Coincidence Spectra Research Articles

First measurement of the 2.4 MeV and 2.9 MeV 6He three-cluster resonant states via the 3H(4He, pα)2n four-body reaction

Two new low-lying 6 He levels at excitation energies of about 2.4 MeV and 2.9 MeV were observed in the experimental investigation of the p-α coincidence spectra obtained by the 3 H (4 He , p α)2 n four-body reaction at E4 He beam energy of 27.2 MeV. The relevant E* peak energy and Γ energy width spectroscopic parameters for such 6 He * excited states decaying into the α+ n+n channel were obtained by analyzing the bidimensional (E p , Eα) energy spectra. The present new result of two low-lying 6 He * excited states above the 4 He +2 n threshold energy of 0.974 MeV is important for the investigation of the nuclear structure of neutron-rich light nuclei and also as a basic test for theoretical models in the study of the three-cluster resonance feature of 6 He .

Conformational and nuclear dynamics effects in molecular Auger spectra: fluorine core-hole decay in CF4

In a molecular Auger spectrum information on the decaying state is implicitly ensemble-averaged. For a repulsive core-ionized state, for example, contributions from all parts of its potential curve are superimposed in the Auger spectrum. Using carbon tetrafluoride (CF4, tetrafluoromethane), we demonstrate for the first time that these contributions can be disentangled by recording photoelectron–Auger electron coincidence spectra with high energy resolution. For the F K-VV spectrum of CF4, there are significant differences in the Auger decay at different intermediate state (single core hole) geometries. With the help of calculations, we show that these differences result primarily from zero-point fluctuations in the neutral molecular ground state, but are amplified by the nuclear dynamics during Auger decay.

Measurement of the background in Auger-photoemission coincidence spectra (APECS) associated with inelastic or multi-electron valence band photoemission processes

Auger photoelectron coincidence spectroscopy (APECS), in which the Auger spectra are measured in coincidence with the core level photoelectron, is capable of pulling difficult to observe low energy Auger peaks out of a large background due mostly to inelastically scattered valence band (VB) photoelectrons. However the APECS method alone cannot eliminate the background due to valence band photoemission processes in which the initial photon energy is shared by two or more electrons and one of the electrons is in the energy range of the core level photoemission peak. Here we describe an experimental method to determine the contributions from these background processes and apply this method in the case of copper M3VV Auger spectrum obtained in coincidence with the 3p3/2 photoemission peak. A beam of 200eV photons was incident on a Cu(100) sample and a series of coincidence measurements were performed using a spectrometer equipped with two cylindrical mirror analyzers (CMAs). One CMA was set at series of fixed energies that ranged between the energy of the core and the VB peaks. The other CMA was scanned over a range corresponding to electrons leaving the surface between 0eV and 70eV. The set of measured spectra were then fit to a parameterized function which was extrapolated to determine the background in the APECS spectra due to multi-electron and inelastic VB photoemission processes. The extrapolated background was subtracted from the APECS spectrum to obtain the spectrum of electrons emitted solely as the result of the Auger process. A comparison of the coincidence spectrum with the same spectrum with background removed shows that in the case of Cu M3VV the background due to the inelastic scattering of VB electrons is negligible in the region of the Auger peak but is more than half the total signal down in the low energy tail of the Auger peak.

Kinetic Energy Release in molecular dications fragmentation after VUV and EUV ionization and escape from planetary atmospheres

The measurements of the Kinetic Energy Release (KER) for various ionic species originating from two-body dissociations reactions, induced by double photoionization of CO2, C2H2 and N2O neutral molecular precursors of interest in planetary atmospheres, are reported. The KER distributions as a function of the ultraviolet (UV) photon energy in the range of 30–65eV (Vacuum and Extreme UV photons) are extracted from the electron–ion–ion coincidence spectra obtained by using tunable synchrotron radiation coupled with ion imaging techniques. This experimental method allows assessing the probability of escape for some simple ionic species in the upper atmosphere of Mars and Titan. In fact, the KER measured for H+, C+, CH+, CH2+, N+, O+, CO+, N2+ and NO+ fragment ions are ranging between 1.0 and 5.5eV (only for H+ the maximum value reaches 6.0eV), and these translational energies are large enough to allow these ionic species in participating in the atmospheric escape from Mars and Titan into space (for CO+, N2+ and NO+, the measured KER of 0.5–2.5eV, 0.5–2.8eV and 1.0–2.5eV, respectively, allows the possible escape only from the Titan atmosphere).

