gal-1 at bones
Mon Feb 20 20:58:27 EST 1995
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?
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