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A cryogenic magnetic detector for storage ring experiments
Nuclear Instruments and Methods 204 (1983) 379-383 North-Holland Publishing Company
A CRYOGENIC
MAGNETIC
DETECTOR
379
FOR STORAGE
RING EXPERIMENTS
L . M . B A R K O V , G . A . B L I N O V , V.S. O K H A P K I N , N . M . R Y S K U L O V , A.I. S H E H T M A N , V.P. S M A K H T I N , E.P. S O L O D O V , B.I. H A Z I N , V . M . H O R E V a n d V . N . Z A I T S E V
Nuclear Physics Institute, Novosibirsk, U.S.S.R. Received 13 April, 1982
A cryogenic magnetic detector (CMD) for colliding beams experiments with VEPP-2M storage ring has been made. A distinctive feature of this detector is its operation at low temperature to get a higher density of the gas mixture in the optical spark chamber, which allows a spatial resolution of about 50 #m. A momentum resolution op/p =O.05p(GeV/c) has been reached at the 32 kG magnetic field generated by the superconducting solenoid. Only charged particles are detected.
1. General description To study the reaction products from e + e - collisions at the storage ring facility VEPP-2M [1] a cryogenic magnetic detector has been made. A schematic view of the detector is shown in fig. 1. The optical spark chamber (6) with six gaps is mounted inside a superconducting solenoid (7). The solenoid and the spark chamber axes are parallel to the particle motion direction in the
I L d ~ L 4 ®-"
storage ring. The spark chamber operates in the track m o d e at temperature of 1 7 0 - 1 8 0 K and pressure of about 2 atm. The high gas density (3.5 times greater than under normal conditions) allows a spatial resolution of ~ 50 ~ m to be obtained. The chamber is triggered by the two cylindrical proportional wire chambers (MWPC)(8,9). The M W P C s operate at the same gas mixture as the spark chamber. The solid angle covered by the detector is 0.6 × 4rr. Two compensating superconducting solenoids (5) are employed to cancel the influence of the main solenoid field on the circulating beams.
~ ~
2. Superconducting solenoids
......
l-
~.,
FA Fig. 1. The horizontal schematic cross section of the detector. (1) - high voltage feeding, (2) - yoke, (3) - nitrogen shell, (4) magnetic lens of the storage ring, (5) - compensating solenoid, (6) - spark chamber, (7) - main solenoids, (8) - outer MWPC, (9) - inner MWPC, (10) - optic lens, (1 l) - mirror. 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland
The magnetic system of C M D with helium dewar is presented in fig. 2. The magnetic system consists of three superconducting solenoids - the main (8) and two compensating (9) ones. The basic features of the solenoids are listed in table 1. The solenoids were wound with partially stabilized N b T i cable, which gave coils with high current density (9 × 103 A / c m 2 ) . The solenoids are mounted inside a vacuum volume formed by the flux return yoke (6). Transversal and longitudinal temperature displacements of the solenoids are prevented by the support (11). A dewar (3) of 100 litres liquid helium capacity is situated on the yoke. It takes 15 litres for one hour operation. The helium vapor cooling current leads go through the dewar and were constructed in the same manner as the ones described in ref. 2. The magnetic field in the spark chamber volume has been measured and calculated with an accuracy about 0.1%.
