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The fissioning isomer 237Npm
Volume 43B, number 1
PHYSICS LETTERS
8 January 197"3
T H E F I S S I O N I N G I S O M E R 237Npm *
K.L. WOLF and J.P. UNIK Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Received 1 December 1972 A spontaneously fissioning isomer of ZaTNp with a half-life of 40 + 12 ns has been discovered. Threshold measurements indicate that the isomer excitation energy is 2.7 + 0.3 MeV. It is concluded that spontaneous fission is not the principal decay mode for this shape isomer.
Fission isomerism has previously been observed in many trans-neptunium isotopes [ 1 - 4 ] . Most of the isomeric half-lives follow systematic trends as a function of neutron number and proton number in general agreement with theoretical calculations of fission barrier heights and depths of the secondary minima in the nuclear mass surfaces [3, 5]. However, previous searches for fissioning isomers of neptunium have been negative [1,2, 6, 7], contrary to expectations based on theoretical calculations and the systematic trends of known isomers. Several authors have suggested that fissioning isomers of neptunium have not been observed due to competing modes of decay from the secondary minima, the most probable being "r-ray emission. If this interpretation is correct, one might still expect to observe weak fission branches in competition with other modes of deexcitation. Thus the purpose of the work reported here was to search for weak delayed fission branches in neptunium isotopes with a much higher experimental sensitivity than used in the past. Delayed fission activity was electronically measured between the beam microstructure of the Argonne 152 cm cyclotron which consisted of -~4 ns wide beam bursts at an 87 ns period [4]. A new fissioning isomer of 237Np with a half-life of 40 -+ 12 ns was discovered during bombardment of a 238U target with protons. The measured cross section of 16 nb at 11.5 MeV is substantially lower than previously established limits for short-lived neptunium isomers. The isotopic assignment of the 40 ns activity was made on the basis of the sharp reaction threshold shown in fig. 1 which is characteristic of the (p, 2n) reaction. An * Based on work performed under the auspices o f the U.S. Atomic Energy Commission.
I0.0 --
238U(p,2n)Z37Npm ~
.
,
~
~
2 e, o
"
2.7 + 0.3 MeV
,.a, .J w
i.o
9.0
~
I I
I
I0.0 I1.0 PROTON ENERGY(MeV)
J
I 2.0
Fig. 1. Delayed to prompt fission ratio as a function o f proton 238 237 m energy for the U(p, 2n) Np reaction. The solid curve is from the calculation described in the text.
isomer excitation energy of 2.7 -+ 0.3 MeV was extracted from the data with an evaporation model calculation performed using the formalism similar to that described by J~igare [ 8 ] . Level density parameters for the primary and secondary minima were set equal and were taken from the systematics of Gilbert and Cameron [9]. Level density parameters were again set equal for the inner and outer fission barriers and were obtained by fitting systematic values of m / l - ' f similar to the procedure of Britt et al. [3]. Fission barrier heights and widths are not known for 237Np but were assumed in this calculation to be equal to those obtained for 238Np from an analysis of (n, f) and (n,3') data [10]. From this analysis, heights of the inner and outer barriers are 6.0 MeV and 5.7 MeV, respectively, with corresponding curvatures of 1.0 MeV and 0.6 MeV. Calculations using this model quanti25
Volume 43B, number 1
PHYSICS LETTERS
tatively fit measured excitation functions for transneptunium isomers and yield excitation energies from 2.5 MeV for 239pum to 2.85 MeV for 240Am m. However the calculated values of the delayed to prompt fission ratio for 237Npm must be multiplied by a factor of 6.9 × 10 -4 to obtain the agreement with experiment shown in fig. 1. This attenuation factor for the observed yield can be interpreted as the branching ratio for spontaneous fission from the isomeric state, i.e. the isomeric state is probably populated by the calculated amount in the evaporation process but decay of this shape isomer is dominated by another mode of decay other than fission. An independent estimate of the branching ratio can also be made from the measured half-life of 40 ns compared to the expected half-life for spontaneous fission of ~0.1 ms obtained by extrapolation of half-life systematics or from semiempirical calculations [ 11 ]. The branching ratio obtained from this latter comparison is ~-4 X 10 -4, in good agreement with the estimate obtained from the cross-section data. From this comparison as well as the fact that the measured value of the isomer excitation energy is approximately the same as those of other known fissioning isomers, we conclude that the observed weak fission branch of 237Npm is most likely from the lowest-lying state in the secondary minimum. The principal decay mode of 237Npm is probably penetration of the inner barrier from the secondary minimum and 7-ray deexcitation through the levels of the primary minimum. The magnitude of the apparent enhancement of 7-ray branching for 237Npm relative to known trans-neptunium isomers is not
26
8 January 1973
understood at this time. It is possible that the inner fission barrier of 237Np is anomalously low ( ~ 5 MeV) compared to the barrier heights of nearby nuclides (-~6 MeV) or that there is an accidental matching of levels in the two minima which can also lead to an unusually large "/-ray decay width [10]. Further experimental studies must be made before it can be concluded whether one of the above possibilities is correct or if a more general explanation is necessary, i.e. factors commonly used in expressions for 7-ray lifetimes which are independent of barrier penetrability [10] may be too large or the odd-nucleon hindrance effect [2] may not influence 3,-ray decay probabilities.
