Passive multi-layer neutron spectrometer for neutron radiation fields dosimetry

Paper describes the dosimetry methods of neutron radiation field using polyethylene moderator and thermoluminescent detectors. Mechanisms of neutrons’ slowing down, detection and measurement steps using TLD are presented. There is an analysis and discussion of TL glow curve interpretation methods. The paper contains a description of the construction and the arrangements of the multi-moderator with possibility of placing passive detectors at different thicknesses in the sphere.

Dosimeter developed in this work has the double construction - in the centre of the moderator there is a place for active detector, while in the moderator, at the different depths, passive detectors. In principle, the active detector can provide continuous monitoring of a field, and passive detectors should enable the evaluation of the neutron spectrum. The central detector is a recombination chamber filled with BF3, and the passive detectors are TLD detectors, types 7LiF and 6LiF. Measurements using the prepared detector were performed at the National Centre for Nuclear Research in Świerk. Measurements were made for two sources of radiation: Californium 252Cf and 238Pu-Be Plutonium-Beryllium at the different time of exposure. For the interpretation of the results it was necessary to use some additional TLD detectors aside from the measuring – the calibration detectors and the background measuring detectors. The calibration detectors made it possible to calculate the absorbed dose from the known dose rate in the radiation field of the source at a given distance. The background measuring detectors were placed in the room in which the radiation from the source does not affect. Their application was to determine the signal collected by the not irradiated detectors. Diagrams showing the results in different places in the spherical moderator and the protocol of measurements are presented. Monte Carlo calculations of TLD response based on lithium and thermal neutron nuclear reaction were performed. Obtained results enable creation of the thermoluminescent response matrix, which is necessary for the unfolding procedures. Deconvolution techniques and corresponding codes were analysed and preliminary tests were performed.

Based on the measurement results, their analysis is possible, taking into account the TLD detectors type used, their position in the moderator and the type of radiation. There is a possibility to extract the gamma and neutron components from the results by means of different thermoluminescent detector types containing two isotopes of lithium and neutron spectra using response matrices can be reconstructed.