Skip Navigation

Physics & Astronomy

Paper by Dr. Katz


 

TRACK STRUCTURE IN RADIOBIOLOGY, PHYSICS AND CHEMISTRY

Robert Katz
Department of Physics and Astronomy, University of Nebraska, Lincoln NE 68588-0111 USA

Key Words: Ion Track, track formation, radio biology, chemistry, physics

The words "track structure" have different meaning in different contexts. To some they imply the spatial distribution of excitations and ionizations about an ion track, to others they imply the microdosimetric (or nanodosimetric) energy depositions in appropriately small volumes. Here we are interested in the radial distribution of affected "targets" about the path of an energetic ion. The model which we have called TRACK THEORY, is based on biological target theory, and uses as a prototype our model of particle tracks in nuclear emulsions.

All detectors are said to be composed of targets. Some are discrete, readily observable physical entities, as in the case of grains of nuclear emulsion, or enzyme or virus molecules, or bacteria. Other targets are not clearly identifiable, as in the case of biological cells having a nucleus, the eucaryotic cells, where the targets are known to be in the cell nucleus but are of uncertain composition, identity, structure, size, and precise location. In these cells more than 1 target must be "hit" in order that the cell be "killed" or "transformed". In chemical systems, in scintillation counters, in thermoluminescent dosimeters, the target may be a region in which deposited energy may act collectively, defined by a difusion length.

The response to secondary electrons from gamma rays is treated statistically via a multi-hit model (as in the cumulative Poisson distribution) or a multi-target model where several targets must be hit to yield an observed end-point The model is parametric, in which a characteristic dose, D0 is the dose at which there is an average of 1 hit per target. Additionally we need to know the number of hits per target required in order to lead to an observed end point, as in many hit detectors.

In order to calculate the response of a detector to heavy ions we first must know the radial distribution to the average "local dose" from delta rays about the ion's path. This is translated into the radial distribution of probability for the activation of a target through application of the response to gamma rays. In turn this may be integrated radially to yield the activation cross section, a. If the stopping power (Linear Energy Transfer, L) and the volume density,N, of targets are known, we may then calculate the G value, the number of affected targets per unit absorbed energy as G aNiL, of greatest interest in chemistry.

Most detectors are 1-hit detectors, having an exponential response to the dose of gamma rays. We have found many hit nuclear emulsions and 2 hit response in chemistry. Some bacteria exhibit 1-hit response. Other biological cells are more complex, requiring special treatment because of their complex internal structure. Here we have used a many target model, but have suppressed both the size and identity of the target in a parametric representation. Two size parameters are required, one to describe the internal targets, and a second to describe the size of the cell nucleus. Parameters have been evaluated for over 40 sets of radiobiological data, including cell killing, mutation and transformation. Once our parameters are measured from a few irradiations with gamma rays and heavy ions, we are able to calculate response for arbitrary heavy ion bombardment, even to "thindown", and for mixed radiation fields.

It is somewhat surprising that a single model is capable of describing the response of so many different detectors, of different composition, different radiation response, and widely varying end points. Perhaps it is because the response of each detector to both gamma rays and heavy ions is attributed to secondary electrons, because the response to heavy ions is calibrated by the response to gamma rays, and because the model is mechanistic only in regard to the physics of radial dose production by delta rays.

 

R. Katz, S.C. Sharma, and M. Homayoontar, THE Structure OF PARTICLE TRACKS, in Topics in Radiation Dosimetry, Supplement 1, F. H. Attix, Ed., Academic Press, NY 1972

R. Katz and B. S. Kobetich, Particle Tracks in Emulsion, Phys. Rev. 186,344-351(1969).

R. Katz and Guo-Rong Huang, TRACK "CORE" EFFECTS IN HEAVY ION RADIOLYSIS, Radiation Physics and Chemistry 33, 345-349 (1989)

R. KaV DOSE Radiat. Res. 137 410-413 (1994).

 

note: this paper was scanned in and so may contain mistakes.