Showing papers in "Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment in 1993"

Magnetic scanning system for heavy ion therapy

Abstract: Beams of heavy ions have favourable physical and biological properties for the use in radiotherapy. These advantages are most pronoucced if the beam is delivered in a tumor-conform way by active beam scanning. A magnetic scanning technique is used to spread the beam laterally. The range of the particles in tissue is controlled by the variation of the beam energy in the accelerator. Computer simulations were used to compare a discrete scan mode (pixel scan) with a continous scan mode (raster scan). It was found that both methods lead to nearly identical results. The design and technical realization of the magnetic scanning system at GSI combines features of both scan techniques. First results using the lateral beam scanning method as well as the combination of the active energy variation with the magnetic beam scanning are presented.

Positron emission particle tracking - a technique for studying flow within engineering equipment

Abstract: The use of the Birmingham positron camera for tracking positron-emitting particles within engineering structures has been investigated, using particles containing typically 4 MBq (100 μCi) 18 F or 22 Na. An algorithm for discarding γ-ray trajectories corrupted by scattering, etc., and using the remaining trajectories to compute the particle's location in three dimensions has been developed, and its performance explained. A slowly moving particle can be located to within 1 mm several times per second, while a particle moving at 1 ms −1 can be located to within 5 mm 50 times per second, through a considerable thickness of surrounding material. An example of results obtained from tracking a particle in a rotating drum of powder illustrates the potential value of the technique.

Analysis of positron lifetime spectra using quantified maximum entropy and a general linear filter

Abstract: Two new approaches are used to analyze positron annihilation lifetime spectra. A general linear filter is designed to filter the noise from lifetime data. The quantified maximum entropy method is used to solve the inverse problem of finding the lifetimes and intensities present in data. We determine optimal values of parameters needed for fitting using Bayesian methods. Estimates of errors are provided. We present results on simulated and experimental data with extensive tests to show the utility of this method and compare it with other existing methods.

Testing and operation of the 15UD Pelletron at NSC

Abstract: The results of the performance demonstration tests and the subsequent operational experiences of the 15UD Pelletron (upgraded to 16 MV by using compres

Fundamental limits of scintillator performance

Abstract: In this paper we consider the basic physical processes involved in the scintillation process and discuss the limitations imposed on two important performance parameters, namely efficiency (light output) and speed. Light output is determined by the product of efficiencies of energy conversion, energy transfer and luminescence processes. We propose a procedure by which these partial efficiencies can be obtained for any scintillator and use it to evaluate some known materials. Limits of speed are set by the value of the Einstein coefficient A for the luminescent emission and by transfer rates. The singular significance of the transfer step is illustrated by some Ce-based scintillators. A figure of merit and an “efficiency-speed” diagram are introduced in order to compare different scintillators.

Beam tests of the ZEUS barrel calorimeter

Abstract: A fully compensating uranium—scintillator calorimeter was constructed for the ZEUS detector at HERA. Several of the barrel calorimeter modules were subjected to beam tests at Fermilab before shipping them to DESY for installation. The calibrations of the modules used beams of electrons and hadrons, measuring the uniformity of the response, and checking the resolution. The runs also provided opportunity to test a large fraction of the actual ZEUS calorimeter readout system in an integrated beam environment more than one year before HERA turn on. The experiment utilized two computer controlled mechanical structures, one of which was capable of holding up to four modules in order to study shower containment, and a magnetic spectrometer with a high resolution beam tracking system. During two running periods, beams of 6 to 110 GeV containing e, μ, π, and p were used. The results show energy resolutions of 35%/ E for hadrons and 19%/ E for electrons, uniformities at the 1% level, energy nonlinearity less than 1%, and equal response for electrons and hadrons.

