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Yields of trapped hydrogen atoms and temperature increase in radiolysis of solid hydrogen at 4.2 K
Radiat. Phys. Chem. Vol. 36, No. 3, pp. 501-503, 1990 Int. J. Radiat. Appl. lnstrum., Part C
0146-5724/90 $3.00+ 0.00 Copyright © 1990 PergamonPress plc
Printed in Great Britain.All rightsreserved
NOTES AND LETTERS YIELDS OF TRAPPED H Y D R O G E N ATOMS A N D TEMPERATURE INCREASE IN RADIOLYSIS OF SOLID H Y D R O G E N AT 4.2 K TETSUO MIYAZAKI,t MASUNARIKATO and KENJI FUErd Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan (Received 23 May 1989; in revised form 21 June 1989)
Abstract--Initial yields of trapped hydrogen atoms in the radiolysis of solid hydrogen were measured by ESR at 4.2 K. G(H) in H2, G(D) in D 2, and G(D) and G(H) in a D2-H 2 (1 mol%) mixture were 6.2, 7.0, 4.1, and 1.3, respectively. The yields of H atoms in irradiated H 2 increase steeply with increasing dose and attain a plateau value above 2 kGy, while those of D atoms in D 2 increase roughly linearly with increasing dose. The dose dependence of the H-atom yields in H2 was interpreted in terms of acceleration of decay reaction of H atoms during ~,-irradiation, caused by the temperature rise of the irradiated samples. The increase of temperature was estimated as 1.3 K from the enhanced decay-rate constant of H atoms.
INTRODUCTION
EXPERIMENTAL
It has been found recently that hydrogen atoms produced by radiolysis of solid hydrogen react with hydrogen molecules by a tunneling effect and diffuse through solid hydrogen by repetition of the tunneling reaction at 4.2 K (Miyazaki et al., 1989; Lee et al., 1987). The clear evidence for the tunneling reaction in the solid hydrogen has stimulated a theoretical study (Takayanagi et al., 1987). It is significant in the study of the tunneling phenomena to determine the yields of trapped hydrogen atoms in the solid hydrogen. Fast injection of pellets of solid deuterium or tritium into future fusion reactors is considered to be the most promising way of refuelling a plasma device. The radiation effect on solid hydrogen is an important problem to be explored in fusion research (Valkealahti et al., 1988). Since hydrogen atoms are the sole main product in the radiolysis of solid hydrogen, the yields of hydrogen atoms are important values for an understanding of radiation effects on solid hydrogen. Though hydrogen atoms in the radiolysis of solid hydrogen at 4.2 K were observed by E S R spectroscopy, their yields were not reported previously. This paper presents the initial yields of hydrogen atoms in the irradiated solid hydrogen. It is found here that yields of D atoms in solid D 2 increase with increasing dose, while those of H atoms in solid H2 attain a plateau value at high dose.
H 2 and D 2 were more than 99.999 and 99.5 mol% pure, respectively. The sample was irradiated at 4.2 K with v-rays from a 6°Co source at a dose rate of 0 . 8 5 k G y h r -~. The trapped hydrogen atoms produced were measured at 4.2 K by a JES-FE2XG E S R spectrometer at a microwave power level of l0 -3/tW. The amounts of hydrogen atoms were obtained by double integration of the signals with a personal computer. The details of the experimental procedure were described in the previous paper (Lee et al., 1987, and references therein).
~'To whom correspondence should be addressed. slc 36/3--Q
501
RESULTS AND DISCUSSION
Initial yields o f trapped hydrogen atoms
Figure 1 shows the yields of trapped hydrogen atoms in the radiolysis of solid hydrogen and methane at 4.2 K against the irradiation time (or dose). The amounts of D atoms in D2, D and H atoms in the D2-H2 (1 mol%) mixture and H atoms in CH4 increase with increasing dose. The amounts of H atoms in H2, however, become a plateau value at high dose. It was reported previously that G(H) in irradiated CH4 at 4.2 K is 3.3 (Bhattacharya et al., 1981) and 3.7 (Takeuchi et al., 1986). If G(H) in the irradiated methane is taken as 3.5, the initial yields of trapped hydrogen atoms can be estimated from the initial slope of the yield-dose plot in Fig. 1. The values obtained are as follows: G ( H ) = 6 . 2 in H2, G(D) = 7 . 0 in D2, G ( D ) = 4 . 1 and G ( H ) = 1.3 in D2-H 2 (1 mol%).
Notes and Letters
502
t-
~5
*.3
k~ ¢tl tr
2
A, t
/! /,D
A
t j
1
/
V.
