Bart >> Diffracted intensity goes up by the cube of the wavelength, but so >> does absorption and I don't know exactly about radiation damage.
I think this statement should be "As an approximation, diffracted intensity (integrated) goes up by the square of the wavelength, but so does absorbed energy." See for example Arndt, U. W. (1984) Optimum X-ray wavelength for protein crystallography. J. Appl. Cryst. 17, 118-119. Fig. 1. Plot of Ie (not Ip). Note that this includes loss of signal due to absorption through the sample. Subsequent calculations have included Compton scattering and other factors. I believe Zachariasen first specifically pointed out the wavelength dependence of the integrated intensity (one has to include a Lorentz factor). For the second factor, the absorption of a photon approximately follows the cube of the wavelength but the absorbed energy itself follows the square of the wavelength. Regards Colin -----Original Message----- From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Bart Hazes Sent: 16 February 2012 15:01 To: ccp4bb Subject: Re: [ccp4bb] choice of wavelength Hi Andrew, I completely agree and it is what I meant by "(assuming the full photon energy is captured)". If the fraction of photons counted goes up at longer wavelengths than the relative benefit of using longer wavelength is even more pronounced on Pilatus. So for native data sets the wavelength sweet spot with a pilatus detector may be a bit longer then what used to be optimal for a given beamline on a previous generation detector. Bart On 12-02-16 02:09 AM, A Leslie wrote: > > On 15 Feb 2012, at 23:55, Bart Hazes wrote: > >> Diffracted intensity goes up by the cube of the wavelength, but so >> does absorption and I don't know exactly about radiation damage. One >> interesting point is that on image plate and CCD detectors the signal >> is also proportional to photon energy, so doubling the wavelength >> gives 8 times diffraction intensity, but only 4 times the signal on >> integrating detectors (assuming the full photon energy is captured). >> So it would be interesting to see how the equation works out on the >> new counting detectors where the signal does not depend on photon >> energy. > > > You make a good point about the variation in efficiency of the > detectors, but I don't think your comment about the "new counting > detectors" (assuming this refers to hybrid pixel detectors) is > correct. The efficiency of the Pilatus detector, for example, falls > off significantly at higher energies simply because the photons are > not absorbed by the silicon (320 microns thick). The DQE for the > Pilatus is quoted as 80% at 12KeV but only 50% at 16KeV and I think > this variation is entirely (or at least mainly) due to the efficiency > of absorption by the silicon. > > Andrew > > > >> Another point to take into account is that beamlines can have >> different optimal wavelength ranges. Typically, your beamline guy/gal >> should be the one to ask. Maybe James Holton will chime in on this. >> >> Bart >> >> On 12-02-15 04:21 PM, Jacob Keller wrote: >>> Well, but there is more scattering with lower energy as well. The >>> salient parameter should probably be scattering per damage. I >>> remember reading some systematic studies a while back in which >>> wavelength choice ended up being insignificant, but perhaps there is >>> more info now, or perhaps I am remembering wrong? >>> >>> Jacob >>> >>> On Wed, Feb 15, 2012 at 5:14 PM, Bosch, Juergen<jubo...@jhsph.edu> >>> wrote: >>>> No impact ? Longer wavelength more absorption more damage. But >>>> between the choices given no problem. >>>> Spread of spots might be better with 1.0 versus 0.9 but that >>>> depends on your cell and also how big your detector is. Given your >>>> current resolution none of the mentioned issues are deal breakers. >>>> >>>> Jürgen >>>> >>>> ...................... >>>> Jürgen Bosch >>>> Johns Hopkins Bloomberg School of Public Health Department of >>>> Biochemistry& Molecular Biology Johns Hopkins Malaria Research >>>> Institute >>>> 615 North Wolfe Street, W8708 >>>> Baltimore, MD 21205 >>>> Phone: +1-410-614-4742 >>>> Lab: +1-410-614-4894 >>>> Fax: +1-410-955-3655 >>>> http://web.mac.com/bosch_lab/ >>>> >>>> On Feb 15, 2012, at 18:08, "Jacob >>>> Keller"<j-kell...@fsm.northwestern.edu> wrote: >>>> >>>>> I would say the better practice would be to collect higher >>>>> multiplicity/completeness, which should have a great impact on maps. >>>>> Just watch out for radiation damage though. I think the wavelength >>>>> will have no impact whatsoever. >>>>> >>>>> JPK >>>>> >>>>> On Wed, Feb 15, 2012 at 4:23 PM, Seungil Han<shan06...@gmail.com> >>>>> wrote: >>>>>> All, >>>>>> I am curious to hear what our CCP4 community thoughts are.... >>>>>> I have a marginally diffracting protein crystal (3-3.5 Angstrom >>>>>> resolution) >>>>>> and would like to squeeze in a few tenth of angstrom. >>>>>> Given that I am working on crystal quality improvement, would >>>>>> different wavelengths make any difference in resolution, for >>>>>> example 0.9 vs. 1.0 Angstrom at synchrotron? >>>>>> Thanks. >>>>>> Seungil >>>>>> >>>>>> -------------------------------------------- >>>>>> >>>>>> Seungil Han, Ph.D. >>>>>> >>>>>> Pfizer Inc. >>>>>> >>>>>> Eastern Point Road, MS8118W-228 >>>>>> >>>>>> Groton, CT 06340 >>>>>> >>>>>> Tel: 860-686-1788, Fax: 860-686-2095 >>>>>> >>>>>> Email: seungil....@pfizer.com >>>>>> >>>>>> >>>>> >>>>> >>>>> -- >>>>> ******************************************* >>>>> Jacob Pearson Keller >>>>> Northwestern University >>>>> Medical Scientist Training Program >>>>> email: j-kell...@northwestern.edu >>>>> ******************************************* >>> >>> >
