Function Of Excitation Energy Research Articles

Enhanced γ-Ray Emission from Neutron Unbound States Populated in β Decay.

Total absorption spectroscopy is used to investigate the β-decay intensity to states above the neutron separation energy followed by γ-ray emission in (87,88)Br and (94)Rb. Accurate results are obtained thanks to a careful control of systematic errors. An unexpectedly large γ intensity is observed in all three cases extending well beyond the excitation energy region where neutron penetration is hindered by low neutron energy. The γ branching as a function of excitation energy is compared to Hauser-Feshbach model calculations. For (87)Br and (88)Br the γ branching reaches 57% and 20%, respectively, and could be explained as a nuclear structure effect. Some of the states populated in the daughter can only decay through the emission of a large orbital angular momentum neutron with a strongly reduced barrier penetrability. In the case of neutron-rich (94)Rb the observed 4.5% branching is much larger than the calculations performed with standard nuclear statistical model parameters, even after proper correction for fluctuation effects on individual transition widths. The difference can be reconciled by introducing an enhancement of 1 order of magnitude in the photon strength to neutron strength ratio. An increase in the photon strength function of such magnitude for very neutron-rich nuclei, if it proves to be correct, leads to a similar increase in the (n,γ) cross section that would have an impact on r process abundance calculations.

Density and temperature in heavy-ion collisions: A test of classical and quantum approaches

Different methods to extract the temperature and density in heavy ion collisions are compared using a statistical model tailored to reproduce many experimental features at low excitation energy. The model assumes a sequential decay of an excited nucleus and a Fermi gas entropy. We first generate statistical events as function of excitation energy but stopping the decay chain at the first step. In such a condition the 'exact' model temperature is determined from the Fermi gas relation to the excitation energy. From these events, using quantum and classical fluctuation methods for protons and neutrons, we derive temperature and density (quantum case only) of the system under consideration. Additionally, the same quantities are also extracted using the double ratio method for different particle combinations. A very good agreement between the "exact" temperatures and quantum fluctuation temperatures is obtained, the role of the density is discussed. Classical methods give a reasonable estimate of the temperature when the density is very low as expected. The effects of secondary decays of the excited fragments are discussed as well.

Shell-gap-reduced level densities inY89,90

Particle-$\gamma$ coincidences from the $^{89}$Y($p,p' \gamma$)$^{89}$Y and $^{89}$Y($d,p \gamma$)$^{90}$Y reactions were utilized to obtain $\gamma$-ray spectra as function of excitation energy. The Oslo method was used to extract the level density from the particle-$\gamma$ coincidence matrices. The impact of the $N=50$ shell closure on the level densities is discussed within the framework of a combinatorial quasi-particle model.

Electromagnetic excitation of nuclei and neutron evaporation in ultrarelativistic ultraperipheral heavy ion collisions

We present a new approach for calculating electromagnetic excitation of nuclei as well as probabilities of emission and distributions of neutrons from decays of excited nuclear systems for ultrarelativistic, ultraperipheral heavy ion collisions. Excitation functions for $\gamma$ + Pb $\to$ Pb$^*$ are parametrized using physics-motivated components: excitation of giant resonances, quasi-deuteron absorption mechanism, excitation of nucleon resonances as well as high-energy dissociation of protons and neutrons. Neutron emission (up to 10 neutrons) from low-energy excitations of $^{208}$Pb is modelled in terms of the Hauser-Feshbach formalism. The probabilities of a given number of neutrons are calculated as a function of excitation energy in a Monte-Carlo code GEMINI. These functions are parametrized by smooth analytical functions. The results are compared to appropriate data for the $\gamma$ Pb $\to$ Pb$^*$ $\to$ k n reaction. As an example the approach is used for calculating electromagnetic excitation in UPC processes. Both single photon and double photon excitations are included and discussed. Topological cross sections with a given number of neutrons in forward and backward directions are calculated in the calculation at the LHC energies. Excitation functions are presented. The results of calculation are compared with SPS and more recent ALICE experimental data and good agreement is achieved.

