Ambient neutrons may cause significant background for underground experiments. Therefore, it is necessary to investigate their flux and energy spectrum in order to devise a proper shielding. Here, two sets of altogether ten moderated 3 He neutron counters are used for a detailed study of the ambient neutron background in tunnel IV of the Felsenkeller facility, underground below 45 m of rock in Dresden/Germany. One of the moderators is lined with lead and thus sensitive to neutrons of energies higher than 10 MeV. For each 3 He counter moderator assembly, the energy-dependent neutron sensitivity was calculated with the FLUKA code. The count rates of the ten detectors were then fitted with the MAXED and GRAVEL packages. As a result, both the neutron energy spectrum from 10 −9 to 300 MeV and the flux integrated over the same energy range were determined experimentally. The data show that at a given depth, both the flux and the spectrum vary significantly depending on local conditions. Energy-integrated fluxes of (0.61 AE 0.05), (1.96 AE 0.15), and ð4.6 AE 0.4Þ × 10 −4 cm −2 s −1 , respectively, are measured for three sites within Felsenkeller tunnel IV which have similar muon flux but different shielding wall configurations. The integrated neutron flux data and the obtained spectra for the three sites are matched reasonably well by FLUKA Monte Carlo calculations that are based on the known muon flux and composition of the measurement room walls.
Recent astronomical data have provided the primordial deuterium abundance with percent precision. As a result, big bang nucleosynthesis may provide a constraint on the universal baryon to photon ratio that is as precise as, but independent from, analyses of the cosmic microwave background. However, such a constraint requires that the nuclear reaction rates governing the production and destruction of primordial deuterium are sufficiently well known. Here, a new measurement of the 2 H(p, γ ) 3 He cross-section is reported. This nuclear reaction dominates the error on the predicted big bang deuterium abundance. A proton beam of 400-1650 keV beam energy was incident on solid titanium deuteride targets, and the emitted γ rays were detected in two high-purity germanium detectors at angles of 55 • and 90 • , respectively. The deuterium content of the targets has been obtained in situ by the 2 H( 3 He, p) 4 He reaction and offline using the elastic recoil detection method. The astrophysical S factor has been determined at center of mass energies between 265 and 1094 keV, addressing the uppermost part of the relevant energy range for big bang nucleosynthesis and complementary to ongoing work at lower energies. The new data support a higher S factor at big bang temperatures than previously assumed, reducing the predicted deuterium abundance.
The field of nuclear astrophysics is devoted to the study of the creation of the chemical elements. By nature, it is deeply intertwined with the physics of the Sun. The nuclear reactions of the proton-proton cycle of hydrogen burning, including the 3 He(α,γ) 7 Be reaction, provide the necessary nuclear energy to prevent the gravitational collapse of the Sun and give rise to the by now wellstudied pp, 7 Be, and 8 B solar neutrinos. The not yet measured flux of 13 N, 15 O, and 17 F neutrinos from the carbon-nitrogen-oxygen cycle is affected in rate by the 14 N(p,γ) 15 O reaction and in emission profile by the 12 C(p,γ) 13 N reaction. The nucleosynthetic output of the subsequent phase in stellar evolution, helium burning, is controlled by the 12 C(α,γ) 16 O reaction.In order to properly interpret the existing and upcoming solar neutrino data, precise nuclear physics information is needed. For nuclear reactions between light, stable nuclei, the best available technique are experiments with small ion accelerators in underground, low-background settings. The pioneering work in this regard has been done by the LUNA collaboration at Gran Sasso/Italy, using a 0.4 MV accelerator.The present contribution reports on a higher-energy, 5.0 MV, underground accelerator in the Felsenkeller underground site in Dresden/Germany. Results from γ-ray, neutron, and muon background measurements in the Felsenkeller underground site in Dresden, Germany, show that the background conditions are satisfactory for nuclear astrophysics purposes. The accelerator is in the commissioning phase and will provide intense, up to 50 µA, beams of 1 H + , 4 He + , and 12 C + ions, enabling research on astrophysically relevant nuclear reactions with unprecedented sensitivity.
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