Design of an improved neutron dose monitor

An instrument for neutron dosimetry has been designed and constructed. It has the uniform response to wide energy of neutrons, and has the equal sensitivity to neutron comes from any direction. The monitor is spherical shaped, based on neutron moderation-absorption technique. It consists of four layers of spherical shell. A Boron nitride (BN) shell was placed between Polymethyl methacrylate (PMMA) and Polyethylene (PE) layers which acts as neutron absorber. The TLD detectors are placed between layers on 12 radiate axis at even intervals so that the monitor is equally sensitive in all directions. The calculation of TLD detectors response for each energy group wa done using MCNP5. The incident energy was considered for 52 energy groups from 0.414eV to 15MeV. In order to know the dose response, ISO spectrum of D2O moderated 252Cf source was chosen, because it is the standard calibration spectrum which might be considered as the representative of workplace field. The dose response was found better than the existing instruments. In order to demonstrate the performance of dose response of the monitor, comparing measured to expected dose, the instrument was irradiated in the reference field at JAEA, FRS. The neutron ambient dose equivalent H*(10) was calculated from the linear combination of 5 groups of TLD reading, which was derived using measured data, linear co-factors and standard irradiated dose of 1.48mSv from the D2O moderated 252Cf source. The dose H*(10) was found 1.43mSv for the 252Cf bare source corresponding to expected dose of 1.25mSv. Within a considerable error, neutron dose from low, intermediate and fast energies can be measured by this monitor at various workplace fields. (author)