380
L.M. Barkov et al. / Cryogenic magnetic detector
Table 1 Main solenoid Inner diam. of coil (mm) Outer diam. of coil (mm) Length of coil (mm) Diam. of superconducting cable (mm) Number of turns Critical current of coil (A) Magnetic field at center of solenoid at the critical current (kG) Maximum working field (kG) Stored energy (kJ)
323 404 320
1.5 4161 290 35 32 170
Compensating solenoids 36 62 153 0.7 3040 220 55 2
3. Spark and proportional chambers 3.1. Construction The spark chamber and M W P C s design is illustrated in fig. 3. The cylindrical spark chamber's electrodes (3) have been soldered by gallium from 0.05 m m aluminium foil. The gap b e t w e e n the electrodes is 17.5 mm. The spark chamber has 6 gaps. The inner electrode is 51 m m in diameter and the outer one 260 mm. The full length of the spark chamber working volume is 220 mm. All spark chamber's c o m p o n e n t s have proportions such that no mechanical deformations occur after cooling. The volume of the chamber is photographed through a plexiglass wall (12). The M W P C have been wound with 28 /xm gold plated tungstem wires with 4 m m wire spacing and 2 X 3 m m gap width and m o u n t e d on the vacuum pipe of the storage ring. The sizes of the plexiglass isolating rings (11) and 0.15 m m gold plated ahiminium cathodes (8) which has the different heat expansion coefficients were selected to give a constant wires tension after cooling. The high voltage is led to the sense wires. The stainless steel pipe (7) with 0.05 m m wall thickness is used as a storage ring's vacuum pipe near the beams' interaction point. A beryllium pipe (9) with 1 m m wall thickness was put inside the vacuum pipe 0.15 mm. The whole thickness of the vacuum pipe and the electrodes of the M W P C is equal to 0.3 g / c m 2 or 0.009 radiation length. Fig. 2. The magnetic system of the detector. (1) - liquid helium supply, (2) - current lead, (3) - dewar, (4) - liquid nitrogen, (5) - liquid helium, (6) - yoke, (7) - nitrogen shell, (8) - main solenoid, (9) - compensating solenoid, (10) - magnetic lens, (11) - solenoid support, (12) - helium supply pipe.
3.2. Gas filling The same gas mixture is used for the spark chamber and the MWPCs. A mixture of Ne + 8% Ar + 2.5% CO 2 was found to be suitable at low temperature [5]. The
L.M. Barkov et al. / Cryogenic magnetic detector
1
2
3
4 5
0
6
50
IOOMM
381
I M4:1 g
8
7
\
I_
i: r_
-...1_5
/
I
Fig. 3. Elements of the spark chamber and MWPC. (1, 4, 5) - isolating rings, (2) - outer electrode of the spark chamber, (3) - foil electrode, (6) - aluminium ring, (9) - beryllium pipe, (8) - electrode (7) - vacuum pipe, (10) - sense wire, (11) - isolating ring, (12) front wall, (13) - prisms, (14) - HV electrode.
presence of CO 2 in the gas mixture limits the lowest working temperature of the chamber to 170-180K. With this mixture the MWPCs have their plateau width near 100V with nearly 100% efficiency. The spark chamber memory time in a magnetic field is about 5 #s. The M W P C time resolution is 2~" = 100 ns.
3.3. Optical system Photographs of events in the spark chamber are taken by the camera that is situated at the focus of the spherical glass lens (10) (see fig. 1). The lens is located directly in front of the chamber (6). The images of two parts of the spark chamber are taken out from the detector bounds and are brought together on the camera with the help of a mirrors system (11). The spark coordinates along the chamber are measured with the help of a prisms system that was proposed in ref. [3]. That system (13 in fig. 3) is placed before the front glass of the spark chamber so that part of the spark light hits the camera after going through prisms. Each cylindrical gap has 20 prisms.
3.4. Pulse high voltage feeding The pulse high voltage of the spark chamber is supplied by means of three Marx generators that are similar to the one described in ref. 4. Each generator feeds two cylindrical gaps. A generator is built up from 5 steps with 20 kV top voltage on each step. Every generator is situated in a cylindrical box which is filled with nitrogen up to a pressure of 3 atm. The cutting discharger is located inside the box to form the back slope of the pulse. Such a generator can produce pulses
with amplitudes up to 100 kV and a minimum pulse duration of 20 ns.
