Po‐Poster ‐ 13: A physical explanation for the failure of chamber calibrations for low energy brachytherapy seeds
Bibliographic record
Abstract
In the last few years it has become apparent that for low energy sources, chamber calibration factors are in error when used at sites with significant elevations. Empirical correction factors have been measured and published, but not a physical explanation of the effect. As the effect is only for low energy and not high energy sources, the explanation must lie in the relative ranges of the electrons set in motion in the wall. If all electrons stop in the chamber cavity, no correction for temperature and pressure need be applied. If all electrons cross the cavity then the usual Ctp correction is appropriate. For Ir‐192 or Cs‐137 sources, applying Ctp to account for changes of ambient pressure is appropriate. However, for lower energy sources (I‐125 or Pd‐103) a simple model predicts a significant number of electrons stop in the chamber and Ctp over‐corrects for the difference in ambient pressure. An analytic solution for a semi‐infinite parallel plate chamber predicts a behaviour similar to that described by the empirical correction factor. A simple numerical model of a well chamber shows that geometry is a second order effect, and that the ratio of cavity dimension to electron range is the principle physical consideration.
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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 | 0.008 | 0.032 |
| 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 | 0.003 |
| Scholarly communication | 0.003 | 0.003 |
| Open science | 0.003 | 0.002 |
| Research integrity | 0.002 | 0.003 |
| Insufficient payload (model declined to judge) | 0.008 | 0.004 |
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".