Showing papers in "Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment in 1989"

New photoelectric injector design for the Los Alamos National Laboratory XUV FEL accelerator

Abstract: The injector for the Los Alamos National Laboratory XUV FEL accelerator has been redesigned to provide more charge to the wiggler. The new design can deliver 8nC of charge within 20 ps with a normalized 90% emittance of π mm mrad to the wiggler at an energy of 200 MeV. In addition to the new design of the injector, we analyze the emittance growth and subsequent reduction through the injector, including the different mechanisms for emittance growth and the methods used to eliminate the correlated emittance.

Rf and space-charge effects in laser-driven rf electron guns

Abstract: The evolution of the electron-beam phase space distribution in laser-driven rf guns is studied by taking into account both the time variation of the rf field and space-charge effects. In particular, simple formulas are derived for the transverse and longitudinal emittances at the exit of the gun. The results are compared and found to agree well with those from simulation.

Vertex Triggering Using Time Difference Measurements in the ZEUS Central Tracking Detector

Abstract: A method of vertex triggering which uses a time difference measurement to determine the z coordinate of a track is under development for the ZEUS Central Tracking Detector. This incorporates a pipelined readout system. We describe the system and report on its performance.

A method for picosecond lifetime measurements for neutron-rich nuclei: (1) Outline of the method

Abstract: A general method of nuclear level lifetime measurements in the low-picosecond range for neutron-rich nuclei is proposed. This new method, designed for use at fission product mass separators, is based on standardized detector systems and procedures, and a comprehensive approach to data analysis for the whole decay scheme. Measurements utilize β-γ-γ coincidences where the fast timing information is derived from β-γ coincidences in timing detectors while the selection of the desired decay branch is obtained by using coincident γ-transitions recorded in a Ge detector with high energy resolution. The key idea is to use a thin ΔE β-detector to provide a uniform timing response for energetic betas. A BaF2 crystal is used as the timing γ-detector due to its superior energy resolution and efficiency in comparison to fast plastic scintillators. The outline of the method, which includes the experimental set-up and general techniques of data analysis, is discussed. A number of lifetime measurements in the subnanosecond range are presented to show the high selectivity of the method in identifying delayed γ-transitions.

Use of DCO ratios for spin determination in γ-γ coincidence measurements

Abstract: The use of large detector arrays allows to measure γ-γ angular correlations and to determine spins and multipolarities, even of weak and unresolved transitions The method utilizing directional correlations of γ-rays deexciting oriented states (DCO ratio method) has been described and applied to experimental data on 178 W taken with the γ-γ coincidence spectrometer OSIRIS The spin of the 35 ns isomer in 178 W has been established to be 14

Argus: A universal detector at DORIS II

Abstract: The detector ARGUS has been designed as a universal tool to investigate final states from e + e − annihilation processes in the energy range of the ϒ resonances. ARGUS started operation in October 1982 and has since successfully taken data at the ϒ(1S), ϒ(2S) and ϒ(4S) energies, and in the nearby continuum. The detector combines excellent charged particle identification and good photon energy resolution over more than 90% of the full solid angle. A particle originating from the interaction vertex and leaving the beam tube traverses the following components: the vertex drift chamber, the main drift chamber which determines its momentum and specific ionization, the time-of-flight system through which its velocity is determined, and the electromagnetic calorimeter. Muons pass through the magnet coils and the flux return yoke and finally hit the muon chamber system which surrounds the detector. The momentum resolution of ARGUS is σ (p T ) P T = (0.01 2 + (0.009p T [ GeV /c]) 2 ) 1 2 , the photon energy resolution in the barrel shower counters is σ (E) E = ( 0.072 2 +0.065 2 E[ GeV ] ) 1 2 . Combining the information from all p devices, more than 80% of all charged hadrons can be recognized unambiguously. The electron-hadron and muon-hadron rejection rates are 1:200 and 1:50 respectively.

A multiterm Boltzmann analysis of drift velocity, diffusion, gain and magnetic-field effects in argon-methane-water-vapour mixtures

Abstract: The effect of adding water vapour to argon-methane mixtures used in TPC-type detectors is investigated using the exact multiterm Boltzmann expansion. The transverse and longitudinal diffusion coefficients and the drift velocity are calculated with an accuracy of ± 3, ± 2 and ± 1% respectively. The calculations are extended into the avalanche region and compared to avalanche gain measurements. The effect of misalignment of the electric and magnetic fields on the drift direction is also investigated.

