Low-energy Tail Research Articles

Single atom catalysts on amorphous supports: A quenched disorder perspective.

Phenomenological models that invoke catalyst sites with different adsorption constants and rate constants are well-established, but computational and experimental methods are just beginning to provide atomically resolved details about amorphous surfaces and their active sites. This letter develops a statistical transformation from the quenched disorder distribution of site structures to the distribution of activation energies for sites on amorphous supports. We show that the overall kinetics are highly sensitive to the precise nature of the low energy tail in the activation energy distribution. Our analysis motivates further development of systematic methods to identify and understand the most reactive members of the active site distribution.

Absorption spectra and aqueous photochemistry of β-hydroxyalkyl nitrates of atmospheric interest

Molar absorption coefficients were measured for select alkyl nitrates and β-hydroxyalkyl nitrates in methanol. The presence of the β-hydroxyl group has a relatively minor effect on the absorption spectrum in the vicinity of the weak n → π* transition, which is responsible for photolysis of organic nitrates in the atmosphere. For both alkyl nitrates and β-hydroxyalkyl nitrates, there is an enhancement in the absorption coefficients in solution compared to the gas-phase values. The effect of the β-hydroxyl group on the spectra was modelled with molecular dynamics simulations using an OM2/GUGA-CI Hamiltonian for ethyl nitrate and β-hydroxyethyl nitrate. The simulation provided a qualitatively correct shape of the low energy tail of the absorption spectrum, which is important for atmospheric photochemistry. The role of direct aqueous photolysis in removal of β-hydroxyalkyl nitrates in cloud and fog water was modelled using a relative rate approach, and shown to be insignificant relative to gas-phase photochemical processes and aqueous OH oxidation under typical atmospheric conditions.

Monolithic Multi-Element HPGe Detector Equipped With CMOS Preamplifiers: Construction and Characterization of a Demonstrator

Multi-element germanium detectors for X-ray fluorescence are widely used in synchrotron experiments and in particular in XAFS experiments. This paper presents the construction and characterization of a demonstrator built to investigate the viability of multi-element monolithic germanium detectors equipped with CMOS front-end electronics. Semikon Detector GmbH fabricated a germanium sensor segmented with a pad pattern with pad size 1×1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . 16 channels were instrumented with the CUBE preamplifiers developed at XGLab Srl. The detector was tested with radioactive sources and with the synchrotron X-ray beam of Diamond. The results showed spectra with energy resolution satisfactory for XAFS experiments but with a considerable tail. The low energy tail was proved to be due to charge sharing. In addition the spectra showed the peak stability as a function of counting rate better than 1% for rate up to 838 kcps. The non-linearity of the peak position vs. energy was estimated to be a maximum of 0.13% No evidence of charge loss in the crystal was identified. This work proved that this technology is a viable option to improve the throughput of germanium fluorescence detectors as long as methods to reduce events leading to charge sharing are in place.

Optical studies of non‐polar m‐plane () InGaN/GaN multi‐quantum wells grown on freestanding bulk GaN

We report on the optical properties of non‐polar m‐plane InGaN/GaN multi‐quantum wells (MQWs) grown on ammonothermal bulk GaN substrates. The low temperature continuous wave (CW) photoluminescence spectra are broad with a characteristic low energy tail. The majority of the emission bands decay with a time constant ~300 ps, but detailed photoluminescence time decay and time resolved spectroscopy measurements revealed the existence of a distinct slowly decaying emission band. This slowly decaying component is responsible for the low energy tails observed in the CW spectra. Scanning electron microscopy–cathodo­luminescence (SEM‐CL) studies show that the low energy emission band originates from regions across step‐bunches, which are associated to the GaN substrate miscut. Subsequent scanning transmission electron microscopy imaging demonstrates that semi‐polar QWs had formed continuous layers on the step bunches between the m‐plane QWs and were responsible for the slower decaying, low energy emission band. Thus we assign the asymmetric low energy emission tails observed in photoluminescence studies to the formation of semi‐polar facet QWs across the step bunches associated with the GaN miscut.

