Stochastic self-oscillations in an extrinsic semiconductor under intense illumination

Summary We show that an extrinsic semiconductor (n-type), containing a deep level from impurity centres, may produce stochastic self-oscillations in the electron density and in the electric field, under intense illumination which ionizes the deep traps and heats the electrons. The feedback mechanism is the dependence of the capture coefficient of the deep centres on the electron temperature and the dependence of the deep energy level on electron temperature and concentration. The conditions for the occurrence of periodic and stochastic self-oscillations are explicitly given.

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Riassunto Si mostra che in un semiconduttore estrinseco (di tipon), contenente impurezze con un livello profondo, si possono produrre auto-oscillazioni stocastiche nella densità di elettroni in modo da ionizzare le impurezze e riscaldare gli elettroni. Il meccanismo responsabile è la dipendenza della sezione di cattura degli elettroni nel livello profondo della temperatura degli elettroni e la dipendenza dell’energia del livello dalla temperatura degli elettroni e dalla loro densitá. Si danno le esplicite condizioni per l’insorgere di auto-oscillazioni periodiche e stocastiche.

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Резюме Мы показываем, что собственный полупроводник (n-типа), содержащий глубокий уровень, связанный с примесными центрами, может производить стохастические автоколебания в электронной плотности и в электрическом поле при интенсивном облучении, которое ионизует глубокие ловушки и нагревает электроны. Этот механизм обратной связи представляет зависимость коэффициента захвата для глубоких центров от электронной температуры и зависимость глубокого энергетического уровня от температуры и концентрации электронов. В явном виде приводятся условия для возникновения периодических и стохастических автоколебаний.

     

On positron annihilation in zinc

The purpose of this work is to understand Mogensen's and Petersen's positron annihilation curves for zinc. Mijnarends' approach is used as an auxiliary method of localizing inhomogeneities of the electronic density in momentum space, as defined in the paper. Evidence is found for a new effect consisting of a strong enhancement of the annihilation probability in the lenses obtained by the intersection of the Fermi surface with HMC surfaces. This effect, not the anisotropy of the Fermi surface, is the main reason for the anisotropy of the annihilation curves. Paper presented at 3rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

N Law for bosons in 2D

A rigorous upper bound is derived for the exact ground-state energy of N negative charged bosons and N motionless, i.e. fixed, positive charges with Coulomb interactions in 2D for arbitrary N ? 4 giving rise to an N²-upper bound. The consistency of such an N² behaviour is also investigated by examining a lower bound to the ground-state energy.     

Measurement of cosmogenic radionuclides in meteorites with a sensitive gamma-ray spectrometer

Summary A large-volume HPGe gamma-ray spectrometer in a NaI(Tl) well has been set up underground at 70 m.w.e. depth for whole body counting of cosmogenic radionuclides in meteorites. The detectors are housed in a 20 cm thick lead shield with a lining of cadmium and OFHC copper. The scintillator is simultaneously operated in anticoincidence as well as in coincidence in selected energy channels to achieve low background levels (in the range of counts per day) and high specificity. In this way a large number of radionuclides such as²⁶Al,⁴⁴Ti,⁶⁰Co,²²Na,⁵⁴Mn, and shorter-lived nuclides produced in extraterrestrial materials like meteorites and lunar rocks can be analysed. Results on Bouvante and Bereba achondrites and Dhajala and Torino chondrites are presented. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Measurement of44Ti(44Sc) in meteorites

Summary The measurement of⁴⁴Ti produced by spallation reaction of cosmic rays in meteorites fell in the last hundred years can be used for investigations on century-scale variations of solar activity. The low specific activity and the interferences of natural background and of meteorite gamma emitters are the main difficulties to be surmounted in this measurement. A selective γ-spectrometer consisting of a HP Ge and a NaI(T1) detectors in coincidence and anticoincidence has been set up in the underground (70 m w.e.) Laboratory of Monte dei Cappuccini, which allows to obtain a reliable measurement of⁴⁴Ti(⁴⁴Sc).     

