HK3 - Thermal neutron analytical methods

Three instruments HC3-a HC3-b and HC3-c operate at the thermal neutron beam formed by short neutron guide tube. The neutron guide ensures an efficient transport of thermal neutrons from a Ć 100 mm horizontal channel of the reactor to small target areas of the HC3-a, HC3-b and HC3-c spectrometers. The neutron guide is built of a mirror type tube of rectangular cross-section, cylindrically bent in the vertical direction. It consists of 15 mirror sections made of glass plates of float type. The surface of these plates is coated with Ni reflecting layer with thickness of 2000 Ă. The internal cross-section of each mirror section is 4×150 mm2. The overall length of the guide is 5.63 m, the curvature radius being 825 m. In order to suppress a background due to a direct beam of gamma rays and fast neutrons, the guide is tightly surrounded by a combined shielding consisting of lead and polyethylene pellets. Unlike thermal neutrons, gamma rays and fast neutrons are not subject to reflections from the Ni coating and penetrate the guide walls. In the shielding around they are scattered and absorbed and only collimated thermal neutrons pass through. In addition, a biological shielding, formed by boron-doped polyethylene and lead bricks, is built along the whole guide. The shape of the incoming neutron beam at the entrance of the guide matches the cross-section of the guide. This has been achieved using a 90 cm long collimator made of lead with a rectangular aperture. Immediately behind the guide exit, the neutron beam is tailored by an additional collimator made of 6Li2CO3 to reduce the beam cross-section to 4×60 mm2. The flux of thermal neutrons at the guide exit averaged over the beam cross-section is (1.5±0.2)·107n cm-2 s-1. The cadmium ratio is equal approximately to 105. The above mentioned fluxes refer to the reactor power of 8 MW. Beyond the guide exit, the vertical divergence of neutron trajectories is characterized by angular deviations below 0.5°.

  • Neutron Depth Profiling

    Thermal Neutron Depth Profiling (NDP) facility was set up just behind the neutron guide at HK3. The multidetector spectrometer consists of a large vacuum chamber, automatic target holders and several different data acquisition systems which can be used at the same time. NDP is the nuclear analytical technique available to profile light elements in solids. It utilizes the existence of isotopes of elements that produce prompt monoenergetic charged particles upon capture of thermal neutrons.

  • Prompt Gamma Activation Analysis

    The facility for the measurements of 10B concentrations in biological samples includes HPGe detector with 25% relative efficiency and associated Pb - 6Li2CO3 shielding. Described facility is installed at the distance of 1 m from the exit of the neutron guide. At the target position the neutron flux is approximately 3×106 n cm-2 s-1 at the reactor power of 8 MW. This facility makes it possible to determine 10B concentration of 1 ppm in 1 ml samples with statistical uncertainty of 5% within 15 min. This instrument is in a common property with Nuclear Research Institute, plc.

  • Thermal Neutron Capture Facility

    Two-germanium-detector system is employed for the study of γ-γ coincidences from (n,g) reactions. The horizontally oriented detectors are placed above and bellow the horizontally oriented target, their axes being mutually parallel. At present, the bottom detector is of the HPGe type with the 20% relative efficiency and the energy resolution of 1.9 keV at Eg= 332 keV, while the Ge(Li) detector with the 12% relative efficiency and 2.1 keV energy resolution at 1332 keV is located above the target. These detectors can be easily replaced by another ones. The distance between cylindrical surfaces of the germanium crystals is 4 cm only. The neutron flux at the target position is (2.8 ± 0.5)·106 n cm-2 s-1 at the reactor power of 8 MW. The coincidence efficiency for a g-cascade from the 60Co source is 3·10 -5, including the effect of the solid angle. The detectors are shielded from g-ray and neutron background in the reactor hall by a combined shielding. The electronic system is based on the standard fast/slow coincidence arrangement. The events are accumulated by a PC computer.