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Experimental investigation of proposed transition effect in particle ionisation loss
:NUCLEAR
INSTRUMENTS
AND
METHODS
I3I
(I975)
I3-I5;
© NORTH-HOLLAND
PUBLISHING
CO.
E X P E R I M E N T A L I N V E S T I G A T I O N OF P R O P O S E D T R A N S I T I O N E F F E C T I N PARTICLE IONISATION LOSS G.C.
SMITH and E. MATHIESON
Physics Department, The University, Leicester, LE1 7RH England Received 23 September 1975 Measurements of the ionisation loss of relativistic particles in traversing thin layers of gas have revealed a discrepancy between observation and theoretical prediction. Garibian has proposed a theoretical transition mechanism to account for this situation.
The present experiment, with 2.4 GeV positrons and thin layers of argon, attempted to measure this transition effect. No significant effect was found under the particular conditions of the experiment.
1. Introduction
2) Particles entered the detector after passing through a solid window, or cathode. In one experiment we used a multi-wire proportional chamber (MWPC) (fig. 1) which contained two different types of cathode plane, one being a 100/~m sheet of aluminium, the other a multiwire plane, transparency 98% consisting of 1 m m spaced, 20 #m diameter gold-plated tungsten. The chamber window (50/~m aluminized melinex) was about 2.5 cm behind the wire cathode and all the anode wires were connected together and attached to a charge-sensitive amplifier; the chamber was continuously flushed with a gas mixture 90% argon, 10% methane by volume. The two conditions for measurement of energy loss referred to above were thus: 1) particles entered the chamber through the transparent multi-wire cathode as shown in fig. 1; 2) (after the chamber had been turned through
Several experiments (e.g. refs. 1, 2) have shown that the ionisation loss of high energy charged particles traversing a small thickness of gas is less than the result predicted by theory3). Garibian 4) has been able to formulate a mechanism for this reduction in ionisation in terms of a transition effect as the particle emerges from the high density detector wall into the gas. Provided the wall, or window, has a thickness >~ 10 -4 cm, the particle field in the detector wall is different from the particle field in the detector gas; Garibian's theory shows that, when the particle leaves the wall, a finite distance is required for the particle field to establish its normal value in the detector gas. The ionisation due to the particle may therefore be lower close to the wall than at distances of the order of centimeters from the wall. During the course of a series of experiments at the Daresbury Laboratory electron synchrotron concerning the detection of X-ray transition radiation from relativistic charged particles we were able to obtain some accurate measurements of ionization loss in a small thickness of gas, and our results also showed a similar discrepancy with theoretical prediction. We therefore took the opportunity to modify our apparatus somewhat in order to examine directly the efficacy of Garibian's theory.
Cathode Plane
Perspex Frame
~ . ( 2 0 ~ m diameter, 1turn // spaced go~d plated
X~ i
i
I[1
L
tungsten)
Positron Beam
2. Experimental arrangement We carried out two experiments, in each of which we measured energy loss in a particle detector under two different conditions: 1) Particles entered the active volume of the detector through an interface in which there was no significant change in dielectric constant.
50)am aluminized melinex window
/// 2.5cm
//
m
Cathode 1.2cm
Fig. l. Multi-wire proportional chamber with asymmetric cathodes.
