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Ionization discrimination in helium-oxygen spark chambers
N U C L E A R I N S T R U M E N T S AND METHODS
169 (1980)
121:124;
(~) N O R T H - H O L L A N D
P U B L I S H I N G CO.
IONIZATION DISCRIMINATION IN HELIUM-OXYGEN SPARK CHAMBERS M.J. ASHBURN, T. G. CALDWELL, R. N. C. GRAY, D.C. STUTELEY* and P. C. M. YOCK
Department of Physics, University of Auckland, Auckland, New Zealand Received 3 July 1979 and in revised form 27 September 1979 It was found that helium-oxygen filled track spark chambers may be systematically operated so that the efficiency for minimum ionizing particles is low whilst for highly ionizing particles it. is high. The threshold ionization for 50% efficiency may be set anywhere from less than minimum ionization to very high values of ionization by simple adjustments of the operating conditions of the spark chamber. Typical charge resolutions attainable for individual fast particles using a multigap system are - 3 0 % .
1. Introduction Some investigations may require a detector, possibly of large area, which is sensitive to highly ionizing particles only. For example, such a situation may arise in heavy-ion physics. Searches currently planned j'2) for magnetic monopoles and related particles 3) provide further examples. The present investigation of helium-oxygen filled spark chambers was prompted by a search for highly ionizing, penetrating particles 3) in the cosmic radiation at sea level which is presently being conducted at the University of Auckland. The purpose of this paper is to report our findings on the operating characteristics of helium-oxygen filled spark chambers, because such chambers may be of use in other applications. Our investigations involved the measurement of spark chamber efficiencies under a variety of situations. The oxygen content of the helium-oxygen filled chambers was varied in the range ½ to 1%, and chamber efficiencies were measured using cosmic rays and also low energy protons having 15 to 3,0 times minimum ionization. Chamber efficiencies were measured as a function of the delay, height and width of the high voltage pulse, and as a function of the angle of the particles relative to the chamber axis. At all times the chambers functioned as track-chambers, with sparks aligned along the particle trajectories. It was found that, under suitable operating conditions as detailed below, significant discrimination in the chamber efficiency could be attained between cosmic rays and protons of 15×minimum ionization, and between protons of 15 × and 30 × minimum ionization. The plan of the paper is as follows. In the following section the experimental work is described and Present address: Cornell University, Ithaca, NY, USA.
in section 3 the experimental results are analyzed. Here an expression is obtained which may be used to predict chamber efficiencies. Conclusions and general discussion are given in section 4. We note that related investigations of spark chamber performance using mixed gas filling have been carried out by Schneider and Hohne 4) and by Burnham and ThompsonS). However these authors did not investigate the dependence of the chamber efficiency on ionization. The efficiency of narrow gap argon filled chambers for cosmic ray muons and o~ particles (from Po decay) was investigated by Grieder and some discrimination was found with this pure nobe gas filling6). Our early studies were made with pure noble gas fillings and we did obtain qualitatively similar results as Grieder's. However, only the results obtained with He-O2 fillings are reported here, because we found that the addition of a small amount of oxygen greatly enhanced the discrimination in ionization. 2. Experimental details Two 30 c m × 3 0 cm chambers were constructed with glass walls, one having aluminium foil electrodes (49 mm gap) and the other stainless steel electrodes (45 mm gap). The electrodes were attached with epoxy resin (Araldite). The heliumoxygen gas filling, at atmospheric pressure, was maintained by a continuous flow of - 1 cm 3 s -1. The oxygen content could be varied in the range 0.5-1.06% and was monitored by gas flow rates and, independently, by gas chromatography. Commercial grade gases were used and impurities at the level of a few parts per million including CO2, H2, Ne and H20 were probably present, but were assumed to have negligible effect in view of the large amount of oxygen present.
122
M.
J.
