Free ion yield, low LET

The free ion yield at zero field strength refers to the number of electron/ion pairs which achieved an initial separation distance larger than the Onsager distance, r (see Section 4.4). Theoretically, electron escape from spurs (spherical geometry) leads to a finite value of G /E = 0). From the track of a high LET particle (cylindrical geometry), escape at E = 0 is zero. The observed small Gfi(E = 0) values are probably due to escape via 5-rays. 

In this section, we shall discuss the free ion yield produced by low LET radiation and make use of the Onsager theory of escape in order to rationalize the observed values. The free ion yield depends on the temperature of the liquid, on external hydrostatic pressure, on the externally applied electric field strength, and on the molecular structure of the molecules comprising the liquid. In some liquids, where electron trapping is involved in the transport, the yield can be influenced by simultaneous illumination with infrared light (see Section 3.8.3). Free ion yields produced by low LET radiation have been determined for many liquids. The discussion of all the details would exceed the frame of this book. The interested reader is referred to the reviews by Hummel and Schmidt (1974) or Freeman (1987). 

According to ideas developed by Magee (1977) and Mozumder (1969), the free ion yield is determined by three processes  

The probability of escape as a function of temperature is given as (see Chapter 4, Equation 40), 

In general, a distribution of separation distances will be attained. The total probability is then obtained as 

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