Lifetime measurements in 71 Ge

An experimental investigation of the structure of 71 Ge has been undertaken at the FN Tandem Van de Graaff accelerator of IFIN-HH. The reaction 71 Ga(p,nγ) has been used to populate levels in the 71 Ge nucleus and deduce level lifetimes through the DSA (Doppler Shift Attenuation) method. The reaction has been performed at the incident energies of 2.5, 3.1 and 3.5 MeV, close to the threshold of the levels of interest. The beam intensities were around 10 nA. The target was a pellet of isotopically enriched 71 Ga stuck to a tantalum foil and cooled; its thickness was about 30 mg/cm2. The gamma rays were detected with a 20% efficiency HPGe detector of 2.2 keV FWHM resolution at 1.33 MeV, placed at 30 cm from the target, at angles between 0 angle and 142 angle with respect to the beam axis. The variation of the energy of the peak centroid of the transitions of interest with the detection angle led to the experimental attenuation factor Fexp(τ). The stability of the energy calibration of the spectrometer was better than the precision in the centroid determinations and was continuously monitored with radioactive sources and activation lines. In evaluating the F(τ) data, we have used different stopping powers estimations. Since for all levels the observed shifts are rather small, close to the limit of method's sensitivity, the measurements have been repeated recently with better energy resolution and special care for the stability of the electronics. A table is given containing the 71 Ge energy levels (from 808.1keV to 1792.2 Kev), the γ ray transitions, the attenuation factors and the corresponding life-times. (authors)