ASHBURN
The performance of.these chambers was investigated using both cosmic rays and low energy protons (IO-20 MeV) as the source of ionization. The low energy proton beams were provided by a system that has been described previously’). The trigger for the high voltage pulse was derived from coincident signals from two scintillators adjacent to the spark chamber. An RC pulse was used with R = 330 a. A barium titanate capacitor (Sprague) having a nominal capacitance of 2.5 nF was used for most of the investigation and a similar capacitor, nominally 3.6 nF, for the remainder. The high voltage pulse was applied at a delay ranging from 1.7-100 ,us via a triggered spark gap. No clearing fields were applied. Efficiency-delay runs for each spark chamber were carried out with the proton beam inclined at angles of O”, 10” and 20” relative to the chamber axis and with cosmic rays at 10”. For all runs the oxygen concentration was either (0.55 f 0.0 1)% or (1.06&0.01)% and the electric field strength was in the range 5-6 kVcm-’ and was known. Some precautionary measures that were taken included the following. The dielectric constant of barium titanate is’) temperature dependent and consequently the high voltage capacitors were placed in an enclosure maintained at (22.5*0.5)“C. To minimize possible effects’) due to conditioning and aging of the dielectric, old capacitors were used
\
n \ .\
Fig. 1. Typical effciencyAelay curves obtained with the 49 mm gap chamber. The operating conditions are explained in the text. The curves are hand-drawn tits to the data only. The shallower slope of the cosmic ray curve is assumed to be due to the significant spread in zenith angles of cosmic rays studied here (mean = 10”. standard deviation ~5”). and also due to their significant spread in ionizations. The nrcan cosmic ray ionization is 0.54 keV cm ’ here9).
et al
. +
= 0.56 .
e
.
keVcm-’
PROTONS = 7.06
F
9’
COSMIC RAYS
k&cm-’
PROTONS z 10.01
keVcm-’
[02] z 1.06 % ez100
r” ‘\
Fig. 2. Typical
efficiency-delay
gap chamber. text.
The
operating
curves conditions
obtained are
with
the 45 mm
explained
in
the
and these were maintained at high voltage for at least 12 h prior to each run. A dead time of at least 2.7 s was maintained between triggers to ensure full charging of the capacitors (time constant for charging -0.05 s). Check runs were held with the gas flow rate increased considerably to ensure that contamination and leakages were negligible. Check proton runs were also held with the beam intensity increased from its normal value of - 10 protons s ’ to -200 protons s-’ to ensure the independence of the efficiency on this figure. Finally, a satisfactory reproducibility check was made over an interval of three months. In all runs the spark-chamber efficiency was measured by counting bright sparks in a normally lighted room. Very dim bluish streaks, visible in a darkened room only, were discounted. In all the runs reported here the field strength was low enough to ensure that spurious sparks (due to dust etc.) did not occur, or only very rarely, and that the chambers operated in the track mode with sparks following particle tracks. In all a total of - 10’ sparks were observed. Typical efficiency-delay curves are shown in fig. 1. These were obtained with the spark chamber with aluminium electrodes at a field strength of 5. 94 kV cm-’ using the 2.5 nF charging capacitor. The oxygen content was 0.55% and the particles traversed the chamber at 10” relative to the chamber axis. Fig. 2 shows a similar set of data for the spark chamber with stainless steel electrodes operated at 5.76 kV cm-’ via the 2.5 nF capacitor with the oxygen concentration at 1.06%. The particles were again incident at 10”.
IONIZATION
DISCRIMINATION
It is evident from figs. 1 and 2 that helium-oxygen filled spark chambers can be used for ionization discrimination. The demonstration of this was the main aim of this work. In the following sections the effect is analyzed with a view to obtaining a systematic procedure for general utilization of the effect.
3. Analysis of results The efficiency of a spark chamber is dependent on the number and distribution of free electrons in a cylinder of diameter - 1 mm (the spark formation zone) at the time of application of the high voltage pulse 1°'~). The number no of electrons deposited by an ionizing particle immediately after traversal of the chamber is proportional to its rate of energy loss dE/dx and is subject to statistical fluctuations from event to event. In helium filled chambers these electrons are 12) thermalized in ~200 ns, and with the small admixture of molecular impurity (02) used here this thermalization time would be reduced. In the presence of an electronegative gas, oxygen in this case, the number of free electrons remaining in the spark formation zone at time t (t>thermalization time, tth) is assumed for the present analysis to take the form n = fno exp [ - (~-~--h)] .
