Search

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.

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.

Ternary fission of the standard reaction 235U(nth,f) induced by cold polarized neutrons has been investigated. Fission fragments and light charged particles were recorded in coincidence. Following cold neutron capture the compound nucleus 236U* has spin 3− or 4−. At the saddle point of the fissioning 236U* nucleus these states are collective. They are expected to retain a sizable collectivity down to the scission point. In fact, a collective rotation has been sensed by the shift in the angular distribution of the light charged particles which depends on the orientation of neutron polarization. Direct observation of the rotation of 236U* excited in a cold neutron reaction is reported here for the first time. It is proposed to call the new phenomenon the “ROT‐effect”.

In the present paper we describe an experiment performed on the reaction 238U + 4He (40 MeV) using a two‐arm TOF‐E (time‐of‐flight vs. energy) spectrometer with micro‐ channel plate detectors and mosaics of PIN diodes. This experiment has aimed at searching for fine structures in the total kinetic energy vs. fragment mass (TKE ‐ M) distributions of binary events, and disclosing rare ternary decay modes others than already known from ternary fission. Details of the procedures of data handling are briefly described as well.

International audience; A novel method to search for the anisotropic emission of prompt neutrons in the center-of-mass system of fission fragments is presented. The anisotropy is conjectured to be due to the large spins of fission fragments are known to carry. Triple neutron- neutron-fragment correlations in spontaneous fission of $^{252}$Cf were investigated in an exploratory experiment dubbed CORA-DEMON experiment. Fission fragments were intercepted in a double ionization chamber while neutrons were spotted in 2 two-dimensional cylindrical walls of Demon detectors with the target on the vertical cylinder axis. A new method of analysis of triple angular correlations between 2 neutrons and a fission fragment was applied. Preliminary results are reported.

P‐odd and T‐odd asymmetries in the emission of fragments and light particles have been investigated in ternary fission induced by polarized neutrons. P‐odd asymmetries unambiguously point to a violation of parity in the fission process. By contrast, T‐odd asymmetries do not necessarily imply a violation of time reversal invariance. The asymmetries observed are rather due to a final state interaction between the light ternary particle and the nucleus from which they are ejected. New results with interesting information on the ternary fission process are presented.

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 particles are available. For the present contribution in particular the yields of Be‐isotopes for the two reactions studied are compared and discussed. For 252Cf(sf) these isotopic yields were hitherto not known.

Fission of 233U induced by thermal neutrons and with detectors for fission fragments and ternary particles covering large solid angles has been studied. Evidence has been found for events where two light charged particles are emitted in coincidence with fission fragments.

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.

Already several years ago the idea has been put forward that a reaction well suited for tests of Time Reversal Invariance (TRI) might be ternary fission [1] [2]. In ternary fission, besides the two main fission fragments, a third (usually light) charged particle is emitted. For a test of TRI a triple correlation has to be studied involving on one hand the momenta of a fission fragment pf and the ternary particle pt, and on the other hand e.g. the spin of the neutron inducing fission s. The correlation coefficient B=ŝ⋅[pf×pt] for the respective unit vectors ŝ, pf and pt reverses sign upon time reversal and a non-vanishing expectation value for B could possibly be due to TRI being violated. However, final state interactions could equally well lead to a non-zero B with TRI being perfectly conserved. A first experiment of this type has been performed in early 1998 at the ILL. Placing fragment and ternary particle detectors at right angles both relative to each other and relative to a longitudinally polarized neutron beam, the observable B assumes the values B=±1. For a fixed set of detectors the sign of B is reversed upon flipping the neutron spin. The expected count rates for the two spin orientations are N=N0⋅(1±D) with N0 the count rate for an unpolarized beam. The asymmetry D measures the expectation value 〈B〉 of the observable B. The reaction chosen was 233U(n,f). An unexpectedly large correlation coefficient passing all tests of fake asymmetries was observed. From the raw data the expectation value for B is D=−(2.35±0.05)⋅10−3 with the sign corresponding to light fragments. Corrections for finite solid angles subtended by the detectors are not included in the above figure. The corrections will further increase the correlation coefficient. At the moment the mere size of D is believed to rule out a failure of TRI as the origin of the effect. But even a less spectacular interpretation—which as yet is not available—should give a detailed and quite new insight into the interplay between the angular momenta of all reaction partners and the emission direction of the ternary particles.

HENDES is a new device for correlation measurements of fission fragments, neutrons and light charged particles produced in HI induced reactions. Double differential neutron spectra are measured with the help of Position Sensitive Neutron Detectors, fission fragment spectra are recorded with large Position Sensitive Avalanche Counters, and light charge particle spectra are measured with arrays of dE-E telescopes. Collective dynamical parameters of nuclear matter are extracted from the data. New results were obtained from 40Ar+180Hf reaction at EAr=190–250 MeV. Work is in progress to study the influence of neutron excess on the fusion-fission dynamics in different combinations of Ni beams on Sn targets.