Instrumental Dead Time Research Articles

Nobody's Perfect: Choice of the Buffer and the Rate of Cu2+ Ion-Peptide Interaction.

The choice of correct pH buffer is crucial in chemical studies modeling biological processes involving Cu2+ ions. Popular buffers for physiological pH are known to form Cu(II) complexes, but their impact on kinetics of Cu(II) complexation has not been considered. We performed a stopped-flow kinetic study of Cu2+ ion interactions with four popular buffers (phosphate, Tris, HEPES, and MOPS) and two buffers considered as nonbinding (MES and PIPPS). Next, we studied their effects on the rate of Cu2+ reaction with Gly-Gly-His (GGH), a tripeptide modeling physiological Cu(II) sites, which we studied previously at conditions presumably excluding the buffer interference [Kotuniak, R.; Angew. Chem., Int. Ed. 2020, 59, 11234-11239]. We observed that (i) all tested pH 7.4 buffers formed Cu(II) complexes within the stopped-flow instrument dead time; (ii) Cu(II)-peptide complexes were formed via ternary complexes with the buffers; (iii) nevertheless, Good buffers affected the observed rate of Cu(II)-GGH complex formation only slightly; (iv) Tris was a competitive inhibitor of Cu(II)-GGH complexation; while (v) phosphate was a reaction catalyst. This is particularly important as phosphate is a biological buffer.

The effects of instrumental deadtime on NICER timing products

Abstract The X-ray timing instrument as part of the neutron star interior composition explorer has the potential to examine the time-domain properties of compact objects in regimes not explored by previous timing instruments, due to its combination of high effective area and timing resolution. We consider the effects of instrumental deadtime at a range of effective countrates in a series of observations of the X-ray binary GX 339-4 to determine what effect deadtime has on photometric and Fourier frequency-domain products. We find that there are no significant inconsistencies across the functional detectors in the instrument, and that in the regimes where instrumental deadtime is a limiting factor on observations that previous approaches to dealing with deadtime, as applied to RXTE and other detectors, are still appropriate, and that performing deadtime corrections to light curves before creating Fourier products are not necessary at the count rates considered in our analysis.

High-throughput capillary electrophoresis analysis of biopharmaceuticals utilizing sequential injections.

The complexity of biotherapeutic products implies an ever-increasing list of product quality attributes that need to be monitored and characterized. In addition, the growing interest in implementing process analytical technology in biopharmaceutical production has further increased the testing burden, together with the need for rapid testing that can facilitate real-time or near-real-time decision-making. Capillary electrophoresis (CE) has made a place in biopharmaceutical analysis but is regarded as a low-throughput method, with the instrument dead time constituting more than 80% of the total time of analysis. In this study, the dead time of CE was utilized to analyse 3mAb samples in a single-CE run. This approach resulted in an up to 77% reduction in the total analysis time and increased the productivity by up to 300%, compared to traditional single CE-ultraviolet runs, without compromising resolution or relative peak areas. Additionally, good method reproducibility was observed. The compatibility of the method has been demonstrated with protein A eluate and cation exchange chromatography fractions. We, thus, propose that sequential injections can be applied for fast and robust CE analysis of biopharmaceuticals.

Determination of uranium isotopes in marine sediments and seawaters by SF ICP-MS after rapid chemical separation using TK200 resin.

This work provided a novel analytical procedure for rapid and precise uranium isotopic determination in marine sediment and seawater, using a new type of extraction resin, TK200 resin, in combination with microwave digestion (for marine sediments), Fe(OH)3 co-precipitation (for seawater), and single collector sector field-inductively coupled plasma mass spectrometry (SF ICP-MS) measurement. The removal ability of TK200 extraction chromatography for the interfering elements (IEs) Hg, Pb, Th, Pt, Tl, and the matrix rare earth elements (REEs) was carefully investigated. High decontamination factors (DFs) were obtained for IEs and REEs. Accurate quantification of uranium isotope ratios was accomplished based on a "double-cycle" ICP-MS measurement method. The analytical method was optimized and validated with isotopic standards (IRMM-187), matrix-containing certified reference marine sediments (IAEA-384, IAEA-385, and IAEA-412), and seawater reference material (IAEA-443). A stable chemical recovery of ~ 90% was obtained for both types of marine environmental samples, and the method showed great efficiency with a total analytical time of less than 6h. The proposed procedure was validated following ISO/IEC 17025 guidelines. The important factors affecting the isotope ratio results (instrument background, procedural blank, memory effects, peak tailing, mass discrimination, dead time, and hydride interferences) were considered in the estimation of combined uncertainties. This work provides an alternative way for the determination of trace uranium isotope ratios and can be applied in the emergency monitoring of nuclear accidents and marine environmental analysis.