(I,
tlere f denotes the fractional loss of electrons during thermalization and T-~ the fractional rate of loss of thermal electrons. The value of r depends on the probabilities for two-body electron attachment, three body attachment (e + 02 + He --, O~-* + He --, O~- + He and e,+02 + 0 2 --' 02" + 0 2 --' 02 +02), and on the rate of electron diffusion out of the spark formation zone. These quantities are not all known for the conditions of these studies~3). In what follows we verify that eq. (1) does however give an adequate fit to our data with an empirically determined value of r. Since the. average value of no is proportional to dE/dx, we deduce that the average number of free electrons in the spark formation zone at time t > 200 ns is if(t) oc ~--~Ex I exp [-- ( ~ ) ] .
(2)
The efficiency of a track spark chamber is a function of ~(td) where ta denotes the delay between the
123
particle's passage through the chamber and the time of application of the high voltage pulse. It is a monotonic function but not a step function because, for a given value of dE/dx, n(td) is subject to statistical fluctuations due to the statistical natures of the energy-loss mechanism, the electron attachment and diffusion processes, and also because of the statistical nature of the spark formation process. The delays, t A and ta, for which the efficiency is the same (say 50%) at two different values of ionization, (dE/dX)A and (dE/dx)a, are related by the equation In
((dE/dx)A~_ tA--tB \(dE/dx)BJ 7
(3)
provided of course no other parameters (i.e., high voltage pulse, oxygen content and particle direction relative to the chamber axis) are altered. We have analysed the efficiency-delay curves obtained in the manner described in the previous section and find that they are consistent with eq. (3) with z = (3.75+0.10) ~s (4) at an oxygen concentration of 0.55%, and z = (2.59_+0.10)/~s
(5)
at a concentration of 1.06%. Eqs. (3-5) give an adequate description of the spark chamber efficiency for purposes of carrying out ionization discrimination. This is discussed in section 4. The studies made here were not aimed at determining the various electron attachment coefficients for thermal electrons in O2-He mixtures at atmospheric pressure, but we note that the values of r determined above are consistent with smooth extrapolations of the data of Charin et al. 13) obtained at low pressures. This suggests that at the atmospheric pressure used here the electron attachment process is probably a three-body process. Further analysis of the present studies is rendered difficult because of the complicated nature of the diffusion process. We note however that we have found eq. (3) to be applicable with r given by eq. (4) and (5) over a reasonably wide range of experimental conditions. These included spark chambers having stainless steel and aluminium electrodes, with gaps of 45 mm and 49 mm, with particle angles up to 20 ° and ionization up to 30 × minimum ionization, with electric field strengths in the range 5-6 kV cm- 1, and RC pulse widths of 0.6-1.0 ps and pulse delays up to - 2 0 p s . For conditions lying outside these ranges we have not tested eq. (3).
124
M. J, ASHBURN et al.
4. Conclusion and discussion It is apparent from efficiency-delay curves such as those shown in figs. 1 and 2 that H e - O 2 filled spark chambers can be used to carry out ionization discrimination. The method shares the advantages of spark chambers, e.g.., ease of large-area coverage at minimal expense combined with low stopping power and acquisition of track location. Only readily available equipment such as commercial grade helium and commercially available capacitors are required. The method is quite flexibile in that the ionization for 50% efficiency may be readily changed from less than minimum to very high values of ionization by simple adjustments of the spark chamber operating conditions (such as adjustment of the delay time of the high voltage pulse). The efficiency of any H e - O 2 filled track chamber for particles of various ionizations may be predicted by eq. (3) provided the operating conditions lie within the ranges listed at the end of the previous section, and also provided that the delay for 50% efficiency has been determined at one particular value of ionization. In particular, cosmic rays may be used to calibrate such a chamber. If the operating conditions lie outside the ranges listed above then eq. (3) may not be reliable. In this case the chamber could be calibrated using particles with various known ionizations. Because the cut-off in efficiency is not a stepfunction of the pulse-delay, more than one gap is required to obtain sharp ionization discrimination for individual particles. With a multigap system, efficiences, and therefore ionizations, may be determined for individual particles. The results shown in figs. 1 and 2 imply that with - 8 gaps, charge resolutions - 3 0 % could be attained for fast particles. The method may be applicable in those situations where several particles are incident simultaneously
