Channeling Radiation Experiments between 10 and 100 MeV

Abstract

When a relativistic charged particle passes through a single crystal very nearly parallel to a major crystalline plane or axis so that it is channeled in that direction, it undergoes periodic motion in the plane transverse to this direction, and hence it can radiate. Quantum mechanically, this channeling radiation corresponds to a radiative crystalline potential; when the transition occurs between two bound states, a sharp spectral line is emitted. In the forward direction in the laboratory frame of reference, the radiation is transformed upwards in energy. In part, this is because of the relativistic velocity of the charged particle that leads to a factor of γ = E/mc ², where E is the total energy of the particle and m is its rest mass (this can also be thought of as a deepening of the crystalline potential well by a factor of γ). The Doppler shift gives rise to an additional factor of 2γ. This combined factor of 2γ² (equal to 2 × 10⁴ for γ = 100, corresponding to electrons or positrons of about 50 MeV, for example) brings channeling radiation up into the interesting keV-to-MeV energy region. This, in turn, makes it relatively easy to observe using the methods of x- and γ-ray spectroscopy, and relatively easy to tune by varying the incident particle energy. The same relativistic transformation folds the radiation forward in the laboratory into a narrow cone having a characteristic half-angle of 1/γ (equal to 10 mrad for the above example), and thus makes it very intense within that solid angle. For the case of planar channeling, the radiation is linearly polarized.