First Measurements of Surface Nuclear Magnetic Resonance Signals Without an Oscillating Excitation Pulse – Exploiting Non‐Adiabatic Prepolarization Switch‐Off

AbstractRecently, small‐scale surface nuclear magnetic resonance (SNMR) measurements with a footprint of about a few square meters have gained interest in the soil geophysics community as they directly provide water content and pore geometry information. Here, the application of strong prepolarization (PP) fields enables the detection of water in the vadose zone. Introducing PP into SNMR shifts the reference for the decay of the SNMR signal from the excitation pulse to the end of the PP, effectively increasing the instrumental dead time, which is already too long to measure short signals associated with unsaturated fine‐grained soil. In an approach to overcome this limitation, we present the first measurements of SNMR signals from a water reservoir without an oscillating excitation pulse. Instead, we use the non‐adiabatic, that is, imperfect, switch‐off of the PP‐field as an effective excitation mechanism. We complement our field experiments with numerical simulations of the corresponding SNMR spin dynamics.

Apsu: a wireless multichannel receiver system for surface nuclear magnetic resonance groundwater investigations

Abstract. Surface nuclear magnetic resonance (surface NMR) has the potential to be an important geophysical method for groundwater investigations, but the technique suffers from a poor signal-to-noise ratio (SNR) and long measurement times. We present a new wireless, multichannel surface-NMR receiver system (called Apsu) designed to improve field deployability and minimize instrument dead time. It is a distributed wireless system consisting of a central unit and independently operated data acquisition boxes each with three channels that measure either the NMR signal or noise for reference noise cancellation. Communication between the central unit and the data acquisition boxes is done through long-distance Wi-Fi and recordings are retrieved in real time. The receiver system employs differential coils with low-noise preamplifiers and high-resolution wide dynamic-range acquisition boards. Each channel contains multistage amplifiers, short settling-time filters, and two 24 bit analog-to-digital converters in dual-gain mode sampling at 31.25 kHz. The system timing is controlled by GPS clock, and sample jitter between channels is less than 12 ns. Separated transmitter/receiver coils and continuous acquisition allow NMR signals to be measured with zero instrument dead time. In processed data, analog and digital filters cause an effective dead time of 5.8 ms including excitation current decay. Synchronization with an independently operated transmitter system is done with a current probe monitoring the NMR excitation pulses. The noise density measured in a shorted-input test is 1.8 nV Hz-1/2. We verify the accuracy of the receiver system with measurements of a magnetic dipole source and by comparing our NMR data with data obtained using an existing commercial instrument. The applicability of the system for reference noise cancellation is validated with field data.

Decoupling of excitation and receive coils in pulsed magnetic resonance using sinusoidal magnetic field modulation

