Mo vs Cu K-alpha radiation
hhope at ucdavis.edu
Fri Nov 24 22:01:14 EST 1995
23 Nov 1995 Dr H.R. Powell wrote:
>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).
With weakly diffracting light-atom crystals Mo radiation is not very
useful in comparison with Cu radiation. There are two main reasons for
1. Around 50 kV the X-ray generation efficiency for Mo is only about half
of that for Cu.
2. The lambda cubed¾ dependence of scattering favors Cu by about a
factor of 10.
Air absorption of Cu radiation is not negligible, but is commonly
counteracted by use of He chambers (or vacuum in the beam tunnel).
The Lp factor also tends to work against you with Mo, but this depends on
the resolution, and may or may not play a role.
The result of these factors is typically that a detector receives 10-20
times as many desirable x-ray quanta with Cu as with Mo.
This of course does not mean that a shorter wavelength never works.
If the crystal is large enough and diffracts well enough or the beam
intensity is high enough, a shorter wavelength may well be preferable.
Synchrotron radiation about 1 A is often a good choice.
>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
About 11 years ago I did the suggested experiment (almost). I collected
both Mo and Cu data on the same crambin crystal. In about two days I
collected a much stronger data set with Cu than I obtained in a week with
Mo. Data were measured with the crystal at 120 K.
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