The design of a Nai(Tl) crystal in a system optimised for high-throughput and emergency measurement of iodine 131 in the human thyroid

•New shape of NaI(Tl) crystal optimised for thyroid measurement of I-131 activity.•Twice higher detection efficiency, better MDA and resolution then old system.•Only 30% less effective than three times larger cylindrical crystal with dimension 76.2×76.2 mm.

In the case of an accidental release of 131I, a system for large-scale monitoring of the population for the radionuclide intake is needed. A monitoring system is required to be capable of measuring adult as well as child subjects across a wide range of ages. Such system has been developed by the National Radiation Protection Institute in Prague (NRPI) and the Evinet company (member of the Nuvia Group). This paper describes the optimisation of the NaI(Tl) detector chosen for this system. The design of the crystal was based on Monte Carlo (MC) simulations, and supported by literature. These simulations examined three different crystal shapes and several dimensions. Based on the MC study, two prototype detectors, with crystal diameters 80 and 73 mm, were manufactured and compared with the crystals having dimensions ∅45×40 mm used for thyroid measurement at NRPI and with a standard NaI(Tl) probe (∅76.2×76.2 mm). The detector with a crystal of 80 mm diameter gave the best results and was chosen for further production.