In pulsed magnetic resonance, the excitation power is many orders of magnitude larger than that induced by the spin system in the receiving coil or resonator. The receiver must be protected during and immediately after the excitation pulse to allow for the energy stored in the resonator to dissipate to a safe level. The time during which the signal is not detected, the instrumental dead-time, can be shortened by using magnetically decoupled excitation and receive coils. Such coils are oriented, with respect to each other, in a way that minimizes the total magnetic flux produced by one coil in the other. We suggest that magnetically decoupled coils can be isolated to a larger degree by tuning them to separate frequencies. Spins are excited at one frequency, and the echo signal is detected at another. Sinusoidal magnetic field modulation that rapidly changes the Larmor frequency of the spins between the excitation and detection events is used to ensure the resonance conditions for both coils. In this study, the relaxation times of trityl-CD3 were measured in a field-modulated pulsed EPR experiment and compared to results obtained using a standard spin echo method. The excitation and receive coils were tuned to 245 and 256.7MHz, respectively. Using an available rapid-scan, cross-loop EPR resonator, we demonstrated an isolation improvement of approximately 20–30dB due to frequency decoupling. Theoretical analysis, numerical simulations, and proof-of-concept experiments demonstrated that substantial excitation-detection decoupling can be achieved. A pulsed L-band system, including a small volume bi-modal resonator equipped with modulation coils, was constructed to demonstrate fivefold dead-time reduction in comparison with the standard EPR experiment. This was achieved by detuning of the excitation and receive coils by 26MHz and using sinusoidal modulation at 480kHz.

Reference measurements for total mercury and methyl mercury content in marine biota samples using direct or species-specific isotope dilution inductively coupled plasma mass spectrometry

The analytical procedures for reference measurements of the total Hg and methyl mercury (MeHg) mass fractions at various concentration levels in marine biota samples, candidates for certified reference materials (oyster and clam Gafrarium tumidum), were evaluated. Two modes of application of isotope dilution inductively coupled plasma mass spectrometry method (ID ICP-MS), namely direct isotope dilution and species-specific isotope dilution analysis with the use of two different quantification mass spectrometry techniques were compared.The entire ID ICP-MS measurement procedure was described by mathematical modelling and the combined uncertainty of measurement results was estimated. All factors influencing the final results as well as isotopic equilibrium were systematically investigated. This included the procedural blank, the moisture content in the biota samples and all factors affecting the blend ratio measurements (instrumental background, spectral interferences, dead time and mass discrimination effects as well as the repeatability of measured isotopic ratios). Modelling of the entire measurement procedures and the use of appropriate certified reference materials enable to assure the traceability of obtained values to the International System of Units (SI): the mole or the kilogram.The total mass fraction of mercury in oyster and clam biota samples, after correction for moisture contents, was found to be: 21.1 (1.1) 10−9 kg kg−1 (U =5.1% relative, k=2) and 390.0 (9.4) 10−9 kg kg−1 (U=2.4% relative, k=2), respectively. For the determination of mercury being present as methyl mercury, the non-chromatographic separation on anion-exchange resin AG1-X8 of the blended samples was applied. The content of MeHg (as Hg) in oyster sample was found: 4.81 (24) 10−9kgkg−1 (U=5.0%, k=2) and 4.84 (21) 10−9kgkg−1 (U=4.3%, k=2) with the use of quadrupole (ICP QMS) or sector field (ICP SFMS) inductively coupled plasma mass spectrometers, respectively. In the case of clam sample, the concentration of MeHg (as Hg) was found to be: 61.0 (2.3) 10−9kgkg−1 (U=3.8%, k=2) and 61.3 (2.2) 10−9kgkg−1 (U=3.6%, k=2), respectively.The mass fractions for total Hg and MeHg determined in this study were used as a contribution of the International Atomic Energy Agency (IAEA) Environment Laboratories in the characterisation of the IAEA 461 and IAEA 470 certified reference materials. The obtained good agreement with the reference values further validated the methods developed in this study.

Determination of cadmium, copper, mercury, lead and zinc mass fractions in marine sediment by isotope dilution inductively coupled plasma mass spectrometry applied as a reference method

