Abstracts

Contributions from the following institutes are confirmed:
CERN, DESY-Hamburg, DESY-Zeuthen, DIAMOND, ELETTRA, Fermilab,GANIL, GSI, HIT, INFN-LNS (Catania), INFN-LNF (Frascati), LBNL, MPQ, MSL Stockholm, NSCL/Michigan State University, PSI, Technical University Darmstadt and University of Tartu

Conceptual ideas on single shot transverse diagnostics for electron bunches at REGAE
Shima Bayesteh
Deutsches Elektronen Synchrotron (MIN)

A new linac is under construction at DESY as the electron source for “Relativistic Electron Gun for Atomic Exploration (REGAE)”. The idea is to use the high quality electron bunches for Femtosecond Electron Diffraction (FED). Specifications of electron bunches at REGAE are very demanding; sub pC-charge, 2 to 5 MeV energy, few micrometre length and coherent length of ?. According to the scientific case of REGAE, single shot FED is desired. For diagnostics of transverse profile of the bunches one method is to use a scintillator material to yield photons. Detection will be done by a specially designed ICCD after an efficient collection of light from scintillator. Investigation on scintillation material will be presented in this paper.

Usage of screens at CERN
Enrico Bravin
CERN

Beam imaging screens are heavily used at CERN with more than 200 stations installed at present on many different beam lines. Over the past 50 years the usage has evolved from a qualitative observation of the beam on TV screens to a quantitative measurement of beam parameters. This evolution triggered the need for better understanding the properties of the different materials and radiation types. This contribution will cover the evolution and the present state of beam imaging at CERN.

Scintillating screens for laser-accelerated relativistic electron bunch diagnostics
Alexander Buck
Max-Planck-Institut für Quantenoptik

Laser-driven plasma waves, produced by focusing an ultra-high intensity laser in a gas jet, have been proposed as particle accelerating medium to generate relativistic electron bunches as an alternative to state-of-the-art linear accelerators because they are not limited by material breakdown and thus can reach 3-4 orders of magnitude stronger accelerating fields. To date, monoenergetic electron bunches in the so-called laser wakefield acceleration (LWFA) regime, which is the most efficient and successful regime of laser-driven electron acceleration, can exhibit strong fluctuations in mean energy as well as in energy spread. Thus, diagnostics for these electron bunches need to be single-shot and cover a broad range of electron energy (typically Emax/Emin ~ 100) and charge (pC to nC). In LWFA experiments, the accelerated electrons are typically deflected energy dependently by strong (~1 T) dipole magnets. The exit plane of the magnet, which is typically on the order of 100 cm^2, of the magnet is covered with a scintillating (“Lanex”) screen to measure both the energy spectrum and the divergence of the electron beam. Several screens with different layers of the phosphor powder have been studied at the ELBE accelerator in Dresden. The presentation will cover specific problems in using scintillating screens for laser-accelerated electrons and the absolute calibration and linearity test of different screens at the ELBE accelerator.

Radiation hardness and commercial cameras
Daniel Diezemann
IDS

In this presentation we will introduce to state-of-the-art CMOS sensors that we use in our current industrial camera lines. We will discuss advantages of these sensors also in relation to scintillating screen applications and high radiation environments. The first sensor features a CMOS technology with High Dynamic Range (HDR) capabilities. Thanks to a patented pixel design which is completely different from commonly used sensors, this image is capable of capturing an impressive 120 dB of dynamic range. This dynamic range exceeds the range of current CCD sensors by factor 1000, making the HDR sensor particularly suitable for beam diagnostic applications with high contrast scenes. The second CMOS sensor we will focus on aims at replacing current CCD imagers. CCD sensors were often chosen for their high sensitivity and global exposure modes. With a new generation of high-sensitivity global shutter CMOS sensors, the same results can be achieved at a much lower price. In addition, this CMOS sensor type features a high radiation hardness.

Screen for low current beams
Paolo Finocchiaro
INFN - Laboratori Nazionali del Sud

The demand of scintillating screens for ion beam imaging has been growing in the last years, for various applications including the real time monitoring of ion beam profiles in nuclear physics research laboratories. R&D activities, started at INFN-LNS many years ago, have been focused to develop ion beam diagnostic devices working in the very low energy range (below 100keV), for beam intensity below 10^5 pps (particles per second). These are the typical characteristics of the radioactive ion beams produced with ISOL facilities, such as EXCYT at LNS. The simplest and most efficient technique we have usually exploited for such purposes, is based on a scintillating screen intercepting the beam and producing a light spot whose shape corresponds to the 2-dimensional transverse beam profile. A high sensitivity CCD camera is positioned close to the screen, thus allowing to watch the beam profile in real time and being extremely useful during the beam transport procedures.

