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Emission of light nuclei in thermal neutron fission of 239Pu
Nuclear Physics A128 (1969) 219 --223; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
E M I S S I O N OF LIGHT N U C L E I IN T H E R M A L N E U T R O N FISSION OF 239pu T. KROGULSKI, J. CHWASZCZEWSKA, M. DAKOWSKI, E. PIASECKI, M. SOWI/'~SKI and J. TYS
Institute of Nuclear Research, Department of Physics, Warsaw, Poland Received 20 January 1969 Abstract: The relative intensities and energy spectra of 1H, 2H, all, 4He, SHe and SHe particles from thermal neutron fission of-°a~Pu have been measured. The comparison with analogous data on the spontaneous fission of zs-'Cf and the thermal neutron fission of ~asU reveals a pronounced similarity.
El
N U C L E A R FISSION 239pu(n), measured relative yields and energy spectra Of IH, 2H, 3H, 4He, 6He and SHe.
[
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In fission the two heavy fragments are accompanied by various light particles with a probability of about one in several hundred. The comparison of spontaneous fission of zsacf and thermal neutron fission of 235U (refs. 1,2)) shows that, although the emission of light particles is similar, there are some differences especially in the yields of protons and 6He, and in the energy spectra of the latter. Besides, for 23SU, the presence of SHe was barely detected. Some experiments have also been made on 239pu but the intensities and the energy spectra of the hydrogen isotopes were not measured and 6He was observed with rather poor statistics 3,,). Hence more complete measurements of the tripartition process were needed for thermal neutron fission of 239pu"
The experimental arrangement was, in general, like that described in ref. 2). However, the detection of helium isotopes was improved by using a thinner AE semiconductor detector (25 pm). Measurements of the hydrogen isotopes were performed both with a full pile neutron spectrum and with epicadmium neutrons only. By subtracting one corrects for background due to (n, p) processes induced by fast neutrons. The background from fission neutrons from the target, which also contribute to the proton spectrum still remains. As stated in ref. 2) this background is negligible above 7 MeV. Since one can expect that the spectra of fast pile neutrons and fission neutrons from the target are similar, the spectra of protons from the (n, p) reaction induced by pile and fission neutrons should also be similar. We have found the ratio of the background due to all neutrons to that due to epicadmium neutrons from the pile to be constant within reasonable limits and equal to 2.0+0.2. Therefore in the final result for protons we multiplied the background by a factor of 2 before subtracting. '
219
220
T. KROGULSKIe t aL
The spectra of hydrogen isotopes are presented in fig. l, and those of helium isotopes in fig. 2. A Gaussian distribution was fitted to the spectra by the least-squares method. The fit was performed in several energy ranges. We conclude that all the spectra are sufficiently well described by a Gaussian distribution although there are small deviations from it. i
co
z~
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× X. X X
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300!.~ !
, \o e
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{000
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8
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t6 E/Mev]
Fig. 1. The energy spectra o f protons, deuterons and tritons.
Generally speaking, the peaks are too sharp or the wings too wide compared to the Gaussian distribution. However the dependence o f the parameters of the Gaussian on the intervals used in the fit is not very pronounced and does not exceed 0.2 MeV for 6He and 0.1 MeV for the other particles. The parameters of the final fits are presented in table 1. The errors quoted are the limits of the overall uncertainty estimated from:
221
THERMAL NEUTRON FISSION OF 239Pu
(i) the upper limit of calibration errors which does not exceed 0.05 MeV, (ii) the uncertainty defined by the fitting prgcedure. The intensities relative to the emission of 100 alphas have been calculated by assumq~
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Fig. 2. The energy spectra of ~-particles, 8He and SHe.
