RADIATION PROTECTION - ILEA

6 | Beta Particles 23
Z = atomic number of absorber
NZ = number of absorber electrons per cm 3
c = speed of light
I = mean excitation and ionization potential of absorber atom
The equation shown here was derived for heavy particles, such as alpha
particles and protons. The term in brackets is more complex for electrons.
Obviously, the greater the density of intercepting electrons, the greater
will be the rate of energy loss per centimeter. Of particular interest is the
fact that the rate of energy loss (and therefore energy imparted to the medium)
varies as the square of the charge on the ionizing particle and inversely
as the square of its velocity. Thus, the charge of +2 on the alpha
particle has the effect of increasing the stopping power by a factor of 4
(i.e., 2 2 ) over the stopping power of the electron (with its single negative
charge). Note that the stopping power is the same whether there is a repulsive
force between the incident particle and the electrons (that is, they have
the same sign) or whether there is an attractive force (that is, they have opposite
signs).
A property that is closely related to the stopping power is the linear energy
transfer (LET) of the radiation, the rate at which energy is transferred
to the medium along the track of the particle. The LET is equal to the
stopping power when all the energy lost by the particle is treated as absorbed
locally along the track. This relationship is generally assumed to
hold for charged particles emitted by radionuclides. The greater the linear
energy transfer, the greater is the damage produced by the particle. The
LET of fast electrons is used as the reference LET, and a great deal of radiation
research has been done to study the relative effectiveness of radiations
with higher LETs, such as alpha particles and protons, in producing damage.
As a result, numerical values called radiation weighting factors (W R ) or
quality factors (Q) have been assigned to measures of the biological effectiveness
of radiation as a function of the type and energy of the radiation.
Beta particles, as the reference radiation, have a radiation weighting factor
of 1. Alpha particles cause about twenty times as much damage as beta particles
for the same energy absorbed, so their radiation weighting factor is
assigned a value of 20.
6 Beta Particles—A Major Class of Charged
Ionizing Particles
Carbon-14, tritium (hydrogen-3), sulfur-35, calcium-45, phosphorous-
32, strontium-90—these names are familiar to investigators who use ra-