Steve R. Domen

Absorbed dose water calorimeter

Advantage was taken of the low thermal diffusivity of water and the imperviousness of polyethylene film to water to construct a calorimeter for directly measuring absorbed dose in that medium. An ultrasmall bead thermistor was sandwiched between two thin films stretched on polystyrene rings and immersed in an unregulated water bath. Ten cobalt-60 irradiation runs were made with a precision of 0.5% mean error of the mean at a dose rate of 66 mGy/s. Further development is directed toward a standard instrument that can be used in a medical therapy beam.

Comparison of dosimetry calibration factors at the NRCC and the NIST

In early 1998, three transfer ionization chambers were used to compare the air-kerma and absorbed-dose-to-water calibration factors measured by the National Research Council of Canada (NRCC) and the National Institute of Standards and Technology (NIST). The ratios between the NRCC and NIST calibration factors are 0.9950 and 1.0061 in the case of the absorbed-dose-to-water and air-kerma standards, respectively. In the case of the standard of absorbed dose to water, the combined uncertainty of the ratio between the standards of the two laboratories is about 0.6% and consequently, the observed difference of 0.5% is not significant at the one sigma level. In the case of the standard of air kerma, the combined uncertainty of the ratio between the standards of the two laboratories is about 0.4%, and so the observed difference of 0.61% is significant at the one sigma level. However, this discrepancy is due to the known differences in the methods of assessing the wall correction factor at the two laboratories. Taking into account changes implemented in the standards that form the basis of the calibrations, the present results are consistent with those of the previous comparison done in 1990/91. As a direct result of these differences in the calibration factors, changing from an air-kerma based protocol following TG-21 to an absorbed-dose-to-water based protocol following TG-51, would alter the relationship between clinical dosimetry in Canada and the United States by about 1%. For clinical reference dosimetry, the change from TG-21 to TG-51 could result in an increase of up to 2% depending upon the ion chamber used, the details of the protocol followed and the source of traceability, either NRCC or NIST.

Comparisons of calorimetric and ionometric measurements in graphite irradiated with electrons from 15 to 50 MeV

Extensive experimental comparisons of calorimetric and ionometric measurements have been made that cover a broader range of electron energies and depths in graphite than previously reported. Electron beams of 15, 20, 25, 30, 40, and 50 MeV were used. Calorimetric absorbed-dose measurements and ionometric specific-charge measurements in air were compared in graphite at depths from 1 to 51 g/cm2. The medium was irradiated with uncollimated electron beams produced by scattering after passing through a 0.1-g/cm2 aluminum vacuum window, various thicknesses of lead foils, and air. The variation in the quotient of the two measurements was studied as a function of lead-foil thickness, depth in the medium, beam energy, foil-to-detector distance, and off-axis distance. These studies permitted the measurements to be corrected and compared with theoretical calculations that assume a broad medium irradiated with broad, parallel, monoenergetic electron beams. The overall experimental uncertainty is estimated to be 1%. The results are generally in good agreement with theoretical and experimental results of other investigators. The calorimeter received close to 1 Mrad during preliminary measurements and from 1 to 2 Mrad during the measurements reported. The results showed no detectable heat defect in graphite after prolonged periods of exposing the calorimeter to air at atmospheric pressure.

A polystyrene absorbed-dose-rate calorimeter

A simple portable calorimeter was constructed by embedding a calibrated thermistor on the axis of a polystyrene rod. This instrument was positioned on the axis of a cylindrical array of 60 Co rods. The duration of the measurements was ∼ 70 s. Absorbed dose rates of ∼ 70 Gy/min were measured to a daily precision of several tenths of a percent standard deviation. The results are in good agreement with earlier measurements made with a graphite calorimeter. An absorbed dose of 620 kGy resulted in no detectable change is thermistor sensitivity. The specific heat capacity of the polystyrene presumably increased ∼ 0.0024%/kGy. The use of a chart recorder was not necessary.

A-150 plastic radiometric calorimeter for charged particles and other radiations

Abstract A local absorbed dose calorimeter with certain novel features has been designed and constructed of A-150 tissue-equivalent (TE) plastic. The radiation absorption properties of this material and the relative absence of core impurities make the calorimeter suitable for use in a wide variety of radiation beams. The low thermal diffusivity of A-150 plastic led to the development of a unique spiral electrical calibration heater that has a mass of only 0.1% that of the core. The calorimeter can be calibrated in the quasiadiabatic or the heat-loss-compensated mode to test for possible effects caused by temperature gradients. The details of construction and operation are described.

A Polystyrene-Water Calorimeter

A new type of calorimeter consisting of polysterene discs in water is desrcibed for the measurement of absorbed dose in polystyrene, which is reported (for some irradiation conditions) to have a heat defect of <1%. This calorimeter provides another investigative tool for comparison with absorbed dose measurements in water, which is the standard reference material. Measurements using 60Co γ-radiation showed that converting the results to water agreed within 1% with similarly determined results when using a graphite calorimeter. These results are ∼3% lower than those determined with a water calorimeter, suggesting a negative heat defect in water ∼3%-when measuring an absorbed dose rate ∼1 Gy/min that produced an accumulated dose up to 150 Gy.

A temperature-drift balancer for calorimetry

Abstract Calorimeters used in radiation measurements often suffer from errors due to thermal drifts at positions of the heat sensors in a Wheatstone bridge. A simple resistance-capacitance circuit can be connected across a Wheatstone bridge in order to balance recorder pen drifts due to gradually changing temperature gradients that often occur in calorimeters.

Emissivity of aluminized mylar

Abstract Commercially-available aluminized Mylar, 6 ωm thick, is a useful material for reducing heat losses by thermal radiation in some calorimeters, such as those constructed of graphite or A-150 plastic. The present experiment shows that clean aluminized layers have a thermal emissivity near 0.044.