Sensitivity of Measured Fission Yields on Prompt-neutron Corrections

Although the number of emitted prompt neutrons from the fission fragments increases as a function of excitation energy, it is not fully understood whether the increase in ν¯ (A) as a function of En is mass dependent. The share of excitation energies among the fragments is still under debate, but there are reasons to believe that the excess in neutron emission originates only from the heavy fragments, leaving ν¯light (A) almost unchanged. We have investigated the consequences of a mass-dependent increase in ν¯ (A) on the final mass and energy distributions. The analysis have been performed on experimentally measured data on 234U (n, f). The assumptions concerning ν¯ (A) are essential when analysing measurements based on the 2E-technique, and impact significantly on the measured observables. For example, the post-neutron emission mass yield distribution revealed changes up to 10–30 %. The outcome of this work pinpoints the urgent need to determine ν¯ (A) experimentally, and in particular, how ν¯ (A) changes as a function of incident neutron energy. Many fission yields in the data libraries could be largely affected, since their analysis is based on a different assumption concerning the neutron emission.

The key role of geminate electron-hole pair recombination in the delayed fluorescence in rhodamine 6G and ATTO-532.

In this paper we investigate the delayed fluorescence (DF) phenomena in the widely used laser dye, rhodamine 6G, and its derivative ATTO-532 as a function of excitation energy using highly sensitive time-resolved gated nanosecond spectroscopy. Excitation with UV laser radiation results in delayed emission, which arises from singlet states created from geminate pair recombination, not triplet annihilation. For the first time the origins and photo-physical properties of delayed fluorescence in these highly fluorescent molecules are elucidated.

Relaxation dynamics of photoexcited resorcinol: internal conversion versus H atom tunnelling

The excited state dynamics of resorcinol (1,3-dihydroxybenzene) following UV excitation at a range of pump wavelengths, 278 ≥ λ ≥ 255 nm, have been investigated using a combination of time-resolved velocity map ion imaging and ultrafast time-resolved ion yield measurements coupled with complementary ab initio calculations. After excitation to the 1(1)ππ* state we extract a timescale, τ1, for excited state relaxation that decreases as a function of excitation energy from 2.70 ns to ~120 ps. This is assigned to competing relaxation mechanisms. Tunnelling beneath the 1(1)ππ*/(1)πσ* conical intersection, followed by coupling onto the dissociative (1)πσ* state, yields H atoms born with high kinetic energy (~5000 cm(-1)). This mechanism is in competition with an internal conversion process that is able to transfer population from the photoexcited 1(1)ππ* state back to a vibrationally excited ground state, S0*. When exciting between 264-260 nm a second decay component, τ2, is observed and we put forth several possible explanations as to the origins of τ2, including conformer specific dynamics. Excitation with 237 nm light (above the 1(1)ππ*/(1)πσ* conical intersection) yields high kinetic energy H atoms (~11,000 cm(-1)) produced in ~260 fs, in line with a mechanism involving ultrafast coupling between the 1(1)ππ* (or 2(1)ππ*) and (1)πσ* state followed by dissociation. The results presented highlight the profound effect the presence of additional functional groups, and more specifically the precise location of the functional groups, can have on the excited state dynamics of model heteroaromatic systems following UV excitation.

Hot Hole Transfer Increasing Polaron Yields in Hybrid Conjugated Polymer/PbS Blends

We use quasi-steady-state photoinduced absorption (PIA) to study charge generation in blends of poly(3-hexylthiophene-2,5-diyl) (P3HT) with PbS nanocrystal quantum dots as a function of excitation energy. We find that, per photon absorbed, the yield of photogenerated holes present on the conjugated polymer increases with pump energy, even at wavelengths where only the quantum dots absorb. We interpret this result as direct evidence for transfer of hot holes in these conjugated polymer/quantum dot blends. These results help understand the operation of hybrid organic/inorganic photovoltaics.

