Development of a high-efficiency pulsed slow positron beam for measurements with orthopositronium in vacuum

We have developed a high-efficiency pulsed slow positron beam for experiments with orthopositronium in vacuum. The new pulsing scheme is based on a double-gap coaxial buncher powered by an RF pulse of appropriate shape. The modulation of the positron velocity in the two gaps is used to adjust their time-of-flight to a target. This pulsing scheme allows to minimize non-linear aberrations in the bunching process and to efficiently compress positron pulses with an initial pulse duration ranging from 300 to 50 ns into bunches of 2.3 to 0.4 ns width, respectively, with a repetition period of . The compression ratio achieved is 100, which is a factor 5 better than has been previously obtained with slow positron beams based on a single buncher. Requirements on the degree, to which the moderated positrons should be mono-energetic and on the precision of the waveform generation are presented. Possible applications of the new pulsed positron beam for measurements of thin films are discussed.     

The time-of-flight technique for the HERMES experiment

This paper describes the use of the time-of-flight (TOF) technique as a particle identification method for the HERMES experiment. The TOF is measured by two scintillation hodoscopes that initially were designed only for the first-level trigger. However, the suitable time structure of the HERA electron beam allows an extension of their functions to also measure the TOF for low momentum hadron identification. Using only these conventional hodoscopes, good particle identification was achieved for protons and pions in the momentum range up to and for kaons up to .     

Application of large area avalanche photodiodes for alpha liquid scintillation counting

In order to improve the resolution in alpha liquid scintillation and to promote this method for the measurement of actinides in environment, a silicon Large Area Avalanche Photodiode (LAAPD) has been tested as liquid scintillation detector. The set-up consists of a LAAPD (16 mm diameter) coupled to a thin vial containing alpha-emitters and a liquid scintillation cocktail. After optimization of several parameters like bias voltage, temperatures, counting geometry and composition of scintillating cocktail, energy resolutions are found to be better than those obtained with photomultiplier tubes: 5% (200 keV FWHM) for ²³²Th and 3.9% (260 keV FWHM) for ²¹⁶Po. The improvement in energy resolution is not due to higher quantum efficiency of the silicon device but to better uniformity of the spatial photoresponse. A few energy spectra of thorium, plutonium and americium are given as example.     

ILC beam energy measurement based on synchrotron radiation from a magnetic spectrometer

We propose to measure, on a bunch-to-bunch basis, the beam energy at the International Linear e⁺e^- Collider by recording synchrotron radiation (SR) light emitted in the magnets of an energy spectrometer based on beam position monitors. Measuring the width of the horizontal SR fan permits to determine the relative beam energy with a precision better than 10^-4. There are two different measuring schemes possible. The first one is based on edge measurements of the direct SR fan, while the second option includes mirrors to deflect soft SR light to detectors located sufficiently off the beamline. Three possibilities for high-spatial resolution detectors are considered: a standard silicon strip detector, a novel-type Si detector and a gas amplification detector, both with exceptional position resolution. The main issue of the first scheme is the high radiation dose expected in the direct SR fan. If mirrors are used this dose is strongly reduced and allows application of any of the three detectors proposed.     

A new analogue sampling readout system for the COMPASS RICH-1 detector

A new electronic readout for CsI-coated multiwire proportional chambers (MWPC), used as photon detectors in the COMPASS ring imaging Cherenkov (RICH) detector, is described. A prototype system comprising more than 5000 channels has been built and tested in high-intensity beam conditions. It is based on the APV25-S1 analogue sampling chip, and replaces the GASSIPLEX chip readout used previously. The APV25 chip, although originally designed for Silicon microstrip detectors, is shown to perform well even with “slow” signals from an MWPC, maintaining a signal-to-noise ratio (SNR) of 9. For every trigger the system reads out three consecutive amplitudes in time, thus allowing to extract information on both the signal amplitude and its timing. This information is used to reduce pile-up events in a high-rate environment. Prototype tests of the new readout electronics on a central RICH photocathode in nominal COMPASS beam conditions showed that the effective time window is reduced from more than for the GASSIPLEX to less than for the APV25 chip. This leads to a significant improvement of the signal-to-background ratio (SBR) with respect to the original readout. A gain by a factor of 5–6 was experimentally verified in the very forward region of phase space, where pile-up due to the muon beam halo is most significant. Owing to its pipelined architecture, the new readout system also considerably reduces the dead time per event, thus allowing to make use of trigger rates exceeding .     

Magnetic field effects on particle trajectories in the muon-spin relaxation experiment: Towards a high-field spectrometer

As a step towards the design of a muon-spin relaxation (μ⁺SR) spectrometer with a high magnetic field capability, we have carried out an investigation into the effects of applying a large magnetic field on the trajectories of particles in a μ⁺SR experiment. We compare the results of measurements and Geant4 simulations of the influence of applied fields on the incoming muon beam and the outgoing decay positron ensemble. Good agreement between measurements and simulations is found and the limits of this agreement are discussed.     

