[en] An absorbed dose water calorimeter that takes advantage of the low thermal diffusivity of water and the water-imperviousness of polyethylene film. An ultra-small bead thermistor is sandwiched between two thin polyethylene films stretched between insulative supports in a water bath. The polyethylene films insulate the thermistor and its leads, the leads being run out from between the films in insulated sleeving and then to junctions to form a wheatstone bridge circuit. Convection barriers may be provided to reduce the effects of convection from the point of measurement. Controlled heating of different levels in the water bath is accomplished by electrical heater circuits provided for controlling temperature drift and providing adiabatic operation of the calorimeter. The absorbed dose is determined from the known specific heat of water and the measured temperature change

[en] Ionization measurements along the central axis were made in a graphite phantom irradiated with cobalt-60 gamma rays. The measurements were made under the following conditions: phantom diameters of 15, 20, and 30 cm; 15 depths from 1 to 39 g/sq cm; and square field sizes of 8.3, 10.5, 12.4, and 17.4 cm at a fixed detector position of 1 m from the source. Empirical fits to the data aid in correcting calorimeter comparisons to a common geometry

[en] Advantages 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. Further development is directed toward a standard instrument that can be used in a medical therapy beam

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[en] The NIST sealed water calorimeter is intended for direct measurement of absorbed dose to water. This calorimeter was used for a series of approximately 3,700 measurements to investigate the so-called heat defect, that is, anomalous endothermic or exothermic effects caused by dissolved gases. The three systems investigated were high-purity water saturated with N₂, H₂, and mixtures of H₂/O₂. The repeatability of the measurements of absorbed dose rates for the ⁶⁰Co teletherapy beam was studied with different water fillings and accumulated absorbed dose. Measurements with the H₂/O₂ system varied with accumulated absorbed dose. Based on the measurements and theoretical considerations, it appears that the H₂-saturated system is the best choice for eliminating the heat defect. Measurements with both the N₂- and H₂-saturated systems are in good agreement with those determined with a graphite and graphite-water calorimeter (for which there is no heat defect)

No abstract available

[en] In the course of an NBS absorbed-dose standards program, theoretical and experimental determinations have been made of the conversion factor C that relates the absorbed dose from electron beams to the ionization in an air cavity. The average medium/air stopping-power ratio - which is proportional to C - has been calculated as a function of the beam energy (1 to 60 MeV) and of the depth in a water phantom, and similar stopping-power ratios have also been obtained for other phantom materials (carbon, polystyrene, acrylic plastic, muscle). The conversion factor in graphite has been measured at energies between 15 and 50 MeV and depths between 0.9 and 51 g/cm² with the use of a calorimeter and a parallel-plate ionization chamber. These measurements were made with beams broadened by lead scattering foils with various thicknesses from 0.144 to 1.584 g/cm². The extrapolation of the results to zero scattering-foil thickness provided conversion factors that could be compared with theoretical C-values for broad, parallel, monoenergetic electron beams. The agreement was found to be close (mean difference of 0.3% and r.m.s. difference of 0.8%). Comparisons have also been made with other C-values found in the literature and recommended by medical physics organizations. The overall conclusion can be drawn that the conversion factor is known reliably at the 1% level of accuracy. (author)

[en] Extensive experimental comparisons of calorimetric and ionometric measurements have been made that cover a broader range of electrons 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 aluminium 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

[en] The main purpose of this chapter is to discuss the principles of new types of absorbed dose calorimeters along with advances in design, control, measurement techniques, and measurement results. Significant progress has been made in the evolution of the calorimeter, long viewed as a cumbersome instrument confined to a few laboratories, toward its development and wider use as a portable field instrument. In this chapter the basic ideas of absorbed dose calorimetry are described and discussed in their simplest form