the function of -70 for X-ray film

[Eric R. Hugo]

   Stephen Lessnick <stevel at ucla.edu> wrote:

>>I expose my 35S sequencing gels anyway at RT, because Idon't like to wait
>>for thawing the next morning :-)
>
>
>A couple of points...
>
>1)  Why would one expose 35S sequencing gels at -70 degrees in the first 
>place since they don't possess enough energy to make the enhancing screen 
>worthwhile.  On the other hand, soaking the gel in salicylate or PPO 
>would provide the enhancement effect, and then I believe it would make a 
>difference to expose the gel at -70.
>
>2)  You don't really need to thaw the gel after a -70 degree exposure.  
>Just take the frozen cassette into the darkroom, and pull the film off 
>while frozen.  Provided the X-omat is properly warmed up, the autorad 
>will come out just fine.  I know, because I usually had to develop my 
>films immediately before our a.m. lab meetings...
>
>Have fun
>
>Steve
>

I do not believe that the -70C exposure has anything to do with
fluors or intensifying screens.  The -70C exposure helps to reduce
reciprocity failure in the film in a similar way to preflashing
of x-ray film does.  As I understand it a silver halide grain
must be 'hit' several times within a given time period to be
activated.  At room temp the half-life between activating 'hits'
is very short. At -70C the window for receiving the activation
events (photons or beta particles) is much longer thus the film
has a much more linear response to low levels of activity.
				Eric


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