During the past few years significant advances have taken place in the different areas of dosimetry for radiation processing, mainly stimulated by the increased interest in radiation for food preservation, plastic processing and sterilization of medical products. Reference services both by international organizations (IAEA) and national laboratories have helped to improve the reliability of dose measurements. Several dosimeter systems like calorimetry, perspex, and radiochromic dye films are being improved and new systems have emerged, e.g. spectrophotometry of dichromate solution for reference and sterilization dosimetry, optichromic dosimeters in the shape of small tubes for food processing, and ESR spectroscopy of alanine for reference dosimetry. In this paper the special features of radiation processing dosimetry are discussed, several commonly used dosimeters are reviewed, and factors leading to traceable and reliable dosimetry are discussed.

Dosimetry for Radiation Processing

Dosimetry systems for radiation processing

The kinetics of the radiation-induced reduction of hexavalent CrVI to trivalent CrIII, through the unstable states CrV and CrIV, in N2O-saturated aqueous solution in the presence of formate, has been studied by pulse radiolysis with optical measurements in the pH range ca. 2–5 and conductometric measurements at pH 3. The absorption spectrum of CrV has been obtained in the pH range ca. 2–9 by comparing the observed and expected bleaching yields resulting from the one-electron reduction of CrVI by CO2˙– or eaq–. The observed second-order rate constant for the disproportionation of CrV, determined from the rate of partial regeneration of CrVI, shows a bell-shaped dependence on pH with a maximum value at pH 3.3. This pH dependence is consistent with a rearrangement of the coordination shell of CrV from tetrahedral, CrVtet, to octahedral, CrVoct, in two sequential proton assisted steps followed by disproportionation between CrVtet and CrVoct with k= 3.4 × 108 dm3 mol–1 s–1. The intermediate form of CrV generated in the first of these steps does not contribute to the disproportionation reaction. Based on the bell-shaped dependence on pH of the observed rate constant for disproportionation of CrV, the forms of CrV in acidic solution are assigned as H3CrO4(pKa= 2.7), H2CrO4–(pKa= 3.8), and HCrO42–. At pH 6.8 the spectral data indicate the presence of CrO43– and pKa= 7.0 for HCrO42–.

J.H. Barrett

Papers

Acidic aqueous dichromate solutions as reference dosimeters in the 10–40 kGy range

Dichromate solution as a reference dosemeter for use in industrial irradiation plants