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Proportional chambers utilizing the atomic induction mechanism and the Penning effect to search for GUT monopoles
278
Nuclear Instruments and Methods 1n Physics Research 228 (1985) 278-282 North-Holland, Amsterdam
PROPORTIONAL CHAMBERS UTILIZING THE ATOMIC INDUCTION MECHANISM AND THE PENNING EFFECT TO SEARCH FOR GUT MONOPOLES Furniyoshi KAJINO
Institute for Cosmic Ray Research, University of Tokyo, Tanashi, Tokyo 188, Japan
Y.K. YUAN
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China
Received 26 June 1984 An apparatus to search for GUT magnetic monopoles has been constructed. The apparatus consists of proportional chambers, scintillation counters and iron layers . Mixed gas of helium and 10% methane flows inside the proportional chambers, utilizing ionization produced by the monopoles through the atomic induction mechanism and the Penning effect. This type of proportional chamber has been proved to work well and to be very stable.
1 . Introduction Dirac predicted the existence of magnetic monopoles in 1931 . Since that time many experiments have been performed using many techniques to detect such monopoles. Almost all of these experiments have been based on the assumption that the monopoles are not so massive . Grand Unified Theories (GUT) predict the existence of superheavy magnetic monopoles with a mass of about 10 16 GeV. Such monopoles will be accelerated by the galactic magnetic field up to a velocity around 10 -3c. The velocity of the monopoles which are gravitationally bound to the solar system is about 10 -4c. The theoretically expected flux of the monopoles is very small. Therefore, we need to measure the monopoles in a velocity region greater than 10 -4c with large area detectors. The energy loss of the monopoles in materials has not been clearly known for such small velocities . Recently, however, Drell et al . [1] presented an excellent theory for the energy loss of the monopoles in the range of velocity from 10 -4c to 10 -3 c in helium . We call their theory "the atomic induction mechanism" hereinafter. We use proportional chambers filled with a mixed gas of helium and 10% methane, and plastic scintillation counters for the monopole search . In this paper we * Present address: Department of Physics, Versima Politechnic Institute and State University, Blacksburg, Virginia 24061, USA. 0168-9002/85/$03 .00 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
report the basic performance of the proportional chambers using a mixed gas of helium and methane.
2. Ionization loss of monopoles in helium + methane gas Drell et al . have shown that large energy losses occur in helium gas in the region of monopole velocity from 10 -4c to 10 -3c. After checking various mixtures of helium and methane, we decided to use a mixed gas of helium and 10% methane for the monopole search . When the monopole goes through the helium gas, the helium is excited by the process
He - He*, where He* is a metastable state of the helium. The collision of the metastable helium and a methane molecule causes an ionization of the methane molecule through the Penning effect as follows, He* + CH 4 - He + CH + + e- . The ionization potential of the methane molecule (13 eV) is smaller than the energy level of the metastable helium (20 eV). The lifetime of the metastable helium is larger than 10 -3 s and the mean collision time between the metastable helium and the methane is about 30 ns . Therefore, the above Penning effect virtually always occurs before the metastable helium radiatively decays . This has been clearly shown by Bortner and Hurst [2] in any arbitrary mixture of helium and methane. The calculated curve for ionization loss of the monopoles in He + 10% CH 4 is shown in fig. 1 . The new
F Kajino, Y K. Yuan / Proportional chamber to search for GUT monopoles
279
layers of scintillation counters and 14 layers of iron . The effective area of each PRC is 246 cm x 92 cm and its thickness is 2 cm . The PRCs use 50 um diameter gold plated tungsten wire for the anodes with 2 cm spacing between adjacent wires. Details of the PRC are also shown in fig. 2. Other details of the apparatus have already been reported elsewhere [6,7]. The results of our experiment of the monopole search utilizing the atorruc induction mechanism and the Penning effect have been reported elsewhere [8].
