Showing papers in "Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment in 1995"

A Multidimensional unfolding method based on Bayes' theorem

Abstract: Bayes' theorem offers a natural way to unfold experimental distributions in order to get the best estimates of the true ones. The weak point of the Bayes approach, namely the need of the knowledge of the initial distribution, can be overcome by an iterative procedure. Since the method proposed here does not make use of continuous variables, but simply of cells in the spaces of the true and of the measured quantities, it can be applied in multidimensional problems.

ESCL8R and LEVIT8R: Software for interactive graphical analysis of HPGe coincidence data sets

Abstract: Programs for analysis of γ-γ matrices and γ-γ-γ cubes from HPGe coincidence experiments are described. The programs are intended primarily for high-spin spectroscopy studies. Users are able to inspect background-subtracted gated spectra, or combinations of such spectra, quickly and easily. The programs also make use of a proposed level scheme, provided by the user, to calculate an expected spectrum for comparison with the observed spectrum. Electron conversion coefficients, detection efficiency and γ-ray energy calibrations are included in the calculation. A graphics-based editor is included to allow fast and easy modification of the proposed level scheme, and least-squares fits to the matrix or cube can be performed to extract optimum values for the energies and intensities of the level scheme transitions.

Performance of the ALEPH detector at LEP

Abstract: The performance of the ALEPH detector at the LEP e+e− collider is reviewed. The accuracy of the tracking detectors to measure the impact parameter and momentum of charged tracks is specified. Calorimeters are used to measure photons and neutral hadrons, and the accuracy obtained in energy and angle is given. An essential property of the detector is its ability to identify particles; the performance in identification of electrons, muons, neutrinos (from missing energy), charged hadrons, π0's and V0's is described.

Demonstration of phase-contrast X-ray computed tomography using an X-ray interferometer

Abstract: Phase-contrast X-ray computed tomography (PCX-CT) using an X-ray interferometer is introduced for observing a density distribution inside an organic material. PCX-CT images are compared with an absorption-contrast X-ray CT image and shown to be highly sensitive. To convert an interference pattern into an image of phase-shift distribution, which is put into a CT algorithm, the author applied subfringe analysis techniques, such as the Fourier-transform method and the fringe scanning method. In the case presented here, a plastic sphere is used as a test sample, and the resulting spatial resolution of the PCX-CT image is less than 40 μm. The signal-to-noise ratio (S/N) for the PCX-CT image is increased to ten times that for an absorption-contrast CT image. The S/N can be further increased by suppressing the movement of the interference pattern caused by air flow around the interferometer.

Lead tungstate (PbWO4) scintillators for LHC EM calorimetry

Abstract: This report describes the work carried out in order to analyse the properties of PbWO4 crystals as scintillators and to determine the perspectives of their use in calorimetry in Large Hadron Collider (LHC) experiments. The scintillation mechanism in PWO crystals is explained and the properties connected with their use as scintillators are analysed both for undoped and Nb doped crystals. The specific problems concerning the physical parameters in the case of large scale production of PWO scintillators are discussed.

Where to stick your data points : the treatment of measurements within wide bins

Abstract: When a data point, measured over a bin of finite width, is to be compared to theoretical or model frequency distributions, neither the central value of the bin nor the weighted mean value (the barycentre) of the abscissa within the bin is the appropriate place to plot the data point. It is shown that such data points ought to appear where the value of the predicted function is equal to its mean value over the wide bin. Some consequences of commonly used but incorrect alternative data presentation methods in particle physics are discussed.

INDRA, a 4π charged product detection array at GANIL

Abstract: INDRA, a new and innovative highly segmented detector for light charged particles and fragments is described. It covers geometrically 90% of the 4π solid angle and has very low detection thresholds. The detector, operated under vacuum, is axially symmetric and segmented in 336 independent cells allowing efficient detection of high multiplicity events. Nucleus identification down to very low energy threshold (≈ 1 A MeV) is achieved by using ionization chambers operated with low pressure C 3 F 8 gas. Residual energies are measured by a combination of silicon (300 μm thick) and cesium iodide (5 to 14 cm in length) detectors. Very forward angles are covered by fast counting phoswich scintillators (NE102/NE115). Charge resolution up to Z = 50 is achieved on a large energy dynamic range (5000 to 1 for silicon detectors). Isotopic separation is obtained up to Z = 3. The treatment of the signals is performed through specifically designed and highly integrated modules, most of which are in the new VXIbus standard. Full remote control of parameter settings, including visualization of signals, is thus allowed. The detector is continuously monitored with a laser source and electronic pulsers and is found stable over several days. Energy calibration procedures, making use of specific detectors and the ability of the GANIL accelerator to deliver secondary beams, have been developed. First experiments were performed in the spring of 1993.

