  The aim of this study was to optimize the design and performance of parallel
plate ion chambers for use in ultra-high dose rate (UHDR) dosimetry
applications, and evaluate their potential as reference class chambers for
calibration purposes. Three chambers were designed and produced: the A11-VAR
(0.2-1.0 mm electrode gap, 20 mm diameter collector), the A11-TPP (0.3 mm
electrode gap, 20 mm diameter collector), and the A30 (0.3 mm electrode gap,
5.4 mm diameter collector).The chambers underwent full characterization using
an UHDR 9 MeV electron beam with individually varied beam parameters of pulse
repetition frequency (PRF, 10-120Hz), pulse width (PW, 0.5-4us), and pulse
amplitude (0.01-9 Gy/pulse). The response of the ion chambers was evaluated as
a function of the dose per pulse (DPP), PRF, PW, dose rate, electric field
strength, and electrode gap. The chamber response was found to be dependent on
DPP and PW, whose dependencies were mitigated with larger electric field
strengths and smaller electrode spacing. At a constant electric field strength,
we measured a larger charge collection efficiency (CCE) as a function of DPP
for ion chambers with a smaller electrode gap in the A11-VAR. For ion chambers
with identical electrode gap (A11-TPP and A30), higher electric field strengths
were found to yield better CCE at higher DPP. A PW dependence was observed at
low electric field strengths (500 V/mm) for DPP values ranging from 1-5 Gy at
PWs ranging from 0.5-4 {\mu}s, but at electric field strengths of 1000 V/mm and
higher, these effects become negligible. This study confirmed that the charge
collection efficiency of ion chambers depends strongly on the electrode spacing
and the electric field strength, and also on the DPP and the PW of the UHDR
beam. The new finding of this study is that the PW dependence becomes
negligible with reduced electrode spacing and increased electric field.
