Energy deposition in different diameter sites can help characterize the incident radiation

The energy deposited by many types of radiation, including the high energy heavy ions found in galactic cosmic rays, is more damaging to biological systems than equal amounts of energy delivered by conventional X-ray irradiation. Average quantities such as dose and LET do not provide the information necessary to predict the effect of these types of irradiation at low doses. Charged particle fluence, as a function of the particle charge and velocity, is the ideal way to characterize the radiation, but in many practical situations the fluence spectrum is not known, and cannot be easily obtained. Low pressure proportional counters, simulating tissue volumes a few micrometers in diameter, provide a convenient alternative to dose and fluence measurements. Such instruments are used routinely on the space shuttle and space station to evaluate radiation exposures. However, the energy deposited does not relate directly to energy transferred because a significant amount of energy is carried away from the particle track by delta rays. Using experimental data and simple approximations we show that energy deposited in targets 0.02, 2, and 20μm in diameter differs by easily measurable amounts. These results suggest that it is possible to estimate the fraction of the events that are due only to delta rays, and, thus, the average velocity of the particles that deposited the energy, from measurements of energy imparted in different size sites.