A study of the spontaneously-fissioning isomer of 242Am through the 241Am(n,γ) reaction

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Nuclear Physics A102 (1967) 443--448; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

A STUDY OF THE S P O N T A N E O U S L Y - F I S S I O N I N G I S O M E R OF 242Am THROUGH THE 241Am(n, 7) REACTION G. N. FLEROV, A. A. PLEVE, S. M. POLIKANOV and S. P. TRETYAKOVA

Joint Institute for Nuclear Research, Dubna and I. BOCA, M. SEZON, I. VILCOV and N. VILCOV

Institute for Atomic Physics, Bucharest Received 31 May 1967 Abstract: The excitation function for the ~41Am(n, 7)242mAre reaction leading to the excitation

of the 14 ms, spontaneously fissioning isomer has been studied in the range 0-6.5 MeV. The experimental results can be well explained taking into account the hypothesis of Strutinsky concerning the "shape isomerism".

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N U C L E A R REACTION, FISSION 24~Am(n, 7)2~2Am, (sf-isomer), E : 0-6.5 MeV; measured or(E). 244Am (sf-isomer) [from 24aAm (n, 7)l; measured T~.

1. Introduction Several recent experimental works have undertaken the study of the properties of the spontaneously fissioning isomer of 242Am with T~ = 14 ms. The threshold measurement in the 243Am(n, 2n)242mAm reaction used for the excitation of the spontaneously fissioning isomer 242Am allowed Flerov et al. 1) to find an excitation ,energy for the isomeric state equal to 2.9___0.4 MeV. Ref. 2) reported the isomer ratios for the same isomer, measured in several reactions, which have practically identical values for quite different average angular momenta of the compound nucleus. Hence it was concluded 2) that the spin of the spontaneously fissioning isomer must be close to the spin value of the 242Am ground state with a difference not larger than some units. The calculation 3) of the 242Am states shows the presence of a large number of .excited states with spin values up to ~ 11 h in the energy range below 1 MeV. For this reason the strongly forbidden radiative transitions from the isomeric state with a 3 MeV excitation energy to lower states cannot be exclusively explained by the spin value. Consequently, it was assumed 2) that the radiative transition hindrance for the spontaneously fissioning isomer 242roAm is due to the large difference between the isomeric and ground-state deformations. It should be noted that the large deformation hypothesis was also needed to explain the substantial increase in the spontaneous fission probability of the 242Am isomeric state 3, 4). Recently, Strutinsky s) showed that in the region of the transuranium nuclei equilibrium states with deformations close to the saddle-point deformation can appear; this could explain the spontaneous fission from isomeric states. 443

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G.N. FLEROV

As the existing experimental data are too poor to allow an explanation of the spontaneously fissioning isomer nature and particularly of the so-called "shape isomerism", new experimental data concerning the spontaneously fissioning isomers appear to be extremely useful. The excitation of the spontaneously fissioning isomer 242roAmwas also attempted 6), through the slow neutron capture in 241Am. The measurements only permitted assignment of an upper limit to the isomer ratio of 5 x 10-7. At the same time the possibility of exciting the spontaneously fissioning isomer 242roAm through the capture of 1 to 3 MeV neutrons in 241Am was suggested 1). The purpose of the present work is a detailed study of the 241Am(n, 7)242roAm reaction leading to the excitation of the 14 ms spontaneously fissioning isomer, using neutrons with energies in the range 0-6.5 MeV.

2. Experimental procedure The reaction 7Li(p, n)7Be was used as a neutron beam source. The Li target was in the form of metallic foils of different thicknesses from 100 #m to 500/~m (for the range

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Fig. 1. Lithium target arrangement. of higher energies); it was held between two Ta plates (see fig. 1). With water cooling we could work with proton beam currents of 10-15 #A. The variation of the neutron beam energy was achieved by changing the energy of the proton beam incident on the Li target. For this purpose, the working conditions at the Cyclotron of I.A.P.-Bucharest included the three energies of the accelerated protons 4.5, 7.5 and 9.8 MeV, the energy being changed further by means of Ta absorbers. The fission fragment detectors arrangement is similar to that used in studying 1) the reaction Z43Am(n, 2n)ZgzmAm. To increase the mica detector sensitivity, they were given a preliminary treatment of heating at high temperature and a long etching before the irradiation 7). The energy spread of the neutron beam imposed by the Li target thickness and the solid angle was about +450 keV for neutron energies within 0-3 MeV and + 1.3 MeV for the energies 3-6.5 MeV.

445

2ltAm (n, 7) REACTION

There were two ways to monitor the neutron beam. (i) The Li target y-activity was measured after every irradiation; it was proportional to the total number of neutrons emitted from the target during the irradiation and was corrected for the neutron angular distribution variation with the incident proton energy. (ii) We measured the activity of a copper foil (placed near the Z41Am target) following the 63Cu(n,T)64CH reaction. For incident neutron energies higher than 5 MeV, Mg monitors 1) were also used. Since the half-lives of the measured activities for Cu as well as for Mg were about of the order of the irradiation-time intervals, we considered also the current diagram of the beam on the target and made suitable corrections of the monitor data.

