Search Results (13 total)

The energy spectrum of cosmic rays above 2.5×10¹⁸ eV, derived from 20 000 events recorded at the Pierre Auger Observatory, is described. The spectral index γ of the particle flux, J∝E^-γ, at energies between 4×10¹⁸ eV and 4×10¹⁹ eV is 2.69±0.02(stat)±0.06(syst), steepening to 4.2±0.4(stat)±0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz’min.

The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth’s crust. Tau leptons from ν_τ charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of ν_τ at EeV energies. Assuming an E_ν^-2 differential energy spectrum the limit set at 90% C.L. is E_ν²dN_ντ/dE_ν<1.3×10^-7  GeV cm^-2 s^-1 sr^-1 in the energy range 2×10¹⁷  eV<E_ν<2×10¹⁹  eV.

Signatures of isospin effects were investigated for neutron-rich (¹²⁴Sn+⁶⁴Ni) and neutron-poor (¹¹²Sn+⁵⁸Ni) systems at 35 MeV/nucleon for noncentral collisions. The centrality dependence of these signatures was tested for several impact parameter estimators. Our main observations are (i) the yields of ¹H and ³He particles in the neutron-poor system are strongly enhanced with respect to the neutron-rich system, and the yields of ³H, ⁶He, and ^7,8Li are suppressed at all impact parameters, (ii) the yields of ²H, ⁴He, and ⁶Li particles are almost the same for both systems, (iii) the N/Z ratio of intermediate mass fragments is correlated with the neutron richness of the system and is weakly dependent on the centrality of the collision, and (iv) the neutron richness of the detected fragments increases strongly with decreasing rapidity in the range from that of the projectile-like fragment to the c.m. region. The gross features of experimental data are reproduced by quantum molecular dynamics model calculations. A comparison between model calculations and the data indicates that the fragments produced in the c.m. regions are weakly excited.

Multi-breakup processes for the ¹²⁴Sn+⁶⁴Ni system at 35 MeV/nucleon have been studied with the forward part of the CHIMERA detector. An extensive comparison between experimental data corresponding to almost complete ternary events and constrained molecular dynamics (CoMD-II) calculations suggests different characteristic times in the selected processes. This is in agreement with previous studies of the same reaction already published concerning the prompt intermediate-mass-fragment emission. Stimulated by CoMD-II calculations, we investigate the existence of more complex dynamical multi-breakup processes occurring on the same time scale. A detailed study of the rotational dynamics leading to slower dynamical fission processes is also presented.

Some properties of fast, nonequilibrium splitting of projectiles in the ¹²⁴Sn+⁶⁴Ni reaction at 35A MeV were determined using the 4π CHIMERA detector system. In particular the charge distributions, in- and out-of-plane angular distributions, and relative velocities of projectilelike fragments were measured. The time scale of the process was estimated and it turned out that the process is sequential but much faster than the ordinary, equilibrated fission.

Semiperipheral collisions in the ¹²⁴Sn+⁶⁴Ni reaction at 35 MeV/nucleon were studied using the forward part of the Charged Heavy Ion Mass and Energy Resolving Array. Nearly completely determined ternary events involving projectilelike fragments (PLF), targetlike fragments (TLF), and intermediate mass fragments (IMF) were selected. A new method of studying the reaction mechanism, focusing on the analysis of the correlations between relative velocities in the IMF+PLF and IMF+TLF subsystems, is proposed. The relative velocity correlations provide information on the time sequence and time scale of the neck fragmentation processes leading to production of IMFs. It is shown that the majority of light IMFs are produced within 40–80 fm/c after the system starts to reseparate. Heavy IMFs are formed at times of about 120 fm/c or later and can be viewed as resulting from two-step (sequential) neck rupture processes.

Doubly differential electron velocity spectra induced by 95-MeV/u ³⁶Ar¹⁸⁺ from thin carbon foils were measured at GANIL (Caen, France) by means of the ARGOS multidetector and the time-of-flight technique. The spectra allow us to determine absolute singly differential cross sections as a function of the emission angle. Absolute doubly differential cross sections for binary encounter electron ejection from C targets are compared to a transport theory, which is based on the relativistic electron impact approximation for electron production and which accounts for angular deflection, energy loss, and also energy straggling of the transmitted electrons. For the thinnest targets, the measured peak width is in good agreement with experimental data obtained with a different detection technique. The theory underestimates the peak width but provides (within a factor of 2) the correct peak intensity. For the thickest target, even the peak shape is well reproduced by theory.

In-plane and out-of-plane angular correlations of fission fragments detected in coincidence with projectile residues produced in nuclear collisions ¹⁹F+²³⁸U at 115, 120, and 125 MeV have been investigated. The data are described assuming sequential fission decay of targetlike nuclei produced in peripheral transfer reactions.

Experimental information has been derived from the neutron-proton correlation function in order to deduce the time sequence of neutrons and protons emitted at 45° in the E/A  =  45 MeV ⁵⁸Ni+²⁷Al reaction.

The strength of the neutron-neutron correlation function from the E=45A MeV ⁵⁸Ni+²⁷Al, ^natNi, and ¹⁹⁷Au reactions depends on the neutron parallel velocity. This indicates the presence of multiple sources of neutron emission. We find these sources consistent with a dissipative, binary reaction mechanism as it is described by, e.g., Boltzmann-Uehling-Uhlenbeck calculations.

We report absolute cross sections for binary encounter and convoy electron emission. Fast-electron velocity spectra were measured for atomic collisions induced by a 45 MeV/u ⁵⁸Ni beam impinging on solid targets by means of the multidetector ARGOS in a large angular range (from 1.5° to 165°). Different conducting elemental targets (¹²C, ²⁷Al, ⁵⁸Ni and ⁶⁴Ni, ^natAg, ¹⁹⁷Au) and polystyrene were used. Characteristics of electrons with a velocity close to the beam velocity (convoy electrons) were found to be very sensitive to the incoming charge state of the projectile. Their yield increases with the target atomic number. The yield of binary encounter electrons with a velocity of almost twice the beam velocity at small ejection angles is roughly proportional to the area density of encountered target electrons. The velocity spectra centroids of these electrons are shifted towards lower velocities than predicted by simple two-body relativistic kinematics. Surprisingly, this effect seems to be mostly independent of target material and thickness. With increasing target atomic number, the high-energy tails of the spectra exhibit an unexpectedly large number of very fast electrons. A multiple-scattering mechanism involving multiple collision sequences of electrons between projectile and target atoms is invoked to explain this effect. The data also show evidence for an excess of fast electrons in the backward direction.

Small angle neutron-neutron correlations have been measured for the E/A=45 MeV ⁵⁸Ni+²⁷Al, ^natNi, and ¹⁹⁷Au reactions. Two-neutron correlation functions, both integrated and gated on the total momentum of the neutron pair, have been constructed. In order to explain these data, a fraction of fast “dynamical” emission is needed in addition to slower evaporative emission. The overall emission time scale is shorter for the symmetric system, indicating that the dynamical component is stronger in this case.

We measured fast electron velocity spectra with the multidetector ARGOS in a large angular range, for atomic collisions induced by a 45A MeV ⁵⁸Ni beam on different solid targets. The velocity centroids of binary encounter electrons are shifted towards lower velocities than predicted by simple two-body relativistic kinematics, independently of target material and thickness. With increasing target atomic number, the high energy tails of the spectra exhibit an unexpectedly large number of very fast electrons. A multiple scattering (Fermi-shuttle) mechanism is invoked to explain this effect.