Perturbation correction factors for the NACP-02 plane-parallel ionization chamber in water in high-energy electron beams
Bibliographic record
Abstract
Recent dosimetry protocols for clinical high-energy electron beams recommend measurements of absorbed dose-to-water with a plane-parallel or cylindrical ionization chamber. For well-guarded plane-parallel ionization chambers, the ionization chamber perturbation factor in water, p(Q), has a recommended value of unity in all protocols. This assumption was investigated in detail in this study for one of the recommended ionization chambers in the protocols: the Scanditronix NACP-02 plane-parallel ionization chamber. Monte Carlo (MC) simulations of the NACP-02 ionization chamber with the EGSnrc code were validated against backscatter experiments. MC simulations were then used to calculate p(wall), p(cav) and p(Q) perturbation factors and water-to-air Spencer-Attix stopping powers in 4-19 MeV electron beams of a calibration laboratory (NPL), and in 6-22 MeV clinical electron beams from a Varian CL2300 accelerator. Differences between calculated and the currently recommended (Burns et al 1996 Med. Phys. 23 383-8) stopping powers, water-to-air, were found to be limited to 0.9% at depths between the reference depth z(ref) and the depth where the dose has decreased to 50% of the maximum dose, R50. p(wall) was found to exceed unity by 2.3% in the 4 MeV NPL calibration beam at z(ref). For higher energy electron beams p(wall) decreased to a value of about 1%. Combined with a p(cav) about 1% below unity for all energies at z(ref), this was found to cause p(Q) to exceed unity significantly for all energies. In clinical electron beams all three perturbation factors were found to increase with depth. Our findings indicate that the perturbation factors have to be taken into account in calibration procedures and for clinical depth dose measurements with the NACP-02 ionization chamber.
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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 | 0.001 | 0.005 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
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".