J. von Kalben

Rotation of the compound nucleus 236U∗ in the fission reaction 235U( n , f ) induced by cold polarised neutrons

A model for ternary fission is discussed in which a third particle (α-particle) is emitted due to non-adiabatic change of the nuclear potential at neck rapture. An expression for energy and angular distribution of α-particles is proposed. It is shown that an interaction between spin of fissioning system and orbital momentum of α-particle (spin-orbit interaction in the final state) results in recently observed asymmetry of α-particle emission, which can be formally related to T-odd correlation. No strong dependence of the asymmetry on the angle of α-particle emission with respect to the fission axis is predicted by the model in accordance with the experimental data.

Tri And Rot Effects In Ternary Fission: What Can Be Learned?

Inducing fission by polarized neutrons allows studying subtle effects of the dynamics of the process. In the present experiments ternary fission of 235U and 239Pu was investigated with cold neutrons in the (n,f) reaction at the Institut Laue‐Langevin, Grenoble. Asymmetries in the emission of ternary particles were discovered by making use of the neutron spin flipping. It was found that two effects are interfering. There is first an asymmetry in the total yields of ternary particles having been called the TRI‐effect. Second, it was observed that the angular distributions of ternary particles are shifted back and forth when flipping the neutron spin. This shift was named ROT effect. Guided by trajectory calculations of the three‐body decay, the signs and sizes of the ROT effect are interpreted in terms of the K‐numbers of the transition states at the saddle point of fission.

Rotation of scissioning nuclei observed in ternary fission induced by polarized neutrons

Angular distributions of light charged particles relative to the momentum of the light fission fragment were studied in ternary fission induced by cold polarized neutrons. It was discovered that the angular distribution is asymmetric with respect to a plane determined by the direction of incident neutron spin and the light fragment momentum. The violation of the mirrorlike symmetry was established at the level of ∼10−2–10−4 for three reactions studied: 233U(n,f), 235U(n,f) and 239Pu(n,f). In spite of the smallness of the effects the neutron spin flip method allowed to make reliable measurements. The current status of the experimental studies of the effect and its interpretation are presented. Perspectives for further studies of the new phenomenon are discussed.

Energy distribution of ternary {alpha} particles in spontaneous fission of {sup 252}Cf

The energy distribution of the ternary {alpha} particles in spontaneous fission of {sup 252}Cf was measured. For the first time an energy threshold as low as 1 MeV was reached. The experiment used an array of unshielded silicon detectors measuring energy and time-of-flight (TOF) of ternary particles in coincidence with fission fragments. The TOF resolution of the system was sufficient for clear separation of {sup 6}He and tritons from {sup 4}He. The statistics were adequate to extract the {sup 6}He/{sup 4}He yield ratio. For both {sup 4}He and {sup 6}He, an excess in the yield (as compared to a Gaussian shape) was observed at energies below 9 MeV. The measured ternary {alpha} spectrum was corrected for the distortion induced by the detection geometry covering equatorial particle emission only. The emission angle was found to affect mainly the width of the energy distribution by up to 1 MeV.

Spontaneous ternary fission of 252Cf: New experiment with GSI super clover detectors

A new multiparameter experiment on ternary fission of 252Cf has recently been successfully performed by using the CODIS2 detector system and two GSI segmented Super Clover Ge γ-ray detectors. The experimental setup and the main subjects of interest are briefly described.

Studies On Particle‐Accompanied Fission Of 252Cf(sf) And 235U(nth,f)

In recent multi‐parameter studies of spontaneous and thermal neutron induced fission, 252Cf(sf) and 235U(nth,f) respectively, the energies and emission angles of fission fragments and light charged particles were measured. Fragments were detected by an energy and angle sensitive twin ionization chamber while the light charged particles were identified by a series of ΔE‐Erest telescopes. Up to Be the light particle isotopes could be disentangled. In addition, in the 252Cf(sf) experiment, gammas emitted by the fragments were analyzed by a pair of large‐volume segmented clover Ge detectors. Here the main interest is to study the γ‐decay and the anisotropy of gammas emitted by fragments and light particles. On the other hand, the high count rates achieved in the U‐experiment performed at the high flux reactor of the ILL, Grenoble, should allow to explore fragment‐particle correlations in very rare events like quaternary fission. At the present stage of data evaluation, yields and energy distributions of light...

Can Time Reversal be tested in ternary fission

Ternary fission of 233U and 235U induced by cold polarized neutrons has been investigated. Several correlations between neutron spin and the momenta of fission fragments and ternary particles were analyzed. These correlations are probing time reversal invariance, parity non-conservation and left-right asymmetries. Results for all three correlations from the reaction 233U(n,f) are presented. Especially the outcome in the searches for time reversal correlations and left-right asymmetries is unexpected. A huge effect observed formally as a violation of time reversal is most probably simulated by specific properties of the emission mechanism for ternary particles.