Ion recombination correction factor in scanned light-ion beams for absolute dose measurement using plane-parallel ionisation chambers.
Phys Med Biol
; 62(13): 5365-5382, 2017 Jul 07.
Article
in En
| MEDLINE
| ID: mdl-28504642
ABSTRACT
Based on international reference dosimetry protocols for light-ion beams, a correction factor (k s) has to be applied to the response of a plane-parallel ionisation chamber, to account for recombination of negative and positive charges in its air cavity before these charges can be collected on the electrodes. In this work, k s for IBA PPC40 Roos-type chambers is investigated in four scanned light-ion beams (proton, helium, carbon and oxygen). To take into account the high dose-rates used with scanned beams and LET-values, experimental results are compared to a model combining two theories. One theory, developed by Jaffé, describes the variation of k s with the ionization density within the ion track (initial recombination) and the other theory, developed by Boag, describes the variation of k s with the dose rate (volume recombination). Excellent agreement is found between experimental and theoretical k s-values. All results confirm that k s cannot be neglected. The solution to minimise k s is to use the ionisation chamber at high voltage. However, one must be aware that charge multiplication may complicate the interpretation of the measurement. For the chamber tested, it was found that a voltage of 300 V can be used without further complication. As the initial recombination has a logarithmic variation as a function of 1/V, the two-voltage method is not applicable to these scanned beams.
Full text:
1
Collection:
01-internacional
Database:
MEDLINE
Main subject:
Radiation Dosage
/
Radiometry
Type of study:
Prognostic_studies
Language:
En
Journal:
Phys Med Biol
Year:
2017
Document type:
Article
Affiliation country:
Bélgica