Ionizing particle, track

Mainly, this method is employed with ionization chambers filled with high mobility warm liquids or with liquid argon, krypton, or xenon. Since only the electronic component is observed, the pulse shape depends on the position of the ionizing particle track in the ionization chamber. Basically, two variants can be distinguished with a parallel plate ionization chamber (1) the track of the ionizing particle is parallel to the field lines or (2) the track runs perpendicular to the lines of field (Figure 32). 

Dolgoshein, B. A., Lebedenko, V. N., and Rodionov, B. U., New method of registration of ionizing-particle tracks in condensed matter, JETP Lett., 11, 351, 1970. 

Ionization Path (Track)—The trail of ion pairs produced by an ionizing particle in its passage through matter. 

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). 

At the end of the physical stage, which is within about 10 sec of the passage of the ionizing particle through the liquid, the track made by the particle contains H20", subexcitation electrons e , and electronically excited water molecules H2O in small clusters called spurs. From about 10 to 10 sec, the following processes are thought to occur and comprise the physicochemical stage [9,10] ... 

With hadrons (i.e., neutrons, protons, and heavy ions), new radiation qualities are introduced in therapy. The distributions of the ionizations (and energy deposition events) along the particle tracks are different, and, as a result, different and increased biological effects (at equal absorbed dose) may be expected compared with the conventional photon beams. Fig. 1 illustrates the differences in dose necessary to produce a given biological effect as a function of radiation quality [15]. 

Fig. 2 illustrates schematically the distribution of the ionizations along the particle tracks after photon and neutron irradiation [1,16]- In a more quantitative way, Fig. 3 presents the microdosimetric lineal energy, y, spectra for two clinical fast neutron beams the highest and lowest energy used in cancer therapy (measurements performed at Louvain-la-Neuve and Essen, respectively) [17]. 

Alpha radiation differs from gamma or electron radiation primarily in the much denser distribution of reactive entities along the alpha particle track with ultraviolet light at 254m often requiring the presence of a sensitizer, one is dealing primarily with excitation and not ionization. 

Particle Tracks. Thus, irrespective of the particulate or photon nature of the primary radiation, the net effect is the formation of tracks consisting of ionized and excited molecules. These tracks, and their detailed structure can be revealed by the Cloud Chamber invented by Wilson in 1911(11). For fast electrons (low LET) the tracks mainly consist of spherical regions called spurs which contain from one to four ion-pairs which are separated in condensed phases by about 10 A. For more highly ionizing particles such as a-particles the tracks are essentially cylindrical columns of ionized and excited molecules. 

In cloud chambers (Wilson chambers) the tracks of ionizing particles are visible by condensation of droplets on the ions produced. The gas in the chamber is saturated with the vapour of water, alcohol or other volatile liquids. By sudden expansion supersaturation is obtained and condensation occurs along the ion tracks. Dust or other condensation centres must be eliminated, to avoid interferences. Cloud chambers can be operated in cycles by a piston or diaphragm (expansion chamber) or by diffusion of a saturated vapour into a colder region (diffusion cloud chamber). 

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