ABSTRACT
The mean energy loss of charged particles per length in medium is important in the ‚elds of radiation physics and radiation dosimetry. This quantity, noted by –dE/dx, is called the stopping power of the medium for the particle-or from the viewpoint of the particle, it is frequently represented by the linear energy transfer (LET). The unit of LET is usually keV µm-1. The stopping power and LET are closely related to the dose given by the recoiled charged particles produced by the uncharged particles, such as photons and neutrons. In addition, those are related to the biological effect of various radiations. The stopping power is de‚ned by the product of the mean energy loss per collision, Qav, and the collision probability per unit length, µ,
in which µ is the macroscopic cross section whose dimension is the inverse of the length. The mean energy loss Qav is then given by
( )dav
Q QW Q Q Q
Q∫= (8.1) where W(Q) is the energy loss spectrum for a collision. The minimum energy loss, Qmin, in the collision between charged particle and electron seems to be ∼0. The maximum energy loss, Qmax, can be roughly estimated from the kinematical relationship. If a charged particle having the mass M and the speed V collides with an electron having the mass m in rest, the energy transfer reaches maximum for the head-on collision. Solving the conservation laws of energy and momentum, Qmax is obtained as
4 ( )max 2
Q mME M m
=
+ (8.2)
in which /22E MV= is the initial kinetic energy of the charged particle. If the incident particle is an electron or positron, Qmax is E. It is experimentally con‚rmed that the energy transfer continuously distributes in the range
min maxQ Q Q< < and Qav is ∼20 eV. The linear stopping power is de‚ned by
d d
( )dav min
maxE x
Q QW Q Q Q
Q∫− = = (8.3) The unit of the linear stopping power is generally MeV cm-1. The quan-
tity divided by the material density, –dE/(ρdx), is called the mass stopping power, whose unit is MeV cm2 g-1. The values of the stopping power vary according to particle, energy, and medium.
