Nuclear Pulse Research Articles

Vibrational echoes: A new approach to condensed-matter vibrational spectroscopy

This review describes the first ultrafast infrared vibrational echo experiments, which are used to examine liquids, glasses and proteins. Like the nuclear magnetic resonance (NMR) spin echo and other NMR pulse sequences, the vibrational echo can extract dynamical and spectroscopic information that cannot be obtained from a vibrational absorption spectrum. The vibrational echo measures the homogeneous vibrationallinewidtheven if the absorption line is massively inhomogeneously broadened. When combined with pump-probe (transient absorption) experiments, the homogeneous pure dephasing (energy level fluctuations) is obtained. Conducting these experiments as a function of temperature provides information on dynamics and intermolecular interactions. The nature of the method and the experimental procedures are outlined. Experimental results are presented for the metal carbonyl solutes, W(CO) and Rh(CO) acac, in several glassy and liquid solvents. The dynamics of the CO ligand bound at the active site of the protein myoglobin are examined and compared with those in several myoglobin mutants. The results provide insights into protein dynamics and how protein structural fluctuations are communicated to a ligand bound at the active site. In addition,two new vibrational echo methods are reviewed. One method involves using multilevel vibrational coherences, which gives rise to vibrational echo beats, to measure vibrational anharmonicities and excited-state dephasing. The other method, in which a vibrational echo spectrum is taken, is demonstrated to be capable of the suppression of unwanted background that dominates the normal vibrational absorption spectrum.

A novel method for determining pulse counting circuitry dead time using the Nuclear Weapons Inspection System

A novel method for measuring dead time in nuclear pulse processing circuitry has been developed using the autocorrelation measurement capability of the Nuclear Weapons Inspection System (NWIS). Initially developed for active neutron interrogation of nuclear weapons and other fissile assemblies, NWIS employs a custom gallium arsenide application specific integrated circuit and a new signature analysis software package to simultaneously acquire and display the autocorrelation and cross-correlation spectra of up to five detector/electronics systems. The system operates at clock frequencies up to 1 GHz, permitting the collection of timing pulses in bins as narrow as 1 ns. In normal operation NWIS uses well characterized detectors and constant fraction discriminators, but it may also be configured to accept pulses from any circuit and to use the autocorrelation spectrum to accurately determine dead-time. Unlike traditional dead-time assessment techniques that typically require multiple sources and an assumed dead-time model, NWIS provides single measurement assessment of circuit dead time and does not require an assumed dead-time model or a calibrated high count-rate source.

How to derive the optimum filter in presence of arbitrary noises, time-domain constraints, and shaped input signals: A new method

The optimum filter constrained to finite width and other key features in the time domain, suitable for nuclear pulse spectrometry as well as for many other applications, is derived by means of a new method. A pattern of typical noise sources, including series and parallel Lorentzian packets, white and f n noises with n positive or negative integer are considered. Such a pattern permits also to treat the N-order fitting to an arbitrary spectral density of noise. The new method features a fast-convergence capability, and, in contrast to other methods, accepts either δ-like or arbitrarily shaped input signals. Flat tops, finite width, and/or zero-area constraints are met by a ‘shift, sum and weigh’ procedure, which uses as core function the non-flat-topped, non-time-limited minimum-noise weight function. This latter is determined, for all the specified input noises, in closed form. The optimum filter for the measurement of a DC signal is also provided as a by product of the method. A computer program, running in seconds in the Matlab environment, is available, and this demonstrates the effectiveness of the method.

5614826 Methods for prepulsing in nuclear magnetic resonance pulse sequencing

5614826 Methods for prepulsing in nuclear magnetic resonance pulse sequencing

5570019 Method for magnetic resonance spectroscopic imaging with multiple spin-echoes

A nuclear magnetic resonance pulse sequence to provide spectral encoding so that the resulting series of spin-echoes each include both spatial and spectral information for spectroscopic imaging. Atoms within the object are excited and may then be spatially encoded, as by a phase encoding gradient. A series of refocusing pulses is then applied, inducing a respective series of spin-echoes. Spectral information is directly encoded in the spin-echo signals. The multiple spin-echoes may be used for sampling different points of k-space, and/or for increasing the signal-to-noise ratio by averaging. In an alternative embodiment, the present invention produces compound weighted spectroscopic images by selecting the period between refocussing pulses according to the coupling constant of a group contained in the compound; thereby, the signal of the selected compounds modulate with a known frequency different for compounds with different coupling constants.

5596274 Determining bound and unbound fluid volumes using nuclear magnetic resonance pulse sequences

A borehole logging tool comprises a Nuclear Magnetic Resonance (NMR) tool and is pulsed according to a selected sequence. An interpretation is performed which involves a singular value decomposition and compression of data obtained by the tool. The compressed data is then subjected to a nonegative, linear least square fit to obtain a distribution function. The distribution function is used in determining bound fluid volume, unbound fluid volume, total porosity, spin-lattice relaxation time, spin-spin relaxation time, and mean relaxation time using only the pulse NMR tool.

