C U L L I G A N
A N D T. W. Q U I R K
7 6
Refractive 5 4
Constant r l x 70_3
jr
/J j
3 2 1 0
2
3
4
5
6
7
Pressure Atmospheres Fig. 1.
The refractive constant ~/ vs gas pressure for propane at 17°C.
are shown in figs. 2 and 3. (~erenkov light produced in the pressure vessel was reflected by a double-sided mirror through perspex lenses into the photomultipliers in a manner previously employed by Lindenbaum and Yuan6). Photons produced in the front half of the counter were reflected directly into the top photomultiplier, those produced in the back half were reflected at the rear window of the counter and then into the bottom photomultiplier. A photomultiplier, its housing and a mu-metal screen formed a single removable unit with the mu-metal screen fixed to the photomultiplier housing with insulating screws. The conical mirror fitted snugly into the mu-metal screen and located the photomultiplier. The photomultiplier base was held in place by grub screws. The pressure vessel, a mild-steel casting machined to a reasonable finish and lined with aluminium foil, was 10" long, with walls one inch thick. Its internal dimensions were six inches by eight inches. Bushes were placed in the various bolt holes to increase the holding strength. The vessel was filled and emptied through a coupling tapped into its side. The equiconvex lenses were made from ultra-violet passing perspex machined to an 8" radius of curvature and 7" aperture. Each lens was 8" in dia., 2.5" thick at the centre and 0.5" thick at the circumference, where it was clamped, with O-rings, between the casting and the steel support by eight steel bolts (fig. 3). A rubber gasket between the support and the casting made a light-tight seal. The perspex conical mirrors were machined to a cone half-angle of 36 ° with inner surfaces polished and aluminized. This angle was defined by the distance from the centre of the lens to the photocathode (4"). These mirrors increased the transport efficiency by reflecting light on to the photocathode.
The general arrangement of the optics gave a reasonably uniform transport efficiency from all parts of the counter for all directions. It is not the best arrangement for a parallel beam but is a compromise which takes account of possible wide divergence. The double sided mirror was made from two 0.001" melinex films stretched and glued on a steel frame. Holes were drilled into the top and bottom edges of the frame to equalize the pressure on the films. The mirror frame was held in place by screws that went through the sides of the casting. The melinex windows were clamped between two aluminium frames as shown in fig. 4. The windows consisted of four layers of 0.01" ordinary melinex with four layers of 0.001" aluminized melinex on the inner surface. The aluminized melinex provided a first reflection for the (~erenkov light and made the counter light tight. Steel bolts, which passed through the melinex, held the windows and their fibre gaskets against the casting. Particles going through the counter passed through 0.017 radiation lengths of material, i.e. 0.009 radiation lengths of melinex and 0.008 radiation lengths of propane.
Fig. 2. The ~erenkov counter with a detached photomultiplier unit.
263
A C O M P A C T (~ERENKOV C O U N T E R
Photomultiplier B a s e . ~ . ~ . ~ Grub Screws
plier Housing ~,
Lomultiplier EMI 6 2 5 5 A Insulating o
Conical
Cathode Screen
Metal ~l
Perspex
Support
Steel Bolts, Ring Seals
DoubleSided
ium
Frame
Mirror
Window
I
1 I I
1 1I r
1)
. . . . .
i I
). . . . . . . .
l
o
I
I
I
i riches
I
I 5
Fig. 3. Schematic diagram of the Cerenkov counter.
1/4' B.S.E.. B o l t s
.
Aluminium
- "/32 Radius
Wire Port 6"x
8"
Fig. 4. Detail of the melinex window and aluminium frame.
Two EMI 6255A quartz-window photomultipliers were used. These were selected tubes with a peak quantum efficiency of greater than 20%. They had a ~;enetian-blind dynode structure with a risetime of 7 nsec. A conventional dynode chain was used with a negative photocathode. The gain and timing of the tubes were checked by light diodes mounted inside the conical mirrors. 4. Safety Safety regulations at the Rutherford Laboratory require that pressure vessels withstand four times the
264
e . C U L L I G A N A N D T. W. QUIRK
10000
1
a { b{
t
1000 ffl "0 cO k) ¢1 ffl 0
t-
Z eJ
(5 r~
100
4~ c-
Pions t.. I1) cl
E Z
10
i ,/
T °'
1
to
/
\
/
~
A~,
i
l
°
°
'
'
'2'0
I
i/o
o
="
3'5 Flight Time in Nanoseconds
4'o
Fig. 5. The time-of-flight spectrum of the beam. (a) without the (~erenkov counter; (b) with the (~:erenkov counter signal in coincidence; (c) with the (~erenkov counter signal in anticoincidence.
