Background and Aims

Preliminary measurements using a water calorimeter in PTB’s UHPDR electron beam have been done. The aim of this work is to calculate the heat correction factor of PTB’s water calorimeter in the UHDR electron beam [1].

Methods

Finite Element Method (FEM) has been used to calculate heat correction factors due to the heat transport occurring during and after irradiation for the PTB water calorimeter in UHDR electron beam. The resulting simulated temperature-time traces are compared to preliminary measurement of 5 pulses at 1 Gy per pulses (PRF of 5Hz).

Results

The simulated temperature-time trace agrees with the preliminary measurement (Figure 1). There is a large temperature gradient between the temperature sensor (thermistor) and the surrounding water shortly after the irradiation due to the heating-up of the glass pipette in which the thermistor is embedded. The thermistor is in equilibrium with the surrounding water within 40 seconds after irradiation. The heat correction factors were estimated to be between 0.999(2) and 1.003(3) for the reference UHDR beams [1].

Figure 1: Temperature-time traces of measurement compared to simulated temperature of the thermistor (FEM – Thermistor), at the equivalent position without thermistor (FEM - Water), and water simulated without heat transport occurring (FEM - Ideal). The right panel presents a zoom of the post-drift curve.

Conclusions

Although the glass pipette significantly impact the temperature measurement during and few 10 seconds after irradiation, the heat correction factors remained comparable to the one obtained at conventional dose rate and does not exceed a correction of 0.3(3)% for reference UHDR pulsed electron beams.

Reference: [1] Bourgouin et al. 2022 Phys. Med. Biol. 67 085013

Acknowledgement: 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.