Development of fIREBall for Coincidence Measurements

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.

核结构研究中，变形核中0+态的性质一直是一个悬而未决的问题。这一问题源于在变形核上叠加振动的可行性。E0跃迁长期以来被视为解答此问题的最后一块拼图。然而，由于观察0+态的困难，实验数据匮乏。考虑到所需的反应、光谱学和E0跃迁，挑战在于从去激发过程中发射的其他粒子中筛选出转换电子。由于重核中能级密度高，产生的光谱也相当复杂，从单次测量中提取必要信息变得尤为困难。为了克服这些挑战，近期的研究发展主要致力于使用巧合测量。本研究旨在通过巧合测量测量156Gd中的E0跃迁。为了提高观察巧合的几率，进行了大量的技术发展工作，以提升电子检测效率。将ICEBall（内部转换电子球）改造为新的fIREBall（f内部转换电子球），以提高电子检测效率。采用了一种创新的方法，结合模拟技术，优化了用于光谱仪中的磁过滤器几何形状。光谱仪内部六个Si(Li)探测器的阵列也被六个新的、更厚的Si(Li)探测器所替代。通过模拟成功设计了磁体形状，将单个过滤器与探测器对的绝对效率从2.8%提升至5.3%，在482 keV的能量下。然而，由于边缘附近不适当的Li漂移，新的探测器被发现效率降低了20%。为了进一步提高实验装置的其他部分，进行了额外的技术工作，包括开发了一种新的纯薄金属箔靶制备方法，启动了一对新的铋酸锗(BGO)康普顿抑制屏蔽，并实施了一个新的数据采集(DAQ)系统，以用于所有未来的fIREBall实验。最后，创建了一个名为TROPIC（TRansitiOn ProbabIlity Calculator）的新程序，以提供一种现代且高效的方法，从寿命测量中提取B(pl)值。通过进行154Sm(a, 2n)156Gd反应的启动实验，以验证改进是否允许进行更好的巧合测量。通过低能范围内的K/(L+M)比率与BrICC的理论计算结果良好一致，证实了该概念的可行性和有效性。然而，由于新的Si(Li)探测器条件不佳，电子计数仅为伽马射线计数的30%，导致高能范围内统计稀疏，而156Gd的E0跃迁就位于这一能区，因此无法确认E0跃迁的观察。显然，为了使巧合测量成为观察E0跃迁的可行方法，必须具备足够质量和数量的探测器。