In addition, other conventional active real-time detectors, such as solid-state detectors based on semiconductors, start to fail at UHDR. For this reason, passive dosimeters, such as alanine, radiochromic films, or luminescence dosimeters (TLDs and OSLDs), are mainly used under these conditions. However, the use of passive dosimeters is complex, very time-consuming, and usually lacks the traceability to primary standards. There is therefore a need for active real-time dosimeters and traceability. It is also desirable to use ionization chambers as a secondary standard under UHDR conditions.
To find a solution to this challenge, the role of the distance between the electrodes of vented ICs and its impact on ion recombination has been analyzed experimentally and by numerical simulation2. The results show that this parameter is the most relevant to achieve negligible recombination losses. Using a very small electrode distance of 0.25 mm in vented ICs (called ultra-thin ICs (UTIC)) leads to a charge collection efficiency higher than 99% for a 2.5 µs pulse of 5 Gy (Figure 1). Ionization chambers are therefore excellent candidates as secondary standard for reference dosimetry in UHDR beams because they are also waterproof, easy to handle and can be used according to the existing methodology in current codes of practice.
flashDiamond – the first detector for FLASH dosimetry
However, ICs are limited in spatial resolution due to the diameter of the sensitive volume, especially when measuring lateral dose distributions and in small fields.
A detector that is particularly well suited for these applications, which require high spatial resolution, is the well-established microDiamond detector by PTW (type T60019).It can also be used for electron radiation, including high dose per pulse (DPP) applications such as used in IOeRT3,4. Due to the high water equivalence of its sensitive volume in terms of effective atomic number, no conversion from ion dose to absorbed dose to water is necessary when determining depth dose curves for electron beams. This characteristic, along with its good stability of response as a function of the accumulated dose, made the microDiamond detector a promising detector for use in UHDR1 environments. However, the detector has shown to exhibit saturation behavior at UHDR pulsed electron beams5. A thorough investigation under ultra-high pulse dose rate (UHPDR) conditions using different samples and modified designs was performed by Marinelli et al.6 and Kranzer et al.7. These studies confirmed the observed saturation behavior (Figure 2) and investigated the influencing parameters.