Nuclear Resonance Fluorescence Research Articles

Transmission-based detection of nuclides with nuclear resonance fluorescence using a quasimonoenergetic photon source

We provide a detailed experimental validation of the concept of transmission-based isotope detection. The dominant background processes in this class of systems were measured by studying the detection of U238 with a quasimonochromatic (ΔE∕E∼3%) photon beam. A notch develops in the spectrum transmitted through our test objects due to the preferential attenuation of photons with an energy that resonantly excites a bound nuclear state in U238 near 2 MeV. The notch was measured downstream of our test objects by means of resonant photon scattering from a secondary U238 target. The dominant backgrounds measured in the notch detector due to radioactive decay and elastic scattering of the transmitted beam are presented. Processes that refill the notch with off-resonance photons will obscure the signal and result in a higher probability of false negatives. A measurement of the refill process produced a null result, and we report an upper limit on the magnitude of the notch fill factor.

New measurements of the lifetimes of excited states ofMn55below 2.7 MeV

The lifetimes of the excited states of $^{55}\mathrm{Mn}$ between 1.5 and 2.7 MeV were measured using nuclear resonance fluorescence. The absolute lifetimes of the excited levels were determined from simultaneous measurements of manganese and aluminum. In this approach, the precisely known aluminum state serves as a means to normalize the results. Our findings differ from the evaluated level lifetimes in the Evaluated Nuclear Structure Data File (ENSDF), but agree with earlier nuclear resonance fluorescence measurements.

Splitting of the pygmy dipole resonance inBa138andCe140observed in the(α,α′γ)reaction

The N = 82 nuclei Ce-140 and Ba-138 have been investigated by means of the (alpha,alpha',gamma) coincidence method to study the pygmy dipole resonance (PDR). The experiments have been performed at the AGOR cyclotron at KVI, Groningen, at a primary beam energy of E-alpha = 136 MeV. The Big-Bite Spectrometer and seven large-volume high-purity germanium detectors were used in coincidence to perform a simultaneous spectroscopy of the scattered alpha particles and the gamma decay. The comparison with results of nuclear resonance fluorescence experiments reveals a splitting of the PDR into two components. Up to about 6 MeV the same states that could be observed in (gamma,gamma') are also excited in alpha-scattering experiments, whereas the higher-lying states are missing in the (alpha,alpha',gamma) reaction. This indicates a structural splitting of the PDR into two modes with different underlying structure.

A New Series of 1,3‐Dihidro‐Imidazo[1,5‐c]thiazole‐5,7‐Dione Derivatives: Synthesis and Interaction with Aβ(25‐35) Amyloid Peptide

Deposition of senile plaques composed of fibrillar aggregates of Abeta-amyloid peptide is a characteristic hallmark of Alzheimer's disease. A widely employed approach in the study of anti-Alzheimer agents involves the identification of substances able to prevent amyloid aggregation, or to disaggregate the amyloid fibrils through a direct structural interaction with the soluble or aggregated forms of the peptide. Here, we report the synthesis of a set of 1,3-dihydro-3,6-disubstituted-imidazo[1,5-c]thiazole-5,7-dione derivatives supporting different alkyl, aryl and alkylamine side chains. The ability of these compounds to interact with the Abeta(25-35) peptide was evaluated using circular dichroism, nuclear magnetic resonance and thioflavin fluorescence spectroscopy. A molecular model for Abeta(25-35)-ligand interactions was calculated by molecular docking procedures. Our data show that the ability of the synthesized compounds to modify the structural behaviour of Abeta(25-35) varies as a function of the overall structural features of the ligands rather contributions from specific individual substituents.

PYGMY DIPOLE RESONANCES

Electric dipole strength below the particle emission threshold both in stable nuclei and short-lived isotopes has received increasing interest due to its astrophysical impact. In analogy to the giant dipole resonance, this strength is commonly referred to as pygmy resonance. Coulomb dissociation of neutron-rich unstable isotopes and nuclear resonance fluorescence photon scattering have begun to provide systematic data on electric dipole strength in various isotope chains. We review the present state of the art of theoretical approaches and point out some open problems. We emphasize the necessity of a simultaneous theoretical treatment of the nucleon separation energies and the energetically low-lying dipole strength because the presently available data do not exclude a non-collective nature of the pygmy strength.