Isotope Dilution Inductively Coupled Plasma Mass Spectrometry (ID ICP-MS) has been applied for the determination of the total mass fractions of five trace elements (Cd, Cu, Hg, Pb and Zn) in marine sediment candidate reference material IAEA-458. Because of the rather complex matrix of the sample and the expected spectral interferences, special care was taken for the validation of the applied method, particularly for its measurement step. Reference isotopic measurements were carried out by quadrupole inductively coupled plasma mass spectrometer (ICP-QMS) or by sector field inductively coupled plasma mass spectrometer (ICP-SFMS). Comparative studies were performed with the aim to examine the degree of equivalence between employed techniques in terms of the high requirements for the quality of reference measurement data.In order to reduce the number of analytical steps multiple spiking approach was applied. The measurements were performed after a microwave digestion of the blend samples assuring complete isotopic equilibration. Cd was determined after matrix separation by ICP-QMS or ICP-SFMS at low resolution mode. Additionally Cd isotopic ratios were measured by ICP-QMS operated in collision reaction mode, without separation of the matrix. ICP-SFMS at medium resolution and ICP-QMS methods were used for Zn and Cu determinations. Pb isotopic ratios were measured by ICP-QMS operated in standard mode. Because of the low Hg content in the sample Hg isotopic ratios measurements were carried out only by ICP-SFMS.The entire ID ICP-MS measurement process was described by mathematical equations and the combined uncertainty was estimated. All factors influencing the final results and their uncertainties were systematically investigated. This included procedural blank, moisture content in the sediment material and parameters affecting the blend ratio measurements (instrumental background, spectral interferences, dead time and mass discrimination effects as well as the repeatability of measured isotopic ratios).The excellent agreement between mass fraction values determined by different measurement techniques proved that all potential problems coming from the complex sediment matrix and the spectral interferences were solved. The consistency of the obtained results confirmed that ICP-QMS can be the method of choice even for the reference measurements, when the element content in a sample is sufficiently high and all steps in the analytical procedure are well described and understood.The modeling of the analytical process resulted in adequate validation of measurement procedure, establishing traceability of the measurement results and estimating the realistic final expanded uncertainty. The combined uncertainties associated to the obtained mass fractions were considerably small (2% for Cd, Cu, Zn and Pb and to 4.9% in the case of Hg, k=2) and in line with the primary assumption to the reference measurements.The developed procedure has been successfully applied for the characterisation of the IAEA-458 candidate reference material as well as used in the calculation of the assigned values in the frame of wide world IAEA-458 inter-laboratory comparisons study.

Increasing the resolution and the signal-to-noise ratio of magnetic resonance sounding data using a central loop configuration

SUMMARY Surface nuclear magnetic resonance technique, also called magnetic resonance sounding (MRS), is an emerging geophysical method that can detect the presence and spatial variations of the subsurface water content directly. In this paper, we introduce the MRS central loop geometry, in which the receiver loop is smaller than the transmitter loop and placed in its centre. In addition, using a shielded receiver coil we show how this configuration greatly increases signal-to-noise ratio and improves the resolution of the subsurface layers compared to the typically used coincident loop configuration. We compare sensitivity kernels for different loop configurations and describe advantages of the MRS central loop geometry in terms of superior behaviour of the sensitivity function, increased sensitivity values, reduced noise level of the shielded receiver coil, improved resolution matrix and reduced instrument dead time. With no extra time and effort in the field, central-loop MRS makes it possible to reduce measurement time and to measure data in areas with high anthropogenic noise. The results of our field example agree well with the complementary data, namely airborne electromagnetics, borehole data, and the hydrologic model of the area.

Analysis of COSIMA spectra: Bayesian approach

Abstract. We describe the use of Bayesian analysis methods applied to time-of-flight secondary ion mass spectrometer (TOF-SIMS) spectra. The method is applied to the COmetary Secondary Ion Mass Analyzer (COSIMA) TOF-SIMS mass spectra where the analysis can be broken into subgroups of lines close to integer mass values. The effects of the instrumental dead time are discussed in a new way. The method finds the joint probability density functions of measured line parameters (number of lines, and their widths, peak amplitudes, integrated amplitudes and positions). In the case of two or more lines, these distributions can take complex forms. The derived line parameters can be used to further calibrate the mass scaling of TOF-SIMS and to feed the results into other analysis methods such as multivariate analyses of spectra. We intend to use the method, first as a comprehensive tool to perform quantitative analysis of spectra, and second as a fast tool for studying interesting targets for obtaining additional TOF-SIMS measurements of the sample, a property unique to COSIMA. Finally, we point out that the Bayesian method can be thought of as a means to solve inverse problems but with forward calculations, only with no iterative corrections or other manipulation of the observed data.