13
14
G. C. S M I T H A N D E. M A T H I E S O N
180 °) particles entered the chamber through the aluminium cathode. The chamber was operated with an earthed cathode and a positive potential was applied to the aluminized melinex window such that there was a field of about 150 V/cm between it and the wire cathode, lonization created by a particle in this 2.5 cm region was therefore prevented from drifting into the active volume of the MWPC. The 1 mm spaced wire cathode did not provide a well defined geometric boundary between the drift region and the proportional chamber proper so that measurements obtained with the above two conditions could not be regarded as yielding distributions of absolute ionization loss. A second experiment was also carried out for which we built a special coaxial proportional counter, shown in fig. 2. This counter consisted of a cylindrical stainless steel cathode, diameter 2.5 cm and a 50/~m diameter tungsten anode which was divided into two insulated sections by a small glass bead 12.7 cm from one end of the counter and 2.5 cm from the other. Each section of wire was supplied with E H T via two independent charge sensitive amplifiers. The counter was placed in, and parallel to, the positron beam so that particles traversed the whole length of the counter. Energy loss measurements were taken only with the 2.5 cm anode wire, so that our two conditions this time were: 1) particles entered the region with the 2.5 cm anode wire after having been in the same gaseous medium for over 12 cm (as in fig. 2), 2) (after the counter had been turned through 180 °) particles entered the region with the 2.5 cm anode wire after emerging from the aluminium end wall of the counter. 3. Results and discussion Ionisation loss distributions of 2.4 GeV positrons in the MWPC for conditions (1) and (2) are shown in
fig. 3. (The peak at channel 70 corresponds to saturation of the main amplifier.) Each distribution is normalised to 10 000 events and channel 29 corresponds to the peak of the 5.9 keV X-ray spectrum; this latter spectrum is shown in fig. 4, where the resolution at 5.9 keV is about 20%. If the transition effect is to explain completely the reported discrepancy between observation and theory then the most probable energy loss in fig. 3 (i) should be greater than that in fig. 3 (ii) by a factor of about 23%. It can be seen from the distributions in fig. 3 that there is no stastically significant change in most probable energy loss between conditions (l) and (2). Although Garibian's theory refers specifically to the most probable energy loss, the average energy loss 1200-
1000~
(i)
800-
\
600-
400-
200-
{/I
0
E
I'0
2'0
3~0
4'0
50
60
70
10
20
30
AO
50
60
70
1200
1000-
800.
600-
LO0-
Aluminium End-wall
Cathode
\ , / :o0.
Insulating Glass Bead
/Glass _~
(
200-
Seal 0
Channel No. 12.7cm
7gcm
Fig. 2. Divided anode wire proportional counter.
Fig. 3. Charge spectra from M W P C from traversal o f 2.4 GeV positrons (i) entering c h a m b e r through 2.5 cm A / 1 0 % CH4 and transparent wire cathode, (ii) entering c h a m b e r t h r o u g h 100 Hm AI cathode.
P R O P O S E D T R A N S I T I O N EFFECT
is agreement, within the limits of experimental error, between the average energy losses for the two different conditions of ionisation loss measurement. lonisation loss distributions from 2 GeV positrons were obtained with the single-wire proportional counter and again there was no statistically significant difference between the most probable energy losses (or average energy losses) for the two different conditions of measurement. From these results, therefore, we conclude that under the conditions of the experiments described above the proposed Garibian transition effect is negligibly small. A further explanation is therefore required to account for experimental absolute ionisation losses being smaller than theoretical prediction.
1400-
1200-
1000
8000 600-
4,00-
200-
0
15
L. I'0
2'0
3'0
40
s'0
go
40
Channel No.
We are grateful to the Director of the Daresbury Physics Laboratory (S.R.C.) for making available the facilities to carry out this work.
Fig. 4. Charge spectrum from M W P C from 5.9 keV X-ray source.
References
for the distributions in fig. 3 have been calculated. The distribution in fig. 3 (i) has an average energy loss (3.77+_0.05) keV and the distribution in fig. 3 (ii) has an average energy loss (3.76-t-0.05) keV. Thus there
1) p. v. Ramana Murthy, Nucl. Instr. and Meth. 68 (1968) 79. 2) Z. Dimcovksy, J. Favier, G. Charpak and G. Amato, Nucl. Instr. and Meth. 94 (1971) 151. a) R. M. Sternheimer and P. F. Peierls, Phys. Rev. B3 (1971) 3681. 4) G. M. Garibian, Sov. Phys. JETP Lett. 16 (1972) 413.