Optical and electronic properties of undoped La_2Be_2O_5 single crystals in the far ultraviolet energy range

The optical and electronic properties of undoped La2Be2O5 single crystals were determined using low-temperature (T=10&#x2009;&#x2009;K) far ultraviolet (4.9&#x2013;33&#xA0;eV) synchrotron radiation spectroscopy, optical absorption spectroscopy (T=80 and 320&#xA0;K), and calculations for the dispersions of the optical functions. On the basis of the obtained data we determined the Urbach&#x2019;s formula parameters and the cut-off energy Ec=5.49&#x2009;&#x2009;eV at 80&#xA0;K for the low-energy tail of the host absorption, the bandgap Eg=6.78&#x2009;&#x2009;eV at 10&#xA0;K, the energy threshold for the creation of unrelaxed excitons Eex=6.28&#x2009;&#x2009;eV, and other electronic properties.

Achieving the photon up-conversion thermodynamic yield upper limit by sensitized triplet-triplet annihilation.

Triplet-triplet annihilation (TTA) based up-conversion is a promising strategy for light harvesting the low-energy tail of the solar spectrum with photovoltaic technologies. Here, we present a bi-component system for photon managing via TTA that allows bypassing the classical statistic limit of 2/5 in the singlet generation, achieving a near unitary conversion efficiency. This result is obtained because of the peculiar relative position of the triplet and singlet energy levels of perylene, used as up-converter and emitter. The system shows a record red-to-blue external up-conversion yield of ∼10% under an irradiance of 1 sun.

Observation of low-lying resonances in the quasicontinuum of195,196Pt and enhanced astrophysical reaction rates

An excess of strength on the low-energy tail of the giant dipole resonance recently has been observed in the γ-decay from the quasicontinuum of 195,196 Pt. The nature of this phenomenon is not yet fully investigated. If this feature is present also in the γ-ray strength of the neutron-rich isotopes, it can affect the neutron-capture reactions involved in the formation of heavy-elements in stellar nucleosynthesis. The experimental level density and γ-ray strength function of 195,196 Pt are presented together with preliminary calculations of the corresponding neutron-capture cross sections.

High resolution alpha particle spectrometry through collimation

Alpha particle spectrometry with collimation is a useful method for identifying nuclear materials among various nuclides. A mesh type collimator reduces the low energy tail and broadened energy distribution by cutting off particles with a low incidence angle. The relation between the resolution and the counting efficiency can be investigated by changing a ratio of the mesh hole diameter and the collimator thickness. Through collimation, a target particle can be distinguished by a PIPS® detector under a mixture of various nuclides.

Optical and luminescence characterization of LiBaAlF6 single crystals

Far ultraviolet optical, luminescence excitation and pulse cathodoluminescence spectroscopy methods were used to determine the electronic structure properties of undoped single crystals of LiBaAlF6 (LBAF). The spectrum of the refraction index in the ultraviolet–visible range was recorded. The photoluminescence excitation spectroscopy was carried out for both the excitonic (4.3eV) and fast vacuum ultraviolet (6.6eV) emission bands of LBAF single crystals at 10K. Modeling of the photoluminescence excitation spectra for excitonic emission was performed to characterize the low-energy tail of the host absorption of LBAF single crystals. The pulse cathodoluminescence spectroscopy method was used to characterize the fast vacuum ultraviolet (6.6eV) emission band and proof its origination from the radiative core-valence transitions F− 2p→ Ba2+ 5p.