104
0 c 102
i
1p' L 16 5
i
10 4
4. Selection
v
10 3
VELOCITY
16 2
10'
10
0
ß
Fig. 1 . The curve shows the calculated ionization loss for monopoles having the Dirac charge as a function of velocity in a mixed gas of helium and 10% methane. calculation for the energy loss by the atomic induction mechanism [3] is used in the range of the velocity from 10 -4c to 10 -3c, and the efficiency of ionization for the Penning effect is estimated to be 83% [4]. Ahlen's formulae [5] for the energy loss of the monopole is used in the region of velocity larger than about 10 - 'c . 3. Experimental apparatus Fig. 2 shows our experimental apparatus which consists of 9 layers of proportional chambers (PRC), 6
of
the mixture
of
helium and methane
It is better to use a gas for which the ratio of helium to methane is high, because the signal to noise ratio will be better in such a gas. The noise in this case will be mainly induced by cosmic ray muons, air showers and soft gamma rays . But if the ratio of the quenching gas such as methane is very small, the proportional chamber will easily discharge and will be unstable, so we must choose a mixture of helium and methane with good stability and a ratio of helium to methane as large as possible . Fig. 3 shows pulse heights as a function of high voltage for various mixtures of helium and methane. The measurement was performed by flowing the gas inside the PRC. The events were triggered by another PRC using cosmic ray muons. Fig. 4 shows counting, rates of the cosmic ray muons as a function of the high voltage for various mixtures . The plateau region becomes wider as the ratio of the methane increases. It is better to use a mixture with wide plateau region for
TOP VIEW
CROSS SECTION
1 Al plate(1mm ) Anode wire(50Pm0) He+10°I°CH4
0 0 0 P1 P2 P3
51 S2
S3
P4 PS P6
S4
Pî P8 P9 S5 56
Fig. 2. Schematic view of the monopole detector and a cross section of the proportional chamber (dimensions are in mm). PI-P9 : proportional chambers, Sl-S6: scintillation counters .
F. Kajmo, Y K Yuan / Proportional chamber to search for GUT monopoles
280
He+ 10°1.CH4 103
'raA 0o
qa~ PM 0~0
5
G lo, 18
20
a
22
0
HIGH VOLTAGE (kV)
Fig. 3. Pulse height as a function of high voltage for cosmic ray muons for various mixtures of helium and methane. Indicated numbers give the percentages of methane in the counter gas.
1O1
I LI a ~ 10°
stability . From figs . 3 and 4, we decided to use the
combination of 90% helium and 10% methane for the monopole search experiment .
, 62
5. Check of saturation of the pulse height As the ionization loss of the monopole is very large,
the pulse height from the PRC may saturate . This is
attributed to the saturation of the multiplication of the avalanche of electrons around the anode wire and to that along the path of heavily ionizing particles in the gas. Fig. 5 shows the charge output from a small test PRC as a function of the high voltage for a t06Ru ß-ray
16
18 HIGH VOLTAGE(kV)
20
Fig. 5. Charge output from the proportional chamber for a 106 Ru /3-ray source and an 241Am a-ray source (5 .5 MeV) as a function of high voltage.
24'Am a-ray source (3 .5 MeV) and an source (5 .5 MeV) . The test PRC has small windows made of aluminium with a thickness of 30 p.m . The ,8-rays were injected
perpendicular to the plane of the PRC through the
10 4 I 2 4 *lol
I
4.8*1. 1
-
90101
15 010'
1
20 *1. l-
10 4 Z
Q
10 3 o 11
.
1
I
1 .5
I
U
0
n
I /
.
I/
I
I-1
2.0
HIGH VOLTAGE (kV)
I
I
I
2 .5
Fig. 4. Counting rate for cosmic ray moons as a function of high voltage for various mixtures of helium and methane . Indicated numbers give the percentages of methane in the counter gas.