3.5. Trigger Two cylindrical M W P C s with sense wires parallel to the beam axes are used to trigger the spark chamber. The wires in both MWPCs are ordered in 16 groups. The signals from every group were taken through dividing capacitors and proceed via 50 f~ cables to the amplifiers, which are outside of the magnet at 5 m distance. The input threshold is adjustable and is normally set at 2/~A. Selection of desirable coincidences is carried out by the trigger logic block, which can be changed. To decrease a background that arises primarily from beam lost particle showers, the majority coincidence circuit has been used. It inhibits the entire logic system when the number of fired sectors of MWPCs is greater than allowed and it decreases lost particle background by a factor of four. The longitudinal magnetic field of C M D depresses the background by a factor of six. The delay time between a particle's passage and a high voltage pulse occurring on the spark chamber is equal to 300 + 350 ns. C M D is shielded against cosmic rays by the scintillation anticoincidence counters. The triggering rate of the detector is less than 1 Hz at -- 1 × 1030 cm -2 s -~ luminosity.
4. Performance of the detector
4.1. Picture processing An automatic measuring system (AMS) was designed for C M D pictures. A picture is projected onto a mecha-
382
L.M. Barkov et al. / Cryogenic magnetic detector
nical rotating disc with holes. These holes are scanning the gaps of the cylindrical spark chamber. A second disc rotating at the slightly different frequency allows the working hole to change consistently. The light-collector assembly and photomultiplier tube are situated after the discs. If the hole crosses the spark the time interval between strobe signal and output signal from the photomultiplier is measured. AMS is on-line with the computer. The measurement rate is six seconds per frame. Twelve coordinates of the track and six coordinates in prisms were measured for each track and written on the magnetic tape. The absolute accuracy of the coordinates measuring is 1/~m on the film which corresponds to about 20 # m in the chamber space. Fig. 4 shows four tracks event picture and the same event on the display after treatment by AMS.
N
G
~600
N ~I~:
~
52P m~O(
600
80C
qo(
~0c
N ~ Z : { 8mm
e
~60C
A ~ : OZ) m r r l
800
~OC
] t ~ 100
200
50
60
t0
20
Fig. 5. The spatial resolution of the detector in xy plane (a) and in perpendicular planes (b). The distances between interaction point and particle trajectories (c).
4.2. Spatial resolution
The particle trajectories were fitted by a helical curve. The -- 100 ~tm displacements of the sparks in the crossing magnetic and electric fields and the 15% non-uniformity of the magnetic field were taken into account. The histograms of mean square displacements of mea-
J
~
"
/
I .
!
J
\"
4.3. Momentum resolution
~ .
Fig. 7 shows the experimental momentum resolution for particles from the reaction e + e - ~ e + e in the VEPP-2M energy region with 32 kG at the center of the detector. The calculated momentum resolution curve with o R = 52 #m, is shown in the same picture. Multiple scattering in the spark chamber electrodes is taken into
/
/
1 1
_
f
,
1
U
I
sured track points from the reconstructed helical curve for the coordinates in the transvere xy plane (o R) and for the coordinate along the chamber (o~) are shown in figs. 5a, b. Fig. 5c shows the displacements ( A R ) of the obtained helical curves from the point of interaction in the xy plane. The angular resolutions of the detector for tracks in the xy plane ( % ) and in the perpendicular planes (oa) are illustrated by the distributions of discollinearity angles for electron-positron events in fig. 6a, b.
m
115
N
i .
.
•
4"
.
6's: 2
~P-.Iml m,,
"
7q
I|ff,~
-.ml
i
BI]
~]l~ ~-Jei*~f, cut -t0 %
Fig. 4. Four track event picture and the same event on the display after coordinate measurement by scanning system,
0
t0
-10
t0
,, e, d~
Fig. 6. The angular resolution of the detector for two particles in the xy plane (a) and in the perpendicular planes (b).
383
L.M. Barkov et al. / Cryogenic magnetic detector
p,Z
account. By using the interaction point for the trajectory fit the m o m e n t u m resolution can be improved (open circles a n d the lower curve in fig. 7). The m o m e n t u m distribution for detected particles at the energy 2 E = 430 M e V is shown in fig. 8.
