Shallow-underground accelerator sites for nuclear astrophysics: Is the background low enough?

Abstract

Abstract. In order to reliably estimate the rate of a charged particle induced nuclear reaction in a nonexplosive 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 0.4 MV 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 1000 m 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 47 m 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.

Felsenkeller 5 MV underground accelerator: Towards the Holy Grail of Nuclear Astrophysics 12C(α, γ)16O

“…Background intercomparison using one and the same escape-suppressed HPGe detector, subsequently transported to different sites. The data [16,17] show that using a muon veto, the Felsenkeller background is not far from the deep underground (Gran Sasso) case.…”

“…Detailed measurements at Felsenkeller [16,17] have shown that the background in the crucial 7-8 MeV γ-energy range (affecting inter alia the 12 C(α, γ) 16 O reaction) is only a factor of 3 higher in Felsenkeller than deep underground at Gran Sasso (Figure 1). …”

“…The feasible energy range for direct total and partial cross section data at Felsenkeller, based on the measured background [16,17] and assuming inverse kinematics, and the Gamow peak for 0.35 GK temperature are also shown.…”

“…One particularly challenging case is the 12 C(α, γ) 16 O reaction. Its rate affects the carbon to oxygen abundance ratio at the end of stellar helium burning and, hence, the abundances of a host of other nuclides that are produced during subsequent burning stages [2].…”

Felsenkeller shallow-underground accelerator laboratory for nuclear astrophysics

“…This system was moved successively to several sites. The data show that with a muon veto, the background in the 6-8 MeV energy region is only a factor of 2-4 higher in Felsenkeller than deep underground at LUNA, enabling highly sensitive experiments [14]. The exercise was repeated with a 3" LaBr 3 detector, and again the background in Felsenkeller was less than one order of magnitude higher than deep underground at LUNA, if a muon veto was used [14].…”