10 2
1.7
18
19
2.0
HIGH VOLTAGE (kV)
21
241Am Fig. 6. Ratio of the charge output for an a-ray source to 106Ru that for a ,8-ray source as a function of the high voltage . The ratio at which no saturation of the pulse height is expected is also shown.
281
F Kajmo, Y.K Yuan / Proportional chamber to search for GUT monopoles
window using a collimator . Therefore the path length of the /3-rays in the gas is 2 cm . The 24 'Am a-source was located inside the PRC, so the total energy loss of the 5.5 MeV a-rays in the gas was measured . The ratio of the charge output of 24 'Am a-rays to that of 106 Ru ß-rays is calculated from the values in fig. 5 and is plotted in fig. 6 . The ratio at which no saturation effect is expected is also shown. The PRCs have been working at a high voltage of 1 .8 kV, at which the saturation by heavily ionizing particles such as monopoles is estimated to be negligible .
z 0
6. Pulse height distribution
20
0
The standard deviation for the pulse height distribution shown in fig. 5 is plotted in fig. 7 where a, is defined as (fwhm/2) gh ,
where fwhm : full width at half-maximum in channel, (fwhm/ 2 ) gh1 : right hand side width at half-maximum, H: channel number at peak . A schematic explanation of a, is also shown in fig. 7. The systematic error for the measurement of the a, value for 24 'Am a-rays is estimated to be larger than the statistical error. The reason why we use the right hand value for the standard deviation is that we could not measure the left hand side width at half-maximum at a high voltage larger than about 1.8 kV for 24'Am a-rays . Fig. 8 shows an example of the distribution of the
He + 10°I.CH4 muons
40
1 2 PULSE HEIGHT (arbitrary units)
3
Fig . 8. An example of the distribution of pulse heights for cosmic ray muons. pulse heights at a high voltage of 1 .9 kV for cosmic ray muons which are triggered by the scintillators in the apparatus shown to fig. 2. 7. Drift time distribution The time of flight of the monopole is measured by our apparatus. It is necessary to limit the drift time of the ionized electrons in the gas to a maximum value of about 1 its for our experiment . Fig. 9 shows the drift time distribution of electrons in the PRC at a high voltage of 1 .8 kV . The measurement system is also shown schematically in the figure .
stop start muon
e
120
10 t z 0 u
10 2
1.6
1.7
1.8
1.9
HIGH VOLTAGE (kV)
20
Fig. 7. Standard deviation of the pulse height distribution for a 106Ru 8-ray source and an 24 'Am a-ray source as a function of the high voltage. A schematic explanation of a, is also shown.
80 40
0
0
0 .5 1 .0 DRIFT TIME (ps )
15
Fig. 9. The distribution of the drift time of ionized electrons m the PRC measured using cosmic ray muons by the method shown in the figure .
F Kajino, YK. Yuan / Proportional chamber to search for GUT monopoles
282
where, H: pulse height normalized to that at 1000 mbar, P: atmospheric pressure in mbar . From this figure, it is seen that the pulse height decreases linearly with atmospheric pressure in the region from 995 mbar to 1020 mbar . 9. Conclusions
ATMOSPHERIC PRESSURE (mbar) Fig. 10 . The pulse height as a function of atmospheric pressure for cosmic ray muons.
