Production of X-rays 2007 ACA Summer School Illinois
Production of X-rays 2007 ACA Summer School Illinois Institute of Technology T. I. Morrison Physics Department, Center for Synchrotron Radiation Research and Instrumentation, IIT Modified 2007 by Andy Howard Historical Background Wilhelm Conrad Roentgen was German discovered X-rays in 1895 is currently dead http://www.nobel.se/physics/laureates/1901/rontgen-bio.html
X-rays are electromagnetic radiation over a range of energies or wavelengths; the specific range depends on the author http://images.google.com/imgres?imgurl=www.srp-uk.org/gif/emspectrum1.gif&imgrefurl=http://www.srp-uk.org/spectrum.html&h=409&w=755&prev=/ images%3Fq%3Delectromagnetic%2Bspectrum%26start%3D40%26svnum%3D10%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26safe %3Doff%26sa%3DN For this author, the range is from about 1.2 KeV (soft X-rays) to about 1,020KeV (pair production) E=h= hc/ In practical terms, E (keV) =12.3984/ (Angstroms)
Origins of electromagnetic radiation: Acceleration (deceleration) of a charged particle Transitions between electronic (or molecular) states (also interpretable as a change in momentum) Consequence of the constancy of the speed of light: it takes a finite amount of time for the information that a charged particle has changed its velocity to get to another point, thus changing the electric field density at that point. This causes a pulse in the em field; a series of pulses makes up a wave train Deceleration of a charged particle Brehmsstralung Bremsstrhalung Brhemsstrahlung Bremsstrahlung: Braking radiation
Deceleration of a multi-keV electron in a metal target: e - e- Bremsstrahlung spectral output Braking radiation Classical theory Self-absorption Emitted radiation Bremsstrahlung: dependencies on
experimental options http://jan.ucc.nau.edu/~wittke/Microprobe/Xray-Continuum.html From Kramer (1923) Icontinuumconst.) ibeamZtarget(Eaccel-E)/E Transitions between electronic states: Characteristic lines http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xrayc.html#c1 Characteristic lines Characteristic lines are unique to each element. They are caused by decay processes in which a hole in a core-level shell is filled by an
electron from a higher-energy shell. e- X-ray Intensities of Characteristic lines Can be understood as product of hole formation probability times relaxation cross-section: ||2 * |< 1s2 |er|2p(hole)>|2 I = (f(Z)) ielectron beam(E0 Ec)p where p~1.7 for E0 < 1.7 Ec (and smaller for higher values of E0) http://jan.ucc.nau.edu/~wittke/Microprobe/Xray-LineIntensities.html Transitions:
2p -> 1s: 3p -> 1s: K 3p -> 2s: L K Characteristic X-ray Emission Lines:
Atomic Energy Level Transitions http://xdb.lbl.gov/Section1/Sec_1-2.html X-Ray Emission Lines K-level and L-level emission lines in KeV No. Element Ka1 Ka2 Kb1 La1 La2
13.3817 14.7644 Fe Co Ni Cu Zn As Se Br Mo Tc Ru Rh Pd
Ag I Ta W Ir Pt Au Pb 6.40384 6.93032 7.47815 8.04778 8.63886 10 .54372 11.2224
11.9242 17.47934 18.3671 19.2792 20.2161 21.1771 22.16292 28.6120 57.532 59.31824 64.8956 66.832 68.8037 74.9 694 Values are from J. A. Bearden,
-Ray Wavelengths", "X , (January 1967) -99, pp. unless 86 otherwise Review of Modern Physics noted. So far, so what? How are X-ray really produced? Here is the general idea: But it isnt quite this simple. http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xtube.html#c1
For most diffraction studies, the X-ray source should be Intense A point Monochromatic Oh. Anything else? Intensity More photons on sample => shorter acquisition times => more publications/unit time => decreased rate of funding cuts X-ray generation is EXTREMELY inefficient Total power ~ accelerating potential x electron beam current ~ 99% of total power goes into heat production Icontinuumconst.) ielectron beam Ztarget(Eaccel-E)/E
