Showing papers in "Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment in 1994"

Calibration and correction of spatial distortions in 2D detector systems

Abstract: Many photon and other radiation 2D detection systems introduce spatial distortions into recorded data. As long as these distortions are to some degree repeatable it is possible to use a grid of holes of known positions to illuminate the detector and calibrate the distortion. Knowledge of this distortion can be used to correct subsequent images. Aspects of calibration grid design are discussed with the aim of achieving optimum results. Cross-correlation with known hole functions is proposed as a method for estimating grid peak centres giving optimum accuracy and efficiency. Strategies to search efficiently complete grids are also presented. Where a physical model of the distortion process is lacking or inadequate, bi-cubic splines are proposed as suitable interpolating functions for defining approximate spatial distortion functions. Bi-cubic splines allow both adequate flexibility to describe arbitrary distortions and, most importantly, to allow efficient calculation for the correction of data. Calibration and correction algorithms have been developed and applied to 2D detector systems. Sample results are presented.

Digital synthesis of pulse shapes in real time for high resolution radiation spectroscopy

Abstract: Techniques have been developed for the synthesis of pulse shapes using fast digital schemes in place of the traditional analog methods of pulse shaping. Efficient recursive algorithms have been developed that allow real time implementation of a shaper that can produce either trapezoidal or triangular pulse shapes. Other recursive techniques are presented which allow a synthesis of finite cusp-like shapes. Preliminary experimental tests show potential advantages of using these techniques in high resolution, high count rate pulse spectroscopy.

Properties of the YAG:Ce scintillator

Abstract: Light yield, light pulse shape due to γ-rays and α-particles, energy resolution and time resolution of the new YAG:Ce scintillator were studied using a light readout by means of the XP2020Q photomultiplier and the S3590-03 photodiode. The light yield of 20 300 ± 2000 photons/MeV was determined using three independent methods. The light pulse produced by γ-rays consists of two components with the decay time constants of 87.9 ns and 302 ns respectively. The light pulse measured for α-particles exhibits a faster and less intense fast component with the decay time constant of 68.4 ns. The energy resolution obtained was 11.1% and 11.7% for the 662 keV γ-rays from a 137 Cs source as measured with the photomultiplier and the photodiode readout, respectively. A time resolution of 1.3 ns was observed for 60 Co γ-rays (at 100 keV threshold) with the crystal coupled to the XP2020Q photomultiplier. The YAG:Ce scintillator with the peak emission at 550 nm is a good candidate to replace CsI(T1) and BGO scintillators in detection of light charged particles when photodiode readout is of importance. The observed difference in the light pulse shape due to γ-rays and α-particles suggests good performance of the crystal in the pulse shape discrimination method for the particle identification.

Signal processing considerations for liquid ionization calorimeters in a high rate environment

Abstract: We present the results of a study of the effects of thermal and pileup noise in liquid ionization calorimeters operating in a high luminosity environment. The method of optimal filtering of multiply-sampled signals to obtain timing and amplitude from calorimeter signals is described. This method has some advantages over the traditional method of sampling the peak of a shaped signal, which include a reduced sensitivity to channel-to-channel variations in the pre-filter shaping parameters and good performance over a wide range of operating conditions. Analytic expressions for the variance of amplitude and timing measurements are found through a frequency domain approach. Implications for the choice of pre-filter shaping time, number and position of the samples, and digitization accuracy are discussed.

The DØ detector

Abstract: The DO detector is a large general purpose detector for the study of short-distance phenomena in high energy antiproton-proton collisions, now in operation at the Fermilab Tevatron collider. The detector focusses upon the detection of electrons, muons, jets and missing transverse momentum. We describe the design and performance of the major elements of the detector, including the tracking chambers, transition radiation detector, liquid argon calorimetry and muon detection. The associated electronics, triggering systems and data acquisition systems are presented. The global mechanical, high voltage, and experiment monitoring and control systems which support the detector are described. We also discuss the design and implementation of software and software support systems that are specific to DO.

The Experimental Physics and Industrial Control System architecture: Past, present, and future

Abstract: The Experimental Physics and Industrial Control System (EPICS), has been used at a number of sites for performing data acquisition, supervisory control, closed-loop control, sequential control, and operational optimization. The EPICS architecture was originally developed by a group with diverse backgrounds in physics and industrial control. The current architecture represents one instance of the “standard model”. It provides distributed processing and communication from any local area network (LAN) device to the front end controllers. This paper presents the current architecture, performance envelope, current installations, and planned extensions for requirements not met by the current architecture.

