InstroTek
51
Fig 4.2 Penetrating Distances
Beta particles
travel a few
feet in air and are stopped by
an inch of wood or a thin
sheet of aluminum or plastic.
In the nuclear gauge beta
particles are stopped by the
source containment
.
Gamma rays
travel hundreds
of feet in air and can be
shielded by thick lead or
concrete.
Neutron particles
travel hundreds of feet in air and can be
shielded by water, plastic, or special concrete.
Cesium-137 and Americium-241/Be are used in the Elite
portable moisture/density gauge and they produce all four
types of ionizing radiation. The alpha and beta radiation will
be stopped by the stainless steel source containment. Only
neutron and gamma radiation contribute to occupational
radiation exposure.
Neutron radiation
Neutron radiation in the Elite gauge is produced by alpha
particles from the Americium 241 bombarding the Beryllium.
Neutron radiation has no charge and is very penetrating. To
shield against neutron, the first step is to slow down the neutron,
which is called thermalization. Thermalization is achieved by
interaction of neutron with particles of same mass as neutron,
like hydrogen in water or polyethylene. As neutrons collide with
these particles, neutron energy is reduced allowing for effective
absorption by the shielding material.