Why freeze autorads?

Andrew Cockburn afc at NERVM.NERDC.UFL.EDU
Thu Nov 21 04:59:38 EST 1996


I don't have the article here, but:

There was an article written in the late 1970's by William Bonner et al (in 
Biochemistry or some similar journal) on improving autoradiography. They 
showed that for tritium freezing at -80 shortened exposure hundreds of times; 
for C-14 it shortened exposure by about 10 times; for P-32 not at all.

The explanation was that the betas from tritium are so weak that it takes 
many of them to expose a silver grain. The intermediate states of the silver 
grain are unstable and decay back to the initial state in a 
temperature-dependent way. So freezing stabilizes these intermediate states 
and makes it more likely that the next beta will hit an already partially 
exposed grain. It takes fewer C-14 betas, so the effect is not as great. It 
only takes one P-32 beta, so the effect is non-existent.

However, this is without screens. A beta from P-32 is converted into light by 
the screen, and it is this light that exposes the film (the betas usually 
just pass right through the screen). Depending on the intensity of this 
light, it is possible that it would take more than one photon to expose a 
grain. Therefore it is possible that -80 would help.

In my experience, -80 helps a little with P-32 and screens, but not a lot. 
IMHO, the most useful thing about storing autorads in the freezer is that it 
is completely dark so cassettes with a light leak are less likely to ruin the 
film, and it reduces the problem of someone opening your cassette to see if 
it is empty.

Andrew Cockburn



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