An Experimental Method for Determining the Heat Defect of Water Using Total Absorption of High-energy Electrons

Abstract

The experimental procedure described here in detail enables the heat defect of water caused by electron radiation in the energy range from 1 MeV to 5 MeV to be determined with an overall uncertainty of less than 0,5 Yo for the correction factor. The principle of measurement is based on a comparison of the temperature increase caused by the radiation energy of electrons totally absorbed in water with that caused by an ohmic heating of the water. In measuring the radiant energy of the electrons, corrections are needed for backscatter of the electrons entering the water and for bremsstrahlung energy loss during electron slowing down. Precise knowledge of the heat defect is a prerequisite for the establishment of the water absorbed-dose calorimeter as a primary standard of absorbed dose to water.

A sealed water calorimeter for measuring absorbed dose

“…The water is to be contained in clean glass containers, and sealed from the atmosphere [16,17]. It is also to be prepared in such a way as to cause an essentially zero heat defect [16][17][18][19][20], or to determine if a particular type of heat defect could be reproduced and accurately corrected [20]. These studies in the present experiment required saturating the HPW with high purity gases.…”

An Experimental Method for Measuring the Heat Defect of Water Using Total Absorption of Soft X-Rays

“…These conditions are characterized by a minimum preirradiation of the water corresponding to an absorbed dose of about 50 Gy, highly purified degassed water in contact only with metal and mylar and irradiated by soft radiation with generating voltages between 17 kV and 30 kV. Considering the uncertainties stated in [6] and those given here, any energy dependence of the heat defect must be well below 1 O/O.…”

“…The establishment of the water absorbed dose calorimeter as a basis for dosimetry in radiation therapy requires a detailed knowledge of the heat defect of water for all types and qualities of radiation applied in this field. For low-LET radiation under specified conditions a zero heat defect has been determined [6].…”

“…absorption of high-energy electrons has been described in this journal [6]. Soft X-radiation with generating voltages to about 30 kV is also well suited for a total absorption experiment due to the small attenuation length of the radiation in water.…”

Development and application of absorbed dose primary standard for 60Co and high-energy photon beams using water calorimetry

“…k HD corrects for the heat defect. In this work, the glass vessels were filled with high-purity water saturated with H 2 or N 2 , to minimize the radio-chemical reactions; hence, this correction factor was taken as unity [34]. Before the calorimetric experiments, each glass vessel (containing two thermistors) was irradiated at a reference point with a dose of ∼100 Gy, to remove the initial O 2 concentration and ensure that the system reached a stationary state with a zero heat defect.…”