Thermodynamics of simple liquid metals calculated using an analytic pair potential

Summary Pettifor-Ward analytic pair potential parameters are rederived for liquid metals Li, Na, K, Rb, Cs, Mg and Al. The analytic pair potentials are used to calculate the thermodynamic properties. The authors of this paper have agreed to not receive the proofs for correction.     

Studies on elemental charge-changing cross-section for relativistic U+Al collisions using CR-39 (DOP) detector

Summary A search has been made on the charge-changing partial cross-sections by using 0.927 GeV/n²³⁸U ion from LBL BEVALAC projected at an angle 30° on²⁷Al target and CR-39 (DOP) was used as analyser. The irradiated plastic sheets were duly etched for one hour in 6.25N NaOH solution and about 1680 cone lengths at both the surfaces of the CR-39 (DOP) sheets were optically measured. The cone length distribution exhibits the existence of U fragments in the charge range 84≤Z≤91 and the estimated partial cross-sections range from 60 to 400 mb. The present data are in accord with the fit to the extrapolated data of Binnset al.     

Paleoastrophysics and cosmic rays

Summary A discussion is given of the status and potential of studying the cosmic-ray flux level over long time scales by high-precision measurement of cosmogenic isotopes content in natural archives of cosmic radiation. Invited talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

A systematic comparison of two different models of cosmogenic nuclide production in meteorites

Summary A systematic comparison of a variable cosmogenic-production rate model (proposed in a previous paper) with the conventional constant-production rate model is carried out. Attention is focussed on the time-integrated concentrations in meteorites. A graphical method for the estimate of the exposure and terrestrial ages is applied with reference to the ten pairs of the five long-lived cosmogenic nuclides³⁶Cl,²⁶Al,¹⁰Be,⁵³Mn and¹²⁹I of interest in the Accelerator Mass Spectrometry. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

On the cosmogenic radionuclides in meteorites

Summary Different fluxes and spectra of galactic cosmic rays (GCR) and solar cosmic rays result in a wide variety of radioactive and stable-nuclides production rates. The records of cosmic-ray interactions, have been studied in terrestrial archives, in meteorites, cosmic dust and lunar samples. We present here some results obtained in our laboratory on the study of cosmogenic radionuclides produced in meteorites which fell in the last century. The laboratory is situated in the underground station of Monte dei Cappuccini, Torino, at a depth of 70 m water equivalent to reduce the cosmic-ray background level. To measure γ-activity in meteorites we have set up a highly efficient and selective Ge−NaI(Tl) γ-ray spectrometer surrounded by a passive shield. The solar modulation affects the intensity of GCR and therefore the isotopes production rates in meteorites. The 11 y solar cycle is well reproduced by the variations of²²Na (T _1/2=2.6 y) in meteorites which fell during the past three solar cycles. The⁴⁴Ti (T _1/2=66.6 y) activity measured in eight chondrites which fell in the last 110 y is qualitatively consistent with the century-scale solar-activity variation, called the Gleissberg cycle. The increase is roughly four times higher, than expected (about 5%) for a GCR flux calculated by a model in which the solar-activity modulation is determined solely by the sunspot number indexR. This result suggests a much weakened modulation of GCR during the prolonged Gleissberg minimum during which highly ordered magnetic fields set up in the heliosphere.     

Study of large fragments of uranium projectiles of energy 0.927 GeV/n in Al target

Summary A stack consisting of CR-39(DOP) detector with 2.6 cm Al target was exposed to a²³⁸U beam of energy 0.927 GeV/n from LBL BEVALAC to study the large-fragmentation phenomena during U+Al interaction. The diameter distribution of the etch pits formed in the plastic detectors for the first three projectile fragments of chargeZ _F≥89 yields the partial cross-section data which is comparable to the data obtained from the cone length distribution. The present result on the partial cross-section is significantly higher than those expected from the abrasion-ablation model of Wilson, Townsed and Badavi (1987). The authors of this paper have agreed not to receive the proofs for correction.     

