*** For details on how to be removed from this list visit the ***
*** CCP4 home page http://www.ccp4.ac.uk ***
Leif,
How do you measure sample temperature?
Do you have any references on the mobility of protons in hyperquenched
water-cosolvent mixtures vs temperature? I have never heard 115K being
quoted in this context before.
-James Holton
MAD Scientist
Hanson, Bryant wrote:
I feel like we are stealing thunder from Ana and Sean's session at
ACA, but returning to the original question, going to a lower
temperature (i.e. helium cooling) can increase the diffractive
lifetime of your crystal. Experiments done by Unmesh Chinte under the
direction of Connie Schall and I show that XI crystals will have their
diffractive lifetime extended by a third if data measurement is done
at 15K vs 100K. Paper is currently making the final rounds through the
various authors prior to submission. It would get done quicker if we
weren't all reading the BB.
Rant on temperature - never trust any cryostat to deliver at the
crystal the temp it says on the readout. Eddie can say yea or nay on
this but we have found that when the device says 100K the xtal is at a
minimum 112K. Sometimes in the inverted orientation the temp will be
122K. This is important because protons will be mobile at 115K. So
let's say 5 to 10K heating from the beam,at most beamlines you will be
into the mobile proton region, and fast approaching the glass
transition temperature. Again more work by Unmesh has shown there is a
1mm sweet spot in the center of a cold stream. Venturing outside will
result in a 5 to 10K rise. 3mm away and local heating will cause the
crystal to warm past the Tg.
So make sure you are centered in the cold stream during data
measurement. As for using helium cooling, BioCARS and SBCCAT have
hellium cryostats. The cost for a liter of liquid He in the US is less
than a gallon of gas. Pinkerton's device uses about 4L per hour.
Leif Hanson
-----Original Message-----
From: [EMAIL PROTECTED] on behalf of Edward A. Berry
Sent: Tue 4/25/2006 4:13 PM
To: Ethan Merritt
Cc: James Holton; Ian Tickle; [EMAIL PROTECTED]; Edward Snell
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 ***
Ethan Merritt wrote:
>
> Back of the lunch-time napkin thought experiment:
>
> 100 watt light bulb is exposed to air (essentially nitrogen), losing
> heat through radiation and conduction over a surface of 4piR^2 for R
> approximately 30 mm. ===> ~ .01 watt per square mm.
> The light bulb is hot, according to everyday experience
Say light bulb is 70*, or 50* above ambient
Heat dissipation is limitted by convection in the surrounding air,
>
> 100 micron cube protein crystal heated by 2% of a 30 milliwatt
> X-ray beam losing heat through conduction over a surface
> of 6 x 0.1^2mm ===> ~ .01 watt per square mm.
> Crystal is now, one would think, light-bulb temperature.
>
Crystal might be 50* above ambient, but not light-bulb temperature.
And there is the wind-chill factor from the draft of the cold stream.
Still sounds like significant heating, but it is getting too
close for back-of the napkin calculations.
Ed