H = 32kG t~ 3
4.4. Multitrack efficiency 2
0
Fig. 7. The experimental and calculated momentum resolution of the detector. Open circles and lower curve show the momentum resolution with the interaction point taken into account
The multitrack efficiency of the spark c h a m b e r was studied by m e a n s of m a n y particles events. The probability ( E ) of the spark lost in one gap for two, three and four particles in the c h a m b e r are listed in table 2. Only the tracks with angle less than 45 ° with respect to electric field were treated. The multitrack efficiency d a t a listed were o b t a i n e d with the o p t i m u m characters of the high voltage pulse: the amplitude was equal to 80 kV and the pulse d u r a t i o n was 3 0 - 4 0 ns. W i t h such pulse parameters the spark c h a m b e r operates close to the streamer mode, so one can see the streamer structure of the spark.
N .50C
E = ~ISMeV H - 2.q le~
5. Conclusion The C M D was installed in the straight line section of the V E P P - 2 M in the beginning of 1979. Experiments with beams show n o decreasing of the luminosity up to the m a x i m u m magnetic field in the detector. A b o u t one million pictures were o b t a i n e d during two years operation. The e + e - --,~r+~r - , K + K - , ~r+~r-~r+~r crosssections were measured in the V E P P - 2 M energy range. T h e data is u n d e r treatment. Experiments with C M D are in progress.
qOC
30£
20C
400
T M
IY,
450
t'~0
~go
2~0
23O
References
P, M~Vc Fig. 8. The momentum distribution of the particles at 2E =430 MeV.
Table 2 Number of particles in a gap E%
2 0.034
3 0.07
4 0.09
[1] I.B. Vasserman, I.A. Koop, V.P. Kutovoi et al., Proc. of Fifth All-Union Meeting on Charged Particle Accelerators (Moscow, 1976) p. 252. [2] K.R. Efferson, Rev. Sci. Instr. 38 (1967) 1776. [3] S.M. Korenchenko, A.G. Morozow, K.G. Nekrasov, U.V. Rodnow. 0INR, R13-5170, Dubna, 1970). [4] D.D. Grossman, Nucl. Instr. and Meth. 51 (1967) 165. [5] L.M. Barkov et al. III International Meeting on Proportional and Drift Chambers (JINR, D13-11807, Dubna, 1978) p. 225.
A CRYOGENIC
MAGNETIC
DETECTOR
379
FOR STORAGE
RING EXPERIMENTS
L . M . B A R K O V , G . A . B L I N O V , V.S. O K H A P K I N , N . M . R Y S K U L O V , A.I. S H E H T M A N , V.P. S M A K H T I N , E.P. S O L O D O V , B.I. H A Z I N , V . M . H O R E V a n d V . N . Z A I T S E V
Nuclear Physics Institute, Novosibirsk, U.S.S.R. Received 13 April, 1982
A cryogenic magnetic detector (CMD) for colliding beams experiments with VEPP-2M storage ring has been made. A distinctive feature of this detector is its operation at low temperature to get a higher density of the gas mixture in the optical spark chamber, which allows a spatial resolution of about 50 #m. A momentum resolution op/p =O.05p(GeV/c) has been reached at the 32 kG magnetic field generated by the superconducting solenoid. Only charged particles are detected.
1. General description To study the reaction products from e + e - collisions at the storage ring facility VEPP-2M [1] a cryogenic magnetic detector has been made. A schematic view of the detector is shown in fig. 1. The optical spark chamber (6) with six gaps is mounted inside a superconducting solenoid (7). The solenoid and the spark chamber axes are parallel to the particle motion direction in the
I L d ~ L 4 ®-"
storage ring. The spark chamber operates in the track m o d e at temperature of 1 7 0 - 1 8 0 K and pressure of about 2 atm. The high gas density (3.5 times greater than under normal conditions) allows a spatial resolution of ~ 50 ~ m to be obtained. The chamber is triggered by the two cylindrical proportional wire chambers (MWPC)(8,9). The M W P C s operate at the same gas mixture as the spark chamber. The solid angle covered by the detector is 0.6 × 4rr. Two compensating superconducting solenoids (5) are employed to cancel the influence of the main solenoid field on the circulating beams.
