Abstract
In the EXILL campaign a highly efficient array of high purity germanium (HPGe) detectors was operated at the cold neutron beam facility PF1B of the Institut Laue-Langevin (ILL) to carry out nuclear structure studies, via measurements of γ-rays following neutron-induced capture and fission reactions. The setup consisted of a collimation system producing a pencil beam with a thermal capture equivalent flux of about 108 n s−1cm−2 at the target position and negligible neutron halo. The target was surrounded by an array of eight to ten anti-Compton shielded EXOGAM Clover detectors, four to six anti-Compton shielded large coaxial GASP detectors and two standard Clover detectors. For a part of the campaign the array was combined with 16 LaBr3:(Ce) detectors from the FATIMA collaboration. The detectors were arranged in an array of rhombicuboctahedron geometry, providing the possibility to carry out very precise angular correlation and directional-polarization correlation measurements. The triggerless acquisition system allowed a signal collection rate of up to 6 × 105 Hz. The data allowed to set multi-fold coincidences to obtain decay schemes and in combination with the FATIMA array of LaBr3:(Ce) detectors to analyze half-lives of excited levels in the pico- to microsecond range. Precise energy and efficiency calibrations of EXILL were performed using standard calibration sources of 133Ba, 60Co and 152Eu as well as data from the reactions 27Al(n,γ)28Al and 35Cl(n,γ)36Cl in the energy range from 30 keV up to 10 MeV.
Highlights
The field of γ-ray spectroscopy based on neutron-induced reactions is an important part of nuclear physics where many interesting results can be obtained when using efficient arrays of Ge detectors
In this paper we have presented EXILL, an efficient detector array for precise measurements of γ-rays from neutron-capture and neutron-induced fission
The array, placed at the cold neutron beam facility PF1B of ILL Grenoble, consisted of up to 49 germanium crystals mounted in a rhombicuboctahedron geometry
Summary
The ILL research reactor supplies a large number of neutron guides allowing for transportation of neutrons from the institute’s high flux reactor over tens of meters to experimental areas. To suppress the γ-ray background from boron, all according apertures are shielded with 5-cm thick lead absorbers downstream of the neutron beam direction. The following three collimators consist of 5-mm thick 6LiF ceramics mounted on 3-cm thick borated polyethylene, supported by 5 cm thick lead apertures. The simulation models neutron propagation from ILL’s vertical cold source along the H113 guide (according to [21] and taking into account all upgrades), through the collimation system until the beam stop. The average wavelength determined from the simulated capture equivalent spectrum of the neutron beam at the target position is λn = 4.6(2) Å, corresponding to Ekin = 3.9(4) meV, where the error is dominated by uncertainties in modeling the ILL reactor’s vertical cold source
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