ing that the low energy spectrum which is not registered is symmetrical to the corresponding high-energy part. The errors quoted are the limits cf the overall uncertainty estimated from: (i) the standard deviation of the number of registered events, (ii) the systematic error connected withthe accuracy of determining the peak position, (iii) the systematic error in distinguishing the different particles (of importance
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THERMAL NEUTRON FISSION OF 239Pu
223
for the d e u t e r o n yield only). W i t h SHe o n l y a m i n o r p a r t o f the s p e c t r u m was m e a s u r ed, for intensity calculations we therefore a s s u m e d the same shape o f the 8He spect r u m as was m e a s u r e d in ref. i). This seems to be justified because the spectra o f 4He a n d 6He f r o m 239pu a n d 2 5 2 C f a r e very similar. T h e present results for t h e r m a l n e u t r o n fission of Z39pu are quite similar to those for s p o n t a n e o u s fission 252Cf. T h e c o m p a r i s o n with 23SU reveals differences for p r o t o n s a n d 6He ref. z). It s h o u l d be p o i n t e d o u t that the p e a k p o s i t i o n o f the 6He s p e c t r u m in ref. z) was e s t a b l i s h e d by a G a u s s i a n fit to the m e a s u r e d p a r t o f the s p e c t r u m which d i d not extend b e l o w 12.8 M e V t. This explains at least p a r t l y w h y the value o f the e x t r a p o l a t e d yield f o u n d in ref. 2) for Z35U is so small. T h e a u t h o r s express their t h a n k s to Dr. Z. S u j k o w s k i a n d Dr. J. Zylicz for t h e i r c o n t i n u o u s s u p p o r t d u r i n g this work. * Control measurements performed recently for 2asU allowed us to establish a more accurate peak position for 6He at 12.0±0.5 MeV.
References 1) S. W. Cosper, J. Cerny and R. C. Gatti, Phys. Rev. 154 (1967) 1193 2) M. Dakowski, J. Chwaszczewska, T. Krogulski, E. Piasecki and M. Sowifiski, Phys. Lett. 25B (1967) 213 3) D. Bollini, M. Cambiaghi, F. Fossati and T. Pinelli, Nuovo Cim. 51B (1967) 235 4) W. N. Andreev, W. G. Nedopekin and W. I. Rogov, ITEF Report Nr 609 (1968); Yad. Fiz. 9 (1969) 23
E M I S S I O N OF LIGHT N U C L E I IN T H E R M A L N E U T R O N FISSION OF 239pu T. KROGULSKI, J. CHWASZCZEWSKA, M. DAKOWSKI, E. PIASECKI, M. SOWI/'~SKI and J. TYS
Institute of Nuclear Research, Department of Physics, Warsaw, Poland Received 20 January 1969 Abstract: The relative intensities and energy spectra of 1H, 2H, all, 4He, SHe and SHe particles from thermal neutron fission of-°a~Pu have been measured. The comparison with analogous data on the spontaneous fission of zs-'Cf and the thermal neutron fission of ~asU reveals a pronounced similarity.
El
N U C L E A R FISSION 239pu(n), measured relative yields and energy spectra Of IH, 2H, 3H, 4He, 6He and SHe.
[
I
In fission the two heavy fragments are accompanied by various light particles with a probability of about one in several hundred. The comparison of spontaneous fission of zsacf and thermal neutron fission of 235U (refs. 1,2)) shows that, although the emission of light particles is similar, there are some differences especially in the yields of protons and 6He, and in the energy spectra of the latter. Besides, for 23SU, the presence of SHe was barely detected. Some experiments have also been made on 239pu but the intensities and the energy spectra of the hydrogen isotopes were not measured and 6He was observed with rather poor statistics 3,,). Hence more complete measurements of the tripartition process were needed for thermal neutron fission of 239pu"
The experimental arrangement was, in general, like that described in ref. 2). However, the detection of helium isotopes was improved by using a thinner AE semiconductor detector (25 pm). Measurements of the hydrogen isotopes were performed both with a full pile neutron spectrum and with epicadmium neutrons only. By subtracting one corrects for background due to (n, p) processes induced by fast neutrons. The background from fission neutrons from the target, which also contribute to the proton spectrum still remains. As stated in ref. 2) this background is negligible above 7 MeV. Since one can expect that the spectra of fast pile neutrons and fission neutrons from the target are similar, the spectra of protons from the (n, p) reaction induced by pile and fission neutrons should also be similar. We have found the ratio of the background due to all neutrons to that due to epicadmium neutrons from the pile to be constant within reasonable limits and equal to 2.0+0.2. Therefore in the final result for protons we multiplied the background by a factor of 2 before subtracting. '
219
220
T. KROGULSKIe t aL
The spectra of hydrogen isotopes are presented in fig. l, and those of helium isotopes in fig. 2. A Gaussian distribution was fitted to the spectra by the least-squares method. The fit was performed in several energy ranges. We conclude that all the spectra are sufficiently well described by a Gaussian distribution although there are small deviations from it. i
co
z~
.3
/
"
i/
× X. X X
"x X~-X--.--2
~000
H
---o---.-.-o.--- J/--/
IgO 450
/c
\
500
,.x t00
300!.~ !