Fragmentation mechanisms for methane induced by 55 eV, 75 eV, and 100 eV electron impact

The fragmentation of CH4 (2+) dications following 55 eV, 75 eV, and 100 eV electron impact double ionization of methane was studied using a cold target recoil-ion momentum spectroscopy. From the measured momentum of each recoil ion, the momentum of the neutral particles has been deduced and the kinetic energy release distribution for the different fragmentation channels has been obtained. The doubly charged molecular ions break up into three or more fragments in one or two-step processes, resulting in different signatures in the data. We observed the fragmentation of CH4 (2+) dications through different mechanisms according to the momentum of the neutral particles. For example, our result shows that there are three reaction channels to form CH2 (+), H(+), and H, one synchronous concerted reaction channel and two two-step reaction channels. For even more complicated fragmentation processes of CH4 (2+) dications, the fragmentation mechanism can still be identified in the present measurements. The slopes of the peak in the ion-ion coincidence spectra were also estimated here, as they are also related to the fragmentation mechanism.

Nonmesonic Weak Decay Spectra of Light Hypernuclei

The nonmesonic weak decay spectra of light hypernuclei have been evaluated in a systematic way. As theoretical framework we adopt the independent particle shell model with three different one-meson-exchange transition potentials. Good agreement with data is obtained for proton and neutron kinetic energy spectra of $^4_\Lambda$He, and $^5_\Lambda$He, when the recoil effect is considered. The coincidence spectra of proton-neutron pairs are also accounted for quite reasonably, but it was not possible to reproduce the data for the neutron-neutron pair spectra. It is suggested that the $\pi+K$ meson-exchange model with soft monopole form factors could be a good starting point for describing the dynamics responsible for the decays of these two hypernuclei. The $^4_\Lambda$H~ spectra are also presented.

On the use of 127Xe standards for the quality control of CTBTO noble gas stations and support laboratories

127Xe has a longer half-life than 131mXe, it can be easily purely produced and it is present in the environment at very low level. For these reasons, 127Xe is supposed to be a convenient quality control radionuclide for remote noble gas stations of the International Monitoring System (IMS) network. As CEA/DAM has recently developed two new photon/electron setups for low-level detection of 131mXe, 133mXe, 133Xe and 135Xe, we took the opportunity to test these setups for the measurement of a 127Xe standard. The results and a detailed description of these measurements are presented in this paper. They illustrate the complexity of 127Xe decay, emitting simultaneously several γ, X-rays, conversion electrons and Auger electrons; this results in highly summated coincidence spectra. The measurements performed provide precise electron energy calibration of the setups. The count rate of electrons in coincidence with iodine Kα X-rays was found to be surprisingly low, leading to the study of electron-gated photon spectrum. Finally, a comparison of three photon/electron coincidence spectra obtained with three different setups is given. The use of 127Xe as a standard for energy calibration of IMS noble gas station is possible, but it appears to be quite complicated for efficiency check of noble gas station equipped with β/γ detectors.

A Linear Time-Over-Threshold Digitizing Scheme and Its 64-channel DAQ Prototype Design on FPGA for a Continuous Crystal PET Detector

We propose a practical general method, called the linear time-over-threshold (linear TOT) scheme, for nuclear pulse amplitude digitization aiming at providing a high degree of integration for a high-channel count detector system. The most critical technique in the scheme is that for a different particle detector and diverse pulse shaping, how to produce the time-varying dynamic threshold voltage, which enables the linear conversion from the peak amplitude of nuclear pulse to the TOT time width. Compared with the normal time-over-threshold method for pulse height digitizing, the linear TOT has a strict linearity, large measurable signal dynamic range, and good signal-to-noise ratio. Using the scheme, we design a prototype of a field-programmable gate-array (FPGA)-based 64-channel data-acquisition (DAQ) system for a continuous crystal poistron emission tomography detector. The system only consists of 64-channel active RC integrators and voltage comparators, a shared dynamic threshold generating circuit, and an FPGA. The physical size of the system is so small that it could be attached on the base of the PMT. The preliminary test results show that the nonlinearity between pulse amplitude and TOT time width is about 0.01%, and the prototype could achieve an energy resolution of 12.3% by 22Na coincident spectrum measurement, which is better than the result (14.6%) obtained using our previous homemade DAQ system designed with a normal analog-to-digital converter technique. The design proves that the linear TOT scheme is an effective solution for a high-channel count digitizing system in nuclear detection, not only for its high performance, but also for a high degree of integration.

