Study of the PTW microLion chamber temperature dependence.
The use of liquid ionization chambers in radiotherapy has grown during the past few years. While for air ionization chambers the k(TP) correction for air mass density due to pressure and temperature variations is well known, less work has been done on the case of liquid ionization chambers, where there is still the need to take into account the influence of temperature in the free ion yield. We have measured the PTW microLion isooctane-filled ionization chamber temperature dependence in a ~ ±10 °C interval around the standard 20 °C room temperature for three operation voltages, including the manufacturer recommended voltage, and two beam qualities, (60)Co and 50 kV x-rays. Within the measured temperature range, the microLion signal exhibits a positive linear dependence, which is around 0.24% K(-1) at 800 V with (60)Co irradiation. This effect is of the same order of magnitude as the T dependence found in air ionization chambers, but its nature is completely different and its sign opposite to that of an air chamber. Onsager theory has been used to model the results and is consistent with this linear behaviour. However, some inconsistencies in the modelling of the 50 kV x-ray results have been found that are attributed to the failure of Onsager's isolated pair assumption for such radiation quality.
Departamento de Física de Partículas, Universidade de Santiago de Compostela, E-15782-Santiago de Compostela, Spain. Radiation Physics Laboratory, Universidade de Santiago de Compostela, E-15782-Santiago de Compostela, Spain.
F
F
Gómez
D
D
González-Castaño
P
P
Díaz-Botana
J
J
Pardo-Montero
D011874:Radiometry / Q000295:instrumentation*; D013696:Temperature