Purpose: Given the extremely short delivery times of FLASH beams, real-time dosimetry is essential. The poor performance of ion chambers in high dose-per-pulse beams is well known and the alternative proposed to date is absorbed dose calorimetry. However, to-date, such devices are complex to operate and require instrumentation for readout not normally possessed by clinics. This work describes the development and initial characterization of a pyroelectric calorimeter capable of FLASH dosimetry measurements that can be read out with only an electrometer.
Methods: A square pyroelectric crystal (lead-zirconate-titanate; PZT) with a side length of 2.8 mm was mounted in a cylindrical Delrin holder. The holder reproducibly compresses the crystal between two electrodes. The pyroelectric crystal is a current source with a signal magnitude that is directly proportional to a change in temperature, hence a calorimeter. As an initial test, a calibrated thermistor was affixed to the crystal and the assembly was embedded in a copper block. The temperature of the copper was then modulated by exposing it to the heat of a halogen lamp, simulating the effect of a FLASH beam. Pyroelectric readout was performed with an electrometer (Keithley 6517A) in current mode, while thermistor resistance was measured with an 8.5-digit multimeter (HP 3458A).
Results: A set of 6000 data points were acquired spanning rates of temperature change from ± 8 mK/s, or 3.5 Gy/s in terms of equivalent dose rate. Pyroelectric current varied linearly with the time derivative of the thermistor temperature. The proportionality constant, which is the detector’s calibration coefficient, was measured to be 2.300(3) nC/K, a value consistent with the literature on PZT.
Conclusion: This proof of principle demonstrates the feasibility of operating a pyroelectric calorimeter with an electrometer for the purpose of absolute determination of absorbed dose rates greater than a few Gy/s, relevant for FLASH dosimetry.