A new tomographic device based on the detection of fluorescent x-rays

Abstract: Fluorescent X-rays emitted by a small volume (voxel) of an irradiated sample can be employed for obtaining tomographic images. A finely collimated beam from an X-ray tube has been employed, and the X-rays emitted at 90° by the elements in a voxel have been analyzed with a highly collimated detector. Moving the sample in the x−y plane, the elemental distribution can be determined in the slice of the object crossed by the radiation. The intensities of the measured peaks have been corrected for absorption effects in the object. The distribution of iodine in test objects has been determined.

Speg: an energy loss spectrometer for ganil

Abstract: Since July 1985, an energy loss spectrometer (SPEG) is under operation at the National Heavy Ion Laboratory (GANIL), at Caen (France). It has been designed to allow the study of quantum states populated in reactions induced by nuclei accelerated at energies up to 100A MeV. The spectrometer has been designed by P. Birien. The optical properties and the main magnetic features have been calculated by Birien and Valero. A detailed reported of their study is given in ref. [1]. In the first part of the present paper, after recalling the specifications of the spectrometer, we shall give an overall description of the main characteristics, together with indications about the various shimming procedures which have been used to achieve the desired resolution (sections 1–4). In the second part, we shall describe various accessories and the different kinds of detectors which are used during experiments, with several illustrations of experimental results (sections 5 and 6).

Neutron yields from thick Be targets bombarded with deuterons or protons

Abstract: Thick target yields of the neutron producing reactions d + Be and p + Be were investigated for deuteron energies between 9.4 and 13.6 MeV and proton energies between 17.2 and 22.0 MeV. The spectral neutron yield per unit beam charge on the Be target was determined by using time-of-flight techniques and the (n, γ) discrimination properties of an NE213 scintillation detector. In addition, the angular dependence of the spectral neutron yield was measured between 0° and 150° for a deuteron energy of 13.54 MeV and a proton energy of 19.08 MeV. The spectral neutron yield at 0° as a function of the incident particle energy, which is approximated by a simple function obtained from a least-squares fit, is compared with previous data. Fluence-averaged mean neutron energies of the neutron spectra are given as a function of the charged particle energy and neutron emission angle for various neutron energy thresholds. The present neutron spectra differ in the low-energy portion from those published by other authors. Deviations of up to 20% are found in the total neutron yield per unit beam charge at 0°.

A method for picosecond lifetime measurements for neutron-rich nuclei (2) Timing study with scintillation counters

Abstract: A delayed coincidence timing system consisting of a thin NE111A plastic scintillator and a BaF2 crystal coupled to XP2020 and XP2020Q photomultipliers, respectively, and operated in a fast dynode mode was developed Its performance was optimized over a dynamic range from 03 to 16 MeV for γ-rays For this range a walk of ±22 ps and a time resolution of 96 ps (FWHM) were measured for β-rays with a mean energy of about 600 keV coincident with 137 MeV γ-rays from a 24Na source The high sensitivity of this detector system has been demonstrated by using a β-γ delayed coincidence mode to measure known picosecond lifetimes of nuclear states in neutron-rich nuclei at the TRISTAN fission product mass separator Mean-lives as short as 51±3 ps have been measured by the slope method and 10±7 ps by the centroid shift method Lifetimes of several nuclear states in 142Ba, 142,144Ce and 146Nd measured by the slope and by the centroid shift method are reported

Heavy ion separation with a gas-filled magnetic spectrograph

Abstract: Heavy ions passing through a magnetic field region filled with gas experience atomic charge-changing collisions and follow trajectories approximately determined by the mean charge state in the gas. The properties of a gas-filled Enge magnetic spectrograph are studied in detail by measuring focal-plane position spectra of fast heavy ions and their evolution as a function of gas pressure. The method allows physical separation of pairs of isobaric ions in the focal plane. Applications in accelerator mass spectrometry 0experiments are described. At intermediate low pressures, single atomic charge-changing processes can be identified. A Monte Carlo simulation program of the ion transport through the gas-filled magnet is developed and reproduces closely the experimental behavior.

Fabrication of detectors and transistors on high-resistivity silicon

Abstract: A new process for the fabrication of silicon p-i-n diode radiation detectors is described. The utilization of backside gettering in the fabrication process results in the actual physical removal of detrimental impurities from critical device regions. This reduces the sensitivity of detector properties to processing variables while yielding low diode reverse-leakage currents. In addition, gettering permits the use of processing temperatures compatible with integrated-circuit fabrication. P-channel MOSFETs and silicon p-i-n diodes have been fabricated simultaneously on 10 k..cap omega../center dot/cm silicon using conventional integrated-circuit processing techniques. 25 refs., 5 figs.

Vlsi structures for track finding

Abstract: We discuss the architecture of a device based on the concept of associative memory designed to solve the track finding problem, typical of high energy physics experiments, in a time span of a few microseconds even for very high multiplicity events. This “machine” is implemented as a large array of custom VLSI chips. All the chips are equal and each of them stores a number of “patterns”. All the patterns in all the chips are compared in parallel to the data coming from the detector while the detector is being read out.

