In general? No.
I believe a few places put "flux" into the header, but as Andreas just
mentioned that is only one of the bits of information you need to
calculate dose. If all you want is a rough estimate, then the numbers
you need are:
f = flux (photons/s)
t = exposure time (s)
w = wavelength (A)
a = beam area (um^2)
The dose (D) to a sample of protein/water/plastic under a given beam
will be roughly:
D = f*t*w^2/a/2000 (in Gy)
For example: a crystal under a square 100 um x 100 um beam at 1 A
wavelength with flux 1e12 ph/s will get 1 MGy dose in 20 s.
The 2000/w^2 is a fudge factor that fits the true curve of metal-free
protein crystals to within 15% for the wavelength range 0.5 < w < 3.
The error induced by not knowing if the beam was round or square and
just multiplying together the width and height to get the area (a), is
21%. The error from not realizing you had 100 mM uranium in your sample
is about a factor of two at 1 A. Smaller concentrations and lighter
atoms have less impact on accuracy.
If you don't know the flux, or beam size, you can try looking them up at
http://biosync.sbkb.org/ . I scraped these for my little dose calculator
here:
bl831.als.lbl.gov/xtallife.html
Some of the biosync numbers are more accurate than others, however,
depending on how often beamline scientists remember to update the site,
and how well they know themselves. And attenuation is not always
written into the header either.
In a pinch, you can estimate the flux by the total number of photons on
the image (P). This is assuming that you know the sample thickness (L)
in microns. You must also assume that the total scattering cross
section of the atoms in the sample is close to that of oxygen (0.2
cm^2/g), that the sample density is 1.2 g/cm^3 and that about 50% of the
scattered photons reach the detector. None of these are terrible
assumptions. The equation then becomes:
f = P/t/L*1.2e-5
Where 1.2e-5 = 0.2 cm^2/g * 1.2 g/cm^3 * 1e-4 cm/micron * 50%, f=flux
and t=exposure (as above).
Getting P from a pixel array is easy: you just add up all the pixel
values. From a CCD you want to be careful to subtract the baseline
value from each pixel first (40 on ADSC, 10 on Mar/Rayonix), and then
divide by the "gain", which near 1 A is ~1 on Mar/Rayonix, 0.6 for ADSC
Q315 (swbin) and 1.8 for Q315r (hwbin). A few considerations, yes, but
it can be a good sanity check.
-James Holton
MAD Scientist
On 5/5/2020 11:48 AM, Murpholino Peligro wrote:
Do diffraction patterns publicly accessible contain information about
the x-ray absorbed dose?
Thanks
------------------------------------------------------------------------
To unsubscribe from the CCP4BB list, click the following link:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?SUBED1=CCP4BB&A=1
########################################################################
To unsubscribe from the CCP4BB list, click the following link:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?SUBED1=CCP4BB&A=1