4. Recent HRIBF Research -
Discovery of the New Alpha Emitters
109Xe and 105Te
(R. Grzywacz, Spokesperson)
The island of alpha and proton radioactivity above 100Sn has attracted considerable interest over the years. In addition to the strong influence of the N=Z=50 double shell closure and the proximity of the proton drip line, this unique region has also recently been identified as the termination point of the rp-process. Furthermore "super-allowed" alpha decays could occur in this region where the valence nucleons occupy the same orbitals, although no conclusive evidence for this effect has so far been obtained.
Figure 4-1: An example of recorded double pulse induced in the DSSD front (black) and back (red) strips. First "step" of this pulse is due to alpha particle emitted from 109Xe, second "step" is due to 105 Te decay leading to 101Sn.
Two new alpha-emitting nuclides were discovered, 109Xe and 105Te, in an experiment performed at HRIBF using the Recoil Mass Spectrometer (RMS) in November 2005. The 109Xe nuclei were produced via the 3n evaporation channel in reactions of 58Ni with 54Fe and implanted into the Double-sided Silicon Strip Detector (DSSD) at the RMS focal plane. The subsequent alpha decay of the daughter 105Te nuclei was expected to occur within a few microseconds of the 109Xe alpha decay, so a new trigger and readout method had to be developed for the digital data acquisition system allowing the preamplifier traces containing the two closely spaced pulses to be recorded. A typical double trace is shown in the Fig. 4-1. From these traces, correlated with the recoil implantation signals, it is possible to extract the energies of both alpha particles and determine the half-lives of 109Xe and 105Te, see Fig. 4-2. An important discovery in the decay of 109Xe was that it exhibits fine structure, as shown in the upper panel of Fig. 4-2, which allows us to determine the relative energies of the d5/2 and g7/2 neutron states in 105Te.
Figure 4-2: Spectra of alpha particle induced pulse amplitudes extracted from measured pile-up pulses (like in Fig. 4-1), attributed to 109Xe (top) and 105Te (bottom) alpha decays. New pulse shape fitting routines were developed to obtained optimum resolution.