Bayesian neural networks and density networks

Abstract: This paper reviews the Bayesian approach to learning in neural networks, then introduces a new adaptive model, the density network. This is a neural network for which target outputs are provided, but the inputs are unspecified. When a probability distribution is placed on the unknown inputs, a latent variable model is defined that is capable of discovering the underlying dimensionality of a data set. A Bayesian learning algorithm for these networks is derived and demonstrated.

Method of probing inner-structure of geophysical substance with the horizontal cosmic-ray muons and possible application to volcanic eruption prediction

Abstract: One potential use of cosmic-ray muons arriving nearly horizontally along the earth is a probe of the inner-structure of a gigantic geophysical substance, such as a volcanic mountain. A simple detection system comprising a plastic scintillator hodoscope which is expandable to a larger scale was developed. The first successful measurement of the inner-structure of Mt. Tsukuba is described. The future perspective of the application of the present method towards the prediction of volcanic eruption is discussed.

Scintillation properties of YAP:Ce

Abstract: Yttrium aluminium perovskite activated by cerium (formula YAlO 3 :Ce, abbreviated YAP:Ce) is systematically investigated in view of its use as efficient scintillator for various applications. The basic physical and chemical properties of YAP:Ce monocrystals are summarized in this paper. The luminescence, scintillation and absorption characteristics, the γ-ray detection capability, energy resolution in the range of 30–600 keV, light yield and radiation hardness are presented and compared with existing literature.

Aberration correction in a low voltage SEM by a multipole corrector

Abstract: We have set up a high resolution SEM dedicated for low beam energies with a quadrupole/octupole corrector for the correction of spherical and chromatic aberration The correction of these aberrations allows theoretically for a resolution limit of 1 nm at beam energies between 05 and 1 keV At the present state we were already able to reduce the resolution limit in low voltage SEM to below 2 nm at 1 keV

Large, low background well-type detectors for measurements of environmental radioactivity

Abstract: During the two last decades, the development of γ counting techniques has led to the application of natural radioactivity to the study of numerous processes occurring in the environment. Such work flourished particularly when large HP germanium detectors with low background became available. For some studies, the sample to be measured is small (e.g. sediment, BaSO4 precipitate). In such cases, the use of high efficiency well-type detectors, when compared with similar detectors of conventional shape, provides the best choice. This paper describes three large, well-type detectors used for environmental studies. The background of the crystals has been lowered by: i) placing the detectors in an underground laboratory (Laboratoire Souterrain de Modane, LSM), shielded from cosmic radiation by 1700 m of rocks; ii) using only very low activity materials for the detector assembly and copper-lead shield. Results from various oceanic samples such as particles, sediments, water and hydrothermal sulphides are presented to illustrate the usefulness of this kind of detector for the measurement of environmental radioactivity. For sample volumes of about 10 cm3, these installations are shown to be the most sensitive way to measure nuclides as 226Ra, 228Ra, 228Th and 137Cs.

Principles of photon colliders

Abstract: Future linear colliders offer unique opportunities to study γγ, γe interactions. Using the laser backscattering method one can obtain γγ, γe colliding beams with energy and luminosity comparable to the electron-position luminosity or even higher. In this review physical principles of photon colliders are described and various problems, concerning the accelerator, laser, interaction region and luminosity are discussed. Some examples of physical processes are given.

RMC: progress, problems and prospects

Abstract: Materials that are disordered are far more common that those that have well-ordered crystalline structures, yet our understanding of them still remains at a very basic level. Since many technologically important materials are disordered it is vital that we improve our detailed knowledge of their atomic structures and how those structures are related to other physical properties. Reverse Monte Carlo (RMC) is a general method for creating structural models of disordered systems based on experimental data, predominantly from diffraction experiments. The development of RMC is described in the context of the scientific problems that have driven it. Some of the present problems are outlined and current and possible future developments are discussed.

