WALL EFFECTS OBSERVED IN TISSUE-EQUIVALENT PROPORTIONAL-COUNTERS FROM1.05 GEV NUCLEON FE-56 PARTICLES/

Tissue-equivalent proportional counters (TEPCs) have been used to meas ure energy deposition in simulated volumes of tissue ranging in diamet er from 0.1 to 10 mu m. There has been some concern that the wall used to define the volume of interest could influence energy deposition wi thin the sensitive volume because it has a density significantly great er than that of the cavity gas. These effects become important for hig h-velocity heavy ions. Measurements of energy deposition were made for 1 GeV/nucleon iron particles in a TEPC simulating a 1-mu m-diameter s phere of tissue. The TEPC was nested within a particle spectrometer th at provided identification and flight path of individual particles. En ergy deposition was studied as a function of pathlength through the TE PC. Approximately 30% of the energy transfer along trajectories throug h the center of the detector escapes the sensitive volume. The respons e of the TEPC, for trajectories through the detector, is always larger than calculations for energy loss in a homogeneous medium. This enhan cement is greatest for trajectories near the cavity/wall interface. An integration of the response indicates that charged-particle equilibri um is essentially achieved for a wail thickness of 2.54 mm. However, e stimates of the linear energy transfer for the incident particles are influenced by these wall effects. (C) 1998 by Radiation Research Socie ty.