1.7 Target composition studies relevant to a proposed ⁷Be(p,)⁸B cross section measurement

E. G. Adelberger, L. De Braeckeleer, S. M. Graff,^* K. A. Snover and Z. Zhao

We propose to carry out a remeasurement of the astrophysically interesting ⁷Be(p,) cross section in the energy range E_p 1 MeV. The experiment will be a collaboration between NPL and TRIUMF. The radioactive ⁷Be target will be fabricated at TRIUMF and the cross section measurements will be carried out here. We plan to use a ⁷Be target of approximately 10¹⁷ atoms/cm² with an area of about 3 mm², and a proton beam which is swept in x and y coordinates to ensure a uniform beam flux on target. In this report we describe layered target composition studies relevant to the proposed experiment.

The ⁷Be activity will be produced by the ⁷Li(p,n)⁷Be reaction, chemically separated and evaporated onto a solid backing at TRIUMF. The total number of ⁷Be atoms will be determined by radioactive counting of the 478-keV gamma ray from the decay of the first excited state of ⁷Li, following electron capture by ⁷Be. The idea of a layered target is to use a second material such as aluminum, which has a narrow resonance in the (p,) reaction at a convenient proton energy, as a monitor of the energy thickness of the Be target. To this end, we tested several targets consisting of 5 - 7 µg/cm² Al evaporated onto solid Cu backings, with 45 µg/cm² natural Be evaporated onto the Al. Both the shift of the 992 keV resonance in ²⁷Al(p,) and the width of the 1083 keV resonance in ⁹Be(p,) were used to deduce the Be thicknesses, with good agreement. The bare Al resonance was about 1.5 keV wide, due to beam energy spread and target thickness, while the shifted Al resonance was 5 - 6 keV wide, due mainly to straggling. With a spot (3 mm²) Be on Al/Cu target approximately 35% of the beam was focused onto the Be, and the shifted Al resonance was clearly visible above the high energy tail of the unshifted Al resonance due to beam striking outside of the spot area.

These tests clearly demonstrate the feasibility of the proposed technique. Two other methods are currently being considered as alternatives to the layered target: (1) the 1083 keV ⁹Be(p,) resonance could serve as a good energy thickness monitor if the target is fabricated with at least 10% ⁹Be. (2) The narrow 1376 keV ⁷Be(,) resonance would also be an excellent monitor, but would require a modification of our terminal ion source to permit a rapid change from proton to alpha beams.