Thermal Cross Section Research Articles

그래핀을 적용한 고출력 반도체 광원의 열특성 분석

The heat flow characteristics of a high-power optical semiconductor source have been analyzed using a 3D CFD commercial tool, and the thermal resistance values for each of the layers revealed the places for thermal bottlenecks to be improved. As the heat source of a LD (Laser Diode) has a small volume and a narrow surface, the effective thermal cross-sectional area near it is also quite small. It was possible to expand the cross-sectional area effectively by using graphene layers on the TIM (Thermal Interface Material) layers of a LD chip. The effective values of heat resistance for the layers are compared to confirm the improvement effect of the graphene layers before and after, which can be considered to expand the thermal cross section of the heat transfer path. Keywords: LD, Thermal resistance, Graphene, TIM, Heat flow OCIS codes: (230.0230) Optical devices; (230.3670) Light-emitting diodes; (140.3320) Laser cooling

Particle Dark Matter Searches Outside the Local Group.

If dark matter (DM) is composed by particles which are nongravitationally coupled to ordinary matter, their annihilations or decays in cosmic structures can result in detectable radiation. We show that the most powerful technique to detect a particle DM signal outside the Local Group is to study the angular cross-correlation of nongravitational signals with low-redshift gravitational probes. This method allows us to enhance the signal to noise from the regions of the Universe where the DM-induced emission is preferentially generated. We demonstrate the power of this approach by focusing on GeV-TeV DM and on the recent cross-correlation analysis between the 2MASS galaxy catalogue and the Fermi-LAT γ-ray maps. We show that this technique is more sensitive than other extragalactic γ-ray probes, such as the energy spectrum and angular autocorrelation of the extragalactic background, and emission from clusters of galaxies. Intriguingly, we find that the measured cross-correlation can be well fitted by a DM component, with a thermal annihilation cross section and mass between 10 and 100GeV, depending on the small-scale DM properties and γ-ray production mechanism. This solicits further data collection and dedicated analyses.

Direct Detection Phenomenology in Models Where the Products of Dark Matter Annihilation Interact with Nuclei.

We investigate the direct detection phenomenology of a class of dark matter (DM) models in which DM does not directly interact with nuclei, but rather, the products of its annihilation do. When these annihilation products are very light compared to the DM mass, the scattering in direct detection experiments is controlled by relativistic kinematics. This results in a distinctive recoil spectrum, a nonstandard and/or even absent annual modulation, and the ability to probe DM masses as low as a ∼10 MeV. We use current LUX data to show that experimental sensitivity to thermal relic annihilation cross sections has already been reached in a class of models. Moreover, the compatibility of dark matter direct detection experiments can be compared directly in Emin space without making assumptions about DM astrophysics, mass, or scattering form factors. Lastly, when DM has direct couplings to nuclei, the limit from annihilation to relativistic particles in the Sun can be stronger than that of conventional nonrelativistic direct detection by more than 3 orders of magnitude for masses in a 2-7 GeV window.

Validation of 241Am Capture Cross Section through Integral Test Using Criticality Data of Light-water Moderated MOX Cores

Abstract In the development of JENDL-4.0, the thermal and resonance capture cross section of 241 Am was increased by about 8-9% to reduce the overestimation of criticality observed in the MOX-Fueled light-water-moderated cores using 241 Am-rich plutonium-fuel.The revised data gives the better results of criticality prediction for those cores than that with JENDL-3.3. The Pu ageing problem was also investigated using the revised data. This problem is that the calculated k-effs for critical MOX cores with different experiment dates showed the tendency of increase with time. The change of k-eff in seven years was about 0.2%dk/k with JENDL-3.3 and it was reduced to about 0.1%dk/k with JENDL-4.0. These results show that the revision of 241 Am capture cross section seemed to be successful, although there still remains the tendency of k-eff increase with time. On the other hand, the recent measurement of thermal and resonance capture cross section of 241 Am indicates that the data of JENDL-3.3 should be increased by about 18%, that is about 8% greater than that of JENDL-4.0. In the present study, the possibility of further revision of 241 Am capture cross section was studied through the integral test for Pu ageing problem by reflecting the recent experimental data. Three criticality data of a light-water moderated MOX core with different experiment date (7 years interval at maximum) have been analyzed with JENDL-3.3 and 4.0 libraries. In addition, the calculations with the number density of 241 Am multiplied by 1.18 for JENDL-3.3 and by 1.08 for JENDL-4.0 were also conducted to simulate the increase of 241 Am capture cross section suggested by the recent measurement. The results show that the increase tendency of k-eff as a function of time observed in the calculations with JENDL-4.0 is not found in those with modified 241 Am number density. This means that the improvement of 241 Am capture cross section will be achieved by employing the recent experimental data.

