Γ-ray Anisotropy Research Articles

TANGRA – an experimental setup for basic and applied nuclear research by means of 14.1 MeV neutrons

For investigation of the basic characteristics of 14.1 MeV neutron induced nuclear reactions on a number of important isotopes for nuclear science and engineering, a new experimental setup TANGRA has been constructed at the Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research in Dubna. For testing its performance, the angular distribution of γ-rays (and neutrons) from the inelastic scattering of 14.1 MeV neutrons on high-purity carbon was measured and the angular anisotropy of γ-rays from the reaction 12 C(n, n′ γ)12 C was determined. This reaction is important from fundamental (differential cross-sections) and practical (non-destructive elemental analysis of materials containing carbon) point of view. The preliminary results for the anisotropy of the γ-ray emission from the inelastic scattering of 14.1- MeV neutrons on carbon are compared with already published literature data. A detailed data analysis for determining the correlations between inelastic scattered neutron and γ-ray emission will be published elsewhere.

Detecting particle dark matter signatures by cross-correlating γ-ray anisotropies with weak lensing

The underlying nature of dark matter still represents one of the fundamental questions in contemporary cosmology. Although observations well agree with its description in terms of a new fundamental particle, neither direct nor indirect signatures of its particle nature have been detected so far, despite a strong experimental effort. Similarly, particle accelerators have hitherto failed at producing dark matter particles in collider physics experiments. Here, we illustrate how the cross-correlation between anisotropies in the diffuse γ-ray background and weak gravitational lensing effects represents a novel promising way in the quest of detecting particle dark matter signatures.

Fermi-LAT γ-ray anisotropy and intensity explained by unresolved radio-loud active galactic nuclei

Radio-loud active galactic nuclei (AGN) are expected to contribute substantially to both the intensity and anisotropy of the isotropic γ-ray background (IGRB). In turn, the measured properties of the IGRB can be used to constrain the characteristics of proposed contributing source classes. We consider individual subclasses of radio-loud AGN, including low-, intermediate-, and high-synchrotron-peaked BL Lacertae objects, flat-spectrum radio quasars, and misaligned AGN. Using updated models of the γ-ray luminosity functions of these populations, we evaluate the energy-dependent contribution of each source class to the intensity and anisotropy of the IGRB. We find that collectively radio-loud AGN can account for the entirety of the IGRB intensity and anisotropy as measured by the Fermi Large Area Telescope (LAT). Misaligned AGN provide the bulk of the measured intensity but a negligible contribution to the anisotropy, while high-synchrotron-peaked BL Lacertae objects provide the dominant contribution to the anisotropy. In anticipation of upcoming measurements with the Fermi-LAT and the forthcoming Cherenkov Telescope Array, we predict the anisotropy in the broader energy range that will be accessible to future observations.

γ-ray anisotropies from dark matter in the Milky Way: the role of the radial distribution

The annihilation of dark matter particles in the halo of galaxies may end up into gamma rays, which travel almost unperturbed till to their detection at Earth. This annihilation signal can exhibit an anisotropic behavior quantified by the angular power spectrum, whose properties strongly depend on the dark matter distribution and its clumpiness. We use high resolution pure dark matter N-body simulations to quantify the contribution of different components (main halo and satellites) to the global signal as a function of the analytical profile adopted to describe the numerical results. We find that the smooth main halo dominates the angular power spectrum of the gamma-ray signal up to quite large multipoles, where the sub-haloes anisotropy signal starts to emerge, but the transition multipole strongly depends on the assumed radial profile. The extrapolation down to radii not resolved by current numerical simulations can affect both the normalization and the shape of the gamma-ray angular power spectrum. For the sub-haloes described by an asymptotically cored dark matter distribution, the angular power spectrum shows an overall smaller normalization and a flattening at high multipoles. Our results show the criticality of the dark matter density profile shape in gamma-ray anisotropy searches, and evaluate quantitatively the intrinsic errors occurring when extrapolating the dark matter radial profiles down to spatial scales not yet explored by numerical simulations.

A study of the induced magnetism in the Au spacer layer of Co/Au/CoO exchange-bias trilayers and related systems

We report the first observation of the effects of exchange bias on the nuclear spin polarization and induced magnetic moments at a magnetic/non-magnetic interface, applying low temperature nuclear orientation (LTNO) to Co/Au(x)/CoO trilayer systems. This technique allows us to determine simultaneously the average alignment of the nuclear moments for the two radioactive probe isotopes 198Au and 60Co with respect to an external magnetic field axis. The total average Au γ-ray anisotropy measured was found (i) to decrease with increasing Au thickness, indicating that large hyperfine fields are restricted to the interfacial Au layers and (ii) to be canted away from the applied field axis even when the Co layers are magnetically saturated. This canting was found to originate at the CoO/Au interface as could be shown from comparative measurements on CoO/Au/CoO trilayers containing two AFM CoO/Au interfaces and on a Co/Au/Co trilayer with two FM Co/Au interfaces. In the case of CoO/Au/CoO, the observed canting was found to be dependent on the Au layer thickness.

