Track ionizing radiation

It has been reported for many years that condensation nuclei can be produced by ionizing radiation. Recent studies have improved the measurement of the activity size distribution of these ultrafine particles produced by radon and its daughters (Reineking, et al., 1985 Knutson, et al., 1985). It seems that the Po-218 ion is formed by the radon decay, is neutralized within a few tens of milliseconds, and then attached to an ultrafine particle formed by the radiolysis generated by the polonium ion recoil. Although there will be radiolysis along the alpha track, those reactions will be very far away (several centimeters) from the polonium nucleus when it reaches thermal velocity. The recoil path radiolysis therefore seems to be the more likely source of the ultrafine particles near enough to the polonium atom to rapidly incorporate it. 

Katz, R. and W. Hofmann, Biological Effects of Low Doses of Ionizing Radiations Particle Tracks in Radiobiology, Nuclear Instruments and Methods 203 433-442 (1982). 

Nuclear track detectors are very simple and very efficient detectors of rare events that produce highly ionizing radiation. Carefully prepared and scanned track detectors have been used to identify individual rare decays. The detectors are integrating in that the damage caused by a track is not spontaneously repaired. The drawback to track detectors is that the tracks are small and can only be observed with a microscope. In the past, scanning by eye was extremely labor intensive and prone to error. Modern computer-controlled scanning has improved the speed and reliability of the analysis. Plastic track detectors that are sensitive to a particles are used extensively in commercial radon detectors. 

However, this information is absolutely insufficient for explaining the changes that occur in a molecular medium exposed to ionizing radiation. The final chemical transformations are determined by the microstructure of the short-lived primary excitation and ionization regions of the track and by the spatial distribution of the radicals produced. For instance, in order to make the theoretical model of water radiolysis agree with experimental data, it was necessary to give the nonhomogeneous distribution of radicals in electron tracks as initial conditions.7... 

The electron produced in reactions (1) and (3) may still contain enough energy to cause further ionizations in the very near neighborhood. These areas, containing a number of ionization and occasionally also electronic-excitation events [reactions (2) and (4)], are called spurs. In the case of sparsely ionizing radiation, these spurs do not overlap. In densely ionizing radiation however, they form cylinders of spurs that are called tracks. 

The scope of applications of MF has been broadened in recent years by the introduction of inert membranes, particularly polypropylene, polycarbonate, and Teflon . In general, these materials cannot be made by the methods developed for the cellulosics because of their insolubility. Both Teflon and pol)propylene MF membranes have been made by a controlled stretching procedure in which microtears are introduced Microporous polycarbonate membranes have been prepared by a unique radiation-track-etch method A thin polycarbonate film is exposed to ionizing radiation which leaves labile sites that can later be chemically etched to produce straight-throi channels. The pore size can be controlled by the etching conditions. The pores in these membranes, contrary to those in cellulosic membranes, are quite uniform in diameter. 

---