SU-E-T-82: Change of Ionization Chamber Correction Factors (P<sub>pol</sub>, P<sub>ion</sub>, K<sub>Wall</sub>) with Chamber Walls of Different Materials in Continuous and Pulsed Beams
Bibliographic record
Abstract
Purpose: To study the effect of wall material on correction factors for polarity effect Ppol, collection efficiency Pion and scatter and attenuation in the chamber wall/central electrode Kwall, in continuous and pulsed beams. Methods: An Exadin A12 ionization chamber was modified to have geometrically identical chamber walls built from aluminum and copper. Measurements were performed using the different walls in both 60Co, and Varian-Clinac 21EX 6 MV beams. Ppol was measured using AAPM TG51 protocol. Pion was obtained form measurement data, where the collected charge was measured at 10 voltage settings and used to form Jaffe-plots. Measurements were compared against Monte Carlo simulated data (egs++/egs_chamber). Kwall values were also calculated using CAVRZnrc. Results: For all beams and all wall materials, Ppol was found to be less than the 0.3 % limit recommended in TG-51. Saturation charges (extrapolated from the Jaffe-plots) were observed to increase with wall materials of increasing atomic number Z. The breakdown of the Boag-predicted linearity of Jaffe-plots in the near-saturation region was observed for all beam types and wall materials. The ratio of the saturation charges (relative to the C552 wall) were predicted by cavity theory, and agreed with simulations to within 9.8 % for 60Co and 5.2 % for 6 MV. Pion measured by the two-voltage technique and from Jaffe-plots were all found to be less than the 1.05 accepted upper limit recommended by TG-51. Kwall showed that attenuation and scatter for the Cu wall influences the measured signal by 1.9 % for 60Co. Conclusion: Ppol and Pion were measured for an Exradin A12 chamber with C552, Al, and Cu wall materials. The basic behavior of Jaffe-plots is independent of wall material. As expected, Kwall increases drastically for high Z walls with decreasing beam energies.
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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 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| 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".