operating pressure, in our case 24 atm. The strength of the lenses is more than adequate. The casting was Xrayed and found to be satisfactory. The melinex windows were designed to be consistent with the above requirement. They were tested 7) by applying pressure hydraulically. It was found that the strength of a window of given thickness increased with the number of laminates used, e.g. eight layers of 0.005"
melinex were stronger than four layers of 0.01". However, the thinner the melinex layer, the more liable it became to creep through the clamp and tear at the bolts. 5. Performance The double-sided mirror divides the detector into two independent t~renkov counters, hence it is possible
A COMPACT
(~ERENKOV COUNTER
265
TABLE 2 (~erenkov c o u n t e r efficiencies at 6 a t m o f propane.
Test
E1
Cosmic ray muons z-#-e beam max. efficiency min. efficiency for side 1 for side 2 Averaged efficiency for a divergent beam of 8° half angle
~11
81B ~ e l ~ S
e
0.85 +0.07
0.56 +0.04
0.47 _+0.06
0.94 +0.08
0.992 + 0.005
0.954 + 0.005
0.945 + 0.005
0.996 + 0.005
0.74 +0.02 0.994 + 0.005
0.75 +0.02 0.54 +0.05
0.56 +0.03 0.53 +0.05
0.944 + 0.005 0.994 + 0.005
0.95 +0.06
0.73 +0.02
0.70 +0.02
0.990± 0.006
to calculate efficiencies from coincidences between the two sides. I f some relativistic particles produce n2 counts in the second of these counters and n12 coincidences, then the efficiency of the first counter is:
trons knocked out of the front melinex window into the lenses gave roughly equivalent efficiencies. I f the two sides are used in coincidence then the efficiency for detecting pious and mucus was found to be less than 4 x 10 -5.
81 = n12/t12;
similarly the efficiency of the second counter is: 82 m n12/n 1
The combined efficiency is: 8 ~-~ 81 "~8 2 --8182,
with a coincidence efficiency 812 ~ 81B2.
The counter was tested initially using cosmic ray mucus, and later at the Liverpool 400 MeV synchrocyclotron using a 200 MeV/c beam of positive pious, muons and electrons. Fig. 5 shows the time-of-flight composition of the beam (a) without the (~erenkov counter signal, (b) with the (~erenkov counter signal in coincidence and (c) with the (~erenkov counter in anticoincidence. Table 2 lists the efficiencies obtained. The maximum efficiency was for electrons passing normal to the windows and through the centre of the counter, while the minimum for electrons was obtained from a beam passing diagonally across the counter. The efficiency for detecting pious at the same momentum was (0.33 + 0.02)%. The random coincidence rate was (0.10_0.01)% which left a rate independent efficiency of (0.23_+0.02)%. A similar efficiency for mucus at the same m o m e n t u m was observed in a subsequent experiment, while a calculation based on elec-
6. Conclusion The gas (~erenkov counter described has proved to be efficient and convenient. It was designed for an experimental study of the semi-leptonic decays of the K + mesons. The experiment was successful and the counter performed as expected. The authors acknowledge the assistance of their colleagues R. M. Brown, R. C. Field, G. L. Salmon and W. S. C. Williams in testing the counter. Also they thank the Department of Nuclear Physics at Liverpool University for their generous support. The counter was constructed in the Oxford University workshops under the direction of Mr. S. A. Tolan.
