Speaker
Description
ASET, α-setup, is a new detection system designed and built to perform particle decay spectroscopy at ISOLDE. It is based on a movable ladder system instead of a wheel, previously used in the Windmill system $^{[1]}$. The working principle is the same as Windmill’s: the ladder moves up and down between two silicon detectors (an annular and a full one), which are placed at each side of the ladder. The main improvement from the Windmill system is the higher efficiency for the silicon detectors as they can be placed closer to the foils, and the reliability of the system. Different ladders can be used, depending on the type of implantation foils applied. Up to now, carbon foils of $20\,μg/cm_2$ thickness, and $Si_3N_4$ membranes of several thicknesses were used. Beside the α spectroscopy, ASET also allows the use of HPGe detectors. These can be placed outside the chamber, but close to the implantation point thanks to two recessed flanges.
The feature of having both α and γ spectroscopy opens the possibility to different kind of experiments: one example is the recent IS456 campaign about α spectroscopy of $^{219,220}$Po $^{[2]}$.
Silicon detectors can be used also to detect fission fragments, enabling β-delayed fission (βDF) to be studied with ASET. Two campaigns were performed on βDF during the last couple of years: IS665 about βDF in $^{176,178}$Au $^{[3]}$ (August 2021), and LOI216 about βDF in $^{230,232,234}$Ac $^{[4]}$ (May 2022). From this last campaign upper limits for the βDF probabilities of $^{230}$Ac and $^{232}$Ac were preliminarily estimated, and the value for $^{230}$Ac was found to be two orders of magnitude lower than the one reported in literature $^{[5]}$.
Moreover, during the IS665 campaign data on $^{221}$Ra and $^{219}$Fr were collected, to conduct investigations on the $^{221}$Ra and $^{215}$At half-lives. In [6], their half-lives were found to diverge substantially from literature values (16(2) s and 37(3) μs instead of 28(2) s $^{[7]}$ and 100(20) μs $^{[7]}$ respectively). A preliminary analysis of the data collected at ISOLDE agreed with the 37 μs half-life for $^{215}$At but confirmed the literature value $^{[7]}$ for $^{221}$Ra.
This contribution will discuss into more details the ASET features and strengths, describe the different experiments that were performed with it, and report some of the preliminary results obtained so far.
[1] L. Ghys, Beta-delayed fission in proton-rich nuclei in the lead region (Doctoral thesis), KU Leuven (2015.)
[2] T. E. Cocolios et al., INTC-SR-089 (2019).
[3] B. Andel et at., INTC-P-558 (2020).
[4] A. N. Andreyev et al., INTC-I-216 (2020).
[5] Y. Shuanggui et al., Eur. Phy. J. A 10, 1-3 (2001).
[6] Såmark-Roth, A. et al. Phys. Rev. C 98, (2018).
[7] Evaluated nuclear structure data file. Available at: https://www.nndc.bnl.gov/ensdf/.