on the spark chamber provided there is only one particle of interest present, and also provided that the experimental conditions can be arranged so that the ionization columns of the other particles have effectively disappeared at the time of application of the high voltage pulse. With this in mind we are presently operating an eight gap 2 m 2 sr He-O2 spark chamber stack to search for highly charged subnucleonic particles -~) in the cores of cosmic ray showers. This search will complement those referred to in refs. 1 and 2. The authors thank A. Chisholm, M. J. Keeling, W. Wood and H. Naylor for assistance in providing the low intensity proton beam, and J. R. Storey for useful discussions. References I) D. Fryberger and P. B. Price, Proposal to search for highly ionizing particles at PEP, SLAC report (1978}. 2) p. Mussett, M. Price, J. P. Vialle and B. Aubcrt. Proposal to search for magnetic monopoles at the p~ colliding ring, CERN report (1978). 3} p. C. M. York, Ann. Phys. (N.Y.) 61 11970) 315. 4) F. Schneider and K.H. H6hne, Nucl. Instr. and Meth. 20 (1963) 152. s) J. U. Burnham and M.G. Thompson, J. Sci. Instr. 41 (1964) 108. 6) p. K. F. Grieder, Rev. Sci. Instr. 37 (1966) 80. 7) j. F. Clare, Nucl. Instr. and Meth. 116 (1974) 525. ~) Sprague Engineering Bulletin. no. 6311C, Sprague Electric Company, North Adams, Massachusetts. 9) W. Blum, K. S6chting and U. Stierlin, Phys. Rev. AI0 (1974) 491. m) O.C. Allko[~er, Spark chambers (Verlag Karl Thiemig, MOnchen, 1969). I L} p. Rice-Evans, Spark, streamer, porportional and drift chambers, (Richelieu Press, London, 1974). 12) V.A. Davidenko, B.A. Dolgoshein, S.V. Somorand and V. N. Starosel'tsev, Soy. Phys.-JETP 30 (1970) 49. I.~) L.M. Chanin, A . V . Phelps and M.A. Biondi, Phys. Rev. 128 (1962) 219.
169 (1980)
121:124;
(~) N O R T H - H O L L A N D
P U B L I S H I N G CO.
IONIZATION DISCRIMINATION IN HELIUM-OXYGEN SPARK CHAMBERS M.J. ASHBURN, T. G. CALDWELL, R. N. C. GRAY, D.C. STUTELEY* and P. C. M. YOCK
Department of Physics, University of Auckland, Auckland, New Zealand Received 3 July 1979 and in revised form 27 September 1979 It was found that helium-oxygen filled track spark chambers may be systematically operated so that the efficiency for minimum ionizing particles is low whilst for highly ionizing particles it. is high. The threshold ionization for 50% efficiency may be set anywhere from less than minimum ionization to very high values of ionization by simple adjustments of the operating conditions of the spark chamber. Typical charge resolutions attainable for individual fast particles using a multigap system are - 3 0 % .
1. Introduction Some investigations may require a detector, possibly of large area, which is sensitive to highly ionizing particles only. For example, such a situation may arise in heavy-ion physics. Searches currently planned j'2) for magnetic monopoles and related particles 3) provide further examples. The present investigation of helium-oxygen filled spark chambers was prompted by a search for highly ionizing, penetrating particles 3) in the cosmic radiation at sea level which is presently being conducted at the University of Auckland. The purpose of this paper is to report our findings on the operating characteristics of helium-oxygen filled spark chambers, because such chambers may be of use in other applications. Our investigations involved the measurement of spark chamber efficiencies under a variety of situations. The oxygen content of the helium-oxygen filled chambers was varied in the range ½ to 1%, and chamber efficiencies were measured using cosmic rays and also low energy protons having 15 to 3,0 times minimum ionization. Chamber efficiencies were measured as a function of the delay, height and width of the high voltage pulse, and as a function of the angle of the particles relative to the chamber axis. At all times the chambers functioned as track-chambers, with sparks aligned along the particle trajectories. It was found that, under suitable operating conditions as detailed below, significant discrimination in the chamber efficiency could be attained between cosmic rays and protons of 15×minimum ionization, and between protons of 15 × and 30 × minimum ionization. The plan of the paper is as follows. In the following section the experimental work is described and Present address: Cornell University, Ithaca, NY, USA.
