At present, NIST’s primary standard for absorbed dose (SI unit is the gray, Gy) is water calorimetry: Typically, a radioactive cobalt-60 source irradiates a volume of water, and the energy received is calculated from the rise in water temperature as measured by immersed thermistors to determine an estimated depth-dose profile.
However, there is an urgent and growing interest in making dosimetry measurement at smaller scale. There is currently no method for direct measurement of radiation dose in those dimensions – a capability that is needed for important medical and industrial applications such as microbeam therapy, microelectronics, and cellular dosimetry.
The NIST on a Chip program is researching a solution based on commercial silicon chip fabrication and telecommunications technology. The goal is micro-scale calorimetry with photonic thermometers – in the form of fiber Bragg gratings or silicon ring resonators -- embedded in a radiation-resistant substrate. These could be arranged to perform real-time dose measurements in a phantom, and eventually even in vivo.
The technology offers the possibility of increased sensitivity, spatial resolution, optical readout, and multiplexing capabilities. Eventually, it could redefine the meaning of “dose,” reduce dependence on Co-60 sources, enable new portable sensors, and help close the loop on quantitative nuclear medicine. One of the challenges would be to independently measure the response of thousands of photonic thermometers through a single fiber optic interface.
NIST research into fabricating sensors that can withstand exposure to ionizing radiation is also of great interest for device use in harsh environments, such as space or energy-generation.
Recently, a collaborative research project between the Radiation Physics Division and Sensor Science Division has begun measuring the impact of ionizing radiation on the performance of silicon photonic devices. In the first round of testing, the team irradiated chips with up 1 MGy of gamma radiation and 250 kGy of beta radiation—10,000 times higher than medical radiation treatment levels--- with little to no damage to the photonic devices. These results suggest that baseline drift in individual experiments is negligible.