Measuring the Gy – preventing convection in water calorimetry using a low duty cycle modulated beam

Published

October 1, 2011

Author(s)

Huaiyu H. Chen-Mayer, Ronald E. Tosh

Abstract

The most fundamental method for determining the absorbed dose to water by radiation beams is by measuring the consequent temperature increase of the water. Water calorimeters used for this purpose are not irradiated uniformly; thus, spatial dose gradients within the calorimeter give rise to significant heat transport effects that interfere with the measurement of the small temperature rise associated with radiotherapy doses (e.g. 240 uK per Gy at a typical dose rate of 1 Gy/min). When subject to periodic exposure to radiation, the calorimeter response (registered as temperature at a point in the water) reaches a steady-state oscillation in which effects due to heat conduction and convection may be studied as a function of modulation frequency, so that appropriate correction factors can be derived for desired operating conditions. Theoretical treatment of the behavior is greatly simplified if experimental conditions can be arranged so that convection is negligible. While this is usually done by refrigerating the calorimeter to 4 deg C, where the thermal expansion coefficient of water is zero (thus, buoyancy forces due to density fluctuations effectively vanish), we present evidence suggesting that such conditions can be achieved at room temperature by reducing the duty cycle, or effective “on” time, of the radiation at a given shutter frequency. Results are presented here in the form of system transfer functions obtained under conditions of 50% duty cycle and 3.5% duty cycle. Comparison of these measured transfer functions with the output of a 3- dimensional finite-element model indicates that convection is greatly suppressed in the lower duty- cycle case. Experimental uncertainty in these measurements is too large to conclude that convection is eliminated altogether, but more extended runs using this technique should enable a more definitive judgment as to whether the simpler conduction corrections alone can be applied.

Proceedings Title

2010 Symposium on Radiation Measurements and Applications.

Conference Dates

May 24-28, 2010

Conference Location

Ann Arbor, MI

Conference Title

Nuclear Instruments and Methods A

Pub Type

Conferences

Keywords

absorbed dose to water, water calorimetry, frequency domain analysis, heat conduction correction, convection, finite element modeling of heat transfer, system response, water calorimetry

Citation

Chen-Mayer, H. and Tosh, R. (2011), Measuring the Gy – preventing convection in water calorimetry using a low duty cycle modulated beam, 2010 Symposium on Radiation Measurements and Applications. , Ann Arbor, MI (Accessed April 25, 2024)