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.     

Surface and bulk investigations at the high intensity positron beam facility NEPOMUC

The NEutron-induced POsitron source MUniCh (NEPOMUC) at the research reactor FRM II delivers a low-energy positron beam (E = 15-1000 eV) of high intensity in the range between 4 × 10⁷ and 5 × 10⁸ moderated positrons per second. At present four experimental facilities are in operation at NEPOMUC: a coincident Doppler-broadening spectrometer (CDBS) for defect spectroscopy and investigations of the chemical vicinity of defects, a positron annihilation-induced Auger-electron spectrometer (PAES) for surface studies and an apparatus for the production of the negatively charged positronium ion Ps⁻. Recently, the pulsed low-energy positron system (PLEPS) has been connected to the NEPOMUC beam line, and first positron lifetime spectra were recorded within short measurement times. A positron remoderation unit which is operated with a tungsten single crystal in back reflection geometry has been implemented in order to improve the beam brilliance. An overview of NEPOMUC's status, experimental results and recent developments at the running spectrometers are presented.     

A positron remoderator for the high intensity positron source NEPOMUC

A remoderator for the high intensity positron source NEPOMUC was developed and installed at the beam facility. A beam of remoderated positrons could be produced with different energies and a diameter of less than 2 mm was obtained. The efficiency of the remoderation setup was determined to be 5%. Due to the brilliance of the remoderated beam, the measurements at the coincidence Doppler broadening spectrometer (CDBS) and at the positron annihilation induced Auger electron spectrometer (PAES) could be improved. The setup and functionality of the remoderation device is presented as well as the first measurements at the remoderator, CDBS and PAES.     

Determination of positron beam parameters by various diagnostic techniques

Various diagnostic techniques have been applied at the neutron-induced positron source Munich NEPOMUC in order to determine the positron beam parameters such as intensity, beam shape and energy distribution. The positron beam intensity is determined by the detection of the annihilation radiation of positrons, which annihilate in a movable target. The use of a micro-channel plate (MCP) detector with a CCD-camera allows a direct measurement of the positron beam shape and the lateral resolved intensity distribution. At NEPOMUC a movable MCP-assembly inside the evacuated beam line enables a quick examination of the beam shape during operation. A retarding grid was mounted inside the homogeneous magnetic guiding field in order to determine the distribution of the longitudinal positron momentum, and hence estimate the energy distribution of the positrons.     

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.     

Compact positron beam for measurement of transmission moderator efficiencies and positron yields of encapsulated sources

We have constructed a simple positron beam for measuring both the efficiencies of positron moderator foils and the positron yields of encapsulated positron beam sources. The slow-positron emission rate from the moderator is determined from the positron annihilation radiation. This technique is also applied to measure source yields. In addition to the coincidence technique, the source yield is determined by measuring electric currents generated by the annihilating positrons. The performance of the system is demonstrated with different tungsten moderator foils before and after heat treatments, and several sources with primary activities in the range 1-50 mCi.     

A study of defects in deformed FeSi alloys using positron annihilation techniques

Steels with high amounts of silicon are used in electrical applications due to their low magnetostriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. The slow positron beam of Gent is used to investigate defects in different deformed FeSi alloys. It was found that the concentration of defects for the alloys deformed at high temperatures are different from the ones related to the alloys deformed at room temperature. These results are correlated to the results of positron annihilation lifetime spectroscopy (PALS).     

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.     

A Magnetic Transport Middle Eastern Positron Beam

A magnetically guided slow positron beam is being constructed at Qatar University and is currently being optimised for regular operation. This is the first positron beam in the Middle East, as well as being the first Arabic positron beam. Novel features in the design include a purely magnetic in-line deflector, working in the solenoid guiding field, to eliminate un-moderated positrons and block the direct line of sight to the source. The impact of this all-magnetic transport on the Larmor radius and resultant beam characteristics are studied by SIMION simulations for both ideal and real life magnetic field variations. These results are discussed in light of the coupled effect arising from electrostatic beam extraction.     

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.     

Study of the effect of annealing on defects in Fe-Mn-Si-Cr-Ni-C alloy by slow positron beam

FeMnSi shape memory alloys (SMAs) have received much attention as one-way SMAs due to their cost-effectiveness. Variable-energy (0-30 keV) positron beam studies have been carried out on a Fe-Mn-Si-Cr-Ni-C alloy with different degrees of deformation. Doppler broadening profiles of the positron annihilation as a function of incident positron energy were shown to be quite sensitive to defects introduced by deformation. The variation of the nature and the concentration of defects are studied as a function of isochronal annealing temperature. These results are correlated with the data measured with the positron annihilation lifetime spectroscopy (PALS). The positron annihilation results are compared to XRD and optical microscopy (OM).     

