Absolute x-ray dosimetry on a synchrotron medical beam line with a graphite calorimeter

• Published in: Medical physics (Lancaster) 2014-05, Vol.41 (5), p.052101-n/a
• Main Authors: Harty, P. D., Lye, J. E., Ramanathan, G., Butler, D. J., Hall, C. J., Stevenson, A. W., Johnston, P. N.
• Format: Article
• Language: English
• Subjects: ABSORBED RADIATION DOSES; Air; Algorithms; Biological material, e.g. blood, urine; Haemocytometers; biomedical equipment; Biomedical instrumentation and transducers, including micro‐electro‐mechanical systems (MEMS); CALORIMETERS; CALORIMETRY; Calorimetry - instrumentation; Devices sensitive to very short wavelength, e.g. x‐rays, gamma‐rays or corpuscular radiation; diagnostic radiography; DOSE RATES; DOSEMETERS; dosimetry; Dosimetry/exposure assessment; Error analysis; Field size; filters; free‐air ionization chamber; GRAPHITE; graphite calorimeter; Measuring temperature; Measuring quantity of heat; Thermally‐sensitive elements not otherwise provided for; Medical imaging; MONTE CARLO METHOD; Monte Carlo methods; Monte Carlo simulations; Non‐adjustable resistors formed as one or more layers or coatings; Non‐adjustable resistors made from powdered conducting material or powdered semi‐conducting material with or without insulating material; Photons; Pressure; protocols; Radiation Dosage; RADIATION PROTECTION AND DOSIMETRY; radiation therapy; Radiation therapy equipment; Radiography; RADIOLOGY AND NUCLEAR MEDICINE; Radiometry - methods; RADIOTHERAPY; Scintigraphy; SPECIFIC HEAT; synchrotron medical beam line; synchrotron radiation; SYNCHROTRONS; Synchrotrons - instrumentation; Temperature; Therapeutic applications, including brachytherapy; THERMISTORS; Transforming x‐rays; Uncertainty; Water; Water vapor; X-RAY DOSIMETRY; X-Rays; X‐ray apparatus; X‐ray technique
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.4870387

Purpose: The absolute dose rate of the Imaging and Medical Beamline (IMBL) on the Australian Synchrotron was measured with a graphite calorimeter. The calorimetry results were compared to measurements from the existing free-air chamber, to provide a robust determination of the absolute dose in the synchrotron beam and provide confidence in the first implementation of a graphite calorimeter on a synchrotron medical beam line. Methods: The graphite calorimeter has a core which rises in temperature when irradiated by the beam. A collimated x-ray beam from the synchrotron with well-defined edges was used to partially irradiate the core. Two filtration sets were used, one corresponding to an average beam energy of about 80 keV, with dose rate about 50 Gy/s, and the second filtration set corresponding to average beam energy of 90 keV, with dose rate about 20 Gy/s. The temperature rise from this beam was measured by a calibrated thermistor embedded in the core which was then converted to absorbed dose to graphite by multiplying the rise in temperature by the specific heat capacity for graphite and the ratio of cross-sectional areas of the core and beam. Conversion of the measured absorbed dose to graphite to absorbed dose to water was achieved using Monte Carlo calculations with the EGSnrc code. The air kerma measurements from the free-air chamber were converted to absorbed dose to water using the AAPM TG-61 protocol. Results: Absolute measurements of the IMBL dose rate were made using the graphite calorimeter and compared to measurements with the free-air chamber. The measurements were at three different depths in graphite and two different filtrations. The calorimetry measurements at depths in graphite show agreement within 1% with free-air chamber measurements, when converted to absorbed dose to water. The calorimetry at the surface and free-air chamber results show agreement of order 3% when converted to absorbed dose to water. The combined standard uncertainty is 3.9%. Conclusions: The good agreement of the graphite calorimeter and free-air chamber results indicates that both devices are performing as expected. Further investigations at higher dose rates than 50 Gy/s are planned. At higher dose rates, recombination effects for the free-air chamber are much higher and expected to lead to much larger uncertainties. Since the graphite calorimeter does not have problems associated with dose rate, it is an appropriate primary standard detector for the synchrotron