4. Recent HRIBF Research - Discovery of the Alpha Decay of 109I
(C. Mazzocchi, spokesperson)
Particle-decay spectroscopy is often the only option to measure separation energies for short-lived nuclei with high precision and using only tens of ions. Moreover, alpha and proton decay are an incredibly rich source of nuclear structure information and can be used to probe the wave functions of the nuclear levels involved and to probe nuclear-shell effects, see e.g. . Of particular interest is the region of the Segre chart above the doubly magic nucleus 100Sn, where an island of alpha and proton emission exists. The occurrence of this alpha- and proton-radioactive nuclei directly reflects the strong N=Z=50 shell closures and the presence of the proton drip-line. Particularly interesting is the case of 109I, which was discovered as a ~100% proton emitter  with 100 μs half-life and is expected to present a small but measurable alpha-decay branch . Several attempts were carried on to observe the alpha-decay branch without any success. The measurement of its decay energy allows to indirectly and independently set the value of the proton separation energy (Sp=-Qp) in the daughter nucleus 105Sb, see Fig.4-1. The measurement of Sp(105Sb) is of interest for the astrophysical rapid proton-capture process (rp-process), since the isotope is located on the predicted path for the process termination around the Sn-Sb-Te cycle, see Fig. 4-1.
Figure 4-1: Portions of the Segre chart above 100Sn. Left panel: The nuclei of interest are represented by a filled box. The known alpha and proton decays of 108Te and 109I, respectively, are represented by solid arrows, while the proton decay of 105Sb and the hitherto unobserved alpha-decay branch of 109I through a dashed arrow. Right panel: Path followed by the rp-process in the 100Sn region as predicted in .
At the HRIBF the 109I nuclei were produced in the fusion-evaporation reaction 58Ni54(Fe,p2n)109I. The recoiling evaporation residues were separated according to their mass-to-charge ratio by means of the Recoil Mass Spectrometer  and implanted into a double-sided silicon strip detector. The preamplifier signals were read out through the digital signal processing-based acquisition system . The ion implantation and decay events were correlated in space and time.
In Fig.4-2, a portion of the alpha-decay energy spectrum of decay events is displayed. The two peaks at 3107- and 3317 keV corresponding to the alpha decays of 109Te and 108Te, respectively. The third peak at an energy of 3774±20 keV is assigned to the alpha decay of 109I. The identification of these alpha events is supported by the observed decay pattern, in agreement with the remeasured 93.5(3) ms 109I half-life. The energy of the alpha decay of 109I corrected for the recoil effect gives Qa=3918±22 keV. The branching ratio is ~0.0001.
Figure 4-2: Portion of the energy spectrum of decay events following implantation of an ion within 400 μs. The peaks are labeled with their precursor.
The measurement of the alpha decay energy of 109I allows for the indirect measurement of the Qp( 105Sb): Qa(109I)+Qp( 105Sb)=Qp(109I)+Qa( 108Te), leading to Qp(105Sb)=356±22 keV. The new Qp value of 105Sb is in clear disagreement with that of 491±15 keV previously reported by Tighe et al. . The Qp value is though in agreement with the non-observation and relative limits set by other experiments searching for this decay branch of 105Sb over the past 20 years.
In order to explore the impact of the new Qp(105Sb) on the rp-process, we ran reaction network calculations using the one-zone X-ray burst model described in . The measured value of Qp(105Sb) excludes the formation of a Sn-Sb-Te cycle at tin isotopes with mass lower than 105.
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