the function of -70 for X-ray film

[Michael Szardenings]

In article <bjanssen-031195103641 at hall-quadra950.tamu.edu>,
bjanssen at bio.tamu.edu (bart janssen) wrote:

> In article <1995Nov1.145830 at molbiol.ox.ac.uk>, rpgrant at molbiol.ox.ac.uk
> (Mad Dan Eccles) wrote:
> 
> > In article <199510281917.OAA13346 at visar.wustl.edu>,
nikolaic at VISAR.WUSTL.EDU (Nikolai Chitaev) writes:
> > >> 
> > >> I have a question to ask.   What is the real function of exposure
> > >> the p32 X-ray film in -70 c?
> > >> 
> > >> lili
> > > 
> > > I guess that at -70oC your ENHANCEMENT SCREENs will reflect emmited 
> > > b-particles toward the film, so providing more energy otherwise being 
> > 
> > No.  The screen actually phosphoresces (or fluoresces); the b-particle hits 
> > the screen, a photon(s) of light then comes back and darkens the film.
> 
> I always thought that the advantage of -70 C was simply that at colder
> temperatures nuclei and beta particles have less energy and hence are more
> likely to interact with the film and/or the intensifier screen. 

Not quite, but besides the latent image effect, the same thing is going on
for the intensifier. When a fluorescent/phosphorescent molecule in the
screen absorbs a quant of energy from radiation, this energy can be
returned in the form of light/radiation. As some time passes until the
energy is released by fluorescens or phophorescens (latter if this takes
longer than >10E-3 sec), there is some chance for the excited molecule to
loose the energy by contact to a neighbouring molecule without the
emission of light/radiation. As the molecular movements are slowed down at
-70 C, this helps to improve the yield to secondary radiation.
Many X-ray films actually seem to contain such intensifiers and its quite
depending on the relaxation rate (or stability of the excited state)
whether there is a real effect at -70 C.
I expose my 35S sequencing gels anyway at RT, because Idon't like to wait
for thawing the next morning :-)
Regards
            Michael

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