3. Results

The excitation function for the 241Am(n, 7)24ZmAm reaction leading to the excitation of the 14 ms spontaneously fissioning isomer 242mAm has been measured in the

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energy range 0-6.5 MeV. In the same energy range we performed several measurements to check the background due to associate processes (induced fission of heavy impurities in mica detectors, induced fission of the A m target by the background neutrons between the beam pulses and the effect due to neutrons emitted from Ta etc). But in all experiments the contribution of these processes to the total effect was less than 5-10 %. Fig. 2 shows the fission fragment distribution in the mica detectors for the 241Am and 238U target for the same incident proton energy Ep -- 4.5 MeV and the same total beam charge. Fig. 3 shows the dependence of the 242Am, spontaneously fissioning, isomer-yield cross section on the neutron energy.

446

G.N. FLEROV e t a l .

4. Discussion The 242Am compound nucleus formed by neutron capture in the target nucleus 241Am has an excitation energy of E* = En+5.5 MeV, where E n is the incident neutron energy (in the c.m. system). The populating of the spontaneously fissioning isomer occurs only through radiative transitions in the compound nucleus. In the energy range considered in this paper, the probability of the spontaneously-fissioning isomer excitation is determined by the weights of the competitive processes: radiative transition to the 24ZAm ground state (or to the 5itg-:~c~ :)

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Fig. 3. T h e ~42mAm yield cross section versus n e u t r o n energy.

isomeric state at 48 keV), neutron scattering and induced fission of 242Am. The induced fission of 242Am becomes important only at incident neutron energies higher than 1 MeV. As for the former competitive processes, there are no previous experimental data f o r 2 4 1 A m in this range of energies, therefore we cannot attempt a detailed analysis of our data. A closer examination of fig. 3 shows a fast increase of the 2 4 2 m A m spontaneously fissioning isomer-yield cross section in the excitation energy range 5.5-7.5 MeV. But, it is known 6) that for 5.5 MeV excitation energy the isomer ratio does not exceed 5 • 10- 7. Though the cross section of the neutron radiative capture process has not been measured previously at 1.5 MeV energy, it cannot exceed 2-3 b (actually taking into account the existing data 8) for 238 U and 232Th ' the neutron capture cross section is likely to be less than 2 . 1 0 .25 cruZ). In this way, for an increase of the 24ZAm compound nucleus excitation energy from 5.5 to 7 MeV (En = 0-1.5 MeV), the isomeric-yield ratio increases almost with a factor of 20. Such a fast increase of the isomeric-yield ratio

241Am(n,y) REACTION

447

points out the appearance of some excited states of the 242Am nucleus in the energy range 5.5-7.5 MeV, which favours the radiative transition to the spontaneously fissioning isomer. But this conclusion does not contradict the assumption that 24amAin could be a case of "shape isomerism", when equilibrium states with deformations corresponding to the critical form of the nucleus are separated from the ground state by a potential barrier 5). The increasing part of the excitation function in fig. 3 corresponds to incident neutron energies of 0-1.5 MeV and may be associated with a 6 MeV high potential barrier. The decrease in the spontaneously fissioning isomer-yield cross section at

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Fig. 4. The 24amAinisomer decay curve. energies above 1.5 MeV is probably due to the competitive processes of the inelastic neutron scattering. In a closer examination of the excitation function, an increase of the isomeric-yield cross section may be observed also at neutron energies above 4 MeV t. In order to try to explain this increase we ought to continue the excitation function in the range of higher neutron energies. We attempted also an irradiation of 243Am with neutrons of 2.6_+ 1.2 MeV energy. Fig. 4. represents the decay curve of the spontaneously fissioning isomer and the corresponding decay curve for the irradiation of an 238U target in similar beam conditions. The resulting half-life T~ = 0.9-t-0.3 ms is in good agreement with the known 9) value for the spontaneously-fissioning isomer 244mAre. In this way it is established that this isomer, which has been reported only for proton-induced reactions, can be excited enough well through neutron capture in the energy range 1.4-3.8 MeV. * T h e possible i m p u r i t y o f 1-2 M e V n e u t r o n s f r o m L i + p (others t h a n (p, n)) reactions, u n f o r t u nately, c a n n o t be completely excluded in this energy range.

448

G.N. FLEROVet al.

References 1) 2) 3) 4) 5) 6) 7)

G. N. Flerov e t al., Nuclear Physics 97A (1967) 444 G. N. Flerov et al., 1UCN Dubna report P7-3065 (1966) L. A. Malov, S. M. Polikanov and V. G. Soloviev, Yad. Fiz. 4 (1966) 528 G. N. Flerov and V. A. Druin, Structura slojn~h iader (Moskva, 1966) p. 249 V. M. Strutinsky, Nuclear Physics 95A (1967) 420 B. N. Markov, A. A. Pleve, S. M. Polikanov and G. N. Flerov, Yad. Fiz. 3 (1966) 455 A. Kapuscik, V. P. Perelygin, V. I. Swiderski and S. P. Tretyakova, IUCN Dubna report P-2705 (1966) 8) D. J. Hughes and R. B. Schwartz, BNL-325 (1958) 9) S. Bjornholm, private communication