Track finding with neural networks

Abstract: A neural network algorithm for finding tracks in high energy physics experiments is presented. The performance of the algorithm is explored on modest size samples with encouraging results. It is inherently parallel and thus suitable for execution on a conventional SIMD architecture. More important, it naturally lends itself to direct implementations in custom made hardware, which would permit real time operations and hence facilitate fast triggers. Both VLSI and optical technology implementations are briefly discussed.

Fast polarization changes in ferroelectrics and their application in accelerators

Abstract: Several mechanisms are described which change the polarization in ferroelectric material. Provided the change is too rapid for the related surface charges to be screened or neutralized, the high charge density can lead to strong electric fields. The fields may possibly be used for emission and acceleration of electrons. First results of experiments are reported in which fast spontaneous polarization changes by reversal or phase transition have been demonstrated. Electrons of 25 keV energy have been observed, emitted from triglycine sulfate (TGS) crystals during phase transition while being slowly heated across the Curie temperature. With fast polarization changes, electron beams of even higher energy and density are expected.

A study of ionization electrons drifting over large distances in liquid argon

Abstract: We have measured the lifetime of electrons drifting in liquid argon using a gridded ionization chamber with a drift gap of 153 mm. The electrons were produced by the passage of cosmic rays through the chamber. We find a best fit for the electron lifetime τ at 5 V/cm of 9.2 ms (with 13.2 >τ > 7.1 ms at 95% CL). This value is consistent with all the data below 15 V/cm. This corresponds to an impurity concentration ρ = 0.03 ppb oxygen equivalent. The attenuation length calculated from the above lifetime is 18 m for a field of 1 kV/cm. The liquid remained in the chamber for 14 weeks with no recirculation system and suffered no noticeable deterioration with respect to the lifetime or the pulse height. The low field mobility μ0 has been measured for fields up to 150 V/cm and we find a value of 545 ± 4 cm2 V−1 s−1 which is consistent with measurements by other authors. The electron yield as a function of the electric field strength has been measured down to 3 V/cm. The results seem to indicate the need for a modification of the geminate recombination theory.

MARS: A momentum achromat recoil spectrometer

Abstract: We are building a Momentum Achromat Recoil Spectrometer (MARS) for use with the new K500 superconducting cyclotron at Texas A&M University. MARS uses a unique optical design utilizing two dispersive planes to combine a momentum achromat with a recoil mass spectrometer. This configuration makes MARS applicable to a broad range of nuclear reaction studies utilizing inverse kinematics. It also leads to a system that is well matched to the range of secondary particle energies that will be produced in reactions with K500 beams. MARS will have a typical mass resolution of δM M ≅ 1 300 , with an energy acceptance of ± 9% ΔE E and a geometric solid angle of up to 9 msr. A beam swinger system will allow reaction products in the angular range 0° to 30° to be studied. MARS will be used to study both the excited states and decay properties of very proton- and neutron-rich nuclei. MARS will also be used to provide a reaction mechanism filter to assist investigations of the dynamics of heavy ion collisions and to produce secondary radioactive beams for reaction and spectroscopic studies of particular interest for nuclear astrophysics. We describe the design of MARS, together with a brief discussion of the scientific program planned for it.

Upper limits in experiments with background or measurement errors

Abstract: It is shown that upper limits can be calculated in classical statistics for measurements contaminated by background or distorted owing to the finite resolution of the apparatus. The a posteriori probability distributions, as fixed by the experimental results, are used for the background and the measurement errors, respectively. Contrary to the Bayesian approach, assumptions on prior distributions of unknown parameters are avoided.

Measurement of the activity of radioactive samples in Marinelli beakers

Abstract: Problems arising in germanium detector measurements of radioactive samples in Marinelli beakers (re-entrant beakers) are discussed. Particular attention is paid to corrections for self-attenuation and coincidence-summing effects.

A “TDPAC-camera”

Abstract: An advanced spectrometer for time differential perturbed γ-γ-angular correlation measurements is presented. It consists of six detectors with BaF 2 -scintillators and is designed to reduce the data collection time as much as possible. Experimental results with “illumination” times of the order of 100 s are presented. This “camera” storing 30 coincidence subgroups simultaneously is compared with a conventional four-detector system with eight subgroups.

