ASTM/ISO 51401-03

4.1 The dichromate system provides a reliable means for measuring absorbed dose in 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). A calibrated spectrophotometer is used to measure the absorbance.

4.3 Effect of Irradiation Temperature:

4.3.1 The dosimeter response has a temperature dependence during irradiation that is approximately equal to -0.2 % per degree Celsius between 25 and 50°C. At temperatures below 25°C, the dependence is smaller. The dosimeter response between 5 and 50°C is shown in Table 1, where the response at a given temperature is tabulated relative to the response at 25°C (4,5).

4.3.2 The data in Table 1 may be fitted with an appropriate formula for convenience of interpolation as follows: Rt 5 b0 1 b1tb2 (2) where: Rt = dosimeter response at temperature t relative to that at 25°C. The curve generated from the fitted data is shown in Fig. 1.

4.4 No effect of ambient light (even direct sunlight) has been observed on dichromate solutions in glass ampoules (6).

4.5 For calibration with photons, the dichromate dosimeter shall be irradiated under conditions that approximate electron equilibrium.

4.6 The absorbed dose in materials other than water irradiated under equivalent conditions may be calculated using the procedures given in ASTM Practices E 666, E 668 and ISO/ASTM Guide 51261.

4.7 The dosimeter response is dependent on the type and energy of the radiation employed. For example, the response in high energy (10 MeV) electron beams is reported to be approximately 3 % lower than the response in cobalt-60 radiation (2). The dosimeter shall be calibrated in a radiation field of the same type and energy as that in which it is to be used.

4.8 Provided the dosimeter solution is prepared as described in this document, and steps are taken to avoid contamination, the dosimeter solution stored or sealed in glass vessels (for example, ampoules) is stable before and after irradiation.

1.1 This practice covers the preparation, testing, and procedure for using the acidic aqueous silver dichromate dosimetry system to measure absorbed dose in 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. It is classified as a reference standard dosimetry system (see ISO/ASTM Guide 51261).

1.2 This practice describes the spectrophotometric analysis procedures for the dichromate system.

1.3 This practice applies only to γ-rays, x-rays/bremsstrahlun, and high energy electrons.

1.4 This practice applies provided the following conditions are satisfied:

1.4.1 The absorbed dose range is from 2 X 10 ³ to 5 X 10⁴ Gy.

1.4.2 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).

1.4.3 For radionuclide gamma-ray 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).

1.4.4 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 4.3). Use outside this range requires determination of the temperature coefficient.

1.5 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 8.3.