Fission Signatures for Nuclear Material Detection

Detection and interdiction of nuclear materials in all forms of transport is one of the most critical security issues facing the United States and the rest of the civilized world. Naturally emitted gamma rays by these materials, while abundant and detectable when unshielded, are low in energy and readily shielded. X-ray radiography is useful in detecting the possible presence of shielding material. Positive detection of concealed nuclear materials requires methods which unequivocally detect specific attributes of the materials. These methods typically involve active interrogation by penetrating radiation of neutrons, photons or other particles. Fortunately, nuclear materials, probed by various types of radiation, yield very unique and often strong signatures. Paramount among them are the detectable fission signatures, namely prompt neutrons and gamma rays, and delayed neutrons gamma rays. Other useful signatures are the nuclear states excited by neutrons, via inelastic scattering, or photons, via nuclear resonance fluorescence and absorption. The signatures are very different in magnitude, level of specificity, ease of excitation and detection, signal to background ratios, etc. For example, delayed neutrons are very unique to the fission process, but are scarce, have low energy, and hence are easily absorbed. Delayed gamma rays are more abundant but featureless, and have a higher background from natural sources and more importantly, from activation due to the interrogation sources. The prompt fission signatures need to be measured in the presence of the much higher levels of probing radiation. This requires taking special measures to look for the signatures, sometimes leading to a significant sensitivity loss or a complete inability to detect them. Characteristic gamma rays induced in nuclear materials reflecting their nuclear structure, while rather unique, require very high intensity of interrogation radiation and very high resolution in energy and/or time. The trade off of signatures, their means of stimulation, and methods of detection, will be reviewed.

Detection of radioactive isotopes by using laser Compton scattered [formula omitted]-ray beams

Non-destructive detection and assay of nuclear materials is one of the most critical issues for both the management of nuclear waste and the non-proliferation of nuclear materials. We use laser Compton scattered (LCS) γ -ray beams and the nuclear resonance fluorescence (NRF) for the non-destructive detection of radioactive materials. Quasi-monochromatic and energy-tunable LCS γ -ray beams help improve the signal-to-noise ratio during NRF measurements. We developed the conceptual design of a high-flux γ -ray source with an energy-recovery linac, which produces a γ -ray beam at the flux of 10 13 photons / s . In this paper, we discuss the execution of simulation studies using a Monte Carlo code, results of a proof-of-principle experiment for isotope detection, and the status of the development of LCS X-ray and γ -ray facilities.

Nondestructive detection of hidden chemical compounds with laser Compton-scattering gamma rays

A nondestructive assay method for measuring a shielded chemical compound has been proposed. The chemical compound is measured by using a nuclear resonance fluorescence (NRF) measurement technique with an energy tunable laser Compton-scattering (LCS) gamma-ray source. This method has an advantage that hidden materials can be detected through heavy shields such as iron plates of a thickness of several centimeters. A detection of a chemical compound of melamine, C(3)H(6)N(6), shielded by 15-mm-thick iron and 4-mm-thick lead plates is demonstrated. The NRF gamma-rays of (12)C and (14)N of the melamine are measured by using the LCS gamma-rays of the energies of up to 5.0 MeV. The observed ratio ((12)C/(14)N)(exp)=0.39+/-0.12 is consistent with (C/N)(melamine)=0.5.