Scintillation Screen Investigations for High-Current Ion Beams
Eiko Gütlich, P. Forck and B. Walasek-Höhne
GSI Darmstadt
W. Ensinger
TU-Darmstadt

Scintillation screens are widely used for qualitative transverse beam profile monitoring in accelerator facilities. However, precise measurements might yield ambivalent results, especially for high beam current operation. At GSI, Helmholtz Centre for Heavy Ion Research, the optical properties of different inorganic scintillating screens under irradiation with ion beams are studied. Various ion beams in the energy range from 4.8 to 11.4 MeV/u are applied with currents up to some mA. Investigations are not only focused on the well-known sensitive scintillators but also on ceramics offering a higher radiation tolerance. The obtained results as light yield, beam width and higher statistical moments are discussed. The light yield and the beam width show a dependence on the used scintillator material and change significantly with the screen temperature, which increases during beam impact. As a comparison to the obtained screen profiles, scraper based method and SEM-Grid were used. Al2O3 seems to represent the ion beam properly up to a certain beam density. Additionally, a model is proposed which could explain the scintillation response of Al2O3. Furthermore the optical spectra in the range of 300-800 nm of the scintillating screens were recorded with an imaging spectrograph. For some materials the spectra of scintillating light differs significantly for light and heavy ion impact.

Scintillators for SwissFELs
Rasmus Ischebeck
PSI

Scintillators are used in various locations at the SwissFEL Injector Test Facility, a linear accelerator that serves as a prototype injector for the proposed X-ray free electron laser SwissFEL. Beam profile monitors for the ultraviolet photocathode laser and the electron beam use scintillating crystals that emit visible light. The light is imaged onto semiconductor area detectors (CCD and CMOS cameras, respectively). Scintillating fibers are used to detect particle losses along the accelerator. Stray electrons excite an optical signal, which is detected by avalanche photodiodes (MPPC elements).

Scintillation Screen Investigations for 300MeV/u Heavy Ion Beams at GSI
Renuka Krishnakumar, C. Andre, F. Becker, P. Forck, R. Haseitl and B. Walasek-Höhne
GSI Darmstadt
W. Ensinger
TU-Darmstadt

For the beams extracted from the GSI heavy ion synchrotron SIS18 the imaging properties of various scintillating screens were studied with respect to light yield and statistical moments of the light intensity distribution, i.e. important parameters for precise transversal beam profile measurements. Uranium ion beams with the energy of 300MeV/u were applied with beam currents ranging from 1E4 to 1E9 particles per pulse. For the lower beam currents sensitive scintillators, namely CsI:Tl, YAG:Ce, P43 and Ce-doped glass, were investigated. Ceramics like Al2O3, Al2O3:Cr, ZrO2 and ZrO2:Mg as well as Herasil-glass were studied for higher beam currents. At the Facility for Antiproton and Ion Research (FAIR) the transverse beam profile will be qualitatively determined by the screens. The obtained results serve as a basis for the related diagnostics.

Resolution Studies of inorganic Scintillation Screens for high energetic and high brilliant Electron Beams
Gero Kube
DESY Hamburg

Luminescent screens are widely used for particle beam diagnostics, especially in transverse profile measurements at hadron machines and low energy electron machines where the intensity of optical transition radiation (OTR) is rather low. The experience from modern linac based light sources showed that OTR diagnostics might fail even for high energetic electron beams because of coherence effects in the OTR emission process. An alternative way to overcome this limitation is to use luminescent screens, especially inorganic scintillators. However, there is only little information about scintillator properties for applications with high energetic electrons. Therefore a test experiment has been performed at the 855 MeV beam of the Mainz Microtron MAMI (University of Mainz, Germany) in order to study the spatial resolution. The results of this experiment will be presented and discussed in view of scintillator material properties and observation geometry.

