Fast Transient Recorder Research Articles

Digital liquid-scintillation counting and effective pulse-shape discrimination with artificial neural networks

Abstract A typical problem in low-level liquid scintillation (LS) counting is the identification of α particles in the presence of a high background of β and γ particles. Especially the occurrence of β-β and β-γ pile-ups may prevent the unambiguous identification of an α signal by commonly used analog electronics. In this case, pulse-shape discrimination (PSD) and pile-up rejection (PUR) units show an insufficient performance. This problem was also observed in own earlier experiments on the chemical behaviour of transactinide elements using the liquid-liquid extraction system SISAK in combination with LS counting. α-particle signals from the decay of the transactinides could not be unambiguously assigned. However, the availability of instruments for the digital recording of LS pulses changes the situation and provides possibilities for new approaches in the treatment of LS pulse shapes. In a SISAK experiment performed at PSI, Villigen, a fast transient recorder, a PC card with oscilloscope characteristics and a sampling rate of 1 giga samples s−1 (1 ns per point), was used for the first time to record LS signals. It turned out, that the recorded signals were predominantly α, β-β and β-γ pile up, and fission events. This paper describes the subsequent development and use of artificial neural networks (ANN) based on the method of “back-propagation of errors” to automatically distinguish between different pulse shapes. Such networks can “learn” pulse shapes and classify hitherto unknown pulses correctly after a learning period. The results show that ANN in combination with fast digital recording of pulse shapes can be a powerful tool in LS spectrometry even at high background count rates.

Evaluation of Switching Surge Risks on the Low-Voltage Auxiliary System of Frequent Start-Stop Plant

In this paper, the switching surge risks on the low-voltage auxiliary system of the frequent start-stop plant are evaluated based on switching surge simulations, field measurements, and the investigation of a heavy damage case. First of all, modeling and switching surge voltage simulations by the Alternative Transient Program (ATP) of the Electromagnetic Transient Program for an actual plant are presented, where the switching surge voltages produced by the operation of load switches and source breakers of the low-voltage auxiliary system are simulated with respect to switch-close and switch-open operations with different switching modes, and different power-supply sources are considered. Then, the field measurement of switching surge voltages for a load circuit of an auxiliary system is implemented by the fast transient recorder; therefore, the ATP simulation correctness is verified. Finally, the affections of the switching surge voltages on insulation failure of equipment and the extended risks are evaluated by the comparisons between the results of simulation and measurement and the recommendations of the IEEE standard, and the investigation result of an actual heavy damage case. With the conduction of the evaluation, the enhanced measures for switching surge protection are proposed.

Fast digitizing techniques applied to scintillation detectors

A 200 MHz 12-bit fast transient recorder card has been used for the digitization of pulses from photomultipliers coupled to organic scintillation detectors. Two modes of operation have been developed at ENEA-Frascati: a) continuous acquisition up to a maximum duration of ∼ 1.3 s corresponding to the full on-board memory (256 MSamples) of the card: in this mode, all scintillation events are recorded; b) non-continuous acquisition in which digitization is triggered by those scintillaton events whose amplitude is above a threshold value: the digitizing interval after each trigger can be set according to the typical decay time of the scintillation events; longer acquisition durations (>1.3 s) can be reached, although with dead time (needed for data storage) which depends on the incoming event rate. Several important features are provided by this novel digital approach: high count rate operation, pulse shape analysis, post-experiment data re-processing, pile-up identification and treatment. In particular, NE213 scintillators have been successfully used with this system for measurements in mixed neutron (n) and gamma ( γ ) radiation fields from fusion plasmas: separation between γ and neutron events is made by means of a dedicated software comparing the pulse charge integrated in two different time intervals and simultaneous neutron and γ pulse height spectra can be recorded at total count rates in the MHz range. It has been demonstrated that, for scintillation detection applications, 12-bit fast transient recorder cards offer improved performance with respect to analogue hardware; other radiation detectors where pulse identification or high count rate is required might also benefit from such digitizing techniques.

ITER relevant developments in neutron diagnostics during the JET Trace Tritium campaign

During the JET Trace Tritium campaign a few new neutron diagnostic systems were deployed under different plasma scenarios to provide information on the total neutron emission and its spatial and energy distribution. The 14MeV neutron yield was measured with a chemical vapour deposited (CVD) diamond detector. Comparison with the JET 14MeV monitors (Si diodes) illustrates the good performance of the CVD device. Key information on the tritium transport and the behaviour of fast particles in the plasma was obtained from the spatially and temporally measurements of neutron emission from the upgraded neutron profile cameras which also provide an independent measure of the total neutron yield. Spatial asymmetries in the neutron emission were observed which is evidence for the influence of fast particles on the plasma. With regard to the energy distribution of the neutron emission, a spectrometer based on a liquid organic scintillator with n-γ pulse shape discrimination (PSD) features was installed. The results demonstrate that such system can operate in real fusion experiments as compact broadband neutron (1.5 MeV < E n <20MeV) spectrometer with high energy resolution. A fast transient recorder has been successfully applied for digital pulse shape discrimination (DPSD) of neutron and gamma events acquired with an organic scintillator at high count rate operation (MHz range). The experience gained at JET indicates that these neutron measurement systems are suitable for large fusion devices such as JET-EP and ITER where fusion neutron diagnostics will play an increasingly important role.

