A large area calorimeter was developed at the National Physical Laboratory (NPL)

to determine the dose-area-product from irradiation using proton and small field

photon beams less than 4 cm diameter. This thesis describes the first measurements

carried out with a dose-area-product calorimeter in proton pencil beams as

well as an analysis of the calorimeter data in small field photon beams. In doing

this analysis, a greater understanding of the response and heat transfer within

the calorimeter was achieved. These measurement results are compared to simulations

carried out using COMSOL Multiphysics®, a finite element simulation

software package. A model of the calorimeter was built in TOPAS and EGSnrc

Monte Carlo platforms for radiation transport simulations. These models were

used to calculate two correction factors: the gap correction factor and the impurity

correction factors using optimised beam models of both the NPL Elekta

Agility linear accelerator and the OncoRay research fixed proton beamline.

Calibration factors were determined and validated, showing promising results.

The calibration factor for the PTW T34073 ionization chamber was found to be

1.625 x 10⁸ Gy cm2/C, with a beam quality correction factor, k_Q.Q0 , of 1.0183 from

literature and 0.9615 from experimental measurements, indicating a 5.9% difference.

Experiments at OncoRay and NPL revealed DAP differences of 3.9% and

0.8% when compared to film dosimetry and similar devices, respectively. Beam

quality correction factors for photons showed a 1.5% difference between experimental

(0.970) and literature (0.985) values. Monte Carlo simulations quantified

correction factors, revealing that k_gap increased with larger air gaps and smaller

field sizes, while k_imp was negligible for small-field photon beams. These findings

confirm the calorimeter’s accuracy, with potential for further development to

enhance its precision and utility in clinical dosimetry.