References 1) R. Cester, P. T. Eschstruth, G. K. O'Neill, B. Quassiati, D. Yount, J. M. Dobbs, A. K. Mann, W. K. McFarlane and D. H. White, Physics Lett. 21 (1966) 343. 3) D. O. Caldwell, J. P. Dowd, K. Heinloth and M. D. Rousseau, Roy. Sci. Instr. 36 (1965) 283. a) j. Frankel, V. Highland, T. Sloan, O. Van Dyck, W. Wales, and D. Wolfe, Rev. Sci. Instr. 37 (1965) 15. 4) F. Din, Thermodynamic functions of gases 2 (Butterwortl~., London, 1956) p. 136. 5) H. Fr6hlich, Theoryof dielectric loss (Clarendon Press, Oxford, 1958) p. 108. e) S. J. Lindenbaum, Methods ofexperimentalphysics Sa (Academic Press, New York, 1961) p. 188. v) A. J. Middleton, Rutherford Laboratory Report, NIRL/R/93 (1965).
(I967) 261--265, ~) N O R T H - H O L L A N D P U B L I S H I N G
CO.
A COMPACT (~ERENKOV COUNTER FOR DETECTING ELECTRONS G. C U L L I G A N and T. W. Q U I R K
Department of Nuclear Physics, University of Oxford, Oxford, U.K. Received 3 April 1967 A compact gas (~erenkov counter for detecting electrons is described. The counter is 0.017 radiation lengths thick along the
beam direction with" an efficiency greater than 0.95 when subtending a solid angle at the electron source of 0.02 of 4n.
1. Introduction
a relativistic electron in a radiator of length 1, with refractive index (1 +r/) is given by the expression
Threshold ~erenkov counters are widely used in high energy physics for detecting eleetronsX-3). However, the thickness of the radiator required for efficient detection produces significant radiation losses and multiple scattering which makes measurements of momentum and direction difficult. This limitation has been overcome in some cases by measuring the direction and momentum of the electron before it passes into the Cerenkov counter 1' 2). However, if the solid angle subtended by the counter is required to be greater than one per cent of 4n, the physical dimensions of the counter make construction difficult and can result in a low efficiency with a high background counting rate. An alternative solution to the problem is described below. It is a transmission ~erenkov counter with an efficiency greater than 95% which presents less than 0.02 radiation lengths of material to particles traversing it. The counter was designed for use in an experiment in which electrons with momenta between 50 MeV/c and 250 MeV/c were produced in the presence of a background of muons and pions in the same momentum range.
n = 800 hi,
(r/,~ 1).
For an average quantum efficiency of 15% and a transport efficiency to the photocathode of 25%, 30 (~erenkov photons produce on average one photoelectron. Thus approximately 150 photons are required for efficient operation, i.e. for 5 photoelectrons. Three possible radiators are compared in table 1. The fraction of a radiation length required to produce 150 photons is given. It is seen from this table that liquid hydrogen is the most efficient radiator. However, its advantages are offset by the technical problems associated with liquid hydrogen. Also, it has a low velocity threshold and is therefore more susceptible to background radiation. Propane at six atmosphere pressure and room temperature is a more suitable choice. The corresponding properties of freon 12 at the same pressure are given for comparison. The refractive constant q for propane at 17°C is given as a function of pressure in fig. I. The refractive constant has been obtained from the density4) and refractive index of propane at NTP, using the ClausiusMosotti equationS).
2. Radiator
The number of (~erenkov photons n, with wavelength between 3500 A units and 5000 A units, produced by
3. ConsU'uetion The structure and apperance of the (~erenkov counter
TABLE I Radiator materials.
Material
Liquid hydrogen Propane :reon 12
Refractive constant r/ at NTP
97 × 10-a l . l O x 10-a 1.15 x 10-a
Density at NTP (rag/era3)
70 2.01 6.33
Radiation length (g/C4"fl2)
58.0 45.0 21.7
* At 17°C; + At 0°C.
261
Fraction of radiation length for 150 photons
Gas pressure (atm)
0.0026 0.008 0.044
6 6
Radiator thickness (cm)
2 29* 25 +
Velocity threshold
0.915 0.994* 0.993 +
262
G. C U L L I G A N
A N D T. W. Q U I R K
7 6
Refractive 5 4
Constant r l x 70_3
jr
/J j
3 2 1 0
2
3
4
5
6
7
Pressure Atmospheres Fig. 1.
