[ RadSafe ] Reference source re-calibration

[John G. Hunt]

Perhaps Radasfers could help with a question that I have not been able to 
answer:

The problem:
For the rectangular reference sources (tracable) for calibrating hand-held 
surface contamination monitors - alpha emitters or beta-gamma emitters;
For sources with long lived radionuclides (Am-241 for example);
For sources that are well looked-after;

The question:
How often do these sources have to be re-calibrated or renewed?    
Is there an international or national or "good practice guide" that refers to 
this problem, and to the (re-)calibration interval?

John Hunt.
 
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