On the use of the covariance matrix to fit correlated data

Abstract: Best fits to data which are affected by systematic uncertainties on the normalization factor have the tendency to produce curves lower than expected if the covariance matrix of the data points is used in the definition of the χ2. This paper shows that the effect is a direct consequence of the hypothesis used to estimate the empirical covariance matrix, namely the linearization on which the usual error propagation relies. The bias can become unacceptable if the normalization error is large, or a large number of data points are fitted.

Digital techniques for real-time pulse shaping in radiation measurements

Abstract: Recursive algorithms for real-time digital pulse shaping in pulse height measurements have been developed. The differentiated signal from the preamplifier (exponential pulse) is amplified and then digitized. Digital data are deconvolved so that the response of the high-pass network is eliminated. The deconvolved pulse is processed by a time-invariant digital filter which allows trapezoidal/triangular or cusp-like shapes to be synthesized. A prototype of a digital trapezoidal processor was built which is capable of sampling and processing digital data in real time at clock rates up to 50 MHz.

Quantities and units in radiation protection dosimetry

Abstract: A new report, entitled Quantities and Units in Radiation Protection Dosimetry, has recently been published by the international Commission on Radiation Units and Measurements. That report (No. 51) aims to provide a coherent system of quantities and units for purposes of measurement and calculation in the assessment of compliance with dose limitations. The present paper provides an extended summary of that report, including references to the operational quantities needed for area and individual monitoring of external radiations.

The L3 silicon microvertex detector

Abstract: The design and construction of the silicon strip microvertex detector (SMD) of the L3 experiment at LEP are described. We present the sensors, readout electronics, data acquisition system, mechanical assembly and support, displacement monitoring systems and radiation monitoring system of the recently installed double-sided, double-layered SMD. This detector utilizes novel and sophisticated techniques for its readout.

Analyses for a planar variably-polarizing undulator

Abstract: A new undulator for generating variably-polarized radiation was proposed in an earlier paper. This device consists of two pairs of planar permanent magnet rows above and below the electron orbit plane. The magnetic field generated with this undulator, stronger than any other existing planar helical device, induces various types of electron motion such as vertically or horizontally sinusoidal motion and helical motion. The analyses of magnetic field and undulator radiation spectra are made on this undulator. For calculating the spectra and angular distributions in various modes, the SPring-8 storage ring parameters are assumed. This undulator generates brilliant circularly-polarized undulator radiation comparable in intensity with linearly-polarized radiation from a conventional undulator.

The design and construction of the ZEUS central tracking detector

Abstract: The mechanical, electrical and electronic design and construction of the ZEUS central tracking detector are described, together with the chamber monitoring and environmental control. This cylindrical drift chamber is designed for track reconstruction, electron identification and fast event triggering in a high beam-crossing rate, high magnetic field application.

VIKING, a CMOS low noise monolithic 128 channel frontend for Si-strip detector readout

Abstract: A low noise Si-strip detector readout chip has been designed and built in 1.5 μm CMOS technology. The chip is optimized w.r.t. noise. Measurements with this chip connected to several silicon strip detectors are presented. A noise performance of ENC = 135 e− + 12 e−/pF and signal to noise ratios between 40–80, depending on the detector, for minimum ionizing particles traversing 280 300 μ m silicon has been achieved.

Absolute calibration and monitoring of a spectrometric channel using a photomultiplier

Abstract: A method for measuring the photomultiplier gain using a realistic photomultiplier response function is described. A precision of about 1% for the deconvoluted gain parameter can be achieved.

An imaging plate neutron detector

Abstract: We have developed two kinds of imaging plate neutron detectors (IP-ND), where the neutron converters, 6 Li (tentatively, Nat Li was used) and Gd were mixed with photostimulated luminescence (PSL) materials on a flexible plastic support, and the dynamic range and spatial resolution of our IP-ND were successfully obtained as 1:10 5 and less than 0.2 mm, respectively, which are comparable to the ones of X-ray. The distinctive features of the IP-ND, where the IP-NDs of different neutron detection efficiency and converters were positively used, were found and discussed. The application of the IP-ND for neutron radiography was demonstrated.

Response of TAPS to monochromatic photons with energies between 45 and 790 MeV

Abstract: The Two Arm Photon Spectrometer TAPS - comprising 384 plastic-BaF2 scintillator telescopes - was tested with monochromatic photons in the energy range between 45 and 790 MeV. The energy resolution for a collimated photon beam hitting the central detector module was determined to sigma/E = 0.59% X E(gamma)-1/2 + 1.9% (E, given in GeV). For the the fast scintillation component alone sigma/E = 0.79% X E(gamma)-1/2 + 1.8% has been measured. The position resolution of the point of impact amounts to DELTAx = 2 cm (FWHM) at the highest energies which corresponds to 30% of the diameter of an individual module. Monte Carlo simulations using the code GEANT3 are in good agreement with the experimental results.

