Proof-of-Principle of Absolute Dosimetry Using an Absorbed Dose Portable Calorimeter with Laser-Driven Proton Beams  
[PERSON]  
1 Centre for Light-Matter Interactions, Queen's University Belfast, Belfast BT7 1 NN, UK  
such as the laser-driven approach, which typically leads to pulse dose rates several orders of magnitude higher than the threshold characterised as the FLASH regime [7; 8; 9]. The laser-driven acceleration of protons has successfully resulted in the generation of beams with energies lying within the range of interest for medical applications [10; 11; 12], with recent demonstrations of the generation of proton beams up to \(\sim\)100 MeV [13]. Since laser-driven proton beams can reach dose rates in excess of \(10^{9}\) Gy/s, dosimetry becomes increasingly complicated, with conventional detectors, such as ionisation chambers, often unusable because of ion recombination effects [14; 15; 16], which leads to increased levels of uncertainty. Furthermore, for laser-driven beams, additional complications arise from the harsh experimental environment, where a large electromagnetic pulse (EMP) can be generated during the laser-target interaction, in addition to non-ideal beam parameters, such as beam inhomogeneity and wide energy spread. For potential applications, accurate dosimetric methods and new approaches must be investigated [17; 18; 19]. Despite the significant metrological challenges faced when performing dosimetry measurements of UHDR beams, potential solutions have been proposed, and a number of these have already been explored. This includes dosimetry based on the use of Faraday cups [20; 21] and ionisation chambers, although the latter of these detectors cannot be used satisfactorily due to recombination effects [22]. Passive detectors such as radiochromic films also have an extensive history of use in the measurement of laser-accelerated beams [23]. Furthermore, a number of measurements have been demonstrated and reported using calorimeters, including in the dosimetry of very high energy electron (VHEE) beams to characterise ion collection losses in ionisation chambers [18]. The National Research Council Canada (NRC) also developed a portable calorimeter that has been tested with UHDR electron beams, aiming to eliminate inhomogeneities in the absorber material by constructing this of pure aluminium [24]. Additionally, a Graphite Probe Calorimeter (GPC) known as 'Aerrow' [25; 26] was built with a waterproof design, allowing its use in a conventional water phantom. The Aerrow has so far been utilised within high-energy photon and electron beams, suggesting its potential usage with ultra-high dose per pulse beams [24; 25; 26]. Furthermore, the ion recombination losses experienced in ionisation chambers were addressed through the development of a prototype secondary standard calorimeter [27]. Developed at the NPL, this device utilises a single sensing core thermistor encased within a 3D-printed body mimicking the shape of a standard Roos-type chamber. Calorimeters have several advantages which make them ideally suited for UHDR dosimetry. They can provide instant readout, have a dose-rate independent response and allow for direct determination of the absorbed dose from fundamental principles. Calorimeters rely on measuring the radiation-induced temperature rise in an absorber material as the initial energy of the impinging particle degrades to heat. Through knowledge of the material properties, including the specific heat capacity of the absorber, the rise in temperature can be converted to a measure of the absorbed dose. Although various formats and applications are available, graphite calorimeters have an already well-established usage as primary standard level instruments for conventional radiotherapy treatments [28; 29; 30; 31; 32; 33]. Calorimetry techniques have been tested with proton beams at dose rates up to 60 Gy/s using clinical scanning beams [34; 33], but have so far not been employed to conduct measurements of the absorbed dose generated by laser-driven proton beams at UHDRs.