NO TIME FOR DEAD TIME: TIMING ANALYSIS OF BRIGHT BLACK HOLE BINARIES WITHNuSTAR

Timing of high-count rate sources with the NuSTAR Small Explorer Mission requires specialized analysis techniques. NuSTAR was primarily designed for spectroscopic observations of sources with relatively low count-rates rather than for timing analysis of bright objects. The instrumental dead time per event is relatively long (~2.5 msec), and varies by a few percent event-to-event. The most obvious effect is a distortion of the white noise level in the power density spectrum (PDS) that cannot be modeled easily with the standard techniques due to the variable nature of the dead time. In this paper, we show that it is possible to exploit the presence of two completely independent focal planes and use the cross power density spectrum to obtain a good proxy of the white noise-subtracted PDS. Thereafter, one can use a Monte Carlo approach to estimate the remaining effects of dead time, namely a frequency-dependent modulation of the variance and a frequency-independent drop of the sensitivity to variability. In this way, most of the standard timing analysis can be performed, albeit with a sacrifice in signal to noise relative to what would be achieved using more standard techniques. We apply this technique to NuSTAR observations of the black hole binaries GX 339-4, Cyg X-1 and GRS 1915+105.

Time-resolved Raman and polyacrylamide gel electrophoresis observations of nucleotide incorporation and misincorporation in RNA within a bacterial RNA polymerase crystal.

The bacterial RNA polymerase (RNAP) elongation complex (EC) is highly stable and is able to extend an RNA chain for thousands of nucleotides. Understanding the processive mechanism of nucleotide addition requires detailed structural and temporal data for the EC reaction. Here, a time-resolved Raman spectroscopic analysis is combined with polyacrylamide gel electrophoresis (PAGE) to monitor nucleotide addition in single crystals of the Thermus thermophilus EC (TthEC) RNAP. When the cognate base GTP, labeled with (13)C and (15)N (*GTP), is soaked into crystals of the TthEC, changes in the Raman spectra show evidence of nucleotide incorporation and product formation. The major change is the reduction of *GTP's triphosphate intensity. Nucleotide incorporation is confirmed by PAGE assays. Both Raman and PAGE methods have a time resolution of minutes. There is also Raman spectroscopic evidence of a second population of *GTP in the crystal that does not become covalently linked to the nascent RNA chain. When this population is removed by "soaking out" (placing the crystal in a solution that contains no NTP), there are no perturbations to the Raman difference spectra, indicating that conformational changes are not detected in the EC. In contrast, the misincorporation of the noncognate base, (13)C- and (15)N-labeled UTP (*UTP), gives rise to large spectroscopic changes. As in the GTP experiment, reduction of the triphosphate relative intensity in the Raman soak-in data shows that the incorporation reaction occurs during the first few minutes of our instrumental dead time. This is also confirmed by PAGE analysis. Whereas PAGE data show *GTP converts 100% of the nascent RNA 14mer to 15mer, the noncognate *UTP converts only ∼50%. During *UTP soak-in, there is a slow, reversible formation of an α-helical amide I band in the Raman difference spectra peaking at 40 min. Similar to *GTP soak-in, *UTP soak-in shows Raman spectoscopic evidence of a second noncovalently bound *UTP population in the crystal. Moreover, the second population has a marked effect on the complex's conformational states because removing it by "soaking-out" unreacted *UTP causes large changes in protein and nucleic acid Raman marker bands in the time range of 10-100 min. The conformational changes observed for noncognate *UTP may indicate that the enzyme is preparing for proofreading to excise the misincorporated base. This idea is supported by the PAGE results for *UTP soak-out that show endonuclease activity is occurring.

