Mo vs Cu K-alpha radiation

Dr H.R. Powell hrp1000 at
Thu Nov 23 07:53:25 EST 1995

I've been wondering about whether copper radiation has a big advantage (if
any) over molybdenum apropos data collection on oligonucleotide crystals
(or even on protein crystals). I know that for a given generator power,
the copper radiation will be more intense, but it will also be attenuated
more by the air path (and scattered more by the air) and will also be
absorbed more by the crystal. For an area detector, to get the same spot
separation, the "film" has to be roughly twice as far away for Mo than for
Cu (so the air scatter/absorptioneffect will be compensated for to some
extent). However, the spots will thus be less spread out due to oblique
incidence on the detector, and the peak intensity should stand out higher 
above background. 

There are a lot of other arguments that I could bring up, but what I was 
interested in is:

"In these days when people freeze and re-use their nucleotide/protein
crystals on a regular basis, has anyone done the experiment where they
collect a dataset using one radiation, and then KEEPING ALL OTHER
VARIABLES CONSTANT (as far as possible), collecting a dataset on the same
sample with the other radiation?". 

Ideally, this would be on an installation where both copper and molybdenum
targets are interchanged. 

I remember Peter Moody from York talking about this, but as I recall, he 
didn't actually do the experiment with the same crystal.

# Harry Powell
# Cambridge Crystallographic Data Centre
# 'phone: +44 1223 336015
# fax:    +44 1223 336033

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