Strengthened electric field technique implemented on CZT detectors

This paper presents the development of a simple electrode structure which only requires a simple readout and is suitable for a large cube CZT crystal, such as a 10×10×10mm3 crystal. A technique named the strengthened electric field (SEF) is investigated in detail and implemented to improve the performance of the detector. Signal processing was also studied to demonstrate its feasibility to further improve the detector’s performance. A SEF line anode (SEFLA) prototype and an SEF point anode (SEFPA) prototype were designed, fabricated and tested. Experimental results demonstrated the effectiveness of the SEF technique. The SEFLA detector achieved an energy resolution of 1.6% (FWHM)@662keV with 4.0keV noise (FWHM) and SEFPA 1.8% with 5.0keV noise. Cathode signal is used to do both the rejection and the correction in the SEFLA prototype. At the cost of detection efficiency, the low energy tail is reduced, while the energy resolution and the P/C ratio are further improved. Possible improvements of the detectors are discussed.

Pygmy resonance and low-energy enhancement in theγ-ray strength functions of Pd isotopes

Background: An unexpected enhancement in the $\ensuremath{\gamma}$-ray strength function, as compared to the low-energy tail of the giant dipole resonance (GDR), has been observed for Sc, Ti, V, Fe, and Mo isotopes for ${E}_{\ensuremath{\gamma}}l4$ MeV. This enhancement was not observed in subsequent analyses on Sn isotopes, but a pygmy dipole resonance (PDR) centered at ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}8$ MeV was however detected. The $\ensuremath{\gamma}$-ray strength functions measured for Cd isotopes exhibit both features over the range of isotopes, with the low-energy enhancement decreasing and PDR strength increasing as a function of neutron number. This suggests a transitional region for the onset of low-energy enhancement, and also that the PDR strength depends on the number of neutrons.Purpose: The $\ensuremath{\gamma}$-ray strength functions of $^{105--108}\mathrm{Pd}$ have been measured in order to further explore the proposed transitional region.Method: Experimental data were obtained at the Oslo Cyclotron Laboratory by using the charged particle reactions $(^{3}\mathrm{He},\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{He}{}^{\ensuremath{'}}\ensuremath{\gamma})$ and ($^{3}\mathrm{He},\phantom{\rule{0.16em}{0ex}}\ensuremath{\alpha}\ensuremath{\gamma}$) on $^{106,108}\mathrm{Pd}$ target foils. Particle-$\ensuremath{\gamma}$ coincidence measurements provided information on initial excitation energies and the corresponding $\ensuremath{\gamma}$-ray spectra, which were used to extract the level densities and $\ensuremath{\gamma}$-ray strength functions according to the Oslo method.Results: The $\ensuremath{\gamma}$-ray strength functions indicate a sudden increase in magnitude for ${E}_{\ensuremath{\gamma}}g4$ MeV, which is interpreted as a PDR centered at ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}8$ MeV. An enhanced $\ensuremath{\gamma}$-ray strength at low energies is also observed for $^{105}\mathrm{Pd}$, which is the lightest isotope measured in this work.Conclusions: A PDR is clearly identified in the $\ensuremath{\gamma}$-ray strength functions of $^{105--108}\mathrm{Pd}$, and a low-energy enhancement is observed for $^{105}\mathrm{Pd}$. Further, the results correspond and agree very well with the observations from the Cd isotopes, and support the suggested transitional region for the onset of low-energy enhancement with decreasing mass number. The neutron number dependency of the PDR strength is also evident.

Photonuclear reactions of calcium isotopes calculated with the nuclear shell model

Photoabsorption cross sections in calcium isotopes are investigated theoretically with the most advanced nuclear-structure calculations using the nuclear shell model. We show that the photoabsorption cross sections in 48Ca dominated by a giant dipole resonance (GDR) are well reproduced with the shell model and predict that a pygmy dipole resonance (PDR) appears in 52Ca in the low-energy tail of GDR. Excellent agreement with the experiment is achieved due to highly correlated many-body wave functions obtained by treating a huge matrix with the dimension of ∼1010.