Chemical characteristics of chromophoric dissolved organic matter in rainwater

Proton nuclear magnetic resonance ( 1H-NMR), UV absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy were used to define the chemical characteristics of chromophoric dissolved organic matter (CDOM) in whole and C 18 extracted rainwater. The average total recovery of fluorescence determined from the sum of extract and filtrate fractions relative to the whole was 86% suggesting that 14% of fluorescent CDOM in rainwater is comprised of very hydrophobic material that cannot be eluted from the column. Half the fluorescence of rainwater was recovered in the filtrate fraction which is important because it suggests that 50% of the chromophoric material present in precipitation is relatively hydrophilic. The average spectral slope coefficient was smaller in extracted samples (16.3 ± 9.0 μm −1) relative to whole samples (18.9 ± 2.8 μm −1) suggesting that the extracted material contains larger molecular weight material. Approximately one-third of the total dissolved organic carbon (DOC) in rainwater exists in the extract fraction suggesting that a large percentage of the uncharacterized DOC in rainwater can be accounted for by these hydrophobic macromolecular species. The fluorescence of extracted samples is strongly correlated with total NMR integration and is most sensitive to aromatic protons suggesting that molecules in this region are the most important in controlling the optical properties of rainwater. The lower removal efficiency of CDOM in rainwater relative to surface waters or the water-soluble fraction of aerosols during solid phase extraction (SPE) suggests that rainwater contains significantly more hydrophilic chromophoric compounds which are compositionally different than found in these other aquatic matrices.

A multisignal detection of hazardous materials for homeland security

The detection of hazardous materials has been identified as one of the most urgent needs of homeland security, especially in scanning cargo containers at United States ports. To date, special nuclear materials have been detected using neutron or gamma interrogation, and recently the nuclear resonance fluorescence has been suggested. We show a new paradigm in detecting the materials of interest by a method that combines four signals (radiography/computer tomography, acoustic, muon scattering, and nuclear resonance fluorescence) in cargos. The intelligent decision making software system is developed to support the following scenario: initially, radiography or the computer tomography scan is constructed to possibly mark the region(s) of interest. The acoustic interrogation is utilized in synergy to obtain information regarding the ultrasonic velocity of the cargo interior. The superposition of the computer tomography and acoustic images narrows down the region(s) of interest, and the intelligent system guides the detection to the next stage: no threat and finish, or proceed to the next interrogation. If the choice is the latter, knowing that high Z materials yield large scattering angle for muons, the muon scattering spectrum is used to detect the existence of such materials in the cargo. Additionally, the nuclear resonance fluorescence scan yields a spectrum that can be likened to the fingerprint of a material. The proposed algorithm is tested for detection of special nuclear materials in a comprehensive scenario.

Synthesis and Gelation Behavior of a Pyrene-Containing Glucose Derivative

A pyrene-containing glucose derivative(PSDAPG) was synthesized and characterized. Its gelation behavior in 36 solvents was evaluated and the compound geled 16 of the solvents tested. Interestingly,PSDAPG was a super gelator for 1-decanol and its lowest gelation concentration(LGC) was 7.0×10-4 g·mL-1. PSDAPG was found to behave as a "bifunctional gelator"as it gels in both water and organic solvents. Scanning electron microscopy(SEM) ,Fourier transform infrared spectroscopy(FTIR) ,nuclear magnetic resonance(1H NMR) spectroscopy and fluorescence spectroscopy reveal that the gelator adopts different supramolecular network structures in different solvents. Additionally,hydrophobic and π-π stacking interactions exist between the pyrene structures of the gelator molecules. Hydrogen bonding between the glucose residues also plays a crucial role in the spontaneous formation of gel networks. Fluorescence emission spectra of PSDAPG in the solution state and in the gel state were characterized by monomer emission and excimer emission. Profiles of the two types of spectra were quite different. We found that the monomer emission increased and the excimer emission decreased as the gelation process proceeded. This indicates that formation of gel networks greatly suppresses the mobility of pyrene units in the gelator and results in lower excimer formation efficiency via Birks′ scheme.