The GEANT-CALOR interface and benchmark calculations of ZEUS test calorimeters

Abstract: The simulation of large scale high energy physics experiments is based mainly on the GEANT package. In the current version 3.15 the simulation of hadronic interacting particles is based on GHEISHA or FLUKA. Both programs miss an accurate simulation of the interaction of low energetic neutrons ( E kin

An Antimatter spectrometer in space

Abstract: We discuss a simple magnetic spectrometer to be installed on a satellite or space station. The purpose of this spectrometer is to search for primordial antimatter to the level of antimatter/matter ≈10 −9 , improving the existing limits obtained with balloon flights by a factor of 10 4 to 10 5 . The design of the spectrometer is based on an iron-free, NdFeB permanent magnet, scintillation counters, drift tubes, and silicon or time projection chambers. Different design options are discussed. Typically, the spectrometer has a weight of about 2 tons and an acceptance of about 1.0 m 2 sr. The availability of the new NdFeB material makes it possible for the first time to put a magnet into space economically and reliably.

The BES detector

Abstract: The Beijing Spectrometer (BES) is a general purpose solenoidal detector at the Beijing Electron Positron Collider (BEPC). It is designed to study exclusive final states in e + e − annihilations at the center of mass energy from 3.0 to 5.6 GeV. This requires large solid angle coverage combined with good charged particle momentum resolution, good particle identification and high photon detection efficiency at low energies. In this paper we describe the construction and the performance of BES detector.

Electron emission from ferroelectrics - a review

Abstract: The strong pulsed emission of electrons from the surface of ferroelectric (FE) materials was discovered at CERN in 1987. Since then many aspects and properties of the method of generation and propagation of electron beams from FE have been studied experimentally. The method is based on macroscopic charge separation and self-emission of electrons under the influence of their own space-charge fields. Hence, this type of emission is not limited by the Langmuir-Child law as are conventional emission methods. Charge separation and electron emission can be achieved by rapid switching of the spontaneous, ferroelectric polarization. Polarization switching may be induced by application of electrical-field or mechanical-pressure pulses, as well as by thermal heating or laser illumination of the ferroelectric emitter. At higher emission intensities plasma formation assists the FE emission and leads to a strong growth of emitted current amplitude, which is no longer limited by the FE material and the surface properties. The most attractive features of FE emission are robustness and ease of manipulation of the emitter cathodes which can be transported through atmospheric air and used without any problems in vacuum, low-pressure gas or plasma environments. Large-area arrangements of multiple emitters, switched in interleaved mode, can produce electron beams of any shape, current amplitude or time structure. The successful application of FE emission in accelerator technology has been demonstrated experimentally in several cases, e.g. for triggering high-power gas switches, for photocathodes in electron guns, and for electron-beam generators intended to generate, neutralize and enhance ion beams in ion sources and ion linacs. Other applications can be envisaged in microwave power generators and in the fields of electronics and vacuum microelectronics.

SHADOW: A synchrotron radiation and X-ray optics simulation tool

Abstract: We announce the release of SHADOW 2.0 - the computer ray-tracing program widely used in the synchrotron radiation community. We discuss new physical models and system extensions recently introduced. The physical models added are capillaries and Kumakhov lenses, general compound mirrors, and transmission crystals. We have extended the base of operating systems available to SHADOW users, overhauled the UNIX graphics, and written a MENU mode for UNIX. We also discuss new physical models in progress - a shaped absorber, segmented mirrors, and extensions to higher orders in the polynomial calculations.

Theory of ring imaging Cherenkov counters

Abstract: A short review of Cherenkov radiation and its use for particle identification with threshold and differential counters is presented. The focusing principles of Ring Imaging Cherenkov (RICH) counters are then developed and applied successively to gas and liquid radiators. An analysis of proximity focused images from liquid or solid radiators follows. A method to reconstruct the Cherenkov angle from the experimental data is developed for spherical as well as for flat detector surfaces. These formulas define the Cherenkov angle sensitivity due to intrinsic as well as extrinsic (measurement) errors. Particle identification limits are then given for each of the different imaging methods.

A γ-ray detection system for 3-D particle tracking in multiphase reactors

Abstract: A system employing eight NaI detectors has been developed for tracking particles moving in a three-phase fluidized bed reactor. One particle is tagged with typically 2 MBq of 46Sc which emits high energy γ-rays. The system is calibrated by measuring the count-rates in the 8 detectors with the tagged particle placed at a number of locations in the reactor and then calculating the count-rates, using the Monte Carlo technique, at 19 200 locations. In a tracking experiment, data are accumulated in the multiscaling mode and the coordinates of the moving particle are calculatedby least-squares using the calibration map. With 30 ms counting intervals, the location of the particle is determined with a typical precision of 5 mm. The use of these data for determining velocity flow fields in multiphase reactors is illustrated.

