Eric Hugo <e_hugo at dsu1.dsu.nodak.edu\> wrote:
> I do not believe that the -70C exposure has anything to do with
> fluors or intensifying screens. The -70C exposure helps to reduce
> reciprocity failure in the film in a similar way to preflashing
> of x-ray film does. As I understand it a silver halide grain
> must be 'hit' several times within a given time period to be
> activated. At room temp the half-life between activating 'hits'
> is very short. At -70C the window for receiving the activation
> events (photons or beta particles) is much longer thus the film
> has a much more linear response to low levels of activity.
This explanation is absolutely right except for two words which makes
the initial thesis wrong.
Let's consider 32P beta particles as the example. They have maximal
energy 1.7 MeV and travel in solid media aprox. 1 cm, i.e. beta
particles loose 170 eV (electronvolts) of energy in every micrometer.
Silver halide crystals in X-ray film emulsion are several micrometers
size, blue light photons have energy aprox. 3 eV . So if a beta
particle hits the silver halide crystal, it produces in this crystal
various ionizations, excitations and photochemical reactions
equivalent to absorbtion of tens or hundreds of visible light
photons. Therefore one hit by a beta particle if sufficient to
activate the silver halide grain, and exposition of photographic
emulsion by beta particles does not suffer reciprocity failure and
doesn't need -70C
Dept. of Biochem. and Biophys., Vilnius University, Lithuania
Rimantas.Plaipa at GF.VU.LT