Panasonic UD-716 badge reader

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

Dave Arnold wrote:

<Recently we have had a couple of false high dose readings. Readout of 
<the backup elements fails to confirm the doses and the wearers' 
<of the badges confirm that they have done nothing out of the ordinary, 
<work wise.

<Has anyone out there had similar experiences of spurious dose 
<readings?  Any comments/explanations welcome.

It is difficult to give a meaningful answer with the information given. I 
will attempt to list similar occurrences that we experienced, using our 
UD710 readers, and the causes for them. It is assumed that with "high 
doses" is meant anything higher than 1 mSv (100 mrem). Doses below 0.5 
mSv (50 mrem) are normally much more influenced by statistical noise of 
the dosimetry system.

Dosimeters:
We found that in most cases where we could identify some anomally for 
doses above 1 mSv (100 mrem), it usually was due to the dosimeter itself 
(e.g. damaged element(s), incorrect Element correction factor(s), not 
properly annealed, very high single previous exposure, high total 
accumulated exposure). Our standard checks in such a case will be the 
following:
a) Check the dosimeter element readings, and glow curves if they were 
recorded, for anomalies
b) Check the dosimeter element correction factors
c) Check the dosimeter readout history: are there high previous doses, is 
the accumulated dose high, are there previous similar occurrences for the 
dosimeter
d) Check the dosimeter physically: are there signs of damage, chemical 
contamination, loose phosphor grains

Reader:
Check the reader for spurious behaviour (we have had cases where the 
Carbon-14 check sources had started to give inconsistent values, causing 
inconsistent reader sensitivity correction, and hence element values)

Dose algorithm:
We also had a few (very embarassing) cases where our dose algorithm did 
something weird with the dose calculation, because the actual computer 
program doing the calculation did not have certain built-in computer 
related checks.
For certain boundary conditions, a dose algorithm may give wrong results 
in any case, unless care is taken to enforce certain limitations (e.g. a 
second order polynomial was used to calculate contributing fractions to 
dose, but the polynomial would only be valid over the range of 
independant variable for which the data were fitted originally in 
establishing the dose algorithm)

Wearer:
There were cases where  wearers did nothing out of the ordinary 
personally, but left their dosimeters in a radiation area, or close to 
radiation sources.
There also were cases where one person wore another person's badge 
(although their names are clearly printed on the holders), and the other 
person actually received some exposure.

Unfortunately, we also have had cases where we are still in dispute with 
wearers over the dose allocated to them. The wearers did nothing out of 
the ordinary, nothing wrong could be found on the processing side, but 
yet the dosimeters in question gave every indication, beyond statistical 
doubt, that they were exposed

I hope this does not confuse you any more.

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