Physics and Engineering of Radiation Detection

2.4. Interaction of Heavy Charged Particles with Matter 105

Solution:

According to equation 2.3.47, the intensity of the photons leaving the detector

is given by

Iout=I 0 e−(μ

atda+μgtdg+μatda)

,

where the subscriptsaandgstand for aluminum and fill gas respectively. The

percentage of photons absorbed in the detector can then be written as

Nabs =

I 0 −Iout

I 0

× 100

=

(

1 −e−(μ

atda+μgtdg+μatda))

× 100.

The mass attenuation coefficient ofCO 2 can be calculated by taking the

weighted mean of the given mass attenuation coefficients of carbon and oxygen.

μgm = wcμcm+woμom

=

(

12

44

)

0 .1514 +

(

32

44

)

0. 1551

=0. 1541 cm^2 g−^1

The required percentage of absorbed photons is then given by

Nabs =

(

1 −e−(2×^0.^1704 ×^2.^699 ×^100 ×^10

− (^4) +0. 1541 × 1. 833 × 10 − (^3) ×6))
× 100.
=1.08%.

2.4 Interaction of Heavy Charged Particles with Matter

By heavy charged particles we mean particles withA≥1 such as protons andα-
particles. The reason for differentiating between heavy and light charged particles
is that the former, due to their heavier mass and higher charge, experience stronger
Coulomb force of nuclei than the latter. Furthermore if they come close enough
to nuclei, they can also be affected by the strong nuclear force, which on lighter
particles, such as electrons, does not act. Consequently the heavy charged particles
behave quite differently in matter than the light charged particles. At low to mod-
erate energies the most important type of interaction for heavy charged particle s is
the so calledRutherford scattering. We will look at it in some detail before we go
on to the discussion of the overall effect of passage of such particles through matter.

2.4.A RutherfordScattering

Rutherford scattering, first discovered by Lord Rutherford, refers to the elastic scat-
tering of a heavy charged particle (such as anα-particle) from a nucleus. In his
famous scattering experiment, Rutherford bombarded a thin gold foil byα-particles
and studied how many of them deflected from their original direction of motion. He
noticed that most of theα-particles passed through the foil undeflected while very