Page 201 - Radiochemistry and nuclear chemistry

Radiation Effects on Matter 185

window ~m cose

_.-
~,,~ ........... .~ ......... ,-/~ .......... ~-;-,,,- ....... ...-,.-- ,: .- ..... _ ........... . :., ~ _ _ ~
~, + A-Insuloting ring
B: Chorgin 9 rod ( hollow to odmit light from window)
B
Side view showing Rxed heovy metol cooled quortz fiber
orrongement of fixed _" . ~, + D-Movoble line metol cooled quortz fiber
ond movoble fibers C E- Metol cylinder
F- Tronsporent scole
G-Metol support for fibers
FIG. 7.8. Pen dosimeter with direct readout of the dose. (From Lapp and Andrews, 1948.)

NaSCN, leading to the formation of the intensely red complex ion Fe(SCN)63- upon
irradiation.
Photographic emulsions are sensitized by ionizing radiation resulting in darkening upon
development. This is used in the film dosimeter for measurement of fl-, "t-, or n-doses. In
order to differentiate between various types of radiation, the film is surrounded by filters
or transfer screens. Although any type of film may be used, special nuclear emulsions have
been designed. The dose received is directly proportional to the optical density of the
exposed film. Film dosimeters are useful in the same range as the pen dosimeter, and both
are used for personnel measurements. While the pen dosimeter can be read directly, the
film dosimeter requires development.
The glass dosimeter is made of phosphate glass containing 5 - 10 % of (AgPO3) n polymer.
The small piece of glass, a few cubic centimeters in size, is protected from light by means
of a coating. When radiation strikes the glass, trapped electrons are produced which can
be released by irradiation with ultraviolet light after removal of the protective coating. This
results in the emission of fluorescent radiation which can be measurexi photometrically. The
amount of fluorescent radiation is proportional to the dose received for doses up to 10 Gy.
The thermoluminescent dosimeter (TLD) covers the range 10 -6 - 10 Gy. The detector
consists of a crystalline powder of CaF 2, LiF, or similar compound, either pure or
incorporated in a plastic material like teflon. The irradiation leads to ionization and trapping
of the electrons in imperfections in the crystal lattice. Upon heating, recombination occurs
with light emission, which is measured photometrically. The electrons are trapped at
different energy levels, and slow heating releases the electrons in order of increasing energy
of the trapping levels. Consequently it is possible to take a dose reading on the
lowest-energy trapped electrons and still retain a memory of the dose through the electrons
left in more energetic traps. The dose can be read at a later time by releasing the remaining
electrons at a higher temperature. The TLD can be designed like the pen dosimeter in
which it is surrounded by screens to differentiate between different kinds of radiation. By
using a lead filter the dosimeter can be made energy independent in the range 0.02-20 MeV
for X-rays and "y-radiation.
This is also the basis for thermoluminescence dating. When geologic minerals like quartz,
feldspars, etc as well as ceramic materials like fired clay are exposed to high energy cosmic