The H 1 liquid argon calorimeter system

Abstract: The liquid argon calorimeter of the H 1 detector presently taking data at the HERA ep collider at DESY, Hamburg, is described. The main physics requirements and the most salient design features relevant to this calorimeter are given. The aim to have smooth and hermetic calorimetric coverage over the polar angular range 4° ≤ θ ≤ 154° is achieved by a single liquid argon cryostat containing calorimeter stacks structured in wheels and octants for easy handling. The absorber materials used are lead in the electromagnetic part and stainless steel in the hadronic part. The read-out system is pipelined to reduce the dead time induced by the high trigger rate expected at the HERA collider where consecutive bunches are separated in time by 96 ns. The main elements of the calorimeter, such as the cryostat, with its associated cryogenics, the stack modules, the read-out, calibration and trigger electronics as well as the data acquisition system are described. Performance results from data taken in calibration runs with full size H 1 calorimeter stacks at a CERN test beam, as well as results from data collected with the complete H 1 detector using cosmic rays during the initial phase of ep operations are presented. The observed energy resolutions and linearities are well in agreement with the requirements.

Scintillation characteristics of PbWO4 single crystals at room temperature

Abstract: Single crystals of PbWO 4 at room temperature show a narrow excitation peak around 325 nm with the FWHM as small as 10 nm and a broad emission peak around 420–440 nm. The light output is about 4.4% of that in BGO. The luminescence is fast with the decay constant less than 10 ns for 84% and as fast as 40 ns for the rest. Degradation in optical transmittance due to γ-ray irradiation is not large up to at least 10 7 rad in the present crystals which are slightly coloured yellow. Spontaneous recovery of the radiation damage occurs to some extent with a time constant much shorter than 18 days. The above result indicates a possibility that this material may be resistive enough against radiation if colourless crystals could be grown.

Spatial resolution of silicon microstrip detectors

Abstract: The spatial resolution of silicon microstrip detectors is studied as a function of the main detector parameters and of the track angle. Several algorithms for finding the position of particle hits are presented and analysed. Analytic expressions of the spatial resolution are derived for the main algorithms. Using a Monte Carlo simulation, the spatial resolution is calculated for each algorithm and, for each detector design and track geometry, the algorithm that gives the best resolution is determined.

A fission ionization detector for neutron flux measurements at a spallation source

Abstract: The construction of a neutron flux monitor that can measure absolute neutron intensities in the neutron energy range from below 1 MeV to over 500 MeV is described. The detector consists of an ionization chamber with several thin deposits of fissionable material. The ionization chamber is thin enough that it does not significantly affect the neutron beam and may be left in the neutron flight path during experimental measurements to continuously monitor the beam flux. The use of this monitor at the continuous-energy spallation neutron source at the WNR target area at LAMPF is described.

Transition radiation detectors

Abstract: The use of transition radiation (TR) as a means of identifying high energy particles has now become a subject of intensive experimental investigations and applications. Our intention is first to study the physics of these phenomena and to describe ways of building detectors which can efficiently identify particles.

The OPAL silicon microvertex detector

Abstract: A silicon strip microvertex detector has been designed, constructed and commissioned in the OPAL experiment at the LEP electron-positron collider. The microstrip devices incorporate a new FoxFET biassing scheme developed together with Micron Semiconductor Ltd., UK. The devices digitise with a precision close to 5 μm and have an exceptionally high signal-to-noise ratio. The associated microelectronics were all custom made for the OPAL project. The detector began operation in 1991 and has since continued to be part of the OPAL experiment, performing to a very high standard and opening up new areas of physics studies.

Simultaneous time resolved SAXS and WAXS experiments using synchrotron radiation

Abstract: Instrumentation has been developed which offers the possibility of performing simultaneous time resolved small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) experiments in the range from approximately 0.007–0.21 A −1 and 0.31–4.2 A −1 . A synchrotron SAXS beamline is therefore equipped with two gas filled proportional detectors. The ultimate system time resolution will be of the order of 1 μs. Successful test experiments have been performed on block copolymer samples and polyethylene.

