S.P. Møller

ELISA, and electrostatic storage ring for atomic physics

Abstract The design of a new type of storage ring for heavy ions using electrostatic deflection and focusing devices is described. At low energy, where the velocity is low for heavy ions, electrostatic bends and quadrupoles are more efficient than magnetic ones. Furthermore, electrostatic devices are more compact and easier to construct than magnetic devices. These and other features, e.g. no magnetic fields, makes such storage rings attractive for many atomic-physics experiments, and also for basic research in neighboring fields such as chemistry and biology.

The combination of an electrospray ion source and an electrostatic storage ring for lifetime and spectroscopy experiments on biomolecules

An electrospray ion source has been coupled to an accelerator that injects ions into an electrostatic heavy-ion storage ring. Since the dc ion current produced by electrospray ionization is low (∼106 ions/s), ions are accumulated in a cylindrical ion trap filled with a helium buffer gas. The ions are collisionally damped in the buffer gas and confined to the central trap region by a rf field. Extraction from the trap occurs within a few microseconds and after acceleration through 22 kV, the ions of interest are selected by a magnet according to their mass to charge ratio. The ion bunch is subsequently injected into the ring. Both positive and negative ions have been stored, with masses ranging over 3 orders of magnitude (∼102–104 Da). From a pickup signal in the ring, the number of ions in a bunch is estimated to be of the order of 103–104 when the accumulation time is 0.1 s. Our first measurements show that we can store a sufficient number of ions to study the decay of metastable ions and to determine re...

First results on proton extraction from the CERN SPS with a bent crystal

Abstract The feasibility of extracting protons from the halo of a high energy beam by means of a bent silicon crystal has been investigated. Protons diffusing from a GeV beam circulating in the SPS at CERN have been extracted at an angle of 8.5 mrad. Efficiencies of abour 10 percent, orders of magnitude higher than the values achieved previously, have been measured. The present results are promising in view of beam extraction from future multi-TeV proton accelerators.

Detailed investigation of the channeling phenomena involved in bending of high-energy beams by means of crystals

Abstract Strong bending effects for 12 GeV/ c positive particles traversing 2–3 cm long silicon crystals are reported, and results for crystal bendings of 4, 20, and 52 mrad are presented. Bending efficiencies up to 20% are seen. A combination of axial and planar channeling is responsible for the effects observed. Computer simulations, based on binary collisions, can explain in detail the experimental findings, and also elucidate important channeling phenomena in the transition region between axial and planar channeling. For negative particles a weak bending effect, caused by doughnut scattering, is demonstrated. Interesting results for planar feed-in and dechanneling are found in an experiment where three detectors are placed along the crystal.

Channeling of high-energy particles in bent crystals - Experiments at the CERN SPS

Abstract During the latest decade, experiments have been performed at the CERN SPS to investigate the use of high-energy channeled nuclei in bent crystals for extraction, beam splitting and beam bending. An understanding of channeling in a bent crystal with extraction and deflection efficiencies for different energies, crystal types and ions has been developed. Furthermore, the long-standing question of radiation damage has been addressed with encouraging outcome. This makes extrapolations possible for the construction of, e.g., an extraction device for the LHC at CERN, RHIC at Brookhaven or new splitting elements in high-energy beams.We present the main results obtained and discuss existing and future applications of bent crystals in high-energy physics.

Large Departures From Landau Distributions for High-energy Particles Traversing Thin Si and Ge Targets

Abstract Energy losses of 2 and 8 GeV/c positive and negative electrons, pions and protons transmitted through 32–1040 μm thick germanium and silicon targets are measured using the targets as semiconductor detectors. The measured energy-loss distributions are well-reproduced by calculations when the binding of the target electrons is taken into account. In particular, the increasing width, up to twice as large as the width of the Landau distribution, and the decreasing most probable energy loss as a function of decreasing target thickness, agree with calculations. The same two quantities increase as a function of the relativistic βγ for βγ ≳ 4 until a saturation is reached for very large values of βγ. The constancy of the Fermi plateau, the saturation value of the most probable energy loss, is confirmed to the one percent level for the considered βγ values and thicknesses.

Channeling radiation from 2–55 GeV/c electrons and positrons: (I). Planar case

Abstract Channeling radiation emitted from 2–55 GeV/ c electrons and positrons traversing Si and Ge single crystals has been measured for planar directions. The intensity is found to be 30 (Si) to 10 (Ge) times the intensity of normal bremsstrahlung. For channeled electrons, no structure is found in the spectrum, whereas strong and sharp peaks are found for positrons. This peak structure is extremely sharp around a certain “magic” beam momentum between 1 and 10 GeV/ c . The spectra are compared to calculated ones based on classical electrodynamics. The agreement is good but for positrons the calculated photon energies are around 5% higher than the measured ones. This discrepancy also exists for practically all previous positron experiments but could be explained in the present investigation by errors in beam momentum calibration. The possibility of subharmonic peaks in the photon spectra is discussed and compared to Monte Carlo calculations. The radiation from {110}, {111} and {100} planes was measured for different momenta between 1 and 10 GeV/ c and for different temperatures. Also the thickness dependence of channeling radiation is presented together with a comparison of photon spectra from Si and Ge crystals. The possibility of making a monoenergetic γ-source in the energy region of 1–100 MeV is discussed.

Ionization of helium and molecular hydrogen by slow antiprotons

Measurements of the cross sections for single and double ionization of helium as well as for the creation of H2+ and H+ ions from H2 for impact of antiprotons in the energy range 13-500 keV are presented. The results are compared with our earlier, less accurate data and with data for equivelocity proton impact. The single ionization cross section of helium agrees remarkably well with a continuum distorted wave (CDW-EIS) calculation. The ratio between the double and the single ionization cross sections of helium increases dramatically with decreasing projectile energy. New theoretical calculations are called for.

Channeling radiation from 2 to 20 GeV/c electrons and positrons (II).: Axial case

Abstract Radiation spectra for GeV electrons and positrons have been measured in a broad range of incidence angles in Si and Ge crystals. An enhancement up to a factor of 50 is found for axially channeled particles. In nearly all cases, the very low-energy part of the photon spectra is strongly reduced either due to equalization in the doughnut or due to the strong multiple scattering from the atomic rows. For large incident angles to the axis, the reduction is due to the so-called Landau-Pomeranchuk effect. For proper channeled positrons, stable oscillations give rise to a peak in the photon spectra. In the transition region from axial to planar channeling, the photon spectra change from being structureless to showing pronounced peaks from the first harmonics. The experimental results are compared to both classical and quantal calculations.

Observation of high deflection efficiency and narrow energy loss distributions for 450 GeV protons channeled in a bent silicon crystal

Abstract A 450 GeV proton beam has been deflected by various angles from 1 to 11 mrad using planar channeling in a (111) silicon crystal which was mechanically bent to achieve the desired beam deflection. High deflection efficiencies of up to 50% have been measured, in good agreement with present theoretical estimates. It is shown that bent crystals are also a unique tool for measurements of energy loss and straggling of channeled particles, without any influence from random particles: Selecting protons which are deflected by increasing angles corresponds to decreasing the transverse energy at the crystal entrance. With this technique energy loss and straggling was measured for protons channeled in the wide and narrow (111) planes in silicon for the first time.