Time bunching of slow positrons for annihilation lifetime and pulsed laser photon absorption experiments

Methods are described for gathering positrons from an extended region and causing them to impact upon a target surface within a small time interval. The most optimistic of these proposed schemes suggests that one should be able to produce ∼10⁻⁹s positron pulses with peak intensities of ∼10¹¹s⁻¹ starting with a ∼200 m Ci⁵⁸Co β⁺ source. These pulses should be useful for studying time-dependent interactions of positrons at surfaces.     

Amorphous hydrogenated silicon studied by positron lifetime spectroscopy

Hydrogenated amorphous silicon (a-Si:H) prepared by dc glow discharge in silane was investigated by positron lifetime measurements at room temperature. The lifetime spectrum shows considerably longer lifetimes than in simultaneously measured Si single crystals. The dominant component with the time constant ₂=402 ps is discussed thoroughly in conjunction with positron trapping at microvoids containing more than 10 to 15 vacancies. Positron trapping at H-saturated dangling bonds cannot be ruled out. The long-lived component with ₃=1800 ps (I ₃=0.06) indicates positronium formation at larger voids.     

A scheme to produce a dense positronium plasma for an antihydrogen experiment

A 6 MeV industrial electron linac with 0.2 mA average current will be installed in December 2007 in CEA-Saclay. Equipped with a tungsten target and moderator, it is aimed at producing rates of order 10⁸ s⁻¹ slow positrons. This setup is part of a project to demonstrate the feasibility of an experiment to produce the ion for a free fall measurement of . The energy is below the neutron activation threshold. Its small size and cost could be of interest for a university laboratory or industry, and could be envisaged as a replacement source for the antihydrogen experiments at CERN.     

Comment on “Amorphous hydrogenated silicon studied by positron lifetime spectroscopy”

H.-E. Schaefer, R. Wurschum, R. Schwarz, D. Slobodin, and S. Wagner [Appl. Phys. A40, 145 (1986)] have recently assigned positron lifetimes to various vacancy sizes. In this comment we will show that their discussions are ill founded.     

Performance of the Beijing pulsed variable-energy positron beam

In order to study the depth-dependent characteristics of open-volume defects in thin surface layers, the variable-energy positron lifetime spectroscopy (VEPLS) has been enabled by pulsing a continuous positron beam. The buncher is a quarter-wave coaxial resonator and the RF-signal is fed in by a coupling loop with a frequency of 149.89 MHz and the reflection factor of 0.05 measured by a Network Analyzer. Three synchronic signals with their phases and amplitudes adjusted independently are supplied for start signal of the positron lifetime measurement and the power signal by an electronic system. The stop signal is derived from a detector, a BaF₂ scintillator coupled to a photomultiplier-tube (Hamamatsu). The time resolution of 295 ps (FWHM) was achieved for a Kapton film and a Ti sample at positron energies in the range between 1 keV and 30 keV.     

Magneto-optical trap and mass-separator system for the ultra-sensitive detection of <Superscript>135</Superscript>Cs and <Superscript>137</Superscript>Cs

We report the first magneto-optical trapping of radioactive ¹³⁵Cs and ¹³⁷Cs and a promising means for detecting these isotopes at ultra-sensitive levels by coupling a magneto-optical trap (MOT) to a mass separator. A sample containing both isotopes was placed in the source of a mass separator, ionized, mass-separated, and implanted in a Zr foil within the trapping cell. After implantation, atoms were released from the foil by inductive heating and then captured in a MOT that used large-diameter beams and a dry-film-coated cell to achieve high trapping efficiency. Trapped-atom numbers in the case of either isotope ranged from 10⁴ to 10⁷, as determined from the MOT fluorescence signal. Over this trapped-atom range, the MOT fluorescence signal was found to increase linearly with the number of atoms implanted in the foil and without isotopic bias to within 4%. In principle, this method can then provide a measurement of the ¹³⁷Cs/¹³⁵Cs ratio accurate to within 4% through the direct ratio of MOT fluorescence signals. The fluorescence signal from stable ¹³³Cs, when implanted and released from the foil, was suppressed relative to MOT signals by more than seven orders of magnitude when the system was tuned to trap ¹³⁵Cs or ¹³⁷Cs. When combined with the isotopic selectivity of ≥10⁵ for the mass separator, the overall suppression of ¹³³Cs is expected to exceed 10¹². At present our system delivers atoms from sample to MOT with an efficiency of 0.5%, has a trapped-atom detection limit of 4000 atoms, and achieves a sample-detection sensitivity of one million atoms. Received: 23 August 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +1-505/667-0440, E-mail: mdd@lanl.gov