Collective atomic recoil laser (CARL) optical gain without inversion by collective atomic recoil and self-bunching of two-level atoms

Abstract: We suggest a novel tunable laser concept, the Collective Atomic Recoil Laser (CARL) which unifies the physics of the FEL and of the atomic lasers. We demonstrate that a cold beam of two-level particles driven coherently by a counter-propagating resonant wave can amplify exponentially a co-propagating optical probe up to a saturation value through an instability very similar to that of a high gain FEL. In addition, the two level atoms undergo collective recoil and exponential self-bunching in space and form a longitudinal grating on the scale of the wavelength of the amplified optical signal.

The OPAL silicon strip microvertex detector with two coordinate readout

Abstract: The OPAL experiment at the CERN LEP collider recently upgraded its silicon strip microvertex detector from one coordinate readout (φ only) to two coordinate readout (φ and z). This allows three dimensional vertex reconstruction and should improve lifetime measurements as well as b quark jet identification. This paper describes the new microvertex detector system with emphasis on the novel techniques and new components used to obtain the second coordinate information. These include the use of back-to-back single-sided detectors with orthogonally oriented readout strips, a gold printed circuit on a thin glass substrate to route the z strip signals to the electronics at the end of the detector, and the use of MX7 readout chips. Results on the performance of the new detector are presented.

Test beam results for an upgraded forward tagger of the L3 experiment at LEP II

Abstract: We have tested new scintillator modules with silicon photodiode readout for the upgraded Active Lead Rings (ALR) of the L3 detector at LEP II. Results are presented from data recorded in muon and electron test beams with particular emphasis on the light production and collection as a function of the particle impact position on the scintillator modules. The results from the beam test data will be used for the design of the readout and trigger electronics in conjunction with the required ALR performance as an electron tagger and beam background monitor at LEP II.

Ce3+ doped inorganic scintillators

Abstract: There is a strong interest in the development of inorganic scintillators with a fast response for applications in high-energy physics, medical diagnostics and a number of other fields. One of the main lines of research concerns luminescence from the allowed 5d-4f transitions of lanthanide ions in appropriately chosen host lattices. Results are presented of research on Ce-doped scintillators.

Design of a Compton spectrometer experiment for studying scintillator non-linearity and intrinsic energy resolution

Abstract: A Compton spectrometer experiment has been designed and modeled for the purpose of studying the light yield non-linearities and intrinsic energy resolution of scintillation materials that are used to detect gamma rays. This coincidence method is used to create a nearly monoenergetic internal electron source within the scintillator by recording pulses from the primary detector only when a simultaneous pulse is generated by the coincidence detector. Such an electron source is necessary to accurately quantify the electron response of a scintillator, and has been previously identified as a requirement for quantifying scintillator non-linearity and intrinsic energy resolution. The ability to quantify these scintillator characteristics using this technique and the characterization of the Compton spectrometer geometry, including collimation of primary and scattered gamma rays, using Monte Carlo simulation are discussed.

Generation of femtosecond X-rays by 90° Thomson scattering

Abstract: We propose Thomson scattering of short pulse laser beams by low energy electron beams at a right angle for generation of femtosecond X-rays. The basic idea is the observation that a low emittance electron beam can be focussed much more tightly in a transverse dimension than in the longitudinal dimension. Therefore much shorter pulses of X-rays can be generated (in the direction of the electron beam) by arranging the laser beam to meet the electron beam at a right angle rather than head on as in the Thomson backscattering configuration. Simple analysis of the process is presented by noting the similarity between the Thomson scattered radiation and the well-understood undulator radiation. Using the parameters of the recently developed femtosecond visible lasers and the high brightness electron guns, it is shown that 1 A X-ray pulses, of 300 fs duration, containing several 10 5 photons within 10% bandwidth per collision, can be generated.

Implementation of the Doppler broadening of a Compton-scattered photon into the EGS4 code

Abstract: A modification to the general-purpose Monte Carlo electron-photon transport code EGS4 [1] was made in order to include Doppler broadening of Compton-scattered photon energy due to electron pre-collision motion. The Compton-scattered photon energy is sampled from a cross section formula based on the Compton profile, and the Compton scattering is sustained if the energy imparted to the electron is less than its binding energy. The electron binding effect modifies the scattered photon energy, angular distribution, and total cross section of the Compton scattering, and affects the photon mean free path used in the calculations. In the improved EGS4 code, all of these electron binding effects in Compton scattering are treated consistently. A simulation of 40 keV photon scattering by C and Cu samples was performed using the improved EGS4 code; the calculated scattered photon spectra agreed well with the measurements.