Coincidence Spectra Research Articles

Experimental gamma coincidence spectra recorded in prompt gamma neutron activation analysis

Experimental gamma coincidence spectra recorded in prompt gamma neutron activation analysis

Valence photoelectron spectra of aminobenzoic acid molecules: a combined theoretical and experimental study

Abstract The valence photoelectron spectra (PES) of gas-phase aminobenzoic acids (meta-, ortho- and para-isomers) were measured using synchrotron radiation and calculated from first principles using Density Functional Theory (DFT) with popular hybrid exchange-correlation functionals and many-body perturbation theory using the perturbative one-shot (G0W0) and eigenvalue self-consistent (GnW0) approaches within the GW approximation. The vibrational structures and line shapes found in the PES were modeled using Time-Dependent DFT. Theory can reproduce the experimental results very well. The photoelectron-photoion coincidence spectra of the ortho- and para-isomers were also measured. They reveal interesting differences in the fragmentation patterns and the influence of metastable states at the onset of fragmentation.

Characterizing Water Composition with an Autonomous Robotic Team Employing Comprehensive In Situ Sensing, Hyperspectral Imaging, Machine Learning, and Conformal Prediction

Inland waters pose a unique challenge for water quality monitoring by remote sensing techniques due to their complicated spectral features and small-scale variability. At the same time, collecting the reference data needed to calibrate remote sensing data products is both time consuming and expensive. In this study, we present the further development of a robotic team composed of an uncrewed surface vessel (USV) providing in situ reference measurements and an unmanned aerial vehicle (UAV) equipped with a hyperspectral imager. Together, this team is able to address the limitations of existing approaches by enabling the simultaneous collection of hyperspectral imagery with precisely collocated in situ data. We showcase the capabilities of this team using data collected in a northern Texas pond across three days in 2020. Machine learning models for 13 variables are trained using the dataset of paired in situ measurements and coincident reflectance spectra. These models successfully estimate physical variables including temperature, conductivity, pH, and turbidity as well as the concentrations of blue–green algae, colored dissolved organic matter (CDOM), chlorophyll-a, crude oil, optical brighteners, and the ions Ca2+, Cl−, and Na+. We extend the training procedure to utilize conformal prediction to estimate 90% confidence intervals for the output of each trained model. Maps generated by applying the models to the collected images reveal small-scale spatial variability within the pond. This study highlights the value of combining real-time, in situ measurements together with hyperspectral imaging for the rapid characterization of water composition.

Photoelectron spectrum of the pyridyl radical.

We report the photoelectron spectrum of the pyridyl radical (C5H4N), a species of interest in astrochemistry and combustion. The radicals were produced via hydrogen abstraction in a fluorine discharge and ionized with synchrotron radiation. Mass-selected slow photoelectron spectra of the products were obtained from photoelectron-photoion coincidence spectra. A Franck-Condon simulation based on computed geometries and vibrational frequencies identified contributions of the o- and p-pyridyl radicals. For the o-isomer an adiabatic ionisation energy of 7.70 eV was obtained, in excellent agreement with a computed value of 7.72 eV. The spectrum of o-pyridyl is characterized by a long progression in an in-plane bending mode and the N-C stretch that contains the radical site.

Primary standardization and Monte Carlo modeling of (243Am + 239Np) by means of a 4π(PC)-γ coincidence counting system

The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN for the primary standardization of a (243Am + 239Np) solution, in secular equilibrium, is described. The measurement was carried out in a 4π(PC) (α,β)−γ coincidence system. The total activity per unit mass of the solution was determined by the extrapolation technique, using a software coincidence counting systsem. The extrapolation curves were compared with Monte Carlo calculations by means of Code ESQUEMA, used in previous works, which, was improved and applied in order to calculate the alpha, beta, gamma, X-rays and coincidence spectra.

