Extracts from the Internet


T-symmetry in neutron decays

A new checking of the T-symmetry in β-decays of nuclei has been conducted at the Neutron Research Center of the National Institute of Standards and Technology (NIST) (USA). The experiment measured the correlation between the directions of neutron spins in the beam and the momenta of decay products in the reaction n → p + e- + anti-ν e. Protons and electrons were recorded by coincidence technique in an array of proton and electron detectors. The obtained value of the parameter characterizing the T-symmetry was D = ( -0.96 ± 1.89(stat) ± 1.01(syst)) × 10-4. In other words, the two configurations of decay with oppositely oriented neutron spins are equiprobable at the accuracy achieved in the experiment and therefore T-symmetry and CP-invariance are conserved within experimental errors. At the moment this result has the best accuracy achieved in experiments with β-decay of nuclei. It limits the applicability of some of the models suggested as generalization of the Standard Model of elementary particles and attempting to explain the prevalence of matter over antimatter in the universe. Source: Phys. Rev. Lett. 107 102301 (2011)

Gamov – Teller transitions in 56Ni nuclei

M. Sasano (Michigan State University, United States) and his colleagues have studied nuclear reactions 56Ni(p,n)56Cu involving the Gamov – Teller transition, i.e. allowed transitions in which the spin of a nucleus changes by unity. Prediction of various theoretical models for these processes differ by about 30 %. The energy distribution of intensity of the B(GT) Gamov – Teller transition obtained by M. Sasano et al. is best reproduced by the software package GXPF1A based on the shell model of the nucleus. In accordance with the predictions of GXPF1, the distribution of B(GT) in the nucleus 56Ni is fragmented over reaction channels in the same way this happens for other nuclei with similar masses, even though 56Ni is a doubly magic nucleus. In this experiment unstable isotopes 56Ni collided with a liquid hydrogen target and fragments reaction were studied using nuclear spectrometer. What was observed in reality was the inverse Gamov – Teller transition converting a proton into a neutron; however, the probability of this process for the nucleus 56Ni equals the probability of direct transition involving electron capture; however, the study of the inverse transition is considerably simpler. The obtained value of B(GT) is important for predicting the rate of electron capture by nuclei which in its turn is important for simulation of supernova explosions. Source: Phys. Rev. Lett. 107 202501 (2011)

Metallic hydrogen?

M.L. Eremets and L.A. Troyan (Institute for Chemistry of the Max Planck Society, Mainz, Germany) have conducted an experiment which allegedly produced metallic hydrogen at room temperature. Creation of metallic hydrogen at low temperatures was reported in the past too but the results remained ambiguous. In this new experiment molecular hydrogen was compressed in a diamond anvil. The diamond culet was covered with a semi-transparent metal film which prevented diffusion of hydrogen into the diamond anvil and thus protected diamond against destruction up to about 300 GPa. The electric resistance was measured directly using contacts on the anvil and an insulating layer. Hydrogen became opaque and semiconducting at pressures above 220 GPa; this followed from the decrease of resistance under laser irradiation. At 240 GPa hydrogen remained electrically conducting even without irradiation and at 260 GPa the gap in electron spectrum disappeared and hydrogen transformed to purely metal phase which was stable in the course of refrigeration from the initial room temperature to 30 K. The reverse transition to molecular phase involved a hysteresis at 200 GPa. The reported results need independent verification. The possibility of metallic phase of hydrogen was predicted in 1935; conditions for its formation may exist in the depths of planets. Source: Nature Materials 10 927 (2011)

Study of H2- anions

B. Jordon-Thaden (Max-Planck-Institut fur Kernphysik, Heidelberg, Germany) and his colleagues for the first time explored in detail the structure of ions H2- using the method of Coulomb explosion imaging. On passing through a very thin carbon foil, anions in the beam lost all electrons and electric repulsion made the resulting identically charged protons flow apart. The closer these protons were to one another initially, the greater was the kinetic energy with which they flew apart. This effect relates the initial wave function of the nucleus to the energy distribution of the ejected protons. The observed distribution revealed peaks corresponding both to H2- (at energy ≈ 5 eV) and to the neutral molecule H2. This result demonstrated that the distance between protons in the anion H2- equals approximately six atomic units which is several time greater than separation in the neutral molecule H2. In addition, it was found that the H2- decay is driven by autoionization, not by spontaneous dissociation, and anions possess high angular momentum J = 25 ± 2 which gives a measure of their metastability caused by the minimum in their interaction potential. H2- anions are an intermediate state in a chain of important chemical reactions, for example, H + H- → H2- → H2 + e-. Source: Phys. Rev. Lett. 107 193003 (2011)

Kneelike structure in the spectrum of cosmic rays

The KASCADE-Grande detector data revealed for the first time a kneelike bend in the spectrum of the heavy component of primary cosmic rays (nuclei with Z > 13 up to iron) at energy 8 × 1016 eV where the slope changes jumpwise: Δγ = -0.48. This bend referred to as knee was well known for some time in the spectrum of the lighter component at the energy 4 × 1015 eV. Predictions were made as for the position of the knee as a function of the mass of primary nuclei but detection of this peak in the spectrum of heavy nuclei was so far elusive. KASCADE-Grande is located at the Karlsruhe Research Center (Germany) and was built as a combination of two detectors covering the area of 700 × 700 m2; it includes the muon trecker and a calorimeter. Independent measurements of various components of cosmic rays were conducted in broad air showers cascades of secondary particles generated in collisions between primary cosmic rays and atoms of the atmosphere. Observation of broad air showers offer a method of testing models of hadron interactions at high energies which remain inaccessible to particle accelerators. Source: Phys. Rev. Lett. 107 171104 (2011)

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