Principle and Application of Scintilators
Paul Lecoq
CERN, Geneva

This tutorial will describe the fundamental phenomena underlying the scintillation process in luminescent media excited by ionizing radiation. The energy transfer mechanisms as well as the sequence of relaxation of electronic excitations will be discussed in detail: creation of electron-hole pairs, energy transfer to luminescent centers and quantum efficiency of these luminescent centers. The theoretical limit of the light output being usually much higher than the experimental one the limiting factors at each step of relaxation will be considered in self-activated, doped and cross-luminescent crystals. Different quenching mechanisms, charge transfer and non-radiative relaxation processes will be also discussed in the context of coupling phenomena of the luminescent centers with the host crystal lattice.

Comparisons and Applications of Scintillator and OTR Screens for Bright Electron Beams
Alex H. Lumpkin
Fermilab*, Batavia, Illinois 60510 USA

Characterization of the transverse and longitudinal emittances of bright electron beams is an ongoing interest at many accelerator facilities including the photoinjected linear accelerators at Fermilab and Argonne National Laboratory (ANL). A program on comparisons of YAG:Ce powder screens, YAG:Ce single crystals, and optical transition radiation (OTR) screens has resulted in the performance upgrades needed for our experiments. Converter efficiency, spectral content, saturation issues, response time, screen geometry, and spatial resolution were among the factors considered for the converter screens. As an example, the emittance exchange (EEX) experiments at Fermilab rely on accurate measurements of these beam properties upstream and downstream of the exchanger beamline. At gamma ~30 the nominal transverse beam sizes of 1 mm were not an imaging challenge, but the use of an array of 50-micron wide slits to sample the phase spaces to measure divergences of less than 100 microradians resulted in 20 times smaller images with positions distributed over several mm (which approached the resolution and depth-of-focus limits of the initial powder screen configuration, respectively). Improvements in the screen resolution term and reduction of the system depth-of-focus impact by using YAG:Ce single crystals normal to the beam direction will be described. Additional applications of beam-profiling diagnostics in the generation of sub-ps pulse trains via EEX at Fermilab and in the mitigation of the coherent OTR effects from the microbunching instability in compressed beams at the ANL linac will be reported. Finally, the choices of screens and geometries of diagnostics for the 1-GeV superconducting linac under construction at Fermilab will be presented.

*Operated by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the United States Department of Energy.

Luminescence and defects creation at the relaxation of various electronic excitations in wide-gap materials
Aleksandr Lushchik
Institute of Physics, University of Tartu, Estonia

Convenient and reliable detection of swift heavy ions with increasingly high energy becomes especially urgent in radiation physics and applications. Related radiation phenomena in wide-gap materials (WGMs, Eg = 5-14 eV) based on some radiation-resistant metal halides and oxides will be briefly considered. The irradiation of WGMs causes the formation of various electronic excitations (EEs) the peculiarities of which by now have been thoroughly studied (at Tartu University as well) using the methods of VUV, electron, EPR and thermoactivation (4-800 K) spectroscopy. The energy absorbed at the formation of EEs undergoes further transformation via three main channels: (i) various types of luminescence, (ii) creation of Frenkel defects and their associations and (ii) heat release (incl. local heating). The following main types of emission have been considered: at the recombination of relaxed or "hot" conduction electrons with band and self-trapped holes; at the tunnel recombination between localized electrons and holes; luminescence of free and self-trapped excitons; emissions connected with impurity and structural defects; as well as the so-called core-valence or cross-luminescence due to the recombination of valence electrons with core holes. Particular emphasis has been placed on a weak, continuous, temperature-independent and fast (subnanosecond range) emission discovered earlier in Tomsk (Russia) and Tartu. This intraband luminescence is connected with the radiative relaxation of electrons and holes within complex conduction and valence bands down to the bottom of c-band or the top of v-band, respectively. The mechanisms of EEs multiplication, when the energy excess of a conduction electron formed by a VUV-photon of 6-70 eV is spent on the creation of a secondary anion exciton or a secondary electron-hole pair, have been considered. In WGMs, where the formation energy of a Frenkel pair exceeds Eg and stable defects are not formed by the recombination of relaxed electron-hole pairs, these defects can be created at the recombination of hot conduction electrons with localized holes. The creation of stable F-H pairs due to the hot recombination will be demonstrated, in particular, via the creation spectra of Frenkel defects measured for the first time in WGMs at 6 K. Possible advantages of a joint use of heat-resistant intraband luminescence and the emissions of some impurities for the detection of swift-ion beams as well as for the interaction of swift ions with both matrix ions and impurities will be discussed.