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.
Accordingly, the Dosimetry Group is exploring the use of ultrasonic and optical probes for extracting and reconstructing thermal distributions in water for dosimetric applications. With the acquisition in 2011 of the 2D circular ultrasonic array developed by Luna Innovations, LLC under a NIST SBIR contract, development work and testing of its thermal imaging capabilities could continue. Improvements in hardware (use of differential amplifiers and expanded scanning capabilities) resulted in a dramatic reduction in noise and enhancement of its spatial resolution, as evidenced by images obtained of a spot from a focused heat lamp (left). Subsequent testing in electron beams led to our first ultrasonic imaging of absorbed dose fields in a water phantom due to 1) heating by a 1.6 MeV electron beam from the NIST Van de Graaff accelerator, passed through various apertures (middle), and 2) cooling by a convective plume resulting from a 12 MeV Clinac electron beam, demonstrating its potential for capturing time evolution of radiation-induced thermal phenomena in a water phantom (right). The detection principle employed in the instrument used to acquire these images exploits temperature dependence of the speed of sound to measure small changes in time-of-flight of ultrasonic pulses. Such phenomena are also suitable for detection by highly sensitive interferometric means, and this is being explored with both ultrasonic and optical probe radiations. A short study conducted this summer with a student intern verified much higher thermal sensitivity of optical interferometry (as compared to ultrasonic interferometry), thereby motivating further work on developing an imaging instrument that would use an optical probe for ultra-high sensitivity thermometry suitable for radiation dosimetry