Photochemical Charge Separation in Poly(3-hexylthiophene) (P3HT) Films Observed with Surface Photovoltage Spectroscopy

Surface photovoltage spectroscopy (SPS) was used to probe photon induced charge separation in thin films of regioregular and regiorandom poly(3-hexylthiophene) (P3HT) as a function of excitation energy. Both positive and negative photovoltage signals were observed under sub-band-gap ( 2.0 eV) excitation of the polymer. The dependence of the spectra on substrate work function, thermal annealing, film thickness, and illumination intensity was investigated, allowing the identification of interface, charge transfer (CT), and band-gap states in the amorphous and crystalline regions of the polymer films. The ability to probe these states in polymer films will aid the development and optimization of organic electronic devices such as photovoltaics (OPVs), light-emitting diodes (OLEDs), and field effect transistors (OFETs). The direction and size of the observed photovoltage features can be explained using the depleted semiconductor model.

A new fission-fragment detector to complement the CACTUS-SiRi setup at the Oslo Cyclotron Laboratory

An array of Parallel Plate Avalanche Counters (PPAC) for the detection of heavy ions has been developed. The new device, NIFF (Nuclear Instrument for Fission Fragments), consists of four individual detectors and covers 60% of 2π. It was designed to be used in conjunction with the SiRi array of ΔE−E silicon telescopes for light charged particles and fits into the CACTUS array of 28 large-volume NaI scintillation detectors at the Oslo Cyclotron Laboratory. The low-pressure gas-filled PPACs are sensitive for the detection of fission fragments, but are insensitive to scattered beam particles of light ions or light-ion ejectiles. The PPAC detectors of NIFF have good time resolution and can be used either to select or to veto fission events in in-beam experiments with light-ion beams and actinide targets. The powerful combination of SiRi, CACTUS, and NIFF provides new research opportunities for the study of nuclear structure and nuclear reactions in the actinide region. The new setup is particularly well suited to study the competition of fission and γ decay as a function of excitation energy.

Communication: Angular momentum alignment and fluorescence polarization of alkali atoms photodetached from helium nanodroplets

The theory of photofragments angular momentum polarization is applied to the photodetachment of an electronically excited alkali atom from a helium nanocluster (N = 200). The alignment of the electronic angular momentum of the bare excited alkali atoms produced is calculated quantum mechanically by solving the excited states coupled equations with potentials determined by density functional theory (DFT). Pronounced oscillations as a function of excitation energy are predicted for the case of Na@(He)200, in marked contrast with the absorption cross-section and angular distribution of the ejected atoms which are smooth functions of the energy. These oscillations are due to quantum interference between different coherently excited photodetachment pathways. Experimentally, these oscillations should be reflected in the fluorescence polarization and polarization-resolved photoelectron yield of the ejected atoms, which are proportional to the electronic angular momentum alignment. In addition, this result is much more general than the test case of NaHe200 studied here. It should be observable for larger droplets, for higher excited electronic states, and for other alkali as well as for alkali-earth atoms. Detection of these oscillations would show that the widely used pseudo-diatomic model can be valid beyond the prediction of absorption spectra and could help in interpreting parts of the dynamics, as already hinted by some experimental results on angular anisotropy of bare alkali fragments.

Information content of the low-energy electric dipole strength: Correlation analysis

We study the information content carried by the electric dipole strength with respect to isovector and isoscalar indicators characterizing bulk nuclear matter and finite nuclei. To separate isoscalar and isovector modes, and low-energy strength and giant resonances, we analyze the E1 strength as a function of excitation energy $E$ and momentum transfer $q$. We use the self-consistent nuclear density functional theory with Skyrme energy density functionals, augmented by the random phase, to compute the E1 strength, and covariance analysis to assess correlations between observables. Calculations are performed for spherical, doubly-magic nuclei $^{208}$Pb and $^{132}$Sn. We demonstrate that E1 transition densities in the low-energy region below the giant dipole resonance exhibit appreciable state dependence and multi-nodal structures, which are fingerprints of weak collectivity. The correlation between the accumulated low-energy strength and symmetry energy is weak, and dramatically depends on the energy cutoff assumed. On the other hand, a strong correlation is predicted between isovector indicators and the accumulated isovector strength at $E$ around 20 MeV and momentum transfer $q\sim 0.65$ fm$^{-1}$. Momentum- and coordinate-space pattern of the low-energy dipole modes indicate a strong fragmentation into individual particle-hole excitations. The global measure of low-energy dipole strength poorly correlates with the nuclear symmetry energy and other isovector characteristics. Consequently, our results do not support the suggestion that there exists a collective "pygmy dipole resonance," which is a strong indicator of nuclear isovector properties.

