the function of -70 for X-ray film

kitchingman at mbcf.stjude.org kitchingman at mbcf.stjude.org
Mon Oct 30 08:20:34 EST 1995


In article <ba1-2910951557210001 at donnabuang.mrc-lmb.cam.ac.uk>, ba1 at mrc-lmb.cam.ac.uk (Brian Ayre) writes:
> In article <46rn3m$d8b at sifon.cc.mcgill.ca>, lIQIN WANG
> <CZLQ at PO-Box.McGill.CA> wrote:
> 
>> Hi,  Dear netters,
>> 
>> I have a question to ask.   What is the real function of exposure the
> p32 X-ray film in -70 c?
>> 
>> lili
> 
> Contact the people at Amersham and ask for the publication "Guide to
> Autoradiography."  It is good all purpose reference, and gives an
> explanation slightly different from those posted.  Particularly, that the
> silver halide emulsion responds differently to photons (generated by your
> screen), and the beta particles from your sample.  The low temp.
> stabilises the silver excited by photons.
> 
> If you do not use a screen, exposure at -70 will not do anything except
> frost-bite your fingers!.

To elaborate a little on this, it takes two hits to deposit a silver grain; one
to excite to a silver ion, and one to generate silver metal.  The half-life of
the silver ion at room temperature is on the order of milliseconds, so unless
you have a very hot sample most of the decays end up being non-productive.  The
half-life of the silver ion at -70 is very long so that there is a good chance
that a second decay will deposit a silver grain.  The silver salt is not so
densely packed as to stop every decay from 32P or 35S, i.e., some beta
particles go right through the film without encountering the silver.  With
intensifying screens, these are captured and turned into light by the rare
earths on the screen.  The light will probably be scattered at a different
angle than the incoming beta particle, hence the size of the band will be
increased relative to direct autoradiography.  At least this is what I was
taught 20 years ago.

Geoff K.> 
> It also gives an interesting discussion on why band densities on
> autoradiograms produced with screens are *NOT* linear, unless preflashed.
> 
> Brian Ayre



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