~ ~
2. Superconducting solenoids
......
l-
~.,
FA Fig. 1. The horizontal schematic cross section of the detector. (1) - high voltage feeding, (2) - yoke, (3) - nitrogen shell, (4) magnetic lens of the storage ring, (5) - compensating solenoid, (6) - spark chamber, (7) - main solenoids, (8) - outer MWPC, (9) - inner MWPC, (10) - optic lens, (1 l) - mirror. 0167-5087/83/0000-0000/$03.00 © 1983 North-Holland
The magnetic system of C M D with helium dewar is presented in fig. 2. The magnetic system consists of three superconducting solenoids - the main (8) and two compensating (9) ones. The basic features of the solenoids are listed in table 1. The solenoids were wound with partially stabilized N b T i cable, which gave coils with high current density (9 × 103 A / c m 2 ) . The solenoids are mounted inside a vacuum volume formed by the flux return yoke (6). Transversal and longitudinal temperature displacements of the solenoids are prevented by the support (11). A dewar (3) of 100 litres liquid helium capacity is situated on the yoke. It takes 15 litres for one hour operation. The helium vapor cooling current leads go through the dewar and were constructed in the same manner as the ones described in ref. 2. The magnetic field in the spark chamber volume has been measured and calculated with an accuracy about 0.1%.
380
L.M. Barkov et al. / Cryogenic magnetic detector
Table 1 Main solenoid Inner diam. of coil (mm) Outer diam. of coil (mm) Length of coil (mm) Diam. of superconducting cable (mm) Number of turns Critical current of coil (A) Magnetic field at center of solenoid at the critical current (kG) Maximum working field (kG) Stored energy (kJ)
323 404 320
1.5 4161 290 35 32 170
Compensating solenoids 36 62 153 0.7 3040 220 55 2
3. Spark and proportional chambers 3.1. Construction The spark chamber and M W P C s design is illustrated in fig. 3. The cylindrical spark chamber's electrodes (3) have been soldered by gallium from 0.05 m m aluminium foil. The gap b e t w e e n the electrodes is 17.5 mm. The spark chamber has 6 gaps. The inner electrode is 51 m m in diameter and the outer one 260 mm. The full length of the spark chamber working volume is 220 mm. All spark chamber's c o m p o n e n t s have proportions such that no mechanical deformations occur after cooling. The volume of the chamber is photographed through a plexiglass wall (12). The M W P C have been wound with 28 /xm gold plated tungstem wires with 4 m m wire spacing and 2 X 3 m m gap width and m o u n t e d on the vacuum pipe of the storage ring. The sizes of the plexiglass isolating rings (11) and 0.15 m m gold plated ahiminium cathodes (8) which has the different heat expansion coefficients were selected to give a constant wires tension after cooling. The high voltage is led to the sense wires. The stainless steel pipe (7) with 0.05 m m wall thickness is used as a storage ring's vacuum pipe near the beams' interaction point. A beryllium pipe (9) with 1 m m wall thickness was put inside the vacuum pipe 0.15 mm. The whole thickness of the vacuum pipe and the electrodes of the M W P C is equal to 0.3 g / c m 2 or 0.009 radiation length. Fig. 2. The magnetic system of the detector. (1) - liquid helium supply, (2) - current lead, (3) - dewar, (4) - liquid nitrogen, (5) - liquid helium, (6) - yoke, (7) - nitrogen shell, (8) - main solenoid, (9) - compensating solenoid, (10) - magnetic lens, (11) - solenoid support, (12) - helium supply pipe.
3.2. Gas filling The same gas mixture is used for the spark chamber and the MWPCs. A mixture of Ne + 8% Ar + 2.5% CO 2 was found to be suitable at low temperature [5]. The
L.M. Barkov et al. / Cryogenic magnetic detector
1
2
3
4 5
0
6
50
IOOMM
381
I M4:1 g
8
7
\
I_
i: r_
-...1_5
/
I
Fig. 3. Elements of the spark chamber and MWPC. (1, 4, 5) - isolating rings, (2) - outer electrode of the spark chamber, (3) - foil electrode, (6) - aluminium ring, (9) - beryllium pipe, (8) - electrode (7) - vacuum pipe, (10) - sense wire, (11) - isolating ring, (12) front wall, (13) - prisms, (14) - HV electrode.
presence of CO 2 in the gas mixture limits the lowest working temperature of the chamber to 170-180K. With this mixture the MWPCs have their plateau width near 100V with nearly 100% efficiency. The spark chamber memory time in a magnetic field is about 5 #s. The M W P C time resolution is 2~" = 100 ns.