The scintillator under the PRC generates a start signal by the cosmic ray muon and the proportional chamber generates a stop signal when the drift electrons make avalanches around the anode wire . The time difference between the time of the start signal and that of the stop signal is recorded in the pulse height analyser. The cosmic ray muons uniformly penetrate the PRC along the distances from the anode wires. Fig. 9 shows that almost all events are included within 1 ps . 8. Dependence of pulse height on atmospheric pressure We are using a flow type PRC. Therefore the output pulse height from the PRC depends on the atmospheric pressure . Fig. 10 shows the pulse heights averaged for all 36 PRCs at a high voltage of 1.9 kV as a function of the pressure, which are sampled in a running time of 72 days. The fitted line to the plots is expressed as H = - 0.00400P + 5.00,
A detector which consists of proportional chambers and scintillation counters has been constructed to search for GUT monopoles. We used a mixed gas of helium and 10% methane after testing various mixtures . Basic performances of the proportional chamber, such as pulse height distribution, drift time distribution, pressure dependence, saturation effect etc ., have been measured . The detector has been very stable and working well since July 1983 . We wish to thank professor T. Kitamura and other members of the MUTRON group for their encouragement and support of this work . Useful suggestions by Profs. J. Arafune, S. Ozaki, T. Takahashi and K. Hayashi, and reading of this manuscript by Prof. M. Crouch and Dr. M.R . Krishnaswamy are also acknowledged. References [1] S.D . Drell, N.M . Kroll, M.T . Mueller, S.J. Parke and M.A . Ruderman, Phys . Rev. Lett . 50 (1983) 644. [2] T.E . Bortner and G.S . Hurst, Phys . Rev. 93 (1954) 1236 . [3] N.M . Kroll presented a new calculation of the energy loss of the monopoles at the Monopole '83 Conf., Ann Arbor, Michigan. It is a factor of 2 smaller than the old one. [4] W.P . Jesse, J. Chem . Phys . 41 (1964) 2060. [5] S.P . Ahlen, Phys. Rev. D17 (1978) 229 . [6] K. Mitsui et al ., Nucl . Instr. and Meth . 169 (1980) 97 . [7] F. Kajino, T. Kitamura, K. Mitsui, Y. Ohashi, A. Okada, Y.K . Yuan, T . Aoki and S. Matsuno, 18th Int Cosmic Ray Conf., Bangalore, vol. 5 (1983) p. 56 ; J. Phys. G 10 (1984) 447. [8] F . Kajino, S. Matsuno, Y.K . Yuan and T. Kitamura, Monopole '83 Conf ., Ann Arbor; Phys. Rev. Lett . 52 (1984) 1373 .
Nuclear Instruments and Methods 1n Physics Research 228 (1985) 278-282 North-Holland, Amsterdam
PROPORTIONAL CHAMBERS UTILIZING THE ATOMIC INDUCTION MECHANISM AND THE PENNING EFFECT TO SEARCH FOR GUT MONOPOLES Furniyoshi KAJINO
Institute for Cosmic Ray Research, University of Tokyo, Tanashi, Tokyo 188, Japan
Y.K. YUAN
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China
Received 26 June 1984 An apparatus to search for GUT magnetic monopoles has been constructed. The apparatus consists of proportional chambers, scintillation counters and iron layers . Mixed gas of helium and 10% methane flows inside the proportional chambers, utilizing ionization produced by the monopoles through the atomic induction mechanism and the Penning effect. This type of proportional chamber has been proved to work well and to be very stable.
1 . Introduction Dirac predicted the existence of magnetic monopoles in 1931 . Since that time many experiments have been performed using many techniques to detect such monopoles. Almost all of these experiments have been based on the assumption that the monopoles are not so massive . Grand Unified Theories (GUT) predict the existence of superheavy magnetic monopoles with a mass of about 10 16 GeV. Such monopoles will be accelerated by the galactic magnetic field up to a velocity around 10 -3c. The velocity of the monopoles which are gravitationally bound to the solar system is about 10 -4c. The theoretically expected flux of the monopoles is very small. Therefore, we need to measure the monopoles in a velocity region greater than 10 -4c with large area detectors. The energy loss of the monopoles in materials has not been clearly known for such small velocities . Recently, however, Drell et al . [1] presented an excellent theory for the energy loss of the monopoles in the range of velocity from 10 -4c to 10 -3 c in helium . We call their theory "the atomic induction mechanism" hereinafter. We use proportional chambers filled with a mixed gas of helium and 10% methane, and plastic scintillation counters for the monopole search . In this paper we * Present address: Department of Physics, Versima Politechnic Institute and State University, Blacksburg, Virginia 24061, USA. 0168-9002/85/$03 .00 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
report the basic performance of the proportional chambers using a mixed gas of helium and methane.
