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Operation of a gas-filled proportional counter for CEMS at temperatures between 15–77 K
127
Nuclear Instruments and Methods in Physics Research B61 (1991) 127-131 North-Holland
Operation of a gas-filled proportional counter for CEMS at temperatures between 15-77 K K. Fukumura a, A. Nakanishi a, T. Kobayashi a, R. Katano b and Y. Isozurni ’ Department of Physics, Shiga University of Medical Science, Oisu, Shiga 520-21, Japan b Institute for Chemical Research, Kyoto University,
b
Uji, Kyoto 611, Japan
Received 2 January 1991
A proportional counter was operated by filling purified helium, purified neon and two gas mixtures, He+ 58N, and He+ lO%CO. It MS confirmed that these gases and gas mixtures make it possible to operate a proportional counter for the observation of CEMS spectra at any temperature between 1.75-300 K.
1. Introduction A proportional counter has been used frequently in conversion electron Massbauer spectroscopy (CEMS). A counter of the gas-filled type is particularly advisable for investigating the temperature dependence of CEMS spectra, because the sample temperature is not well controlled in a gas-flow counter. Recently, commercially available pure helium gas was used to operate a counter at temperatures below 15 K [1,2], where the counter works with another mechanism different from that for a counter filled with a gas mixture [3]. It was confirmed that gas mixtures such as He + 2%CH, are available to operate a proportional counter above 77 K [4]. However, no reliable investigation has been carried out to operate a proportional counter in the temperature range between 15-77 K. In our previous work on a proportional counter [2], we pointed out the possibility of extending the temperature range by using a gas of higher purity in order for the counter to work stably. It may be also possible to further extend the range by filling other kinds of pure gases or gas mixtures in the counter. In this low temperature range, only a few substances can be candidates of counter gas, i.e., three rare gases (He, Ne and Ar), four molecular gases (Hz, N,, CO and CH,) and their mixtures [S]. In the present work, two purified gases, helium and neon, and two gas mixtures, He + lO%CO and He + 5%N,, were examined with the aim to make possible CEMS measurements at any low temperature.
2. Experiment The counter assembly elsewhere [2]. The counter 0168-583X/91/$03.50
and cryostat was installed
are described in the cryostat
after being cleaned in acetone, distilled water and ethanol successively with a ultrasonic washer and after being dried in air. A 57Co source (approximately 1 kBq) electroplated on a 15 pm thick aluminum foil was mounted on the cathode plate of the counter. Energy spectra of emitted electrons from the 57Co source were recorded by filling purified helium, purified neon, He + lO%CO and He + 54&N, into the counter. The filled gas pressure was 1 atm at room temperature. Helium and neon gases were purified with a trap of molecular sieves. After the gas line with the molecular sieves trap heated to 200° C and evacuated through a liquid nitrogen trap, the gas was introduced into the counter through the molecular sieves trap cooled with liquid nitrogen In order to examine the counter system performance for CEMS studies, CEMS spectra were observed with a 57Co Mbssbauer source in an Rh matrix (1.85 GBq) and with a natural ion foil mounted on the cathode plate.
3. Counter operation 3. I. Helium The counter operation was examined at temperatures above 14.5 K. By applying bias voltages of 730-1160 V, the counter works well at temperatures between 14.5-22 K. An example of the observed energy spectra of low energy electrons from 57Co is shown in fig. 1. The peak around the 150th channel is probably caused from superimposed 5.6 keV K-LL Auger electrons and 7.2 keV K-conversion electrons, and the coincidence detection of these electrons results in the other broad peak corresponding to 12.8 keV around the 300th channel
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
128
K. Fukumura et al. / Gas-filledproportional
a
200
400
600
CHANNEL NUMBER
Fig. 1. Energy spectrum of low energy electrons obtained at 22 K by filling purified helium gas in the counter. The bias voltage is 1150 V. and the wide base above the 400th channel. A similar spectrum was obtained as well at other temperatures in this temperature range. At 23 K the counter operation
counter for CEMS at 15-77 K
becomes unstable with time, i.e., the voltage range in which the counter works becomes narrower and narrower with time. For example, about 40 min from the start of counter operation, voltage discharge occurs at 1050 V. At very low temperatures, most of the impurity gas is frozen and counting mechanism different from that for a counter filled with a gas mixture functions [3]. The impurity molecules evaporate at higher temperatures and eventually block the counter from working by the low temperature mechanism, which leads to the operational instability of the counter. There are other reports on the counter operation with pure helium gas at higher temperatures [6]. In our opinion, the counter in these cases works with the mechanism for a gas mixture, where impurity molecules and outgoing gas may act as a kind of quenching gas. Actually, the counter operation was possible at temperatures between 60-300 K by directly filling with commercially available pure helium gas [2]. Taking account of our previous results [1,2], therefore, it can be concluded that a proportional counter works well at temperatures between
2000
800
Neon 22K
1500
,
.