NMR/ON of 60Co Experiments on the Neutron Activated Fe-Co Alloy

The irradiation by thermal neutrons is a commonly used procedure for the preparation of radioactive samples for low-temperature nuclear orientation (LTNO) experiments. Such an irradiation is a potential source of irradiation damage. The ferromagnetic alloy Fe+0.5 Co was selected for this kind of radiation damage study by LTNO and the nuclear magnetic resonance of oriented nuclei (NMR/ON), respectively. It was found by LTNO that in an as-irradiated sample, smaller amount of 60Co nuclei is subject to regular hyperfine field Bhf=29.01 T in comparison with the annealed sample. The same LTNO single resonance line on the frequency f=165.6(2) MHz was observed with both as-irradiated and annealed samples, respectively. The resonant destruction of γ-ray anisotropy is significantly smaller in as-irradiated sample and higher rf field is to be used to obtain a sizable NMR/ON effect. The experimental data indicates a great spread of hyperfine field values in non-regular positions of 60Co nuclei without contribution to the NMR/ON.

Low-Temperature Nuclear Orientation of 144Pm in Metamagnetic (RE)NiAl4 Single Crystals

Magnetic properties of Pm ions in NdNiAl4 were investigated by low-temperature nuclear orientation of 144Pm. The observed γ-ray anisotropy as a function of external fields revealed a change in the magnetic structure of Pm ions at the metamagnetic phase transition of NdNiAl4. The extracted hyperfine field was 50(5) T which is substantially smaller than those previously reported.

Decay of the 67-μs Isomer in 142Pm

Excited states in 142Pm are investigated via the128Te(19F, 5n)142Pm reaction at beamenergies from 75 to 95 MeV by using techniques of in-beam γ-rayspectroscopy. Measurements of γ-ray excitation function, X–γ andγ–γ–t coincidences are performed with 10 BGO(AC)HPGe detectors.Decay of the previously known 67-μs isomer in 142Pm is observedand added into the new level scheme. This isomer is definitely placedat the excitation energy of 2828.5 keV. The spin of this isomer issuggested to be 13 ℏ according to the measured γ-ray anisotropies.

Multi-quasiparticle excitations in 145Tb

High-spin states in 145Tb have been investigated by means of in-beam γ-ray spectroscopy techniques with the 118Sn(32S, 1p4n) reaction. Excitation functions, X–γ–t and γ–γ–t coincidences and γ-ray anisotropies were measured. A level scheme of 145Tb was established up to Ex≈ 7 MeV. The level structure shows characteristics of a spherical nucleus. Based on the systematics of level structure in the odd-A N = 80 isotones, the level structure below 2 MeV excitation is interpreted by coupling an h11/2 proton to the excitations in the even–even 144Gd core. Above 2 MeV excitation, most of the yrast levels are interpreted with multi-quasiparticle shell-model configurations.

Rotational bands in 169Re

High-spin states in 169Re have been investigated by means of in-beam γ-ray spectroscopy techniques with the 144Sm(28Si, 1p2nγ)169Re reaction. X-γ and γ-γ-t coincidences, γ-ray anisotropies, and DCO ratios were measured. A strongly coupled band based on the 9/2-[514] Nilsson state and a decoupled band built on the h9/2 intruder proton orbital (nominally 1/2-[541]) have been established. The AB neutron crossings are observed at ω = 0.23 and 0.27 MeV for the 9/2-[514] and 1/2-[541] bands, respectively. The 9/2-[514] band in 169Re shows the largest signature splitting at low spin among the odd-mass Re isotopes. Band properties of the AB neutron crossing frequencies, alignment gains, and signature splittings are discussed, and compared with those in the heavier odd-A Re isotopes. Additionally, a three-quasipariticle band is observed, and the π9/2 -[514] ⊗ νAE configuration is proposed tentatively.

Shape co-existence and structural evolution of the yrast band in 157Yb

The high-spin states of 157Yb have been studied via the 144Sm(16O, 3n) reaction at 16O energy of 90 MeV using techniques of in-beam γ-ray spectroscopy. Measurement of γ-γ-t coincidences was performed with 11 BGO(AC)HPGe detectors. Based on the measured results of γ-γ coincidences, γ-ray anisotropies and DCO ratios, the level scheme for 157Yb was established. The shape co-existence and structural evolution of the νi 13/2 band with increasing angular momentum in 157Yb have been discussed. The systematics of the νi 13/2 bands in the N = 87 odd-A isotones have been compared.

