Intensifying Screens

[Warren Gallin]

On Mon, 20 Feb 1995, Colin Rasmussen wrote:

> In article <199502202034.OAA06083 at mail.utexas.edu>,
> kbrowning at MAIL.UTEXAS.EDU (Karen Browning) wrote:
> 
> > screen was the Quanta III by Dupont.  Exposing at -70 was necessary for 
> > enhancement of 32P and 125I. 
> 
> ...because the film becomes more sensitive to photons at low temperature
> if I remember right....

Not quite.  The film only becomes more sensitive to low photon flux at low 
temperature.  The low temperature prevents the thermal decay of unstable 
intermediate states in the formation of the "holes" in the individual 
silver halide crystals in the emulsion.  At room temperature the damage 
done by one photon to the crystal will decay before a second hit occurs 
that could stabilize the exposed state.  That's why the film response is 
non-linear at very low exposures (reciprocity failure).  Astronomers have 
been using the technique of cooling film for years to get good exposures 
of low intensity images in multi-hour exposures.  What this means is, 
even if you are not using enhancing screens, low temperature exposure 
will make it possible to see bands with a very low photon or particle 
flux per unit area.

Hi Colin, how's it going?

Warren Gallin