Lifetime measurement of the first excited state of Be-7 using the Doppler shift attenuation method applied to an exothermic nuclear reaction

Abstract

The lifetime of the first excited state of Be-7 (E-x = 0.429 MeV; J(pi) = 1/2(-)) was measured by mean of the Doppler shift attenuation method. An exothermic B-10(rho,alphagamma) Be-7 nuclear reaction was used to populate 7 Be using 2-MeV protons delivered from the Tandetron accelerator at Seoul National University. Two Compton-suppressed germanium gamma-ray detectors were used to reduce the Compton background around the 429-keV photopeak. A Monte Carlo simulation code specific to the present exothermic reaction kinematics was developed in order to predict the velocity history of a recoiled nucleus as a function of time, The mean lifetime of the first excited state of Be-7 has been deduced to be 169+/-51 fs. Despite the nature of the exothermic reaction, which makes assessment of the velocity distribution difficult especially for a backward recoiled nucleus, our experimental value is in fair agreement with the adopted value of 192+/-25 fs, obtained from the endothermic Li-7(rho,ngamma) Be-7 nuclear reaction.