VEPP-2M Storage Ring Research Articles

Upgrade of the KEDR tagging system

An upgrade of a special system to tag scattered electrons from two-photon processes, based on triple-GEM detectors, is described. The system is intended for the experiments at the VEPP-4M storage ring. Tests of the detector prototype with two gas mixtures confirm the feasibility of the chosen upgrade layout.

Absolute calibration of X-ray semiconductor detectors in the 0.3−1.5 keV photon energy range on synchrotron radiation from VEPP-2M storage ring

The paper presents the results of absolute spectral responsivity measurements for SPPD11 and SPPD11-04 pulse-type silicon detectors, carried out in the X-ray energy range from 0.3 to 1.5 keV with a relative uncertainty of about 8% (1 σ). The measurements were performed using the detector exposure to a well-calculable synchrotron radiation through the selective filters with well-known transmittance. Synchrotron radiation from the VEPP-2M storage ring was utilized for these measurements. The spectral responsivity of the detectors was restored by solving the proper system of integral equations on the base of measurement data. For increasing the calibration accuracy, the X-ray transmittance near the L-absorption edges of the filters used and the angular spread in the positron beam of the storage ring were experimentally determined.

SR front ends of VEPP-4M storage ring

The VEPP-4M storage ring system of SR front ends is described. SR is released by means of 14 front ends. Eleven of them are intended for beamlines of experimental stations. One front end is technical. For the permanent stabilization of an orbit of a beam with respect to a coordinate and angle in the vertical direction, two monitoring front ends are used. They take out SR from emission regions, which are at a large distance from one another.

Magnetic and electric field effects on the photoelectron emission from prototype LHC beam screen material

This paper describes the experimental studies of the effect of a dipole field on photoelectron emission and on the photon reflectivities from LHC beam screen material. These studies were performed using synchrotron radiation from the VEPP-2M storage ring at BINP (Novosibirsk). The particular surface roughness and geometry of the prototype LHC beam screen material requires dedicated experimental measurements. The experiments were performed under conditions close to those expected in the LHC. An important result obtained is that a dipole magnetic field attenuates the photoelectron emission from the surface by more than two orders of magnitude with the magnetic field aligned parallel to the surface. The measurements of photon reflectivities, forward scattered and diffuse, and the azimuthal distribution of emitted photoelectrons from the same material are reported. These experimental results are important input for the final design of the LHC beam screen.

Reflection of photons and azimuthal distribution of photoelectrons in a cylindrical beam pipe

In a cryogenic proton accelerator, such as the LHC, the creation of an electron cloud and generated heat loads resulting from electron bombardment are strongly dependent on the azimuthal distribution of created photoelectrons. In this context, photon reflection and photoelectron yield measurements have been performed using a beam line on the VEPP-2M storage ring. Six electrodes, covering the complete vacuum chamber perimeter, were mounted such that they could be suitably biased, and while one electrode was irradiated with synchrotron radiation the resulting electron current of all the others could be measured. A detailed description of the experimental apparatus and the results of the measurements of photon reflection and the azimuthal distribution of generated photoelectrons are presented.

Detector KEDR tagging system for two-photon physics

A special system to tag scattered electrons from γγ-processes is described. The system is intended for the experiments at the VEPP-4M storage ring. This system has a high detection efficiency and good invariant mass resolution.

Beam energy measurement and stabilization at the storage ring VEPP-2M

Adequate use of storage rings as synchrotron radiation sources requires exact knowledge of parameters like beam energy. This article discusses the long term experience of beam energy measurements at the storage ring VEPP-2M.

The station for detector calibration in the soft X-ray range at the VEPP-2M storage ring

The experimental station for detector calibration with synchrotron radiation (SR) in the photon energy range of 0.1–1 keV has been designed and constructed at the storage ring VEPP-2M. SR from a positron beam is extracted from a bending magnet of the storage ring and monochromatized by a multilayer mirror. The first calibration procedures are to use the good calculability of the SR photon flux and the measured parameters of the multilayer monochromator when the calibration is performed in a monochromatic beam, or the reconstruction of a detector spectral sensitivity from detector signals measured in “white” SR beam through a set of calibrated absorbing filters and at different positron energies. A description of the station, the estimations of the calibration uncertainties and preliminary results of the monochromator testing are presented.