INSTRUMENTS
AND
METHODS
I3I
(I975)
I3-I5;
© NORTH-HOLLAND
PUBLISHING
CO.
E X P E R I M E N T A L I N V E S T I G A T I O N OF P R O P O S E D T R A N S I T I O N E F F E C T I N PARTICLE IONISATION LOSS G.C.
SMITH and E. MATHIESON
Physics Department, The University, Leicester, LE1 7RH England Received 23 September 1975 Measurements of the ionisation loss of relativistic particles in traversing thin layers of gas have revealed a discrepancy between observation and theoretical prediction. Garibian has proposed a theoretical transition mechanism to account for this situation.
The present experiment, with 2.4 GeV positrons and thin layers of argon, attempted to measure this transition effect. No significant effect was found under the particular conditions of the experiment.
1. Introduction
2) Particles entered the detector after passing through a solid window, or cathode. In one experiment we used a multi-wire proportional chamber (MWPC) (fig. 1) which contained two different types of cathode plane, one being a 100/~m sheet of aluminium, the other a multiwire plane, transparency 98% consisting of 1 m m spaced, 20 #m diameter gold-plated tungsten. The chamber window (50/~m aluminized melinex) was about 2.5 cm behind the wire cathode and all the anode wires were connected together and attached to a charge-sensitive amplifier; the chamber was continuously flushed with a gas mixture 90% argon, 10% methane by volume. The two conditions for measurement of energy loss referred to above were thus: 1) particles entered the chamber through the transparent multi-wire cathode as shown in fig. 1; 2) (after the chamber had been turned through
Several experiments (e.g. refs. 1, 2) have shown that the ionisation loss of high energy charged particles traversing a small thickness of gas is less than the result predicted by theory3). Garibian 4) has been able to formulate a mechanism for this reduction in ionisation in terms of a transition effect as the particle emerges from the high density detector wall into the gas. Provided the wall, or window, has a thickness >~ 10 -4 cm, the particle field in the detector wall is different from the particle field in the detector gas; Garibian's theory shows that, when the particle leaves the wall, a finite distance is required for the particle field to establish its normal value in the detector gas. The ionisation due to the particle may therefore be lower close to the wall than at distances of the order of centimeters from the wall. During the course of a series of experiments at the Daresbury Laboratory electron synchrotron concerning the detection of X-ray transition radiation from relativistic charged particles we were able to obtain some accurate measurements of ionization loss in a small thickness of gas, and our results also showed a similar discrepancy with theoretical prediction. We therefore took the opportunity to modify our apparatus somewhat in order to examine directly the efficacy of Garibian's theory.
Cathode Plane
Perspex Frame
~ . ( 2 0 ~ m diameter, 1turn // spaced go~d plated
X~ i
i
I[1
L
tungsten)
Positron Beam
2. Experimental arrangement We carried out two experiments, in each of which we measured energy loss in a particle detector under two different conditions: 1) Particles entered the active volume of the detector through an interface in which there was no significant change in dielectric constant.
50)am aluminized melinex window
/// 2.5cm
//
m
Cathode 1.2cm
Fig. l. Multi-wire proportional chamber with asymmetric cathodes.
