Characterization and dosimetry of a practical X-ray alternative to self-shielded gamma irradiators

The Insect Pest Control Laboratory of the Joint FAO/IAEA1 Division of Nuclear Techniques in Food and Agriculture recently purchased an X-ray irradiator as part of their programme to develop the sterile insect technique (SIT). It is a self-contained type with a maximum X-ray beam energy of 150 keV using a newly developed 4π X-ray tube to provide a very uniform dose to the product. This paper describes the results of our characterization study, which includes determination of dose rate in the centre of a canister as well as establishing absorbed dose distribution in the canister.The irradiation geometry consists of five canisters rotating around an X-ray tube—the volume of each canister being 3.5 l. The dose rate at the maximum allowed power of the tube (about 6.75 kW) in the centre of a canister filled with insects (or a simulated product) is about 14 Gy min−1. The dose uniformity ratio is about 1.3. The dose rate was measured using a Farmer type 0.18-cm3 ionization chamber calibrated at the relevant low photon energies.Routine absorbed dose measurement and absorbed dose mapping can be performed using a Gafchromic® film dosimetry system. The radiation response of Gafchromic film is almost independent of X-ray energy in the range 100–150 keV, but is very sensitive to the surrounding material with which it is in immediate contact. It is important, therefore, to ensure that all absorbed dose measurements are performed under identical conditions to those used for the calibration of the dosimetry system.Our study indicates that this X-ray irradiator provides a practical alternative to self-shielded gamma irradiators for SIT programmes.