2. Ionization loss of monopoles in helium + methane gas Drell et al . have shown that large energy losses occur in helium gas in the region of monopole velocity from 10 -4c to 10 -3c. After checking various mixtures of helium and methane, we decided to use a mixed gas of helium and 10% methane for the monopole search . When the monopole goes through the helium gas, the helium is excited by the process
He - He*, where He* is a metastable state of the helium. The collision of the metastable helium and a methane molecule causes an ionization of the methane molecule through the Penning effect as follows, He* + CH 4 - He + CH + + e- . The ionization potential of the methane molecule (13 eV) is smaller than the energy level of the metastable helium (20 eV). The lifetime of the metastable helium is larger than 10 -3 s and the mean collision time between the metastable helium and the methane is about 30 ns . Therefore, the above Penning effect virtually always occurs before the metastable helium radiatively decays . This has been clearly shown by Bortner and Hurst [2] in any arbitrary mixture of helium and methane. The calculated curve for ionization loss of the monopoles in He + 10% CH 4 is shown in fig. 1 . The new
F Kajino, Y K. Yuan / Proportional chamber to search for GUT monopoles
279
layers of scintillation counters and 14 layers of iron . The effective area of each PRC is 246 cm x 92 cm and its thickness is 2 cm . The PRCs use 50 um diameter gold plated tungsten wire for the anodes with 2 cm spacing between adjacent wires. Details of the PRC are also shown in fig. 2. Other details of the apparatus have already been reported elsewhere [6,7]. The results of our experiment of the monopole search utilizing the atorruc induction mechanism and the Penning effect have been reported elsewhere [8].
104
0 c 102
i
1p' L 16 5
i
10 4
4. Selection
v
10 3
VELOCITY
16 2
10'
10
0
ß
Fig. 1 . The curve shows the calculated ionization loss for monopoles having the Dirac charge as a function of velocity in a mixed gas of helium and 10% methane. calculation for the energy loss by the atomic induction mechanism [3] is used in the range of the velocity from 10 -4c to 10 -3c, and the efficiency of ionization for the Penning effect is estimated to be 83% [4]. Ahlen's formulae [5] for the energy loss of the monopole is used in the region of velocity larger than about 10 - 'c . 3. Experimental apparatus Fig. 2 shows our experimental apparatus which consists of 9 layers of proportional chambers (PRC), 6
of
the mixture
of
helium and methane
It is better to use a gas for which the ratio of helium to methane is high, because the signal to noise ratio will be better in such a gas. The noise in this case will be mainly induced by cosmic ray muons, air showers and soft gamma rays . But if the ratio of the quenching gas such as methane is very small, the proportional chamber will easily discharge and will be unstable, so we must choose a mixture of helium and methane with good stability and a ratio of helium to methane as large as possible . Fig. 3 shows pulse heights as a function of high voltage for various mixtures of helium and methane. The measurement was performed by flowing the gas inside the PRC. The events were triggered by another PRC using cosmic ray muons. Fig. 4 shows counting, rates of the cosmic ray muons as a function of the high voltage for various mixtures . The plateau region becomes wider as the ratio of the methane increases. It is better to use a mixture with wide plateau region for
TOP VIEW
CROSS SECTION
1 Al plate(1mm ) Anode wire(50Pm0) He+10°I°CH4
0 0 0 P1 P2 P3
51 S2
S3
P4 PS P6
S4
Pî P8 P9 S5 56
Fig. 2. Schematic view of the monopole detector and a cross section of the proportional chamber (dimensions are in mm). PI-P9 : proportional chambers, Sl-S6: scintillation counters .
