EPOS—An intense positron beam project at the ELBE radiation source in Rossendorf

EPOS, the acronym of ELBE Positron Source, describes a running project to build an intense pulsed beam of mono-energetic positrons (0.2-40 keV) for materials research. Positrons will be created via pair production at a tungsten target using the pulsed 40 MeV electron beam of the superconducting linac electron linac with high brilliance and low emittance (ELBE) at Forschungszentrum Rossendorf (near Dresden, Germany). The chosen design of the system under construction is described and results of calculations simulating the interaction of the electron beam with the target are presented, and positron beam formation and transportation is also discussed.     

Progress of the intense positron beam project EPOS

EPOS (the ELBE POsitron Source) is a running project to build an intense, bunched positron beam for materials research. It makes use of the bunched electron beam of the ELBE radiation source (Electron Linac with high Brilliance and low Emittance) at the Research Centre Dresden-Rossendorf (40 MeV, 1 mA). ELBE has unique timing properties, the bunch length is <5 ps and the repetition time is 77 ns. In contrast to other Linacs made for Free Electron Lasers (e.g., TTF at DESY, Hamburg), ELBE can be operated in full cw-mode, i.e., with an uninterrupted sequence of bunches. The article continues an earlier publication. It concentrates on details of the timing system and describes issues of radiation protection.     

Construction and timing system of the EPOS beam system

The Forschungszentrum Dresden-Rossendorf provides an intense pulsed 40 MeV electron beam with high brilliance and low emittance (ELBE). The pulse has a length of 1-10 ps and a repetition time of 77 ns, or in slow mode 616 ns. The EPOS system (ELBE Positron Source) generates by pair production on a tungsten converter and a tungsten moderator an intense pulsed beam of mono-energetic positrons. To transport the positrons to the laboratory (12 m) we constructed a magnetic beam guidance system with a longitudinal magnetic field of 75 G. In the laboratory outside the cave, the positron beam is chopped and bunched according to the time structure, because the very sharp bunch structure of the electron pulses is broadened for the positron beam due to transport and moderation.     

Spontaneous e+e− pair creation in heavy ion collisions

We demonstrate that vibrational perturbations of supercritical “quasimolecules” which may occur in collisions of heavy ions can, within the context of first-order perturbation,theory, result in the production of coincident narrow e⁺e⁻ peaks, in place of and with a larger yield than that predicted for spontaneous positron emission from static supercritical systems. Certain features of this phenomenon coincide with those of the e⁺e⁻ peaks observed by the EPOS collaboration although some important aspects of the observations cannot be reproduced within our approximation. A detailed attempt to reconcile this theoretical model with the EPOS experiment must go beyond perturbation theory.     

Transmission positron images using imaging plates

As an image recording medium for transmission positron microscopes, imaging plates are quite useful and powerful. Imaging plates are also quite sensitive and the photon-stimulated luminescence (PSL) is linearly proportional to the positron intensity in six digits (10⁶). No bulky or expensive equipment is necessary to accommodate in vacuum. Imaging plates can be set under bright lights, this is different from the photographic films. Darkness is only required during exposure and transfer to a reader. Slow Positron Facility at KEK, Japan was used to study the effect of “mono-chromatic” positron beam. Specimens were set just in front of an imaging plate. After a certain time of exposure, the imaging plates were processed by a reader. Used imaging plates can be used repeatedly after erased by ultra-violet lights. Images through samples can be obtained. Similar experiments using non-monochromatic (white) positrons and electrons have been performed at Teikyo University of Science and Technology (TUST) and Research Reactor Institute, Kyoto Univ. (RRI). Sealed ²²Na positron source can be conveniently used for non-destructive tests.     

Planned positron experiments at FRM-II

The new research reactor FRM-II near Munich has a strong positron source, which delivers an intense, nearly monoenergetic positron beam. Our positron systems, the pulsed low energy positron source (PLEPS) and the scanning positron microscope (SPM) will be operated at this beam. Some aspects of matching these systems to the new positron source will be discussed.Considerable improvements are expected, e.g. more than 10⁵ s⁻¹ recorded events at PLEPS and sub-micrometre resolution at SPM. They will enable investigations in so far inaccessible problems like the evaluation of annihilation characteristics and trapping constants of individual defects or studies of fast dynamical processes. In applied materials science complex defect structures will be studied which demand a resolution into many differing lifetimes, e.g. fractured specimens, wear, corrosion, etc. Also large series of measurements at small systematic modifications are planned. There is also the opportunity to analyse in addition the chemical microstructure of the specimens by means of a hydrogen microprobe and other ion beam techniques available close to FRM-II at the Technical University of Munich.     

