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CDF Radiation Measurement and Dosimetry |
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Thermal Luminescent Dosimeters (TLDs)
One common type of dosimeter uses a process known as thermal
luminescence to record a radiation dose. Ionizing radiation
passing through the thermal luminescent material stimulates an
transition to a meta-stable state. Heating the material produces
phonons which stimulate another transition to another energy level.
This new level has an allowed transition back to the ground state
emitting a photon. Counting the number of photons gives a measure
of the meta-stable state population and hence the radiation. For
a thermal luminescent device to be effective, the lifetime of the
meta-stable state must be long compared to the length of the
exposure or time between readings (heating).
FIG 1: Energy level diagram showing the thermal
luminescent mechanism.
One common type of TLD material is LiF with trace amounts of
other Ti and Mg. In LiF, the lifetime of this meta-stable state
at room temperature is several years. LiF also has the advantage
that 6Li has a large neutron capture cross section compared with
7Li. The recoil products from the reaction 6Li(n,3H)4He contributes
to the population of the above metastable state. By making
dosimeters of isotopically pure 6Li and 7Li and comparing the
results form a pair of dosimeters allows one to measure radiation
from low energy neutrons.
FIG 2: Neutron absorbtion cross section for two lithium
isotopes as a function of neutron energy.
Questions, comments:
tesarek@fnal.gov
Last modified: Fri Aug 30 10:59:05 CDT 2002