A machine learning approach to predict radioxenon isotopes concentrations using experimental data

Monitoring atmospheric radioxenon isotope concentration is crucial for verifying compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The most sensitive method for radioxenon isotope detection is beta-gamma coincidence measurement. Quantities of activity concentrations and, more specifically, their ratios are vital for analyzing spectra from atmospheric radioactivity to characterize the emission source. This study aims to develop a novel approach utilizing a machine learning algorithm to estimate the activity concentrations of radioxenon isotopes from real spectra in the format of experimental two-dimensional raw spectra without the need for extensive mathematical calculations for subtracting interferences and backgrounds. This method can be applied as a pre-screening method in a fast way to highlight spectra with concentration anomalies and help analysts in the interactive review step. Traditional methods like region-of-interest (ROI) and simultaneous decomposition analysis tool (SDAT) have been used to estimate net counts for each specific ROIs. However, this study employs deep convolutional neural networks (CNNs) to extract fine structure and patterns from the complex spectra, capturing both local and global features from entire beta-gamma coincidence spectra without relying on predefined ROIs.

A fully digital coincidence Doppler broadening spectrometer based on FPGA

In this article, we implemented a fully digital Coincidence Doppler Broadening (CDB) spectrometer consisting of two HPGe detectors, a fully digital processing and acquisition board, and a dedicated software system. The Field Programmable Gate Array (FPGA) on the board extracts energy and time stamp of digitized pulses from HPGe detectors, and communicates with the dedicated software by which the single channel spectrums and 2D coincidence spectrum were produced. We designed an instruction set dedicated to the CDB Spectrometer and the controller module in FPGA so that the FPGA subsystem is fully controllable and configurable. The trapezoidal pulse shaping algorithm was implemented in FPGA whose arguments are all configurable through the dedicated software by the instruction set. We examined the long-term stability of the spectrometer by 511 keV peak position of both channels. The 2D Gaussian fitting is performed on coincidence spectrum by the software to correct the peak drift of both channels. The results show that this method effectively improves the Doppler-broadened spectrum, by which the peak-to-background ratio increases from 7.0 × 105 to 1.9 × 106.

Clustering effects in Ar36 nuclei produced via the Mg24 + C12 reaction

The nuclear properties of $^{36}\mathrm{Ar}$ composite $\ensuremath{\alpha}$-like nuclei produced at 46.72 MeV of excitation energy via the $^{24}\mathrm{Mg}+^{12}\mathrm{C}$ reaction were investigated. In the past, at this excitation energy, resonant structures with peak-to-valley variation of 50--100 mb have been observed for this system and associated with fusion cross sections. To reveal the nature of this phenomenon the fusion channel observable was investigated. Exclusive measurements of $\ensuremath{\alpha}$ particles and evaporation residues were carried out at Laboratori Nazionali di Legnaro using the $8\ensuremath{\pi}\mathrm{LP}$ apparatus coupled to an evaporation residue detector. Then the experimental data were interpreted through the comparison with statistical model predictions. The energy spectra in coincidence with evaporation residues evidence the limitation of the statistical model assuming nuclear shape of the compound nucleus according to the rotating liquid drop model. To reproduce the experimental data very elongated nuclear shapes and reduced barriers for light particle emission have to be considered, with a major to minor axis ratio up to 3 at higher angular momenta. This large value for the axis ratio is selected in agreement with the predictions of the cranked cluster model and is consistent with previous findings for $\ensuremath{\alpha}$-like nuclei.

Methane Cluster Fragmentation by Fast Electron Impact

We investigate the fragmentation of the CH4 cluster by fast electron impact at stagnation pressures from 0.5 bar to 16 bar. By measuring the time of flight spectrum (TOF), two types of ions, including (CH4)n−1CH5+ and (CH4)n−2(C2Hm)+, are observed. In the 1D TOF spectrum, it is shown that for the stagnation pressure larger than 4 bar, the former ion is predominant for each n, similar to the previous experimental result. However, as the pressure decreases to 0.5 or 2 bar, the contribution of the C2Hm+ ion is dominant over that of the CH4CH5+ ion. In the 2D coincident TOF spectrum, the above two patterns of ions are also distinguished, and the enhancement of C2Hm+ is observed at 4 bar pressure. The phenomena appearing in 2D and 1D TOF spectra imply that the C2Hm+ ion prefers to survive in a smaller cluster, while the stabilization of the protonated ion needs a more massive cluster environment.

