Stopping power Linear Energy Transfer

Stopping power or linear energy transfer (LET) is the energy lost per unit path length. Equation 6-2 expresses this relationship. 

S = stopping power LET = linear energy transfer AE = energy lost AX = path length of travel... 

The stopping power of a material for a particular radiation is commonly expressed as the rate of energy loss (R.E.L.) or the linear energy transfer (L.E.T.) of the radiation in the material. These quantities are assumed to be proportional to the linear ion density and the specific ionization. Stopping powers range from approximately 106 e.v./cm. for fast electrons (1 Mev.) in water to 1011 e.v./cm. for fission recoils. The ranges of particles are frequently expressed in mg./cm.2, which when multiplied by the density of the material yields the range. 

One of the characteristics of radiation considered in radiation chemistry and in radiobiology is the linear energy transfer (LET). For fast charged particles the LET practically equals the ionization losses (or polarization losses, in condensed media) and is given by the formulas for the stopping power presented in Section V.A. 

The yield of H2 in the radiolysis of polymers with y-rays is well known for several types of polymers [2], However, transuranic waste materials are a-particle emitters. The radiation chemistry induced by a-particles can be very different than that due to y-rays because of the difference in energy deposition density [13], The high linear energy transfer (LET, equal to the stopping power) of heavy particles leads to an increase in second order reactions, which may change the yields of some products. 

We see that the LET is approximately inversely proportional to the square of the velocity and proportional to the square of the charge z, but that the mass of the fast-moving particle does not appear. The linear energy transfer is also called the stopping power. 

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