2
3
Irradiation I I
I
2
4
time, I
3
these systems, however, increase approximately linearly with increasing dose (cf. Fig. 1). Therefore, the plateau yield of H atoms in H2 may not be ascribed to a recombination reaction of mobile hydrogen atoms. The decay rate of H atoms may be accelerated during y-irradiation and most of the H atoms produced decay during y-irradiation. It was found previously that the decay rate of H atoms in solid H2 is accelerated by tungsten-lamp illumination probably because of a slight increase of temperature (Miyazaki et al., 1989). The remarkable acceleration of the decay rate by light illumination was not observed in D atoms in D2 and H and D atoms in D2-H2 mixtures. If it is assumed that the rate constant (k) for decay of H atoms in solid H 2 is enhanced during y-irradiation, the rate of H-atom formation during the irradiation is given by equation (1):
5
d[H] _ I - k[H] 2 dt
hr
Irradiation dose, kGy
I
(1)
4
Fig. 1. Yields of trapped hydrogen atoms in the radiolysis of solid hydrogen and methane at 4.2 K against the irradiation time (or dose). ( 0 ) H in H2; (IS]) D in D2; (/X) D in D2-H 2(1 mol%); (V) H in D2-H 2(1 mol%); (O) H in CH4; ( ) yields of H atoms in H 2 calculated by equation (2) in text.
Significant dose dependence of H-atom yields in solid
1-12 Though the initial yields of H atoms in irradiated solid H 2 increase sharply with increasing dose, they become a plateau value above 2 kGy (cf. closed circles in Fig. 1). When the irradiated solid H2 is stored at 4.2 K for 5 h after 7-irradiation, the amount of decay of H atoms is less than 10% of the initial amount (Miyazaki et al., 1989). If the decay rate of H atoms during y-irradiation is the same as that of H atoms after 7-irradiation, the yields of H atoms should increase with increasing dose up to the irradiation time of 5 h. The yields, however, become a plateau value above the irradiation time of 2h. There is one possibility that H atoms produced initially can migrate through solid hydrogen until they are trapped and that the mobile H atoms recombine with other trapped H atoms at high dose, resulting in a plateau yield of H atoms. D atoms also may be mobile before trapping. The initial yields of trapped D atoms in D2 and trapped H and D atoms in D2-H2 (1 mol%) are roughly the same as that of trapped H atoms in H2. Thus, if the recombination of mobile H and/or D atoms with trapped hydrogen atoms play a role at high dose, both the yields of D atoms in D 2 and those of H and D atoms in D2-H2 (1 mol%) should show the plateau value at the high dose as observed for H atoms in H2. The yields in
where I is the rate of H-atom production by y-irradiation. The amount of H atoms, [H], is obtained by integration of equation (1):
[H] = \ ~ ] \exp(2(kl)l/2t)+ll)
(2)
I is taken here as the initial rate of H-atom formation, that is the initial slope of the plot of H-atom yield against irradiation time in Fig. 1. k can be obtained by fitting equation (2) to experimental yields of H atoms. When the value of k is 2.1 × 103cm3s ~mol -~, the yields of H atoms, calculated from equation (2) and represented by a solid line in Fig. 1, agree well with the experimental values, denoted by closed circles• The rate constant (k = 2.1 x 10 3 c m 3 S-I mol -j) for recombination reactions of H atoms in solid H2 during y-irradiation is 26 times as large as that (8.2 x 10 cm 3 s -l mo1-1) of H atoms in solid H 2 at 4.2K after y-irradiation (Miyazaki et al., 1989). The enhancement is perhaps due to the increase of temperature of the irradiated samples• The temperature dependence of the recombination rate constant of H atoms in solid H2 was reported previously (Katunin et al., 1981). When compared with their results, the large rate constant during 7-irradiation corresponds to the value at 5.5 K. This implies that the temperature of the sample increases from 4.2 to 5.5 K during y-irradiation at a dose rate of 0.85 kGy h-~. The results presented here are the first estimate of temperature increase during y-irradiation at ultralow temperature. REFERENCES
Bhattacharya D., Wang H. and Willard J. E. (1981) J. Phys.
Chem. 85, 1310.
Katunin A. Ya., Lukashevich I. I., Orozmamatov S. T., Sklyarevskii V. V., Suraev V. V., Filippov V. V., Filippov
Notes and Letters N. I. and Shevtsov V. A. (1981) Pis'ma Zh. Eksp. Teor. Fiz. 34, 375. Lee K., Miyazaki T., Fueki K. and Gotoh K. (1987) J. Phys. Chem. 91, 180. Miyazaki T., Iwata N., Lee K. and Fueki K. (1989) J. Phys. Chem. 93, 3352.
503
Takayanagi T., Masaki N., Nakamura K., Okamoto M., Sato S. and Schatz G. C. (1987) J. Chem. Phys. 86, 6133. Takeuchi A., Miyazaki T., Fueki K. and Gotoh K. (1986) J. Phys. Chem. 90, 1204. Valkealahti S., Schou J., Sorensen H. and Nieminen R. M. (1988) Nucl. lnstrum. Meth. B34, 321.