New insight into dissociative photoionization of N2O at ∼20 eV using threshold photoelectron–photoion coincidence velocity imaging

Dissociative photoionization (DPI) of N2O at ∼20eV has been reinvestigated with threshold photoelectron–photoion coincidence (TPEPICO) velocity imaging. In threshold photoelectron spectrum, a shoulder peak at 20.045eV is observed close to the ground vibrational level of C2Σ+ state at 20.100eV. Through comparing the coincident mass spectra recorded at 20.045 and 20.100eV, the assignment of the shoulder band is corrected to a vibrational excited D2Π ionic state from the previous conclusions of the vibrationless level of b4Π or hot band of C2Σ+ state. For the dominant photofragment of NO+ at 20.045eV, TPEPICO time-sliced velocity image is measured to obtain the corresponding total kinetic energy and angular distributions. Interestingly, both the bimodal vibrational population and angular distribution of NO+ fragment from dissociation of N2O+(D2Π) are very similar to those of N2O+(C2Σ+) ions. With the aid of potential energy curves, the DPI mechanisms of N2O via D2Π ionic state at 20.045eV along the NO+(X1Σ+)+N(2D) and NO+(X1Σ+)+N(2P) dissociation channels are clarified, in which the internal conversion from D2Π to B2Π state is the rate-determined step.

Line-narrowing effects in the near-infrared spectrum of water and precision determination of spectroscopic parameters.

A dual laser absorption spectrometer operating at 1.39 μm was employed to acquire high-quality absorption spectra in coincidence with the 44,1 → 44,0 line of the H2 (18)O ν1 + ν3 band, at the temperature of the triple point of water. A rather sophisticated global fitting procedure was developed and used to simultaneously fit spectra across the explored pressure range, roughly between 1 and 4Torr. Adopting the speed-dependent Galatry profile with a hypergeometric dependence of the collisional width and shift on the absorbers' velocity, we could take into account narrowing effects ascribed to collision-induced velocity changes and speed-dependence of relaxation rates. The global approach allowed us to implement and exploit some relevant physical constraints at the aim of reducing significantly statistical correlation issues among free parameters. Consequently, we demonstrated the possibility of determining pressure broadening and shifting parameters with high precision and accuracy, at the level of one part over 10(3). Furthermore, even being in the Doppler regime, it was possible to determine the effective frequency of velocity-changing collisions for self-colliding H2 (18)O molecules.

Determination of isotopic ratios of uranium samples using passive gamma spectroscopy with multiple detectors

Uranium samples of various enrichments have been passively counted on the University of Texas detector gamma–gamma coincidence system. By observing gamma rays emitted from 235U and its daughters compared to gamma rays emitted by 238U daughters and comparing the data to standards of known enrichments, a technique has been developed to take a uranium sample of unknown enrichment and passively count it to determine its uranium isotopic concentration. Because the gamma rays from 235U are generally in the low-energy regime, there is a strong susceptibility to background interferences, especially from the Compton background produced from higher energy gamma rays. Other interferences, such as those from the decay series of uranium also exist for 235U gamma rays. In this light, we have collected data using list-mode to produce two-dimensional gamma–gamma coincidence spectra, which allows us to gate the low-energy gamma rays from 235U with gamma rays that are in coincidence. In doing this, much of the low energy interferences are reduced, and one can analyze the 235U gamma rays with high precision. Because of the high density of uranium, self-shielding has significant effects especially in the low-energy regime. To correct for this attenuation the detector system has been modeled by MCNP and self-shielding factors have been calculated across the energy spectrum. A big advantage to this method is the capability of performing this analysis with small (<1 g) samples in a non-destructive and relatively inexpensive manner. If necessary, this analysis can be performed within 24 h if an urgent nuclear forensics scenario arises.

Imaging photoelectron circular dichroism of chiral molecules by femtosecond multiphoton coincidence detection

