x-ray science and applications - postechphome.postech.ac.kr/user/atl/note/chapter2-1.pdf · x-ray...

X-Ray Science and Applications

2008 Fall SemesterLecturer; Yang MO KOO

Tuesday and Thursday 14:45~16:00

X-ray & AT Laboratory, GIFT, POSTECH

2.1 Electron impact x-ray sources- Types of x-ray source- Bremsstrahlung emission- Characteristic emission

2.2 Synchrotron radiation sources- Introduction- Characteristics of bending magnet radiation- Characteristics of undulator radiation- Undulator radiation: undulator equation, harmonics, and radiation power - Characteristics wiggler radiation

2. X-ray sources


Types of X-ray Source

Gas-filled tube

Modern X-ray tubes: William D. Coolidge(1913)

2.1 Electron impact x-ray sources

electrons focus to shaped :CathodeMetal :Anode

torr :pressure Gas100kV-30 :HV

410−~

filament heated :CathodeMetal :Anode

torr :pressure Gas100kV-30 :HV

610−~


Since more than 99% of incident electron beam power is dissipated by heat, the removal of heat without melting or distorting the anode is main consideration to design X-ray tubes.

Most of tubes are operating at up to 1000KV of applied voltage and 3kW of impact power. Operating voltage and power are decided by the variety of target metal, electron beam energy density, etc.



X-ray tube for Diffractometer

Water

Electron irradiatedArea: 1mm x 10mm



X-ray tube for Spectroscopy- Forward emission- Characteristic X-ray; change of target

metals

X-ray tube for Microfocus:- ~10μm diameter electron spot- low power high voltage (air cooling)

Rotating anode type X-ray tube- power dissipation can be increased

by rapidly rotating cooled anode

Rigaku: 18kW, 60kW x-ray generator.



X-ray source size;Line focus X-ray source: 0.1mm x 10mm at take off angle ~6 degree

- ideal for use with powder diffractometerPoint focus X-ray source: 1mm x 1mm at take off angle ~6 degree

- ideal for use with single crystal diffractometer



Bremsstrahlung Emission (from the German bremsen, to brake and Strahlung, radiation, thus, "braking radiation" or "deceleration radiation"), is electromagnetic radiation produced by the acceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus. Strictly speaking, bremsstrahlung refers to any radiation due to the acceleration of a charged particle, which includes synchrotron radiation; however, it is frequently used (even when not speaking German) in the more literal and narrow sense of radiation from electrons stopping in matter.

When the accelerated electrons stop at metal target, the decelerated electronsradiate electron magnetic wave.


appearance toroidal D-3 (b) pattern. radiation The (a) Θ2sin

http://en.wikipedia.org/wiki/German_language

http://en.wikipedia.org/wiki/Electromagnetic_radiation

http://en.wikipedia.org/wiki/Acceleration

http://en.wikipedia.org/wiki/Electron

http://en.wikipedia.org/wiki/Atomic_nucleus

http://en.wikipedia.org/wiki/Synchrotron_radiation




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Characteristic Emission:2.1 Electron impact x-ray sources


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2.2 Synchrotron radiation sources

Advance Light Source(ALS) Beam lines

I. Bending MagnetII. UndulatorIII. Wiggler

Introduction


Structure of SynchrotronI. Injection system

- Linear accelerator + Booster ring (ALS, APS, Spring-8 etc)- Linear accelerator (PLS, Photon factory)

II. Storage RingIII. Beam Lines

Electron injection

Bending magnet

UndulatororWiggler

RF Cavity

Storage Ring Structure of undulator



Linac Accelerator Tunnel Energy Doubler

Storage ring Quadrupole of PAL

Synchrotron AcceleratorComponents of Synchrotron Accelerator



Synchrotron AcceleratorBeam lines


Beamline nDiffractio Powder Resolution-High Beamline Microproberay -X


Bending magnet radiation


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Bending magnet radiation occurs when a relativistic electron travels in a uniform magnetic field, executing a circular motion with acceleration directed toward the center. The radiation is directed tangentially outward in a narrow radiation cone, giving the appearance of sweeping ‘searchlight’. The radiation spectrum is very broad, analogous to a ‘white light’ x-ray light bulb. The emission angle is typically 1/ γ, where γ Lorentz contraction factor.


Undulator radiation is generated as highly relativistic electron travels a periodic magnetic field. In the undulator limit, the magnetic field is relatively weak and resultant angular excursions of electron are smaller than the angular width of natural radiation cone, 1/ γ, normally associated with synchrotron radiation. The frequency spread of undulatorradiation can be very narrow, and the radiation can be extremely bright and partially coherent, under certain circumstances. The characteristic emission angle is narrowed by a factor , where N is the number of magnetic periods. Typically N is of order 100. Depending on the magnet strength, harmonic radiation may be generated.

N/1

Undulator radiation



Wiggler radiation is generated from periodic magnetic structure, but in the strong magnetic field limit where in at least one plane the angle excursions are significantly greater than the natural(1/γ) radiation cone. Because accelerations are stronger in this limit, the radiation generated peaks at higher photon energies and is more abundant (high photon flux and more power is radiated, wiggler radiation is less bright because of the substantially increased radiation cone.

Wiggler radiation



Comparison of X-ray Photons

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Comparison of old synchrotron and modern synchrotron

Early synchrotron radiation facilities were basically circular rings for bending magnet radiation. Modern storage rings are dedicated to broad scientific use and optimized for high spectral brightness through the inclusion of many long straight sections for undulators and wigglers as well as tightly confined electron beams.


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