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Search for fission shape isomers in (n,f) reactions induced by neutrons of about 1 MeV
Volume 48B, number 1
PHYSICS LETTERS
7 Januar~¢ 1974
SEARCH FOR FISSION SHAPE ISOMERS IN (n, f) REACTIONS INDUCED
BY NEUTRONS
OF ABOUT
1 MeV
R. MULLER and F. GONNENWEIN Phystkahsches Instttut der Untversttat Tubtngen, German)'
F. KAPPELER and A. ERNST Kernforschungszentrum Karlsrulle, German)' J SCHEER Hahn-Mettner-lnstttut Berhn, Germany
Received 29 November 1973 Upper hmlts for the isomer to prompt fissmn ratio were measured for the compound nuclm 236Uand 24°pu excited by neutrons with energies m the 0 5-2.5 MeV region. From these data the differences m the barrier hmghts of the double-humped fission potential were calculated with a modified stat~sncal theory
The existence o f fissioning shape isomers is one o f the most striking phenomena interpreted in terms of a double-humped fission barrier. This type o f potential barrier was shown by Strutmsky [1 ]i to,be expected when applying shell corrections to the classical liquid drop model. Fission induced by neutrons o f about 1 MeV proves to be an especially well suited reaction to study the relative heights o f the two barriers. The isotopes 234,235,238U and 239pu were used as target nuclei. Experiments on charged particle induced fission indicate that only for the compound nuclei 236U and 240pu shape isomers may show up with halflives o f 130 ns and 4 ns, respectively. For neutron induced fission Elwyn and Ferguson [2] measured unusually high cross sections for isomer production. The present experiment was in part stimulated by this work. A more detailed description o f the present experiment and the calculations is given in refi [3]. The measurements were performed with the pulsed beam Van de Graaff accelerators o f the K F Z Karlsruhe and the HMI Berlin. The neutrons were produced b) the reactions 7Li(p, n) and T(p, n). In the fission decay mode o f the compound nuclei excited by the neutron burst one expects besides the p r o m p t component delayed events due to isomeric fission. The fission fragments were identified by surface barrier detectors. A major problem in this type o f experiments are
false events due to scattered and therefore delayed neutrons. This is especially true for the highly fissionable target nuclei 235U and 239pu. For 234U and 238U it is possible to discriminate against neutrons produced at backward angles and rescattered onto the fission target By a suitable choice of neutron producnon reactions and fissioning nuclei these neutrons may not have enough energy to excite the compound nuclei beyond the fission threshold As seen m fig 1 the time spectrum obtained for 234U Is indeed superior to those for 235U and 239pu In order to minimize the influence of neutron scattering and the propagation time differences, care was taken to keep the distances between neutron target, fission target and fission counter as small as possible. These distances were 5 mm and 1.5 mm, respectively As shown by a Monte Carlo calculation, this compact geometrical arrangement not only ensures small differ. ences in propagation time, but indeed reduces the influence of neutrons scatterd, e.g., from the beam tubes of the accelerator The thickness of the fission targets was 1 2 mg/cm 2 The time spectra have been considerably improved as compared to older measurements. For the even isotopes they are now comparable to those from charged-pamcle Induced fission. From fig 1 the halfwidth o f the 234U spectrum is read to be 1 ns and within 5 ns the number of events drops by a factor of 105 relative to the peak value. 25
Volume 48B, number 1
PHYSICS LETTERS
107
Table 1 Isomer to prompt fission ratio (× 10-s).
j'
106
.
234u
/
+
23Su
I
v 239pu
10-c
7 January 1974
Isomeric nuclei (half-hves) '
L!
A m
Neutron
236U
240p u
235U
240p u
energy
130 ns
4 ns
20 ns
28 ns
055 MeV 2 5 MeV 2 2 MeV
<27 <5 31 < 5 < 6
<7 < 24 390
<23
<30 <8 41
slow
10~
<20
o
/ °
0.5 MeV
3
21
Ref
This work [2] [4] [5 ] [6] [7,8]
103
,:~ ,/.-+;' ? i/)
z
0 102
;/,v
/
10
.
1
30
25
~r
20
15
.
.
.
.
.
.
10
.
5
.