Results from pion calibration runs for the H 1 liquid argon calorimeter and comparisons with simulations

Abstract: We present results on calibration runs performed with pions at the CERN SPS for different modules of the H 1 liquid argon calorimeter which consists of an electromagnetic section with lead absorbers and a hadronic section with steel absorbers. The data cover an energy range from 3.7 to 205 GeV. Detailed comparisons of the data and simulation with GHEISHA 8 in the framework of GEANT 3.14 are presented. The measured pion induced shower profiles are well described by the simulation. The total signal of pions on an energy scale determined from electron measurements is reproduced to better than 3% in various module configurations. After application of weighting functions, determined from Monte Carlo data and needed to achieve compensation, the reconstructed measured energies agree with simulation to about 3%. The energies of hadronic showers are reconstructed with a resolution of about 50% √E ⨸ 2% . This result is achieved by inclusion of signals from an iron streamer tube tail catcher behind the liquid argon stacks.

The JET neutron emission profile monitor

Abstract: This paper provides a technical description of the neutron emission profile monitor as used routinely at the Joint European Torus (JET), and includes representative examples of its operational capabilities. The primary function of this instrument is to measure the neutron emission as a function of both position and time in a poloidal (vertical along major radius) section through the torus. For the first time the spatially localised effects of sawteeth (magnetic relaxation phenomena) have been observed using a neutron diagnostic. The total (global) neutron emission can be obtained from the profile monitor data by performing a volume integral over the plasma; the absolute neutron emission rates agree with those obtained from the JET time-resolved neutron monitor to within ±15%. This was the first such instrument routinely in use on any tokamak. It provides unique data which are independent of all other diagnostic measurements.

First supermodule of the MACRO detector at Gran Sasso

Abstract: In this paper the design, construction and performance of the lower part of the first supermodule of the MACRO detector is described.

The HERA polarimeter and the first observation of electron spin polarization at HERA

Abstract: Electron spin polarizations of about 8% were observed at HERA in November 1991. In runs during 1992, utilizing special orbit corrections, polarization values close to 60% have been achieved. In this paper the polarimeter, the machine conditions, the data analysis, the first results and plans for future measurements are described.

A novel charge sensitive preamplifier without the feedback resistor

Abstract: A novel charge sensitive preamplifier which has no resistor in parallel with the feedback capacitor is presented. No external device or circuit is required to discharge the feedback capacitor. The detector leakage and signal current flows away through the gate of the first JFET which works with its gate to source junction slightly forward biased. The DC stabilization of the preamplifier is accomplished by an additional feedback loop, which permits to equalize the current flowing through the forward baised gate to source junction and the current coming from the detector. An equivalent noise charge of less than 20 electrons r.m.s. has been measured at room temperature by using an input JFET with a transconductance to gate capacitance ratio of 4 mS/5.4 pF.

Temperature dependence of CsI(Tl) gamma-ray excited scintillation characteristics

Abstract: The gamma-ray excited, temperature dependent scintillation characteristics of CsI(Tl) are reported over the temperature range of −100 to + 50°C. The modified Bollinger-Thomas and shaped square wave methods were used to measure the rise and decay times. Emission spectra were measured using a monochromator and corrected for monochromator and photocathode spectral efficiencies. The shaped square wave method was also used to determine the scintillation yield as was a current mode method. The thermoluminescence emissions of CsI(Tl) were measured using the same current mode method. At room temperature, CsI(Tl) was found to have two primary decay components with decay time constants of τ1 = 679±10 ns (63.7%) and τ2 = 3.34±0.14 μs (36.1%), and to have emission bands at about 400 and 560 nm. The τ1 luminescent state was observed to be populated by an exponential process with a resulting rise time constant of 19.6±1.9 ns at room temperature. An ultra-fast decay component with a

Point source search techniques in ultra high energy gamma ray astronomy

Abstract: Searches for point astrophysical sources of ultra high energy (UHE) gamma rays are plagued by large numbers of background events from isotropic cosmic rays. Some of the methods that have been used to estimate the expected number of background events coming from the direction of a possible source are found to contain biases. Search techniques that avoid this problem are described. There is also a discussion of how to optimize the sensitivity of a search to emission from a point source.