in section 3 the experimental results are analyzed. Here an expression is obtained which may be used to predict chamber efficiencies. Conclusions and general discussion are given in section 4. We note that related investigations of spark chamber performance using mixed gas filling have been carried out by Schneider and Hohne 4) and by Burnham and ThompsonS). However these authors did not investigate the dependence of the chamber efficiency on ionization. The efficiency of narrow gap argon filled chambers for cosmic ray muons and o~ particles (from Po decay) was investigated by Grieder and some discrimination was found with this pure nobe gas filling6). Our early studies were made with pure noble gas fillings and we did obtain qualitatively similar results as Grieder's. However, only the results obtained with He-O2 fillings are reported here, because we found that the addition of a small amount of oxygen greatly enhanced the discrimination in ionization. 2. Experimental details Two 30 c m × 3 0 cm chambers were constructed with glass walls, one having aluminium foil electrodes (49 mm gap) and the other stainless steel electrodes (45 mm gap). The electrodes were attached with epoxy resin (Araldite). The heliumoxygen gas filling, at atmospheric pressure, was maintained by a continuous flow of - 1 cm 3 s -1. The oxygen content could be varied in the range 0.5-1.06% and was monitored by gas flow rates and, independently, by gas chromatography. Commercial grade gases were used and impurities at the level of a few parts per million including CO2, H2, Ne and H20 were probably present, but were assumed to have negligible effect in view of the large amount of oxygen present.
122
M.
J.
ASHBURN
The performance of.these chambers was investigated using both cosmic rays and low energy protons (IO-20 MeV) as the source of ionization. The low energy proton beams were provided by a system that has been described previously’). The trigger for the high voltage pulse was derived from coincident signals from two scintillators adjacent to the spark chamber. An RC pulse was used with R = 330 a. A barium titanate capacitor (Sprague) having a nominal capacitance of 2.5 nF was used for most of the investigation and a similar capacitor, nominally 3.6 nF, for the remainder. The high voltage pulse was applied at a delay ranging from 1.7-100 ,us via a triggered spark gap. No clearing fields were applied. Efficiency-delay runs for each spark chamber were carried out with the proton beam inclined at angles of O”, 10” and 20” relative to the chamber axis and with cosmic rays at 10”. For all runs the oxygen concentration was either (0.55 f 0.0 1)% or (1.06&0.01)% and the electric field strength was in the range 5-6 kVcm-’ and was known. Some precautionary measures that were taken included the following. The dielectric constant of barium titanate is’) temperature dependent and consequently the high voltage capacitors were placed in an enclosure maintained at (22.5*0.5)“C. To minimize possible effects’) due to conditioning and aging of the dielectric, old capacitors were used
\
n \ .\
Fig. 1. Typical effciencyAelay curves obtained with the 49 mm gap chamber. The operating conditions are explained in the text. The curves are hand-drawn tits to the data only. The shallower slope of the cosmic ray curve is assumed to be due to the significant spread in zenith angles of cosmic rays studied here (mean = 10”. standard deviation ~5”). and also due to their significant spread in ionizations. The nrcan cosmic ray ionization is 0.54 keV cm ’ here9).
et al
. +
= 0.56 .
e
.
keVcm-’
PROTONS = 7.06
F
9’
COSMIC RAYS
k&cm-’
PROTONS z 10.01
keVcm-’
[02] z 1.06 % ez100
r” ‘\
Fig. 2. Typical
efficiency-delay
gap chamber. text.
