XRAYS What are they How are they made

X-RAYS!! • What are they? • How are they made? • X-ray diffraction

HISTORY • Discovered by Roentgen in 1895 • Resulted from firing a cathode ray (beam of electrons) at a metal target • He saw the bones in his hand!

What they are: • Short wavelengths of electromagnetic radiation (2 x 10 -12 to 1 x 10 -8 m)

X-ray production • Electrons are accelerated through a potential difference gaining large amounts of KE (typically several thousand e. V) • When electrons smash into a metal target xrays are emitted. TARGET (anode) cathode

X-RAY SPECTRUM Characteristic x-rays Cutoff wavelength Bremsstrahlung (“braking”) radiation

Brehmsstrahlung: èbraking radiation èwhen electron strikes target it slows down to a halt…suddenly or gradually èclassical EM theory predicts that accelerated charge emits radiation èspectrum of wavelengths is continuous

CHARACTERISTIC X-RAYS èOccur at discrete wavelengths èThe wavelength is characteristic of the target material èWhen the electron beam is accelerated through larger potential difference more lines appear èClassical theory also fails to explain why a cutoff wavelength exists

X-Ray Diffraction

X-RAY DIFFRACTION • Characteristic x-rays can serve as monochromatic sources of radiation • When x-rays are scattered off of a crystal bright and dark spots result • This is an interference pattern! (bright=constructive, dark=destructive) • What is small enough to create an interference pattern for x-rays?

BRAGG’S LAW = 2 a = ml sin = a/d ml = 2 dsin

QUESTIONS…. • How can x-rays be produced artificially? • What is the difference between Bremsstrahlung radiation and characteristic x-rays • Does x-ray diffraction provide evidence that light is a wave? • Why are x-rays a useful probe for investigating the structure of solids?

www. ndt-ed. org/Education. Resources/High. School/ Radiography/ www. tulane. edu/~sanelson/