CMB constraints on the thermal WIMP mass and annihilation cross section

A thermal relic, often referred to as a weakly interacting massive particle (WIMP), is a particle produced during the early evolution of the Universe whose present (relic) abundance depends only on its mass and its thermally averaged annihilation cross section (annihilation rate factor) $⟨\ensuremath{\sigma}v{⟩}_{\text{ann}}$. Late time WIMP annihilation has the potential to affect the cosmic microwave background (CMB) power spectrum. Current observational constraints on the absence of such effects provide bounds on the mass and the annihilation cross section of relic particles that may be, but need not be, dark matter candidates. For a WIMP that is a dark matter candidate, the CMB constraint sets an upper bound to the annihilation cross section, leading to a lower bound to its mass that depends on whether or not the WIMP is its own antiparticle. For a self-conjugate WIMP, ${m}_{\mathrm{min}}=50f\text{ }\text{ }\mathrm{GeV}$, where $f\ensuremath{\le}1$ is an electromagnetic energy efficiency factor. For a non--self-conjugate WIMP, the minimum mass is a factor of two larger. For a WIMP that is a subdominant component of the dark matter density there is no bound on its mass and the upper bound to its annihilation cross section imposed by the CMB transforms into a lower bound to its annihilation cross section. These results are outlined and quantified here using the latest CMB constraints for a stable, symmetric (equal number of particles and antiparticles), WIMP whose annihilation is s-wave dominated, and for particles that are, or are not, their own antiparticle.

Validation of JEFF-3.1.1 Thermal and Epithermal Neutron-Induced Capture Cross Sections through MELUSINE Experiment Analysis

Three similar experiments performed in the 8-MW(thermal) MELUSINE experimental power pool reactor aimed at analyzing 1 GWd/tonne HM spent fuel pellets doped with several actinides and fission products. The goal was to measure the energy-integrated neutron-induced capture cross section in three different neutron spectra (from pressurized water reactor–like to undermoderated ones). This paper summarizes the combined deterministic APOLLO2 and stochastic TRIPOLI4 analysis using the JEFF-3.1.1 European nuclear data library. Very good agreement is observed for most neutron-induced capture cross sections of actinides except for a clear underestimation of 241Am(n,γ). An accurate value of its associated isomeric branching ratio is also suggested. A resonant fluctuation (factor of 2.7 between the two available excited levels regarding the l = 0 total orbital momenta) is suggested for this isomeric branching capture ratio. Finally, a precise value (more accurate than the reported JEFF-3.1.1 one) of the decay branching ratio of 242gAm(β+/ε) is deduced: 0.171 ± 0.001.

Thermal neutron scattering kernels for sapphire and silicon single crystals

Sapphire and silicon are materials usually employed as filters in facilities with thermal neutron beams. Due to the lack of the corresponding thermal cross section libraries for those materials, necessary in calculations performed in order to optimize beams for specific applications, here we present the generation of new thermal neutron scattering kernels for those materials. The Debye model was used in both cases to represent the vibrational frequency spectra required to feed the NJOY nuclear data processing system in order to produce the corresponding libraries in ENDF and ACE format. These libraries were validated with available experimental data, some from the literature and others obtained at the pulsed neutron source at Centro Atómico Bariloche.

A Novel Orange-Red Emitting ZnB4O7:Eu3+Phosphor with Urchin-Like Nanostructure

A novel phosphor, ZnB4O7:Eu3+, with urchin-like structure consisting of radially arranged high density nanorods was successfully synthesized by hydrothermal process at 150°C for 24 h. The nanorods were measured from 200 to 400 nm in diameter and severalµm in length. The urchins were fewµm to 40 µm in diameter. The ZnB4O7:Eu3+phosphors were efficiently excited by ultraviolet (UV ~ 254 nm) to visible light of ~ 220 to 450 nm and exhibited intense orange-red emission consisting of main peaks at 590, 615, and 695 nm due to the charge transfer in the host andf→ftransitions (5D0to7F1,2,4) of the Eu3+ions. Effect of the Eu3+ions concentration on the photoluminescence (PL) emission intensity was investigated and it was found that 5 at% Eu3+is the optimum concentration. Meanwhile, the concentration quenching mechanism was discussed. The key parameters, such as temperature dependent PL and CIE values of ZnB4O7:Eu3+phosphors, were studied. The ZnB4O7:Eu3+phosphor exhibited good thermal stability and better absorption cross section compared to the commercial Y2O2S:Eu3+phosphor. All these characteristics indicate that the phosphor will be a potential candidate for the UV based white LEDs.