: t .. t .
i
600
1
m
He+ 1 OWCO 47K
P ZJ 1000 G 500 600
1000
i
50K
I cn 600 !S 2 Ll
400 200 0 600
..
&&A . 2
400 P 5 g
50K
200
0 P 0
_ 100
200
CHANNEL
300
400
NUMBER
Fig. 2. Energy spectra observed at 22, 23 and 50 K by filling purified neon gas in the counter. The bias voltage is 920, 980 or 960 V at 22, 23 or 50 K, respectively.
0
200 CHANNEL
400
600
800
NUMBER
Fig. 3. Energy spectra observed at 47, 50 and 60 K by filling the gas mixture He+ lO%CO. The bias voltage is 450, 550 or 1500 V at 47, 50 or 60 K, respectively.
K. Fukumura et al. / Gas-filled proportional 1.75-22
described
K by filling with helium gas purified above.
in the way
3.2. Neon The low energy electron spectra were observed at temperatures between 22-53 K, three examples of which are given in fig. 2. Between 23-50 K, the spectra are similar to each other. Above 51 K, however, the energy resolution becomes poorer with increasing temperature. Owing to liquefaction of neon gas at temperatures below 21 K, the counter operation becomes unstable. The evaporation of impurity molecules may again cause the operational instability of the counter at temperatures higher than 53 K. When commercially available pure neon gas is filled in the counter, the temperature range for stable operation was 22-30 K and, moreover, the quality of the spectra is poorer than that with purified neon. The operational mechanism with pure neon gas is unknown, though some discussions for pure helium have been put forward previously [3].
counter for CEMS at 15- 77 K
129
3.3. He + IO%CO The counter operation with this gas mixture goes well at temperatures higher than 47 K. The bias voltage to operate the counter stably was around 1500 V at any temperature above 60 K, while, below 59 K, the operating bias voltage decreased with decreasing temperature due to partial liquefaction of CO gas. The high bias voltage above 60 K compared with the cases for the pure gases may be due to the counting mechanism for a gas mixture. Some of the observed spectra are shown in fig. 3. The spectra observed above 60 K were very similar to those at 60 K. 3.4. He + SWN, The operating behaviour of the counter with He + 5% N, is similar to that with He + lO%CO, except that the counter works above 46 K and liquefaction of nitrogen gas happens below 53 K. Fig. 4 gives several examples of the spectra obtained.
4. Applications to CEMS
He+5WNi
i t
trogen 46K
f
800-1;
400 p
300-
5 0 v
200-
47K
.. &
60K
t
.:-‘ 0
4 200 CHANNEL
Fig. 4. Energy spectra
.
l
loo-
400
600
800
NUMBER
observed at 46, 47 and 60 K by filling the gas mixture H + 5%N,. The bias voltage is 570, 630 or 1200 V at 46,47 or 60 K, respectively.
