Mag fields and GMs -Reply

[SIMPSOND@ORAU.GOV]

I have enjoyed the current thread of comments on EM field effects on
radiation instruments as I have seen these effects confuse people for a
number of years.  These effects are well worth demonstrating to
newcomers in radiation protection.  At our facility we have two
demonstrations that have worked very well with students.  The first
uses a Tesla coil and and an old Victoreen "cutie pie" ion chamber. 
Using the "zero adjust" we raise the needle on the instrument to 15-20
mR/hr, then turn on the Tesla coil.  From 6-8 feet away, the needle will
drop to zero. (Actually it drops below zero, which is a good indicator for
this instrument that its being effected by an EM field) 

In the second demo, we use a bar magnet and a NaI portable scintillation
detector.  When the magnet is held near the point on the probe where the
crystal contacts the PMT, the count rate will drop from its usual value of
a few 1000 cpm to almost zero.

Dave Simpson
Oak Ridge Institute for Science and Education
Professional Training Programs