13
14
G. C. S M I T H A N D E. M A T H I E S O N
180 °) particles entered the chamber through the aluminium cathode. The chamber was operated with an earthed cathode and a positive potential was applied to the aluminized melinex window such that there was a field of about 150 V/cm between it and the wire cathode, lonization created by a particle in this 2.5 cm region was therefore prevented from drifting into the active volume of the MWPC. The 1 mm spaced wire cathode did not provide a well defined geometric boundary between the drift region and the proportional chamber proper so that measurements obtained with the above two conditions could not be regarded as yielding distributions of absolute ionization loss. A second experiment was also carried out for which we built a special coaxial proportional counter, shown in fig. 2. This counter consisted of a cylindrical stainless steel cathode, diameter 2.5 cm and a 50/~m diameter tungsten anode which was divided into two insulated sections by a small glass bead 12.7 cm from one end of the counter and 2.5 cm from the other. Each section of wire was supplied with E H T via two independent charge sensitive amplifiers. The counter was placed in, and parallel to, the positron beam so that particles traversed the whole length of the counter. Energy loss measurements were taken only with the 2.5 cm anode wire, so that our two conditions this time were: 1) particles entered the region with the 2.5 cm anode wire after having been in the same gaseous medium for over 12 cm (as in fig. 2), 2) (after the counter had been turned through 180 °) particles entered the region with the 2.5 cm anode wire after emerging from the aluminium end wall of the counter. 3. Results and discussion Ionisation loss distributions of 2.4 GeV positrons in the MWPC for conditions (1) and (2) are shown in
fig. 3. (The peak at channel 70 corresponds to saturation of the main amplifier.) Each distribution is normalised to 10 000 events and channel 29 corresponds to the peak of the 5.9 keV X-ray spectrum; this latter spectrum is shown in fig. 4, where the resolution at 5.9 keV is about 20%. If the transition effect is to explain completely the reported discrepancy between observation and theory then the most probable energy loss in fig. 3 (i) should be greater than that in fig. 3 (ii) by a factor of about 23%. It can be seen from the distributions in fig. 3 that there is no stastically significant change in most probable energy loss between conditions (l) and (2). Although Garibian's theory refers specifically to the most probable energy loss, the average energy loss 1200-
1000~
(i)
800-
\
600-
400-
200-
{/I
0
E
I'0
2'0
3~0
4'0
50
60
70
10
20
30
AO
50
60
70
1200
1000-
800.
600-
LO0-
Aluminium End-wall
Cathode
\ , / :o0.
Insulating Glass Bead
/Glass _~
(
200-
Seal 0
Channel No. 12.7cm
7gcm
Fig. 2. Divided anode wire proportional counter.
Fig. 3. Charge spectra from M W P C from traversal o f 2.4 GeV positrons (i) entering c h a m b e r through 2.5 cm A / 1 0 % CH4 and transparent wire cathode, (ii) entering c h a m b e r t h r o u g h 100 Hm AI cathode.
P R O P O S E D T R A N S I T I O N EFFECT
is agreement, within the limits of experimental error, between the average energy losses for the two different conditions of ionisation loss measurement. lonisation loss distributions from 2 GeV positrons were obtained with the single-wire proportional counter and again there was no statistically significant difference between the most probable energy losses (or average energy losses) for the two different conditions of measurement. From these results, therefore, we conclude that under the conditions of the experiments described above the proposed Garibian transition effect is negligibly small. A further explanation is therefore required to account for experimental absolute ionisation losses being smaller than theoretical prediction.
1400-
1200-
1000
8000 600-
4,00-
200-
0
15
L. I'0
2'0
3'0
40
s'0
go
40
Channel No.
We are grateful to the Director of the Daresbury Physics Laboratory (S.R.C.) for making available the facilities to carry out this work.
Fig. 4. Charge spectrum from M W P C from 5.9 keV X-ray source.
References
for the distributions in fig. 3 have been calculated. The distribution in fig. 3 (i) has an average energy loss (3.77+_0.05) keV and the distribution in fig. 3 (ii) has an average energy loss (3.76-t-0.05) keV. Thus there
1) p. v. Ramana Murthy, Nucl. Instr. and Meth. 68 (1968) 79. 2) Z. Dimcovksy, J. Favier, G. Charpak and G. Amato, Nucl. Instr. and Meth. 94 (1971) 151. a) R. M. Sternheimer and P. F. Peierls, Phys. Rev. B3 (1971) 3681. 4) G. M. Garibian, Sov. Phys. JETP Lett. 16 (1972) 413.