On the Radiation-Acoustic Effect in Radiobiology

Abstract

A fundamental problem in radiobiology is the contradiction between the spatially limited deposition of the energy of ionizing radiation and a biological effect which manifests itself in the cell as a whole. This contradiction has not been resolved on the basis of the short-range concept. An attempt to introduce a long-range mechanism of radiation action was made by Apfel et al. (1). It is based on the thermal spike in the track (2) which generates the elastic wave. This is the radiation-acoustic effect which is well known to radiation physicists but practically unknown to radiobiologists. It was described in 1957 by Askarian (3), who for the first time pointed out the possible biological implications of this effect. Since this time, the radiation-acoustic effect has been investigated both experimentally and theoretically in solids and liquids [see, e.g., (4-6)]. It is well established that the adiabatic expansion produced by the heat deposition in the track leads to the generation of a detectable acoustic signal, with amplitude proportional to the ionization loss and dependent on the heat capacity and volume expansivity of the medium. In the near field of the track the signal generated is a shock wave with high gradients of temperature and pressure at its front. In the far field this signal has a power law of damping, and its detectable range is limited by the thermal acoustic noise of the medium. The main problems in the acceptance of the radiationacoustic effect in radiobiology are related to its very short transition time, the relatively short attenuation length of the generated wave, and the low efficiency of the transformation of the thermal energy to the acoustic radiation. The thermoelastic stresses associated with the acoustic wave generated in the near field can exceed the static fracture limits of the macromolecules (e.g., for L = 400 keV/Am, and the radius of the acoustic antenna of the track of 1.79