Sub-microsecond alpha-particle emitters studied with fast sampling ADCs
Atomic nuclei north-east to the doubly magic nucleus 208Pb (Z=82, N=126) in the nucleidic chart are very short-lived α-decayers. The properties of these nuclei were rather thoroughly studied before the 1980s, however so called pile-ups aggravated the analysis for the nuclei with half-lives on the order of µs. The interplay between modern electronics and a novel algorithm, developed in my master thesis, open up for a deeper study of the short-lived nuclei. This project aims for a continued thorough analysis of the master thesis data for more reliable half-lives, new decay modes and branching ratios, to in the end improve our understanding of the nuclear structure of these short-lived α-decayers.
In nuclear physics, atomic nuclei having the following N,Z = 2, 8, 20, 28, 50, 82 and N = 126 display an enhance in stability. These are called magic numbers (indicated in Figure 1) and are nucleon compositions that assume a special gain in binding energy and hence become more stable . Some theories exist concerning the evolution of the magic numbers towards the superheavy elements, SHE, such as Z = 114 and N = 184 . However, these have not been firmly established. Superheavy are those elements with the number of protons, Z, exceeding 103.
In November 2012 an experiment at the GSI Helmholtzzentrum für Schwerionenforschung was conducted in Darmstadt, Germany. The experiment was led by a group of scientists from Lund. The aim of the experiment was to get a first glimpse of the nuclear structure along α-decay chains originating from superheavy elements (SHE) [3,4]. Alpha-decay chains are sequences of α-decays all originating from the same mother nucleus. In the conducted experiment, a beam of 48Ca impinged on a target material of 243Am to create the element with Z = 115, E115 (recently named Moscovium), through a fusion-evaporation reaction. The detector set-up was developed by the nuclear structure group in Lund. It consisted of different semiconductor detectors based on silicon and germanium with the purpose of detecting α-particles and photons, respectively. Alpha-photon coincidence spectroscopy was successfully performed. For the first time, a measure of energy levels provided detailed nuclear structure information in the superheavy element region.
There is a very low production cross section for creating SHE and only 30 decay chains were proposed to originate from E115 in the experiment. The stored data was mostly the result of background radiation. This background radiation stems primarily from other nuclear reaction products which reached the detector set-up. The creation of these reaction products is believed to be a result of nucleon transfers between the beam and the target material. In this process typically the target material transferred some of its nucleons to the beam such that isotopes, denoted transfer reaction products (TRPs), more neutron deficient were produced .
Fast sampling ADCs directly connected to the preamplifiers were employed for the core silicon detectors in the E115-experiment. The preamplified signals were digitised directly and recorded in traces which contained 4000 samples which corresponded to a time of ~70 µs. The experiment proceeded for over two weeks with an event rate of ~100/s and a large volume of data was generated.
The transfer reaction mechanism with similar beam-target combinations to that in the E115-experiement, is not all that trivial to understand and even so complicated to analyse due to a vast amount of reaction product outcomes. However, α-decays of nuclei just above the Z=82, N=126 magic numbers have been measured. Due to this, α-decay mother nuclei have short half-lives (down to a few ns). These fast decaying nuclei are indicated in Figure 1. The subsequent fast α-decays result in pile-ups with analogue pulse processing electronics. Pile-up signals appear when two or more events occur in the detector within a very short range of time, on the order of µs, and cannot be treated in a good way with former analogue electronics. As a consequence, times and energies are not measured correctly and information is typically lost. However, with modern fast sampling ADCs, as the ones employed in the E115-experiment it is possible to store digitised traces and perform offline processing of all detector signals. A proof-of-concept of such an analysis was accomplished with my master thesis.
The objective of this project is to thoroughly study the E115-experiment data set with the developed pile-up pulse analysis algorithm to investigate the properties of the fast α-decaying nuclei. The analysis takes off with a close examination of the α1-α2 correlation 2D-spectrum, cf. Figure 2. Since previous measurements of these nuclei have typically been performed before the 1980s, as e.g. in [7,8], with now outdated techniques there is potential for improvement of nuclear data and discovery of new nuclear structure. Especially, more reliable half-lives, new decay modes and improved branching ratios are foreseen to in the end improve the understanding of the underlying nuclear structure of these unstable nuclei.
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