A highly-segmented ΔE-time-of-flight wall as forward detector of the 4π-system for charged particles at the SIS/ESR accelerator

Abstract: At the SIS/ESR accelerator facility at GSI in Darmstadt the 4π-detector system FOPI is under construction at present. It is designed for the investigation of central collisions of heavy ions in the energy range up to 2 A GeV. As phase I of this detector a forward wall has been built and used in various experiments. It comprizes a total number of 764 scintillators with an additional shell of 188 thin ΔE -detectors in front of it and covers the full azimuth of the polar angles from 1° to 30°. The velocity and the nuclear charge of the fragments are determined by a combined time-of-flight and ΔE measurement.

Fresnel and refractive lenses for X-rays

Abstract: We present a Gaussian beam analysis of X-ray refractive and Fresnel lenses. The X-ray refractive lens is featured by an intrinsic soft (Gaussian) aperture due to strong absorption of X-rays by materials. We defined a parameter N0, the critical Fresnel number (CFN), to describe this optical property. The values of N0 for all practical materials are below 1000 for photon energies exceeding 30 eV, still lower for high-Z materials. The maximum effective Fresnel number of a lens is determined by its material to be 2N0 and its maximum enhancement of X-ray intensity is limited to (2πN0)2, independent of its shape. We found that the refractive lens is likely to be useful for manipulating nearly diffraction limited beam in the hard X-ray region and its application is severely restricted by available fabrication capabilities today. X-ray Fresnel lenses, both in cylindrical and linear forms, are proposed as superior focusing elements for hard X-rays. Their high efficiency, up to 100% in optimal construction, will enable us to manipulate beams with multiple lenses and obtain higher performance optics. Their design and fabrication are discussed in reference to those of X-ray Fresnel zone plates and micro Fresnel lenses for optoelectronics.

The micro-gap chamber

Abstract: The micro-gap chamber (MGC), a new type of position sensitive proportional gas counter, is introduced. The device is built using microelectronics technology. In this detector the separation between the electrodes collecting the avalanche charge (the anode-cathode gap) is only a few microns. The time it takes to collect the positive ions is therefore very short ( ≈ 10 ns). The speed of the device now equals that of solid state detectors but it is more than three orders of magnitude higher than in standard proportional counters and one order of magnitude higher than in the recently introduced microstrip gas chamber (MSGC). As a result, the rate capability is extremely high (> 9×10 6 c /mm 2 s). The amplifying electric field around the thin anode microstrip extends over a small volume but is very intense (270 kV/mm). It provides a gas gain of 2.5 × 10 3 at 400 V with 14% (FWHM) energy resolution at 5.4 keV. The anode pitch is 100 μm and the readout is intrinsically two-dimensional. Because there is practically no insulating material in view, charging was not observed even at the highest rate. This device seems very well suited for instrumentation of the tracking system at the new hadron colliders (LHC/SSC) as well as in many other fields of research.

The differential decay curve method for the analysis of Doppler shift timing experiments

Abstract: The differential decay curve method (DDCM) is shown to be a general tool for analyzing data from Doppler-shift timing experiments. The application for recoil-distance Doppler-shift measurements is outlined. An extension of this method for the analysis of Doppler-shift attenuation method (DSAM) measurements both in singles and coincidence mode is worked out. Improvements are expected for detecting and avoiding systematic errors which are likely to appear in the analysis of DSAM data. The application of the DDCM to DSAM coincidence measurements would solve problems associated with observed and unobserved feeding and their time behavior. Quantitative relations are derived for the improved data analysis.

Analysis of multiphase flows using dual-energy gamma densitometry and neural networks

Abstract: Dual-energy gamma densitometry offers a powerful technique for the non-intrusive analysis of multiphase flows. By employing multiple beam lines, information on the phase configuration can be obtained. Once the configuration is known, it then becomes possible in principle to determine the phase fractions. In practice, however, the extraction of the phase fractions from the densitometer data is complicated by the wide variety of phase configurations which can arise, and by the considerable difficulties of modelling multiphase flows. In this paper we show that neural network techniques provide a powerful approach to the analysis of data from dual-energy gamma densitometers, allowing both the phase configuration and the phase fractions to be determined with high accuracy, whilst avoiding the uncertainties associated with modelling. The technique is well suited to the determination of oil, water and gas fractions in multiphase oil pipelines. Results from linear and non-linear network models are compared, and a new technique for validating the network output is described.