The
operating
curves conditions
obtained are
with
the 45 mm
explained
in
the
and these were maintained at high voltage for at least 12 h prior to each run. A dead time of at least 2.7 s was maintained between triggers to ensure full charging of the capacitors (time constant for charging -0.05 s). Check runs were held with the gas flow rate increased considerably to ensure that contamination and leakages were negligible. Check proton runs were also held with the beam intensity increased from its normal value of - 10 protons s ’ to -200 protons s-’ to ensure the independence of the efficiency on this figure. Finally, a satisfactory reproducibility check was made over an interval of three months. In all runs the spark-chamber efficiency was measured by counting bright sparks in a normally lighted room. Very dim bluish streaks, visible in a darkened room only, were discounted. In all the runs reported here the field strength was low enough to ensure that spurious sparks (due to dust etc.) did not occur, or only very rarely, and that the chambers operated in the track mode with sparks following particle tracks. In all a total of - 10’ sparks were observed. Typical efficiency-delay curves are shown in fig. 1. These were obtained with the spark chamber with aluminium electrodes at a field strength of 5. 94 kV cm-’ using the 2.5 nF charging capacitor. The oxygen content was 0.55% and the particles traversed the chamber at 10” relative to the chamber axis. Fig. 2 shows a similar set of data for the spark chamber with stainless steel electrodes operated at 5.76 kV cm-’ via the 2.5 nF capacitor with the oxygen concentration at 1.06%. The particles were again incident at 10”.
IONIZATION
DISCRIMINATION
It is evident from figs. 1 and 2 that helium-oxygen filled spark chambers can be used for ionization discrimination. The demonstration of this was the main aim of this work. In the following sections the effect is analyzed with a view to obtaining a systematic procedure for general utilization of the effect.
3. Analysis of results The efficiency of a spark chamber is dependent on the number and distribution of free electrons in a cylinder of diameter - 1 mm (the spark formation zone) at the time of application of the high voltage pulse 1°'~). The number no of electrons deposited by an ionizing particle immediately after traversal of the chamber is proportional to its rate of energy loss dE/dx and is subject to statistical fluctuations from event to event. In helium filled chambers these electrons are 12) thermalized in ~200 ns, and with the small admixture of molecular impurity (02) used here this thermalization time would be reduced. In the presence of an electronegative gas, oxygen in this case, the number of free electrons remaining in the spark formation zone at time t (t>thermalization time, tth) is assumed for the present analysis to take the form n = fno exp [ - (~-~--h)] .
(I,
tlere f denotes the fractional loss of electrons during thermalization and T-~ the fractional rate of loss of thermal electrons. The value of r depends on the probabilities for two-body electron attachment, three body attachment (e + 02 + He --, O~-* + He --, O~- + He and e,+02 + 0 2 --' 02" + 0 2 --' 02 +02), and on the rate of electron diffusion out of the spark formation zone. These quantities are not all known for the conditions of these studies~3). In what follows we verify that eq. (1) does however give an adequate fit to our data with an empirically determined value of r. Since the. average value of no is proportional to dE/dx, we deduce that the average number of free electrons in the spark formation zone at time t > 200 ns is if(t) oc ~--~Ex I exp [-- ( ~ ) ] .
(2)
The efficiency of a track spark chamber is a function of ~(td) where ta denotes the delay between the
123
particle's passage through the chamber and the time of application of the high voltage pulse. It is a monotonic function but not a step function because, for a given value of dE/dx, n(td) is subject to statistical fluctuations due to the statistical natures of the energy-loss mechanism, the electron attachment and diffusion processes, and also because of the statistical nature of the spark formation process. The delays, t A and ta, for which the efficiency is the same (say 50%) at two different values of ionization, (dE/dX)A and (dE/dx)a, are related by the equation In
((dE/dx)A~_ tA--tB \(dE/dx)BJ 7
(3)
provided of course no other parameters (i.e., high voltage pulse, oxygen content and particle direction relative to the chamber axis) are altered. We have analysed the efficiency-delay curves obtained in the manner described in the previous section and find that they are consistent with eq. (3) with z = (3.75+0.10) ~s (4) at an oxygen concentration of 0.55%, and z = (2.59_+0.10)/~s
(5)
at a concentration of 1.06%. Eqs. (3-5) give an adequate description of the spark chamber efficiency for purposes of carrying out ionization discrimination. This is discussed in section 4. The studies made here were not aimed at determining the various electron attachment coefficients for thermal electrons in O2-He mixtures at atmospheric pressure, but we note that the values of r determined above are consistent with smooth extrapolations of the data of Charin et al. 13) obtained at low pressures. This suggests that at the atmospheric pressure used here the electron attachment process is probably a three-body process. Further analysis of the present studies is rendered difficult because of the complicated nature of the diffusion process. We note however that we have found eq. (3) to be applicable with r given by eq. (4) and (5) over a reasonably wide range of experimental conditions. These included spark chambers having stainless steel and aluminium electrodes, with gaps of 45 mm and 49 mm, with particle angles up to 20 ° and ionization up to 30 × minimum ionization, with electric field strengths in the range 5-6 kV cm- 1, and RC pulse widths of 0.6-1.0 ps and pulse delays up to - 2 0 p s . For conditions lying outside these ranges we have not tested eq. (3).
