Wall Effects Observed in Tissue-Equivalent Proportional Counters
COLORADO STATE UNIV FORT COLLINS DEPT OF RADIOLOGICAL HEALTH SCIENCES
Pagination or Media Count:
Tissue equivalent proportional counters TEPC have been used to measure energy deposition in simulated volumes of tissue ranging in diameter from 0.1 to 10 microns. There has been some concern that the wall used to define the volume of interest could influence energy deposition within the sensitive volume. These wall effects occur because the wall has a density significantly greater than the cavity gas. Energy deposition measurements were made for 1 GeVnucleon Fe ions in a TEPC simulating 1, 2, and 3 microns diameter spheres of tissue. The detector was nested within a particle spectrometer that provided identification and flight path of individual particles. Energy deposition was studied as a function of pathlength through the detector. Approximately 25 of the energy transfer along trajectories through the center of the detector escapes the sensitive volume. The response of the detector, for trajectories through the detector, is always larger than calculations for energy loss in a homogenous medium. This enhancement is greatest for trajectories near the cavitywall interface. An integration of the response indicates that charged particle equilibrium is essentially achieved for a wall thickness of 2.54 mm. However, estimates of LET and quality factor are influenced by these wall effects. Detector response for fragment particles through the detector was found to scale closely to Z2 for ions of 18 Z 26, as expected by theory for ions of high velocity. The mean detector response to particle trajectories where a charge-changing nuclear interaction occurred in the detector wall was lower than that of the response to particle trajectories not involved in charge-changing nuclear interactions.
- Nuclear Instrumentation