Beta-delayed charged-particle spectroscopy using TexAT

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Abstract

β-delayed charged-particle emission is a sensitive probe of three-body decays in light nuclei. Time Projection Chambers (TPCs) offer a significant advantage over traditional charged-particle spectroscopy techniques due to a low-energy threshold and a high-geometric efficiency (≈4π) which are essential for use with radioactive ion beams where the beam intensities are limited. The technique for high-sensitivity spectroscopy of β-delayed charged-particle emission is shown to be possible using the Texas Active Target (TexAT) TPC in conjunction with the General Electronics for TPCs (GET) system. The benchmark case studied was that of 12N β-decay to the first α-unbound state in 12C, the Hoyle state. Half-life and branching ratio measurements are presented and are in good agreement with previous studies. The efficacy of using TPCs to study such a near-threshold state and disentangle the three-body dynamics of the decay products is demonstrated.

Section snippets

Beta-delayed particle decay

β-delayed particle decay can occur where the daughter nucleus has states, populated by β± decay, which are above the particle decay threshold. This emission can be used for the study of a selective population of states as allowed by the β-transition rules. For instance, in 12C, one can use the decay of 12N∕12B to populate states above the α-decay threshold. Isospin symmetry can also be measured here by examining the relative branching ratios of states. Additionally, the strength of the

TexAT detector

The β-delayed charged-particle emission was studied using the Texas Active Target detector system. It is a state-of-the-art active target time projection chamber that was designed to study structure of exotic nuclei, clustering phenomena, nucleon transfer and fusion reactions with rare isotope beams. The detailed technical description of TexAT is given in Ref. [10]. The detector was commissioned in August of 2017 and the results of the commissioning run, in which the structure of the exotic

Experimental details

The experiment was performed using the K500 cyclotron at the Cyclotron Institute at Texas A&M University. A 12N beam (typical intensity of 60 pps) was created via the interaction of a 11 AMeV 10B primary beam undergoing a 3He(10B,12N)n reaction in a gas cell placed upstream of the MARS (Momentum Achromatic Recoil Spectrometer) [14]. MARS can then select and deliver the 12N secondary beam to the TexAT chamber. The beam contaminant was 7Be which was fully separated in the offline analysis. This

Event merging

The employed L1A/L1B (implant/decay) trigger logic will throw away events where the decay time exceeds 30 ms. Therefore, unpaired L1A events were merged to increase the statistics of the overall 3-α events as well as to constitute an initial pruning of events. At this stage, the IC waveform was fitted and the events corresponding to the lower amplitude 7Be contaminant were removed. For the full L1A/L1B events, there was an issue with the MFM [18] (Multi-Frame MetaFormat) file where the L1A and

GEANT4 simulations

A GEANT4 [27] simulation package was used to model the response of TexAT to different reactions. This TexAtSim program allows for different fill gases and pressures requiring inputs from Magboltz [28] calculations which give the drift velocity as well as the time and position spread of a drift electron. Following the primary event generation, the energy loss of the reaction products throughout the medium is then used to generate electrons along the path consistent with the work function of the

Branching ratios

It is possible in our setup to calculate the branching ratios from 12N β-decay as the total number of L1A triggers (12N events) is recorded and the beam purity is also monitored. For the partial L1A events, the stopping point of the beam can be reconstructed and events selected where the beam stops in the ‘safe region’ (more than 47 mm from the edge of the Micromegas). By counting the amount of beam ions that stop in this region and comparing this to the number of Hoyle events measured inside

Conclusion

The TexAT device in conjunction with GET is able to measure the β-delayed particle decay and measure the phase space of subsequent three-body reactions. The time separation between the implant and decay can extend up to 1.677 s with GET and using the decay-by-decay method in conjunction with correlating the beam stopping position with the decay vertex, a very low background can be achieved. The results for the branching ratio of 12N to states in 12C above the 3-α threshold agree with previous

CRediT authorship contribution statement

J. Bishop: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. G.V. Rogachev: Conceptualization, Methodology, Validation, Investigation, Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. S. Ahn: Conceptualization, Methodology, Software, Validation, Investigation, Writing - review & editing. E. Aboud: Investigation, Writing - review & editing. M.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

TexAT project was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Science, under Award No. DE-FG02-93ER40773 and by National Nuclear Security Administration through the Center for Excellence in Nuclear Training and University Based Research (CENTAUR) under grant number #de-na0003841. G.V.R. also acknowledges the support of the Nuclear Solutions Institute.

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    Present address: Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA.

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    Present address: Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.

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