【国外标准】 Standard Practice for Use of a Dichromate Dosimetry System

4.1 The dichromate system provides a reliable means for measuring absorbed dose to water. It is based on a process of reduction of dichromate ions to chromic ions in acidic aqueous solution by ionizing radiation.4.2 The dosimeter is a solution containing silver and dichromate ions in perchloric acid in an appropriate container such as a sealed glass ampoule. The solution indicates absorbed dose by a change (decrease) in optical absorbance at a specified wavelength(s) ((3), ICRU Report 80). A calibrated spectrophotometer is used to measure the absorbance.1.1 This practice covers the preparation, testing, and procedure for using the acidic aqueous silver dichromate dosimetry system to measure absorbed dose to water when exposed to ionizing radiation. The system consists of a dosimeter and appropriate analytical instrumentation. For simplicity, the system will be referred to as the dichromate system. The dichromate dosimeter is classified as a type I dosimeter on the basis of the effect of influence quantities. The dichromate system may be used as either a reference standard dosimetry system or a routine dosimetry system.1.2 This document is one of a set of standards that provides recommendations for properly implementing dosimetry in radiation processing, and describes a means of achieving compliance with the requirements of ISO/ASTM Practice 52628 for the dichromate dosimetry system. It is intended to be read in conjunction with ISO/ASTM Practice 52628.1.3 This practice describes the spectrophotometric analysis procedures for the dichromate system.1.4 This practice applies only to gamma radiation, X-radiation/bremsstrahlung, and high energy electrons.1.5 This practice applies provided the following conditions are satisfied:1.5.1 The absorbed dose range is from 2 × 10 3 to 5 × 104 Gy.1.5.2 The absorbed dose rate does not exceed 600 Gy/pulse (12.5 pulses per second), or does not exceed an equivalent dose rate of 7.5 kGy/s from continuous sources (1).21.5.3 For radionuclide gamma sources, the initial photon energy shall be greater than 0.6 MeV. For bremsstrahlung photons, the initial energy of the electrons used to produce the bremsstrahlung photons shall be equal to or greater than 2 MeV. For electron beams, the initial electron energy shall be greater than 8 MeV.Note 1—The lower energy limits given are appropriate for a cylindrical dosimeter ampoule of 12 mm diameter. Corrections for displacement effects and dose gradient across the ampoule may be required for electron beams (2). The dichromate system may be used at lower energies by employing thinner (in the beam direction) dosimeter containers (see ICRU Report 35).1.5.4 The irradiation temperature of the dosimeter shall be above 0°C and should be below 80°C.Note 2—The temperature coefficient of dosimeter response is known only in the range of 5 to 50°C (see 5.2). Use outside this range requires determination of the temperature coefficient.1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in 9.3.