Cosmic ray induced ionisation of a molecular cloud shocked by the W28 supernova remnant

Cosmic rays are an essential ingredient in the evolution of the interstellar medium, as they dominate the ionisation of the dense molecular gas, where stars and planets form. However, since they are efficiently scattered by the galactic magnetic fields, many questions remain open, such as where exactly they are accelerated, what is their original energy spectrum, and how they propagate into molecular clouds. In this work we present new observations and discuss in detail a method that allows us to measure the cosmic ray ionisation rate towards the molecular clouds close to the W28 supernova remnant. To perform these measurements, we use CO, HCO$^+$, and DCO$^+$ millimetre line observations and compare them with the predictions of radiative transfer and chemical models away from thermodynamical equilibrium. The CO observations allow us to constrain the density, temperature, and column density towards each observed position, while the DCO$^+$/HCO$^+$ abundance ratios provide us with constraints on the electron fraction and, consequently, on the cosmic ray ionisation rate. Towards positions located close to the supernova remnant, we find cosmic ray ionisation rates much larger ($\gtrsim100$) than those in standard galactic clouds. Conversely, towards one position situated at a larger distance, we derive a standard cosmic ray ionisation rate. Overall, these observations support the hypothesis that the $\gamma$ rays observed in the region have a hadronic origin. In addition, based on CR diffusion estimates, we find that the ionisation of the gas is likely due to $0.1 -1$ GeV cosmic rays. Finally, these observations are also in agreement with the global picture of cosmic ray diffusion, in which the low-energy tail of the cosmic ray population diffuses at smaller distances than the high-energy counterpart.

SU-E-T-605: A New Design for a Rotating Gamma Knife. Monte Carlo Simulation

Purpose: to determine the characteristics of the 60Co beam emerging from a new design of a rotating Gamma Knife system and to calculate 3D dose distributions at the isocenter for different source configurations and collimator openings. Methods: We employed the BEAM-Monte Carlo code to realistically model the geometry design, including 30 60Co source capsules, two circular primary collimators (diameter of 6.6mm and 6.1mm) and four different changeable collimators. The shielding of the head design was also simulated. The sources (2.8mm diameter) are distributed in six groups in the spherical geometry. Each source is individually collimated to obtained four different circular fields at the isocenter (3mm, 3.5mm, 6mm and 8mm).The phase-space particles reaching the scoring plane below the primary collimation assembly were recorded and the BEAMDP code was used to determine the fluence and energy spectra of the particles emerging from each source-collimator configuration. The dose distributions at the isocenter plane (397.6mm from the source) were calculated in a spherical component module for the circular field sizes studied. Results: The energy spectra below the head assembly and primary collimators have been obtained, which exhibited the typical 60Co peaks and a small low-energy tail due to scattered photons (from about 200keV to 1MeV). The scattered component of the spectra represents about 8 % of the total number of photons reaching the scoring plane. The radial photon fluence does not vary significantly inside the collimator openings. The spectra of particles from different source groups are compared. Conclusion: The 60Co beam emerging from each source configuration was characterized, which can be used to establish a generic source model for all the sources for fast MC dose calculation. Further investigations are needed to determine the dose variations as a result of partial switching on/off different groups of sources for advanced Gamma Knife SRS/SBRT planning.

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.

Investigation of charge-collection efficiency of Kyoto׳s X-ray astronomical SOI pixel sensors, XRPIX

We are developing a monolithic active pixel sensor referred to as XRPIX for X-ray astronomy on the basis of silicon-on-insulator CMOS technology. A crucial issue in our recent development is the impact of incomplete charge collection on the spectroscopic performance. In this paper, we report the spectral responses of several devices having different intra-pixel structures or produced from different wafers. We found that an emission line spectrum exhibits large low-energy tails when the size of the buried p-well, which acts as the charge-collection node, is small. Moreover, in charge sharing events, the peak channels of the emission lines shift toward channels lower than those without charge sharing. This peak shift is more pronounced as the distance between the pixel center and the position of incident photon increases. This suggests that the charge-collection efficiency is degraded at the pixel boundary. We also found that the charge-collection efficiency depends on the strength of the electric field at the interface of the depletion and insulator layers.

