[ RadSafe ] TLD questions

[Kai Kaletsch]

Friends,

So far, I have always just taken the dose value that the TLD readout service 
generates and assumed it is correct. Recently, it has become apparent that I 
need  some education regarding TLDs.

I am the radiation safety consultant for a uranium mine in a developing 
country. The local readout service did not have enough TLDs, so we purchased 
300 cards. (I was not involved at that time.) They were TLD-100 LiF:Ti,Mg 
Type "0110" #53196# (don't know what that means) from Thermo Electron for a 
model 4500 reader. I think there are 2 TLDs per card. The cards are on site 
(on a different continent). So, I can't confirm the configuration right now.

The readout service took some time making enough cards available to us. The 
reason was that some were not annealing properly.

Question 1: What is the annealing process and is it reasonable that there 
should be difficulty with this process? Keep in mind that we don't know in 
what shape the equipment used by the service is.

The TLDs were on a monthly changeover schedule. I suggested that we go to a 
quarterly changeover, instead. That way, we could keep the TLDs that were 
issued for another 2 months while the annealing issue was being sorted out. 
(Expected gamma exposures were very low.) The readout service indicated that 
these were monthly TLDs and should not be used on a quarterly basis. We 
ended up getting enough TLDs by the end of the month. However, I would like 
to eventually go to a quarterly change over for other reasons.

Question 2: What makes a TLD monthly, rather than quarterly?

The results we get have headings of: Name, TLD No. Roii, Roiii, D mSv. I 
assume that Roii and Roiii are the raw light outputs of the 2 TLDs in the 
card, I.e. "Region of Interest i and ii".

Question 3: Does my interpretation of Roii and Roiii make sense?

The ore grade is low and there has been very little activity near the ore 
lately. The control dosimeters are reading around 0.05 mSv and most worker's 
dosimeters are below 0.1 mSv. (Control readings are not subtracted from 
worker's readings.)

Question 4: Is there consensus on whether control dosimeter readings should 
or should not be subtracted from worn readings.

Some readings are considerably higher (up to 1 mSv). It is pretty much 
mathematically impossible for anyone to get these exposures in a month. 
Furthermore, the individuals that seem to get the highest readings are the 
ones that spend the least time near the ore. For the "well behaved" 
dosimeters, Roii is approximately equal to Roiii and is between 5 and 10. 
For the outliers, there is normally a large difference between Roii and 
Roiii and either Roii or Roiii can be larger. For example, one worker had 
Roii = 84.51 and Roiii = 14.31 and was assigned 0.98 mSv while another had 
Roii = 14.84 and Roiii = 24.15 and was assigned 0.33 mSv.

Question 5: Would one expect these Roii and Roiii readings from properly 
functioning dosimeters, or is this an indication that something is wrong.

Some of the people with higher readings spend more time outside than others 
and it can get pretty cold on site.

Question 6: Are TLDs cold sensitive?

I only have a subset of the data here and am not sure if the same people 
always end up with elevated results or if it is the same TLDs that always 
end up with elevated results. My next steps would be to get all the data and 
look more closely at these correlations. However, I would appreciate input 
from other people before I go on a wild goose chase. (...and, yes, I am also 
planning to talk to the TLD manufacturer...)

Question 7: Is it possible to permanently damage a TLD so that it 
consistently reads high.

Thank You in advance and Best Regards,
Kai

Kai Kaletsch
Environmental Instruments Canada Inc.
#202 135 Robin Cr.
Saskatoon, Saskatchewan
Canada S7L 6M3
Tel: 406 686 0081