Neutronic feasibility study of U–Th–Pa based high burnup fuel for pebble bed reactors

Neutronic feasibility study of U–Th–Pa based high burnup fuel for the pebble bed reactors has been carried out. An additional content of P231a in a mixed U233–T232h fuel pebble was optimized for simultaneously controlling long-term excess reactivity and attaining high fuel burnup. Because of a large thermal absorption cross section, P231a has a potential to absorb thermal neutrons and act as a burnable absorber in the early stage of burnup. In the latter stage, P231a is transmuted to U233 and act as a fertile fuel. Parametric study was performed for heavy metal (HM) amount, U233 enrichment and P231a content. The objective is to attain a target burnup greater than 100 GWd/T with a low uranium enrichment and a small content of P231a. Six fuel compositions with the HM amount of 9–18 g per pebble, the U233 enrichment of 8 and 10%, and the P231a content of 3.2–5.5% were selected for discussion and comparison. The burnup levels of about 130–170 GWd/t are achieved for the six fuel compositions. The results of temperature coefficient show that negative reactivity feedback during burnup with the increase of temperature is assured, which is one of the important safety parameters.

Dynamical collisional energy loss and transport properties of on- and off-shell heavy quarks in vacuum and in the quark gluon plasma

In this study we evaluate the dynamical collisional energy loss of heavy quarks, their interaction rate, as well as the different transport coefficients (drag and diffusion coefficients, $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{q}$, etc). We calculate these different quantities for (i) perturbative partons (on-shell particles in the vacuum with fixed and running coupling) and (ii) for dynamical quasi-particles (off-shell particles in the QGP medium at finite temperature $T$ with a running coupling in temperature as described by the dynamical quasiparticles model). We use the perturbative elastic $[q(g)Q\ensuremath{\rightarrow}q(g)Q]$ cross section for the first case and the infrared enhanced hard thermal loop cross sections for the second. The results obtained in this work demonstrate the effects of a finite parton mass and width on the heavy-quark transport properties and provide the basic ingredients for an explicit study of the microscopic dynamics of heavy flavors in the QGP--as formed in relativistic heavy-ion collisions--within transport approaches developed previously by the authors.

Fitting the Galactic Center gamma-ray excess with cascade annihilations

The apparent excess of gamma rays in an extended region in the direction of the Galactic Center has a spatial distribution and amplitude that are suggestive of dark matter annihilations. If this excess is indeed due to dark matter annihilations, it would indicate the presence of both dark matter and an additional particle beyond the Standard Model that mediates the interactions between the dark matter and Standard Model states. We introduce reference models describing dark matter annihilation to pairs of these new mediators, which decouples the SM-mediator coupling from the thermal annihilation cross section and easily explains the lack of direct detection signals. We determine the parameter regions that give good descriptions of the gamma-ray excess for several motivated choices of mediator couplings to the SM. We find fermion dark matter with mass 7--26 GeV and a dark vector mediator, or scalar dark matter in the 10--50 GeV range (Higgs portal mediator) or 10--65 GeV range (gluophilic mediator) can provide a comparable or improved fit, compared to the case of direct annihilation. We demonstrate that these models can easily satisfy all constraints from collider experiments, direct detection, and cosmology.

Neutron capture cross sections of 194Hg and the decays of 195Hg

The thermal cross section and resonance integral have been determined for radiative neutron capture by radioactive 194Hg by observing the γ rays emitted in the decay of 195Hg. Captures leading to the low-spin isomer of 195Hg give σ=877b and I=10,270b, while captures leading to the high-spin isomer give σ=49b and I=608b. Energies and intensities of the γ rays emitted in the decays of 195Hg produced by neutron capture have been obtained with increased precision relative to previous studies with sources produced by other reactions, leading to correspondingly improved values for β-decay branching intensities and energy levels in 195Au. New values for the half-lives of 195gHg and 195mHg are also reported.

