Researchers at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) have measured the half-lives of 100Sn and 96Cd, two nuclei with equal numbers of protons and neutrons that are close to the proton drip line – the proton-rich limit of stability. The result for 100Sn narrows the error range of previous half-life measurements, while the half-life of 96Cd, measured here for the first time, casts a light on the role of the isotope in the rapid proton-capture (rp) process – a key part of heavy-element synthesis in the cosmos. The result for 96Cd also implies a new, as-yet-unknown origin for 96Ru in the solar system, where its abundance has long remained unexplained.
Daniel Bazin and colleagues at MSU used the same fast-beam fragmentation scheme to create both species. Using the facility’s coupled cyclotrons, the team generated a primary beam of 120 MeV/nucleon 112Sn and fragmented it on a beryllium target. The resulting radioactive beam was filtered through the A1900 Fragment Separator and newly commissioned RF Fragment Selector. Finally, the filtered secondary beam implanted itself in NSCL’s Beta Counting System, a series of silicon beta-particle detectors flanked by detectors of the laboratory’s segmented germanium array. To track the beta-decay of implanted nuclei, Bazin’s team monitored decay events at the impact site and neighbouring pixels in the detector for 10 s after implantation.
For 100Sn, the team observed a half-life of 0.55+0.70–0.31s. This result is similar to previous measurements made at GSI and yields an average of 0.86+0.37–0.20s when combined. The increased precision may bolster understanding of this isotope, which is one of the few “doubly magic” nuclei close to the proton drip line. Its protons and neutrons both form a closed-shell configuration, which affords extra stability to the nucleus.
The measured half-life of 96Cd, which was previously unknown, was 1.03+0.24–0.21. This is within the range of several theoretical predictions but it is too short to make 96Cd a critical “waiting point” in the rp process. This process, along with slow neutron capture and rapid neutron capture, probably accounts for many of the universe’s heavy elements. It occurs in supernovae, X-ray bursts and perhaps other astrophysical environments where seed nuclei join with free protons to form nuclei of increasing atomic number. Build-up stalls at specific stages when the binding of another proton is energetically unfavourable. Nuclei accumulate at these so-called waiting points, generating a spike in the observed isotope abundance. Such a spike exists at 96Ru, the product of beta-decay from 96Cd, which suggests a waiting point at 96Cd.
With the result for 96Cd, the half-lives of all expected waiting points along the proton drip line, up to the rp-process’s predicted endpoint, are now known experimentally. However, the half-life that Bazin and collaborators have measured is approximately a tenth of the value required to account for the observed abundance of 96Ru. There must be a different explanation – perhaps an unexplored astrophysical process.
Further reading
D Bazin et al. submitted Phys. Rev. Lett. http://arxiv.org/pdf/0810.3597v1 [nucl-ex].
