LLD for TLD and ED

[DUTOIT DuToit Volschenk <dutoit@mail.sabs.co.za>]

George Wu wrote:

>>I would like to know the Lower Limit of Detection values for common TLD
badges and electronic dosimeters, and the procedures used to determine
these values. ....

There are a number of methods and formulae for determining the LLD for a 
TLD system.

In the Health Physics Society HPS N13.11-1993 standard  (Personnel 
Dosimetry Performance - Criteria for testing) there is one such 
mathematical method (based on experimental observations). 

It is also important to realise that the LLD is not the only factor to 
consider when reporting low doses.

There are basically three quantities of interest:
a) the detection threshold (above which there is a certain statistical 
confidence that the signal is not due to background)
b) the lower limit of detection (where there is a statistical confidence 
that the "dose" is a dose)
c) the determination limit (basically a reporting level with an 
acceptable statistical confidence that the reported dose is correct)

One could have an LLD with a low value, but with a very high statistical 
uncertainty. So - one should also consider the determination limit before 
just accepting the LLD at face value.


Regarding the actual LLD values used by vendors, these would depend on a 
number of factors. Two of these are:
a) the type of dosimetry system used
b) the dosimetry application (if you know what your radiation environment 
is, generally you should be able to have a lower LLD, while if you are 
monitoring "blind", your LLD has to be higher)



Du Toit Volschenk
Radiation Protection Service
South African Bureau of Standards
Tel: 012 428 6493 (international: 27 12 428 6493)
Fax: 012 344 1568 (international: 27 12 344 1568)
mailto: DUTOIT@SABS.CO.ZA

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