Staplex Particle Redistribution

[Robert_A_Jones@RL.gov]

     Howdy Radsafers,
     
     I have had an interesting problem given to me.  Our sample counter guy 
     ran a staplex planchet on our Surface Barrier Alpha Spec on day, and 
     around one week later, was asked to count it again.  When he did, the 
     number of counts (In the Pu region) dropped to 1/3-1/4 the original 
     value.  It is doubtfull that this is because of Radon because 1) the 
     Staplex has about a 5% radon collection efficiency and 2) This is in 
     the Plutonium region on the spectrum (PHA analysis).  
     
     These are my assumptions, which have been verified by review:
     
     a)  The sample was taken according to procedure (one drop oil, spread 
     around the planchett).
     
     b)  The sample was not disturbed after the first count (rubbing would 
     remove some of the activity)
     
     c)  The sample was not stored in temperature extremes.
     
     
     My thoughts are that, over time, the particulates that are captured 
     have re-dispursed within the oil.  The oil may appear to be thin film 
     to us, but may appear to the (self-aware and always fashionable) 
     particle like a small lake.  This would cause self shielding of the 
     alpha particles if this is true, and if the thickness is there to 
     justify this possibility.  
     
     This may seem to have an easy answer, "Don't wait a week to count a 
     Staplex," but the problem suggests a minimum transfer time (from the 
     field to the detector) if this is possible.
     
     Any thougts for this would be helpfull, especially those with 
     experience with Staplex samplers for Pu.
     
     Robert A. Jones
     Health Physicist - PFP
     Hanford, WA
     (509)376-8528
     Robert_A_Jones@RL.gov