Analysis of positron diffusion data

In this paper we present and discuss experimental methods to determine the positron diffusion coefficient from slow positron beam measurements. We also evaluate the use of the annihilation line Doppler-broadening technique in positron diffusion measurements, as compared on the more commonly used method of positronium fraction. The effects of incomplete positron thermalization and uncertainties of the positron implantation profile at low-positron incident energies to the measured data are discussed. We apply the presented methods to the model case of A1(110) system in the temperature range from 20 to 500 K. This data shows that Doppler-broadening and positronium fraction measurements give consistent results for the positron diffusion coefficient in A1(110), where D₊(300 K) = 1.7(2) cm²/s with the temperature dependence D₊ T ^–0.62(3).     

Sticking coefficient of nitrogen on solid N2 at low temperatures

The sticking coefficient of nitrogen gas on a thick solid nitrogen film on a copper cold finger was studied at low temperature. For surface temperatures of about 12 K the sticking coefficient is measured to be 99.0 ± 0.6%. Our result implies that it will be possible to make a intense and high brightness slow positron source starting from a small diameter deposit of the gaseous positron emitter ¹³N₂ produced in the reaction ¹²C(d,n)¹³N.     

A mobile positron-lifetime spectrometer for field applications based on β+-γ coincidence

+ -γ coincidence measurement, which has been applied to in-situ examinations of microstructural evolution processes during the fatigue of copper single crystals, is described. Since no sandwich-type geometry is required, it is applicable to all specimen geometries commonly used in materials testing and to the non-destructive testing of engineering parts in service. As a radioactive source ⁷²Se generates the positron-emitting ⁷²As, which provides two positron spectra with maximum energies of 2.5 MeV and 3.3 MeV and a prompt γ quantum of 835 keV. The positrons emitted in the direction towards the specimen pass through a fast plastic scintillator and produce a scintillation signal, thereby losing about 150 keV of their energy. This signal serves as a start signal for the positron-lifetime measurement and is measured in coincidence with the subsequent 511 keV annihilation quantum. After passage through the plastic scintillator the remaining positron energy is still high enough to penetrate deep into the material and to allow for real bulk examinations. The prompt γ quantum may serve as an on-line control of the stability of the electronic system which will be useful under non-constant service conditions in a proposed field application. Received: 8 January 1997/Accepted: 14 March 1997     

Development of radiation medicine at DLNP, JINR

The Dzhelepov Laboratory of Nuclear Problems’ activity is aimed at developing three directions in radiation medicine: 3D conformal proton therapy, accelerator techniques for proton and carbon treatment of tumors, and new types of detector systems for spectrometric computed tomography (CT) and positron emission tomography (PET). JINR and IBA have developed and constructed the medical proton cyclotron C235-V3. At present, all basic cyclotron systems have been built. We plan to assemble this cyclotron at JINR in 2011 and perform tests with the extracted proton beam in 2012. A superconducting isochronous cyclotron C400 has been designed by the IBA-JINR collaboration. This cyclotron will be used for radiotherapy with proton, helium and carbon ions. The ¹²C⁶⁺ and ⁴He²⁺ ions will be accelerated to an energy of 400 MeV/amu, the protons will be extracted at the energy 265 MeV. The construction of the C400 cyclotron was started in 2010 within the framework of the Archarde project (France). Development of spectrometric CT tomographs may allow one to determine the chemical composition of a substance together with the density, measured using traditional CT. This may advance modern diagnostic methods significantly. JINR develops fundamentally new pixel detector systems for spectrometric CT. The time-of-flight (TOF) system installed in the positron emission tomograph (PET) permits essential reduction in the detector noise from occasional events of different positron annihilations. The micropixel avalanche photodiodes (MAPDs) developed at JINR allow a factor of 1.5 reduction in the resolution time for the PET TOF system and suppression of the noise level as compared to commercial PET. The development of a combined PET/MRI is of considerable medical interest, but it cannot be made with the existing PET tomographs based on detectors of compact photomultipliers due to strong alternating magnetic field of MRI. Change-over to detectors of micropixel avalanche photodiodes permits making a combined PET/MRI.     