Lipid Transfer Kinetics from Nanolipoprotein Particles to Bicelles

Nanolipoprotein particles (NLPs), also known as nanodiscs, are lipid bilayers bounded by apolipoprotein. We recently showed that lipids and membrane proteins cannot exchange between NLPs. However, addition of bicelles opens NLPs and transfers their contents to bicelles, which freely exchange lipids and proteins. Because monomeric membrane proteins can be prepared in NLPs by cell-free protein synthesis, the bicelle-induced transfer process may provide a new method for studying membrane protein oligomerization. The mechanism of the NLP-bicelle interaction is unknown. We have now tested the effects of bicelle detergent (DHPC), apolipoprotein (MSP1E3D1), and temperature on lipid transfer from NLPs to bicelles, using stopped-flow kinetics. NLPs were prepared with fluorescent lipids (0.02 to 0.05 mole fraction), consisting of FRET donors (NBD-PE) and acceptors (LR-PE) at approximately equal concentrations. NLPs were mixed with a 200-fold molar excess of DHPC/DMPC bicelles (equimolar 6- and 14-carbon acyl chains) in a stopped-flow fluorometer. The rate of lipid transfer was monitored by the appearance of unquenched NBD fluorescence at 520 nm. The observed pseudo-first-order rate constant was surprisingly small (0.26/sec). NLPs did not react with DHPC alone below its critical micelle concentration (cmc). Above the cmc, the reaction was complete within the instrument dead time. Thus, the rate-limiting step is not the reaction of NLPs with DHPC monomers or micelles. Added MSP1E3D1 had no effect on the rate, ruling out free apolipoprotein involvement. The NLP-bicelle mixing rate showed a strong temperature dependence (activation energy = 28 kcal/mol). Near or below the DMPC phase transition temperature, the kinetics were biphasic. The results suggest NLP-bicelle mixing kinetics may be mechanistically similar to lipid mixing via fusion pores.

Research and Realization of Short Dead-Time Surface Nuclear Magnetic Resonance for Groundwater Exploration

Surface nuclear magnetic resonance is a geophysical method designed for direct groundwater exploration, but its applications for conducting research on contamination analysis in the vadose zone are limited due to instrumental dead time. To address this challenge, we have modeled and simulated the phenomenon and impact of the transmitter loop discharge, relay oscillation, and delay of the amplifier caused by the instrumental dead time. Next, we propose and develop a Q-toggle dead-time shortening circuit for suppressing oscillation and improving signal quality. Laboratory and field tests demonstrate that this circuit is sensitive to low-amplitude and fast-decay signals and can successively increase the detection resolution of the instrument receiver. Finally, the inversion results confirm that this improvement can be used to accurately characterize the water content in the vadose area.

Pulse properties of terrestrial gamma‐ray flashes detected by the Fermi Gamma‐Ray Burst Monitor

AbstractThe Gamma‐ray Burst Monitor (GBM) on board the Fermi Gamma‐ray Space Telescope has triggered on over 300 terrestrial gamma‐ray flashes (TGFs) since its launch in June 2008. With 14 detectors, GBM collects on average ∼100 counts per triggered TGF, enabling unprecedented studies of the time profiles of TGFs. Here we present the first rigorous analysis of the temporal properties of a large sample of TGFs (278), including the distributions of the rise and fall times of the individual pulses and their durations. A variety of time profiles are observed with 19% of TGFs having multiple pulses separated in time and 31 clear cases of partially overlapping pulses. The effect of instrumental dead time and pulse pileup on the temporal properties are also presented. As the observed gamma ray pulse structure is representative of the electron flux at the source, TGF pulse parameters are critical to distinguish between relativistic feedback discharge and lightning leader models. We show that at least 67% of TGFs at satellite altitudes are significantly asymmetric. For the asymmetric pulses, the rise times are almost always shorter than the fall times. Those which are not are consistent with statistical fluctuations. The median rise time for asymmetric pulses is ∼3 times shorter than for symmetric pulses while their fall times are comparable. The asymmetric shapes observed are consistent with the relativistic feedback discharge model when Compton scattering of photons between the source and Fermi is included, and instrumental effects are taken into account.

