Authors(s):B. W. Ridley Publication:Nuclear Physics Publication Date: Publisher: Citation: Link:
Individual angular correlations can be interpreted uniquely only when either Fermi or G.T. nuclear matrix elements are eliminated by selection rules. In the pure G.T. decay of Hes, Rustad and Ruby ‘) found that CI= +0.36f0.10 and +0.31&0.14 in two measurements, implying a predominantly T interaction. Pure Fermi emitters are comparatively rare and so far none has been found suitable for recoil experiments. However, mixed transitions (occurring when dJ = 0, not 0 + 0) can yield information about the Fermi interaction if the ratio of Fermi to G.T. matrix elements is known and the nature of the G.T. interaction assumed. Recoil experiments on two such decays, those of the free neutron (Robson 8)) and Ne23 (Maxson et al. Q),Good and Lauer lo) and Alford and Hamilton ii) ) gave values of a clustering around zero, showing that the Fermi interaction is mainly S if the G.T. interaction is assumed to be T. In view of the importance of Rustad and Ruby’s He6 experiment in identifying both the Fermi and G.T. interactions, we felt is desirable to check their result by experiments on another essentially pure G.T. emitter, Nea. Interest in such a measurement has been further enhanced by recent evidence from decay of A35 that the Fermi interaction is not S after all, but mainly V), 
If we assume that both the major transitions from Ne23 are pure G.T., then the value a = -O.OS&O.lO implies that 0.86 5 X 5 1.62. This appears to be in serious disagreement with the result -0.5 5 X s +0.3 quoted by Rustad and Ruby 7) for another pure G.T. emitter, He6. The He6 result, however, was evaluated on the basis of the old parity conserving beta decay theory in which an admixture of T and A interactions necessarily implies the existence of a Fierz interference term. If the Fierz term is assumed to be zero, as required by experiments on allowed spectrum shapes and K-capture to positon emission ratios, the He6 results imply that X < 0.6 and X < 0.4 for the two values of a quoted.
Though the discrepancy is reduced, it can hardly be ignored. If we force an agreement between the He6 and Nes3data by supposing that the almost certainly pure G.T. ground state transition from Nea3is dominated by the T interaction, then we find that for the 3.95 MeV transition of Nes8, a = -0.85f0.3. This would require the latter transition to be substantially a Fermi one, dominated by an S interaction. This is in contradiction both with Elliott’s 14) conclusions on the effective magnitude of the Fermi matrix element and with evidence from decay of A35 that the Fermi interaction is mainly V 12). xx Scott 16) has pointed out that the expression (3) for a mixed Fermi - G.T. decay may be rewritten in the form [equation] where aF and aGT are the angular correlation coefficients for pure Fermi and G.T. interactions respectively and R = [equation]. If a is plotted against R for a variety of different allowed decays, all points should lie on a straight line. The intercepts aGT and aF at R = 0 and 1 respectively determine the combination of invariants in the G.T. and Fermi interactions. Angular correlation data measured up to date are presented in this way in fig. 8. A straight line through the existing points is clearly out of the question. The diagram is, however, highly suggestive in the direction of a (V, T, A) or (V, A) combination.