K.A. Snover, D.W. Storm and D. Wright

In order to perform an accurate test of CVC by comparing the isovector M1 strength in the gamma decay of ⁸Be with the weak magnetism determined from the analogue beta decays of ⁸Li and ⁸Be, it is necessary to determine the spectral strength of the gamma decay, because the final state is broad. For details see DeBraeckeleer, et al¹. It is particularly important to be able to determine the gamma ray spectrum at energies considerably lower than the main transition energy of about 14 MeV. Since the excited state in ⁸Be is made by a ⁴He+⁴He resonant collision, one uses a long gas cell with well shielded windows and does a measurement at 90°.

In preliminary measurements we found a background produced by He beam particles scattering in the windows and gas and then hitting the walls of the cell where they interacted and produced photons. This background depends on the kind of gas present, and is not well reproduced when we fill the cell with hydrogen. In order to reduce the background we plan to take two steps. First, we will line the cell with tantalum, so the Coulomb barrier will discourage interactions with beam ions that have already lost significant energy in gas scattering. Second, we can make a large enough diameter cell and can intercept the particles scattered from the windows with an aperture located in the cell behind the gamma ray shield. Thus, particles scattered from the window will not hit the cell wall in the visible region. The large diameter, combined with careful placement of the aperture, requires that any beam particles that scatter from the gas into the part of the cell wall that is visible to the detector will scatter through an angle large enough to reduce the energy of such particles from the 34 MeV beam energy to well below Coulomb barrier.

The details of the geometry are being worked out in order to build a cell that can be used as soon as the gamma ray detection system is ready.