Spectroscopy of high rate events during active interrogation

Pulsed accelerators are being used for radiographic inspections in homeland security applications. Adding spectroscopic detection capability enables additional information to be extracted but challenges signal processing hardware. This work compares two approaches to cost-effective digital spectroscopy systems designed to handle these high rate scenarios. In the first system, the signal from a PMT is directly digitized and then deconvolved on a PC using standard techniques. Bypassing the preamplifier, this system can handle high count rates (>10 6 cps) from an LaCl 3 detector with no dead time and an energy resolution of 5% at 1.33 MeV. Spectra acquired from fission product gamma rays between interrogation pulses are shown. At higher rates, this approach is limited by the DAQ transfer rate to the PC. Instead, we tested a fast ADC (125 MSPS) and an FPGA that performed onboard, real-time digital signal processing (DSP). The energy spectrum is generated on the FPGA board during the measurement and transferred later. This system was tested in nuclear resonance fluorescence (NRF) experiments. Resonance peaks from NRF reactions are shown.

Parity assignments inCe140up to 7 MeV using Compton polarimetry

Parity quantum numbers of $J=1$ states up to 7 MeV in the region of the Pygmy Dipole Resonance in $^{140}\mathrm{Ce}$ were determined model independently by combining the methods of Nuclear Resonance Fluorescence and Compton polarimetry. For the first time the well-established method of Compton polarimetry was applied at such high energies. The experiment was performed using a fourfold segmented HPGe clover detector for the detection of the scattered photons. For all investigated dipole transitions asymmetries are found which correspond to negative parity of the excited states.

Nuclear resonance fluorescence excitations near 2 MeV inU235andPu239

A search for nuclear resonance fluorescence excitations in {sup 235}U and {sup 239}Pu within the energy range of 1.0- to 2.5-MeV was performed using a 4-MeV continuous bremsstrahlung source at the High Voltage Research Laboratory at the Massachusetts Institute of Technology. Measurements utilizing high purity Ge detectors at backward angles identified 9 photopeaks in {sup 235}U and 12 photopeaks in {sup 239}Pu in this energy range. These resonances provide unique signatures that allow the materials to be non-intrusively detected in a variety of environments including fuel cells, waste drums, vehicles and containers. The presence and properties of these states may prove useful in understanding the mechanisms for mixing low-lying collective dipole excitations with other states at low excitations in heavy nuclei.

Proposal of Nondestructive Radionuclide Assay Using a High-Flux Gamma-Ray Source and Nuclear Resonance Fluorescence

A nondestructive assay system for radioactive waste management is proposed. The system utilizes nuclear resonance fluorescence triggered by a quasi-monochromatic high-flux gamma ray generated from the Compton scattering of laser photons by relativistic electrons. We employ an energy-recovery linac as an electron source and a mode-locked fiber laser followed by a laser supercavity as a photon source. The combination of these novel technologies realizes a gamma-ray flux much higher than existing sources using electron storage rings. The proposed gamma-ray source produces a quasi-monochromatic gamma ray with a flux of 1010/s/keV, which is high enough for industrial applications such as the nondestructive analysis of radionuclides in nuclear waste and the interrogation of fissile material in cargoes. The nuclear resonance fluorescence triggered by quasi-monochromatic gamma rays provides a versatile method of nondestructive analysis of both radioactive and stable nuclides.

Sulfate stabilizes the folding intermediate more than the native structure of endostatin

Endostatin, a potent angiogenesis inhibitor, is an acid resistant protein with compact tertiary structure. Nuclear magnetic resonance, circular dichroism, and tryptophan emission fluorescence were used to monitor the structural changes of endostatin during acid-, heat-, and urea-induced unfolding processes. Results show that sulfate anions sensitize endostatin to acid, but specifically stabilize it against heat or urea. Moreover, the disappearance of the tertiary structure and the formation of the folding intermediate of endostatin at pH 3.0 are sulfate concentration dependent. These phenomena indicate that sulfate anions stabilize the folding intermediate more than the native structure of endostatin. In addition, heparin shows stronger effect than sodium sulfate on sensitizing endostatin against acid, and very limited stabilizing effect against urea. The loose structure of endostatin upon heparin binding may imply that the physiologically favorable structure for endostatin exerting its biological functions is not as compact as what was reported.