A high-energy electron beam ion trap for production of high-charge high-Z ions

Abstract: We have developed a new high-energy electron beam ion trap, the first laboratory source of low-energy, few-electron, high- Z ions. We describe the device and report measurements of its performance, including the electron beam diameter, current density and energy, and measurements of the ionization balance for several high- Z elements in the trap. This device opens up a wide range of possible experiments in atomic physics, plasma physics, and nuclear physics.

An advanced high-current low-emittance dc microwave proton source

Abstract: The high-current low-emittance microwave proton source introduced two years ago act the Chalk River Laboratories has been extensively redesigned. The 2.45 GHz microwave line was modified to incorporate a 50 kV dc break and a rectangular-to-ridged waveguide transition. The layered microwave window has been replaced by a single plate of aluminum nitride. The solenoids were electrically isolated from the plasma chamber. Only the plasma chamber remains at the extraction potential. The total beam current and the proton beam current have been measured as a function of the microwave power, the hydrogen mass flow and the solenoid positions and currents. More than 95 mA dc of hydrogen ions with a proton fraction in excess of 85% were extracted from a single 2.5 mm radius aperture with only 500 W of microwave power and 2.0 sccm (3 μg/s) of hydrogen mass flow. The perveance at minimum divergence and the minimum emittance have been determined for various extraction geometries. The variation of the perveance with the aspect ratio of the extraction system was found to be virtually identical for microwave and arc discharge ion sources. The normalized rms emittance was less than 0.13 p mm mrad whatever the extraction geometry.

Design of a new type of planar undulator for generating variably polarized radiation

Abstract: A small prototype model of a new undulator is under construction for generating variably polarized radiation. This undulator consists of two pairs of planar permanent magnet arrays above and below the electron orbit plane in a storage ring. This device has no magnetic structures in the horizontal orbit plane in order to avoid the conflict with the aperture of the electron orbit. The on-axis magnetic field generated by shifting the relative position of pairs of magnet arrays in this device induces helical and sinusoidal electron orbit motions. It leads to the generation of circularly and linearly polarized undulator radiation. This new undulator will be installed in the JAERI storage ring (JSR) in order to examine the performance for generating variable polarization.

The pn-CCD on-chip electronics

Abstract: A new pn-CCD with an activa area of 3 × 1 cm2 was recently fabricated for ESA's X-ray Multi Mirror Mission (XMM). The front-end electronics has been integrated on the same chip as the detector, and its noise behaviour was investigated. X-rays from a 55Fe source have been used for the absolute calibration. The measured electronic Equivalent Noise Charge (ENC) of the on-chip amplifier was 8.8 e− at room temperature and 2.2 e− at the CCD operating temperature of 150 K. The improvements with respect to the last version with noise figures of 4.8 e− (at 150 K) are due to the reduction of the total input capacitance by a factor of 1.6, the improvement of the transistor transconductance by a factor of 2, and the reduction of 1 ƒ noise because of the different p-well implant with a better thermal annealing.

A new cooling and focusing device for ion guide

Abstract: A highly efficient focusing device which uses a RF multipole field (sextupole-ion-beam guide (SPIG)) has been developed for the ion guide isotope separator on-line (IGISOL) The SPIG, placed after the nozzle, consists of six circular rods uniformly distributed on a circle in the plane perpendicular to the axis of symmetry Under the combined action of the sextupole RF electric field produced by the rods and helium gas flow, well focused ion beams with low energy spread can be transmitted efficiently by use of the SPIG The experimental results, using a discharge ion source, show that almost nearly 90% of the ions which leave the nozzle can be transported through the SPIG, and that the width of the kinetic energy distribution after the SPIG is about 08 eV (FWHM) These experimental results agree with the results of Monte Carlo simulations; the simulation studies also indicate that the size of the spatial distribution in the plane perpendicular to the symmetry axis is less than 1 mm in diameter at the end of the SPIG