James Mitchell Allmond

This manuscript reports on the commissioning of the Resonance Ionization Spectroscopy Experiment (RISE) at the BEam COoler and LAser spectroscopy (BECOLA) facility at Facility for Rare Isotope Beams (FRIB). The instrument implements the collinear resonance ionization spectroscopy technique for sensitive measurements of isotope shifts and hyperfine structure of short-lived isotopes produced at FRIB. The existing BECOLA beamline was extended to integrate an electrostatic ion-beam bender and an ion detector at ultrahigh vacuum. An injection-seeded titanium-sapphire laser and a multiharmonic pulsed neodymium-doped yttrium aluminum garnet laser were installed to perform resonant excitation and selective ionization. Commissioning tests were performed to demonstrate the capabilities of the instrument by measuring the hyperfine structure of stable Al 27 produced in an offline ion source. The RISE instrument is ready and operational for future studies of short-lived isotopes at FRIB.

In an experiment performed at the Facility for Rare Isotope Beams (FRIB) using the FRIB Decay Station initiator, 15 new half-lives of isotopes near ^{54}Ca were measured. A new method of extracting lifetimes from experimental data, taking into account the unknown β-delayed neutron emission branches of very neutron-rich nuclei, was developed to enable systematic uncertainty analysis. The experiment observed a dramatic change in the half-life systematics for the isotopes with neutron number N=34. Beyond N=34, the decline of nuclear lifetime is much slower, leading to longer than anticipated lifetimes for near-dripline nuclei. State-of-the-art shell-model calculations can explain the experimental results, revealing the imprint of shell effects and the need for modification of single-particle neutron states. The results from a newly developed quasirandom-phase approximation model with potential for making global predictions were also tested against the experimental results and good agreement was found.

Alpha particle radioactivity is one of the most striking evidence for the existence of cluster structures in atomic nuclei. During the decay process, a preexisting α particle tunnels through the potential barrier formed by the residual nucleus [1,2]. The degree of preformation of the α particle, a strongly bound system of two protons and two neutrons, is extracted from the data by dividing the α-decay probability by the barrier penetrability for a given particle energy. The preformation probability changes rapidly near nuclear shell closures, which is direct evidence that clustering is connected to nuclear structure [3]. Enhanced preformation was observed in the lightest α-particle emitters, spherical tellurium and xenon isotopes decaying to magic isotopes of tin. Here, we show the most extreme case of α-particle preformation from the measurement of the decay of tellurium-104. With a half-life of 7.2 ^+2.3 _-1.5 nanoseconds, tellurium-104 is the fastest ground state α-emitting nucleus known to date. The deduced preformation demonstrates that the enhancement is greater for tellurium-104 than for any other nucleus. One nuclear model that can explain our observation postulates that the α particle can exist only in the low nuclear matter density regions on the surface of the nucleus. The uniquely high preformation for tellurium-104 is attributed to its relation to doubly-magic tin-100, creating conditions conducive to form an α particle.