Large Angular Region Research Articles

Limits on dark matter annihilation in prompt cusps from the isotropic gamma-ray background

Recent studies indicate that thermally produced dark matter will form highly concentrated, low-mass cusps in the early universe that often survive until the present. While these cusps contain a small fraction of the dark matter, their high density significantly increases the expected γ-ray flux from dark matter annihilation, particularly in searches of large angular regions. We utilize 14 years of Fermi-LAT data to set strong constraints on dark matter annihilation through a detailed study of the isotropic γ-ray background, excluding with 95% confidence dark matter annihilation to bb¯ final states for dark matter masses below 120 GeV. Published by the American Physical Society 2024

Parametric Modeling of Radiation Patterns and Scattering Parameters of Antennas

This paper describes a data-driven method to model the radiation patterns (over a large angular region) and scattering parameters of antennas as a function of the geometry of the antenna. The radiation pattern model consists of a linear combination of characteristic basis function patterns (CBFPs), where the expansion coefficients of the CBFPs are the functions of geometrical features of the antenna. Scattering parameters are modeled by means of parameterized state-space matrices. The obtained models are quick to evaluate and are thus suitable for design activities where multiple simulations are required. The proposed method is validated through illustrative examples.

Fractal Reflectarray Antennas: State of Art and New Opportunities

Fractal geometries are appealing in all applications where miniaturization capabilities are required, ranging from antennas to frequency selective surfaces (FSS) design. Recently, some fractal patches configurations, giving low losses, reduced size, and quite good phase ranges, have been proposed for the design of reflectarray unit cells. This paper reviews existing fractal-based reflectarrays, highlighting their benefits and limitations. Furthermore, a comprehensive analysis of an innovative reflectarray unit cell, using a fractal-shaped fixed-size patch, is presented. The miniaturization capabilities of the Minkowski fractal shape are fully exploited to obtain a compact cell offering quite good phase agility, by leaving unchanged the patch size and acting only on the fractal scaling factor. Experimental validations are fully discussed on a realized 10 GHz0.3λ×0.3λcell. This is subsequently adopted to synthesize various reflectarray prototypes offering single or multiple-beam capabilities over a quite large angular region (up to 50 degrees). Finally, experimental validations on a realized15×15elements prototype are presented to demonstrate the wide angle beam-pointing capabilities as well as a quite large bandwidth of about 6%.

Near-range point spread function coarray analysis for array imaging

This paper presents an analysis of the near-range imaging performance of an array in terms of the point spread function using a spatially varying coarray formulation. The results demonstrate that the spatially varying coarray formulation allows the prediction of near-range imaging performance over large angular regions.

Radar cross-section reduction for a microstrip patch antenna using PIN diodes

The authors investigate in this study using PIN attenuator diodes to reduce the radar cross-section (RCS) of a microstrip patch antenna. A simple design guideline and a useful loading scheme are developed. In the investigation, PIN diodes are equivalent to capacitances and resistances when diodes are reverse-biased and forward-biased, respectively. According to the magnitude distribution of the induced electric field under the patch for a specific incidence angle, the loading positions of PIN diodes are selected near to where the maximum electric field appears. First, three typical incident angles are separately considered. As a result, RCS reduction for the microstrip patch antenna is realised by utilising the variable resistance characteristic (energy dissipation) of PIN attenuator diodes. Secondly, the loading positions for that three incident angles are combined to realise RCS reduction over a large angular spatial region and within a wideband frequency range, and thus an electronically controllable low RCS status can be successfully realised. The proposed design guideline is demonstrated. It is worth mentioning that, by real-time adjusting the bias status of each PIN diode, the antenna realised gain as well as the relative bandwidth is well maintained when the antenna is at work.

StressTextureCalculator: a software tool to extract texture, strain and microstructure information from area-detector measurements

Modern materials science diffractometers are generally equipped with area detectors that allow a high time efficiency to be achieved by simultaneously collecting the scattering pattern over large angular regions. These area-detector-based instruments, however, produce a huge amount of data, especially if they are located at large-scale neutron or synchrotron sources. The softwareStressTextureCalculator(STeCa) was designed to facilitate fast, easy and automated access to such area-detector data. Its outstanding features are direct calculation of diffraction patterns from different types of area-detector measurements, automatic data treatment and peak fitting using several implemented fit options. The resulting information on intensity, peak shift and broadening can then be exported into several data formats. These in turn can be used as input for a wide range of texture, stress and microstructure analysis software packages without additional prior treatment.

The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer

Aims. The brown dwarf (BD) formation process has not yet been completely understood. To shed more light on the differences and similarities between star and BD formation processes, we study and compare the disk fraction among both kinds of objects over a large angular region in the Taurus cloud. In addition, we examine the spatial distribution of stars and BD relative to the underlying molecular gas Methods. In this paper, we present new and updated photometry data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this region. We also useed recently available CO mm data to study the spatial distribution of stars and BDs relative to the cloud's molecular gas. Results. We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a value statistically consistent with the one among TTS (58 \pm 9%). We find that BDs in transition from a state where they have a disk to a diskless state are rare, and we study one isolated example of a transitional disk with an inner radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR excess compared to most members of Taurus that have disks. We find that BDs are statistically found in regions of similar molecular gas surface density to those associated with stars. Furthermore, we find that the gas column density distribution is almost identical for stellar and substellar objects with and without disks.