SPIDER: A new instrument for fission fragment research at the Los Alamos Neutron Science Center

The study of fission fragment yields and how they behave as a function of excitation energy provides insight into the process in which they are formed. Fission yields are also important for nuclear applications, as they can be used as a diagnostic tool. A new instrument, SPIDER (Spectrometer for Ion DEtermination in fission Research), is being developed for measuring fission yields as a function of incident neutron energy at the Los Alamos Neutron Science Center. The instrument employs a time-of-flight mass spectrometry method in which the velocity and kinetic energy of the fragments are measured in order to determine their mass. Additionally, by using Bragg peak spectroscopy, the charge of the fragments can be identified. A prototype instrument has been developed and preliminary results indicate that ∼ 1 mass unit resolution is feasible using this approach. A larger detector array is currently being designed, and will be used at study fission yields from thermal neutron energies up to at least 20 MeV.

High-spin structure in40K

High-spin states of $^{40}$K have been populated in the fusion-evaporation reaction $^{12}$C($^{30}$Si,np)$^{40}$K and studied by means of $\gamma$-ray spectroscopy techniques using one AGATA triple cluster detector, at INFN - Laboratori Nazionali di Legnaro. Several new states with excitation energy up to 8 MeV and spin up to $10^-$ have been discovered. These new states are discussed in terms of J=3 and T=0 neutron-proton hole pairs. Shell-model calculations in a large model space have shown a good agreement with the experimental data for most of the energy levels. The evolution of the structure of this nucleus is here studied as a function of excitation energy and angular momentum.

Exciton Dynamics in Disordered Poly(p-phenylenevinylene). 1. Ultrafast Interconversion and Dynamical Localization

The disordered Frenkel-Holstein model is introduced to investigate dynamical relaxation and localization of photoexcited states in conformationally disordered poly(p-phenylenevinylene). It is solved within the Ehrenfest approximation, in which the excited state is treated fully quantum mechanically, but the nuclear displacements are treated classically. The following are shown: (i) Lower energy local exciton ground states (LEGSs) adiabatically relax to vibrationally relaxed states (VRSs) in the time scale of one or two vibrational periods (ca. 40 fs). The relaxation of LEGSs is accompanied by localization and fluorescence depolarization, as the transition dipole moment reduces and rotates. The amount of dynamical localization increases as the torsional disorder decreases, causing an increase in the fluorescence depolarization. (ii) Higher energy quasi-extended exciton states (QEESs) interconvert to VRSs via three distinct episodes. A brief initial period of adiabatic relaxation is followed by the time-evolving eigenstate becoming a linear superposition of instantaneous eigenstates of the Frenkel-Holstein Hamiltonian. Typically, after a few hundred femtoseconds, one of the instantaneous eigenstates dominates the linear superposition, and the remaining dynamics is again adiabatic relaxation to a VRS. (iii) Very high energy QEESs, which are delocalized over many chromophores, sometimes exhibit a splitting of the wave function into more than one VRS. This self-localization onto more than one chromophore is assumed to be a failure of the Ehrenfest approximation, as this approximation neglects quantum mechanical coherences between the electronic and nuclear degrees of freedom. (iv) QEESs exhibit larger, but slower, fluorescence depolarization than LEGSs. Thus, ultrafast fluorescence depolarization is a function of excitation energy and conformational disorder.

