DOI: 10.7274/25593117.v1
title: Development of fIREBall for Coincidence Measurements

description: The nature of 0+ states in deformed nuclei has been an open question in nuclear structure studies. This question arose from the viability of superimposed vibrations on a deformed nucleus. E0 transitions have long been considered the final piece needed to answer this question. However, there is a lack of experimental data due to the difficulty observing 0+ states. Given the reactions, spectroscopy, and E0 transitions that are required, the challenge revolves around the need to single out the conversion electrons from other particles that are emitted during de-excitation. The resulting spectra are also complex, due to the high density of levels in heavy nuclei, making it difficult to extract the necessary information from singles measurements. To overcome these challenges, recent developments have been aimed towards using coincidence measurements. 

This work aimed to measure E0 transitions in 156Gd through coincidence measurements. A significant amount of technical developmental work was done to improve the electron detection efficiency to increase the odds of observing coincidences. ICEBall (Internal Conversion Electron Ball) was transformed into the new fIREBall (fInternal conveRsion Electron Ball) to improve electron detection efficiency. An innovative method involving simulations was used to optimize the geometries for the magnet filters used in the spectrometer. The array of six Si(Li) detectors inside the spectrometer was also replaced with six new, thicker Si(Li) detectors. The simulations were successfully used to design magnet shapes that improved the absolute efficiency from 2.8% to 5.3% at 482 keV for a single filter and detector pair. However, the new detectors were found to be 20% less efficient due to improper Li-drifting near the edges.

Additional technical work was done to improve the rest of the experimental setup. This includes the development of a new method of fabricating pure thin metal foil targets, the commissioning of a new pair of Bismuth Germanate (BGO) Compton Suppression shields, and the implementation of a new data (DAQ) system for all future fIREBall experiments. Finally, a new program, TROPIC (TRansitiOn ProbabIlity Calculator), was created to provide a modern and efficient method to extract B(pl) values from lifetime measurements. 

Commissioning experiments with the 154Sm(a, 2n)156Gd reaction were done to see if the improvements allowed for better coincidence measurements. The viability and validity of the concept was confirmed by K/(L+M) ratios in the low energy range that were in good agreement with theoretical calculations from BrICC. Unfortunately, the sub-optimal condition of the new Si(Li) detectors resulted in the electron count being only 30% of the ? ray count. This resulted in sparse statistics in the high energy range, where the E0 transitions for 156Gd are, precluding any confirmation on the observance of E0 transitions. It is clear that detectors that are of sufficient quality and quantity are absolutely necessary in order for coincidence measurements to be a viable method for observing E0 transitions.