Positron annihilation studies on the migration of deformation induced vacancies in stainless steel AISI 316 L

Positron lifetime measurements were carried out at room temperature before and after isochronous annealing of cylindrical, machined fatigue specimens and of round slabs of austenitic stainless steel AISI 316 L deformed in compression. Annealing experiments are evaluated in terms of vacancy migration and sinking to grain boundaries and dislocations. The model assumes spherical grains with a homogeneous initial distribution of vacancies. A vacancy migration enthalpy of H^M _V=(0.9±0.15) eV was found. It is concluded that positron trapping at dislocation lines does not significantly contribute to positron lifetime measurements at room temperature and that single vacancies are the dominating positron traps. Positron annihilation depth profiling on cross-sectional areas prepared from machined specimens using a positron microprobe with 10 μm spatial resolution shows that machining of cylindrical specimens creates vacancies up to 5 mm below the surface. Received: 11 August 2000 / Accepted: 13 November 2000 / Published online: 28 February 2001     

Status of the pulsed low energy positron beam system (PLEPS) at the Munich Research Reactor FRM-II

The Munich pulsed low energy positron beam system (PLEPS) is now installed at the high intensity positron source (NEPOMUC) at the Munich Research Reactor FRM-II. In order to enhance the performance of the system several improvements have been implemented: two additional collinear detector ports have been installed. Therefore in addition to the normal lifetime measurements it is now possible to simultaneously perform Doppler-broadening, coincident Doppler-broadening and age momentum correlation experiments. An additional chopper was included to periodically suppress pulses and therefore to extend the standard time window of 20 ns for precise measurements of longer lifetimes. First test-experiments have been performed in May and July 2007. With all pulsing components in operation we achieved a count-rate of 1.4 × 10⁴ counts per second. The total time resolution (pulsing and detector) was about 240 ps (FWHM) with a peak to background ratio up to 6 × 10³:1.     

Experimental study for the production cross sections of positron emitters induced from 12C and 16O nuclei by low-energy proton beams

In proton therapy, positron emitters are induced from ¹²C and ¹⁶O nuclei by protons on the beam path in the patient. Many studies for monitoring positron emitters with beam-induced PET technique have been performed by various groups to verify the proton beam range and the dose in the patient for quality assurance (QA). The QA methods proposed by some groups require accurate production cross sections of the positron emitters produced by protons, especially in the low-energy region. The aim of this study was to develop a method for measuring the production cross sections of positron emitters using standard equipment for proton therapy, and to measure the cross sections of positron emitters produced by low-energy protons and verify them in comparison with data of previous experiments. An 80-MeV proton beam was produced by a synchrotron, and the energy was degraded by polyethylene blocks to obtain various beam energies. The number of protons was estimated from the charge induced in a parallel-plate ionization chamber by protons. Low-energy protons of 14–70 MeV were used to bombard ¹²C-rich and ¹⁶O-rich target materials: namely, polyethylene and gelatinous water. The time-activity curve was then measured by a high-sensitivity PET scanner to extract the number of positron emitters produced in the target. The production cross sections for four reaction channels: ¹⁶O(p, pn)¹⁵O, ¹⁶O(p, 3p3n)¹¹C, ¹⁶O(p, 2p2n)¹³N, and ¹²C(p, pn)¹¹C were then measured. The cross sections for the ¹⁶O(p, pn)¹⁵O reaction channel were consistent with data of previous experiments within the uncertainties, while those of ¹²C(p, pn)¹¹C were generally lower than data of previous experiments. These results suggested that further measurements of the production cross sections will be necessary.     

Demonstration of positron re-emission microscopy using an immersion objective

Hulett et al. have suggested forming a magnified image of the positrons re-emitted from a surface having a negative positron affinity as a new kind of microscope, the positron re-emission microscope (PRM). We have built an immersion objective (cathode lens) for the PRM and obtained the first images using a Ni film back-illuminated with 5 keV positrons from a brightness enhanced slow positron beam. The magnification at the detector plane is 330x and the resolution is better than 1m. With the addition of a projector lens the magnification and resolution should be sufficient to observe defect structures and large molecules on the surface.     

