Abstract
Following the completion of the second neutron beam line and the related experimental area (EAR2) at the n_TOF spallation neutron source at CERN, several experiments were planned and performed. The high instantaneous neutron flux available in EAR2 allows to investigate neutron induced reactions with charged particles in the exit channel even employing targets made out of small amounts of short-lived radioactive isotopes. After the successful measurement of the 7Be(n,α)α cross section, the 7Be(n,p)7Li reaction was studied in order to provide still missing cross section data of relevance for Big Bang Nucleosynthesis (BBN), in an attempt to find a solution to the cosmological Lithium abundance problem. This paper describes the experimental setup employed in such a measurement and its characterization.
Highlights
In July 2014 the second experimental area (EAR2) of the n_TOF spallation neutron-time-of-flight facility at CERN came into operation
The advantage of n_TOF, with respect to other neutron facilities in the world, is the extremely high instantaneous neutron flux delivered in a short time interval at the sample position
The first experiment done at EAR2 was the measurement of the energy-dependent 7Be(n,α)α cross-section [4,5], of relevance for a possible nuclear solution to the cosmological Lithium abundance problem (CLIP) in the Big Bang Nucleosynthesis (BBN) framework [6,7]
Summary
In July 2014 the second experimental area (EAR2) of the n_TOF spallation neutron-time-of-flight facility at CERN came into operation. The neutron flux features of the EAR2 facility and the n_TOF time-energy dynamic range provided the opportunity of a high quality direct measurement of the 7Be(n,p)7Li cross-section. The 7Be(n,p)7Li reaction has a very high thermal cross-section (several 104 b), but it produces rather low-energy protons of 1.44 MeV which could interfere with background from other reaction channels on the sample backing or contaminants. The main advantages of such a setup were the reduction of: (i) the pile-up issues; (ii) the background of 478 keV γ-rays following the natural decay of the 7Be target nuclei into 7Li; and (iii) the huge background due to the so-called γ-flash, i.e. the big prompt burst of γ-rays and relativistic charged particles produced by the n_TOF spallation target These three issues would have posed severe limitations on the performance of an in-beam detector arrangement similar to the one exploited in Refs. The overall shape of the efficiency accounts for the 45◦ tilt of the target with respect to the detector
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