Minimum-noise filter for baseline estimation in radiation detection systems

Abstract Baseline height estimation in nuclear pulse spectrometry can be optimized by using a proper weight-function. The minimum-noise function for baseline estimation in presence of series and parallel white noises, constrained to a finite duration, is theoretically derived in this paper. Baseline restoration can be subsequently performed by subtracting the minimum-noise baseline estimation to the output signal pulse. The overall signal-to-noise performance obtained using the minimum-noise baseline estimation for pulse height correction is compared to that obtained with routinely used Base Line Restorers. An improvement of up to 16% in the Equivalent Noise Charge, with respect to classic baseline restoration, can be theoretically achieved in case of a simple triangular pulse shaping. The minimum-noise baseline weight-function can be synthesized in practice by means of a digital processing unit.

The “switch off” baseline restorer for the 120 channel silicon detector system for EXAFS at NSLS

A new circuit for high precision baseline restoration in multichannel nuclear pulse spectrometers is proposed. It has been designed for the 120 channel silicon detector system for EXAFS research at NSLS (National Synchrotron Light Source), Brookhaven National Laboratory, USA [L.R. Furenlid et al., Nucl. Instr. and Meth. A 319 (1992) 408], and is based on a current “switch off” mechanism self-triggered on the input signal pulses. No auxiliary gate signals are required, and, consequently, the network is particularly compact. As opposed to Robinson restorers, this restorer is not affected by undershoot, and it does not involve the significant complexity of gated solutions. The restorer has been tested with a 7 pole quasi-Gaussian filter permitting very high resolution X-ray spectroscopy (160 eV FWHM on pulser peak). The signal-to-noise degradation due to the restorer is about 7%. The BLR response to 18 μs large trapezoidal input pulses shows an undershoot which is about 0.3%. Baseline is correctly recovered for repetition rates of up to 5 × 104 trapezoidal signals/s, which corresponds to a duty cycle of 60%. The BLR has been layed out in SMT on a 0.8 in. × 0.8 in. area, which is suitable for multichannel applications where the circuit has to be largely used as the last stage of each individual unipolar shaping filter.

A low-power, CMOS peak detect and hold circuit for nuclear pulse spectroscopy

A low-power CMOS peak detecting track and hold circuit optimized for nuclear pulse spectroscopy is presented. The circuit topology eliminates the need for a rectifying diode, reducing the effect of charge injection into the hold capacitor, incorporates a linear gate at the input to prevent pulse pileup, and uses dynamic bias control that minimizes both pedestal and droop. Both positive-going and negative-going pulses are accommodated using a complementary set of track and hold circuits. Full characterization of the design fabricated in 1.2 /spl mu/m CMOS including dynamic range, integral nonlinearity, droop rate, pedestal, and power measurements is presented. The circuit operates with only 250 /spl mu/w for input pulses with 7 /spl mu/s peaking time. Power consumption was increased to 750 /spl mu/w for driving off-chip and test system capacitances. Analysis and design approaches for optimization of operational characteristics are also discussed. >

The impact of ADC specifications on the performance of digital (DL)/sup N/ shaping

Multiple Delay Line Shaping (DL)/sup N/ of nuclear pulses is performed in practice by introducing an Analog to Digital converter ADC and a digital filter within the noise-shaping circuit. The critical component of the processor is the ADC; since its non-idealities can modify the entire filter Weighting Function WF, rather than affect only the final pulse classification in a histogram. In this paper we discuss the specifications that a candidate ADC should fulfill in order to be suitable for (DL)/sup N/ shaping and, conversely, the specifications of the digital portion of (DL)/sup N/ shaper suitable to reduce the impact of ADC nonidealities: say, linearity, conversion time and number of bits, on the obtained spectra. For what concerns the quantization error, new WF allow the use of ADC with up to three bits less than those necessary in previous implementations. This is obtained with little penalty (a factor four) on the required ADC speed. The effects of the lack of synchronization between the incoming pulse and the sampling comb are shown to be useful, since they result in a sort of sliding scale effect, which allows to reduce the effects of ADC differential non linearity by an estimated factor of at least five. >

5389877 Nuclear magnetic resonance pulse sequences for determining bound fluid volume

An NMR pulse sequence for use in a borehole logging tool includes a series of CPMG pulses according to: T r - 90° ±x - (t cp - 180° y - t cp - echo j ) where j is the index of CPMG echoes gathered, T r is wait time, t cp is the Carr-Purcell spacing. This pulse sequence is used to determine Bound Fluid Volume (BFV) which is subtracted from total porosity to yield Unbound Fluid Volume (UFV) of a formation surrounding the borehole. Measuring the BVF, the amount of rapidly relaxing fluid (less than 50 ms), is more efficient than measuring UFV (up to 2 secs), and is insensitive to motion of the logging tool.