Here, we provide a detailed account of novel experiments employing electron-ion coincidence imaging to discriminate chiral molecules. The full three-dimensional angular scattering distribution of electrons is measured after photoexcitation with either left or right circular polarized light. The experiment is performed using a simplified photoelectron-photoion coincidence imaging setup employing only a single particle imaging detector. Results are reported applying this technique to enantiomers of the chiral molecule camphor after three-photon ionization by circularly polarized femtosecond laser pulses at 400 nm and 380 nm. The electron-ion coincidence imaging provides the photoelectron spectrum of mass-selected ions that are observed in the time-of-flight mass spectra. The coincident photoelectron spectra of the parent camphor ion and the various fragment ions are the same, so it can be concluded that fragmentation of camphor happens after ionization. We discuss the forward-backward asymmetry in the photoelectron angular distribution which is expressed in Legendre polynomials with moments up to order six. Furthermore, we present a method, similar to one-photon electron circular dichroism, to quantify the strength of the chiral electron asymmetry in a single parameter. The circular dichroism in the photoelectron angular distribution of camphor is measured to be 8% at 400 nm. The electron circular dichroism using femtosecond multiphoton excitation is of opposite sign and about 60% larger than the electron dichroism observed before in near-threshold one-photon ionization with synchrotron excitation. We interpret our multiphoton ionization as being resonant at the two-photon level with the 3s and 3p Rydberg states of camphor. Theoretical calculations are presented that model the photoelectron angular distribution from a prealigned camphor molecule using density functional theory and continuum multiple scattering X alpha photoelectron scattering calculations. Qualitative agreement is observed between the experimental results and the theoretical calculations of the Legendre moments representing the angular distribution for the two enantiomers. The electron-ion coincidence technique using multiphoton ionization opens new directions in table-top analytical mass-spectrometric applications of mixtures of chiral molecules.

State-resolved predissociation dynamics of the formyloxyl radical

Dissociation dynamics of the deuterated formyloxyl radical, DCO2, were studied by photoelectron–photofragment coincidence spectroscopy of DCO2-. Photodetachment at 4.27eV accesses the three lowest-lying electronic states (2A1, 2B2, and 2A2) of DCO2, leading to vibrational excitation dominated by the 2A1 state O−C−O bending mode. Predissociation leads to bending excitation in the CO2 products as observed in the product translational energy distribution. Gating the coincidence spectra by electron kinetic energy for each predissociated vibrational mode in the radical provides a direct measure of the CO2 bending vibration (ν2) product state distribution, revealing substantially more excitation than ab initio quantum-dynamics calculations predict.

In-Vivo Neutron Activation Analysis for Aluminium in Bone: System Upgrade and Improve Data Analysis

An existing NaI(Tl) multidetector array at McMaster University used for in vivo measurement of aluminum by means of neutron activation has been recently upgraded with an in-house built pulse processing system. The new system is capable of collecting spectra in coincidence and anticoincidence modes which allows for better discrimination between the 1.78 MeV28Al peak and the interfering38Cl peak. The system is interfaced with Matlab and controlled using a custom graphical user interface, which provides periodic spectral data storage during the acquisition. This feature makes it possible to validate the acquired data and the spectral fitting routines through half-life analysis of the radionuclides. A new set of calibration phantoms spanning a narrower range of aluminum concentrations than employed in the previous study, has also been developed. The combined effect of the new improvements resulted in the minimum detectable level of aluminum in aqueous solutions containing Na, Cl, and Ca in physiological levels, equal to approximately 3 µgAl/gCa.

Variations of gamma-ray background in the Belgrade shallow underground low-level laboratory

During the last three years we investigated the variations of background simultaneously in two laboratories, the ground level (GLL) and the underground laboratory. The Forbush-like effect from March 2010 was observed in the GLL using a Ge detector and plastic veto scintillator. The underground plastic scintillator saw the same effect but the coincident veto spectrum did not detect the decrease of cosmic-ray intensity. Using a time series analysis of prominent post-radon lines, a significant radon daily variability was detected in the Ge detector background spectrum, but only in the GLL.

Momentum imaging spectrometer for molecular fragmentation dynamics induced by pulsed electron beam

A momentum imaging spectrometer has been built for studying the electron impact molecular fragmentation dynamics. The setup consists of a pulsed electron gun and a time of flight system as well as a two-dimensional time and position sensitive multi-hit detector. The charged fragments with kinetic energy up to 10 eV can be detected in 4π solid angles and their three-dimensional momentum vectors can be reconstructed. The apparatus is tested by electron impact ionization of Ar and dissociative ionization of CO2. By analyzing the ion-ion coincidence spectra, the complete and incomplete Coulomb fragmentation channels for CO2(2+) and CO2(3+) are identified. The kinetic energy release (KER) and angular correlation for the two-body breakup channel CO2(2+*) → O(+) + CO(+) are reported. The peak value of total KER is found to be 6.8 eV which is consistent with the previous photoion-photoion coincidence studies, and the correlation angle of O(+) and CO(+) is also explicitly determined to be 172.5°.

