Towards an improved measurement of the 4+ - to- 2+ γ-decay in 8Be: a new gas target chamber and detector simulation

The first direct observation of the γ-ray transition between the 4+ and 2+ resonances in 8Be was found to be consistent with both abinitio as well as cluster model calculations. The precision in the measurement of the radiative capture cross section in the 4He(4He, γ)8Be channel was rather poor, about ±30%, and not enough to discriminate between the two models. The earlier measurement was made in a gas target chamber not optimised for the experiment. Six square Si(PIN) diodes were used for detecting the two α-particles following γ-decay in coincidence and a 14-BGO detector array (treated as 2 large detectors to simplify the electronics) for measuring the 4+ to 2+ γ-ray with an energy ∼8 MeV. The main source of error was the large γ-ray background arising from the inelastic excitation of the 4.44 MeV state in 12C which is a constituent of the Kapton window which isolates the 4He gas target from the beam line vacuum. An improved measurement would require better shielding of these γ-rays and more segmentation of the γ-ray and α-detectors. A dedicated new gas target chamber has been designed which will allow better shielding of the 4.4 MeV γ-rays from Kapton. The results of a simulation of the α-α detection, following radiative capture in an extended gas target, are also reported