Damage evolution and acoustic emission of wood at tension perpendicular to fiber

This paper presents the tracing of damage evolution in spruce loaded in tension perpendicular to fiber direction by means of acoustic emission (AE) analysis. The 2 dimensional burst source location in cross-sectional slabs of boards utilized full wave form recording of AE signals monitored by six simultaneously triggered multiple resonant longitudinal (p-)wave sensors and fast transient recorder PC cards. The location algorithm was based on the minimization of the run time differences between experimental and theoretical signal travel times. The latter accounted for the global polar material anisotropy leading to considerably variable off-axis wave velocities along the wave propagation paths, assumed to be straight; the employed orthotropic on- and off-axis p-wave velocities were obtained experimentally from ultrasound pulse measurements. Below 50% of the ultimate load generally very few burst events were recorded. At further increased load, burst agglomerations were predominantly associated to local areas of elevated stresses here resulting from the specific configuration of anisotropy and loading. For all specimens a distinct burst localisation was obtained in the range of 80 to 90% of the ultimate load, coinciding well with the theoretically highest stressed areas of the fracture plane at brittle failure. The correlation of AE event rates with global strain allowed a tracing of the damage evolution especially when events in confined areas are regarded. Distinct damage localisation was then denoted by a clear AE rate increase, however, not accompanied by a global stiffness change. The investigations revealed that failure of spruce - and most probably of similar softwoods - in tension perpendicular to grain, although being very brittle from a macroscopic perspective, is preceded by progressive and localizing damage evolution on the micro-level which can be traced accurately in space and time by acoustic emission burst source analysis.

On-line sampling and intact mass analysis of nanometer-size aerosols via time-of-flight high-mass spectrometry

A device capable of continuously sampling and mass analyzing aerosols in the 1–10 nm diameter size range (masses up to 1 MDa) at part per billion (ppb, 10 10 cm −3) concentrations is described. Small aliquots of an aerosol flowing at atmospheric pressure are sampled into a time-of-flight mass spectrometer (TOFMS) via a pulsed molecular beam valve at 10–50 Hz. The aerosol molecular beam is singly and nondestructively ionized with light from an ultraviolet excimer laser and accelerated across a 20 kV electrostatic field. Ionized particle packets are separated in mass during a free flight and re-accelerated across an additional 30 kV into a stainless steel conversion dynode plate. Signals from ejected electrons and negative ion fragments resulting from the particle-dynode collisions are amplified in a dual microchannel plate detector, digitized, and stored in a fast transient recorder. Sampling of He flow streams bearing benzene, fullerenes, as well as Na, Mg, and CsI particles (nanocrystals) is demonstrated. In addition, the gas-phase reaction between a pre-formed Na nanocrystal and alkane thiols is monitored in real-time.

Analog integrating detection technique for pulsed muon beams

A new analog integrating detection technique for time resolvedμSR measurements at high intensity pulsed muon beams has been developed. Data acquisition is realized by digitizing the integrated anode signal for eachμ burst by means of a fast transient recorder. Details of setup and performance are described.

Time-resolved measurements in spark source mass spectrometry. II. Detection of the ion current by a fast transient recorder

Time-resolved measurements of the ion current of any particular ion, produced in a single discharge, were performed using the electrical detection system of a Jeol SSMS and a very fast digital transient recorder. The maximal time resolution achieved was 10 ns. The arrival of the ions at the detector is discontinuous as a function of time and the absolute intensity; the shape and the duration of the individual time distributions are not reproducible from one discharge to the other. The minimal time of arrival of the atomic ions could, however, be determined rather precisely and could be satisfactorily approximated by a linear relationship with ( m/z) sol1 2 . Even for this minimal arrival time, the molecular ions were retarded with respect to the atomic ions for a given m/z. After averaging the individual time distributions, the following sequence of average appearance of the ions at the detector could be observed: multiply charged atomic, singly charged atomic, C n + molecular, Al-based molecular ions and, finally, S-based molecular ions. The width of the average time distributions was also found to be dependent on the kind of ion studied. It increased from a few hundred ns for the multiply charged ions to some μs in the case of S-based molecular ions.

Laser-induced thermal desorption of CO from clean polycrystalline copper: Time-of-flight and surface diiffusion measurements

Pulsed laser-induced thermal desorption was utilized as a technique to study the dynamics of desorption and surface diffusion of CO on clean polycrystalline copper at 85K in UHV. Single pulses of a focused KrF excimer laser (248nm, 15ns FWHM) were used to desorb the CO molecules, which were subsequently detected by a quadrupole mass spectrometer (effective time constant =4.4μs). The time-of-flight (TOF) plus distribution was recorded using a fast transient recorder (Biomation 610B/8100). The surface diffusion measurements were made using a versatile technique (ref.1) recently developed in our laboratory, which can be used on any system where the adsorbate can be desorbed thermally. In this technique, a small area on the surface is initially depleted of adsorbate with a laser pulse. Measurements of the time dependence of the growth of the integrated desorption flux from subsequent pulses yield a diffusion coefficient. The TOF desorption spectra could be fit to Boltzmann distributions, and effective translational temperatures of the desorbed CO molecules were obtained from these fits. The desorption process was studied as a function of laser power density, and a desorption threshold corresponding to an absorbed power density of 10MW cm −2 was observed. Near threshold, both the desorption flux and velocity distribution varied rapidly with laser power density. At power densities much larger than the threshold (30–75MW cm −2), the peak desorption flux varied approximately linearly with power density. In the range of power densities studied, the most probable velocity of the CO molecules varied between 350 and 500M s −1, corresponding to Boltzmann temperatures between 200 and 400K. The applicability of the classical desorption rate equation for laser induced desorption will be discussed. A preliminary analysis of the diffusion data for CO adsorbed on polycrystalline copper at 110K yielded a value for the diffusion coefficient D = 5 × 10 −7cm 2 s −1 (±2×10 −7cm 2 s −1) (ref.1). The temperature dependence of the diffusion coefficient is currently being investigated in detail in order to accurately determine the activation energy for the diffusion process.