Fast Iterative Method of Power Synthesis for Antenna Arrays

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A fast and simple method of power synthesis for antenna arrays of arbitrary geometry is presented. The method adopts an auxiliary function having the desired amplitude pattern and a generic phase pattern, and determines in closed form the array pattern approximating the auxiliary function in both amplitude and phase. A cost function involving the auxiliary function and the approximating pattern is minimized by modifying the phase pattern of the auxiliary function, using a simple and fast iterative technique. Once the optimal phase pattern has been found, the array pattern approximating the corresponding auxiliary function is the synthesized pattern. The proposed method allows to satisfy stringent requirements on the amplitude pattern, including multibeam synthesis and radiation suppression within large angular regions. Numerical results show the effectiveness of the algorithm and are compared to those obtained using other existing methods. In order to achieve a more significant comparison such methods have been properly modified to improve their performances. Such comparison shows that the presented algorithm allows a significant reduction of CPU time with respect to the other considered methods, as well as a weaker dependence on the starting point. Finally, the presented method is extended in such a way as to reduce the dynamic range ratio (DRR) of the array excitations. </para>

Calibration of millimeter-wave polarimeters using a thin dielectric sheet

We present the theory and application of a novel calibration system for millimeter and microwave polarimeters. The technique is a simple extension of the conventional wire-grid approach, but employs a thin dielectric sheet rather than a grid. The primary advantage of this approach is to obtain a calibration signal that is only slightly polarized, which can be beneficial for certain applications such as astronomical radiometers that measure very low levels of polarization, or systems with a small dynamic range. We compare this approach with other calibration techniques and discuss its successful use in the calibration of the polarization observations of large angular regions experiment, designed to measure polarization in cosmic microwave background radiation.

A Limit on the Large Angular Scale Polarization of the Cosmic Microwave Background

We present an upper limit on the polarization of the cosmic microwave background (CMB) at 7 angular scales in the frequency band between 26 and 36 GHz, produced by the Polarization Observations of Large Angular Regions experiment. The campaign produced a map of linear polarization over the right ascension range 112–275 at declination 43. The model-independent upper limit on the E-mode polarization component of the CMB at angular scales is 10 mK (95% confidence). The corresponding limit for the B-mode is also 10 mK. l p 2–20 Constraining the B-mode power to be zero, the 95% confidence limit on E-mode power alone is 8 mK. Subject headings: cosmic microwave background — cosmology: observations — polarization

Large Angular Scale Polarization of the Cosmic Microwave Background Radiation and the Feasibility of Its Detection

In addition to its spectrum and temperature anisotropy, the 2.7 K cosmic microwave background (CMB) is also expected to exhibit a low level of polarization. The spatial power spectrum of the polarization can provide details about the formation of structure in the universe as well as its ionization history. Here we calculate the magnitude of the CMB polarization in various cosmological scenarios, with both an analytic and a numerical method. We then outline the fundamental challenges to measuring these signals and focus on two of them: achieving adequate sensitivity and removing contamination due to foreground sources. We describe the design of a ground-based instrument (Polarization Observations of Large Angular Regions) that could detect polarization of the CMB at large angular scales in the next few years.

Polarization observables in pi + 6Li scattering: Analyzing powers iT11 and tau 22.

The vector analyzing power ${\mathit{iT}}_{11}$ and the tensor analyzing power ${\mathrm{\ensuremath{\tau}}}_{22}$=${\mathit{T}}_{22}$+${\mathit{T}}_{20}$/ \ensuremath{\surd}6 have been measured for ${\mathrm{\ensuremath{\pi}}}^{+}$${\ensuremath{\rightarrow}}^{6}$Li\ensuremath{\rightarrow} elastic and inelastic scattering (to the ${3}^{+}$0, 2.19 MeV state) at incident pion energies of 120, 150, 180, and 194 MeV. These data and earlier ones from our group are compared with new coupled channels calculations. Within the coupled channels model, the influence of different nuclear wave functions, the strength of the coupling of the ground and first excited states, and the effects of the phenomenological second-order potential were investigated. It was found that the calculated vector analyzing power ${\mathit{iT}}_{11}$ depends strongly on the transverse form factor whichever nuclear wave function is chosen. In the angular region where the ${\mathit{iT}}_{11}$ data were taken the coupling of the ground to the first excited state has a minor effect on the theoretical predictions. Reasonable agreement with the data can be obtained for a large energy and angular region with the first-order potential only.

