Page 162 - Radiochemistry and nuclear chemistry

Absorption of Nuclear Radiation 147

FIG. 6.18. Tracks of electron pair in a H 2 bubble chamber in a strong magnetic field
perpendicular to the plane of the tracks. (Courtesy Lawrence Radiation Laboratory.)

6.6. Absorption of neutrons

A beam of collimated neutrons is attenuated in a thin absorber through scattering and
absorption processes in a similar manner to the attenuation of 7-rays; these processes are
described in previous w In a thick absorber the neutrons are slowed from incident energy
at the absorber face to thermal energies if the absorber is thick enough. The ultimate fate
of the neutron is capture by an absorber atom. Because of the spread in neutron energy and
the energy dependency of the capture cross-sections, no simple relation can be given for
the attenuation of the neutron beam (cf. next section).

6.7. Radiation shielding
The absorption properties of nuclear radiation in material must be known in order to
design shielding to avoid unwanted radiation effects on the surroundings by nuclear
radiation sources.
For charged particles the shielding is usually slightly thicker than that required for the
maximum range of projectiles in the material. Absorption thicknesses of 0.2 mm are
adequate to completely absorb the particles from a-decay. By contrast 15 mm of materials
of low Z such as water, plastic, etc., are required for absorption of B-radiation with
energies up to 3 MeV. Radiation shielding constructed from materials of higher atomic
number require correspondingly thinner thicknesses. The data in Table 6.2 and the curves
in Figs. 6.6 and 6.13 provide information on the thickness of absorber material required
for the energy of various types of radiation.
Since 7-rays and neutrons have no definite range but exhibit a logarithmic relation
between thickness and intensity, only a partial reduction of the radiation can be obtained.
Combining (6.6) and (6.7)