Final PhD Seminar

Measuring g factors of fpg-shell nuclei by the transient-field and recoil-in-vacuum techniques

Mr Brendan McCormick
Department Nuclear Physics

Picosecond-lifetime nuclear-state g factors are challenging to measure, with essentially only the transient-field (TF) and recoil-in-vacuum (RIV) techniques able to probe them. These techniques rely on hyperfine interactions that often require independent calibration, a key limitation in their use. The TF can be used to reliably obtain relative g factors, which can be scaled using an independent measurement. Time-differential (TD) RIV allows for absolute g factors to be determined ab initio, i.e. based on precise atomic-structure calculations. Together, these techniques can allow previously inaccessible g-factor systematics to be obtained along isotope chains stretching into regions containing exotic, radioactive nuclei. With such measurements available, critical comparisons with the nuclear shell model can be made.

This talk will focus on two measurements, one by the TDRIV technique and the other by the TF technique. These served as proof-of-principle measurements for ab initio determination of the hyperfine interaction in the RIV measurement, and to demonstrate a robust method for measuring g-factor ratios using the TF technique. The TDRIV measurement was performed on the 56Fe 21+ state, in which hyperfine interactions with a multi-electron ion ensemble (9 – 13 bound electrons) were investigated using atomic-structure calculations and a Monte-Carlo simulation of atomic-decay cascades. The time-dependent behaviour of the hyperfine interaction was modelled for each ionic species, allowing the dominant atomic states to be identified, and the g factor was determined. The newly obtained g factor’s greatly improved precision allowed for rescaling of g(21+;54Fe)/g(21+;56Fe) and g(21+;58Fe)/g(21+;56Fe) measurements available in the literature, and critical comparisons are made with shell-model predictions.

Using the TF technique the first instance of a simultaneous g-factor measurement on isobaric nuclides by beam-excitation was performed, being the 21+ states of 74Ge and 74Se, along with the even-A isotopes of each element, using a single target. Such measurements reduce systematic uncertainty in the relative g factors. The g-factor systematics are compared to shell-model predictions. The g-factor results from the TDRIV measurement and the present TF measurement are then used to investigate the validity of TF-strength parameterisations, and the TF’s varying behaviour in iron versus gadolinium.

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