Intensifying screens

Michael Schmitz schmitz at
Thu Jun 20 16:21:21 EST 1996

Chi-kuang Wen (ckwen at wrote:
: Laurence Tiley (lst21 at wrote:
: : Paul has it right.
: : Keeping the gel at -70 is necessary if you use preflashing, and for some of 
: : the fluors to work efficiently.

: Just a follow up:

: Keeping the film at -70C is to keep the e- at very low energy orbit, and
: the perception of radiation may excite the low-energy e- to a higher energy
: orbit.  Following a back from higher-energy to low-energy orbit exchange, acc-
: ording to E=Hv, light is emitted onto the film.  At higher temperature, RT, the
: e- is at higher energy orbit and much less excited light may be generated.

I can't believe that: energy levels in atoms are on the eV scale, and
thermal energy is on the milli-eV scale, insufficient to induce transitions
(these can only occur by collision, or by radiation). 
Energy levels in atoms are _not_ Boltzmann-distributed, if I understand the
Pauli-rules right :-). And the intensity of emitted light is not related to
the 'energy' orbit - the _color_ is.

What _is_ dependent on the temperature of the screen is the rate of
collisions of the fluorophores with other atoms in the screen material, and
these may lead to non-radiative decay of the excited state. Fluorescence 
transitions may have long lifetimes in the excited state, so these thermal
relaxation processes may be an issue.


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