Modulation of the cosmogenic nuclide production expected from the galactovertical motion of the solar system

Summary A variable cosmogenic-production rate model is proposed which takes account of the changes in galactic-cosmic-ray irradiation conditions arising from the vertical oscillation of the solar system about the galactic plane. The model incorporates and substantiates several ideas and suggestions previously put forward in the literature. The time distribution of the¹⁰Be concentration predicted by the model appears to be consistent with the data of the deep-sea sediment core RC 12-65. A comparison of our model with the conventional constant-production rate model is carried out, especially with reference to time-integrated concentrations in meteorites.

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Riassunto Si propone un modello di produziones variabile di nuclidi cosmogenici che tiene conto del cambiamento nelle condizioni di esposizione ai raggi cosmici galattici derivante dal moto verticale del sistema solare rispetto al pianogalattico. Il modello sviluppa alcune idee avanzate di recente da vari autori. La distribuzione differenziale della concentrazione di¹⁰Be in riserve terrestri prevista dal modello è consistente con i dati del sedimento marino RC 12-65. Si confrontano le previsioni del modello a produzione variabile con quelle del modello tradizionale a produzione costante, con particolare attenzione alle concentrazioni, integrate sul tempo d'irradiazione cosmica, di nuclidi cosmogenici in meteoriti.

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Резюме Предлагается модель переменной интенсивности космогенного образования, которая учитывает изменения в условиях космического облучения, возникающие от вертикальных осцилляций солнечной системы относительно галактической плоскости. Эта модель включает и подтверждает некоторые идеи и предположения, ранее выдвинутые в литературе. Временное распределение концентрации¹⁰Be, предсказннное в рамках этой модели, согласуется с данными RC 12-65 для глубоководных отложений. Проводится сравнение предложенной модели с общепринятой моделью постоянной интенсивности образования, особое внимание уделяется интегральным по времени концентрациям космогенных нуклидов в метеоритах.

     

On some measurements of the propagation properties of thermal waves in the Nb-H and Ta-H systems

Summary By means of an experimental technique based on a nonsteady-state method,i.e. on the propagation of thermal waves, we have measured at room temperature for oscillation frequencies between 6 and 30 mHz the thermal diffusivity, the lateral thermal-loss coefficient and the ?fractional heat loss? in Nb and Ta wires as a function of hydrogen doping. The appearance of hydride formation and precipitation notably changes the behaviour of these quantities. From these measurements it was also possible to calculate the propagation velocity and attenuation coefficient of thermal waves. It was found that the increase of the hydrogen concentration in the solid solution produces a gradual decrease in the velocity and increase in the attenuation coefficient until the solubility limit is reached. For larger hydrogen concentrations, the velocity showed a tendency to increase towards the value of the pure metal, while the attenuation coefficient decreased below the value of the pure metal. These results as a whole appear quite promising for studying the properties of hydrogenated systems. This work was supported by the Gruppo Nazionale di Struttura della Materia of the Consiglio Nazionale delle Ricerche and by the Centro Interuniversitario di Struttura della Materia of the M.P.I., Roma (Italia).     

A ring-source method to measure the thermal diffusivity of a conducting specimen in forced air flow: Study on aluminium

Summary Thermal diffusivity is obtained with an accuracy of ∼ 1% by means of a new simple method, in which two parallel disks (of chromel and alumel, respectively) surround the middle section of a cylindrical specimen of conducting material. The disks act in the clamped zone as the hot juction of a thermocouple and, at the same time, as a heat source for the specimen, subjected to a laminar air flow. A second thermocouple placed at the base of the cylinder accomplishes acquisition of another set of experimental data from which the thermal diffusivity and the surface heat loss coefficient can be determined. Preliminary measurements on aluminium 99.999% consistent with literature data have been reported. Work supported by the Ministero della Pubblica Istruzione of the Italian Goverment.     