Miissbauer spectra were observed at several temperatures by filling the above mentioned gases and gas mixtures in the counter, and the spectra are given in fig. 5. The relative resonance peak-heights with pure helium and mixed gases are approximately 7-8%, but conspicuously low peak-heights (about 3%) are observed in the case of pure neon gas. Fig. 6 shows the energy spectra recorded at 25 K by filling neon gas in the counter with the Miissbauer source placed outside the counter, in the CEMS conditions, and with the electroplated source mounted in the counter. Distinct and indistinct peaks and a wide base are seen in the spectrum with the Mossbauer source. The peaks in the spectrum with the electroplated source come from the conversion and Auger electrons as described above. In the cases of helium and the gas mixtures, no definite peak is found in the spectrum with the Mijssbauer source; an example for helium is given in fig. 7. Considering the difference in atomic number of neon and helium, the distinct and indistinct peaks in the spectrum with the Mijssbauer source in fig. 6 are probably caused by the 14.4 keV y rays and 6.4 keV K-X-rays from the source, respectively. The 122 keV y rays may be responsible for the wide base in fig. 6. Mijssbauer spectra with neon were observed by using counting signals in the pulse-height ranges indicated by the arrows in fig. 6, which are shown in fig. 8. The resonance peaks with approximately 4% height are evidently seen in the spectrum recorded with signals in the range (A), while the peak-height reduces to approximately 1% in the
K. Fukumura et al. / Gas-filled proportional
counter for CEMS at IS-77
K
electroplated Mksbauer I
I
..
(bl
.
-
I
-;
200
II
400
600
CHANNEL
1 00
800
NUMBER
.
Fig. 7. Energy spectra observed at 20 K with the counter filled with purified helium in the same conditions of the sources as in fig. 6.
_&j
_h
a
i
i
velocity
lmm/sl
Fig. 5. M&batter spectra observed at several temperatures. The counter gases are purified helium at 20 K (a), purified neon at 25 (b) and 45 K (c), and He+ lO%CO(d) or He + S!%N, (e) at 55 K.
support the above consideration of the energy spectrum with the Mijssbauer source in fig. 6. Consequently, when a proportions counter is operated by filling neon gas, the CEMS spectrum should be observed with signals in the range (A) in fig. 6. In conclusion, a proportional counter can be operated stably by filling purified helium and neon gases at temperatures between 1.75-22 K and 22-53 K, respec-
4.61
t
range (B) and no resonance peak is found in the range (C). Even in the range (A), however, the peak-height is rather lower than in the cases with pure helium or the gas mixtures, which suggests a partial wnt~bution of 6.4 keV K-X-rays to signals in the range (A). These results obtained from the MBssbauer spectra in fig. 8
I
.I_
1
0
.
I
200
400
CHANNEL
.
1
I
*e.*-
_
I
9
600
I
800
c”r 1
NUMBER
Fig. 6. Energy spectra observed at 25 K by filling purified neon gas in the counter. One was recorded with the Mksbauer source placed outside the counter and the other with the electroplated source mounted in the counter.
I
:
-8
I
-4 velocity
0
4
8
(mm/s1
Fig. 8. Mossbauer spectra observed at 25 K by using counter signals of the neon-filled counter in the pulse-height ranges (A), (B) and (C) described in fig. 6.
K. Fukumura et al. / Gas-j?NedproportionaI
tively. Above 46 or 47 K, the mixed gas He + 5%N, or He + lO%CO is available, respectively. CEMS measurements at any temperature between 1.75-300 K are possible by using a proportional counter filled with either pure gas or a gas mixture if chosen suitably for the temperature concerned.
References [l] Y. Isozumi, S. Ito, T. Fujii and R. Katano, Rev. Sci Instr. 60 (1989) 3262.
counter for CEMS at IS-77
K
131
121 K. Fukumura, R. Katano, T. Kobayashi, A. Nakanishi and Y. Isozumi, Nucl. Instr. and Meth. A, to be published. 131 S. Kishimoto, Y. Isozurni, R. Katano and H. Takekoshi, Nucl. Instr. and Meth. A262 (1987) 413. and R. Katano, Nucl. Instr. and [41 Y. Isozumi, M. Ku&ado Meth. 204 (1983) 571. PI S. Kishimoto, Y. Isozumi, R. Katano and S. Shimizu, Nucl. Instr. and Meth. A255 (1987) 213. f61A. Kastner, G. Lugert and G. Bayreuther, Hyperfine Interactions 42 (1988) 1145.