Secondary-electron emission induced byin vacuosurface excitations near a polycrystalline Al surface

The double-differential spectrum of coincidences between backscattered electrons and secondary electrons (SEs) emitted from a polycrystalline Al surface bombarded with 100-eV electrons was measured. For energy losses of the scattered electron in between the work function of Al and the bulk plasmon energy, a sharp peak is observed in the SE spectra, corresponding to ejection of a single electron near the Fermi edge receiving the full energy loss and momentum of the primary electron. This process predominantly takes place when the primary electron suffers a surface energy loss in vacuum, and leads to SE ejection from the very surface. At energy losses just above the bulk plasmon energy, a sharp transition is observed, corresponding to a sudden increase in the depth of ejection. The latter is a direct consequence of the complementarity of surface and bulk plasmons, the so-called Begrenzungs effect.

Photodissociation mechanisms of the CO22+ dication studied using multi-state multiconfiguration second-order perturbation theory

Employing the multi-state multiconfiguration second-order perturbation theory (MS-CASPT2) and complete active space self-consistent field (CASSCF) methods, the geometries, relative energies (T(v)') to the ground state (X(3)Σg(-)), adiabatic excited energies, and photodissociation mechanisms and corresponding kinetic energy releases for the lower-lying 14 electronic states of the CO2 (2+) ion are studied. The T(v)' values are calculated at the experimental geometry of the ground state CO2 molecule using MS-CASPT2 method and highly close to the latest threshold photoelectrons coincidence and time-of-flight photoelectron photoelectron coincidence spectrum observations. The O-loss dissociation potential energy curves (PECs) for these 14 states are drawn using MS-CASPT2 partial optimization method at C(∞v) symmetry with one C-O bond length ranging from 1.05 to 8.0 Å. Those 14 states are confirmed to be correlated to the lowest four dissociation limits [CO(+)(X(2)Σ(+)) + O(+)((4)S(u)), CO(+)(A(2)Π) + O(+)((4)S(u)), CO(+)(X(2)Σ(+)) + O(+)((2)D(u)), and CO(+)(X(2)Σ(+)) + O(+)((2)P(u))] by analyzing Coulomb interaction energies, charges, spin densities, and bond lengths for the geometries at the C-O bond length of 8.0 Å. On the basis of these 14 MS-CASPT2 PECs, several state/state pairs are selected to optimize the minimum energy crossing points (MECPs) at the CASSCF level. And then the CASSCF spin-orbit couplings and CASPT2 state/state energies are calculated at these located MECPs. Based on all of the computational results, the photodissociation mechanisms of CO2(2+) are proposed. The relationships between the present theoretical studies and the previous experiments are discussed.

Measurements of alpha–gamma coincidences with an optimized dual-parameter multichannel system

Measurements of alpha–gamma coincidences have usually been carried out using a single channel to detect alpha-particles of a given energy, and a multichannel analyser for the detection of the corresponding coincident gamma-rays. An alpha–gamma coincidence chamber coupled to the electronic chain ending with a dual-parameter multichannel analyser has been developed and optimized. This system simultaneously stores alpha-particle, gamma-ray, and alpha–gamma coincidence spectra, which allows a general analysis to be made of the degree of coincidence between each alpha-particle and each gamma-ray emission. With this technique, a two-dimensional spectrum was obtained and analysed using “contour graphics”. An application to the study of the decay scheme of 241Am is described.

Electronic Properties of Fluorosulfonyl Isocyanate, FSO2NCO: A Photoelectron Spectroscopy and Synchrotron Photoionization Study

The electronic properties of fluorosulfonyl isocyanate, FSO2NCO, were investigated by means of photoelectron spectroscopy and synchrotron based techniques. The first ionization potential occurs at 12.3 eV and was attributed to the ejection of electrons formally located at the π NCO molecular orbital (MO), with a contribution from nonbonding orbitals at the oxygen atoms of the SO2 group. The proposed interpretation of the photoelectron spectrum is consistent with related molecules reported previously and also with the prediction of OVGF (outer valence green function) and P3 (partial third order) calculations. The energy of the inner- and core-shell electrons was determined using X-ray absorption, measuring the total ion yield spectra, and the resonances before each ionization threshold were interpreted in terms of transitions to vacant molecular orbitals. The ionic fragmentation mechanisms in the valence energy region were studied using time-of-flight mass spectrometry as a function of the energy of the incident radiation. At 13 eV the M(+) was the only ion detected in the photoion-photoelectron-coincidence spectrum, while the FSO2(+) fragment, formed through the breaking of the S-N single bond, appears as the most intense fragment for energies higher than 15 eV. The photoion-photoion-photoelectron-coincidence spectra, taken at the inner- and core-levels energy regions, revealed several different fragmentation pathways, being the most important ones secondary decay after deferred charge separation mechanisms leading to the formation of the O(+)/S(+) and C(+)/O(+) pairs.