Light-ion Production in 175 MeV Neutron-induced Reactions on Oxygen

We have measured double-differential cross sections for 175 MeV quasi-monoenergetic neutrons on oxygen. The detector setup used in MEDLEY consists of eight Si-Si-CsI telescopes designed to detect light ions (up to A=4), with a low-energy threshold and over an angular domain ranging from 20∘ to 160∘, in steps of 20∘. The ΔE−E technique is used to identify the light ions. Suppression of events induced by neutrons in the low-energy tail of the neutron field is achieved by time-of-flight techniques. The data are normalised relative to elastic np scattering measured in one of the telescopes at 20∘. We present preliminary double-differential production cross sections for protons, deuterons and α particles and compare them with theoretical reaction model calculations.

Experimental Neutron-induced Fission Fragment Mass Yields of 232Th and 238U at Energies from 10 to 33 Me

Development of nuclear energy applications requires data for neutron-induced reactions for actinides in a wide neutron energy range. Here we describe measurements of pre-neutron emission fission fragment mass yields of 232Th and 238U at incident neutron energies from 10 to 33 MeV. The measurements were done at the quasi-monoenergetic neutron beam of the Louvain-la-Neuve cyclotron facility CYCLONE; a multi-section twin Frisch-gridded ionization chamber was used to detect fission fragments. For the peak neutron energies at 33, 45 and 60 MeV, the details of the data analysis and the experimental results were published in Ref. [I.V. Ryzhov, S.G. Yavshits, G.A. Tutin et al., Phys. Rev. C 83, 054603 (2011)]. In this work we present data analysis in the low-energy tail of the neutron energy spectra. The preliminary measurement results are compared with available experimental data and theoretical predictions.

Synthesis and characterization of emissive mononuclear Cu(I) complexes with 5-tert-butyl-3-(pyrimidine-2-yl)-1H-1,2,4-triazole

A new series of mononuclear copper(I) halide complexes possessing 5-tert-butyl-3-(pyrimidine-2-yl)-1H-1,2,4-triazole (bpmtzH) and PPh3, Cu(bpmtzH)(PPh3)X (X = I (1); Br (2); Cl (3)), have been synthesized and characterized. As revealed via X-ray crystallography, 1–3 show a chiral, distorted tetrahedral N2PX arrangement, in which bpmtzH is a neutral bidentate chelating ligand using the 4-N of the 1,2,4-triazolyl ring and one N donor from the 2-pyrimidyl ring, consistent with the computational results. A comparatively weak low-energy absorption tail is observed between 320 and 450 nm for CH2Cl2 solutions of 1–3 at room temperature, ascribing to charge-transfer transitions with appreciable metal-to-ligand charge transfer (MLCT) character. Complexes 1–3 in the solid state have good luminescence at ambient temperature, although they are non-emissive in solution. The solid-state emission, most likely originating from both 3MLCT and 3XLCT transitions, can be modulated via alteration of the halide bound to {Cu(bpmtzH)(PPh3)} motif.

Effective energy barrier distributions for random and aligned magnetic nanoparticles

Isothermal magnetic relaxation measurements are widely used to probe energy barriers in systems of magnetic nanoparticles. Here we show that the result of such an experiment can differ greatly for aligned and randomly oriented nanoparticles. For randomly oriented cobalt-doped magnetite nanoparticles we observe a prominent low-energy tail in the energy barrier distribution that is greatly attenuated when the particles are magnetically aligned. Monte Carlo simulations show that this behaviour arises for nanoparticles with both cubic and uniaxial magnetic anisotropy energy terms even though for cubic or uniaxial anisotropy alone the energy barrier distribution is independent of nanoparticle orientation.