Physics and Engineering of Radiation Detection

4 Chapter 1. Properties and Sources of Radiation

Example:

Compare the de Broglie wavelengths of a proton and an alpha particle moving

with the same speed. Assume the velocity to be much smaller than the

velocity of light. Consider the mass of theα-particle to be about 4 times the

mass of the proton.

Solution:

Since the velocity is much slower than the speed of light, we can use the

approximation 1.2.5, which for a proton and anα-particle becomes

λp =

h

mpv

and λα =

h

mαv

Dividing the first equation with the second gives

λp

λα

=

mα

mp

Anα-particle comprises of two protons and two neutrons. Since the mass

of a proton is approximately equal to the mass of a neutron, we can use the

approximationmα≈ 4 mpin the above equation, which then gives

λp≈ 4 λα.

This shows that the de Broglie wavelength of a proton is around 4 times larger

than that of anα-particle moving with the same velocity.

1.3 RadioactivityandRadioactiveDecay

Radioactivity is the process through which nuclei spontaneously emit subatomic
particles. It was discovered by a French scientist Henri Bacquerel in 1896 when he
found out that an element, uranium, emitted something invisible that fogged his
photographic plates. The termradioactivitywas suggested by Marie Curie about 4
years later. Originally three types of radiation were discovered:

α-rays (helium nuclei with only 2 protons and 2 neutrons)

β-rays (electrons)

γ-rays (photons)

Later on it was found that other particles such as neutrons, protons, positrons and
neutrinos are also emitted by some decaying nuclei. The underlying mechanisms
responsible for emission of different particles are different. To add to this complica-
tion, a particle can be emitted as a result of different modes of decay. For example,
a common decay mode resulting in the emission of neutrons isspontaneous fission.
During this process a heavy nucleus spontaneously splits into twolighternuclides