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| Issue 4, 2024 |
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Extracts from the Internet | ||
Charge radius of the proton1 August 2010R. Pohl (Max Planck Institute of Quantum Optics) and coworkers established a better value for the charge radius (the mean square radius of charge distribution) of the proton by measuring the Lamb shift (energy difference between the levels 2s1/2F=1 and 2p3/2F=2 which arises due to radiative corrections) in muonic hydrogen atoms μp in which the electron of the ordinary hydrogen atom is replaced with a muon. As compared with the ordinary hydrogen, the advantage of using μp lies in the fact that the muon is on the average 200 times closer to the nucleus than the electron, and therefore the finite size of the nucleus has a greater effect on the wave function of the muon in the s-state. As a result it was possible to improve the accuracy of measuring the charge radius of the proton approximately ten-fold. In the experiment we describe, the μp atoms formed in collisions of the beam of muons with a hydrogen target; in some of of these collisions muons replaced electrons, forming highly excited μp atoms. Laser radiation caused transitions 2s → 2p and the spectrum of x-ray radiation of 2p → 1s transitions was observed using an array of 20 avalanche photodiodes; the Lamb shift was calculated with high accuracy from the measured position of the resonance peak in the spectrum. The charge radius was obtained by combining experimental data and quantum electrodynamics computations. Therefore, the correctness of theoretical calculations significantly affects the result. The new value of the charge radius rp = 0,84184 ± 67 fm was found to be 4% less than previously accepted. The discrepancy with the previous results which were mostly obtained using ordinary hydrogen comes to about five standard deviations. The cause of the discrepancy has not been understood so far but it could tentatively be an unknown systematic error in measurements or insufficient accuracy of theoretical computations. Source: Nature 466 213 Photoemission delay in neon atoms1 August 2010Ì. Schultze (Ludwig-Maximilians-Universitat, Germany) and his colleagues measured the delay time between acts of emission of electrons from 1ð and 1s electron orbitals of the neon atom caused by ultrashort light pulses. The measured interval was (21 ± 5) × 10-18 s. The experiment was carried out by illuminating neon with synchronized IR and UV laser pulses causing photoemission of electrons and measuring the time shift of electron spectrograms relative to the initial pulse. It is usually assumed that photon absorption and electron emission in photoemission occur simultaneously. A more accurate approach requires to take account of the finite time necessary to transform the wave function of the electron in the atom into the wave function of the emitted electron taking into account the complicated sequence of interactions with other electrons. The duration of formation of a wave packet of the emerging electron are different for different orbitals. Theoretical calculations of many-electron interactions cannot so far offer an accurate prediction of time delay in photoemission of atoms heavier than helium -- the calculated values are much shorter than the measured delay. In contrast, the cross section of photoemission in the helium atom, for which reliable calculation is possible, are very small which makes experimental verification of the effect of delay in helium atoms so far unfeasible. Investigation of delays in the photoemission is important for establishing the zero reference point of time in experiments which study the structure of atoms and molecules using photoelectric effect. Source: Science 328 1658 (2010) Bose – Einstein condensate under microgravity conditions1 August 2010T.V. Zoest and his colleagues from Germany and France performed an experiment in which the Bose – Einstein condensate of rubidium atoms was created during the time of free fall of an experimental setup. A capsule with the setup was dropped from a height of 146 m from the tower at the Center of Applied Space Technology and Microgravity of the University of Bremen. The capsule had all the necessary equipment for cooling the gas and remotely monitoring its properties. In this experiment the residual gravitational field was not higher than 10-5 g. During its free fall for more than a second the gas on the atom chip formed the Bose – Einstein condensate. Then the potential of the magneto-optical trap was switched off, and the gas cloud expanded freely. The characteristics of the way the cloud spread were indicative of the physical characteristics of the degenerate gas and of the external fields affecting it. Such experiments with Bose – Einstein condensate may prove useful for ultra-precise measurements, for instance, to test the equivalence principle. Plans for the future include adding a quantum interferometer to the capsule in free fall and in more remote future — to conduct the experiment under weightlessness conditions in space. Source: Science 328 1540 (2010) Paramagnons in palladium1 August 2010R. Doubble and his colleagues in the UK and the US investigated spin fluctuations in palladium — a nearly ferromagnetic metal -- using the method of inelastic neutron scattering. Nearly ferromagnetic materials possessing no stable ferromagnetism are of interest because spin fluctuations significantly modify their electronic properties. The experiment was carried out on a MARI neutron spectrometer at the Rutherford – Appleton Laboratory. Spin excitations — paramagnons — were observed in palladium crystals in a wide range of energies from 25 to 128 meV. The contribution of paramagnons to the heat capacity was studied and found to be about 30-40%, which is consistent with the observed spectrum of paramagnons. Source: Phys. Rev . Lett. 105 027207 (2010) The asymmetry of type Ia supernova explosions1 August 2010Type Ia supernovas are used as “standard candles” in evaluating cosmological distances owing to a well-defined relation between their maximum luminosity and decline rate on their light curve. However, there is a certain spread in spectral properties of supernovae with respect to this function, namely, a distribution is observed in expansion rate gradient dvSi/dt. Ê. Maeda (Tokyo University) and his colleagues compared the gradient dvSi/dt with the expansion rate of matter vneb along the line of sight determined from the Doppler effect, for 20 supernovae of type Ia. Researches came to a conclusion that the observed spread is caused by nonspherical character of thermonuclear explosions of white dwarfs. In this case the appearance of the spectrum depends on the direction from which the explosion is viewed. A possible reason for non-sphericity of the front of thermonuclear burning is an offset of the initial point from the center of the white dwarf because of the convective motion of matter. An alternative scenario to an asymmetrical explosion is a collision of white dwarfs in a binary system (see Phys. Usp. 53 325 (2010)). Source: Nature 466 82 (2010) |
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources. | |
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