Why freeze autorads?

[klenchin at facstaff.wisc.edu]

In article <3293A2DD.6BCF at pt.cyanamid.com>,
   "S. Silverman" <silvermans at pt.cyanamid.com> wrote:
->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?

This, to my great surprise, has been discussed here to death. Briefly:

1. radioactive decay (and thus emission) does not depend on temperature;
2. degree of "diffusion" of bands is a function of particle's energy -
the higher is energy, the more diffusion; compare  32P and 33P and 35S
(emmision direction is random, absorption probability decreases with 
decreasing angle but increases with the energy of the particle; thus, 
32P will excite larger area on screen than 33P)
3. as with any chemical reaction, there is an equilibrium Ag <-> Ag+
(excited state "decays), and as with any chemical reaction, it is 
temperature dependent. Thus, the idea of -70C is to increase sensitivity.
You'd have the same result at +20C if you wait longer. 

->	The absorption and emission properties of the material in 
->intensifying screens depend on temperature.  The screens are more 
->efficient at low temp.  It will take someone with more familiarity with 
->molecular orbital theory than me to give you a more precice reason.  

I don't think so. Intensifying screens emit light upon absorption of
high-energy particles. Light excites silver on film. Freezer is to 
increase sensitivity of latter. 

- Dima