Advances in Water Calorimetry

The NIST water calorimetry effort has been directed toward accurate modeling and detailed measurement of time-varying phenomena within the calorimeter vessel – due principally to heat transfer – with the objective of developing primary standards that address metrological challenges posed by modern radiotherapy beams.  The work has had two primary thrusts: 

Figure 1: Continue work on a second-generation, Domen- type primary standard for standard reference ⁶⁰Co and high-energy x-ray beams.  Procedures developed for obtaining heat-transport correction factors for the second-generation, Domen-type instrument have been tested successfully under a variety of irradiation conditions in ⁶⁰Co beams.  Comparison tests of this instrument with ionization chambers calibrated via the original water calorimeter have shown agreement to within 0.3 %, which is well within the experimental uncertainty of ± 0.47 % associated with the historical value.  Further tests are being conducted in both ⁶⁰Co and high-energy x-ray beams provided by our Clinac 2100C medical accelerator.

Figure 2: Develop a high-precision thermometry system for water based on ultrasonic time-of-flight technology that would leverage the imaging capabilities of ultrasound to enable dosimetry of time-varying, 3D dose fields in water from arbitrary types of beams. An ultrasonic thermometry technology, developedby Luna Innovations as part of a phase-II SBIR contactct with NIST, has progressed from a single-channel implementation capable of resolving radiation-induced average temperature changes with microkelvin-level precision along a 30 cm diameter of a water phantom to a multi-channel array instrument that uses tomographic reconstruction to obtain time-resolved, 2D temperature distributions with a spatial linear resolution of better than 5 mm.  Preliminary studies with the multi-channel instrument yielded sub-millikelvin resolution of temperature changes induced in water by radiation from a heat lamp.  Because it is based on the same technology used in the single-channel prototype, much higher temperature resolution (microkelvin) is expected to be achievable.  The multi-channel array was recently acquired by NIST so that further research may be conducted to investigate its feasibility for use in radiation dosimetry.