R. Rex Denton
DENTON at biomed.med.yale.edu
Fri Jun 28 09:19:49 EST 1996
> > My best answer -
> > The intensifying screens are activated when exposed to -70C
> > temperatures.
> You DO NOT need two of them, just put one on one side of
> > the gel, and a piece of film on the other side. This procedure can be
> > substituted for merely developing 32P gels at room temp., with no
> > intensifying screen. This may suffice if you have a screaming hot gel.
> > INtensifying screens are good if you are looking for something barely
> > there, or if you are using 35S. Just remember, take the saran wrap OFF
> > when exposing 35S gels!
Okay, I've ignored it long enough,
The image obtained during autoradiography id the result of the radiation from
the sample causing a change in the valence of the Silver atoms embedded in the
emulsion of the film from Ag+ to Ag0. (The Ag + is salted with NO3.)
The Ag0 is responsible for the dark image on most films.
Like all reactions, this one is in a state of dynamic equilibrium, and the
valence of the Ag will move back and forth between ground (0charge) and +1
as a natural consequence of this equilibrium. Therefore, placing the cassette
in the freezer slows down the equilibrium, allowing more of the Ag0 to be
created; temperature doesn't alter radioactive decay, but will slow a
chemical reaction down. Thus, storing a film at -70 or colder, allows you to
see an image more quickly. I think I read somewhere, that on average,
an Ag+ requires more than one hit to become exposed and that is why film can be
somewhat non-linear in its' exposure latitude, and some people "pre-flash"
An intensifier "screen" is coated with rare-earth metals, (lanthanides)
that phosphoresce when hit with things that resemble electrons (radiation).
The electrons come in contact with the lanthanide electron shells, causing an
increase in their electron energy levels. The natural entropy of the decrease
in this energy
results in the emission of light at some frequency capable of exposing the
Thus, any radiation that makes it through the film, will cause the emission of
light from the screen. If your gel is on paper, the screen on the paper side
is worthless, as the light from doesn't go from the screen through the paper
supporting the gel. If you are working with 35S screens are nearly useless; the
radiation is not sufficiently energetic to go through the plastic.
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