Background and Aims

The calibration of a dosimeter system to be used in ultra-high pulse dose rate electron beams - allowing FLASH radiotherapy - requires a traceable measurement of absorbed dose to water.
METAS has been using Fricke solution for dosimetry purpose for over twenty years. This chemical dosimeter is based on a closely water-equivalent ferrous ammonium sulfate solution. Irradiation with ionizing radiation causes oxidation of Fe²⁺ to Fe³⁺. The resulting concentration of Fe³⁺ in the Fricke solution is proportional to the absorbed dose to water and can be determined by analyzing the change in absorbance at well-defined wavelengths in the UV spectral range.

Methods

The primary standard is realized by means of the total absorption technique. A thin and monoenergetic electron beam with known charge is totally absorbed in a large volume of Fricke solution. The well-known deposited energy of the beam and the mass of the liquid is used to determine the radiation chemical yield of the Fricke dosimeter.

Results

This factor allows calibrating secondary standards like ionization chambers in reference fields by comparison with small bags filled with Fricke solution used as transfer standard. We will present the status and results of the total absorption experiment.

Conclusions

We show that the Fricke dosimeter is suitable for dosimetry in the (ultra-high) pulse dose rate regime, by comparing Fricke dosimetry to Alanine and ionization chambers.

This project 18HLT04 UHDpulse has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.