Air density correction in ionization dosimetry

Abstract

Air density must be taken into account when ionization dosimetry is performed with unsealed ionization chambers. The German dosimetry protocol DIN 6800-2 states an air density correction factor for which current barometric pressure and temperature and their reference values must be known. It also states that differences between air density and the attendant reference value, as well as changes in ionization chamber sensitivity, can be determined using a radioactive check source. Both methods have advantages and drawbacks which the paper discusses in detail. Barometric pressure at a given height above sea level can be determined by using a suitable barometer, or data downloaded from airport or weather service internet sites. The main focus of the paper is to show how barometric data from measurement or from the internet are correctly processed. Therefore the paper also provides all the requisite equations and terminological explanations. Computed and measured barometric pressure readings are compared, and long-term experience with air density correction factors obtained using both methods is described.

Examining the influence of humidity on reference ionization chamber performance

“…The relation between RH and water content has been investigated in detail. 6,11 As can be seen in Fig. 1 of ref.…”

“…RH is, as the name indicates, a relative quantity and is not a direct measure of the amount of water in the air. The relation between RH and water content has been investigated in detail . As can be seen in Fig.…”

“…[1][2][3][4][5] However, as noted by Poirier and Douysset, 6 the experimental data on which this recommendation is based are rather limited, decades old, and were acquired using detectors that do not bear much resemblance to modern reference-class ionization chambers. [7][8][9][10] That is not to say that the conclusions of those investigations are not valid, but it does serve to point out that the effect of humidity is perhaps the 'forgotten cousin' of temperature and pressure, influence quantities which have been discussed at length (e.g., Christ et al 11 ).…”

“…This ‘constant’ has an approved consensus value for dry air but shows a small variation with the relative humidity . Christ et al have also shown that the standard corrections for temperature and pressure ( P TP in AAPM TG‐51 terminology) are also affected by the water content of the air. The environmental conditions at the time of any ionization chamber measurement therefore impact the dose measurement.…”

Addendum to the AAPMˈs TG‐51 protocol for clinical reference dosimetry of high‐energy photon beams

“…A working definition of equilibrium is that the drift in the water phantom temperature is less than 1 • C/h. Christ et al 68 give much detail on the correction for air density. For megavoltage photon beams, problems with the pressure correction for low-energy photons [69][70][71] are not significant.…”