Dosimetry concepts and measurements in food irradiation processing

The associations between the dosimetry concepts, Minimum absorbed dose (D min), maximum absorbed dose (D max), and average dose and median dose are investigated for the case of a large cobalt-60 plaque source irradiating homogeneous bulk product in a two-pass, two-sided irradiation. It is assumed that to a first approximation the intensity of radiation decreases exponentially with the depth, t, in the product. A series of mathematical relationships is derived for the average dose, the maximum and minimum dose, the median dose [defined as (D max/D min)/2], and the uniformity ratio (defined as U.R. = (D max/D min). The relationships are derived in terms of a constant D0 (the dose on the surface of the product in the pass close to the source) and the relaxation length (mu t) of the radiation in the product. Since the uniformity ratio and other dose parameters can be calculated for certain chosen values of mu t, the individual values of mu (the energy absorption coefficient) and t do not need to be known. By dividing the dose range from D min to D max into 10 equal fractions, the amount of product irradiated to each of the fractions is calculated, and it is shown that, independent of the value of U.R., about a third of the product receives a dose in the first fraction above D min. It is also shown that for a given median dose, the average dose decreases as U.R. increases. The calculated dose relationships are confirmed by measurements in homogeneous dummy product, using the lyoluminescence of glutamine to measure dose. The implications of these results for the regulation of the food irradiation process and for the design of irradiation facilities are discussed.