SR power calorimetry and photon measurements at the photodesorption beam line of the VEPP-2M storage ring

Measurements of synchrotron radiation beam power and its reflectivity from the walls of a transport beam line were performed at the gas photodesorption beam line of the storage ring VEPP-2M. Measurements of radiation power and induced photo-electron current were applied in these experiments. It is shown that the measured power of the synchrotron radiation beam is in agreement with the calculated one within the measurement errors, and effects of radiation reflection from the beam line walls are negligible. The obtained results indicate that the calculated values of the power and the photon flux for a direct synchrotron radiation beam can be used, with a quite acceptable accuracy, in gas photodesorption experiments.

Detector KEDR tagging system for two-photon physics

A special system to tag scattered electrons from γγ-processes is described. The system is intended for the experiments at the VEPP-4M storage ring. This system has a high detection efficiency and good invariant mass resolution.

Cold beam tube photodesorption and related experiments for the Superconducting Super Collider Laboratory 20 TeV proton collider

The next generation of proton colliders that has been contemplated—the Superconducting Super Collider (SSC) in the U.S. and the Large Hadron Collider (LHC) at CERN—will be the first to encounter significant intensities of synchrotron radiation within the cold bore tube of superconducting magnets. Aside from removal of the synchrotron radiation heat load, the main problem encountered is how to deal with the magnitudes of photodesorbed gases. Choosing the beam tube to coincide with the cryosorbing magnet bore tube has the advantages of simplicity and, in principle, of providing very high pumping speed. Tightly bound H, C, and O in the near-surface layer (∼100 Å) are converted by photodesorption to a steadily increasing surface density of physisorbed molecules. However, the effective pumping by the bore tube is greatly reduced by the photodesorption of relatively weakly bound physisorbed molecules. In addition, the saturation vapor density of H2 at the ∼4.2 K temperature of the SSC cryostats exceeds, by a factor of fifty, the upper bound allowed by the nuclear scattering deposition of energy in the magnet cryostats. Consequently, accumulation of a monolayer of physisorbed H2 must be avoided even locally. An alternative approach is to install a coaxial perforated tube or liner inside the magnet bore tube which allows the photodesorbed gases to be pumped out of the view of the synchrotron radiation photons. The purpose of the work described in this paper is to develop a methodology that will allow prediction of the SSC beam tube vacuum for simple, 4.2 K beam tubes and for distributed pump or liner configurations—and to provide the technical data required for choosing among the alternative possibilities. The first photodesorption experiments have been completed on the VEPP2M storage ring at the Budker Institute of Nuclear Physics (BINP). Additional photodesorption experiments are underway at BINP and are being planned for a beamline at the UV ring of the Brookhaven National Laboratory National Synchrotron Light Source (BNL NSLS). Related experiments at BNL measuring molecular sticking coefficients and at the State University of New York-Albany (SUNY-Albany) measuring the depth profile of hydrogen on beam tube surfaces are also beginning to yield data. New ideas for directly measuring molecular density inside a cryosorbing beam tube are under development—neutralization of H− and H+ beams at BINP and positron annihilation at BNL. A status report of these activities is given in this paper.

Photon desorption measurements of copper and copper plated beam tubes for the SSCL 20 TeV proton collider

Relativistic protons circulating in the SSCL 20 TeV collider emit synchrotron radiation (1016 photons/m/s, Ecrit=284 eV). This radiation will photodesorb gases from the walls of the beam tube. The 4.2 K superconducting magnet bore tubes will act as very long distributed pumps. Build up of cryosorbed gases could severely limit collider operation and may require a bore tube liner to hide cryosorbed molecules from the photon flux and to increase the H2 pumping capacity. To predict collider vacuum performance, photon stimulated desorption experiments have been running at the National Synchrotron Light Source (NSLS) at BNL and on the VEPP2M storage ring at BINP, Novosibirsk, Russia. At the U10B beamline of the VUV ring at NSLS, beam tube samples were exposed to over 1023 photons/m with Ecrit=486 eV and incident angle=12 mrad. Unbaked copper and copper plated stainless steel have been tested. In addition, results have been obtained with a 1100 G dipole magnetic field and with a 350 °C vacuum bake. Results are presented and compared with other measurements at the NSLS and at other laboratories.