0
DELAY TIME {ns}
Fig. 1 Tame spectra of fission fragments induced by neutrons (550 keV) from 7Ll(p, n) The time spectra for the odd isotopes studied are very snnllar (fig. 1). Tlus means that the evidence for isomeric fission is not manifest However, from a careful comparison of the two spectra one obtains for an incoming neutron energy of 550 keV the following ratios of Isomeric to prompt fission (0.9 -+ 0.9) × 10 . 5 for the 130 ns 236U Isomer and (0.0 +- 3.5) × 10 . 5 for the 4 ns 240pu isomer At a 2o confidence level the upper hmlts for the above ratios are 2.7 × 10 .-5 and 7 × 10 - 5 , respectively A summary of the results for the ratio of lsomerlc to prompt fission measured so far is shown in table 1. Compared with the present work the figures given m ref. [2] are higher by about a factor of 10. This is also true for the delayed events from 235U and 240pu with halflIves of 20 ns and 28 ns, respectively, only found in that work Presumably the deterioration of the data by neutron scattering was underestimated for the physical setup used m that work. Measurements with slow neutrons [ 4 - 6 ] yield upper limits which are about twice as high as found in the present work for 26
a neutron energy of 550 keV. For excitation of the compound nucleus 236U via the (d, p) reaction [7, 8], the ratio of isomeric to prompt fission at an excitation energy of 7 MeV (corresponding to an incoming neutron energy of 0.5 MeV) is 3 × 10 - 5 . This value agrees within the statistics with the result of the present work, but it leads to a higher upper limit. From the present results one can estimate the relative peak heights of the double-humped fission barher. The reaction widths were calculated in the framework of the statlst]cal theory, as gwen by Jggare [9]. However, in contrast to Jagare It is not assumed that 7-decay m the deformation region of the second well necessarily populates the isomeric state. There may be competing processes like fission and 7-decay into the first well. Furthermore, in the latter decay mode the coupling of class I and class II states as defined by Lynn [10] is described by perturbation theory for low compound excitation energies. The parameters of the level density formula used [1 I] were fitted to reproduce level spacings known from slow neutron work and the o,y/of ratio. From the work of Back et al. [12] the fission barrier heights were taken to be 6.1 MeV and 6.05 MeV for the compound nuclei 236U and 240pu, respectively. This barrier was assumed to be the higher one of the two barhers. On the other hand a m l m m u m height of about 5.3 MeV for the lower b a m e r can be derwed from the existence of vibrational resonances near 5.0 MeV for either nuclei [12]. For 236U the calculations show that the inner barrier has to be identified with the fission barrier. The outer barrier is lower and lies between 5.3 MeV and 5.75 MeV. From the isomeric lifetime the width pa-
Volume 48B, number 1
PHYSICS LETTERS
rameter t/co o f this barrier is b e t w e e n 0.58 MeV and 0.67 MeV. The evidence for 240pu is not so clearcut. Nevertheless, here t o o the inner b a m e r seems to be the higher one. We gratefully acknowledge financial support b y the B u n d e s m l n i s t e r m m ftir F o r s c h u n g und Technologle.
References [1] V.M. Strutmsky, Nucl. Phys. A95 (1967) 420, A122 (1968) 1. [2] A.J. Elwyn and A.T.G Ferguson, Nucl. Phys. A148 (1970) 337.
