Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Radiation of fast positrons interacting with periodic microstructure on
  the surface of a crystal   Radiation of charged particles passing through a set of equidistant ridges on
the surface of a single crystal is calculated. The ridges are rectangular in
shape, each of thickness of half of the particle trajectory period at planar
channeling in a thick crystal. Positively charged particle entering the first
ridge with angle smaller than the critical channeling angle is captured into
channeling and changes the direction of its transversal velocity to reversed.
Between the half-wave ridges the particle moves along a straight line. Passing
through such set of half-wave crystal plates the particle moves on
quasi-undulator trajectories. Properties of the particle radiation emitted
during their passage through such "multicrystal undulator" are calculated. The
radiation spectrum in each direction is discrete, and the frequency of the
first harmonic and the number of harmonics in the spectrum depends on the
distance between the plates, on energy of the particles and on the averaged
potential energy of atomic planes of the crystal. The radiation is bound to a
narrow cone in the direction of the average particle velocity and polarized
essentially in a plane orthogonal to the atomic planes of the crystal.

16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
18
The categories of this paper are: econ
14
The categories of this paper are: math-ph
9
The categories of this paper are: nlin
7
The categories of this paper are: hep-th
24
The categories of this paper are: dg-ga
33
The categories of this paper are: plasm-ph
36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
27
The categories of this paper are: comp-gas
6
The categories of this paper are: astro-ph
17
The categories of this paper are: q-fin
23
The categories of this paper are: solv-int
2
The categories of this paper are: math
26
The categories of this paper are: bayes-an