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.     

Excitation of molecular vibrations by positron impact

Absolute cross sections for the vibrational excitation of CO, CO2, and H2 by positron impact are presented for incident positron energies from 0.5 eV to several electron volts. The measurements use a novel technique that exploits the adiabatic motion of a positron beam in a strong magnetic field. This work is the first systematic experimental study of vibrational excitation by positron impact, and extends to energies where positron measurements have traditionally been difficult. The measured cross sections are compared with available theoretical calculations.     

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.     

北京慢正电子强束流磁场输运系统设计研究

针对基于北京正负电子对撞机的慢正电子强束流系统对输运磁场的设计要求,本文对不同规格的磁场输运线圈模型、长螺线管端口处磁场的补偿以及地磁场和弯管道对正电子束流的影响等进行了计算,提出适用于本系统传输慢正电子束流的输运磁场分布、补偿线圈、调整线圈的加工参数,计算表明,系统的总体磁场不均匀度小于10髎.实际运行束流测试表明,所设计的磁场系统能够很好的将慢正电子束流输运到约16m远的样品测量室,慢正电子束斑尺寸基本没有变化,满足慢正电子束流系统的设计要求.     

Performance of a slow positron beam using a hybrid lens design

The University of Hong Kong positron beam employs conventional magnetic field transport to the target, but has a special hybrid lens design around the positron moderator that allows the beam to be focused to millimeter spot sizes at the target. The good focusing capabilities of the beam are made possible by extracting work-function positrons from the moderator in a magnetic field free region using a conventional Soa lens thus minimizing beam canonical angular momentum. An Einzel lens is used to focus the positrons into the magnetic funnel at the end of transportation magnetic field while at the same time bringing up the beam energy to the intermediate value of 7.5 keV. The beam is E × B filtered at this intermediate energy. The final beam energy is obtained by floating the Soa-Einzel system, E × B filter and flight tube, and accelerating the positrons just before the target. External beam steering saddle coils fine tune the position, and the magnetic field around the target chamber is adjusted so as to keep one of the beam foci always on the target. The system is fully computer controlled. Variable energy-Doppler broadened annihilation radiation (VEDBAR) data for a GaN sample are shown which demonstrate the performance of the positron beam system.     

Back to the future: Polarised positron beams

Plans for the development of a polarised mono-energetic positron beam at the University of Bath are described. Pilot measurements with the existing beam, modified only by increasing the source-moderator separation, have demonstrated that the helicity of the present beam is sufficient to show sensitivity to ferromagnetism in bulk iron. However, an increase in flux and helicity is required if the beam is to be successfully used to probe magnetic structures of current technological interest, and strategies to achieve both are described. Potential applications in magnetic semiconductor structures and magnetic nanoparticles are discussed.     

Some investigations of moderators for slow positron beams

Measurements were made on a low-energy positron beam apparatus in an attempt to increase the efficiency of the slow positron yield from radioisotopes. A study was made to sweep thermalized positrons to the surface of a silicon wafer with an applied electric field at 298 and 140 K. Temperature studies were also made on more conventional Pt and Pt+MgO powder moderators and the results are discussed. The role of the MgO powder has been clarified, though fundamental questions remain. The positron apparatus beam and relevant information regarding sources, temperature and magnetic fields are discussed in sufficient detail so that such a slow positron beam utilizing a “conventional” slow positron moderator could be easily duplicated for use in solid state studies.     

Construction of a positron microbeam in JAEA

We have developed a positron microbeam using magnetic lenses based on the commercial scanning electron microscope (SEM). A slow positron beam was generated using a handmade source with ²²Na and a solid neon moderator. The beam diameter was 3.9 μm on a target. Two-dimensional image of S parameter was successfully obtained. By introducing a beam pulsing section, positron lifetime measurement beam is also available.     

New design of the CDB-spectrometer at NEPOMUC for T-dependent defect spectroscopy in Mg

Crystal defects in magnesium and magnesium-based alloys like AZ31 are of major interest for the understanding of their macroscopic properties. Coincident Doppler broadening spectroscopy with positrons (CDBS) is a well-established technique to investigate lattice defects and their chemical surrounding in solids. However, Mg and its alloys are demanding materials for positron spectroscopy, since the trapping rate in Mg is low and the trapping sites are shallow. In order to increase the trapping rate, the CDBS-facility at the high intense positron beam NEPOMUC is currently redesigned and improved with a cryostat for sample cooling. Furthermore, the recently installed remoderator provides a beam energy between 20 and 200 eV. On the contrary to the previous beam energy of 1 keV, the beam is therefore no longer non-adiabatically released from the magnetic field at the grounded field termination. Hence it is necessary to construct an insulated field termination where voltages up to 5 kV can be applied.     

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.     