0146-5724/90 $3.00+ 0.00 Copyright © 1990 PergamonPress plc
Printed in Great Britain.All rightsreserved
NOTES AND LETTERS YIELDS OF TRAPPED H Y D R O G E N ATOMS A N D TEMPERATURE INCREASE IN RADIOLYSIS OF SOLID H Y D R O G E N AT 4.2 K TETSUO MIYAZAKI,t MASUNARIKATO and KENJI FUErd Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan (Received 23 May 1989; in revised form 21 June 1989)
Abstract--Initial yields of trapped hydrogen atoms in the radiolysis of solid hydrogen were measured by ESR at 4.2 K. G(H) in H2, G(D) in D 2, and G(D) and G(H) in a D2-H 2 (1 mol%) mixture were 6.2, 7.0, 4.1, and 1.3, respectively. The yields of H atoms in irradiated H 2 increase steeply with increasing dose and attain a plateau value above 2 kGy, while those of D atoms in D 2 increase roughly linearly with increasing dose. The dose dependence of the H-atom yields in H2 was interpreted in terms of acceleration of decay reaction of H atoms during ~,-irradiation, caused by the temperature rise of the irradiated samples. The increase of temperature was estimated as 1.3 K from the enhanced decay-rate constant of H atoms.
INTRODUCTION
EXPERIMENTAL
It has been found recently that hydrogen atoms produced by radiolysis of solid hydrogen react with hydrogen molecules by a tunneling effect and diffuse through solid hydrogen by repetition of the tunneling reaction at 4.2 K (Miyazaki et al., 1989; Lee et al., 1987). The clear evidence for the tunneling reaction in the solid hydrogen has stimulated a theoretical study (Takayanagi et al., 1987). It is significant in the study of the tunneling phenomena to determine the yields of trapped hydrogen atoms in the solid hydrogen. Fast injection of pellets of solid deuterium or tritium into future fusion reactors is considered to be the most promising way of refuelling a plasma device. The radiation effect on solid hydrogen is an important problem to be explored in fusion research (Valkealahti et al., 1988). Since hydrogen atoms are the sole main product in the radiolysis of solid hydrogen, the yields of hydrogen atoms are important values for an understanding of radiation effects on solid hydrogen. Though hydrogen atoms in the radiolysis of solid hydrogen at 4.2 K were observed by E S R spectroscopy, their yields were not reported previously. This paper presents the initial yields of hydrogen atoms in the irradiated solid hydrogen. It is found here that yields of D atoms in solid D 2 increase with increasing dose, while those of H atoms in solid H2 attain a plateau value at high dose.
H 2 and D 2 were more than 99.999 and 99.5 mol% pure, respectively. The sample was irradiated at 4.2 K with v-rays from a 6°Co source at a dose rate of 0 . 8 5 k G y h r -~. The trapped hydrogen atoms produced were measured at 4.2 K by a JES-FE2XG E S R spectrometer at a microwave power level of l0 -3/tW. The amounts of hydrogen atoms were obtained by double integration of the signals with a personal computer. The details of the experimental procedure were described in the previous paper (Lee et al., 1987, and references therein).
~'To whom correspondence should be addressed. slc 36/3--Q
501
RESULTS AND DISCUSSION
Initial yields o f trapped hydrogen atoms
Figure 1 shows the yields of trapped hydrogen atoms in the radiolysis of solid hydrogen and methane at 4.2 K against the irradiation time (or dose). The amounts of D atoms in D2, D and H atoms in the D2-H2 (1 mol%) mixture and H atoms in CH4 increase with increasing dose. The amounts of H atoms in H2, however, become a plateau value at high dose. It was reported previously that G(H) in irradiated CH4 at 4.2 K is 3.3 (Bhattacharya et al., 1981) and 3.7 (Takeuchi et al., 1986). If G(H) in the irradiated methane is taken as 3.5, the initial yields of trapped hydrogen atoms can be estimated from the initial slope of the yield-dose plot in Fig. 1. The values obtained are as follows: G ( H ) = 6 . 2 in H2, G(D) = 7 . 0 in D2, G ( D ) = 4 . 1 and G ( H ) = 1.3 in D2-H 2 (1 mol%).
Notes and Letters
502
t-
~5
*.3
k~ ¢tl tr
2
A, t
/! /,D
A
t j
1
/
V.
