Picosecond Lifetimes in Using the 100Pd Recoil Distance Curve Method.

Abstract

Measuring picosecond lifetimes of the yrast sequence in 100Pd and the 15/2− state in 101Pd is the subject of this thesis. The nuclei of interest were produced via the fusion evaporation reactions 24Mg(80Se, 4n)100Pd and 24Mg(80Se, 3n)101Pd in the Wright Nuclear Structure Laboratory (WNSL) at Yale University. The ESTU linear accelerator was utilised for accelerating the 80Se beam at 268 MeV, a 0.8mg/cm2 24Mg target, a 16mg/cm2 thick Cu stopper and ten clover germanium detectors, 4 each placed at forward (θ=41.5◦) and backward angles (θ=138.5◦) and 2 placed at 90◦ with respect to the beam direction. A Cu stopper was used to stop the recoils and both the target and stopper were mounted on the New Yale Plunger device (NYPD). Measurements were taken for ten target-stopper distances ranging from 7 μm-750 μm and the data were sorted in the offline analysis into three two-dimensional Eγ-Eγ coincidence matrices, for forward vs backward, forward vs forward, and backward vs backward detectors respectively. The ‘Recoil Distance Method’ was used for analysing the data and the ‘direct gating’ on the Doppler shifted component of the direct feeding transition was mostly used to determine the lifetime of a state of interest. The lifetimes of the yrast sequence in 100Pd are compared with previously published values and are found to be systematically lower but following the same trend. This could potentially be due to the low recoil velocity of the recoils in the previous work (∼0.8%) in comparison with the much higher v/c (∼5.8%) in the current work. A low recoil velocity means that some of the recoils never leave the production target. This leads to a higher stopped to shifted ratio and hence an elevated apparent lifetime value. The experimental reduced transition probabilities (B(E2)) measured for the yrast sequence of 100Pd were deduced from the lifetime values and were compared to predictions from restricted basis shell model, total Routhian surface (TRS) calculations and Interacting Boson Model (IBM). The shell model and IBM model reproduced the energy states in 100Pd reasonably well, however, the comparison with the measured transition rates was less consistent. The IBM model predicts that 100Pd is a transitional nucleus between a U(5) vibrator and an SU(3) rotor. The β2 values produced by the Total Routhian Surface Calculations for both the yrast sequence states in 100Pd and the 15/2− state in 101Pd were lower compared to the β2 experimental values. The experimental β2 value of the 15/2− state in 101Pd was higher compared to that of the 2+ state in 100Pd, consistent with a coupling of the h11/2 neutron to the 100Pd core, as previous suggested