PHYSICAL REVIEW C VOLUME 35, NUMBER 6 Anisotropies in transfer-induced fission of ' 0 + Th JUNE 1987 F. Videbaek Physics Di Vision, Argonne Rational Laboratory, Argonne, Illinois 60439 S. G. Steadman, G. G. Batrouni, * and J. Karp Laboratory for 1Vuclear Science and Physics Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 24 November 1986) Transfer-induced fission has been measured for ' 0+ Th at 105 MeV. The angular distribu- tions have a small in-plane anisotropy, but a substantial out-of-plane anisotropy, increasing with the mass transfer. The transferred angular momentum needed to describe the distribution is in agree- ment with the semiclassical predictions. Large anisotropies have been observed in the angular distributions of fission fragments induced by the reactions ' O+ Th and ' C+ U near and below the Coulomb barrier. ' The conclusion from the analysis in Refs. 1 and 2 is that larger than expected mean square spin values are reached in the compound nucleus before fission, assuming that all fission originates via complete compound nucleus formation. It has been pointed out ' that transfer- induced fission may contribute significantly to the total fission cross section at below-barrier energies. This would alter the interpretation of anisotropies in terms of spin values, either larger or smaller, from values derived assuming only compound nuclear function. In the present paper we report on probabilities of transfer-induced fis- sion and of in-plane and out-of-plane anisotropies for ' 0+ Th at E/VCB — 1. 16. The in-plane anisotropies are small and the out-of-plane anisotropies are between 0.8 and 0. 4. If these values persist at the lower energies (E/Vc&-0. 85), significant contributions to the fission angular distribution anisotropies will come from the transfer-induced fission. It is shown that the measured anisotropies are consistent with an angular momentum transfer expected from the simplest semiclassical model. The experiment was performed at the Brookhaven Na- tional Laboratory Tandem Facility with a 105-MeV ' 0 beam on a target of Th with a thickness of approxi- mately 1 mg/cm . A AE-E solid-state telescope, used to measure and identify the transfer products, was placed at 70', which is near the grazing angle. The results from the singles experiment have been published in Ref. 5. Fission products in coincidence with transfer reaction products were measured at three angles, sufficient to give approxi- mate in- and out-of-plane anisotropies. Fission was ob- served in coincidence with projectile remnants ranging from ' N to ' Be. The relative distribution among the various channels is given in Table I. Here is also shown the fission probabilities, assuming a 1/sinO distribution. This assumption is not justified, as seen later, but is used as a convenient reference because the anisotropies were not determined for all channels. At the lower bombarding energies (E/VCB-0. 8 — 1.05) (Ref. 4) the dominant chan- nels contributing to fission are ' C (45%%uo), ' C (25%%uo), and '5N (20&o). At the higher beam energy in the present study more transfer channels are being populated, thus spreading the fission strength among more final transfer products. The total transfer cross section is 140 mb while the transfer-induced fission cross section is 62 mb under the 1/sin0 assumption. At the top of Fig. 1 are shown the singles energy distributions for the '"C and ' N transfer channels. The distributions for the same two nu- clei in coincidence with fission are shown in the middle frames. The ' N channel can only contribute to fission for Q & — 7 MeV, so even though the ' N channel has the highest single transfer cross section, it only contributes — 16% to the fission cross section, because Q window ef- fects concentrate the strength at high Q values. The ' C channel is populated at large negative Q, corresponding to high excitation energies in the final U, which lead to fission. In the lower two frames is shown the fission probability Q value, assuming again a 1/sinO distribution. Integration of more realistic angular distributions results in a roughly 30% decrease in the probabilities. The ' N channel exhibits, at least at Q —— 8 and — 19 MeV, peaks in the fission probabilities. These appear close to the 1n- and 2n-binding energies for U. Similar observations have been made with light-ion-induced fission (see, e. g. , Ref. 6, and references therein). Such thresholds can also result in variations in the observed fission anisotropies, usually in the form of a sudden increase. The anisotropies of the fission angular distributions are given in Table II for the channels with sufficient statistics to get meaningful results. The angles are defined (similar to Ref. 7) as follows: P is measured relative to the recoil axis of the heavy fragment in the reaction plane defined by the beam axis and the recoil axis; O is the angle mea- sured from the Z axis perpendicular to the reaction plane. We observe two basic trends in the data: The in-plane an- isotropies are fairly small decreasing with the mass transfer, while the out-of-plane ones are larger and in- creasing with the mass transfer. To gain insight into the properties of the angular distribution, we performed an analysis of the data using the parametrized formalism of transfer-induced fission used in the analysis of Kr+ Bi data. The small number of data points and the good ap- proximation of this method to the exact description justi- fies this choice. In brief, the angular distribution is given 35 2333 1987 The American Physical Society