Icharacteristic (f(Z)) ielectron beam(Eaccel Ec)p It gets worse: Electron beam X-rays are produced nearly isotropically; very few go where you would like them Your experiment How bad is is REALLY? An example: 3 kW X-ray source
1.54 radiation (8.05 keV) 107 photons/sec (107 *8.05 *103 *1.6 *10-19)/(3*103) ~ 0 (=4*10-12) Power supplies: Virtually always the anode floats at 20-80KV; the cathode is grounded This has typically meant big, heavy supplies with big, heavy transformers 18 KW 60 KV 300 mA
http://www.rigaku.com/protein/ruh3r.html which will eventually be replaced by small, lightweight HF supplies 20 KW 200 KV 100 mA http://www.voltronics.com/products/index.html Point sources Stipulate the need for high intensities Smaller source puts more X-rays on sample => More difficulties: high heat loads
Typically acceptable source size ~ 1mm x 1mm 3kW/mm2 exceeds most materials capabilities Multiple approaches required: Spread beam out Active water cooling Move beam along target (or equivalent) Spreading the beam reduces power density Electron beam Actual source size
Projected source is Actual source x sin(takeoff angle) Projected source size Active water cooling removes heat load 3kW heat load would melt 1 kG of Cu in about 3 minutes Electron beam Details of a
typical sealed tube http://www.panalytical.com/images/products/xrdgp.jpg Maximum practical thermal loads dQ/dtmax ~ 80kW Treasonable ~ 80K Cp(water) = 4190 J/kg K mwater/time ~ .02kg/sec But: 3 kW/10mm2 exceeds thermal transport capabilities of most materials! Move beam along target (or equivalent) i.e. rotate target very quickly under electron beam How quickly?
~6000 rpm for 3-12 kW operation http://www.nonius.com/products/gen/fr591/anode.jpg Monochromatic n = 12.4n/E = 2 d sin Take a narrow, bright slice out of emitted spectrum http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xrayc.html#c1 How? Crystal monochromator. How wide a slice (bandwidth)? Dunno depends on experiment. What crystal monochromator to use? Dunno depends on the bandwidth
n = 12.4n/E = 2 d sin E/E= cot and < experimental diffraction linewidth Thus, experimental needs will dictate desired energy and desired energy and angular bandwidths. Materials properties dictate what you can have. K-alpha K-alpha Melting Thermal Heat capacity energy wavelength Point (K) Conductivity (J/(g-K)) (KeV) (Angstroms)
22.16 0.56 1235 429 0.24 Tungsten 59.31 0.21 3683 173 0.13 Beam focusing: Size, angle, and phase space X-rays can be focused using Diffraction Bragg
Laue Mirrors (specular reflection) However, beam size and convergence angle must be conserved Smaller beam, greater convergence angle. Sorry. (cos2/p) 2(cos)/Rc + (cos2/q) = 0 Rc/2 = radius of Rowland circle, on which object, optic and image lie http://www.nanotech.wisc.edu/shadow/SHADOW_Primer/figure711.gif For bent crystal optics: Meridional radius
Rm = (2/sinB)[pq/(p+q)] Sagittal radius Rs = Rm sin2B For specular reflection optics: c = arcsin[(e2e/moc2)1/2] ~ few milliradians where c is the critical angle for total external reflection So: for small angles Meridional radius Rm = (2/B)[pq/(p+q)] Sagittal radius Rs = Rm B2 Thus, we have a system comprising: A 12-60kW transformer
A vacuum on the order of 10-7 torr A heated metal target rotating at 6000 rpm An electron filament at ~20 60KVP above ground Ionizing radiation everywhere A water flow rate from .05 1 l/sec Optical components aligned to fractions of milliradians An efficiency of ~4 x 10-12 Nonetheless: Braggs Law necessitates reflections of x-rays from crystals Relatively few photons are necessary to define the location of a crystal reflection Sources and cooling schemes continue to provide
higher brilliances from rotating anode systems Higher-power generators continue to be developed Detector technology continues to advance It is possible to determine 3-dimensional structural information at the atomic level using x-ray crystallographic techniques
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