H. Dejbakhsh

Signature of the shape coexistence in 72Kr: discontinuities of the moment of inertia at low spin

Abstract The low-lying level structure of the N=Z nucleus 72Kr was studied by 40 Ca ( 35 Cl, p , 2 n γ) 72 Kr at 95 MeV. Gamma ray coincidences are used to construct for the first time a level scheme for this nucleus based on coincidence gating with the previously identified 2+−0+ transition. The new level scheme indicates strong prolate-oblate mixing of two shape coexisting bands. The results support theoretical predictions of a stable oblate minimum in this region.

Continuous distribution of αt relative kinetic energies in 7Li breakup reactions

Abstract A newly developed technique has been used to probe a previously inaccessible region of the continuum in the at channels of 7 Li breakup reactions. This was used to investigate reactions of 63 MeV 7 Li with 58 Ni, 120 Sn, 144 Sm, and 208 Pb targets. Breakup pairs of α-particles and tritons emitted within a finite opening angle were analyzed according to their magnetic rigidity using a K = 150 Enge split-pole spectrograph. The αt coincidences were taken with two single-wire proportional counters mounted at different positions along the focal plane of the spectrograph. The large energy acceptance ( E max / E min ≈ 8) of the spectrograph is crucial to the present coincidence measurements involving two particles with different q / m values. Very small relative energy components were studied without any detection threshold. A pronounced bump was observed immediately above the αt particle threshold. No strong indication of the post Coulomb acceleration was observed. The excitation of the continuum states as well as the 7 − 2 state in 7 Li is discussed in terms of the distorted-wave Born approximation. The astrophysical implications of the data are also discussed regarding the Coulomb excitation of the continuum. The present technique offers the promise of very high sensitivity at small relative energies if it were applied in conjunction with ray-tracing, a thin target, and a dispersion-matched beam.

Observation of α-t continuum states with relative energies of 0–2 MeV in7Li breakup reactions

Abstract The breakup of 7 Li at 63 MeV was studied in an α-t coincidence experiment using an Enge split pole spectrograph. For the first time, the entire spectrum of the α-t continuum states of 7 Li was observed in the range of the relative energy ɛ from zero to 2 MeV. A pronounced bump was found at ɛ ≈ 0.4MeV. The astrophysical implications of the data are discussed.

Probing the tilting mode in fission with γ-ray multiplicity techniques

Abstract The γ-ray multiplicities of the fragments from a number of 4 He-induced fission reactions have been measured as a function of fragment emission angle. The value of M γ is found to vary with angle in qualitative agreement with the predictions of statistical models of fission-fragment angular distributions. The observed variation is rather weak, on the order of 5%. The data are compared with several models. Calculations assuming a rigidly rotating, transition-state nucleus predict a much stronger angular dependence of the fragment spin than indicated by the data. The agreement is significantly improved if a fragment-spin enhancement effect is included in these calculations. The fragment spins are explored within the framework of the statistical scission model and the collective mode model. Both of these models predict large fragment spins and thus a relatively weak angular dependence of the total fragment spin. Neither model provides a completely satisfactory explanation of the data. Depending on the assumptions made in the calculations, the models either predict too strong an angular dependence of the total fragment spin or spins which seem incompatible with M γ .

K-equilibration and spin enhancement effects in 120 MeV 16O- and 214 MeV 32S-induced fission

Abstract The gamma-ray multiplicities (Mγ) of fission fragments have been investigated in a variety of 16O- and 32S-induced reactions. The dependence of Mγ on target mass and excitation energy is well explained by a model which assumes the thermal excitation of angular momentum bearing collective modes. As predicted by the statistical description of fission fragment angular distributions, the value of Mγ is found to vary with fission fragment detection angle. The observed variation is considerably weaker than predicted by simple rigid rotation models. A large part of this discrepancy can be explained by the excitation of collective modes. For heavy targets, sequential fission contributions also weaken the angular dependence of the fragment spins. However, even with these corrections, some systems still seem to exhibit a weaker angular dependence than predicted by the model calculations. Possibly this is due to a suppression of high K-states.

Exploring the k-distribution in fission

Abstract Statistical models of fission angular distributions predict that the spins of the fragments should vary with angle. This effect has been investigated in several 4 He-induced fission reactions using γ-ray multiplicity techniques. While M γ does vary with emission angle, the effect is smaller than expected. The excitation of additional angular momentum bearing modes accounts for much of this discrepancy.

Possible interpretation of configuration dependent deformation in odd-A transitional nuclei in the mass 100 region☆

Abstract A recently proposed formalism which relates the n−p interaction strength to the product of the number of valence neutrons and protons (N n ∗N p ) is extended to the odd-A nuclei in the A ≈ 100 region. The data suggest that the effective n−p force depends on the configuration ( π p 1 2 or πg 9 2 ).

Termination of the 12C12C cluster states revealed in the 12C(20Ne,12C12C) 8Be breakup reaction

Abstract An investigation of the 12C(20Ne,12C12C)8Be reaction at 300 MeV shows the breakup yield disappears at the same excitation energy as observed in a previous measurement at 160 MeV. This would be consistent with the breakup being associated with highly-deformed-shape isomeric bands which model calculations predict will terminate at high excitation energies.

Search for resonant electron positron annihilation in flight

Abstract We have studied the energy dependence of the electron-positron annihilation-in-flight cross section for di-photon invariant masses in the range of 1600–2000 keV. At the 95% confidence level, no resonances are observed implying, for example, Γ γγ Γ ee Γ meV at the location of the anomalous peak in e + e − sum energy at ≈800 keV observed in heavy ion reactions.