5023551 Nuclear magnetic resonance pulse sequences for use with borehole logging tools

An NMR pulse sequence for use in the borehole environment is provided which combines a modified fast inversion recovery (FIR) pulse sequence with a series of more than ten, and typically hundreds, of CPMG pulses according to [W.sub.i -180-τ.sub.i -90-(t.sub.cp -180-t.sub.cp -echo).sub.j ].sub.i where j is the index of the CPMG echoes gathered, i is the index of the wait times in the pulse sequence, Wi are the wait times, i are the recovery times before the CPMG pulses, and tcp is the Carr-Purcell spacing. Measurements are made of the signals induced in the formation as a result of the magnetic fields. Determinations of Mo and/or T1 are then made from the measurements according to relationships which relate Mpo, T1 and T2 to the signal magnitude. Other relationships which provide stretched exponentials or multiple exponentials can also be used. From the Mo and/or T1 determinations, formation parameters such as porosity and permeability may be derived according to equations known in the art. In obtaining the most accurate determinations of formation parameters in the least amount of time, the various pulse sequence parameters (I, J, Wi, and τi) are optimized prior to logging. Additional accuracy is obtained by integrating a gated portion of the echoes rather than by measuring amplitude, and by utilizing a phase alternated CPMG sequence in repetitive measurements in order to eliminate baseline shift error.

Nuclear Electromagnetic Interaction and Coupling Analysis in Buried Cables

A model is presented for numerically calculating the transient currents and voltages Induced in the buried cables by an impinging Nuclear Electromagnetic Pulse (NEMP). The conventional transmission...

CW and pulsed mode transfer impedance measurements in coaxial cables

Transfer impedance measurements have been performed both in frequency and time domains. CW measurements were performed using a matched triaxial system, a quintaxial system, and stripline injection. For nuclear electromagnetic pulse (NEMP) purposes, measurements up to a maximum frequency of 200 MHz are sufficient. For this frequency range the matched triaxial system was found to be the most convenient. Amplitude and phase measurements in different cables are discussed. Direct and indirect injection was employed in the time domain measurements. A delayed coupling is demonstrated when the diffusion term is dominant in the transfer impedance. In a magnetically shielded cable, hysteresis and saturation effects have been observed. At 5 kA the transfer impedance of the magnetically shielded cable increased by an order of magnitude due to saturation. >

In Situ NMR Study of Dislocation Jump Distance During Creep of Pure Sodium Chloride Single Crystals

Nuclear magnetic resonance pulse techniques were used in situ during creep of single crystals of NaCl to evaluate the contribution of mobile dislocations to spin relaxation. 23Na spin‐lattice relaxation rates were measured in the rotating frame (T1p) during compression creep of single crystals of NaCl along the [110] direction at 473 K at an applied stress of 20 MPa. The relaxation rates were evaluated from the spin‐echo height following π/2, locking, and 64° pulse sequence. We describe here the experimental setup and methodology in evaluating the dislocation jump distance from NMR‐creep experiments. The height of the free induction decay decreased as soon as the load was applied followed by a gradual increase until the steady state was reached, at which point a saturation value was observed corresponding to the constant steady‐state creep rate. The mean jump distance of the mobile dislocations, evaluated from the ratio of the signal heights without deformation and during creep, decreased with time/strain, reaching a constant value during the steady‐state creep regime. The results are compared with the dislocation‐dislocation spacing, subgrain size, as well as the jump distance predicted from creep models.

A high precision peak sensing circuit for measuring the integral linearity of nuclear pulse amplifiers

Abstract This paper describes a new instrument that very accurately transforms the peak height of repetitive waveforms into a dc voltage. Used in conjunction with a precision pulse generator, this instrument provides an improved method for establishing the integral linearity of pulse amplifiers. A measured overall accuracy of 0.005% for pulses with a nominal 1.0 μs peaking time has been achieved. This method is particularly useful with very fast amplifiers in the 5–100 ns range. The results of linearity measurements on several typical amplifiers are presented.

Theoretical considerations for optimal positioning of peaking capacitor arms about a Marx generator parallel to a ground plane

The authors develop theoretical considerations useful in determining the electromagnetically optimal positions for a specified number of peaking-capacitor arms about a Marx generator parallel to a ground plane. It is shown that this can be determined by a conformal transformation. A parameter is defined and computed that is indicative of the effectiveness of the peaking-capacitor arms in shielding the Marx from the ground plane. The results of interest in connection with determining the hardness of systems to a nuclear electromagnetic pulse environment. >

4710718 Spatially selective nuclear magnetic resonance pulse sequences

4710718 Spatially selective nuclear magnetic resonance pulse sequences

Automatic microscope focusing system for solid state nuclear track detectors

We describe and evaluate the performance of our automatic focusing system developed for use with an image analysis system for SSNTD readout and analysis. The automatic focusing system is based upon standard nuclear pulse counting instrumentation, and maximization of the high spatial frequency content of the image signal.

Extrapolation of measured power system response data to high-altitude EMP excitation

Pulse injection measurements performed on an electrical generation plant in Sweden are reported. It is shown how these data may be used to calculate the high-altitude nuclear electromagnetic pulse (HEMP) response of the system when it is connected to the powergrid. It is found that the HEMP-induced current stages in overhead transmission lines are significantly attenuated as they propagate from the switch yard to the generator output terminals.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>