Pre-irradiation effects on ionization chambers used in radiation therapy
Bibliographic record
Abstract
Dosimetry protocols recommend that ionization chambers used in radiation therapy be pre-irradiated until they 'settle', i.e., until a stable reading is obtained. Previous reports have claimed that a lack of pre-irradiation could result in errors up to several per cent. Recently, data collected for a large number of commonly used ion chambers at the Institute for National Measurement Standards, NRC, Canada, have been collated and analysed, with additional data contributed by the National Physical Laboratory, UK. With this data set, it was possible to relate patterns of ion chamber behaviour to design parameters. While several mechanisms seem to contribute to this behaviour, the most obvious correlations implicate the type of insulator surrounding the central collector electrode, the extent of collector electrode shielding and possibly the area of the insulator exposed at the base of the active air volume. The results show that ion chambers with electrode connections guarded up to the active air volume settle quickly (approximately 9 min) and the change in response is small (less than approximately 0.2%). For ion chambers where the guard connection surrounding the central collector does not extend up to the active air volume, settling times of 15-20 min and an associated change in response of up to 1% are typical. For some models of ion chambers, the irradiation rate may also play a role in settling behaviour. Settling times for the ion chambers studied here were found to be independent of beam quality.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.003 | 0.014 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.002 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".