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SU‐GG‐T‐195: Ion Chamber Collection Efficiency Considerations for Un‐Flattened X‐Ray Beams
Author(s) -
Johnsen S
Publication year - 2008
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2961947
Subject(s) - linear particle accelerator , physics , ionization chamber , optics , ion , beam (structure) , pulse (music) , x ray , atomic physics , ionization , quantum mechanics , detector
Purpose: To assess the corrections necessary, due to loss of ion chamber collection efficiency, when measuring accelerator‐produced unflattened x‐ray beams. Method and Materials: Ion chambers typically used to map therapeutic x‐ray beams were used to map unflattened beams created by removing the flattening filter from a radiotherapy accelerator. Boag's two‐voltage technique was used to calculate the collection efficiency at the high dose‐per‐pulse attained in such fields. Results: Collection efficiency could be fit with a simple linear function of dose‐per‐pulse. At the center of the unflattened fields the collection efficiency was as much as 2.6% lower than at the lower‐intensity periphery. Different ion chamber types exhibited more or less loss of efficiency. Conclusion: When high intensity unflattened x‐ray beam distributions are measured, appropriate corrections for collection efficiency must be made for accurate determinatation of those distributiions. This can be accomplished using a simple linear correction function. These corrections will not be the same for all locations in the field.