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The Compton Effect

According to the Encyclopedia Americana:

"The next extremely important fundamental contribution which came from the field of X-ray research was made by Arthur Holly Compton in 1923.   It had already been observed by Joseph Alexander Gray (1920)that short wavelength X-radiation, after scattering from carbon and other low atomic number atoms, was somewhat more absorbable than the primary radiation, but still of a "hardness" so clearly related to the primary radiation as to exclude its being a characteristic fluorescent radiation from the scatterer.

"Compton gave the following daring explanation of this effect. He supposed the bundles of radiation energy, "hf ", instead of being associated with spreading waves, to be propagated through space from the source in the form of projectiles. When one of these projectiles (or "photons"), each with momentum "hf/c", was scattered by a loosely bound electron in some low atomic number scattering material such as carbon, the electron would recoil under the impact, the more so the larger the angle of scattering and the higher the quantum energy (and hence the momentum) of the projectile. The kinetic energy thus given to the electron at the expense of the photon explained the "softening" of the scattered radiation in a completely and quantitatively correct way.

"Using characteristic line radiation from a molybdenum target tube, Compton showed that in the spectrum of the scattered radiation there appeared lines each of which was shifted toward longer wavelengths than its corresponding line in the primary radiation by an amount in complete accord with his theoretical explanation.

"The recoil electrons were also detected and shown by Compton and Simon to have the requisite speeds and directions of recoil. Even more than this was ascertained, for Jesse W. M. Dumond and Harry A. Kirkpatrick succeeded in showing that the shifted line above referred to was notably broader than the unshifted line and this was satisfactorily explained by them (with complete quantitative verification in all respects), to be due to the randomly directed velocities possessed initially by the atomic electrons which were the agents that scattered the photons. The presumptively dynamic character of the electronic clouds in atoms both for gases and and for solid metallic bodies was thus experimentally verified."

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