The refractive constant ~/ vs gas pressure for propane at 17°C.
are shown in figs. 2 and 3. (~erenkov light produced in the pressure vessel was reflected by a double-sided mirror through perspex lenses into the photomultipliers in a manner previously employed by Lindenbaum and Yuan6). Photons produced in the front half of the counter were reflected directly into the top photomultiplier, those produced in the back half were reflected at the rear window of the counter and then into the bottom photomultiplier. A photomultiplier, its housing and a mu-metal screen formed a single removable unit with the mu-metal screen fixed to the photomultiplier housing with insulating screws. The conical mirror fitted snugly into the mu-metal screen and located the photomultiplier. The photomultiplier base was held in place by grub screws. The pressure vessel, a mild-steel casting machined to a reasonable finish and lined with aluminium foil, was 10" long, with walls one inch thick. Its internal dimensions were six inches by eight inches. Bushes were placed in the various bolt holes to increase the holding strength. The vessel was filled and emptied through a coupling tapped into its side. The equiconvex lenses were made from ultra-violet passing perspex machined to an 8" radius of curvature and 7" aperture. Each lens was 8" in dia., 2.5" thick at the centre and 0.5" thick at the circumference, where it was clamped, with O-rings, between the casting and the steel support by eight steel bolts (fig. 3). A rubber gasket between the support and the casting made a light-tight seal. The perspex conical mirrors were machined to a cone half-angle of 36 ° with inner surfaces polished and aluminized. This angle was defined by the distance from the centre of the lens to the photocathode (4"). These mirrors increased the transport efficiency by reflecting light on to the photocathode.
The general arrangement of the optics gave a reasonably uniform transport efficiency from all parts of the counter for all directions. It is not the best arrangement for a parallel beam but is a compromise which takes account of possible wide divergence. The double sided mirror was made from two 0.001" melinex films stretched and glued on a steel frame. Holes were drilled into the top and bottom edges of the frame to equalize the pressure on the films. The mirror frame was held in place by screws that went through the sides of the casting. The melinex windows were clamped between two aluminium frames as shown in fig. 4. The windows consisted of four layers of 0.01" ordinary melinex with four layers of 0.001" aluminized melinex on the inner surface. The aluminized melinex provided a first reflection for the (~erenkov light and made the counter light tight. Steel bolts, which passed through the melinex, held the windows and their fibre gaskets against the casting. Particles going through the counter passed through 0.017 radiation lengths of material, i.e. 0.009 radiation lengths of melinex and 0.008 radiation lengths of propane.
Fig. 2. The ~erenkov counter with a detached photomultiplier unit.
263
A C O M P A C T (~ERENKOV C O U N T E R
Photomultiplier B a s e . ~ . ~ . ~ Grub Screws
plier Housing ~,
Lomultiplier EMI 6 2 5 5 A Insulating o
Conical
Cathode Screen
Metal ~l
Perspex
Support
Steel Bolts, Ring Seals
DoubleSided
ium
Frame
Mirror
Window
I
1 I I
1 1I r
1)
. . . . .
i I
). . . . . . . .
l
o
I
I
I
i riches
I
I 5
Fig. 3. Schematic diagram of the Cerenkov counter.
1/4' B.S.E.. B o l t s
.
Aluminium
- "/32 Radius
Wire Port 6"x
8"
Fig. 4. Detail of the melinex window and aluminium frame.
Two EMI 6255A quartz-window photomultipliers were used. These were selected tubes with a peak quantum efficiency of greater than 20%. They had a ~;enetian-blind dynode structure with a risetime of 7 nsec. A conventional dynode chain was used with a negative photocathode. The gain and timing of the tubes were checked by light diodes mounted inside the conical mirrors. 4. Safety Safety regulations at the Rutherford Laboratory require that pressure vessels withstand four times the
264
e . C U L L I G A N A N D T. W. QUIRK
10000
1
a { b{
t
1000 ffl "0 cO k) ¢1 ffl 0
t-
Z eJ
(5 r~
100
4~ c-
Pions t.. I1) cl
E Z
10
i ,/
T °'
1
to
/
\
/
~
A~,
i
l
°
°
'
'
'2'0
I
i/o
o
="
3'5 Flight Time in Nanoseconds
4'o
Fig. 5. The time-of-flight spectrum of the beam. (a) without the (~erenkov counter; (b) with the (~:erenkov counter signal in coincidence; (c) with the (~erenkov counter signal in anticoincidence.
operating pressure, in our case 24 atm. The strength of the lenses is more than adequate. The casting was Xrayed and found to be satisfactory. The melinex windows were designed to be consistent with the above requirement. They were tested 7) by applying pressure hydraulically. It was found that the strength of a window of given thickness increased with the number of laminates used, e.g. eight layers of 0.005"
melinex were stronger than four layers of 0.01". However, the thinner the melinex layer, the more liable it became to creep through the clamp and tear at the bolts. 5. Performance The double-sided mirror divides the detector into two independent t~renkov counters, hence it is possible
A COMPACT
(~ERENKOV COUNTER
265
TABLE 2 (~erenkov c o u n t e r efficiencies at 6 a t m o f propane.