Evaluation of Hydrocarbon Saturation Using Carbon Oxygen (CO) Ratio and Sigma Tool


 The main aim of this study is to evaluate the remaining oil in previously produced zones, locate the water productive zone and look for any bypassed oil behind casing in not previously perforated intervals. Initial water saturation was calculated from digitized open hole logs using a cut-off value of 10% for irreducible water saturation. The integrated analysis of the thermal capture cross section, Sigma and Carbon/oxygen ratio was conducted and summarized under well shut-in and flowing conditions. The logging pass zone run through sandstone Zubair formation at north Rumaila oil field. The zones where both the Sigma and the C/O analysis show high remaining oil saturation similar to the open hole oil saturation, could be good oil zones that do not appear to be water flooded. The zones where the Sigma analysis shows high residual oil saturation, which is close to open hole oil saturation and the C/O analysis, show medium residual oil saturation; this could indicate that these zones were fresh water flooded to a certain extent and they still keep some residual oil. If the C/O analysis shows low residual oil saturation, it indicates that these zones were probably fresh water flooded thoroughly. If both Sigma analysis and the C/O analysis show medium residual oil saturation, most probably these zones were saline water flooded to a certain extent and they still keep some residual oil.

Shallow donor and deep DX-like center in InAlN layers nearly lattice-matched to GaN

Nonintentionally doped 200-nm-thick ${\mathrm{In}}_{0.16}{\mathrm{Al}}_{0.84}\mathrm{N}/{n}^{+}\text{\ensuremath{-}}\mathrm{GaN}$ samples were grown by metal-organic vapor phase epitaxy and used for the electrical characterization of InAlN. In the temperature range 180--400 K, the forward current of Schottky diodes is dominated by a tunneling mechanism below 1.2 V. Capacitance and conductance-temperature characteristics were measured at 1 MHz in the 90--400 K range and at various voltages. The conductance vs temperature reveals two peaks ${\mathrm{D}}_{1}$ and ${\mathrm{D}}_{2}$, which are attributed to bulk states in InAlN. Their characterization by admittance spectroscopy gives thermal activation energies of $\ensuremath{\approx}68$ meV and 290 meV, and thermal capture cross section of $9.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ and $\ensuremath{\approx}6.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$, respectively. The same levels are also revealed by extracting the temperature dependence of the carrier density in the neutral region of InAlN from $I\text{\ensuremath{-}}V\text{\ensuremath{-}}T$ characteristics on the Schottky diode. A partial carrier freeze out is demonstrated and discussed in the framework of an existing theory for DX centers. The use of this approach is supported by the evidence of persistent photoconductivity effects, which strongly indicate the presence of DX centers in our material. It results that each donor in InAlN would exist in two distinct lattice configurations, a substitutional one (${\mathrm{D}}_{1}$, hydrogenic state) and a lattice-distorted one (${\mathrm{D}}_{2}$, DX state). From secondary ion mass spectrometry data, theoretical grounds, and previous experimental evidence in the $\mathrm{Al}{}_{x}\mathrm{Ga}{}_{1\ensuremath{-}x}\mathrm{N}$ system, oxygen is the most probable candidate for such an unintentional dopant.

Nonstationary holographic currents in neutron-irradiated SiC crystal

We report the excitation of nonstationary holographic currents in semi-insulating 6H-SiC crystal preliminarily irradiated by reactor neutrons. The currents are studied for the cases of zero and non-zero external electric fields. The dependences of the signal amplitude versus the frequency of phase modulation, light intensity, spatial frequency and amplitude of the external ac field are measured. The frequency transfer functions of the effect demonstrate unusual behaviour, namely, there are two frequency-independent regions in the diffusion regime of signal excitation and low-frequency maximum in the experiments with the applied ac field. These peculiarities are explained in the frames of a two-level model of semiconductor. Photoconductivity, diffusion length of electrons, thermal excitation rate and ionization cross section for shallow traps are estimated for the wavelength λ = 532 nm.

Neutron capture by 94,96Zr and the decays of 97Zr and 97Nb

Cross sections for radiative neutron capture have been determined for 94Zr and 96Zr using the activation technique with samples of naturally occurring Zr metal. The sensitivity to the correction for epithermal neutrons in the determination of small thermal cross sections is discussed, particularly in view of the variation in the resonance integral at different sites in the reactor. Gamma-ray spectroscopic studies of the decays of 97Zr and its daughter 97Nb have been performed, leading to improved values of the energies and intensities of the emitted γ rays, and correspondingly improved values for the energy levels and β feedings of excited states populated in 97Nb and 97Mo.