Nuclear Instruments and Methods in Physics Research B61 (1991) 127-131 North-Holland
Operation of a gas-filled proportional counter for CEMS at temperatures between 15-77 K K. Fukumura a, A. Nakanishi a, T. Kobayashi a, R. Katano b and Y. Isozurni ’ Department of Physics, Shiga University of Medical Science, Oisu, Shiga 520-21, Japan b Institute for Chemical Research, Kyoto University,
b
Uji, Kyoto 611, Japan
Received 2 January 1991
A proportional counter was operated by filling purified helium, purified neon and two gas mixtures, He+ 58N, and He+ lO%CO. It MS confirmed that these gases and gas mixtures make it possible to operate a proportional counter for the observation of CEMS spectra at any temperature between 1.75-300 K.
1. Introduction A proportional counter has been used frequently in conversion electron Massbauer spectroscopy (CEMS). A counter of the gas-filled type is particularly advisable for investigating the temperature dependence of CEMS spectra, because the sample temperature is not well controlled in a gas-flow counter. Recently, commercially available pure helium gas was used to operate a counter at temperatures below 15 K [1,2], where the counter works with another mechanism different from that for a counter filled with a gas mixture [3]. It was confirmed that gas mixtures such as He + 2%CH, are available to operate a proportional counter above 77 K [4]. However, no reliable investigation has been carried out to operate a proportional counter in the temperature range between 15-77 K. In our previous work on a proportional counter [2], we pointed out the possibility of extending the temperature range by using a gas of higher purity in order for the counter to work stably. It may be also possible to further extend the range by filling other kinds of pure gases or gas mixtures in the counter. In this low temperature range, only a few substances can be candidates of counter gas, i.e., three rare gases (He, Ne and Ar), four molecular gases (Hz, N,, CO and CH,) and their mixtures [S]. In the present work, two purified gases, helium and neon, and two gas mixtures, He + lO%CO and He + 5%N,, were examined with the aim to make possible CEMS measurements at any low temperature.
2. Experiment The counter assembly elsewhere [2]. The counter 0168-583X/91/$03.50
and cryostat was installed
are described in the cryostat
after being cleaned in acetone, distilled water and ethanol successively with a ultrasonic washer and after being dried in air. A 57Co source (approximately 1 kBq) electroplated on a 15 pm thick aluminum foil was mounted on the cathode plate of the counter. Energy spectra of emitted electrons from the 57Co source were recorded by filling purified helium, purified neon, He + lO%CO and He + 54&N, into the counter. The filled gas pressure was 1 atm at room temperature. Helium and neon gases were purified with a trap of molecular sieves. After the gas line with the molecular sieves trap heated to 200° C and evacuated through a liquid nitrogen trap, the gas was introduced into the counter through the molecular sieves trap cooled with liquid nitrogen In order to examine the counter system performance for CEMS studies, CEMS spectra were observed with a 57Co Mbssbauer source in an Rh matrix (1.85 GBq) and with a natural ion foil mounted on the cathode plate.
3. Counter operation 3. I. Helium The counter operation was examined at temperatures above 14.5 K. By applying bias voltages of 730-1160 V, the counter works well at temperatures between 14.5-22 K. An example of the observed energy spectra of low energy electrons from 57Co is shown in fig. 1. The peak around the 150th channel is probably caused from superimposed 5.6 keV K-LL Auger electrons and 7.2 keV K-conversion electrons, and the coincidence detection of these electrons results in the other broad peak corresponding to 12.8 keV around the 300th channel
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
128
K. Fukumura et al. / Gas-filledproportional
a
200
400
600
CHANNEL NUMBER
Fig. 1. Energy spectrum of low energy electrons obtained at 22 K by filling purified helium gas in the counter. The bias voltage is 1150 V. and the wide base above the 400th channel. A similar spectrum was obtained as well at other temperatures in this temperature range. At 23 K the counter operation
counter for CEMS at 15-77 K
becomes unstable with time, i.e., the voltage range in which the counter works becomes narrower and narrower with time. For example, about 40 min from the start of counter operation, voltage discharge occurs at 1050 V. At very low temperatures, most of the impurity gas is frozen and counting mechanism different from that for a counter filled with a gas mixture functions [3]. The impurity molecules evaporate at higher temperatures and eventually block the counter from working by the low temperature mechanism, which leads to the operational instability of the counter. There are other reports on the counter operation with pure helium gas at higher temperatures [6]. In our opinion, the counter in these cases works with the mechanism for a gas mixture, where impurity molecules and outgoing gas may act as a kind of quenching gas. Actually, the counter operation was possible at temperatures between 60-300 K by directly filling with commercially available pure helium gas [2]. Taking account of our previous results [1,2], therefore, it can be concluded that a proportional counter works well at temperatures between
2000
800
Neon 22K
1500
,
.