The Stanford linear accelerator polarized electron source

Abstract: The Stanford 3-km linear accelerator at SLAC has operated exclusively since early 1992 using a polarized electron beam for its high-energy physics programs. The polarized electron source now consists of a diode-type gun with a strained-lattice GaAs photocathode DC biased at high voltage and excited with circularly polarized photons generated by a pulsed, Ti:sapphire laser system. The electron polarization at the source is > 80%. To date the source has met all the beam requirements of the SLC and fixed target programs with < 5% downtime.

Neural network based electron identification in the ZEUS calorimeter

Abstract: We present an electron identification algorithm based on a neural network approach applied to the ZEUS uranium calorimeter. The study is motivated by the need to select deep inelastic, neutral current, electron proton interactions characterized by the presence of a scattered electron in the final state. The performance of the algorithm is compared to an electron identification method based on a classical probabilistic approach. By means of a ponciple component analysis the improvement in the performance is traced back to the number of variables used in the neural network approach.

High resolution measurements of neutron energy spectra from AmBe and AmB neutron sources

Abstract: A 3He sandwich spectrometer incorporating two semiconductor detectors and a proportional counter region has been designed and constructed to perform high resolution measurements of neutron energy spectra in the energy range 100 keV to 15 MeV. The efficiency of the spectrometer was determined experimentally, using a 252Cf spontaneous fission source, in the low-scatter facility of the National Physical Laboratory. The Monte Carlo technique was also used to determine the efficiency and these calculated values were used to extrapolate the measured efficiency to higher energies. The neutron energy spectra from two different sized Am Be neutron sources and an Am B neutron source were measured. Although these spectra have been measured previously, the present work represents an improvement in the energy resolution and extends the energy range. Average values for dose equivalent quantities for neutrons from these sources are also presented. The spectral data is available from the authors, as a printed table or on a computer diskette, on request.

Study of a Threshold Cherenkov Counter Based on Silica Aerogels with Low Refractive Indices

Abstract: To identify π ± and K ± in the region of 1.0−2.5 GeV/ c , a threshold Cherenkov counter equipped with silica aerogels has been investigated. Silica aerogels with a low refractive index of 1.013 have been successfully produced using a new technique. By making use of these aerogels as radiators, we have constructed a Cherenkov counter and checked its properties in a test beam. The results obtained demonstrated that our aerogel was transparent enough to make up for loss of the Cherenkov photon yield due to a low refractive index. Various configurations for the photon collection system and some types of photomultipliers, such as the fine-mesh type, for a read out were also tested. From these studies, our design of a Cherenkov counter dedicated to π K separation up to a few GeV/ c with an efficiency greater than 90% was considered.

Background subtraction from in-beam HPGe coincidence data sets

Abstract: Algorithms for the subtraction of backgrounds from γ-γ matrices and higher-fold data sets, obtained from in-beam HPGe coincidence experiments with heavy-ion-induced nuclear reactions, are described. The backgrounds are parameterized as the cross-products of lower-dimensional projections of the data and a one-dimensional background spectrum. A novel method of correcting for a mixture of different reaction channels in the complete data set, by making use of one or more gates on background channels in the energy region of the “E2 bump”, is presented. In many cases, this new method provides a significantly better description of the background.

Calibration of the new composite ``clover'' detector as a Compton polarimeter for the EUROGAM array

Abstract: The application of a composite Ge detector to the measurement of the linear polarisation of γ-rays has been investigated. The polarisation sensitivity of this device has been determined over the energy range 197–1368 keV. Comparison to a previous design of a similar detector has been made.

Radiation studies and operational projections for silicon in the ATLAS inner detector

Abstract: The current models for bulk damage in high resistivity silicon have been reviewed. The damage constants were obtained from a global data survey. On this basis the degradation of the silicon counters in the ATLAS Inner Detector during a 10 year LHC operation is forecasted.

Comparison of n-γ discrimination by zero-crossing and digital charge comparison methods

Abstract: A comparative study of the n-γ discrimination done by the digital charge comparison and zero-crossing methods was carried out for a 130 mm in diameter and 130 mm high BC501A liquid scintillator coupled to a 130 mm diameter XP4512B photomultiplier. The high quality of the tested detector was reflected in a photoelectron yield of 2300 ± 100 phe/MeV and excellent n-γ discrimination properties with energy discrimination thresholds corresponding to very low neutron (or electron) energies. The superiority of the Z C method was demonstrated for the n-γ discrimination method alone, as well as, for the simultaneous separation by the pulse shape discrimination and the time-of-flight methods down to about 30 keV recoil electron energy. The digital charge comparison method fails for a large dynamic range of energy and its separation is weakly improved by time-of-flight method for low energies.