The development of perturbations in the universe described by the nonstationary equation of state

The growth of the perturbation density in a baryon substance due to a nonstationary character of the equation of state of nonbaryon matter in the Universe is studied. It is shown that the perturbations evolve slower than within the Friedmann cosmological model. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 61–67, March, 2009.     

Spherically symmetric solutions in Macroscopic Gravity

Schwarzschild’s solution to the Einstein Field Equations was one of the first and most important solutions that lead to the understanding and important experimental tests of Einstein’s theory of General Relativity. However, Schwarzschild’s solution is essentially based on an ideal theory of gravitation, where all inhomogeneities are ignored. Therefore, any generalization of the Schwarzschild solution should take into account the effects of small perturbations that may be present in the gravitational field. The theory of Macroscopic Gravity characterizes the effects of the inhomogeneities through a non-perturbative and covariant averaging procedure. With similar assumptions on the geometry and matter content, a solution to the averaged field equations as dictated by Macroscopic Gravity are derived. The resulting solution provides a possible explanation for the flattening of galactic rotation curves, illustrating that Dark Matter is not real but may only be the result of averaging inhomogeneities in a spherically symmetric background.     

Superfluid nuclear matter in BCS theory and beyond

Medium polarization eflects are studied for 1S0 pairing in nuclear matter within BHF approach. The screening potential is calculated in the RPA limit, suitably renormalized to cure the low density mechanical instability of nuclear matter. The self-energy corrections are consistently included resulting in a strong depletion of the Fermi surface. The self-energy effects always lead to a quenching of the gap, whereas it is almost completely compensated by the anti-screening effect in nuclear matter.     

Superfluid nuclear matter in BCS theory and beyond

Medium polarization effects are studied for ¹S₀ pairing in nuclear matter within BHF approach. The screening potential is calculated in the RPA limit, suitably renormalized to cure the low density mechanical instability of nuclear matter. The self-energy corrections are consistently included resulting in a strong     

核物理中的π介子

在强子层次上,原子核或强子物质的基本组元是核子和介子.弄清这些强子的结构,并由基本原理出发研究它们的性质,是当代核物理的重要课题.在各种介子中,π介子是最轻且最重要的介子.关于自由空间中π介子的结构与性质、核介质内π介子的性质、π 核子相互作用与π 核相互作用等问题,始终受到相当多的关注.π介子在核物理中的作用直接联系着手征对称性,汤川秀树关于π介子的最初概念已经大大发展了.有清楚的实验证据表明,核内存在π介子的集体模式,这种集体模式与以前观测到的所有核集体运动模式截然不同.拟对π 核物理的研究现状及值得进一步研究的主要问题予以简要评述. At the hadronic level, nucleons and mesons are constituents of nuclei and hadronic matter. Understanding the structures of hadrons, finding the physics of how the properties of these particles arise from the first principle, are major interests in modern nuclear physics. Among mesons, the lightest and most important one is certainly the pion, thus it is no accident that its structure, properties (both in free space and in nuclear matter) and interactions with nucleons and nuclei have gotten considerable ...     

Optical shadowgraphy and proton imaging as diagnostics tools for fast electron propagation in ultrahigh-intensity laser–matter interaction

This paper reports the results of some recent experiments performed at the LULI laboratory (Palaiseau, France) concerning the propagation of large relativistic electron currents in a gas jet. We present our experimental results according to the type of diagnostics used in the experiments: (1) time resolved optical shadowgraphy and (2) proton imaging. Proton radiography images did show the presence of very strong fields in the gas probably produced by charge separation. In turn, these imply a slowing down of the fast electron cloud as it penetrates in the gas. Indeed, shadowgraphy images show a strong inhibition of propagation and a strong reduction in time of the velocity of the electron cloud from the initial value, which is of the order of a fraction of c.