“Antiblazing” of gratings

A general property of diffraction gratings is established numerically: at a certain values of the groovedepth a grating supporting only two orders acts like a plane mirror—the efficiency of the non-specular order does not exceed 0.1% in a large angular region. This property is shown to be a direct consequence of the reciprocity theorem.

General radiation-pattern synthesis technique for array antennas of arbitrary configuration and element type

An interactive iterative-synthesis technique is presented, which is of particular use for far-field patterns defined in terms of power only. The method is applied in general to three-dimensional array configurations composed of different element types, with the required far-field defined over large angular regions. Suggestions for extending the method to near-field, simultaneous multiparameter or constrained synthesis are also given. The description of the method is presented for three different conditions, depending on whether both the array amplitude and the phase or just the amplitude or the phase are variable. Examples of the method, applied to various practical synthesis problems, and comparisons to other techniques are also given.

Measurement of the pp analyzing power Aoono in a large angular region between 0.88 and 2.7 GeV

The pp analyzing power was measured using the SATURNE II polarized proton beam and the Saclay frozen spin polarized target. The measurements at 0.88 and 1.1 GeV were carried out in the angular region θCM from 28° to ≅50° and complete our previous measurements from 45 ° to 90°. Above 1.1 GeV the measurements presented here cover both regions, extending from θCM = 28° (at the lower energies) or θCM = 18° (at the higher energies) to θCM > 90°. The shape of the angular distribution Aoono(pp) = ƒ(θCM) changes considerably with increasing energy. The new data show the onset of a characteristic t-dependence of the analyzing power, with a minimum at −t ≅ 1.0 (GeV/c)2 followed by a second maximum at −t ≅ 1.5 (GeV/c)2. This structure is present at all energies, from kinematic threshold to 200 GeV.

Scattering Function of Self-Avoiding Chain (n=0 Vector Model); Calculation and Experiments

Using a new method to study a self-avoiding walk problem developed by de Gennes and des Cloizeaux, scattering functions in small and large arigular regions are calculated with an assumption that a subchain in a single chain and a full chain with same steps have a same probability density for correlation. They are compared with experimental results. The result obtained in polystyrene (P.S.)-M.E.K. solution which has a small excess free energy does agree well with the function in small angular region, but the result obtained in P.S.-benzene solution which is thermodynamically good solution does not. On the other hand, the scattering function in large angular region has a dependence of k -3.3 , which is similar to a dependence of k -4 of two phase model in a porous granule, although the results published so far do not agree with this dependence.

Spin Flip in the Inelastic Scattering of Protons

Proton spin-flip probabilities and differential inelastic scattering cross sections were measured over a large angular region for the following ${2}^{+}$ excitations: 4.44 MeV in $^{12}\mathrm{C}$ at 12, 13, 14, 15, and 20 MeV; 1.45 MeV in $^{58}\mathrm{Ni}$ at 9.25, 10.46, 15, and 20 MeV; 1.33 MeV in $^{60}\mathrm{Ni}$ at 10.5 and 14 MeV; and 1.34 MeV in $^{64}\mathrm{Ni}$ at 10.5 and 14 MeV. The results were analyzed in the distorted-wave Born approximation, with collective-model form factors derived from the optical-model potential. The deformed spin-dependent part of the coupling potential was of the full Thomas form, and the data are best described when ${{\ensuremath{\beta}}_{2}}^{\mathrm{SO}}g{\ensuremath{\beta}}_{2}$. Good fits are obtained for elastic polarization data (obtained elsewhere) when the depth of the spin-orbit potential is determined from spin-flip probability measurements.

Inelastic Scattering of 40-MeV Polarized Protons

The left-right asymmetry and differential cross section for the inelastic scattering of 40-MeV polarized protons were measured over a large angular region for each of the following ${2}^{+}$ excitations: 1.78 MeV in $^{28}\mathrm{Si}$, 1.41 and 2.97 MeV in $^{54}\mathrm{Fe}$, 1.45 MeV in $^{58}\mathrm{Ni}$, and 1.33 MeV in $^{60}\mathrm{Ni}$. Some asymmetry and cross-section data were also obtained for ${3}^{\ensuremath{-}}$ states: 6.9 MeV in $^{28}\mathrm{Si}$, 4.8 and 6.4 MeV in $^{54}\mathrm{Fe}$, 4.5 MeV in $^{58}\mathrm{Ni}$, and 4.08 MeV in $^{60}\mathrm{Ni}$. The inelastic scattering was analyzed in the distorted-wave approximation, using the collective-model extension of the optical-model potential determined by fitting elastic polarization and cross-section measurements for each target. The inelastic asymmetry and cross-section data are best reproduced with a collective-model interaction obtained by deforming the complete optical potential, including its imaginary and spin-orbit parts.

Small scattering chamber for large angular region

A description is given of a small vacuum chamber for scattering of charged particles on nuclei at low and intermediate energies (< 30 MeV). The chamber has an angular range of 0° to 160° and can be used without foils in front of the counters.