Li and 
Li both respond to beta
and gamma radiation .
In addition Li responds to slow neutrons
(0.025 eV to 0.6 MeV) via the Li(n, 
)
H reaction. Fig. 6.14
shows a
diagram of a neutron dosimeter badge . The badge
contains a chip each of LiF and
LiF side by side. These are covered by a layer of cadmium
which absorbs very slow moving neutrons incident on the badge when worn on the
body. The LiF TLD therefore measures the slow neutrons
which are generated by higher energy
neutrons incident on the body and which reflect back into the dosimeter. Such
a dosimeter is referred to as an `albedo' dosimeter .
The beta radiation is
also screened out by the cadmium so that the
LiF gives the gamma dose only while the
Li gives the dose due to both gamma rays and neutrons.
The difference in the readings is the neutron dose.
In addition the badge contains a piece of polycarbonate (a plastic called CR-39 also used in the manufacture of eye glass lenses. Fast neutrons which interact with the plastic leave an invisible damage track in the plastic which can be made visible by enlarging the tracks using an etching technique. By counting the number of etch tracks per unit area in the plastic, a measure of the fast neutron dose can be obtained.
These badges are only worn by people who are likely to be exposed to neutron fluxes above the very low levels normally found outside the accelerator shielding.
Figure 6.14: Neutron Dosimeter