Design of a 4 π neutron detector for ß-delay neutron detection experiments.

Abstract

Beta delayed neutron emission probability is one of the main properties of neutron rich nuclei away from the valley of beta stability. The knowledge of this parameter provides the opportunity to extend the information on: the structure of atomic nuclei and its dependence with the N/Z ratio; the rapid neutron capture nucleosynthesis process (r-process), and hence study the formation of nuclei in supernova explosions; new data for reactor technology applications, since is one of the key features for the safe operation of actual nuclear power plants. The objective of the thesis was to design and test a prototype of a 4p neutron detector for DESPEC experiments related with the measurement of beta delayed neutron emission probabilities. As the neutron emission probabilities are low and the neutron energies are not well known, it is necessary to design a detector with high neutron detection efficiency, and a low dependence with neutron energy for a wide neutron energy ranges. The prototype designed has a mean neutron detection efficiency of the order of 27.1 % and is flat for neutron energies ranging from thermal values to 6 MeV. The detector designed is composed by 20 3He proportional counters at 20 atm, inserted in a polyethylene matrix. The counters are distributed in 2 concentric rings, surrounding a beam hole located at the central axis of the matrix This Master Thesis is divided in three main steps: 1) the analysis of different detector configurations and properties; 2) the selection of the definitive configuration of detector prototype, including the electronics and data acquisition system (DAQ); 3) the test of prototype and calibration with neutron sources. The detector configuration analysis has been done by means of Monte Carlo simulations with the code MCNPX. These MC simulations were useful to find: the optimal pressure of 3He counters; the best number of counters, taking in account the cost and the maximum neutron detection efficiency achievable; the number of counters in each concentric ring, and the distance of rings to the center of polyethylene matrix; the optimal counter length; or the shielding thickness for background radiation reduction. All of these studies were basic to obtain the optimal neutron detector configuration. The design of the detector also involved the selection of the most suitable electronics and Data Acquisition System (DAQ), as is described in this Thesis. This activity was done with the collaboration of the "Instituto de Física Corpuscular (IFIC- Valencia)". This PhD. Thesis includes the description of different tests done to check MC simulations of the neutron detector and the DAQ system. The definitive experiment done to check the detector design was done at IGISOL facility (Jyväskylä University, Finland). This experiment allowed to obtain the detector efficiency, measuring the beta-delayed neutron emission of two well known nuclei: 95Rb and 88Br. The results showed a good agreement between experiment and simulations.