| The thermoluminescent dosemeter (TLD) is designed
to measure doses from X-, beta and gamma radiations in terms of the
radiation quantities specified by the Health and Safety Executive (HSE).
The
TL dosimetry service is
approved by the
HSE under
Regulation 35 of the Ionising Radiations Regulations 1999. The
dosemeter consists of two thermoluminescent detectors containing the
radiation-sensitive material lithium fluoride. The detectors are
located in a plate which is identified uniquely by means of an array
of holes. The lithium fluoride stores the energy it receives from
ionising radiations until it is heated during processing (in this
case to about 250°C)
when the energy is released as light. The amount of light released
is proportional to the radiation dose. The plate is supplied to the
wearer in a plastic wrapper which protects the detectors from light
and contaminants. This bears the wearer's name, an establishment
code, the expiry date and an optional personal identifier for each
employee, e.g. department name or a works number. If the name is not
required a serial number is printed instead. The wearer places the
wrapped plate in a plastic holder,which is supplied by the
NRPB
on permanent loan and is available with safety pin or clip
attachments. We also provide 'wear and care' cards for each member
of staff. These are designed to help users understand how and why
they should wear the dosemeter.
The dosemeter measures two quantities. The first is the personal
dose equivalent Hp (10), which is often referred to as the 'whole
body' dose which results from penetrating radiation. It is measured
by the detector behind the domed part of the holder. The second
quantity is the personal dose equivalent Hp (0.07) which is an
assessment of the dose equivalent to the skin from both weakly and
strongly penetrating radiations. This is measured by the detector
behind the circular window. |
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TLD technical
specification
| Detection |
x rays and gamma rays |
beta particles |
| Dose range measured |
0.05
mSv to 10
Sv |
0.05
mSv to 10
Sv |
| Energy range detected |
10
keV to 10
MeV for Hp (0.07)
15
keV to 10
MeV for Hp (10) |
700
keV to 3.5
MeV (Emax) for Hp
(0.07) |
| Periods of use |
2, 4, 8, 13 weekly (calendar issue periods are also
available) |
2, 4, 8, 13 weekly (calendar issue periods are also
available) |
Special features of the
TLD
Tissue equivalence
The detectors absorb radiation energy in the same way and to the same
extent as human tissue. This enables us to evaluate doses of complex
mixtures of radiations in a simple and straightforward manner, thus
keeping errors of measurement to a minimum.
Life span
The detector is capable of retaining the stored dose information for
extended periods before assessment. Even in conditions of relatively
high temperature (40°C) and
high humidity (up to 100%), the information can be stored for up to one
year. Issue periods of up to 13 weeks can be offered thus keeping the
cost of monitoring low.
Reassessment of
TLD
TL glow curves of all
dosemeter readings are kept for at least five years. This allows
retrospective investigation in the event of a customer query. The glow
curves for dosemeters with assessments in excess of 15
mSv are all checked. For doses
over 25
mSv, it is possible to verify the
original assessment using a special technique, at no extra cost.
Image 1& 2 Typical TLD holder & card
The TLD Card |
The TLD Holder |
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Image 3 TLD Badges
Image
4
TLD Ring types 
Image 4 A selection of TLDs from Canada
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Useful Link :http://www.nrpb.org/
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