, \o e
\
{000
50 150
4
•500
6
8
fo
~
~
t6 E/Mev]
Fig. 1. The energy spectra o f protons, deuterons and tritons.
Generally speaking, the peaks are too sharp or the wings too wide compared to the Gaussian distribution. However the dependence o f the parameters of the Gaussian on the intervals used in the fit is not very pronounced and does not exceed 0.2 MeV for 6He and 0.1 MeV for the other particles. The parameters of the final fits are presented in table 1. The errors quoted are the limits of the overall uncertainty estimated from:
221
THERMAL NEUTRON FISSION OF 239Pu
(i) the upper limit of calibration errors which does not exceed 0.05 MeV, (ii) the uncertainty defined by the fitting prgcedure. The intensities relative to the emission of 100 alphas have been calculated by assumq~
,o
,,.)
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1 1
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I00 4000
L3000 50 -2000
//
// fO00
///
%"~ L to
I5
20
25
3"0
Fig. 2. The energy spectra of ~-particles, 8He and SHe.
ing that the low energy spectrum which is not registered is symmetrical to the corresponding high-energy part. The errors quoted are the limits cf the overall uncertainty estimated from: (i) the standard deviation of the number of registered events, (ii) the systematic error connected withthe accuracy of determining the peak position, (iii) the systematic error in distinguishing the different particles (of importance
222
T. K R O G U L S K I
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THERMAL NEUTRON FISSION OF 239Pu
223
for the d e u t e r o n yield only). W i t h SHe o n l y a m i n o r p a r t o f the s p e c t r u m was m e a s u r ed, for intensity calculations we therefore a s s u m e d the same shape o f the 8He spect r u m as was m e a s u r e d in ref. i). This seems to be justified because the spectra o f 4He a n d 6He f r o m 239pu a n d 2 5 2 C f a r e very similar. T h e present results for t h e r m a l n e u t r o n fission of Z39pu are quite similar to those for s p o n t a n e o u s fission 252Cf. T h e c o m p a r i s o n with 23SU reveals differences for p r o t o n s a n d 6He ref. z). It s h o u l d be p o i n t e d o u t that the p e a k p o s i t i o n o f the 6He s p e c t r u m in ref. z) was e s t a b l i s h e d by a G a u s s i a n fit to the m e a s u r e d p a r t o f the s p e c t r u m which d i d not extend b e l o w 12.8 M e V t. This explains at least p a r t l y w h y the value o f the e x t r a p o l a t e d yield f o u n d in ref. 2) for Z35U is so small. T h e a u t h o r s express their t h a n k s to Dr. Z. S u j k o w s k i a n d Dr. J. Zylicz for t h e i r c o n t i n u o u s s u p p o r t d u r i n g this work. * Control measurements performed recently for 2asU allowed us to establish a more accurate peak position for 6He at 12.0±0.5 MeV.
References 1) S. W. Cosper, J. Cerny and R. C. Gatti, Phys. Rev. 154 (1967) 1193 2) M. Dakowski, J. Chwaszczewska, T. Krogulski, E. Piasecki and M. Sowifiski, Phys. Lett. 25B (1967) 213 3) D. Bollini, M. Cambiaghi, F. Fossati and T. Pinelli, Nuovo Cim. 51B (1967) 235 4) W. N. Andreev, W. G. Nedopekin and W. I. Rogov, ITEF Report Nr 609 (1968); Yad. Fiz. 9 (1969) 23