Test
E1
Cosmic ray muons z-#-e beam max. efficiency min. efficiency for side 1 for side 2 Averaged efficiency for a divergent beam of 8° half angle
~11
81B ~ e l ~ S
e
0.85 +0.07
0.56 +0.04
0.47 _+0.06
0.94 +0.08
0.992 + 0.005
0.954 + 0.005
0.945 + 0.005
0.996 + 0.005
0.74 +0.02 0.994 + 0.005
0.75 +0.02 0.54 +0.05
0.56 +0.03 0.53 +0.05
0.944 + 0.005 0.994 + 0.005
0.95 +0.06
0.73 +0.02
0.70 +0.02
0.990± 0.006
to calculate efficiencies from coincidences between the two sides. I f some relativistic particles produce n2 counts in the second of these counters and n12 coincidences, then the efficiency of the first counter is:
trons knocked out of the front melinex window into the lenses gave roughly equivalent efficiencies. I f the two sides are used in coincidence then the efficiency for detecting pious and mucus was found to be less than 4 x 10 -5.
81 = n12/t12;
similarly the efficiency of the second counter is: 82 m n12/n 1
The combined efficiency is: 8 ~-~ 81 "~8 2 --8182,
with a coincidence efficiency 812 ~ 81B2.
The counter was tested initially using cosmic ray mucus, and later at the Liverpool 400 MeV synchrocyclotron using a 200 MeV/c beam of positive pious, muons and electrons. Fig. 5 shows the time-of-flight composition of the beam (a) without the (~erenkov counter signal, (b) with the (~erenkov counter signal in coincidence and (c) with the (~erenkov counter in anticoincidence. Table 2 lists the efficiencies obtained. The maximum efficiency was for electrons passing normal to the windows and through the centre of the counter, while the minimum for electrons was obtained from a beam passing diagonally across the counter. The efficiency for detecting pious at the same momentum was (0.33 + 0.02)%. The random coincidence rate was (0.10_0.01)% which left a rate independent efficiency of (0.23_+0.02)%. A similar efficiency for mucus at the same m o m e n t u m was observed in a subsequent experiment, while a calculation based on elec-
6. Conclusion The gas (~erenkov counter described has proved to be efficient and convenient. It was designed for an experimental study of the semi-leptonic decays of the K + mesons. The experiment was successful and the counter performed as expected. The authors acknowledge the assistance of their colleagues R. M. Brown, R. C. Field, G. L. Salmon and W. S. C. Williams in testing the counter. Also they thank the Department of Nuclear Physics at Liverpool University for their generous support. The counter was constructed in the Oxford University workshops under the direction of Mr. S. A. Tolan.
References 1) R. Cester, P. T. Eschstruth, G. K. O'Neill, B. Quassiati, D. Yount, J. M. Dobbs, A. K. Mann, W. K. McFarlane and D. H. White, Physics Lett. 21 (1966) 343. 3) D. O. Caldwell, J. P. Dowd, K. Heinloth and M. D. Rousseau, Roy. Sci. Instr. 36 (1965) 283. a) j. Frankel, V. Highland, T. Sloan, O. Van Dyck, W. Wales, and D. Wolfe, Rev. Sci. Instr. 37 (1965) 15. 4) F. Din, Thermodynamic functions of gases 2 (Butterwortl~., London, 1956) p. 136. 5) H. Fr6hlich, Theoryof dielectric loss (Clarendon Press, Oxford, 1958) p. 108. e) S. J. Lindenbaum, Methods ofexperimentalphysics Sa (Academic Press, New York, 1961) p. 188. v) A. J. Middleton, Rutherford Laboratory Report, NIRL/R/93 (1965).