Ten years of personal neutron dosimetry with albedo dosemeters in The Netherlands.

Since 1987, the dosimetry service of the Netherlands Energy Research Foundation (ECN) has been certified by the Dutch government to perform personal dosimetry, using thermoluminescence dosemeters (TLDs). Performing neutron personal dosimetry requires a rather large investment in readers, TLDs and personnel to operate the service. About 800 persons are subjected to routine neutron monitoring in The Netherlands and their annual neutron doses are a relatively small fraction (less than 10%) of the annual Hp(10). In general, the measured neutron dose values are low (on average 93% of the users receive an annual neutron dose <0.2 mSv). The collective annual (neutron) dose has tended to decrease since 1992, but incidentally high doses have been observed. Leaving these incidents out, the average collective annual neutron doses for the different users of neutron sources are about the same.

The neutron sensitivity of dosimeters applied to boron neutron capture therapy.

To obtain a high accuracy in the dosimetry of an epithermal neutron beam used for boron neutron capture therapy (BNCT), the neutron sensitivity of dosimeters applied to determine the various dose components in-phantom has been investigated. The thermal neutron sensitivity of Mg(Ar) ionization chambers, TE(TE) ionization chambers, and thermoluminescent dosimeters (TLD) has been experimentally determined in a "pure" thermal neutron beam. Values much higher than theoretically expected were obtained and a variation up to a factor of 2.5 was found between values for the thermal neutron sensitivity of different Mg(Ar) ionization chambers of the same type. The sensitivity of the TE(TE) ionization chamber to intermediate and fast neutrons (kt) has been calculated for the neutron energy spectrum in a phantom irradiated by a clinical epithermal BNCT beam, obtained using Monte Carlo simulations. The kt value for muscle tissue ranged from 0.87 at small depths to 0.93 at larger depths in the phantom. The application of the thermal neutron sensitivities to measurements in a phantom irradiated by the epithermal BNCT beam yielded up to 17% higher gamma-ray dose rate values compared with measurements using 6Li containing caps to shield the detectors from thermal neutrons, due to a substantial perturbation of the in-phantom radiation field by the 6Li cap. The application of the new kt values resulted in a dose from intermediate and fast neutrons about 10% higher than the dose based on currently applied relative neutron sensitivities of TE(TE) chambers in BNCT beams. The resulting improvement in the accuracy of the determination of the dose from gamma rays and intermediate and fast neutrons is important in view of the required accuracy for dosimetry in radiotherapy.

Determination of dose components in phantoms irradiated with an epithermal neutron beam for boron neutron capture therapy.

The application of activation foils, thermoluminescent detectors, and ionization chambers has been investigated for the determination of the different dose components in phantoms irradiated with a mixed gamma-ray and epithermal neutron beam for boron neutron capture therapy. The thermal neutron fluence has been determined using a set of AuAl and MnNi activation foils. TLD-700 and a Mg(Ar) ionization chamber have been used for the determination of the gamma-ray dose. The dose from epithermal neutrons has been determined using a TE(TE) ionization chamber. The detector characteristics and the relative sensitivities of the various detectors to the different dose components in the phantom have been determined. The following accuracies (1 standard deviation) in the determination of the different components have been obtained: thermal neutron fluence rate: 5%; gamma-ray dose rate: 7%; epithermal neutron dose rate: 15%. These values make these detectors suitable for obtaining the complete set of clinical dosimetry data required for patient dose assessment.