Well Chamber Response as a Function of Pressure

1. Well Chamber Response as a Function of Pressure Sheridan L. Griffin Tim D. Bohm John A. Micka Larry A. DeWerd University of Wisconsin Medical Radiation Research Center Department of Medical Physics

2. Air-communicating chambers must be corrected to standard atmospheric temperature (22oC) and pressure (760 Torr or 1 atm) using: At high altitudes for low-energy photons (20 – 100 keV) this correction was found to be insufficient. Truly an air density effect. Introduction • This work investigated an additional correction factor for these types of ionization chambers to be applied after CTP.

3. Apparatus &Technique

4. Sources in Use • High-energy photon sources: • Iridium-192 (192Ir) • Cesium-137 (137Cs) • Beta sources: • Strontium-90 (90Sr) • Phosphorous-32 (32P) • Low-energy photon sources: • Palladium-103 (103Pd) • Iodine-125 (125I) with a silver substrate • Iodine-125 (125I) without a silver substrate

5. Chambers in Use • Two similarly-designed air-communicating well chambers with aluminum collecting electrodes and with aluminum foil lining the inner wall (rough schematic shown at right) • One pressurized (argon) well chamber • One custom RPC-constructed air-communicating well chamber made of C552 conducting plastic

6. High-Energy Photons in Air-Communicating Chambers • The standard CTP correction is sufficient to normalize chamber response to standard atmospheric pressure

7. Beta Radiation in Air-Communicating Chambers • The standard CTP correction is sufficient to normalize the responses of the chambers to beta emitting sources (90Sr, 32P) Photons in Pressurized Chambers • When used with photon sources the pressurized chamber needs no CTP correction and exhibits the same response regardless of the ambient pressure

8. Beta Radiation in Pressurized Chambers • The response of pressurized chambers is not constant • May be due to decreased attenuation of the betas by the low-pressure air

9. Low-Energy Photons in Air-Communicating Chambers • Applying only CTP over-corrects the response • The over-correction increases in magnitude as the photon energy decreases

10. Proposed Altitude Correction, CA • The altitude correction is: where the proposed coefficients for the two chambers used in this experiment are: • Making the total correction for temperature and pressure:

11. Monte Carlo Simulation • One air-communicating chamber was modeled • modeled with MCNP v4c3 transport code with updated photon cross sections • also modeled with DOSRZnrc user code from the EGSnrc code system • Initial investigation with bare point sources • Followed up with models of the seeds used • Results differ at most at 560 Torr by 3.1% (103Pd) and 2.3% (125I w/ silver substrate)

12. Validation of Monte Carlo Analysis • Aluminum walls play a strong role in the over response • Calculations indicate a 2.3% improvement at 560 Torr with modification • Modified chamber exhibited a 3.1% decrease in over-response at 560 Torr • An RPC chamber was also experimentally tested

13. Conclusion • The correction factor for ambient pressure, CA, was determined for two air-communicating re-entrant ionizing well chambers with aluminum components. • The CA correction should be used for the majority of chambers in clinical use and is especially important at higher altitudes. • Air-communicating chambers do not require correction for high-energy photons or beta radiation. • Plastic chambers such as the RPC chamber do not appear to require correction beyond CTP.

14. Future Work • More work using Monte Carlo calculations will be done to attempt to determine the cause of the pressure effect. • The effect of using various materials for the lining of the inner wall and the collecting electrode will be investigated. • Further investigation into the apparent over-response of pressurized chambers to beta emitting sources will be undertaken.

15. My Thanks • I would like to thank Paul Cleven for his construction of the the pressure vessel and Ben Palmer for his assistance with the beta sources.