3.3. Optical system Photographs of events in the spark chamber are taken by the camera that is situated at the focus of the spherical glass lens (10) (see fig. 1). The lens is located directly in front of the chamber (6). The images of two parts of the spark chamber are taken out from the detector bounds and are brought together on the camera with the help of a mirrors system (11). The spark coordinates along the chamber are measured with the help of a prisms system that was proposed in ref. [3]. That system (13 in fig. 3) is placed before the front glass of the spark chamber so that part of the spark light hits the camera after going through prisms. Each cylindrical gap has 20 prisms.
3.4. Pulse high voltage feeding The pulse high voltage of the spark chamber is supplied by means of three Marx generators that are similar to the one described in ref. 4. Each generator feeds two cylindrical gaps. A generator is built up from 5 steps with 20 kV top voltage on each step. Every generator is situated in a cylindrical box which is filled with nitrogen up to a pressure of 3 atm. The cutting discharger is located inside the box to form the back slope of the pulse. Such a generator can produce pulses
with amplitudes up to 100 kV and a minimum pulse duration of 20 ns.
3.5. Trigger Two cylindrical M W P C s with sense wires parallel to the beam axes are used to trigger the spark chamber. The wires in both MWPCs are ordered in 16 groups. The signals from every group were taken through dividing capacitors and proceed via 50 f~ cables to the amplifiers, which are outside of the magnet at 5 m distance. The input threshold is adjustable and is normally set at 2/~A. Selection of desirable coincidences is carried out by the trigger logic block, which can be changed. To decrease a background that arises primarily from beam lost particle showers, the majority coincidence circuit has been used. It inhibits the entire logic system when the number of fired sectors of MWPCs is greater than allowed and it decreases lost particle background by a factor of four. The longitudinal magnetic field of C M D depresses the background by a factor of six. The delay time between a particle's passage and a high voltage pulse occurring on the spark chamber is equal to 300 + 350 ns. C M D is shielded against cosmic rays by the scintillation anticoincidence counters. The triggering rate of the detector is less than 1 Hz at -- 1 × 1030 cm -2 s -~ luminosity.
4. Performance of the detector
4.1. Picture processing An automatic measuring system (AMS) was designed for C M D pictures. A picture is projected onto a mecha-
382
L.M. Barkov et al. / Cryogenic magnetic detector
nical rotating disc with holes. These holes are scanning the gaps of the cylindrical spark chamber. A second disc rotating at the slightly different frequency allows the working hole to change consistently. The light-collector assembly and photomultiplier tube are situated after the discs. If the hole crosses the spark the time interval between strobe signal and output signal from the photomultiplier is measured. AMS is on-line with the computer. The measurement rate is six seconds per frame. Twelve coordinates of the track and six coordinates in prisms were measured for each track and written on the magnetic tape. The absolute accuracy of the coordinates measuring is 1/~m on the film which corresponds to about 20 # m in the chamber space. Fig. 4 shows four tracks event picture and the same event on the display after treatment by AMS.
N
G
~600
N ~I~:
~
52P m~O(
600
80C
qo(
~0c
N ~ Z : { 8mm
e
~60C
A ~ : OZ) m r r l
800
~OC
] t ~ 100
200
50
60
t0
20
Fig. 5. The spatial resolution of the detector in xy plane (a) and in perpendicular planes (b). The distances between interaction point and particle trajectories (c).
4.2. Spatial resolution
The particle trajectories were fitted by a helical curve. The -- 100 ~tm displacements of the sparks in the crossing magnetic and electric fields and the 15% non-uniformity of the magnetic field were taken into account. The histograms of mean square displacements of mea-
J
~
"
/
I .
