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 [1]. Some theories exist concerning the evolution of the magic numbers towards the superheavy elements, SHE, such as Z = 114 and N = 184 [2]. However, these have not been firmly established. Superheavy are those elements with the number of protons, Z, exceeding 103.


Chart of nuclides, E115-experiment
Figure 1. The chart of nuclides. Each small square represents a known atomic nucleus. The different colours indicate the preferred decay mode of a given nucleus. The legend shows all the decay modes. Here EC+β+ stands for electron capture and β+-decays. P and N are decays by emission of a proton or a neutron, respectively. SF is short for spontaneous fission. The nuclides with proton and/or neutron magic numbers can be found within blue and black marked regions. The target material 243Am is marked in the figure with a red square. The region with the fast α-decaying nuclei north east of the double magic 208Pb nucleus is indicated with a large red box. The image has been modified from Ref. [6].
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 [5].

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 α12 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.

Figure 2. For the pile-up signals, displayed in the top-left inset, the energy of the first alpha-particle (x-axis) and the energy of the second alpha-particle (y-axis) are plotted in a 2D histogram. The z-axis indicates the number of counts in each pixel which is 10×10 keV. Ten 2D-peaks are labelled from A-J. Each region represents a certain alpha-decay mother and daughter, i.e. a decay path.

[1] S. G. Nilsson and I. Ragnarsson. Shapes and Shells in Nuclear Structure. Cambridge University Press, 1995.
[2] A. Sobiczewski. Closed Shells for Z>82 and N>126 in a Diffuse Potential Well. Physical Letters, 1966.
[3] Ulrika Forsberg. Element 115. PhD thesis, Faculty of Science, Lund University, 2016.
[4] D. Rudolph et al. Spectroscopy of Element 115 Decay Chains. Physical Review Letters, 2013.
[5] J.M. Gates et al. First Superheavy Element Experiments at the GSI Recoil Separator
TASCA: The Production and Decay of Element 114 in the 244 Pu( 48 Ca, 3-4n) Reaction. Physical Review C, 2011.
[6] National Nuclear Data Center. Chart of Nuclides., 2016.
[7] J. D. Bowman et al. Alpha Spectroscopy of Nuclides Produced in the Interaction of 5 GeV Protons with Heavy Element Targets. Physical Review C, 1982.
[8] K. Valli. Production and Decay Properties of Thorium Isotopes of Mass 221-224 Formed in Heavy-Ion Reactions. Physical Review C, 1970.