| 84Mo is an even-even N = Z nucleus lying on the proton drip line that is thought to be created during explosive hydrogen burning in Type I X-ray bursts in the astrophysical rapid proton capture (rp) process. 84Mo is an important waiting point in the rp-process reaction sequence, determining mass abundance at and procession beyond A = 84 for stable isotopes on the proton-rich side of the valley of stability [1]. A previous experiment established the half-life of 84Mo to be 3.7+1.0-0.8 s [2]. However, treatment of the background and the poor statistics accumulated during that study left questions about the statistical and systematic errors in the measurement. The half-life of 84Mo has been re-measured using a concerted setup of the NSCL beta Counting System (BCS) [3] and 16 detectors from the Segmented Germanium Array (SeGA) [4]. The BCS relies on a highly-segmented Si detector to correlate implantations and subsequent beta decays on an event-by-event basis. The correlation method employed to deduce half-lives and other properties of the beta decay required that the average time between implantations be larger than the half-life of the nuclide under study. Consequently, the overall implantation rate into this detector must be carefully controlled, without negatively affecting the typically low rate of the desired isotope. The recently constructed Radio Frequency Fragment Separator (RFFS) [5] at NSCL was used to purify 84Mo based on relative time-of-flight differences between the beam species of interest, isotonic contaminants, and contaminants due to the overlap of low momentum tails of high-yield beam species. A half-life of 2.2(2) s was deduced for 84Mo, based on a sample of 1037 implantations, more than 30 times larger than the previous study. The new half-life reduced the uncertainty in the amount of 84Mo formed in the rp process, and the consequent amount of 84Sr, to less than a factor 2. Implications of the new half-life on theoretical treatments of nuclear level density near A = 84 along N = Z will also be discussed. The performance capabilities of the RFFS in rejecting unwanted isotopes associated with the production of 84 Mo will be reported as well. |