Intensity correlations in a coherently prepared Rb vapor in a magnetic field

We have studied the intensity correlations between two orthogonally linearly polarized components of a laser field propagating through a resonant atomic medium. These experiments have been performed in a rubidium atomic vapor. We observe that the correlations between the orthogonally polarized components of the laser beam are maximal in the absence of a magnetic field. The magnitude of the correlations depends on the applied magnetic field, and the magnitude first decreases and then increases with increasing magnetic field. Minimal correlations and maximal rotation angles are observed at the same magnetic fields. The width of the correlation function is directly proportional to the excited state lifetime and inversely proportional to the Rabi frequency of laser field.     

Stability of the Driving Bunch in the Plasma Wakefield Accelerator

We investigate the stability of the driving electron or positron beam in the plasma wakefield accelerator (PWFA). Although the beam is subject to self-focusing, filamentation, and two-stream instability, we find that all of these can be stabilized by introducing thermal energy and an axial magnetic field.     

Spin-polarization of an electro-static positron beam

We constructed an electro-static positron beam apparatus. We fabricated a simple spin-polarimeter composed of a permanent magnet with a surface magnetic field of 0.65 T and an iron pole piece. The longitudinal spin-polarization of the positron beam was determined to be 0.3 by analyzing the magnetic field dependence of the Doppler broadening of annihilation radiation from a fused silica specimen. The effect of spin rotation was examined using an iron poly-crystal and a simple E × B filter.     

Some aspects of positronium physics

Some aspects of both theoretical and experimental study of the positronium system to probe physics beyond the Standard Model are reviewed. In particular, new experiments to search for the invisible decay of orthopositronium (o-Ps) with the sensitivity in the branching ratio Br(o-Ps → invisible) ≃ 10⁻⁸–10⁻⁷ are discussed. The experimental technique involves a specially designed high-efficiency pulsed slow positron beam, which is also applicable for other experiments with o-Ps in vacuum. Details of the beam design, as well as the first measurements results are presented. Possible applications of the slow-pulsed positron beam for materials research are discussed. The text was submitted by the authors in English.     

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.     

Emission of positronium in a nanometric PMMA film

Positron beam experiments have been performed for the first time on a self-supporting polymethyl metacrylate (PMMA) film of 310 nm-thick made by spin coating. The positronium (Ps) emission from the PMMA surface is studied as a function of the positron implantation energy by using Doppler profile spectroscopy and Compton-to-peak ratio analysis. When the sample and the Ge-detector are perpendicular to the positron beam, the emission of para-positronium (p-Ps) is detected as a narrow central peak. By rotating the sample 45° with respect to the beam, the emission of p-Ps is detected as a blue-shifted fly-away peak. The bulk Ps fraction, the efficiency for the emission of Ps by picking up an electron from the surface, and the diffusion lengths of positrons (thermal and or epithermal), p-Ps and ortho-positronium (o-Ps) are obtained.     

Gas moderation of positrons

A variety of low-energy positron experiments need an improved brilliance of the beam by means of a remoderator. Conventionally, a tungsten foil or single crystal is used as a remoderator in transmission or reflection geometry. We have developed a new remoderation unit which is based on inelastic positron scattering and the drift of positrons in nitrogen gas. In first measurements we succeeded in detecting fully thermalized positrons. High positron losses occurred at the entrance of the gas cell, and therefore the injection of positrons will be improved for next measurements.     

Positron scattering on helium,neon and argon above 4 eV

The authors have measured the transmission of a positron beam through helium, neon and argon gases from 4 eV up to the positronium formation thresholds. A variable longitudinal magnetic field affects the transmission and allows some degree of phase shift separation, analogously to the performance of an angular distribution experiment. The results for neon and argon are presented.     

Positron emission isotope production cyclotron in DLNP JINR (Status Report)

A C10-cyclotron for radioisotope production is under construction at the Dzhelepov Laboratory of Nuclear Problem, Joint Institute for Nuclear Research (DLNP JINR). It is a compact isochronous cyclotron for accelerating H⁻ ions to the energy of about 10 MeV. The magnetic system, vacuum chamber and accelerating system is being built now. Results of the calculation and forming of the cyclotron magnetic field and the study of the beam dynamics from an ion source to an extraction system in calculated magnetic field are presented. The text was submitted by the authors in English.     

电磁铁摇摆器磁场的二维数值计算

本文根据泊松方程用数值计算给出了电磁铁摇摆器的二维磁场分布,定量地讨论了磁极形状、磁隙大小、周期长短和电流大小等因素对峰值磁场强度的影响,重点讨论了如何根据二维磁场分布选择磁极形状才能提高峰值磁场强度和抑制磁饱和的问题。将计算的二维磁场分布和摇摆器磁场的理想波形相比较,可以帮助我们确定进入摇摆器的束流半径应该控制的范围。在对电磁铁摇摆器磁场二维数值计算的基础上,还探讨了适用ATA电子束的PALADIN摇摆器用于ETA电子束带来的结构设计和材料选择等问题。