F. Kajmo, Y K Yuan / Proportional chamber to search for GUT monopoles
280
He+ 10°1.CH4 103
'raA 0o
qa~ PM 0~0
5
G lo, 18
20
a
22
0
HIGH VOLTAGE (kV)
Fig. 3. Pulse height as a function of high voltage for cosmic ray muons for various mixtures of helium and methane. Indicated numbers give the percentages of methane in the counter gas.
1O1
I LI a ~ 10°
stability . From figs . 3 and 4, we decided to use the
combination of 90% helium and 10% methane for the monopole search experiment .
, 62
5. Check of saturation of the pulse height As the ionization loss of the monopole is very large,
the pulse height from the PRC may saturate . This is
attributed to the saturation of the multiplication of the avalanche of electrons around the anode wire and to that along the path of heavily ionizing particles in the gas. Fig. 5 shows the charge output from a small test PRC as a function of the high voltage for a t06Ru ß-ray
16
18 HIGH VOLTAGE(kV)
20
Fig. 5. Charge output from the proportional chamber for a 106 Ru /3-ray source and an 241Am a-ray source (5 .5 MeV) as a function of high voltage.
24'Am a-ray source (3 .5 MeV) and an source (5 .5 MeV) . The test PRC has small windows made of aluminium with a thickness of 30 p.m . The ,8-rays were injected
perpendicular to the plane of the PRC through the
10 4 I 2 4 *lol
I
4.8*1. 1
-
90101
15 010'
1
20 *1. l-
10 4 Z
Q
10 3 o 11
.
1
I
1 .5
I
U
0
n
I /
.
I/
I
I-1
2.0
HIGH VOLTAGE (kV)
I
I
I
2 .5
Fig. 4. Counting rate for cosmic ray moons as a function of high voltage for various mixtures of helium and methane . Indicated numbers give the percentages of methane in the counter gas.
10 2
1.7
18
19
2.0
HIGH VOLTAGE (kV)
21
241Am Fig. 6. Ratio of the charge output for an a-ray source to 106Ru that for a ,8-ray source as a function of the high voltage . The ratio at which no saturation of the pulse height is expected is also shown.
281
F Kajmo, Y.K Yuan / Proportional chamber to search for GUT monopoles
window using a collimator . Therefore the path length of the /3-rays in the gas is 2 cm . The 24 'Am a-source was located inside the PRC, so the total energy loss of the 5.5 MeV a-rays in the gas was measured . The ratio of the charge output of 24 'Am a-rays to that of 106 Ru ß-rays is calculated from the values in fig. 5 and is plotted in fig. 6 . The ratio at which no saturation effect is expected is also shown. The PRCs have been working at a high voltage of 1 .8 kV, at which the saturation by heavily ionizing particles such as monopoles is estimated to be negligible .
z 0
6. Pulse height distribution
20
0
The standard deviation for the pulse height distribution shown in fig. 5 is plotted in fig. 7 where a, is defined as (fwhm/2) gh ,
where fwhm : full width at half-maximum in channel, (fwhm/ 2 ) gh1 : right hand side width at half-maximum, H: channel number at peak . A schematic explanation of a, is also shown in fig. 7. The systematic error for the measurement of the a, value for 24 'Am a-rays is estimated to be larger than the statistical error. The reason why we use the right hand value for the standard deviation is that we could not measure the left hand side width at half-maximum at a high voltage larger than about 1.8 kV for 24'Am a-rays . Fig. 8 shows an example of the distribution of the
He + 10°I.CH4 muons
40
1 2 PULSE HEIGHT (arbitrary units)
3
Fig . 8. An example of the distribution of pulse heights for cosmic ray muons. pulse heights at a high voltage of 1 .9 kV for cosmic ray muons which are triggered by the scintillators in the apparatus shown to fig. 2. 7. Drift time distribution The time of flight of the monopole is measured by our apparatus. It is necessary to limit the drift time of the ionized electrons in the gas to a maximum value of about 1 its for our experiment . Fig. 9 shows the drift time distribution of electrons in the PRC at a high voltage of 1 .8 kV . The measurement system is also shown schematically in the figure .
stop start muon
e
120
10 t z 0 u
10 2
1.6
1.7
1.8
1.9
HIGH VOLTAGE (kV)
20
Fig. 7. Standard deviation of the pulse height distribution for a 106Ru 8-ray source and an 24 'Am a-ray source as a function of the high voltage. A schematic explanation of a, is also shown.