Differential die-away analysis system response modeling and detector design

Differential die-away-analysis (DDAA) is a sensitive technique to detect presence of fissile materials such as and . DDAA uses a high-energy (14 MeV) pulsed neutron generator to interrogate a shipping container. The signature is a fast neutron signal hundreds of microseconds after the cessation of the neutron pulse. This fast neutron signal has decay time identical to the thermal neutron diffusion decay time of the inspected cargo. The theoretical aspects of a cargo inspection system based on the differential die-away technique are explored. A detailed mathematical model of the system is developed, and experimental results validating this model are presented.     

Production of a low-energy radioactive nuclear beam with high purity using JAERI-RMS

Low-energy (1–2 MeV/nucleon) radioactive nuclear beams (RNBs) of ⁸Li, ¹²B and ¹⁶N have been produced through nuclear transfer reactions using a recoil mass separator (RMS) at Japan Atomic Energy Research Institute (JAERI). The contamination of the primary beam to the RNB was investigated quantitatively and was well suppressed. The typical beam intensities of ⁸Li-, ¹²B- and ¹⁶N-RNB were 1.4×10⁴, 7.8×10³ and , respectively. Their purities were 99%, 98% and 98.5%, respectively.     

A parametric study of a pressurized blister test for an elastic–plastic film-rigid substrate system

A finite element simulation of a blister test of an elastic–plastic film, bonded to a substrate and subject to plane strain conditions, is performed. A traction-separation law models the fracture process ahead of the crack tip at the interface between the thin film and the substrate. Only two parameters are significant in describing the traction-separation law: adhesion energy, Γ₀ and interface strength, . The dependences of the pressure, P, and the product of the pressure with the central deflection, PH, on the adhesion properties (Γ₀ and ), the geometry and material properties of the film are studied. The latter quantity (PH) has the same unit as the adhesion energy, Γ₀, and is “conceptually” appropriate for the analysis. We suggest a method to extract the adhesion energy, Γ₀ and the interface strength, , independently from the total energy dissipated.     

Position sensitivity of the TIGRESS 32-fold segmented HPGe clover detector

Sensitivity to the locations of γ-ray interactions within a 32-fold segmented clover-type HPGe detector has been investigated through 90 Compton scattering of -rays from a collimated source. A mean position sensitivity of 0.44 mm at an energy of 373 keV is deduced by comparing the average pulse shapes for net charge collecting signals, as well as transient induced signals in neighbouring contacts, from 1007 pairs of three-dimensionally localized interaction points. The reconstruction of individual event locations based on a χ² comparison with the measured set of basis waveforms is presented.     

Development of gas electron multiplier foils with a laser etching technique

Fine-pitch gas electron multiplier (GEM) foils have been produced for cosmic X-ray polarimeters using a carbon dioxide laser etching technique. The finest hole pitch of the foil which can be produced repeatedly is and the smallest hole diameter is . The electron amplification factor was measured as a function of applied voltage. The behavior of the factor is almost the same as the -pitch standard foil fabricated by CERN. Our GEMs had no rate-dependent gain instability, which is expected of the GEMs having holes of good cylindrical geometry. The amplification factor of the foil in a mixture of 70% argon and 30% carbon dioxide reaches about 5000 without any micro-discharge at a voltage of 570 V between foil electrodes.     

Response of the GLAST LAT calorimeter to relativistic heavy ions

The CsI calorimeter of the Gamma-Ray Large-Area Space Telescope (GLAST) will be calibrated in flight with cosmic-ray heavy ions. In order to determine the response of the calorimeter to relativistic heavy ions lighter than Fe, an experiment was carried out at the GSI heavy ion facility using the Fragment Separator (FRS). The measured response exhibits an unexpected feature for light ions, opposite to that observed at low incident energy: for a given deposited energy, the observed signal is greater for these ions than for protons (or more generally Z=1 minimum ionizing particles). Pulse shapes are found to be almost identical for carbon ions and Z=1 particles, with a significant slow scintillation component, which constitutes another departure from the low-energy behavior. Data on the energy resolution for the individual CsI crystals and on the loss of ions due to nuclear reactions in the calorimeter are also presented.     

Light collection uniformity of lead tungstate crystals for the CMS electromagnetic calorimeter

A technique has been developed to measure the light collection uniformity of lead tungstate crystals for the CMS electromagnetic calorimeter. The use of a hybrid photomultiplier tube allows a precision measurement despite the intrinsically low light yield. It is found that shading the chamfered edges of the crystals with a pencil can achieve a reduction in the front-non-uniformity of approximately 0.1%/X₀. Similar effects are observed by de-polishing the chamfers.     

