Why freeze autorads?

William Tivol tivol at news.wadsworth.org
Wed Nov 20 17:04:45 EST 1996

Warren Gallin (wgallin at gpu.srv.ualberta.ca) wrote:

: In Article <56vh41$292s at piglet.cc.uic.edu>, blackman at tigger.cc.uic.edu
: (Samuel C. Blackman) wrote:
: >My labmates and I, obviously with too much time on our hands, were 
: >wondering why we put our 32P-labelled autorads in the freezer at -80C.
: >Our advisor hypothesized that the low temp. promotes a more "focused"
: >autorad, but without a good explanation for that.  The decay equation
: >has no mention of temperature, so we're stumped.  Any ideas?
: >
:     The short answer is "reciprocity failure".  At low exposure rates the
: effect of a single radiation event on a single halide crystal in the film
: emulsion will  often decay before subsequent stabilizing events hit.  Ths is
: reflected in the film reciprocity curve, which shows a non-linear
: relationship between exposure and optical density at very low temperatures. 
: The decay of single event effects is temperature dependent.  Thus, by
: lowering the temperature, you prevent the decay and increase the sensitivity
: of the film.  You can get the same effect by pre-flashing the film.  The
: techniques was used by astronomers long before molecular biologists twigged
: to it; they pack their cameras in dry ice for long exposures.
:     Note that this is only a significant factor if the signal is low and the
: exposure has to run for days.

Dear Warren & Sam,
	The short answer is OK for exposure of film to light; however, 32P
is a high-energy (1.7 MeV endpoint) beta-emitter.  X-ray film or other elec-
tron sensitive film needs only one event to render a grain developable.  I
work with a 1.2 MV electron microscope, so I deal with this a lot.  There
may still be some decay in the response due to changes the film undergoes
at room temp over the course of a few weeks, but it is not the same phenom-
enon as with visible light.  It should be fairly easy to do the relevant
experiment if you really have so much time on your hands.
				Bill Tivol

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