Can time reversal invariance be tested in ternary fission?

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=s·[pfxpt] for the respective unit vectors s, 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 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