Coincidence Doppler broadening study on hydrocarbons with pi and sigma valence electrons: positronium correction

The coincidence Doppler broadening (CDB) technique was applied to study the electron momentum distribution in anthracene, diphenyl, naphthalene, and polystyrene. A method for separation of the positron and positronium (Ps) components from the Doppler-broadened annihilation line (DBAL) was developed further to be applicable to hydrocarbons with different π and σ valence electron distributions. This method allows extraction of the electron momentum distribution (EMD) from DBAL for samples when Ps formation occurs. The annihilation on π valence electrons was detected as broadening of the EMD compared to that obtained for a polymer sample only with σ valence electrons. The broadening appeared as a significant change in the shape of the CDB ratio of the corresponding positronium-corrected curves: a slight enhancement above the unity line in the low-momentum region and a drop in the momentum region, 10–20×10⁻³m_oc.     

Study on the Raman scattering measurements of Mn ion implanted GaN

We investigate the temperature dependent Raman spectra of Mn implanted (Ga,Mn)N samples with five Mn implantation doses. A small shoulder at 572.4 cm⁻¹ on the high energy side of the main Raman peak has been attributed to the Mn-related local vibrational mode (LVM). It is found that with the increase of Mn implantation dose the intensity ratio of LVM to that of the increases at first and tends to saturate at high implantation dose. In addition, at high temperature or after rapid thermal anneal treatment, the value of decreases significantly, explaining the reason why it is difficult to observe Mn-related LVM reported in the literature.     

Successive wave crests in Gaussian seas

In this paper, following the theory of quasi-determinism of Boccotti [Boccotti P. Wave mechanics for ocean engineering. Oxford: Elsevier Science.], the necessary and sufficient conditions, for the occurrence of two successive wave crests of large heights in a gaussian sea, are given. It is proven that the first two-peaks part of the autocovariance function ψ(T) describes the structure of two successive-wave patterns. As a corollary, it is shown that the tail probability of the joint distribution of two successive wave crests is given by a bivariate Weibull distribution. The Weibull parameter is equal to . Here, is the abscissa of the second absolute maximum of the autocovariance function ψ(T). The analytical results are in agreement with Monte Carlo simulations. Finally, as an application, the maximum expected wave crest pressure in an undisturbed deep water waves is evaluated by taking into account the stochastic dependence of successive wave crests.     

Dissociated dislocations in Ni: a computational study

A systematic computational study of the behavior of a dissociated screw dislocation in fcc nickel is presented, in which atomic interactions are described through an embedded-atom potential. A suitable external stress is applied on the system, both for modifying the equilibrium separation distance d and moving the dislocation complex. The structure of the dislocation and its corresponding changes during the motion are studied in the framework of the two-dimensional Peierls model, for different values of the ratio d/a^′, where a^′ is the period of the Peierls potential. The distance between the edge and screw components of the partials, as well as their widths, undergo a modulation with period a^′, as the dislocation moves, and the amplitudes of such oscillations are shown to depend on d/a^′. The stress profile acting on the dislocation complex is analyzed and the effective Peierls stress is estimated for different values of d/a^′.     

Performance of a shower maximum detector with avalanche photodiode readout

We have constructed a shower maximum detector (SMD) using 1-cm wide scintillating strips. The SMD measures the position of electrons and photons in the electromagnetic calorimeter, and improves the e/π separation capability. We use avalanche photodiodes (APDs) to read out the scintillation light; they are directly attached to each end of a strip so as to construct a compact detector. We operate the SMD at room temperature. This report describes the structure of the SMD and its performance as measured in beam tests for future linear collider experiments.     

A NaI activation method for the measurement of the weak thermal neutron field around the MEG experiment

A precise measurement of the thermal neutron flux (and an approximate measurement of the non-thermal neutron flux), associated with the high intensity stopping μ⁺ beam (≈) of the MEG experiment, was studied by measuring the delayed radioactivity of an activated NaI detector. This passive method provides a high sensitivity and allows the neutron flux determination at all locations, even in regions of high magnetic field. We present and discuss the results.     

Diffraction field of tightly focused laser beams and its influence to laser accelerator

In this paper, we have studied the dynamic characteristics of relativistic electron injected into stationary intense vacuum laser fields. We found the dynamic trajectories can basically be classified as three categories, namely Inelastic Scattering (IS), Capture and Acceleration Scenario (CAS) and Penetrate into Axial Region and Move (PARM) trajectory. The physical mechanism as to the three kinds of electrons have been examined. In particular, the PARM trajectory which we presented in this paper is different from the CAS and IS trajectory which we had already found in our previous work. We will show the PARM stems from the strong diffraction effect of a tightly focused laser field. In addition, the initial condition for the three kinds of electrons to emerge were detailed investigated. It has been found that there are four factors which chiefly decide the appearance of the three kinds of dynamics trajectories, namely the laser beam width w₀ and intensity a₀, the electrons incident angle θ and initial transversal momentum p_ti. The implication of the PARM electrons to the planned vacuum laser accelerators is illustrated.