INVESTIGATION OF GAMMA-RAYSATTENUATION IN METALLIC POWDERS USING GAMMA- GAMMACOINCIDENCE METHOD

Gamma-rays attenuation in metallic powders is investigated using coincidence technique with two 3ʺx3ʺ NaI(Tl) scintillation detectors. Na-22 source isused with 0.4µCi activity and resolving time registered was 6.7 ns. Linear attenuation coefficient (L.A.C) measurements were carried out at 511keV energy for Fe3O4 and ZrSiO4 metallic powders with uniform 100µm grains size and Fe sheets locally available (for comparison) with different thicknesses. The L.A.C values (in cm-1) calculated from direct spectra were 0.2705, 0.2641and 0.6239 for the materials respectively. The L.A.C values calculated from coincidence spectra using net peak area NPA were 0.2363, 0.2152 and 0.6231 and with using gross peak area GA were 0.2092, 0.1974 and 0.5261 respectively. The mass attenuation coefficient M.A.C (in cm²/g) for Fe sheets from direct spectra was 0.0792 and from coincidence spectra was 0.0791 and 0.0660 for NPA and GA respectively. The ratio of L.A.C from coincidence spectrum / L.A.C from direct spectrum were 0.7733, 0.7474 and 0.8432 using GA and 0.8735, 0.8148 and 0,9987 using NPA respectively.

COMPARISON OF GAMMA–RAYS ATTENUATION MEASURED BY DIRECT AND COINCIDENCE SPECTRA

Measurements of gamma –rays attenuation in lead and iron sheets were carried out using direct spectrum and gamma- gamma fast slow coincidence spectrometer composed of two 3"×3"NaI(Tl) detectors and Na-22 source. Measurements of mass attenuation coefficient at 511keV were carried out and its values (in cm²/g units) for lead from direct spectrum for the cases of without and with using a collimator (1cm diameter and 5 cm length) were 0.12029 and 0.13591and from coincidence spectrum were 0.10721 and 0.10602 respectively. For iron sheets the mass attenuation coefficient values from direct spectrum for the cases of without and with using a collimator were 0.0780 and 0.0766 andfrom coincidence spectrum were 0.0593 and 0.0616 respectively. The present results registered by direct measurement are little lower than the standard values due to making only photoelectric absorption needed for comparison with coincidence results where the latter showed lower values.
 The build-up factor were also measured as function of thickness using lead and iron sheets and found fluctuating around unity for lead and iron coincidence spectra and iron direct spectrum, whereas its increases gradually for lead direct spectrum for the measured thickness range. The coincidence spectrometer greatly reduces scatteringevents in the measured material, and thus its results somewhat resemble that of direct measurements with using the collimator.

USE OF GAMMA–GAMMA COINCIDENCE TECHNIQUE IN INVESTIGATING GAMMA – RAYS ATTENUATION AT DIFFERENT ENERGIES

A gamma–gamma fast –slow coincidence spectrometer composed of two 3"×3"NaI(Tl) detectors and peripheral electronics is used in investigating some effects on the photo peak after the penetration of gamma –rays from lead. Coincidence measurements are carried out using Na-22 source with selecting the 511 keV gate. Linear and mass attenuation (in cm²/g) coefficients measurements are made using lead sheets with dimensions 10×10 cm. Mass attenuation coefficient was measured for coincidence spectrum at 511 keV and found to be 0.114 and 0.140 for direct spectrum. Also coincidence and direct spectra measurements were carried out using Co-60 source with 1332 and 1173 keV gates. Mass attenuation coefficient at 1173 keV was 0.0460 for coincidence spectrum and 0.0522 for direct spectrum. At 1332 keV the same coefficient was0.0432 for coincidence spectrum and 0.0452 for direct spectrum. Mass attenuation coefficients values from coincidence measurement are lower than those in direct measurement at same energies

Electron and ion spectroscopy of azobenzene in the valence and core shells.