Proposal of Nondestructive Radionuclide Assay Using a High-Flux Gamma-Ray Source and Nuclear Resonance Fluorescence

A nondestructive assay system for radioactive waste management is proposed. The system utilizes nuclear resonance fluorescence triggered by a quasi-monochromatic high-flux gamma ray generated from the Compton scattering of laser photons by relativistic electrons. We employ an energy-recovery linac as an electron source and a mode-locked fiber laser followed by a laser supercavity as a photon source. The combination of these novel technologies realizes a gamma-ray flux much higher than existing sources using electron storage rings. The proposed gamma-ray source produces a quasi-monochromatic gamma ray with a flux of 1010/s/keV, which is high enough for industrial applications such as the nondestructive analysis of radionuclides in nuclear waste and the interrogation of fissile material in cargoes. The nuclear resonance fluorescence triggered by quasi-monochromatic gamma rays provides a versatile method of nondestructive analysis of both radioactive and stable nuclides.

Intrinsic motions along an enzymatic reaction trajectory

The mechanisms by which enzymes achieve extraordinary rate acceleration and specificity have long been of key interest in biochemistry. It is generally recognized that substrate binding coupled to conformational changes of the substrate-enzyme complex aligns the reactive groups in an optimal environment for efficient chemistry. Although chemical mechanisms have been elucidated for many enzymes, the question of how enzymes achieve the catalytically competent state has only recently become approachable by experiment and computation. Here we show crystallographic evidence for conformational substates along the trajectory towards the catalytically competent 'closed' state in the ligand-free form of the enzyme adenylate kinase. Molecular dynamics simulations indicate that these partially closed conformations are sampled in nanoseconds, whereas nuclear magnetic resonance and single-molecule fluorescence resonance energy transfer reveal rare sampling of a fully closed conformation occurring on the microsecond-to-millisecond timescale. Thus, the larger-scale motions in substrate-free adenylate kinase are not random, but preferentially follow the pathways that create the configuration capable of proficient chemistry. Such preferred directionality, encoded in the fold, may contribute to catalysis in many enzymes.

The effect of pressure on the nuclear quadrupole resonance and ruby fluorescence in aNiCrAl pressure cell

A pressure of 4.0 GPa was achieved using a piston–cylinder-type NiCrAlpressure cell which possesses a large sample space of 4.4 mm diameter × 15 mm length at ambient pressure. As monitors of the pressure, the63Cu nuclear quadrupoleresonance (NQR) of Cu2O and ruby fluorescence were studied at the same time. The relation between the NQRfrequency and the R1 line of the ruby fluorescence was investigated at pressures below3.2 GPa.

Aggregation Behavior of Ionic Liquids in Aqueous Solutions: Effect of Alkyl Chain Length, Cations, and Anions

Self aggregation of the ionic liquids, 1-butyl-3-methylimidazolium chloride [C4mim][Cl], 3-methyl-1-octylimidazolium chloride [C8mim][Cl], 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4], N-butyl-3-methylpyridinium chloride [C4mpy][Cl], in aqueous solution has been investigated through 1H nuclear magnetic resonance (NMR) and steady-state fluorescence spectroscopy. Aggregation properties were determined by application of mass action theory to the concentration dependence of 1H NMR chemical shifts. Aggregation properties showed fairly good agreement with the previously reported results obtained from small angle neutron scattering, conductivity, and surface tension measurements. A detailed analysis of chemical shifts of water and various protons in ILs has been employed to probe the aggregate structure. Fluorescence spectroscopy provided important information about the critical aggregation concentration (cac) and the microenvironment of the aggregates. We could also observe a break point quite consistent with that of 1H NMR and fluorescence spectroscopy at cac from the concentration dependence of refractive index measurements. Standard free energies of aggregation DeltaGom of various ILs derived using the refractive index/concentration profiles were found comparable to those of classical ionic surfactants.