Free ion yield observed in liquid isooctane irradiated by rays. Comparison with the Onsager theory

Recombination in liquid-filled ionization chambers beyond the Boag limit

PURPOSE

The high mass density and low mobilities of charge carriers can cause important recombination in liquid-filled ionization chambers (LICs). Saturation correction methods have been proposed for LICs. Correction methods for pulsed irradiation are based on Boag equation. However, Boag equation assumes that the charge ionized by one pulse is fully collected before the arrival of the next pulse. This condition does not hold in many clinical beams where the pulse repetition period may be shorter than the charge collection time, causing overlapping between charge carriers ionized by different pulses, and Boag equation is not applicable there. In this work, the authors present an experimental and numerical characterization of collection efficiencies in LICs beyond the Boag limit, with overlapping between charge carriers ionized by different pulses.

METHODS

The authors have studied recombination in a LIC array for different dose-per-pulse, pulse repetition frequency, and polarization voltage values. Measurements were performed in a Truebeam Linac using FF and FFF modalities. Dose-per-pulse and pulse repetition frequency have been obtained by monitoring the target current with an oscilloscope. Experimental collection efficiencies have been obtained by using a combination of the two-dose-rate method and ratios to the readout of a reference chamber (CC13, IBA). The authors have also used numerical simulation to complement the experimental data.

RESULTS

The authors have found that overlap significantly increases recombination in LICs, as expected. However, the functional dependence of collection efficiencies on the dose-per-pulse does not change (a linear dependence has been observed in the near-saturation region for different degrees of overlapping, the same dependence observed in the nonoverlapping scenario). On the other hand, the dependence of collection efficiencies on the polarization voltage changes in the overlapping scenario and does not follow that of Boag equation, the reason being that changing the polarization voltage also affects the charge collection time, thus changing the amount of overlapping.

CONCLUSIONS

These results have important consequences for saturation correction methods for LICs. On one hand, the two-dose-rate method, which relies on the functional dependence of the collection efficiencies on dose-per-pulse, can also be used in the overlapping situation, provided that the two measurements needed to feed the method are performed at the same pulse repetition frequency (monitor unit rate). This result opens the door to computing collection efficiencies in LICs in many clinical setups where charge overlap in the LIC exists. On the other hand, correction methods based on the voltage-dependence of Boag equation like the three-voltage method or the modified two-voltage method will not work in the overlapping scenario due to the different functional dependence of collection efficiencies on the polarization voltage.

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.

Initial recombination in the track of heavy charged particles: numerical solution for air filled ionization chambers

INTRODUCTION

Modern particle therapy facilities enable sub-millimeter precision in dose deposition. Here, also ionization chambers (ICs) are used, which requires knowledge of the recombination effects. Up to now, recombination is corrected using phenomenological approaches for practical reasons. In this study the effect of the underlying dose distribution on columnar recombination, a quantitative model for initial recombination, is investigated.

MATERIAL AND METHODS

Jaffé's theory, formulated in 1913 quantifies initial recombination by elemental processes, providing an analytical (closed) solution. Here, we investigate the effect of the underlying charged carrier distribution around a carbon ion track. The fundamental partial differential equation, formulated by Jaffé, is solved numerically taking into account more realistic charge carrier distributions by the use of a computer program (Gascoigne 3D). The investigated charge carrier distributions are based on track structure models, which follow a 1/r(2) behavior at larger radii and show a constant value at small radii. The results of the calculations are compared to the initial formulation and to data obtained in experiments using carbon ion beams.

RESULTS

The comparison between the experimental data and the calculations shows that the initial approach made by Jaffé is able to reproduce the effects of initial recombination. The amorphous track structure based charge carrier distribution does not reproduce the experimental data well. A small additional correction in the assessment of the saturation current or charge is suggested by the data.

CONCLUSION

The established model of columnar recombination reproduces the experimental data well, whereas the extensions using track structure models do not show such an agreement. Additionally, the effect of initial recombination on the saturation curve (i.e. Jaffé plot) does not follow a linear behavior as suggested by current dosimetry protocols, therefore higher order corrections (such as the investigated ones) might be necessary.

Development of a guarded liquid ionization chamber for clinical dosimetry

Liquid ionization chambers are considered superior to air-filled chambers in terms of size, energy dependence and perturbation effects. We constructed and tested a liquid ionization chamber for clinical dosimetry, the GLIC-03, with a sensitive volume of approximately 2 mm3. We also examined two methods to correct for general ion recombination in pulsed photon beams: that of Johansson et al, which modifies Boag's theory for recombination in gases, and an empirical method relating recombination to dose per pulse. The second method can be used even in cases where the first method is not applicable. The response of the GLIC-03 showed a stable, linear and reproducible decrease of 1% over 10 h. The liquid-filled GLIC-03 had a 1.1 +/- 0.4% energy dependence while that of the air-filled GLIC-03 was 2.1 +/- 0.3% between the 6 and 18 MV beams from a Clinac 21EX. The two methods for recombination correction agreed within 0.2% for measurements at 18 MV, 700 V, 100 MU min(-1). Measurements with the GLIC-03 in Solid Water in the build-up region of an 18 MV beam agreed with extrapolation chamber measurements within 1.4%, indicating that the GLIC-03 causes minimal perturbation.

Development and operation of a pixel segmented liquid-filled linear array for radiotherapy quality assurance

A liquid isooctane (C(8)H(18)) filled ionization linear array for radiotherapy quality assurance has been designed, built and tested. The detector consists of 128 pixels, each of them with an area of 1.7 mm x 1.7 mm and a gap of 0.5 mm. The small pixel size makes the detector ideal for high gradient beam profiles such as those present in intensity modulated radiation therapy (IMRT) and radiosurgery. As the read-out electronics we use the X-ray Data Acquisition System with the Xchip developed by the CCLRC. Studies concerning the collection efficiency dependence on the polarization voltage and on the dose rate have been made in order to optimize the device operation. In the first tests, we have studied dose rate and energy dependences. Dose rate dependence was found to be lower than 2.1% up to 5 Gy min(-1), and energy dependence lower than 2.5% up to 20 cm depth in solid water. Output factors and penumbras for several rectangular fields have been measured with the linear array and were compared with the results obtained with a 0.125 cm(3) air ionization chamber and radiographic film, respectively. Finally, we have acquired profiles for an IMRT field and for a virtual wedge. These profiles have also been compared with radiographic film measurements. All the comparisons show a good correspondence. The device has proved its capability to verify on-line therapy beams with good spatial resolution and signal-to-noise ratio.