SEARCHING FOR DARK MATTER ANNIHILATION IN THE SMITH HIGH-VELOCITY CLOUD

Recent observations suggest that some high-velocity clouds may be confined by massive dark matter halos. In particular, the proximity and proposed dark matter content of the Smith Cloud make it a tempting target for the indirect detection of dark matter annihilation. We argue that the Smith Cloud may be a better target than some Milky Way dwarf spheroidal satellite galaxies and use gamma-ray observations from the Fermi Large Area Telescope to search for a dark matter annihilation signal. No significant gamma-ray excess is found coincident with the Smith Cloud, and we set strong limits on the dark matter annihilation cross section assuming a spatially-extended dark matter profile consistent with dynamical modeling of the Smith Cloud. Notably, these limits exclude the canonical thermal relic cross section ($\sim 3\times10^{-26}{\rm cm}^{3}{\rm s}^{-1}$) for dark matter masses $\lesssim 30$ GeV annihilating via the $b \bar b$ or $\tau^{+}\tau^{-}$ channels for certain assumptions of the dark matter density profile; however, uncertainties in the dark matter content of the Smith Cloud may significantly weaken these constraints.

Current dark matter annihilation constraints from CMB and low-redshift data

Updated constraints on the dark matter cross section and mass are presented combining cosmic microwave background (CMB) power spectrum measurements from Planck, WMAP9, ACT, and SPT as well as several low-redshift data sets (BAO, HST, and supernovae). For the CMB data sets, we combine WMAP9 temperature and polarization data for $l\ensuremath{\le}431$ with Planck temperature data for $432\ensuremath{\le}l\ensuremath{\le}2500$, ACT and SPT data for $l>2500$, and Planck CMB four-point lensing measurements. We allow for redshift-dependent energy deposition from dark matter annihilation by using a ``universal" energy absorption curve. We also include an updated treatment of the excitation, heating, and ionization energy fractions and provide an updated deposition efficiency factors (${f}_{\text{eff}}$) for 41 different dark matter models. Assuming perfect energy deposition (${f}_{\text{eff}}=1$) and a thermal cross section, dark matter masses below 26 GeV are excluded at the $2\ensuremath{\sigma}$ level. Assuming a more generic efficiency of ${f}_{\text{eff}}=0.2$, thermal dark matter masses below 5 GeV are disfavored at the $2\ensuremath{\sigma}$ level. These limits are a factor of $\ensuremath{\sim}2$ improvement over those from WMAP9 data alone. These current constraints probe, but do not exclude, dark matter as an explanation for reported anomalous indirect detection observations from AMS-02/PAMELA and the Fermi gamma-ray inner-Galaxy data. They also probe relevant models that would explain anomalous direct detection events from CDMS, CRESST, CoGeNT, and DAMA, as originating from a generic thermal weakly interacting massive particle. Projected constraints from the full Planck release should improve the current limits by another factor of $\ensuremath{\sim}2$ but will not definitely probe these signals. The proposed CMB Stage IV experiment will more decisively explore the relevant regions and improve upon the Planck constraints by another factor of $\ensuremath{\sim}2$.

Measurements of Thermal Neutron Capture Cross Sections of 136Ce, 156Dy, and 168Yb

For several low abundance stable nuclei, the thermal neutron capture cross sections are not well measured, while the cross sections for isotopes with high abundances are already well measured. Our experiments, different from the commonly used method of using gold foil as reference, are performed using natural foils for which we know the relative abundances of all isotopes and thermal neutron capture cross sections. Therefore, we can obtain the cross sections of low abundance isotopes, which are not known well, by comparing the yields of gammas from the neutron captures by various isotopes in the foils. The advantage of this method is the cancellation of potential systematic errors from thermal neutron flux, flux profile, foil thickness, foil size, and irradiation time. We have measured the thermal capture cross sections of 136Ce, 156Dy, and 168Yb isotopes, using the high thermal neutron flux from the reactor HANARO at KAERI, and have obtained new cross section values of 7.64±0.63 barn for 136Ce, 14.8±2.0 barn for 156Dy, and 1335±43 barn for 168Yb.

Measurement and Analysis of 241Am(n,γ) Cross Sections with C6D6 Detectors at the n_TOF Facility at CERN

The 241Am(n,γ) cross sections have been measured at the n_TOF facility at CERN using C6D6 liquid scintillators and time of flight spectrometry. The results in the resolved resonance range bring new constraints to evaluations below 150 eV, and the energy upper limit can be extended from 150 eV to 320 eV. The analysis goes from thermal energy to 150 keV, and the unresolved resonance range cross section turns out to be larger than expected by evaluations or otherwise measured by previous works. The thermal cross section is found to be σth = 740 ± 74 barns, which is larger than expected by evaluations and most previous measurements.