Researchers at the Nuclotron at the Laboratory for High Energies at the Joint Institute for Nuclear Research (JINR), Dubna, have observed parametric X-ray radiation (PXR) from moderately relativistic nuclei interacting with crystals. Predicted theoretically in 1971, PXR has already been detected and investigated in electron beams at various energies, but this is the first time that it has been observed for heavy charged particles. It could lead to a new diagnostic method for use in nuclear beams at high-energy accelerators.
PXR emission by fast charged particles in crystals occurs when the virtual-photon field of the particles is diffracted by the crystallographic planes. The radiation arises from the uniform straight-line motion of the charged particle in the crystal and the yield depends only weakly on the value of the particle’s relativistic factor γ. It was natural to assume that the observation of PXR from heavy charged particles – relativistic nuclei – was a real possibility. Moreover, PXR from nuclei with a charge Z > 1 should be more intense than PXR from electrons, because the parametric-radiation yield is proportional to the square of Z.
The variation of the yield with γ contrasts with the case for radiation produced by the change of a particle’s velocity, such as bremsstrahlung and synchrotron radiation, where there is a strong dependence on γ. Both bremsstrahlung and synchrotron radiation are practically absent for protons and nuclei with the energies typical of the Nuclotron in contrast with electrons with the same energies.
The measurements at the Nuclotron were performed by a collaboration from JINR, the Institute of Physical-Technical Problems in Dubna, the Nuclear Physics Institute in Tomsk Polytechnical University and the Moscow State Institute of Electronic Technology. The team used silicon and graphite crystals in extracted beams of 5 GeV protons and 2.2 GeV/u carbon nuclei. The beam fell onto a thin (001) silicon-crystal target, inclined to the beam axis at an angle, θB, near 20°. The detector was placed close to an angle 2θB, which is the diffraction angle of virtual photons in a particle field from the (001) planes.
The figures show the X-ray spectra measured for a 5 GeV proton beam and a 2.2 GeV/u carbon-nuclei beam incident on the silicon crystal. The peaks α and β correspond to the characteristic radiation of nickel atoms that were excited in the detector casing by secondary particles. The peaks Eγ are due to parametric radiation.
The angular density of the parametric radiation was found to be 2.25 × 10-6 and 9.76 × 10-5 photon/(particle·sr) from protons and carbon nuclei, respectively, for a crystal inclination angle θB = 22.5°. The considerably higher radiation density from carbon nuclei confirms qualitatively the dependence of the parametric-radiation yield on particle charge Z.
This observation of parametric X-ray radiation from relativistic nuclei in the experiments at the Nuclotron opens possibilities for applications of the effect as a nuclear-beam diagnostic at other high-energy accelerators. The significant advantage is the large angle of PXR photons to the beam direction. The crystal target for the diagnostics can be made very thin – less than 100 μm – to decrease its influence on the beam. The application of bent crystals for collimation of the LHC beams is also under investigation. Detection of PXR generated by the beam halo particles in the crystal collimator could provide information about the stability of its angular position and also, as a by-product, about the structure of the crystal.
Further reading
A D Kovalenko and A M Taratin, JINR.