: t .. t .
i
600
1
m
He+ 1 OWCO 47K
P ZJ 1000 G 500 600
1000
i
50K
I cn 600 !S 2 Ll
400 200 0 600
..
&&A . 2
400 P 5 g
50K
200
0 P 0
_ 100
200
CHANNEL
300
400
NUMBER
Fig. 2. Energy spectra observed at 22, 23 and 50 K by filling purified neon gas in the counter. The bias voltage is 920, 980 or 960 V at 22, 23 or 50 K, respectively.
0
200 CHANNEL
400
600
800
NUMBER
Fig. 3. Energy spectra observed at 47, 50 and 60 K by filling the gas mixture He+ lO%CO. The bias voltage is 450, 550 or 1500 V at 47, 50 or 60 K, respectively.
K. Fukumura et al. / Gas-filled proportional 1.75-22
described
K by filling with helium gas purified above.
in the way
3.2. Neon The low energy electron spectra were observed at temperatures between 22-53 K, three examples of which are given in fig. 2. Between 23-50 K, the spectra are similar to each other. Above 51 K, however, the energy resolution becomes poorer with increasing temperature. Owing to liquefaction of neon gas at temperatures below 21 K, the counter operation becomes unstable. The evaporation of impurity molecules may again cause the operational instability of the counter at temperatures higher than 53 K. When commercially available pure neon gas is filled in the counter, the temperature range for stable operation was 22-30 K and, moreover, the quality of the spectra is poorer than that with purified neon. The operational mechanism with pure neon gas is unknown, though some discussions for pure helium have been put forward previously [3].
counter for CEMS at 15- 77 K
129
3.3. He + IO%CO The counter operation with this gas mixture goes well at temperatures higher than 47 K. The bias voltage to operate the counter stably was around 1500 V at any temperature above 60 K, while, below 59 K, the operating bias voltage decreased with decreasing temperature due to partial liquefaction of CO gas. The high bias voltage above 60 K compared with the cases for the pure gases may be due to the counting mechanism for a gas mixture. Some of the observed spectra are shown in fig. 3. The spectra observed above 60 K were very similar to those at 60 K. 3.4. He + SWN, The operating behaviour of the counter with He + 5% N, is similar to that with He + lO%CO, except that the counter works above 46 K and liquefaction of nitrogen gas happens below 53 K. Fig. 4 gives several examples of the spectra obtained.
4. Applications to CEMS
He+5WNi
i t
trogen 46K
f
800-1;
400 p
300-
5 0 v
200-
47K
.. &
60K
t
.:-‘ 0
4 200 CHANNEL
Fig. 4. Energy spectra
.
l
loo-
400
600
800
NUMBER
observed at 46, 47 and 60 K by filling the gas mixture H + 5%N,. The bias voltage is 570, 630 or 1200 V at 46,47 or 60 K, respectively.