References
[1] N.L. Lark, G. Sletten, J. Petersen and S. Bj~rnholm, NucL Phys. A139 (1969) 481. [2] S.M. Polikanov and G. Sletten. Nucl. Phys. A151 (1970) 656. [3] H.C. Britt et al., Phys. Rev. C4 (1971) 1444. [4] K.L. Wolf and J.P. Unik, Phys. Lett. 38B (1972) 405. [5] M. Bolsterli, E.O. Fiset, J.R. Nix and J.L. Norton, Phys.
Rev. C5 (1972) 1050. [6] K.L. Wolf et al., Phys. Rev. C1 (1970) 2096. [7] R. Repnow, V. Metag, J.D. Fox and P. Von Brentano, Nuci. Phys. A147 (1970) 183. [8] S. J~gare, Phys. Lett. 32B (1970) 571. [9] A. Gilbert and A.G.W. Cameron, Can. J.of Phys. 43 (1965) 1446. [10] J.E. Lynn. in Nuclear structure associated with the fission barrier, AERE M 2505 (1971). [ 11 ] V. Metag, R. Repnow and P. Von Brentano, Nucl. Phys. A165 (1971) 289.
PHYSICS LETTERS
8 January 197"3
T H E F I S S I O N I N G I S O M E R 237Npm *
K.L. WOLF and J.P. UNIK Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Received 1 December 1972 A spontaneously fissioning isomer of ZaTNp with a half-life of 40 + 12 ns has been discovered. Threshold measurements indicate that the isomer excitation energy is 2.7 + 0.3 MeV. It is concluded that spontaneous fission is not the principal decay mode for this shape isomer.
Fission isomerism has previously been observed in many trans-neptunium isotopes [ 1 - 4 ] . Most of the isomeric half-lives follow systematic trends as a function of neutron number and proton number in general agreement with theoretical calculations of fission barrier heights and depths of the secondary minima in the nuclear mass surfaces [3, 5]. However, previous searches for fissioning isomers of neptunium have been negative [1,2, 6, 7], contrary to expectations based on theoretical calculations and the systematic trends of known isomers. Several authors have suggested that fissioning isomers of neptunium have not been observed due to competing modes of decay from the secondary minima, the most probable being "r-ray emission. If this interpretation is correct, one might still expect to observe weak fission branches in competition with other modes of deexcitation. Thus the purpose of the work reported here was to search for weak delayed fission branches in neptunium isotopes with a much higher experimental sensitivity than used in the past. Delayed fission activity was electronically measured between the beam microstructure of the Argonne 152 cm cyclotron which consisted of -~4 ns wide beam bursts at an 87 ns period [4]. A new fissioning isomer of 237Np with a half-life of 40 -+ 12 ns was discovered during bombardment of a 238U target with protons. The measured cross section of 16 nb at 11.5 MeV is substantially lower than previously established limits for short-lived neptunium isomers. The isotopic assignment of the 40 ns activity was made on the basis of the sharp reaction threshold shown in fig. 1 which is characteristic of the (p, 2n) reaction. An * Based on work performed under the auspices o f the U.S. Atomic Energy Commission.
I0.0 --
238U(p,2n)Z37Npm ~
.
