A segmented conical electric lens for optimization of the beam spot of the low-energy muon facility at PSI: a Geant4 simulation analysis

The low-energy muon facility at PSI provides nearly fully polarized positive muons with tunable energies in the keV range to carry out muon spin rotation (LE-μSR)experiments with nanometer depth resolution on thin films,heterostructures,and near-surface regions.The low-energy muon beam is focused and transported to the sample by electrostatic lenses.In order to achieve a minimum beam spot size at the sample position and to enable the steering of the beam in the horizontal and vertical direction,a special electrostatic device has been implemented close to the sample position.It consists of a cylinder at ground potential followed by four conically shaped electrodes,which can be operated at different electric potential.In LEμSR experiments,an electric field at the sample along the beam direction can be applied to accelerate/decelerate muons to different energies (0.5-30 keV).Additionally,a horizontal or vertical magnetic field can be superimposed for transverse or longitudinal field μSR experiments.The focusing properties of the conical lens in the presence of these additional electric and magnetic fields have been investigated and optimized by Geant4 simulations.Some experimental tests were also performed and show that the simulation well describes the experimental setup.     

Pulsed source of ultra low energy positive muons for near-surface μSR studies

We have produced a pulsed beam of low energy (ultra slow) polarized positive muons (LE-μ⁺) and performed several demonstration muon spin rotation/relaxation (μSR) experiments at ISIS RIKEN-RAL muon facility in UK. The energy of the muons implanted into a sample is tuneable between 0.1 keV and 18 keV. This allows us to use muons as local magnetic microprobes on a nanometre scale. The control over the implantation depth is from several nanometres to hundreds of nanometres depending on the sample density and muon energy. The LE-μ⁺ are produced by two-photon resonant laser ionization of thermal muonium atoms. Currently ∼15 LE-μ⁺/s with 50% spin polarization are transported to the μSR sample position, where they are focused to a small spot with a diameter of only 4 mm. The overall LE-μ⁺ generation efficiency of 3 × 10⁻⁵ is comparable to that obtained when moderating the muon beam to epithermal energies in simple van der Waals bound solids. In contrast to other methods of LE-μ⁺ generation, the implantation of the muons into the sample can be externally triggered with the duration of the LE-μ⁺ pulse being only 7.5 ns. This allows us to measure spin rotation frequencies of up to 40 MHz.     

Spin polarization and production rate studies of surface muons in a novel solenoid capture system based on CSNS

A novel surface muon capture system with a large acceptance was proposed based on the China spallation neutron source(CSNS).This system was designed using a superconducting solenoid where a long graphite target was put inside it.Firstly,the spin polarization evolution was studied in a constant uniform magnetic field.As the magnetic field can interact with the spin of the surface muon,both the spin polarization and production rate of the surface muons collected by the new capture system were calculated by the G4beamline.Simulation results showed that the surface muons could still keep a high spin polarization([90%)with different magnetic fields(0–10 T),and the larger magnetic field is,the more surface muons can be captured.Finally,the proton phase space,Courant–Snyder parameters,and intensities of surface muons of different beam fractions were given with magnetic fields of 0 and 5T.The solenoid capture system can focus proton and surface muon beams and collect p?and l?particles.It can also provide an intense energetic positron source.     

Magnetic field measurement of large aperture FFAG magnet for muon-phase-rotation ring

The PRISM-FFAG ring, which consists of 10 scaling-radial-sector magnets, has been designed to store muons with large emittance and momentum spread. The FFAG magnets differ from conventional dipole magnets in that they have a magnetic field that varies strongly with radius and the vertical direction of the magnetic field alternates. Each magnet is composed of three sets of coils to produce a defocussing (outward bending)-focussing-defocussing triplet. After careful design, the PRISM-FFAG magnets were constructed and magnetic field measurements were made. The field maps in the fiducial volume show satisfactory agreement with the design fields based on OPERA 3D/TOSCA calculations.     

Implementation of muon interaction models in PHITS

We have constructed models for muon interactions (i.e., bremsstrahlung, electron–positron pair production, muon photonuclear interaction, and negative muon capture reaction) and implemented these models in the particle and heavy ion transport code system (PHITS). The PHITS2.86 agrees well with experimental data for the vertical intensities of cosmic-ray muons in water and standard rock. The calculated results for neutron production by muon photonuclear interaction and negative muon capture reaction are in good agreement with measured data, except in the case of lead target. PHITS2.86 can also reproduce the cross-section of radionuclide production by muons passing through a concrete wall very well. These results indicate the applicability of PHITS2.86 to the shielding design of muon facilities in which estimations of attenuation length and induced radioactivity are important.     

An ion optical decel system for oblique angular impact of low-energy ions on targets

A beam retardation system is presented for performing sputtering investigations with oblique angles of incidence of the ion beam. The system is designed for ion energies from 90 eV to 3 keV. For oblique ion impact it is necessary to have a field-free space in front of the target to avoid deflection of the beam in front of the tilted target due to an otherwise distorted electric field. A double-lens system yielding a parallel ion beam can meet this requirement if the lenses are movable in the beam direction. Such a system is described for a two-dimensional field. The system was tested with a tungsten ion beam in a tungsten self-sputtering investigation at an energy of 350 eV. For this ion-target combination a large angular dependence of the sputtering yield is expected. The results are compared with Monte Carlo calculations.     