Miissbauer spectra were observed at several temperatures by filling the above mentioned gases and gas mixtures in the counter, and the spectra are given in fig. 5. The relative resonance peak-heights with pure helium and mixed gases are approximately 7-8%, but conspicuously low peak-heights (about 3%) are observed in the case of pure neon gas. Fig. 6 shows the energy spectra recorded at 25 K by filling neon gas in the counter with the Miissbauer source placed outside the counter, in the CEMS conditions, and with the electroplated source mounted in the counter. Distinct and indistinct peaks and a wide base are seen in the spectrum with the Mossbauer source. The peaks in the spectrum with the electroplated source come from the conversion and Auger electrons as described above. In the cases of helium and the gas mixtures, no definite peak is found in the spectrum with the Mijssbauer source; an example for helium is given in fig. 7. Considering the difference in atomic number of neon and helium, the distinct and indistinct peaks in the spectrum with the Mijssbauer source in fig. 6 are probably caused by the 14.4 keV y rays and 6.4 keV K-X-rays from the source, respectively. The 122 keV y rays may be responsible for the wide base in fig. 6. Mijssbauer spectra with neon were observed by using counting signals in the pulse-height ranges indicated by the arrows in fig. 6, which are shown in fig. 8. The resonance peaks with approximately 4% height are evidently seen in the spectrum recorded with signals in the range (A), while the peak-height reduces to approximately 1% in the
K. Fukumura et al. / Gas-filled proportional
counter for CEMS at IS-77
K
electroplated Mksbauer I
I
..
(bl
.
-
I
-;
200
II
400
600
CHANNEL
1 00
800
NUMBER
.
Fig. 7. Energy spectra observed at 20 K with the counter filled with purified helium in the same conditions of the sources as in fig. 6.
_&j
_h
a
i
i
velocity
lmm/sl
Fig. 5. M&batter spectra observed at several temperatures. The counter gases are purified helium at 20 K (a), purified neon at 25 (b) and 45 K (c), and He+ lO%CO(d) or He + S!%N, (e) at 55 K.
support the above consideration of the energy spectrum with the Mijssbauer source in fig. 6. Consequently, when a proportions counter is operated by filling neon gas, the CEMS spectrum should be observed with signals in the range (A) in fig. 6. In conclusion, a proportional counter can be operated stably by filling purified helium and neon gases at temperatures between 1.75-22 K and 22-53 K, respec-
4.61
t
range (B) and no resonance peak is found in the range (C). Even in the range (A), however, the peak-height is rather lower than in the cases with pure helium or the gas mixtures, which suggests a partial wnt~bution of 6.4 keV K-X-rays to signals in the range (A). These results obtained from the MBssbauer spectra in fig. 8
I
.I_
1
0
.
I
200
400
CHANNEL
.
1
I
*e.*-
_
I
9
600
I
800
c”r 1
NUMBER
Fig. 6. Energy spectra observed at 25 K by filling purified neon gas in the counter. One was recorded with the Mksbauer source placed outside the counter and the other with the electroplated source mounted in the counter.
I
:
-8
I
-4 velocity
0
4
8
(mm/s1
Fig. 8. Mossbauer spectra observed at 25 K by using counter signals of the neon-filled counter in the pulse-height ranges (A), (B) and (C) described in fig. 6.
K. Fukumura et al. / Gas-j?NedproportionaI
tively. Above 46 or 47 K, the mixed gas He + 5%N, or He + lO%CO is available, respectively. CEMS measurements at any temperature between 1.75-300 K are possible by using a proportional counter filled with either pure gas or a gas mixture if chosen suitably for the temperature concerned.
References [l] Y. Isozumi, S. Ito, T. Fujii and R. Katano, Rev. Sci Instr. 60 (1989) 3262.
counter for CEMS at IS-77
K
131
121 K. Fukumura, R. Katano, T. Kobayashi, A. Nakanishi and Y. Isozumi, Nucl. Instr. and Meth. A, to be published. 131 S. Kishimoto, Y. Isozurni, R. Katano and H. Takekoshi, Nucl. Instr. and Meth. A262 (1987) 413. and R. Katano, Nucl. Instr. and [41 Y. Isozumi, M. Ku&ado Meth. 204 (1983) 571. PI S. Kishimoto, Y. Isozumi, R. Katano and S. Shimizu, Nucl. Instr. and Meth. A255 (1987) 213. f61A. Kastner, G. Lugert and G. Bayreuther, Hyperfine Interactions 42 (1988) 1145.