Positron spectra from internal pair conversion observed in 238U + 181Ta collisions

We present new results from measurements and simulations of positron spectra, originating from ²³⁸U + ¹⁸¹Ta collisions at beam energies close to the Coulomb barrier. The measurements were performed using an improved experimental setup at the double-Orange spectrometer of GSI. Particular emphasis is put on the signature of positrons from Internal-Pair-Conversion (IPC) processes in the measured e⁺-energy spectra, following the de-excitation of electromagnetic transitions in the moving Ta-like nucleus. It is shown by Monte Carlo simulations that, for the chosen current sweeping procedure used in the present experiments, positron emission from discrete IPC transitions can lead to rather narrow line structures in the measured energy spectra. The measured positron spectra do not show evidence for line structures within the statistical accuracy achieved, although expected from the intensities of the observed γ-transitions ( E _γ∼ 1250-1600 keV) and theoretical conversion coefficients. This is due to the reduced detection efficiency for IPC positrons, caused by the limited spatial and momentum acceptance of the spectrometer. A comparison with previous results, in which lines have been observed, is presented and the implications are discussed. Received: 27 April 2000 / Accepted: 14 September 2000     

High-momentum analysis in Doppler spectroscopy

Unique information about the chemical vicinity of positron annihilation sites is provided by the contribution of high electron momenta to the Doppler spectrum, since this momentum range is characteristic for the annihilation with core electrons and hence element specific. However, the corresponding energy region in the spectrum is overlaid by a huge background caused by the annihilation radiation itself and the Compton spectrum of other gamma lines having an energy above 511 keV. Usually these backgrounds are reduced by measuring both annihilation quanta in coincidence.By mathematically analyzing the background contributions, we open another possibility to obtain the high-momentum region employing one single germanium detector. A necessary precondition is employing either background-free positron beams or a low-background positron source, e.g. ⁶⁸Ge, instead of the widely used positron emitter ²²Na. The ⁶⁸Ge-source emits positrons with an endpoint energy of about 1.9 MeV, where as the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low.When analyzing spectra from metals and semiconductors according to the described background subtraction, the same information contained in the momentum range up to 35 × 10⁻³m₀c or beyond can be extracted, as if the spectra were measured employing a coincidence setup with two Ge-detectors.     

Status of an R&D project of a positron gun at “Horia Hulubei” NIPNE Bucharest

A new positron gun (PG) will enable high sensitivity measurements in applications of positron annihilation spectroscopy in Romania. Some data concerning the design of a modular system for focussing, transport and acceleration of mono-energetic positrons in the range 0.8-50 keV have been obtained and experimenting on moderators and CDBS was performed. We present a short overview of the present status of the project and preliminary results from Coincidence Doppler Broadening Spectroscopy with a ²²NaCl source, on Al samples. The entire positron gun system will be designed as a high-vacuum dedicated extension operating with two options: a 50 mCi ²²NaCl source and in-line with the NIPNE cyclotron or a new intense compact cyclotron.     

Digital signal processing application to <Superscript>252</Superscript>Cf(sf) reaction products spectroscopy

The emission of prompt fission neutrons in spontaneous fission of ²⁵²Cf was studied by the application of digital data processing for the first time. The study was aimed at comparing the obtained results with those from the pioneering work of Budtz-Jorgensen and Knitter. By using a twin Frisch grid ionization chamber for fission fragment detection and a neutron scintillator (NE213) for the detection of neutrons, a total of about 10⁷ neutron coincidences were registered. The fission-fragment kinetic energies were measured using a fast (80 MHz) waveform digitizer that provided 10-bit amplitude resolution. Standard analog NIM modules were used for neutron time-of-flight and pulse shape measurements. A time-to-digital converter (TDC) with 1-ns resolution and a pair of gated charge-to-digital converters with 12-bit amplitude resolution were also used. The fission fragment signals were analyzed by means of digital signal processing algorithms. Data on prompt fission neutron energies were obtained by using a time-of-flight (TOF) spectrum unfolding procedure applicable to list-mode data.     

Muon content of ultrahigh-energy air showers: Yakutsk data versus simulations

A sample of 33 extensive air showers (EASs) with estimated primary energies above 2 × 10¹⁹ eV and high-quality muon data recorded by the Yakutsk EAS array is analyzed. The observed muon density is compared event-by-event to that expected from CORSIKA simulations for primary protons and iron using SIBYLL and EPOS hadronic interaction models. The study suggests the presence of two distinct hadronic components, “light” and “heavy.” Simulations with EPOS are in good agreement with the expected composition in which the light component corresponds to protons and the heavy component to iron-like nuclei. With SIBYLL, simulated muon densities for iron primaries are a factor of ∼ 1.5 less than those observed for the heavy component for the same electromagnetic signal. Assuming a two-component proton-iron composition and the EPOS model, the fraction of protons with energies E > 10¹⁹ eV is 0.52_−0.20^+0.19 at the 95% C.L. The text was submitted by the authors in English.     

Defect study on electron irradiated GaAs by means of positron annihilation

Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was 0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×10¹⁸ cm^–3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs.