Write-up for TARI-35
“Channeling undulator”
As a matter of justification, accelerator-based sources of hard-photon radiation are a rapidly
developing field, as can be seen, e.g., from a topical issue of Beam Line [1]. Coherent radiation of a
particle beam in an undulator is a popular choice. The energy of a photon emitted in an undulator is
in proportion to the square of the particle Lorentz factor  and in inverse proportion to the undulator
period L: ћω=2πћγ2c/L. Typically, at the modern accelerators the period of undulator in the
synchrotron light sources is a few centimeters [1].
With a strong worldwide attention to novel sources of radiation, there has been broad interest [210] to compact crystalline undulators, with some approaches covering also nanotechnology to make
use of nanotubes to guide radiating particles. The crystalline undulators with periodically deformed
crystallographic planes offer electromagnetic fields in the order of 1000 Tesla and could provide a
period L in sub-millimeter range. This way, a hundred-fold gain in the energy of emitted photons
would be reached, as compared to a usual undulator.
The use of bent crystals for channeling extraction of beams from accelerators has been under
development at several laboratories [11-15]. A collaboration of researchers working at the 70-GeV
accelerator of IHEP has recently achieved a substantial progress in the efficiency of crystal-assisted
beam deflection: extraction efficiency larger than 85% has been obtained at intensity as high as
1012 protons [16].
Different ideas were proposed for creation of crystalline undulators [3-10], but they are still
pending realization. Recently [2] our team demonstrated experimentally by means of X-rays that
microscratches on the crystal surface make sufficient stresses for creation of a crystalline undulator
by making a series of scratches with a period of 1 mm. Presently, we have optimized this process
and were able to produce an undulator with a period in sub-millimeter range and with a good
amplitude. A series of undulators was manufactured with the following parameters: the length
along the beam 1 to 5 mm, thickness across the beam 0.3 to 0.5 mm, 10 periods of oscillation with
step from 0.1 to 0.5 mm, the amplitude on the order of 20 - 150 Å.
X-ray tests of the crystal deformation have shown that a sinusoidal-like shape of crystalline planes
goes through the bulk of the crystals. This opens up the possibility for experiments with highenergy particles channeled in crystalline undulator, a new compact source of radiation. The first
experiments on photon emission in crystal undulator has been in preparation for the BTF area of the
LNF with 500-800 MeV positrons, aiming to produce undulator photons in the range of 50 keV.
The team has developed a Monte Carlo code for simulation of positron interaction (channeling,
scattering, radiation) with curved (undulated) crystal lattice. The calculations of radiation intensity
were carried out for the (110) plane of the silicon single crystal. For calculations of the photon
spectrum from 800 MeV positron beam we selected the period and amplitude of deformation of the
silicon single crystal equal to 0.1 mm and 80 Å; these values we recently obtained. From the
calculation for 800-MeV positrons, one can see that the maximum of the distribution corresponds
to approximately 55 KeV. Our calculations take into account the following factors: channeling
radiation and dechanneling process, finite length of the crystalline undulator, equal to 1 mm,
radiation of the above-barrier positrons, absorption of gamma-quanta in the undulator bulk. Our
calculations allow one to make the following conclusions concerning the photon spectrum: clear
peak of photon number can be observed in the range 30-60 KeV; the sum of the photons under this
peak is equal to approximately 20 % of all the photon spectrum (or 0.05 photons per positron);
most of the photons are due to channeling radiation; these photons are distributed in the wide range
(up to 2.5 MeV) and their spectral density (per MeV) is 5 times lower than the undulator-photon
density is in the range where the undulator photons are. Strong absorption below 30 KeV allows us
to obtain better monochromaticity of undulator photons
In the BTF area, part of the experimental set-up has been assembled (actually, the part used for TARI23 channeling project), which included: a collimator specially made for this experiment, a goniometer
with channeling samples to be irradiated and tested, electronics to control the goniometer remotely, a
vacuum pipe with a pump in order to reduce scattering on the way from the sample to detector and
thus improve background situation there, and hodoscope detector to monitor particle profiles in
horozontal and vertical planes. This arrangement was being optimized in the course of accelerator runs
with different channeling samples with positrons during the allocated beam time at the BTF, in order
to ensure positron beam at the entrance to the samples collimated as parallel as possible. This
experimental activity is yet in the early stages, with an upgrade of the experimental set-up needed in
order to observe first undulator photons from the sample. The objective of the experiment is the
observation of undulator photons emitted with expected energy in a crystalline undulator,
measurement of its spectrum, experimental comparison to the case of a usual straight crystal,
demonstration of the crystalline undulator work for the first time in the world.
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