ULTRA HIGH ENERGY COSMIC RAYS: PROPAGATION IN THE GALAXY AND ANISOTROPY

We considered propagation of Ultra High Energy Cosmic Rays (UHECR) through the galaxy. We investigated models with sources of UHECR distributed in the same way as Cold Dark Matter (CDM) in a self-consistent way, taking into account both extra-galactic and Galactic contributions. Using a very simple toy model of galactic magnetic field we showed that in the case of galactic origin of UHECRs the anisotropy can reach considerable values. In the case of extragalactic UHECRs origin, the anisotropy appears to be nonvanishing only for electron and photon components due to synchrotron losses, but it can hardly be reassured. The reason is an extremely low flux of UHE electrons and a too low level of γ-ray anisotropy.

In-beam study of 145Tb

The high-spin states of 145Tb have been studied in the 118Sn(32S, 1p4n) reaction at 32S energies from 161 to 175 MeV using techniques of in-beam γ-ray spectroscopy. Measurements of γ-ray excitation functions and γ-γ-t coincidences were performed with 12 BGO(AC)HPGe detectors. Based on the measured results of γ-γ coincidences, γ-ray anisotropies and DCO ratios, a level scheme for 145Tb was established for the first time. The observed excited states show typical irregular pattern in a spherical nucleus, and the low-lying levels have been interpreted qualitatively with a particle-core coupling.

Low-temperature nuclear orientation studies of metamagnetic (RE)NiAl 4 single crystals

Low-temperature nuclear orientation was applied to study hyperfine interactions of 142Pr, 147Nd and 160Tb nuclei in antiferromagnetic Pr 0.7Nd 0.3NiAl 4 and Pr 0.6Nd 0.3Tb 0.1NiAl 4 single crystals. The γ-ray anisotropy reveals the magnetic anisotropies of each rare earth ion, individually. External field effects on the 147Nd γ-ray anisotropy exhibit an unexpected result associated with the metamagnetic phase transition.

Magnetic Hyperfine Fields of 151PmFe and 147NdNi

Low-temperature nuclear orientation (LTNO) experiments were performed on 151Pm implanted into iron and 147Nd implanted into nickel. The γ-ray anisotropy measurements yielded values of the magnetic hyperfine field and the substitutional fractions of these ions in the host metals. An observed discrepancy between the substitutional fractions obtained from the 91 keV and 531 keV data on 147NdNi is briefly discussed.

Study of high-spin states in 143Pm

High-spin states in 143Pm have been investigated via the 128Te(19F,4nγ)143Pm reaction using techniques of in-beam γ-spectroscopy. γ-ray singles, γ-γ coincidences, γ-ray anisotropies and DCO ratios have been measured. Based on these measurements, the level scheme of 143Pm has been extended up to an excitation energy of 10535.4 keV, including 48 new γ-rays deexciting 28 new levels. The yrast levels in 143Pm can be understood qualitatively in the framework of a weak-coupling model.

Ag hyperfine fields at the interface in Fe/Ag multilayers

Magnetic hyperfine fields in the Ag of Fe/Ag multilayers have been observed with low-temperature nuclear orientation for multilayers [Fe( xML)/Ag( yML)] with ( x, y) values of (9,3), (18,3) and (10,5). The γ-ray anisotropy was measured as a function of the external applied magnetic field. The hyperfine fields from the measurements are then compared with those fields derived by the model developed by M.B. Stearns.

Nuclear orientation studies of Ag hyperfine fields in Fe/Ag multilayers

This paper reports on a low-temperature nuclear orientation (LTNO) study of Fe/Ag multilayers [Fe( x ML)/Ag( y ML)] 20 with ( x, y ) values of (9, 3), (18, 3) and (10, 5). The samples were prepared by molecular beam epitaxy (MBE) and the quality of the multilayers was checked by X-ray diffraction. The γ-ray anisotropy of the neutron activated 110 m Ag was measured as a function of the applied magnetic field. It was found that, in low external applied fields, the induced Ag hyperfine fields are directed out of the plane of the multilayer.

Magnetic anisotropy of Pm impurity in rare earth nickel compound

Low-temperature nuclear orientation was applied to study hyperfine interactions of 142Pr, 147Nd and 143,144Pm nuclei in Pr0.5Nd0.5Ni single crystal. Angular distributions, temperature dependence and external magnetic field effects on the γ-ray anisotropy are presented. A Nd-Pm exchange interaction seems to dominate the magnetic properties of Pm ions in this system.

A study of the induced magnetic hyperfine fields in Fe/Ag multilayers

Near perpendicular magnetic hyperfine fields in 110mAg have been observed in Fe/Ag multilayers. These fields are studied by low temperature nuclear orientation (LTNO) in multilayers [Ag(x ML)/Fe(y ML)]20, with (x,y) monolayer (ML) values of (2,10), (3,9), (5,10) and (3,18). The 110mAg γ-ray anisotropy was measured as a function of applied magnetic field parallel to the multilayer. The average induced hyperfine field of Ag is significantly influenced by the quality of the multilayer as measured by X-ray diffraction.