Low Positive Doses in Personnel Monitoring

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

Sue M. Dupre wrote:
< I presume that we all are suspicious of doses that are close to
<minimum detectable levels, that we've all seen cases in which a dose 
<is reported but the badge wasn't worn, for instance.  Assuming that
<you've dealt with the vendor to eliminate factors such as control badge
<problems,  then how do you proceed?
<...

There is an interesting article entitled "Required accuracy and dose 
thresholds in individual monitoring" in Radiation Protection Dosimetry, 
Vol 54, Nos 3/4, pp. 279-285 (1994), Nuclear Technology Publishing, which 
may help you to make a better decision on the matter.

In quick summary of the article, the following (terminology will 
certainly vary from country to country):

There is a decision limit (with an attached level of confidence), above 
which measurements can be considered to be NOT background.

There is a detection limit, which is the lowest dose that can be detected 
with a specified level of confidence. Nothing is said about the accuracy 
of the measurement in this case.

There is a determination limit, which is the lowest dose that can be 
measured with a given precision. For your legal (or own) requirements 
regarding lowest reported dose, it can be estimated what the precision of 
the measurement will be, based on information obtained from certain 
measurements.

The article assumes that reported dose have been corrected for background 
irradiation.

For our large dosimetry operation it is certainly impractical to 
investigate all non-zero doses. Normally only reported doses  which will, 
if continued, lead to an annual dose exceeding the annual dose limits, 
are investigated routinely. In other cases doses are investigated when 
wearers query them, and also when the distribution of doses for a 
specific place  is abnormal.

Whatever you decide, your decision and the rationale behind it should be 
documented in your quality system.


Du Toit Volschenk
Section Head: Radiation Protection Service
South African Bureau of Standards
Tel: 012 428 6882 (international: 27 12 428 6882)
Fax: 012 344 1568 (international: 27 12 344 1568)
E-mail: DUTOIT@SABS.CO.ZA