Physics and Engineering of Radiation Detection

46 Chapter 1. Properties and Sources of Radiation

results in the production of a large number of particles. The particles produced in
this way help the physicists to understand the fundamental properties of particles
and their interactions.

Radioactive Sources of Positrons

As we described earlier in the Chapter, there are a number of radioactive isotopes
of different materials that decay by positron emission. The only problem is that the
positrons thus produced have very short lives, i.e. they tend to quickly annihilate
with a nearby electron. Table 1.6.3 lists some of the radioisotopes that are commonly
used in laboratories.

Table 1.6.3: Common positron emitters and their half lives.

Element Isotope T 1 / 2

Carbon^116 C 20.39 minutes

Nitrogen^137 N 9.96 minutes

Oxygen^158 O 122.24 seconds

Fluorine^189 F 109.77 minutes

1.6.D Protons

Protons are extremely stable composite particles made up of three quarks. They
carry the same amount of electric charge as electrons but in positive polarity. How-
ever they are about 1836 times heavier than electrons.

Basic Properties of Protons

Rest mass = 1. 67 × 10 −^27 kg= 938. 27 MeV/c^2

Electrical charge = +1. 602 × 10 −^19 C

Mean life > 1025 years

Internal structure : Made up of 3 quarks

It was Eernest Rutherford who, in 1911, gave the idea of the atom being com-
posed of positively charged nucleus and separate negative charges. After a series
of experiments he reached the conclusion that the nuclei of different elements were