Production of X-rays 2007 ACA Summer School Illinois

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

X-rays are electromagnetic radiation over a range of energies or wavelengths; the specific range depends on the author 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 From Kramer (1923) Icontinuumconst.) ibeamZtarget(Eaccel-E)/E Transitions between electronic states: Characteristic lines 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) Transitions:

2p -> 1s: 3p -> 1s: K 3p -> 2s: L K Characteristic X-ray Emission Lines:

Atomic Energy Level Transitions X-Ray Emission Lines K-level and L-level emission lines in KeV No. Element Ka1 Ka2 Kb1 La1 La2

Lb1 Lb2 Lg1 26 27 28 29 30 33 34 35 42 43

44 45 46 47 53 73 74 77 78 79 82 6.39084 6.91530 7.46089 8.02783

8.61578 10.50799 11.1814 11.8776 17.3743 18.2508 19.1504 20.0737 21.0201 21.9903 28.3172 56.277 57.9817 63.2867 65.112 66.9895

72.8042 7.05798 7. 64943 8.26466 8.90529 9.5720 11.7262 12.4959 13.2914 19.6083 20.619 21.6568 22.7236 23.8187 24.9424

32.2947 65.223 67.2443 73.5608 75.748 77.984 84.936 0.7050 0.7762 0.8515 0.9297 1.0117 1.2820 1.37910 1.48043

2.29316 2.4240 2.55855 2.69674 2.83861 2.98431 3 .93765 8.1461 8.3976 9.1751 9.4423 9.7133 10.5515 0.7050 0.7762

0.8515 0.9297 1.0117 1.2820 1.37910 1.48043 2.28985 2.55431 2.6 9205 2.83325 2.97821 3.92604 8.0879 8.3352 9.0995 9.3618

9.6280 10.4495 0.7185 0.7914 0.8688 0.9498 1.0347 1.3170 1.41923 1.52590 2.39481 2.5368 2.68323 2.83441 2.99022

3.15094 4.22072 9.3431 9.67235 10.7083 11.0707 11.4423 12.6137 2.5183 2.8360 3.0013 3.17179 3.34781 4.5075 9.6518

9.9615 10.9203 11.2505 11.5847 12.6226 2.6235 2.9645 3.1438 3.3287 3.51959 4.8009 10.8952 11.2859 12.5126 12.9420

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.

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 which will eventually be replaced by small, lightweight HF supplies 20 KW 200 KV 100 mA 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 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 Monochromatic n = 12.4n/E = 2 d sin Take a narrow, bright slice out of emitted spectrum 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)

(J/(m-sec-K)) Iron 6.4 1.94 1808 73 0.44 Cobalt 6.93 1.79 1768 100 0.42 Nickel 7.48 1.66

1726 91 0.44 Copper 8.05 1.54 1357 401 0.38 Molybdenum 17.48 0.71 2890 138 0.25 Silver

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 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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