Surface NMR instrumentation and methods for detecting and characterizing water in the vadose zone

ABSTRACTA commercially available surface NMR instrument was modified to address the challenges of using earth’s field surface NMR to detect and characterize water in the unsaturated (or vadose) zone. The modified instrument incorporates faster switching electronics to achieve an instrument dead time of 2.8 ms, and higher output power electronics to enable a maximum coil voltage of 8000 volts and coil current of 800 amps. The instrument was used to collect and interpret surface NMR data at several active vadose zone investigation sites in the western US. A 6‐week surface NMR experiment was conducted at a managed aquifer storage and recovery facility in Arizona, to explore the measurement capabilities and limitations of the instrument, during a managed infiltration event. The resulting time lapse surface NMR data were used to map zones of held water prior to the flood event, image the influx of water through the top 15 metres of the subsurface during and after the event, quantify the spatial and temporal distribution of infiltrating water throughout the event, and characterize the distribution of water in different relative pore sizes throughout the event. Data obtained at pseudo‐static vadose zone investigation sites indicate that the surface NMR instrument can detect and image some forms of water held in unconsolidated vadose zone formations, at depths up to 30 metres. Complementary NMR logging data indicate that the surface NMR instrument does not detect all of the water held in these pseudo‐static formations, but that the non‐invasive surface NMR data may yield valuable information nonetheless.

Quantitative SPECT/CT: SPECT joins PET as a quantitative imaging modality

The introduction of combined modality single photon emission computed tomography (SPECT)/CT cameras has revived interest in quantitative SPECT. Schemes to mitigate the deleterious effects of photon attenuation and scattering in SPECT imaging have been developed over the last 30 years but have been held back by lack of ready access to data concerning the density of the body and photon transport, which we see as key to producing quantitative data. With X-ray CT data now routinely available, validations of techniques to produce quantitative SPECT reconstructions have been undertaken. While still suffering from inferior spatial resolution and sensitivity compared to positron emission tomography (PET) imaging, SPECT scans nevertheless can be produced that are as quantitative as PET scans. Routine corrections are applied for photon attenuation and scattering, resolution recovery, instrumental dead time, radioactive decay and cross-calibration to produce SPECT images in units of kBq.ml(-1). Though clinical applications of quantitative SPECT imaging are lacking due to the previous non-availability of accurately calibrated SPECT reconstructions, these are beginning to emerge as the community and industry focus on producing SPECT/CT systems that are intrinsically quantitative.

Field experiment provides ground truth for surface nuclear magnetic resonance measurement

The need for sustainable management of fresh water resources is one of the great challenges of the 21st century. Since most of the planet's liquid fresh water exists as groundwater, it is essential to develop non‐invasive geophysical techniques to characterize groundwater aquifers. A field experiment was conducted in the High Plains Aquifer, central United States, to explore the mechanisms governing the non‐invasive Surface NMR (SNMR) technology. We acquired both SNMR data and logging NMR data at a field site, along with lithology information from drill cuttings. This allowed us to directly compare the NMR relaxation parameter measured during logging,T2, to the relaxation parameter T2* measured using the SNMR method. The latter can be affected by inhomogeneity in the magnetic field, thus obscuring the link between the NMR relaxation parameter and the hydraulic conductivity of the geologic material. When the logging T2data were transformed to pseudo‐T2* data, by accounting for inhomogeneity in the magnetic field and instrument dead time, we found good agreement with T2* obtained from the SNMR measurement. These results, combined with the additional information about lithology at the site, allowed us to delineate the physical mechanisms governing the SNMR measurement. Such understanding is a critical step in developing SNMR as a reliable geophysical method for the assessment of groundwater resources.

COUNT RATE LIMITATIONS FOR PULSE-COUNTING INSTRUMENTATION IN PULSED ACCELERATOR FIELDS

This paper discusses various concepts involved in the counting losses of pulse-counting health physics instrumentation when used within the pulsed radiation environments of typical accelerator fields in order to preestablish appropriate limitations in use. Discussed are the "narrow" pulse and the "wide" pulse cases, the special effect of neutron moderating assemblies, and the effect of pulse fine microstructure on the counting losses of the pulse-counting instrumentation. In the narrow-pulse case, the accelerator pulse width is less than or equal to the instrument's dead time; whereas in the wide-pulse case, the accelerator pulse width is significantly longer than the instrument's dead time. Examples are provided that highlight the various concepts and limitations.