7 January 1974
[3] R. Muller, thesis, to be pubhshed. [4] E Konecny et al, Nucl. Phys. A187 (1972) 426 [5] J C. Browne and C.D. Bowman, Phys. Rev Lett 28 (1972) 617. [6] G.V Valsky, O M. Mrachkovsky, G.A Petrov and Yu. S. Pleva, Yad. Fiz 16 (1972) 667. [7] H.C. Brltt and B H. Erkklla, Phys Rev C4 t 1971) 1441 [8] J Pedersen and B. Rasmussen, Nucl. Phys A178 (1972) 449. [9] S Jagare, Nucl Phys A137 (1969) 241. [10] J.E. Lynn, Physics and chemistry of fission (IAEA, Vienna, 1969) p. 249. [11] A. Gilbert and A.G.W. Cameron, Can. J. Phys. 43 (1965) I446 [12] B B. Back et al, Nucl. Phys. A165 (1971) 449
27
PHYSICS LETTERS
7 Januar~¢ 1974
SEARCH FOR FISSION SHAPE ISOMERS IN (n, f) REACTIONS INDUCED
BY NEUTRONS
OF ABOUT
1 MeV
R. MULLER and F. GONNENWEIN Phystkahsches Instttut der Untversttat Tubtngen, German)'
F. KAPPELER and A. ERNST Kernforschungszentrum Karlsrulle, German)' J SCHEER Hahn-Mettner-lnstttut Berhn, Germany
Received 29 November 1973 Upper hmlts for the isomer to prompt fissmn ratio were measured for the compound nuclm 236Uand 24°pu excited by neutrons with energies m the 0 5-2.5 MeV region. From these data the differences m the barrier hmghts of the double-humped fission potential were calculated with a modified stat~sncal theory
The existence o f fissioning shape isomers is one o f the most striking phenomena interpreted in terms of a double-humped fission barrier. This type o f potential barrier was shown by Strutmsky [1 ]i to,be expected when applying shell corrections to the classical liquid drop model. Fission induced by neutrons o f about 1 MeV proves to be an especially well suited reaction to study the relative heights o f the two barriers. The isotopes 234,235,238U and 239pu were used as target nuclei. Experiments on charged particle induced fission indicate that only for the compound nuclei 236U and 240pu shape isomers may show up with halflives o f 130 ns and 4 ns, respectively. For neutron induced fission Elwyn and Ferguson [2] measured unusually high cross sections for isomer production. The present experiment was in part stimulated by this work. A more detailed description o f the present experiment and the calculations is given in refi [3]. The measurements were performed with the pulsed beam Van de Graaff accelerators o f the K F Z Karlsruhe and the HMI Berlin. The neutrons were produced b) the reactions 7Li(p, n) and T(p, n). In the fission decay mode o f the compound nuclei excited by the neutron burst one expects besides the p r o m p t component delayed events due to isomeric fission. The fission fragments were identified by surface barrier detectors. A major problem in this type o f experiments are
false events due to scattered and therefore delayed neutrons. This is especially true for the highly fissionable target nuclei 235U and 239pu. For 234U and 238U it is possible to discriminate against neutrons produced at backward angles and rescattered onto the fission target By a suitable choice of neutron producnon reactions and fissioning nuclei these neutrons may not have enough energy to excite the compound nuclei beyond the fission threshold As seen m fig 1 the time spectrum obtained for 234U Is indeed superior to those for 235U and 239pu In order to minimize the influence of neutron scattering and the propagation time differences, care was taken to keep the distances between neutron target, fission target and fission counter as small as possible. These distances were 5 mm and 1.5 mm, respectively As shown by a Monte Carlo calculation, this compact geometrical arrangement not only ensures small differ. ences in propagation time, but indeed reduces the influence of neutrons scatterd, e.g., from the beam tubes of the accelerator The thickness of the fission targets was 1 2 mg/cm 2 The time spectra have been considerably improved as compared to older measurements. For the even isotopes they are now comparable to those from charged-pamcle Induced fission. From fig 1 the halfwidth o f the 234U spectrum is read to be 1 ns and within 5 ns the number of events drops by a factor of 105 relative to the peak value. 25
Volume 48B, number 1
PHYSICS LETTERS
107
Table 1 Isomer to prompt fission ratio (× 10-s).
j'
106
.
234u
/
+
23Su
I
v 239pu
10-c
7 January 1974
Isomeric nuclei (half-hves) '
L!
A m
Neutron
236U
240p u
235U
240p u
energy
130 ns
4 ns
20 ns
28 ns
055 MeV 2 5 MeV 2 2 MeV
<27 <5 31 < 5 < 6
<7 < 24 390
<23
<30 <8 41
slow
10~
<20
o
/ °
0.5 MeV
3
21
Ref
This work [2] [4] [5 ] [6] [7,8]
103
,:~ ,/.-+;' ? i/)
z
0 102
;/,v
/
10
.
1
30
25
~r
20
15
.
.
.
.
.
.
10
.
5
.
