Abstract
In order to reliably estimate the rate of a charged particle induced nuclear reaction in a non-explosive astrophysical scenario, its cross-section must be measured far below the Coulomb barrier. However, at the corresponding energies the cross-section values are very low, so that the experimental counting rate is dominated by cosmic-ray induced background, even if a suitable anticoincidence shield is applied. This problem can be overcome by performing an accelerator-based experiment in a deep underground site, as has been done with great success at the LUNA 0.4MV accelerator in Gran Sasso, Italy. Several underground accelerators with higher beam energy are in the planning phase worldwide. All of them are shielded by over 1000m of rock, a depth at which cosmic-ray effects are negligible for the purposes of nuclear astrophysics experiments. It is shown here that a combined approach, using a shallow-underground laboratory below 47m of rock and an active shield to veto surviving muons in simple detectors, results in a background level that is not far from that of deep underground sites. Data have been obtained using two “traveling” γ-detectors. They have been transported both shallow underground, to the Dresden Felsenkeller in Germany, and deep underground, to the Gran Sasso laboratory in Italy. As shallow-underground facilities are more easily accessible than deep-underground ones, the present finding holds the promise of greatly accelerated progress in the field of cross-section measurements for nuclear astrophysics.
Highlights
Many nuclear physics inputs are needed for the modeling of astrophysical scenarios [1,2,3], and progress must be made on two frontiers: Nuclei far from the valley of βstability will become accessible at generation radioactive ion beam facilities
In order for the muon flux to become negligible, depths of 1000 m of rock or more are required [7]. This depth has been selected with great success by the Laboratory for Underground Nuclear Astrophysics (LUNA)
LUNA is placed in the Gran Sasso laboratory in Italy, below 1400 m of rock, and with uniquely low background [8,9,10]
Summary
Many nuclear physics inputs are needed for the modeling of astrophysical scenarios [1,2,3], and progress must be made on two frontiers: Nuclei far from the valley of βstability will become accessible at generation radioactive ion beam facilities. For reactions of stable nuclei, the frontier is given by the lack of precise cross-section data at low energy This limitation can be overcome by placing the entire laboratory in a low-background setting underground. It is shown here that already shallow-underground sites present satisfactory background conditions for a number of nuclear astrophysics experiments, if a suitable active shield, surrounding either just the detector or the whole laboratory, suppresses remaining muons. 7. Before turning to the new experimental data obtained here, in the present section the background situation in detectors without active shielding underground is briefly reviewed based on the literature. Data from nuclear astrophysics related setups are selected for this comparison, where possible At these γ-energies, a thick lead shield is essential to suppress the background from radionuclides. Passively shielded shallow-underground facilities may provide some kind of intermediate solution between surface-based and deep-underground experiments, when it comes to in-beam γ-spectrometric experiments for nuclear astrophysics with detected γ-rays of Eγ < 2.615 MeV
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