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<lurk_mode_off> Thanks James, I've been following this discussion with interest. We're busy writing up the paper but in un-refereed summary as long as you have a cryostream running you remove heat faster than it can build up. The rise in temperature due to the beam for a crystal in a cryostream at 100K will not take it into the phase change region and should not even take it to the point where free radicals are mobile even for the worst case scenario. Having said that we generate this data from a model validated by experimentally testing with time resolved infrared imaging of the sample as the shutter opens, the beam hits it and until it reaches steady state. We have a spatial resolution of ~7 micron^2 pixels so it's possible that on a very small scale the heat may be much more but on the length-scale of our observations we do not see this. As an experimentalist this area is fascinating - a lot can be learned from simple elegant experiments such as the one James described. There have been four superb international workshops spearheaded by Elspeth Garman, Gerd Rosenbaum and Colin Nave (I hope I mentioned everyone) and there are still many questions concerning radiation damage and related areas. The next one will be in 2008 and hopefully by then some more of those questions will be answered and probably even more generated. Cheers, Eddie Edward Snell Ph.D. Assistant Prof. Department of Structural Biology, SUNY Buffalo, Hauptman-Woodward Medical Research Institute 700 Ellicott Street, Buffalo, NY 14203-1102 Phone: (716) 898 8631 Fax: (716) 898 8660 Email: [EMAIL PROTECTED] Telepathy: 42.2 GHz Heisenberg was probably here! <lurk_mode_on> -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of James Holton Sent: Tuesday, April 25, 2006 1:49 PM To: Ian Tickle Cc: [EMAIL PROTECTED] Subject: Re: [ccp4bb]: Radiation damage problem *** For details on how to be removed from this list visit the *** *** CCP4 home page http://www.ccp4.ac.uk *** Ahh yes, the ol' "beam heating hypothesis". I used to believe in it too... James Murray just cited studies that cast a great deal of doubt on the idea that there is any significant heating of protein crystals by x-ray beams. I don't want to steal all the thunder form the RD4 meeting last month, but Eddie Snell and Michael Kazmierczak HAVE done heat transfer calculations and compared their predictions to the results of thermal imaging experiments. http://www.spring8.or.jp/en/users/meeting/rd4/rd4 (last two talks) Very nice work IMHO. And I think they have put the last nail in the coffin of the "beam heating hypothesis" of radiation damage. If you think about it, a 30-degree gradient over 100 microns is 3000 degrees/cm. That's like poking a red-hot nail into ice and expecting it to keep glowing. There are very few substances that have a low enough thermal conductivity to maintain such a strong thermal gradient with only a few milliwatts being deposited at the "hot" end. None of the many phases of water are that good an insulator. Even the most powerful x-ray beams in the world are only milliwatt-class sources of energy, and typically, only about 2% of the energy from the beam is deposited in the sample. Then again, it is always easier to rationalize things once you know the right answer... I don't think the impact of solutes on the phase transition between glassy water, low-density amorphous water and the ultraviscous deeply supercooled water phase between 136 and 160K is something that has been studied all that much. Although there is a very instructive table summarizing much of the work on amorphous water phases here: http://www.lsbu.ac.uk/water/amorph.html One thing I can tell you is that I did an experiment looking at the diffraction pattern of a MPD-PEG8K-water solution as I slowly ramped up the cryostream temperature. I saw a fairly clear "elbow" in the graph of "correlation coefficient to the first image" vs temperature at ~135K, exactly where the glass transition of water is supposed to be. A singular result, I admit, but I think it supports the hypothesis that our cryostream temperature readings are indicative of the sample temperature. -James Holton MAD Scientist Ian Tickle wrote: >Unless of course the crystal isn't as cold as you think it is! >Essentially all of the energy of absorbed X-ray photons must ultimately >be degraded to thermal energy, and unless this is efficiently conducted >to the surface where it can be removed by the cold stream, it's going to >produce some 'hot spots' which given sufficient X-ray flux may be >sufficient to cause local melting of the water glass. There could well >be a steep temperature gradient, liq N2 temperature at the surface and >much warmer towards the centre. Also if low molecular weight solutes >are present the melting point could be locally depressed way below that >of pure water, so there wouldn't be quite so far to go to cause melting. >Admittedly I haven't looked at any calculations of what order of >magnitude the heating and heat conduction effects might be for a typical >flux and I could be way off beam but it's a least a theoretical >possibility. > >-- Ian > <snip>