2
3
Irradiation I I
I
2
4
time, I
3
these systems, however, increase approximately linearly with increasing dose (cf. Fig. 1). Therefore, the plateau yield of H atoms in H2 may not be ascribed to a recombination reaction of mobile hydrogen atoms. The decay rate of H atoms may be accelerated during y-irradiation and most of the H atoms produced decay during y-irradiation. It was found previously that the decay rate of H atoms in solid H2 is accelerated by tungsten-lamp illumination probably because of a slight increase of temperature (Miyazaki et al., 1989). The remarkable acceleration of the decay rate by light illumination was not observed in D atoms in D2 and H and D atoms in D2-H2 mixtures. If it is assumed that the rate constant (k) for decay of H atoms in solid H 2 is enhanced during y-irradiation, the rate of H-atom formation during the irradiation is given by equation (1):
5
d[H] _ I - k[H] 2 dt
hr
Irradiation dose, kGy
I
(1)
4
Fig. 1. Yields of trapped hydrogen atoms in the radiolysis of solid hydrogen and methane at 4.2 K against the irradiation time (or dose). ( 0 ) H in H2; (IS]) D in D2; (/X) D in D2-H 2(1 mol%); (V) H in D2-H 2(1 mol%); (O) H in CH4; ( ) yields of H atoms in H 2 calculated by equation (2) in text.
Significant dose dependence of H-atom yields in solid
1-12 Though the initial yields of H atoms in irradiated solid H 2 increase sharply with increasing dose, they become a plateau value above 2 kGy (cf. closed circles in Fig. 1). When the irradiated solid H2 is stored at 4.2 K for 5 h after 7-irradiation, the amount of decay of H atoms is less than 10% of the initial amount (Miyazaki et al., 1989). If the decay rate of H atoms during y-irradiation is the same as that of H atoms after 7-irradiation, the yields of H atoms should increase with increasing dose up to the irradiation time of 5 h. The yields, however, become a plateau value above the irradiation time of 2h. There is one possibility that H atoms produced initially can migrate through solid hydrogen until they are trapped and that the mobile H atoms recombine with other trapped H atoms at high dose, resulting in a plateau yield of H atoms. D atoms also may be mobile before trapping. The initial yields of trapped D atoms in D2 and trapped H and D atoms in D2-H2 (1 mol%) are roughly the same as that of trapped H atoms in H2. Thus, if the recombination of mobile H and/or D atoms with trapped hydrogen atoms play a role at high dose, both the yields of D atoms in D 2 and those of H and D atoms in D2-H2 (1 mol%) should show the plateau value at the high dose as observed for H atoms in H2. The yields in
where I is the rate of H-atom production by y-irradiation. The amount of H atoms, [H], is obtained by integration of equation (1):
[H] = \ ~ ] \exp(2(kl)l/2t)+ll)
(2)
I is taken here as the initial rate of H-atom formation, that is the initial slope of the plot of H-atom yield against irradiation time in Fig. 1. k can be obtained by fitting equation (2) to experimental yields of H atoms. When the value of k is 2.1 × 103cm3s ~mol -~, the yields of H atoms, calculated from equation (2) and represented by a solid line in Fig. 1, agree well with the experimental values, denoted by closed circles• The rate constant (k = 2.1 x 10 3 c m 3 S-I mol -j) for recombination reactions of H atoms in solid H2 during y-irradiation is 26 times as large as that (8.2 x 10 cm 3 s -l mo1-1) of H atoms in solid H 2 at 4.2K after y-irradiation (Miyazaki et al., 1989). The enhancement is perhaps due to the increase of temperature of the irradiated samples• The temperature dependence of the recombination rate constant of H atoms in solid H2 was reported previously (Katunin et al., 1981). When compared with their results, the large rate constant during 7-irradiation corresponds to the value at 5.5 K. This implies that the temperature of the sample increases from 4.2 to 5.5 K during y-irradiation at a dose rate of 0.85 kGy h-~. The results presented here are the first estimate of temperature increase during y-irradiation at ultralow temperature. REFERENCES
Bhattacharya D., Wang H. and Willard J. E. (1981) J. Phys.
Chem. 85, 1310.
Katunin A. Ya., Lukashevich I. I., Orozmamatov S. T., Sklyarevskii V. V., Suraev V. V., Filippov V. V., Filippov
Notes and Letters N. I. and Shevtsov V. A. (1981) Pis'ma Zh. Eksp. Teor. Fiz. 34, 375. Lee K., Miyazaki T., Fueki K. and Gotoh K. (1987) J. Phys. Chem. 91, 180. Miyazaki T., Iwata N., Lee K. and Fueki K. (1989) J. Phys. Chem. 93, 3352.
503
Takayanagi T., Masaki N., Nakamura K., Okamoto M., Sato S. and Schatz G. C. (1987) J. Chem. Phys. 86, 6133. Takeuchi A., Miyazaki T., Fueki K. and Gotoh K. (1986) J. Phys. Chem. 90, 1204. Valkealahti S., Schou J., Sorensen H. and Nieminen R. M. (1988) Nucl. lnstrum. Meth. B34, 321.