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.