,
~
~
2 e, o
"
2.7 + 0.3 MeV
,.a, .J w
i.o
9.0
~
I I
I
I0.0 I1.0 PROTON ENERGY(MeV)
J
I 2.0
Fig. 1. Delayed to prompt fission ratio as a function o f proton 238 237 m energy for the U(p, 2n) Np reaction. The solid curve is from the calculation described in the text.
isomer excitation energy of 2.7 -+ 0.3 MeV was extracted from the data with an evaporation model calculation performed using the formalism similar to that described by J~igare [ 8 ] . Level density parameters for the primary and secondary minima were set equal and were taken from the systematics of Gilbert and Cameron [9]. Level density parameters were again set equal for the inner and outer fission barriers and were obtained by fitting systematic values of m / l - ' f similar to the procedure of Britt et al. [3]. Fission barrier heights and widths are not known for 237Np but were assumed in this calculation to be equal to those obtained for 238Np from an analysis of (n, f) and (n,3') data [10]. From this analysis, heights of the inner and outer barriers are 6.0 MeV and 5.7 MeV, respectively, with corresponding curvatures of 1.0 MeV and 0.6 MeV. Calculations using this model quanti25
Volume 43B, number 1
PHYSICS LETTERS
tatively fit measured excitation functions for transneptunium isomers and yield excitation energies from 2.5 MeV for 239pum to 2.85 MeV for 240Am m. However the calculated values of the delayed to prompt fission ratio for 237Npm must be multiplied by a factor of 6.9 × 10 -4 to obtain the agreement with experiment shown in fig. 1. This attenuation factor for the observed yield can be interpreted as the branching ratio for spontaneous fission from the isomeric state, i.e. the isomeric state is probably populated by the calculated amount in the evaporation process but decay of this shape isomer is dominated by another mode of decay other than fission. An independent estimate of the branching ratio can also be made from the measured half-life of 40 ns compared to the expected half-life for spontaneous fission of ~0.1 ms obtained by extrapolation of half-life systematics or from semiempirical calculations [ 11 ]. The branching ratio obtained from this latter comparison is ~-4 X 10 -4, in good agreement with the estimate obtained from the cross-section data. From this comparison as well as the fact that the measured value of the isomer excitation energy is approximately the same as those of other known fissioning isomers, we conclude that the observed weak fission branch of 237Npm is most likely from the lowest-lying state in the secondary minimum. The principal decay mode of 237Npm is probably penetration of the inner barrier from the secondary minimum and 7-ray deexcitation through the levels of the primary minimum. The magnitude of the apparent enhancement of 7-ray branching for 237Npm relative to known trans-neptunium isomers is not
26
8 January 1973
understood at this time. It is possible that the inner fission barrier of 237Np is anomalously low ( ~ 5 MeV) compared to the barrier heights of nearby nuclides (-~6 MeV) or that there is an accidental matching of levels in the two minima which can also lead to an unusually large "/-ray decay width [10]. Further experimental studies must be made before it can be concluded whether one of the above possibilities is correct or if a more general explanation is necessary, i.e. factors commonly used in expressions for 7-ray lifetimes which are independent of barrier penetrability [10] may be too large or the odd-nucleon hindrance effect [2] may not influence 3,-ray decay probabilities.
References
[1] N.L. Lark, G. Sletten, J. Petersen and S. Bj~rnholm, NucL Phys. A139 (1969) 481. [2] S.M. Polikanov and G. Sletten. Nucl. Phys. A151 (1970) 656. [3] H.C. Britt et al., Phys. Rev. C4 (1971) 1444. [4] K.L. Wolf and J.P. Unik, Phys. Lett. 38B (1972) 405. [5] M. Bolsterli, E.O. Fiset, J.R. Nix and J.L. Norton, Phys.
Rev. C5 (1972) 1050. [6] K.L. Wolf et al., Phys. Rev. C1 (1970) 2096. [7] R. Repnow, V. Metag, J.D. Fox and P. Von Brentano, Nuci. Phys. A147 (1970) 183. [8] S. J~gare, Phys. Lett. 32B (1970) 571. [9] A. Gilbert and A.G.W. Cameron, Can. J.of Phys. 43 (1965) 1446. [10] J.E. Lynn. in Nuclear structure associated with the fission barrier, AERE M 2505 (1971). [ 11 ] V. Metag, R. Repnow and P. Von Brentano, Nucl. Phys. A165 (1971) 289.