An EGSnrc investigation of the correction factor for ion chambers in kilovoltage x rays
Bibliographic record
Abstract
As part of the standard practice for obtaining consistent ion chamber measurements with cavities open to the surrounding atmosphere, the raw measured response is corrected to the response at a reference temperature and pressure using the standard temperature-pressure correction factor (P(TP)). In this study, the EGSnrc Monte Carlo code was used to investigate the validity of the P(TP) correction factor for kilovoltage x rays incident on various geometrically distinct ion chambers. The calculated P(TP)-corrected chamber response deviated by over 2% relative to expected values for a 40 kV spectrum incident on a graphite thimble chamber at an air density typical of Mexico City. The relative deviation from the expected response was much worse for a large spherical graphite chamber, exceeding 16% at an air density of 0.6 kg/m3 (approximately 0.5 atm at 22 degrees C) for the same beam energy. The breakdown of the P(TP) correction factor was also observed for a 26 kV mammography spectrum incident on two mammography chambers. For 60Co beams, the P(TP) correction factor behaved as expected. For day-to-day variations in pressure, only a negligible of the P(TP) correction factor was observed with low x-ray energies. Factors contributing to the breakdown of the P(TP) correction factor at low x-ray energies and large pressure variations, such as the range of electrons, the material of the wall, the chamber dimensions and air-photon interactions, are discussed in depth.
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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 | 0.011 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.001 | 0.002 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 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".