Figure-8 undulator as an insertion device with linear polarization and low on-axis power density

Abstract: A new type undulator for linearly polarized radiation, figure-8 undulator, is proposed. It has the advantage that the on-axis power density is much lower than that of an ordinary planar undulator. Electrons inside the proposed device move along the trajectory which looks like a figure-8 when projected on the plane perpendicular to the beam axis. The spectral performance and the power density are calculated and compared to those of an ordinary planar undulator.

Detection Efficiency of the Neutron Modular Detector Demon and Related Characteristics

Abstract: DEMON is a detector array for neutrons emitted in heavy ion induced reactions at low to intermediate energies. The neutron detection efficiency of some single DEMON modules has been measured in the energy range 8-21 MeV, with detection thresholds down to 0.1 MeV equivalent-electron (MeVee). The influence on the efficiency of cross talk, neutron-gamma and neutron-charged particle discrimination techniques was found to be negligibly small. Monte Carlo simulations have been performed and are compared with the experimental data. The relative weight of the interaction channel C-12(n,n'gamma) in the organic liquid scintillator was determined experimentally. Taking into account observed saturation effects in the detector output, a calibration method is proposed for high light outputs, based on the energy loss of cosmic ray muons.

A dual detector single readout system for simultaneous small (SAXS) and wide-angle X-ray (WAXS) scattering

Abstract: A data acquisition system is described which allows to simultaneously record X-ray scattering patterns in different angular regimes both with high spatial and temporal resolution. It consists of two linear detectors with delay-line readout connected in series. A few examples illustrate its application in the study of the polymorphism of lipid systems and their phase transitions as well as of synthetic polymers.

The scintillation of CF4 and its relevance to detection science

Abstract: The scintillation properties of CF4 are presented in comparison with those of Xe and CH4. Alpha-particle induced photon emission was measured with vacuum phototubes and with a CsI-based gaseous photomultiplier. The latter method provides an absolute sensitivity of such devices to particle-induced UV-photon background in CF4 and CH4 gaseous Cherenkov radiators. Integrated CF4 scintillation yields over the range of 150–220 nm are, on the average, 315±95 to 242±60 photons/MeV, in the respective pressure range of 0.063 to 0.75 atm, compared to CH4 which emits 0.06±0.01 photons/MeV at 1 atm. The total photon yield, integrated over the full emission spectrum of CF4 (150–500 nm), is of the order of 1200 photons/MeV × 4 π. The primary scintillation photon yield of CF4 is about 16(±5)% of that of Xe. No proportional secondary scintillation was observed in CF4. The avalanche-induced photon yield was measured to be of the order of 0.3 photons per electron. The implications of this considerable photon emission, are discussed.

An analytical approach to the evaluation of the pulse shape discrimination properties of scintillators

Abstract: In this paper a general methodology to compute the performances of different pulse shape discrimination techniques, applied to exploit the intrinsic PSD features of organic and of some inorganic scintillators, is presented. The most popular methods, i.e. the rise time technique and the charge integration approach, are evaluated and compared. As a specific example, detailed calculations are carried out for a binary organic liquid scintillator under consideration for the low energy solar neutrino experiment Borexino. It is demonstrated, in particular, that the charge integration method offers superior performances, the difference being remarkable for pulses comprising a low number of photoelectrons.

CAIN: Conglomérat d'ABEL et d'Interactions Non-linéaires☆

Abstract: We present our plans for a Monte-Carlo code simulating all possible combinations of (electromagnetic) interactions between colliding electron, positron, and both high-energy and laser photon beams, based on the ABEL code for beam-beam interaction. The implementation and first results for the laser-e− interaction are described.

Proton- and deuteron spin targets in bioligical structure research

Abstract: Polarized neutron scattering from polarized nuclear spin targets has been used to study the in situ structure of ribosomal components in solutions. The progress of this technique is substantiated by first results obtained from protonated tRNA and mRNA labels in a deuterated ribosome. The site of the tRNA could be determined uniquely, whereas there are several orientations of the [tRNA]2-mRNA complex which are compatible with the neutron scattering data. Similarly, a small, protonated mRNA fragment was studied. Its size favours a complicated path of the mRNA chain through the ribosome. The role of deuteron spin targets is discussed.