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Calorimetry based upon remote sensing of the temperature field in an irradiated volume would offer distinct advantages over the present approach, employing thermistor probes, for the dosimetry of nonstandard beams used routinely for cancer treatment and, more generally, for spatial mapping of dose distributions within matter.
Temperature dependence of both the speed of sound and the index of refraction of water are being exploited for purposes of measuring heating of water by therapy-level radiation fields, enabling standoff detection of radiation-induced temperature rise, hence absorbed dose, by application of ultrasonic and laser probes, respectively. The project got underway ca. 2006 with the development of a single-transducer, pulse-echo system employing phase-sensitive detection, which evolved a few years later into a circular imaging array that has been used to image heating effects in water subjected to high-dose radiation from a VdG and therapy-level radiation from a Clinac 2100C medical accelerator, both at NIST. Since then, efforts to realize similar precision using more widely available commercial instrumentation have led to the development of techniques based on continuous-wave and pulsed time-of-flight that are capable both of detecting sub-mK changes in temperature, typical of therapy-level dose, and of being deployed in multi-source/multi-receiver arrays that would enable more rapid image acquisition than is currently possible with the existing imaging array. In 2014, a dual-channel 5 MHz ultrasonic system was used to obtain points on depth-dose profiles from 16 MeV therapy beams in water (beam width was varied, and results obtained at two depths were found to be consistent within experimental uncertainties and exhibited good agreement with depth-dose profiles obtained by scanning a calibrated ionization chamber). In 2015, work began on the feasibility of using pseudo-random bit sequences for modulating the ultrasonic carrier, as their orthogonality properties are widely exploited in communications engineering for multi-source/receiver systems.
A parallel effort using laser interferometry is also underway, after initial measurements demonstrated that it could be an order-of-magnitude more sensitive than ultrasound for detecting radiation-induced temperature changes in a typical water phantom under reference conditions. Initial work with a simple Michelson setup was used in 2013 to measure temperature rise in water subjected to multiple 10-minute, 4 Gy/min exposures to 12 MeV electron beams. Instabilities in the interferometer obscured detection of individual exposures in that work, but in 2014 a more stable, Sagnac-type cyclic interferometer was used with a PMMA block as target, and similar pulses were subsequently resolved. Because the cyclic arrangement leaves little room for introducing a sample, work in 2015 has focused on a sturdy Mach-Zehnder configuration employing periscope assemblies mounted on 1” steel rods and digital cameras.