Lifetime Measurement of the First Excited State of 7Be Using the Doppler Shift Attenuation Method Applied to an Exothermic Nuclear Reaction

Lifetime Measurement of the First Excited State of 7Be Using the Doppler Shift Attenuation Method Applied to an Exothermic Nuclear Reaction

The lifetime of the first excited state of $^{7}$Be ($E_x$ = 0.429 MeV; $J^{\pi} = 1/2^{-}$) was measured by means of the Doppler shift attenuation method. An exothermic $^{10}$B(p,$\alpha\gamma$)$^{7}$Be 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 $^{7}$Be has been deduced to be $169 \pm 51$ 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 $192 \pm 25$ fs, obtained from the endothermic $^{7}$Li(p,n$\gamma$)$^{7}$Be nuclear reaction.

The lifetime of the first excited state of $^{7}$Be ($E_x$ = 0.429 MeV; $J^{\pi} = 1/2^{-}$) was measured by means of the Doppler shift attenuation method. An exothermic $^{10}$B(p,$\alpha\gamma$)$^{7}$Be 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 $^{7}$Be has been deduced to be $169 \pm 51$ 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 $192 \pm 25$ fs, obtained from the endothermic $^{7}$Li(p,n$\gamma$)$^{7}$Be nuclear reaction.