Colliding beam polarization measurement using superconducting helical undulator at the VEPP-2M storage ring

Compton scattering of circular polarized photons is an effective and sensitive method for the beam polarization measurement as was shown at different storage rings [1–3]. The application of the synchrotron and undulator radiation [4.5] enlarges the possibilities of this method. This report presents the result of the calculation and measurements of the electron-positron colliding beam polarization at the VEPP-2M using the helical undulator installed in the interaction region as a source of circular polarized photons.

Grazing-incidence monochromator for the 15–800 Å wavelength range at the storage ring VEPP-2M

A new Rowland monochromator with a fixed output slit, which operates according to the grazing-incidence scheme, is described. The device is notable for the capability to change the Rowland radius within 1–3 m. The monochromator was tested using synchrotron radiation from the storage ring VEPP-2M.

Focusing properties of Bragg-Fresnel lens in white spectrum of synchrotron radiation (abstract)

The investigation results on details of the imaging by Bragg-Fresnel lens (BFL) in white spectrum of synchrotron radiation are presented. BFL was fabricated on a silicon single crystal with the reflecting plane (III) by the methods of microlithography and plasma-chemical etching. The relief depth of the zone structure was 3 μm, the last zone was 0.5 μm in size, the total width of the zone plate was 200 μm. The superconducting wiggler installed at the storage ring VEPP-2M was used as a source of synchrotron radiation. The experimental results on the radiation focusing at different wavelengths (λ=1÷2.5 Å) were obtained. The results include the focusing both in- and outside the focal plane of the zone plate. The results are also presented on an observation of the focusing in antibragg diffraction geometry when the orientation of Fresnel zones coincides with the vertical section of initial incident beam of radiation. The obtained results are treated analytically.

A study of the optical properties of semiconductors with synchrotron radiation from the VEPP-2M storage ring

This paper reports on evaluation of measurements of reflectivity on III–V semiconductors GaAs for a broad spectral range. The measurements were made using both a classical source (from 4 to 13 eV) and synchrotron radiation from the VEPP-2M storage ring (from 14 to 26 eV). The method of evaluation is based on a fit of the relative reflectivities as obtained from Fresnel formulas to the measured data for 10 discrete angles of incidence. In the range form 19 to 26 eV, where the optical transitions from atomic 3d-states of Ga to the conduction band of GaAs are predominant, the experimental results were compared with pseudopotential calculations. A good qualitative agreement with the experimental data was found.

Luminescence of AH crystals on excitation by ultrasoft x-ray synchrotron radiation

By means of the synchrotron radiation (SR) from the storage ring VEPP-2M of the Novosibirsk Institute of Nuclear Physics the excitation spectra of the stationary recombination luminescence (SRL) of a number of alkali halide (AH) crystals have been measured in the spectral region of 50–250 eV. The nature of the spectra is shown to be determined by the migration of low-energy electronic excitations from the volume of the crystal to its surface. A near-surface component of luminescence (besides the bulk one) has been found, which is one of the results of this migration. The sum of these two - bulk and near-surface - components determines the complexity of the shape of luminescence excitation spectra in AH crystals.

The width of the ω-meson

The reaction e +e −→ω→π +π −π 0 has been studied with the Neutral Detector at the VEPP-2M storage ring. The value of the ω-meson total width was found to be Г ω = = 8.40 ± 0.10 MeV , that is four standard deviations lower than the rable value.

New experiment on the precise comparison of the anomalous magnetic moments of relativistic electrons and positrons

A comparison of the anomalous magnetic moments of the electron and positron has been performed using the resonance depolarization method for the VEPP-2M storage ring beams. It has been shown that the difference between the anomalous magnetic moments of the electron and positron does not exceed 1.2×10 −7 with 95% confidence level, in agreement with the CPT-theorem and the principle of relativistic invariance.

First experiments with SR from the 75 kG superconducting wiggler on the VEPP-2M storage ring

The 75 kG superconducting wiggler design is described. A special sweeping orbit regime provides irradiation of the large area on the plate in X-ray lithography experiments.