The response of well chamber to pressure variations at high altitudes - a Monte Carlo study for ¹⁶⁹Yb source

Well-type ionization chambers traceable to national standards laboratory are routinely used to measure air-kerma strength of Brachytherapy sources. During measurements, a temperature and pressure correction factor (K_TP) factor is applied to account for the change of air density in chamber volume. Pressure, P is directly proportional to air density at a constant temperature, T (℃). P falls exponentially with height, h (m) and is given by the Barometric formula P=P₀ exp (-Mgh/R (T+273.15)), where P₀ is standard pressure (1013.2 mbar) at h=0, M is molecular mass (29 g/mol), g is acceleration due to gravity (9.8 m/s²), R is the universal gas constant (8.314 J/mol. ⁰K). For example, air density at Shimla (h=2276 m) is 0.94 kg/m³, which is about 78% of standard air density of 1.197 kg/m³