1.1 Measurements of the angular correlations in the beta decays of ⁸Li and ⁸B

M. Beck, D.T. Corcoran, L. De Braeckeleer, M. Felton, H. Lawler, J.P.S. van Schagen,

H.E. Swanson, D.W. Storm, D. Wright and Z. Zhao

The angular correlation in beta decays of ⁸Li and ⁸B are being studied as part of a test of CVC or a search for second class currents. As has been explained previously¹, in each of these beta decays the angular correlation has the form

The coefficient a₂ results from induced weak currents. The a₁ coefficient results from the recoil of the ⁸Be produced in the beta decays. Both of these coefficients are expected to be roughly proportional to the energy of the beta particle. For 10 MeV beta particles the kinematic term, a₁, should be 10% and the term resulting from induced currents, a₂, is expected to be around ±3 to 4%, where the + sign is for ⁸Li decay. Because the decaying states in ⁸B and ⁸Li are members of an iso-triplet, when the a₂ values obtained in each decay are subtracted, the difference results from weak magnetism, weak electricity, and from possible second class currents. If CVC holds and there are no second class currents, this term can be related to the isovector M1 and E2 widths of the decay of the corresponding member of the isotriplet in ⁸Be. This photon measurement is discussed elsewhere in this Annual Report, and our results have been published².

Our measurement of the angular correlation in the decay of ⁸Li has been reported previously^1,3 where an upgrade of the apparatus involving two more beta detectors was mentioned. In addition, the mylar windows of the -particle counters were changed from 200 to 140 µg/cm² thickness. In order to do the ⁸B measurement effectively, the terminal ion source was developed⁴ for intense ³He beams. During the previous year the system with the new beta counters was first tested with ⁸Li and then, using the terminal ion source, we carried out test runs with ⁸B, followed by a long run with ⁸B. We had to change the targetry for the ⁸B production, because the intense ³He beams overheated the nickel foil target wheel. (The ⁸B is produced by the ⁶Li(³He,n) on a rotating wheel coated with ⁶LiF. The ⁸B activity recoils into a carbon catcher foil.) Copper, mounted on a black painted wheel, withstood the beam power and the LiF did not evaporate. While analyzing the new ⁸B data, a new beta detector calibration technique was developed (see the following section).

Because of the symmetry of the experiment, we make two simultaneous measurements of the angular correlation with either of two alpha counters providing the reference. From the ⁸B test run we obtained values for a_1u/E of (-9.7±1.4)×10^-3 MeV^-1 and for a_1d/E of (-9.1±0.9)×10^-3 MeV^-1, while a_2u/E was (-5.2±0.9)×10^-3 MeV^-1 and a_2d/E was (-4.9±0.9)×10^-3 MeV^-1. The subscripts u and d refer to the alpha particle counter upstream or downstream of the catcher, respectively. The agreement between the two pairs of values is encouraging. Although not strictly proportional to beta energy, the values of a₁ do exhibit the expected beta energy dependence, except for the case of a_1d at the highest energies.

The measurement of ⁸B requires more running time than ⁸Li, in spite of the higher ³He beam intensity we have obtained, because of the lower production rate. We now have 2.2×10⁷ events, compared to the total of 1.6×10⁸ for ⁸Li. The final analysis of these data is presently underway.