Advances in Radiative Capture Studies at LUNA with a Segmented BGO Detector

TL;DR

This paper advances radiative capture studies at LUNA by refining a segmented BGO total absorption spectrometer to achieve high efficiency with dramatically reduced backgrounds, enabling direct measurement prospects for the elusive $E^{res.}_{cm}=65~\mathrm{keV}$ resonance in ${}^{17}\mathrm{O}(\mathrm{p},\gamma)^{18}\mathrm{F}$ and demonstrating a novel in-situ activation counting approach. Central to the improvement are (i) targeted changes to target chambers and shielding (Setup A→B→C), which increase add-back efficiency and suppress beam-induced backgrounds, (ii) Monte Carlo validation of detector response to accurately quantify efficiency and systematics, and (iii) segmentation-enabled analysis, including ROI gating, to discriminate cascades and suppress spurious coincidences. The study reports a background reduction by about a factor of $4.3$ relative to prior lead shielding and an overall single-γ efficiency around $60\%$ at $E_\gamma \sim 6~\mathrm{MeV}$, making near-threshold measurements feasible. Additionally, the authors introduce an in-situ activation counting mode, validated through ${}^{14}\mathrm{N}(\mathrm{p},\gamma)^{15}\mathrm{O}$ and ${}^{12}\mathrm{C}(\mathrm{p},\gamma)^{13}\mathrm{N}$ tests, to count β+-decays via the 511 keV annihilation signature with high efficiency, expanding the experimental toolbox for astrophysical reaction studies.

Abstract

Studies of charged-particle reactions for low-energy nuclear astrophysics require high sensitivity, which can be achieved by means of detection setups with high efficiency and low backgrounds, to obtain precise measurements in the energy region of interest for stellar scenarios. High-efficiency total absorption spectroscopy is an established and powerful tool for studying radiative capture reactions, particularly if combined with the cosmic background reduction by several orders of magnitude obtained at the Laboratory for Underground Nuclear Astrophysics (LUNA). We present recent improvements in the detection setup with the Bismuth Germanium Oxide (BGO) detector at LUNA, aiming to reduce high-energy backgrounds and to increase the summing detection efficiency. The new design results in enhanced sensitivity of the BGO setup, as we demonstrate and discuss in the context of the first direct measurement of the 65 keV resonance ($E_{x} = 5672$ keV) of the $^{17}$O($p,γ$)$^{18}$F reaction. Moreover, we show two applications of the BGO detector, which exploit its segmentation. In case of complex $γ$-ray cascades, e.g. the de-excitation of $E_{x} = 5672$ keV in $^{18}$F, the BGO segmentation allows to identify and suppress the beam-induced background signals that mimic the sum peak of interest. We demonstrate another new application for such a detector in form of in-situ activation measurements of a reaction with $β^{+}$ unstable product nuclei, e.g., the $^{14}$N($p,γ$)$^{15}$O reaction.

Figures (9)

• Figure 1: Cross section sketch of the three discussed setups. From top to bottom: Setup A, B and C, see text for more details. Different colors correspond to different materials: green represents stainless steel, red aluminium, yellow brass, white plastic, grey lead, purple borated-Polyethylene, blue water, and cyan the BGO crystal.
• Figure 2: Add-back $\gamma$-ray spectra of the $^{137}$Cs and $^{60}$Co calibration sources, comparing simulation with measurement. The agreement between the integrals in the respective regions of interest (ROI) is within 3%.
• Figure 3: $\gamma$-ray spectra of the ${}^{14}\mathrm{N}$$(\mathrm{p},\gamma)$${}^{15}\mathrm{O}$$E_\mathrm{c.\,m.} = 258keV$ resonance, comparing simulation with measurement. Upper panel: single crystal spectrum (BGO 1). Lower panel: add-back spectrum. The agreement between the expected yield on the basis of imbriani2005 and the simulations is within 3%, see text for details.
• Figure 4: Add-back spectrum efficiency (sum peak efficiency) for single $\gamma$-rays with energies from 500keV up to 10MeV obtained via simulations of Setup A (green) Setup B (red) and Setup C (blue). Simulations were implemented considering the complete target-detector-shielding geometry. Shown in black is a simulation of an idealized setup with no target chamber and target holder, thus with no $\gamma$-ray absorption.
• Figure 5: Measured background with Setup B (black) and C (blue). The inset shows the region of interest for the $E^{res.}_\mathrm{c.\,m.}$ = 65keV resonance of the ${}^{17}\mathrm{O}$$(\mathrm{p},\gamma)$${}^{18}\mathrm{F}$ reaction.