!
J
\"
4.3. Momentum resolution
~ .
Fig. 7 shows the experimental momentum resolution for particles from the reaction e + e - ~ e + e in the VEPP-2M energy region with 32 kG at the center of the detector. The calculated momentum resolution curve with o R = 52 #m, is shown in the same picture. Multiple scattering in the spark chamber electrodes is taken into
/
/
1 1
_
f
,
1
U
I
sured track points from the reconstructed helical curve for the coordinates in the transvere xy plane (o R) and for the coordinate along the chamber (o~) are shown in figs. 5a, b. Fig. 5c shows the displacements ( A R ) of the obtained helical curves from the point of interaction in the xy plane. The angular resolutions of the detector for tracks in the xy plane ( % ) and in the perpendicular planes (oa) are illustrated by the distributions of discollinearity angles for electron-positron events in fig. 6a, b.
m
115
N
i .
.
•
4"
.
6's: 2
~P-.Iml m,,
"
7q
I|ff,~
-.ml
i
BI]
~]l~ ~-Jei*~f, cut -t0 %
Fig. 4. Four track event picture and the same event on the display after coordinate measurement by scanning system,
0
t0
-10
t0
,, e, d~
Fig. 6. The angular resolution of the detector for two particles in the xy plane (a) and in the perpendicular planes (b).
383
L.M. Barkov et al. / Cryogenic magnetic detector
p,Z
account. By using the interaction point for the trajectory fit the m o m e n t u m resolution can be improved (open circles a n d the lower curve in fig. 7). The m o m e n t u m distribution for detected particles at the energy 2 E = 430 M e V is shown in fig. 8.
H = 32kG t~ 3
4.4. Multitrack efficiency 2
0
Fig. 7. The experimental and calculated momentum resolution of the detector. Open circles and lower curve show the momentum resolution with the interaction point taken into account
The multitrack efficiency of the spark c h a m b e r was studied by m e a n s of m a n y particles events. The probability ( E ) of the spark lost in one gap for two, three and four particles in the c h a m b e r are listed in table 2. Only the tracks with angle less than 45 ° with respect to electric field were treated. The multitrack efficiency d a t a listed were o b t a i n e d with the o p t i m u m characters of the high voltage pulse: the amplitude was equal to 80 kV and the pulse d u r a t i o n was 3 0 - 4 0 ns. W i t h such pulse parameters the spark c h a m b e r operates close to the streamer mode, so one can see the streamer structure of the spark.
N .50C
E = ~ISMeV H - 2.q le~
5. Conclusion The C M D was installed in the straight line section of the V E P P - 2 M in the beginning of 1979. Experiments with beams show n o decreasing of the luminosity up to the m a x i m u m magnetic field in the detector. A b o u t one million pictures were o b t a i n e d during two years operation. The e + e - --,~r+~r - , K + K - , ~r+~r-~r+~r crosssections were measured in the V E P P - 2 M energy range. T h e data is u n d e r treatment. Experiments with C M D are in progress.
qOC
30£
20C
400
T M
IY,
450
t'~0
~go
2~0
23O
References
P, M~Vc Fig. 8. The momentum distribution of the particles at 2E =430 MeV.
Table 2 Number of particles in a gap E%
2 0.034
3 0.07
4 0.09
[1] I.B. Vasserman, I.A. Koop, V.P. Kutovoi et al., Proc. of Fifth All-Union Meeting on Charged Particle Accelerators (Moscow, 1976) p. 252. [2] K.R. Efferson, Rev. Sci. Instr. 38 (1967) 1776. [3] S.M. Korenchenko, A.G. Morozow, K.G. Nekrasov, U.V. Rodnow. 0INR, R13-5170, Dubna, 1970). [4] D.D. Grossman, Nucl. Instr. and Meth. 51 (1967) 165. [5] L.M. Barkov et al. III International Meeting on Proportional and Drift Chambers (JINR, D13-11807, Dubna, 1978) p. 225.