Azobenzene is a prototype and a building block of a class of molecules of extreme technological interest as molecular photo-switches. We present a joint experimental and theoretical study of its response to irradiation with light across the UV to x-ray spectrum. The study of valence and inner shell photo-ionization and excitation processes combined with measurement of valence photoelectron-photoion coincidence and mass spectra across the core thresholds provides a detailed insight into the site- and state-selected photo-induced processes. Photo-ionization and excitation measurements are interpreted via the multi-configurational restricted active space self-consistent field method corrected by second order perturbation theory. Using static modeling, we demonstrate that the carbon and nitrogen K edges of azobenzene are suitable candidates for exploring its photoinduced dynamics thanks to the transient signals appearing in background-free regions of the NEXAFS and XPS.

On the Peculiarities of Studying Unbound Excited States of 4He Nucleus by α + 3H Interaction

Due to the alpha-particle beam interaction with tritium, the two-dimensional spectra of tt, t3He, and dd coincidences are obtained using the particle-decay spectroscopy from three-particle 3H(α,tt)p, 3H(α,t 3He)n, and 3H(α,td)d reactions (Eα = 67.2 MeV). Excitation energies and energy widths for seven excited 4He states, as well as the ratio of their different decay modes, namely t + p, n + 3He, and d + d, are obtained by using the Monte Carlo method.

Photodissociation spectroscopy via a rovibrational resonance in intense UV pulses

Photodissociation dynamics via a rovibrational resonance in intense UV pulses is investigated theoretically, using a showcase ${\text{CH}}^{+}$ molecule promoted to the $\text{C}\phantom{\rule{0.16em}{0ex}}^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ valence excited electronic state with a potential barrier, thus giving access to study shape resonance controlled by the pulse frequency. Simulations of the kinetic energy release (KER) and angular distribution of the photofragments (ADP) spectra show dramatic differences for the cases when the pulse is tuned on and off the rovibrational resonance. It shows that as the increasing pulse intensity for the transitions to shape resonance, the KER spectra develop into new and higher energy peaks overlying on the broadened background, which is explained by the involvement of other resonances with higher partial waves through electronic Rabi flopping between the ground and excited electronic states. Those nonlinear contribution increases drastically the probability of photofragmentation along the laser polarization in the ADP spectra. The coincident KER-ADP spectra reveal clearly the correlated dynamics in the vicinity of the dissociation barrier. The present work opens possibilities for the manipulation of ultrafast photodissociation dynamics with the help of resonance states in the continuum.

Coincidence spectroscopy of molecular normal Auger decay by ultrashort x-ray pulses

When a core-shell electron is ionized by coherent ultrashort x-ray pulses with varied duration, different nuclear wave packets of a cationic molecule are formed, which can be effectively studied by detecting the photoelectron-Auger electron-ion fragment as it is proposed in the present paper. We develop the theory for the photoelectron, i.e., Auger electron, ion fragment three-body coincidence spectroscopy, where dissociative Auger final states are considered. Simulations to display the HF molecule show that the coincidence spectra encode the detailed information of coherent nuclear wave-packet dynamics. Our work paves the way for using currently available coherent ultrashort x-ray pulses to investigate and manipulate molecular nuclear wave packets. Moreover, we discuss possibilities for high-resolution coincidence experimental techniques.

A new endstation for extreme-ultraviolet spectroscopy of free clusters and nanodroplets.