124
M. J, ASHBURN et al.
4. Conclusion and discussion It is apparent from efficiency-delay curves such as those shown in figs. 1 and 2 that H e - O 2 filled spark chambers can be used to carry out ionization discrimination. The method shares the advantages of spark chambers, e.g.., ease of large-area coverage at minimal expense combined with low stopping power and acquisition of track location. Only readily available equipment such as commercial grade helium and commercially available capacitors are required. The method is quite flexibile in that the ionization for 50% efficiency may be readily changed from less than minimum to very high values of ionization by simple adjustments of the spark chamber operating conditions (such as adjustment of the delay time of the high voltage pulse). The efficiency of any H e - O 2 filled track chamber for particles of various ionizations may be predicted by eq. (3) provided the operating conditions lie within the ranges listed at the end of the previous section, and also provided that the delay for 50% efficiency has been determined at one particular value of ionization. In particular, cosmic rays may be used to calibrate such a chamber. If the operating conditions lie outside the ranges listed above then eq. (3) may not be reliable. In this case the chamber could be calibrated using particles with various known ionizations. Because the cut-off in efficiency is not a stepfunction of the pulse-delay, more than one gap is required to obtain sharp ionization discrimination for individual particles. With a multigap system, efficiences, and therefore ionizations, may be determined for individual particles. The results shown in figs. 1 and 2 imply that with - 8 gaps, charge resolutions - 3 0 % could be attained for fast particles. The method may be applicable in those situations where several particles are incident simultaneously
on the spark chamber provided there is only one particle of interest present, and also provided that the experimental conditions can be arranged so that the ionization columns of the other particles have effectively disappeared at the time of application of the high voltage pulse. With this in mind we are presently operating an eight gap 2 m 2 sr He-O2 spark chamber stack to search for highly charged subnucleonic particles -~) in the cores of cosmic ray showers. This search will complement those referred to in refs. 1 and 2. The authors thank A. Chisholm, M. J. Keeling, W. Wood and H. Naylor for assistance in providing the low intensity proton beam, and J. R. Storey for useful discussions. References I) D. Fryberger and P. B. Price, Proposal to search for highly ionizing particles at PEP, SLAC report (1978}. 2) p. Mussett, M. Price, J. P. Vialle and B. Aubcrt. Proposal to search for magnetic monopoles at the p~ colliding ring, CERN report (1978). 3} p. C. M. York, Ann. Phys. (N.Y.) 61 11970) 315. 4) F. Schneider and K.H. H6hne, Nucl. Instr. and Meth. 20 (1963) 152. s) J. U. Burnham and M.G. Thompson, J. Sci. Instr. 41 (1964) 108. 6) p. K. F. Grieder, Rev. Sci. Instr. 37 (1966) 80. 7) j. F. Clare, Nucl. Instr. and Meth. 116 (1974) 525. ~) Sprague Engineering Bulletin. no. 6311C, Sprague Electric Company, North Adams, Massachusetts. 9) W. Blum, K. S6chting and U. Stierlin, Phys. Rev. AI0 (1974) 491. m) O.C. Allko[~er, Spark chambers (Verlag Karl Thiemig, MOnchen, 1969). I L} p. Rice-Evans, Spark, streamer, porportional and drift chambers, (Richelieu Press, London, 1974). 12) V.A. Davidenko, B.A. Dolgoshein, S.V. Somorand and V. N. Starosel'tsev, Soy. Phys.-JETP 30 (1970) 49. I.~) L.M. Chanin, A . V . Phelps and M.A. Biondi, Phys. Rev. 128 (1962) 219.