A large solid-angle secondary-beam collector using an axially symmetric superconducting magnetic field

A new type of beam collector for light secondary charged particles (“Large ω” beam course) was designed and constructed at the RIKEN Ring Cyclotron (RRC). It makes use of the focusing properties of an axially symmetric magnetic field produced by three short superconducting coils. The system sees the target at right angles to the primary beam, and covers a solid angle of about 600 msr. It has been used mainly to transport surface muons from the decay of pions produced in nucleus-nucleus collisions. The large collection efficiency of surface muons by this collector helped us to obtain valuable data on the production cross section of low-energy pions (below 10 MeV) in reactions of 135 MeV/u ¹⁴N beam with various target nuclei. Its future development will also be discussed.     

Some thoughts on the production of muons for fusion catalysis

For muon-catalyzed fusion to be of practical interest, a very efficient means of producing muons must be found. We describe here some schemes for producing muons that may be more energy efficient than any heretofore proposed. There are, in particular, some potential advantages of creating muons from collisions of high-energy tritons confined in a magnetic mirror configuration. If one could catalyze 200 fusions per muon and employ a uranium blanket that would multiply the neutron energy by a factor of ten, one might produce electricity with an overall plant efficiency (ratio of electric energy produced to nuclear energy released) approaching 30%.     

Measurements of neutron emission induced by muons stopped in metal deuteride targets

An 80-MeV/c negative muon beam from the Alternating Gradient Synchrotron at Brookhaven National Laboratory was used to investigate the stopping of muons inside Pd, Ti, and Y targets saturated with deuterium. Neutron emission from the targets was measured with an array of³He detectors, and in some runs, the temperature of the target was monitored as a function of time, with and without a flux of muons on the target. The neutron rates were also measured for Pd cathodes in an active electrochemical cell similar in design to those used in so-called “cold fusion” experiments, and the electrolyte solution was analyzed for excess tritium. No evidence was found for muon-catalyzed fusion at rates consistent with those claimed in “cold fusion” experiments. Neutron production from catalyzed fusion due to the presence of deuterium in palladium deuteride, PdD_0.7, exposed to muons was determined to be 0.0±0.03 (stat.) ±0.25 (syst.) neutrons per stopped muon.     

中子垂直聚焦单色器的调试

中子聚焦单色器可有效提高中子散射谱仪样品处的中子注量率。对安装在中国工程物理研究院核物理与化学研究所的中子衍射谱仪中子垂直聚焦单色器进行调节，在样品处利用中子照相确定了束斑的中心位置和大小，通过在水平和垂直方向上的扫描得到了可利用的中子束的面积，测量了该处的最大中子注量率。在样品处得到略大于1倍的增益。这一结果表明，调试是成功的。     

上海光源EPU100中横梁变形的分析与计算

上海光源EPU100是一台APPLE-Ⅱ型可变椭圆极化波荡器,其磁结构由四排磁排列组成,其中两排对角线上的磁排列可沿纵向移动,从而产生不同的磁场分布以满足各种极化模式的要求.随着磁排列在纵向上的相对移动,磁排列间的磁力大小和方向会发生变化,从而引起横梁不仅在垂直方向而且在水平方向产生不同的变形.本文通过对不同相位下不同磁力所引起滑动梁的变形进行分析研究,得到与实测变形一致的等效横梁力学模型.     

Geant4 simulation of production and interaction of muons

A set of models for Monte Carlo simulation of production and interaction of high energy muons is developed in the framework of the Geant4 toolkit. It describes the following physics processes: ionization of high energy muons with radiative corrections, bremsstrahlung, electron-positron pair production, muon induced nuclear reactions, gamma annihilation into muon pair, positron annihilation into muon pair, and into pion pair. These processes are essential for the LHC experiments, for the understanding of the background in underground detectors, for the simulation of effects related with high-energy muons in cosmic ray experiments and for the estimation of backgrounds in future colliders. The applicability area of the models extends to 1 PeV. The major use-cases are discussed.     

Simulation of the Particle Dynamics in Storage Rings with Coupled Transverse Motion

The toroidal low-energy particle accumulator is intended for confining and electronic cooling of a circulating positron beam and generating antihydrogen and positronium in flight. Its special features are a longitudinal magnetic field, a spiral particle winding, and a sectional structure of the optical components, which serve to form a closed orbit. The longitudinal magnetic field simultaneously magnetizes the positron beam and, as a consequence, ensures a long lifetime for the circulating beam. However, such a field results in strong coupling of the horizontal and vertical degrees of freedom and the appearance of additional resonances. The particle dynamics is simulated using the specially developed BETATRON computer code, based on the BOLIDE program package (for producing interfaces for computational and simulation programs). In the present work, the results of the simulation of particle dynamics and a calculation of the structure functions for a positron storage ring are presented.__________Translated from Atomnaya Energiya, Vol. 98, No. 4, pp. 300–306, April, 2005.     