80 40
0
0
0 .5 1 .0 DRIFT TIME (ps )
15
Fig. 9. The distribution of the drift time of ionized electrons m the PRC measured using cosmic ray muons by the method shown in the figure .
F Kajino, YK. Yuan / Proportional chamber to search for GUT monopoles
282
where, H: pulse height normalized to that at 1000 mbar, P: atmospheric pressure in mbar . From this figure, it is seen that the pulse height decreases linearly with atmospheric pressure in the region from 995 mbar to 1020 mbar . 9. Conclusions
ATMOSPHERIC PRESSURE (mbar) Fig. 10 . The pulse height as a function of atmospheric pressure for cosmic ray muons.
The scintillator under the PRC generates a start signal by the cosmic ray muon and the proportional chamber generates a stop signal when the drift electrons make avalanches around the anode wire . The time difference between the time of the start signal and that of the stop signal is recorded in the pulse height analyser. The cosmic ray muons uniformly penetrate the PRC along the distances from the anode wires. Fig. 9 shows that almost all events are included within 1 ps . 8. Dependence of pulse height on atmospheric pressure We are using a flow type PRC. Therefore the output pulse height from the PRC depends on the atmospheric pressure . Fig. 10 shows the pulse heights averaged for all 36 PRCs at a high voltage of 1.9 kV as a function of the pressure, which are sampled in a running time of 72 days. The fitted line to the plots is expressed as H = - 0.00400P + 5.00,
A detector which consists of proportional chambers and scintillation counters has been constructed to search for GUT monopoles. We used a mixed gas of helium and 10% methane after testing various mixtures . Basic performances of the proportional chamber, such as pulse height distribution, drift time distribution, pressure dependence, saturation effect etc ., have been measured . The detector has been very stable and working well since July 1983 . We wish to thank professor T. Kitamura and other members of the MUTRON group for their encouragement and support of this work . Useful suggestions by Profs. J. Arafune, S. Ozaki, T. Takahashi and K. Hayashi, and reading of this manuscript by Prof. M. Crouch and Dr. M.R . Krishnaswamy are also acknowledged. References [1] S.D . Drell, N.M . Kroll, M.T . Mueller, S.J. Parke and M.A . Ruderman, Phys . Rev. Lett . 50 (1983) 644. [2] T.E . Bortner and G.S . Hurst, Phys . Rev. 93 (1954) 1236 . [3] N.M . Kroll presented a new calculation of the energy loss of the monopoles at the Monopole '83 Conf., Ann Arbor, Michigan. It is a factor of 2 smaller than the old one. [4] W.P . Jesse, J. Chem . Phys . 41 (1964) 2060. [5] S.P . Ahlen, Phys. Rev. D17 (1978) 229 . [6] K. Mitsui et al ., Nucl . Instr. and Meth . 169 (1980) 97 . [7] F. Kajino, T. Kitamura, K. Mitsui, Y. Ohashi, A. Okada, Y.K . Yuan, T . Aoki and S. Matsuno, 18th Int Cosmic Ray Conf., Bangalore, vol. 5 (1983) p. 56 ; J. Phys. G 10 (1984) 447. [8] F . Kajino, S. Matsuno, Y.K . Yuan and T. Kitamura, Monopole '83 Conf ., Ann Arbor; Phys. Rev. Lett . 52 (1984) 1373 .