Dissociation dynamics of the low-lying Rydberg states of Xe2: a velocity map imaging study

The photodissociation dynamics of Xe2 Rydberg states in the 76,900–80,000 cm−1 region have been studied using a combination of resonance enhanced multi-photon ionization and velocity map imaging. By monitoring photodissociation product channels as a function of excitation energy, several localized and long range perturbations of the excited states have been identified. In addition, several new band systems have been observed and assigned as the Xe2 5d 2[1/2]0° ← X , 5d 2[1/2]1° ← X , 5d 2[3/2]2° ← X , and 5d 2[7/2]3° ← X transitions.

Temperature dependence of the repulsive core potential in heavy-ion fusion reactions

In this work, we expand upon our previous study, the effect of hot nuclear matter on the calculation of the internuclear potential [Phys. Rev. C 84, 024612 (2011)], to the ${}^{28}\mathrm{Si}+{}^{40}\mathrm{Ca}$, ${}^{35}\mathrm{Cl}+{}^{48}\mathrm{Ti}$, and ${}^{40}\mathrm{Ar}+{}^{74}\mathrm{Ge}$ collision systems. For this purpose, we have employed the equation of state of hot nuclear matter, which is based on density-dependent Seyler-Blanchard formalism at finite temperature, to discuss the energy dependence of a repulsive core potential in the nucleon-nucleon interaction. The obtained values for strength of this potential show a linear dependence as a function of excitation energy ${E}^{*}$ (or temperature $T$) of the compound nucleus. Indeed, it is predictable that the effect of hot nuclear matter on heavy-ion reactions can be formulated by an additional energy-dependent function in the nucleon-nucleon potential.

Study of the Order-to-Chaos transition in 174W with the AGATA-Demonstrator

The transition between order and chaos is studied in the warm rotating nucleus 174W by γ-spectroscopy, focusing on the conservation of selection rules of the K quantum number with the excitation energy, where K is the projection of the total angular momentum on the symmetry axis. The 174W nucleus was populated by the fusion-evaporation reaction of 80Ti (at 217 MeV) on a 128Te backed target. The measurement was performed in July 2010 at Legnaro National Laboratories of INFN using the AGATA Demonstrator HPGe-array coupled to an array of 27 BaF2 scintillators, named Helena.The data analysis concentrates on γ-γ coincidence matrices selecting the γ-decay flow populating low-K and high-K structures. By a statistical fluctuation analysis the total number of low-K and high-K bands can be evaluated as a function of excitation energy. Comparisons with cranked shell model calculations at finite temperature are used to extract information on the onset of the chaotic regime as a function of excitation energy.

Coherent radial-breathing-like phonons in graphene nanoribbons

We have developed a microscopic theory for the generation and detection of coherent phonons in armchair and zigzag graphene nanoribbons using an extended tight-binding model for the electronic states and a valence force field model for the phonons. The coherent phonon amplitudes satisfy a driven oscillator equation with the driving term depending on photoexcited carrier density. We examine the coherent phonon radial-breathing-like mode amplitudes as a function of excitation energies and nanoribbon types. For photoexcitation near the optical absorption edge the coherent phonon driving term for the radial-breathing-like mode is much larger for zigzag nanoribbons where transitions between localized edge states provide the dominant contribution to the coherent phonon driving term. Using an effective mass theory, we explain how the armchair nanoribbon width changes in response to laser excitation.

New single crystal scintillators: CsCaCl3:Eu and CsCaI3:Eu

The current work describes two new Eu2+-activated scintillator materials, CsCaI3:Eu and CsCaCl3:Eu. Single crystals of CsCaI3:Eu and CsCaCl3:Eu were grown from the melt via the vertical Bridgman method. Scintillation properties, including decay time, light output, X-ray excited luminescence and light output, as a function of excitation energy were measured, as were the photoluminescence emission and excitation spectra. In addition, moisture sensitivity was evaluated by measuring the moisture uptake rate and compared with those of NaI:Tl and Cs2LiYCl6:Ce reference scintillators.