Magnetic Sensorless Sensing of Plasma Position in the Superconducting Tokamak HT-7

Magnetic sensorless sensing experiments of the plasma horizontal position have been carried out in the superconducting tokamak HT-7. The horizontal position is calculated from the vertical field coil current and voltage without using signals of magnetic probes placed nearby a plasma. The calculations are focused on the ripple frequency component of the power supply.There is no drift problem with the time integration of magnetic probe signals. The error of the derived plasma position is lower than 2% of the plasma minor radius.     

Fast Electrons Behavior in Presence of External Radial Electric Field in IR-T1 Tokamak

The fact that the mean free path of an electron in plasma is a strongly increasing function of its velocity gives rise to the phenomenon of fast (high energy) electron production. In an electric field, electrons which exceed a critical velocity, for which the collisional drag balances the acceleration by the field, are accelerated freely and can reach very high energies. In low density tokamak discharges a considerable amount of these high energy electrons with energies up to tens of keV to MeV can thus be created. As these energetic electrons are effectively collision-less, they follow the magnetic field lines and can therefore been used to probe the magnetic turbulence in the core of the plasma. In this research, external electric field effects on the discharges which lead to this phenomenon were investigated. Tokamak limiter biasing is one of the methods for controlling the radial electric field and can induce a transition to an improved confinement state.     

Surface amorphization, sputter rate, and intrinsic stresses of silicon during low energy Ga focused-ion beam milling

Transmission electron microscopy (TEM) is a standard technique to characterize microelectronic device structures. As structures shrink to the nanoscale, surface damage produced by focused ion beam (FIB) sample preparation destroying the region of interest and degrading the resolution of TEM images becomes increasingly a problem. The thickness of the damaged layer at the sidewalls of a prepared cross section is around 20-30 nm for silicon at typical beam energies of 30 keV. In order to reduce these artifacts to a minimum low beam energies have been proposed for FIB polishing. We use a combination of molecular dynamics simulations and experiments to assess the influence of the focused ion beam on the surface structure of silicon for beam energies ranging from 1-5 keV and a grazing angle of 10° typically used in low voltage FIB polishing. Under these conditions, the thickness of the amorphous layer depends linearly on the beam energy. Intrinsic surface stresses introduced by FIB are always tensile and of a magnitude of around 1 GPa.     

中子粉末衍射谱仪垂直聚焦单色器的调试方法

介绍了中子散射谱仪用的中子垂直聚焦单色器的基本原理,描述了安装在中国工程物理研究院核物理与化学研究所的中子衍射谱仪的中子垂直聚焦单色器的调节方法.在样品处利用中子照相确定了束斑的中心位置和大小,通过在水平和垂直方向上的扫描得到了可利用的中子束的面积,测量了该处的最大中子注量率,结果表明在样品处得到略大于一倍的增益.     

Proton elastic scattering and proton induced γ-ray emission cross-sections on Na from 2 to 5 MeV

Differential cross-sections for proton elastic scattering on sodium and for γ-ray emission from the reactions ²³Na(p,p′γ)²³Na (E_γ = 440 keV and E_γ = 1636 keV) and ²³Na(p,α′γ)²⁰Ne (E_γ = 1634 keV) were measured for proton energies from 2.2 to 5.2 MeV using a 63 μg/cm² NaBr target evaporated on a self-supporting thin C film.The γ-rays were detected by a 38% relative efficiency Ge detector placed at an angle of 135° with respect to the beam direction, while the backscattered protons were collected by a Si surface barrier detector placed at a scattering angle of 150°. Absolute differential cross-sections were obtained with an overall uncertainty estimated to be better than ±6.0% for elastic scattering and ±12% for γ-ray emission, at all the beam energies.To provide a convincing test of the overall validity of the measured elastic scattering cross-section, thick target benchmark experiments at several proton energies are presented.     

Development of a real-time beam current monitoring system for microbeam scanning-PIXE analysis using a ceramic channel electron multiplier

Microbeam scanning-PIXE (micro-PIXE) analysis is a useful method for obtaining information of multi-elemental distribution in samples by two-dimensional images of sample surfaces such as mammalian cells, tissues, and other environmental monitoring species. In addition to elemental distribution information, quantitative analysis is in demand for further investigations of environmental and biomedical studies concerning heavy metals accumulated in terms of cells and sub-cellular organelles. To make quantitative analysis possible, a real-time beam monitoring system that gives a precise number of ions, and an output independent from a sample that enables one to keep a beam resolution of micrometer size is required. In this paper, we report on the development of beam current monitoring. The beam current was monitored using a ceramic channel electron multiplier (CEM) to detect secondary electrons induced from a 50 nm thick carbon film (10 μg/cm²). This carbon film was attached to a sample holder, which was set at the targeted sample position. The output value of the CEM was proportional to the Faraday cup installed just after the sample position. The beam resolution was measured using off-axis STIM by scanning a copper grid, and was estimated at 1.79 and 1.72 μm for the horizontal and vertical directions, respectively, sufficient for routine micro-PIXE analysis.