Physics and Engineering of Radiation Detection

1.6. General Properties and Sources of Particles and Waves 53

produces around 4 neutrons per decay.

252

98 Cf→

94

38 Sr+

154

60 Nd+4n

1 mgof californium-252 emits around 2. 3 × 109 neutrons per second with an
average neutron energy of 2.1MeV. It also emits a large number ofγ-ray photons
but the intensity is an order of magnitude lower than that of the neutrons. It
therefore does not pose much problem for applications requiring a moderately clean
neutron beam. Table 1.6.5 lists some of the common sources of neutrons and their
decay modes.

1.6.F AlphaParticles..........................

Alpha particles are essentially helium nuclei with 2 protons and 2 neutrons bound
together. The consequence of their high mass and electrical charge is their inability
to penetrate as deep as other particles such as protons or electrons. In fact, a
typical alpha particle emitted with a kinetic energy of around 5MeV is not able
to penetrate even the outer layer of our skin. On the other hand they interact very
strongly with the atoms they encounter on their way, the reason being of course
their highly positive charge. Henceα-particle sources pose significantly higher risk
than other types of sources of equal strength if the particles are able to reach the
internal organs. This can happen, for example if the source is somehow inhaled or
digested.

Basic Properties ofα-Particles

Rest mass = 6. 644 × 10 −^27 kg=3. 727 × 103 MeV/c^2

Electrical charge = 3. 204 × 10 −^19 C

Mean life : Stable

Internal structure : Made up of 2 protons and 2 neutrons

The advantage of their extremely low range is that use of gloves in handling an
α-particle source is generally sufficient to avoid any significant exposure. It should,
however, be noted that because they are readily absorbed by the material, other
penetrating particles might be emitted in the process that could harm the body. As
noted above, ifα-particles somehow enter the body (for example by inhalation of
a radionuclide or through an open wound into the blood stream) they can deliver
large doses to internal organs leading to irreversible damage.
α-particles can not travel more than a few centimeters in air and readily capture
two electrons to become ordinary helium. Because of their low penetration capa-
bilities and very high ionizing powerα-particle beams are rarely used in radiation
therapy. Even if they are somehow administered near a tumor cell it will be very
difficult to control the destruction of cancerous cells while minimizing the damage
to the healthy tissues.