0
DELAY TIME {ns}
Fig. 1 Tame spectra of fission fragments induced by neutrons (550 keV) from 7Ll(p, n) The time spectra for the odd isotopes studied are very snnllar (fig. 1). Tlus means that the evidence for isomeric fission is not manifest However, from a careful comparison of the two spectra one obtains for an incoming neutron energy of 550 keV the following ratios of Isomeric to prompt fission (0.9 -+ 0.9) × 10 . 5 for the 130 ns 236U Isomer and (0.0 +- 3.5) × 10 . 5 for the 4 ns 240pu isomer At a 2o confidence level the upper hmlts for the above ratios are 2.7 × 10 .-5 and 7 × 10 - 5 , respectively A summary of the results for the ratio of lsomerlc to prompt fission measured so far is shown in table 1. Compared with the present work the figures given m ref. [2] are higher by about a factor of 10. This is also true for the delayed events from 235U and 240pu with halflIves of 20 ns and 28 ns, respectively, only found in that work Presumably the deterioration of the data by neutron scattering was underestimated for the physical setup used m that work. Measurements with slow neutrons [ 4 - 6 ] yield upper limits which are about twice as high as found in the present work for 26
a neutron energy of 550 keV. For excitation of the compound nucleus 236U via the (d, p) reaction [7, 8], the ratio of isomeric to prompt fission at an excitation energy of 7 MeV (corresponding to an incoming neutron energy of 0.5 MeV) is 3 × 10 - 5 . This value agrees within the statistics with the result of the present work, but it leads to a higher upper limit. From the present results one can estimate the relative peak heights of the double-humped fission barher. The reaction widths were calculated in the framework of the statlst]cal theory, as gwen by Jggare [9]. However, in contrast to Jagare It is not assumed that 7-decay m the deformation region of the second well necessarily populates the isomeric state. There may be competing processes like fission and 7-decay into the first well. Furthermore, in the latter decay mode the coupling of class I and class II states as defined by Lynn [10] is described by perturbation theory for low compound excitation energies. The parameters of the level density formula used [1 I] were fitted to reproduce level spacings known from slow neutron work and the o,y/of ratio. From the work of Back et al. [12] the fission barrier heights were taken to be 6.1 MeV and 6.05 MeV for the compound nuclei 236U and 240pu, respectively. This barrier was assumed to be the higher one of the two barhers. On the other hand a m l m m u m height of about 5.3 MeV for the lower b a m e r can be derwed from the existence of vibrational resonances near 5.0 MeV for either nuclei [12]. For 236U the calculations show that the inner barrier has to be identified with the fission barrier. The outer barrier is lower and lies between 5.3 MeV and 5.75 MeV. From the isomeric lifetime the width pa-
Volume 48B, number 1
PHYSICS LETTERS
rameter t/co o f this barrier is b e t w e e n 0.58 MeV and 0.67 MeV. The evidence for 240pu is not so clearcut. Nevertheless, here t o o the inner b a m e r seems to be the higher one. We gratefully acknowledge financial support b y the B u n d e s m l n i s t e r m m ftir F o r s c h u n g und Technologle.
References [1] V.M. Strutmsky, Nucl. Phys. A95 (1967) 420, A122 (1968) 1. [2] A.J. Elwyn and A.T.G Ferguson, Nucl. Phys. A148 (1970) 337.
7 January 1974
[3] R. Muller, thesis, to be pubhshed. [4] E Konecny et al, Nucl. Phys. A187 (1972) 426 [5] J C. Browne and C.D. Bowman, Phys. Rev Lett 28 (1972) 617. [6] G.V Valsky, O M. Mrachkovsky, G.A Petrov and Yu. S. Pleva, Yad. Fiz 16 (1972) 667. [7] H.C. Brltt and B H. Erkklla, Phys Rev C4 t 1971) 1441 [8] J Pedersen and B. Rasmussen, Nucl. Phys A178 (1972) 449. [9] S Jagare, Nucl Phys A137 (1969) 241. [10] J.E. Lynn, Physics and chemistry of fission (IAEA, Vienna, 1969) p. 249. [11] A. Gilbert and A.G.W. Cameron, Can. J. Phys. 43 (1965) I446 [12] B B. Back et al, Nucl. Phys. A165 (1971) 449
27