The PTB water calorimeter for determining the absolute absorbed dose to water in ultra-high pulse dose rate electron beams

Objective. The purpose of this investigation is to establish the water calorimeter as the primary standard in PTB's ultra-high pulse dose rate (UHPDR) 20 MeV reference electron beams. Approach. The calorimetric measurements were performed at the PTB research linac facility using the UHPDR reference electron beam setups that enable a dose per pulse between about 0.1 Gy and 6 Gy. The beam is monitored by an in-flange integrating current transformer. The correction factors required to determine the absorbed dose to water were evaluated using thermal and Monte Carlo simulations. Measurements were performed with different total doses delivered per pulse by modifying the instantaneous dose rate within a pulse and by changing the pulse length. The obtained temperature-time traces were compared to the simulated ones to validate the thermal simulations. In addition, absorbed-dose-to-water measurements obtained using the secondary standard alanine dosimeter system were compared to measurements performed with the primary standard. Main results. The simulated and measured temperature-time traces were shown to be consistent, within combined uncertainties, with one another. Measurements with alanine dosimeters proved to be consistent within k = 1 of the total combined uncertainty with the absorbed dose to water determined using the primary standard. Significance. The total relative standard uncertainty of absorbed dose to water determined using the PTB water calorimeter primary standard in UHPDR electron beams was estimated to be less than 0.5%, and the combined correction factors were found to deviate from 1 by less than 1% for both PTB UHPDR 20 MeV reference electron beams. The water calorimeter is therefore considered to be an established primary standard for the higher energy UHPDR reference electron beams.