Performance of The New fIREBall Si(Li) Detectors

Authors: Alina Bennett and John Read

In collaboration with: Kevin Lee, Joey Guerra, Wanpeng Tan, Shelly Lesher, Ani Aprahamian

The measurements of conversion electrons for gamma-rays depopulating an excited nucleus can reveal the multipolarity and the character of the gamma-ray shedding light on nuclear structure. The Internal Conversion Electron Ball (ICEBall) is being upgraded to the fInternal conveRsion Electron Ball (fIREBall) by several ways starting with simulation to determine the best geometric configurations of the magnets to enable the highest possible collection of electrons. As well as the purchase of new detectors to replace the old Si(Li) detectors that are now over 25 years old with new detectors. The detectors were purchased from the Lawrence Berkeley National Laboratory (LBNL) but delivery of the detectors was significantly delayed by two years due to complications related to COVID. This year, we have received two of the detectors and we have assessed their performance. Before this project was started, it was observed that the LBNL detectors had an efficiency deficiency of 20% when compared to the older detectors from Canberra. A 207Bi conversion electron source and an alpha source of mixed radioisotopes of 148Gd and 241Am were used to determine the efficiencies of the new detectors in comparison with the old ones. We suspect that the production of these detectors suffer from a deadlayer which is not smooth and interrupts the performance of the detectors. The deadlayer is not uniform due to the lithium drifting, and there are distinct and measurable edge effects on the new detectors. We expect this to negatively impact the increased efficiency we envisioned with the new fIREBall. We have made several measurements and the results are reported in this report.