X-ray Sources

On This Page:

• Origin of X-rays
• Generic X-ray Tube
• Problems with Traditional X-ray Generators
• Synchrotron
• ``Insertion Devices:'' Wigglers and Undulators
• What can you do with an x-ray synchrotron?
• X-ray Facilities at the University of Pennsylvania Materials Research Laboratory

Origin of X-rays

In general, light is created by accelerating electrical charges, or equivalently (but less obviously) by quantum transitions of charged particles from one energy state to another.

Examples: radio (electrons oscillate in antenna), mercury lamp (transitions between atomic states).

When an electron hits an anode:

1. It bumps into atoms and slows down, creating radiation of a continuous distribution of wavelengths (``bremstrahlung'')
2. It causes sharp atomic transitions, resulting in x-rays with definite wavelengths.

Generic X-ray Tube

Problems with Traditional X-ray Generators

• X-rays emitted isotropically, so you only utilize a small fraction of the radiation.
• Radiation only intense at well-defined wavelength.
• If you turn up the current, the anode melts.
  Fix: water cool the anode.
  But...itt still melts when you turn up the power a bit more.
  Fix: rotate the anode so that the electron beam travels over the surface .

Fix: think of a completely new way of making x-rays.

Synchrotron

A way of keeping high energy charged particles going in a circle.

Particles are accelerated, and therefore they radiate.

Radiation is intense, continuous (tunable), intrinsically collimated, pulsed and polarized.

Disadvantages: big facility mode, heating of optical elements, sample damage by x-rays.

``Insertion Devices:'' Wigglers and Undulators

Wiggler: line of bending magnets which enhances the signal but leaves the net direction of the electron beam unchanged.

Wiggler: strong magnetic field, large excursion, short wavelength.

Undulator: weak magnetic field, small excursion, partial coherence.

What can you do with an x-ray synchrotron?

• Spectroscopy (EXAFS)--yields local structure
• X-ray lithography--the silicon connection
• Traditional crystallography--very fast
• Time-resolved structural studies (t between 100 ps and one week)
• Crystallography using very small samples--e.g., protein crystallo-graphy, 100 micron sample, 10**4 peaks
• High resolution diffraction and diffuse scattering with small or unstable samples--e.g. metal or semiconductor surfaces, liquid surfaces, membranes
• Novel applications--magnetic scattering, inelastic scattering, speckle interferometry, ...?

X-ray Facilities at the University of Pennsylvania Materials Research Laboratory

• Fixed-tube generators (6-8 at around $80K). Routine characterization. Low maintenance.
• Rotating-anode generators (3-4 at around $200K). For in-house, high-precision structural studies. High maintenance.
• Synchrotron XRD at NSLS (Brookhaven National Laboratory). Partial ownership of one line + collaborative research on 2-3 other lines. A beamline costs around $1000K to construct.
• Synchrotron XRD at the Advanced Photon Source, at Argonne National Laboratory. Part of a consortium building a beam-line. Estimated cost $3,500K.

Copyright 1995, 1996, Paul A. Heiney. Individuals should feel free to make links to this document or any images contained in it, or to make a copy for their own personal use. However, you may not further disseminate copies in electronic, printed, or any other form without the express permission of the author, and this copyright notice must appear on any copy.

Last updated December 30, 1996