In this work, we present a new endstation for the AMOLine of the ASTRID2 synchrotron at Aarhus University, which combines a cluster and nanodroplet beam source with a velocity map imaging and time-of-flight spectrometer for coincidence imaging spectroscopy. Extreme-ultraviolet spectroscopy of free nanoparticles is a powerful tool for studying the photophysics and photochemistry of resonantly excited or ionized nanometer-sized condensed-phase systems. Here, we demonstrate this capability by performing photoelectron-photoion coincidence experiments with pure and doped superfluid helium nanodroplets. Different doping options and beam sources provide a versatile platform to generate various van der Waals clusters as well as He nanodroplets. We present a detailed characterization of the new setup and show examples of its use for measuring high-resolution yield spectra of charged particles, time-of-flight ion mass spectra, anion-cation coincidence spectra, multi-coincidence electron spectra, and angular distributions. A particular focus of the research with this new endstation is on intermolecular charge and energy-transfer processes in heterogeneous nanosystems induced by valence-shell excitation and ionization.

Intermolecular Coulombic Decay in Liquid Water.

We report the first observation of intermolecular Coulombic decay (ICD) in liquid water following inner-valence ionization. By combining a monochromatized tabletop high-harmonic source with a liquid microjet, we record electron-electron coincidence spectra at two photon energies that identify the ICD electrons, together with the photoelectrons originating from the 2a_{1} inner-valence band of liquid water. Our results confirm the importance of ICD as a source of low-energy electrons in bulk liquid water and provide quantitative results for modeling the velocity distribution of the slow electrons that are thought to dominate radiation damage in aqueous environments.

Time characterization of cosmic-ray induced events in HPGe detector by Monte Carlo simulations

Detailed studies of background events provide information necessary for the planning of active and passive shielding of the detector in all experiments, especially ones searching for rare nuclear events. An effective approach in understanding the contribution of cosmic rays to the background of the HPGe detector is the use of Monte Carlo simulations. In this study, the Monte Carlo simulations were performed in order to obtain the time spectrum of the events induced by cosmic-ray muons and neutrons in the materials in the vicinity of the detector system and the detector itself. In order to reject background events originating from environmental radionuclides (from 238U and 232Th series, 40K, etc.), a coincidence system based on a large-volume HPGe detector and plastic scintillation detector was developed and the same setup was constructed in simulations. The experimental spectrum was compared with the simulated one. The possibility to distinguish these events based on the time was presented and illustrated through the selection of several time intervals of the time spectrum obtained in the simulation and corresponding spectra.Furthermore, the contribution of the muon-induced neutrons and cosmic-ray neutron-induced events to the background spectrum of the large-volume HPGe detector was analyzed. It was found that neutrons produced in the interaction of cosmic-muons with detector shielding and the detector itself are relatively small contributors to the overall neutron-induced events (i.e. low-energy region up to 100 keV originating dominantly from recoils and gamma lines originating from inelastic neutron scattering) registered in coincidence spectrum of HPGe detector compared to the contribution of cosmic-neutron themselves. The share of neutron components in this low-energy region of the coincidence spectrum of the HPGe detector was estimated to be ≈88%. In experiments searching for hypothetical dark matter particles, expected signals are in this energy region, thus in this type of experiment, it is essential to perform rejection of events induced by neutron interaction, in order to avoid misidentification of registered signals.

Photoemission and photofragmentation of butanoic, hexanoic and octanoic acids in the gas phase

The valence and C 1 s core photoelectron spectra of three carboxylic acids (butanoic, hexanoic and octanoic) as well as the near-edge absorption fine structure spectra and photo-electron photo-ion coincidence spectra have been measured. Changes in the valence spectra are related to the lengthening of the aliphatic chains, and the core spectra have been assigned. Small shifts of the carboxylic carbon 1 s binding energy are related to screening of the core hole by the aliphatic group. The C K edge near-edge absorption fine structure spectra probe the unoccupied density of states and the main changes in the spectra are due to increasing length of the aliphatic chain and related Rydberg states. The coincidence spectra show that soft (near threshold) ionization of all compounds leads to parent ion formation for butanoic and octanoic acids. For hexanoic acid, both parent ions and fragments are produced for near threshold ionization. Fragmentation occurs at the C2C3 bond for all carboxylic acids at the lowest energies, and other channels open for higher internal energy.