Why freeze autorads?

Eric R. Hugo e_hugo at dsu1.dsu.nodak.edu
Sat Nov 30 10:06:13 EST 1996


In article <576c93$vme at raven.cc.ukans.edu>, PGegen at UKans.edu  (Dr. Peter Gegenheimer) wrote:
>In <wgallin.1198981441C at news.srv.ualberta.ca>, wgallin at gpu.srv.ualberta.ca
> (Warren Gallin) writes:
TEXT DELETED

The reason for exposure at -70 C is because of reciprocity failure in the 
photographic emulsion.  For a halide crystal to be activated (that is 
competent for development) it needs several activating photons in a given time 
period.  At room temp the half-life is less than 0.5 seconds.  That means that 
the silver halide crystal needs to get 'hit' several times in less than a 
second (I think 5 photons is about the right number).  At minus 70 the half 
life is greatly extended (I think to over 30 minutes).  Thus for low activity 
areas where one only gets a flash from the intesifying screen every minute or 
so the -70 C exposure will increase sensitivity.

                                                                             
Eric

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