Volume recombination parameter in ionization chambers

Boutillon, M

doi:10.1088/0031-9155/43/8/005

Cited by 80 publications

(67 citation statements)

References 22 publications

(23 reference statements)

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Based on the conclusion of Palmans et al., that a cyclotron proton beam should be treated as a purely continuous beam, and assuming that this also applies to a scanned beam, the k s value was calculated according to the formula derived from Boag's theoretical model:

k_{s} = 1 + \frac{m^{2} d^{4}}{6 v V^{2}} \cdot I_{sat}

where

m^{2}

is a parameter representing volume recombination, as given by Boutillon et al.,

d

is the electrode spacing,

v

is the volume of the ionization chamber,

I_{italicsat}

is the ionization current, and

V

is the chamber voltage. The parameter

m^{2}

is a property of the gas in the cavity and it value depends on the recombination coefficient and on the mobility of negative and positive ions in the gas.…”

Section: Methodsmentioning

confidence: 99%

Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

et al. 2017

View full text Add to dashboard Cite

Purpose: To evaluate the effect on charge collection in the ionization chamber (IC) in proton pencil beam scanning (PBS), where the local dose rate may exceed the dose rates encountered in conventional MV therapy by up to three orders of magnitude. Methods:We measured values of the ion recombination (k s ) and polarity (k pol ) correction factors in water, for a plane-parallel Markus TM23343 IC, using the cyclotron-based Proteus-235 therapy system with an active proton PBS of energies 30-230 MeV. Values of k s were determined from extrapolation of the saturation curve and the Two-Voltage Method (TVM), for planar fields. We compared our experimental results with those obtained from theoretical calculations. The PBS dose rates were estimated by combining direct IC measurements with results of simulations performed using the FLUKA MC code. Values of k s were also determined by the TVM for uniformly irradiated volumes over different ranges and modulation depths of the proton PBS, with or without range shifter. Results: By measuring charge collection efficiency versus applied IC voltage, we confirmed that, with respect to ion recombination, our proton PBS represents a continuous beam. For a given chamber parameter, e.g., nominal voltage, the value of k s depends on the energy and the dose rate of the proton PBS, reaching c. 0.5% for the TVM, at the dose rate of 13.4 Gy/s. For uniformly irradiated regular volumes, the k s value was significantly smaller, within 0.2% or 0.3% for irradiations with or without range shifter, respectively. Within measurement uncertainty, the average value of k pol , for the Markus TM23343 IC, was close to unity over the whole investigated range of clinical proton beam energies. Conclusion: While no polarity effect was observed for the Markus TM23343 IC in our pencil scanning proton beam system, the effect of volume recombination cannot be ignored.

show abstract

k_{s} = 1 + \frac{m^{2} d^{4}}{6 v V^{2}} \cdot I_{sat}

where

m^{2}

is a parameter representing volume recombination, as given by Boutillon et al.,

d

is the electrode spacing,

v

is the volume of the ionization chamber,

I_{italicsat}

is the ionization current, and

V

is the chamber voltage. The parameter

m^{2}

is a property of the gas in the cavity and it value depends on the recombination coefficient and on the mobility of negative and positive ions in the gas.…”

Section: Methodsmentioning

confidence: 99%

Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

et al. 2017

View full text Add to dashboard Cite

show abstract

“…For continuous radiation, previously developed models for deriving the recombination parameters at different dose rates and different polarizations can be used. The ratio of the saturation current I sat and the current I V at operating voltage V for continuous beams can be approximated by 19,20…”

Section: Iia1 Continuous 60 Co Gamma Radiationmentioning

confidence: 99%

“…Expressing Eq. ͑1͒ for two values of the polarizing voltage V and V / n, where n is not necessarily an integer, and dividing both expressions yields approximately 19,20…”

Section: Iia1 Continuous 60 Co Gamma Radiationmentioning

confidence: 99%

Ion recombination for ionization chamber dosimetry in a helical tomotherapy unit

et al. 2010

View full text Add to dashboard Cite

Because of the presence of charge multiplication, it is more accurate to determine the recombination correction at lower operating voltages than are often applied using the two-voltage method. Models and experiments for partial irradiation conditions of the ion chamber show that resulting recombination corrections are reduced compared to those for an open field. A model for helical dose deliveries results in recombination corrections that get lower with smaller slice widths. This model could be adapted to any IMRT delivery where the ion chamber is instantaneously partial and/or inhomogeneously irradiated, and could provide a practical procedure to calculate the recombination for complex deliveries for which it is difficult to be measured.

show abstract

“…However, when a proper correction is required initial recombination in continuous beams should not be neglected. In this case k s can be approximated by using a method described by Boutillon [37]. This method evaluates the recombination correction including both the initial and general recombination component.…”

Section: Recombination Correctionmentioning

confidence: 99%

NCS Report 18: Code of practice for the absorbed dose determination in high energy photon and electron beams

Aalbers¹,

Hoornaert²,

Minken³

et al. 2008

View full text Add to dashboard Cite

Volume recombination parameter in ionization chambers

Cited by 80 publications

References 22 publications

Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

Ion recombination for ionization chamber dosimetry in a helical tomotherapy unit

NCS Report 18: Code of practice for the absorbed dose determination in high energy photon and electron beams

Contact Info

Product

Resources

About