Scattered X-ray beam nondestructive testing

Abstract: X-ray scatter interactions generally dominate the linear attenuation coefficient at the photon energies typical of medical and industrial radiography. Specific advantages of X-ray scatter imaging, including a flexible choice of measurement geometry, direct 3D-imaging capability (tomography) and improved information for material characterization, are illustrated with results from Compton and coherent scatter devices. Applications of a Compton backscatter scanner (ComScan) in the aerospace industry and coherent scatter imaging in security screening are briefly considered.

Investigation of the performance of CsI(Tl) for charged particle identification by pulse-shape analysis

Abstract: We have studied the possibility of particle discrimination by pulse-shape analysis in CsI(Tl) scintillator. In order to investigate the limitations of that process we have analysed the time response of the scintillator for different ions at various energies. It is found that for energies lower than 200 MeV the identification is limited to charges Z < 4. This work also gives an understanding of these limitations.

A resonance ionization mass spectrometer as an analytical instrument for trace analysis

Abstract: An instrument for trace analysis based on resonance ionization mass spectroscopy has been developed. It consists of three tunable pulsed dye lasers pumped by one or two copper vapour lasers and a time-of-flight mass spectrometer. The properties of the laser system, the quartz fibres that are used for the transport of the laser beams, the time-of-flight spectrometer, the atomic beam sources and the data acquisition and laser control electronics are described in detail. A mass resolution of 2700 was obtained with gadolinium evaporated from an oven. The detection efficiency after evaporation from a filament was measured to be a few times 10 −6 for plutonium as well as for technetium at a very low background. Measurements of the isotopic ratios of plutonium samples yielded a satisfactory agreement with mass spectrometric data. Examples are given for some spectroscopic applications of the instrument such as the investigation of autoionizing states.

Ions penetrating through ion-optical systems and matter — non-liouvillian phase-space modelling

Abstract: Desirable and inevitable phase-space modifications for heavy ions propagating through matter and ion-optical systems are described. Some aspects of the interaction of energetic ions with matter incorporated in ion-optical devices and the use of profiled targets at relativistic energies are discussed. A variety of measured and calculated results is presented which demonstrates interesting applications, including the interaction of heavy ions under channeling conditions or with free electron targets, and the separation and implantation of radioactive beams. As examples, some of the new GSI projects are briefly presented which illustrate the growing importance of non-Liouvillian ion-optics.

Properties of lead iodide semiconductor radiation detectors

Abstract: Semiconductor radiation detectors have been fabricated from melt grown crystals of lead iodide (PbI2) and the performance of these detectors as room temperature X-ray spectrometers has been measured. These detectors exhibit good energy resolution (915 eV FWHM for the 5.9 keV peak of 55Fe at 20°C). Preliminary results indicate they are more stable than HgI2 detectors and capable of operating at temperatures over 100°C.

A microstrip gas avalanche chamber with two-dimensional readout

Abstract: A microstrip gas avalanche chamber with a 200 μm anode pitch has been built and successfully tested in our laboratory. A gas gain of 104 and an energy resolution of 18% (FWHM) at 6 keV have been measured using a gas mixture of argon-CO2 at atmospheric pressure. A preliminary measurement of the positional sensitivity indicates that a spatial resolution of 50 μm can be obtained.

The EAS-TOP array at E0 = 1014 − 10 16eV: Stability and resolutions

Abstract: The characteristics of the EAS-TOP extensive air shower array as a detector of very high energy cosmic rays ( E 0 ≥ 10 14 eV) for astrophysical studies are discussed. The array is located on top of the underground Gran Sasso Laboratory in central Italy; a subarray (11 modules of the em detector) has been operating since the end of 1987. From such data the stability of the detector, the timing resolution, the accuracies in the determination of the arrival directions ( δθ = 1.2° at E 0 ∼ 200 TeV in the present configuration) and in the reconstruction of the lateral electron distribution and of the shower size are derived.

Double-sided readout silicon strip detectors for the aleph minivertex

Abstract: Large double-sided readout silicon strip detectors have been fabricated for the Aleph minivertex. We have tested them with a β-source and have observed charge collection and capacitive charge division both on the junction side and on the ohmic side. A charge correlation has been observed between the two faces and can be used to reduce ambiguities in the case of two particles crossing the same wafer.

Advantages and limitations of cadmium selenide room temperature gamma ray detectors

Abstract: State-of-the-art technology of spectrometer grade CdSe gamma ray detectors is presented in this paper. It is shown that high resistivity CdSe single crystals can be reproducibly grown by the temperature gradient solution zoning technique. Zone refining of Se used for crystal growth is found to reduce efficiently the concentration of Cu trace impurities, which are mainly responsible for the electron trapping. The charge carrier transport parameters are studied in detail, and the perspective of further improvement of the energy resolution of CdSe detectors are discussed. A comparison with the performance of CdTe and HgI 2 nuclear radiation detectors is also given.