Unraveling the Nature of Charge Excitations in La2_2CuO4_4 with Momentum-Resolved Cu KK-edge Resonant Inelastic X-ray Scattering

Results of model calculations using exact diagonalization reveal the orbital character of states associated with different Raman loss peaks in Cu $K$-edge resonant inelastic X-ray scattering (RIXS) from La$_{2}$CuO$_{4}$. The model includes electronic orbitals necessary to highlight non-local Zhang-Rice singlet, charge transfer and $d$-$d$ excitations, as well as states with apical oxygen 2$p_z$ character. The dispersion of these excitations is discussed with prospects for resonant final state wave-function mapping. A good agreement with experiments emphasizes the substantial multi-orbital character of RIXS profiles in the energy transfer range 1-6 eV.Comment: Original: 4.5 pages. Replaced: 4 pages and 4 figures with updated content and reference

Astrochemical models of interstellar ices: History matters

Ice is ubiquitous in the interstellar medium. We model the formation of the main constituents of interstellar ices, including H2O, CO2 , CO, and CH3 OH. We strive to understand what physical or chemical parameters influence the final composition of the ice and how they benchmark to what has already been observed, with the aim of applying these models to the preparation and analysis of JWST observations. We used the Nautilus gas-grain model, which computes the gas and ice composition as a function of time for a set of physical conditions, starting from an initial gas phase composition. All important processes (gas-phase reactions, gas-grain interactions, and grain surface processes) are included and solved with the rate equation approximation. We first ran an astrochemical code for fixed conditions of temperature and density mapped in the cold core L429-C to benchmark the chemistry. One key parameter was revealed to be the dust temperature. When the dust temperature is higher than 12 K, CO2 will form efficiently at the expense of H2O, while at temperatures below 12 K, it will not form. Whatever hypothesis we assumed for the chemistry (within realistic conditions), the static simulations failed to reproduce the observed trends of interstellar ices in our target core. In a second step, we simulated the chemical evolution of parcels of gas undergoing different physical and chemical situations throughout the molecular cloud evolution and starting a few 1e7 yr prior to the core formation (dynamical simulations). Our dynamical simulations satisfactorily reproduce the main trends already observed for interstellar ices. Moreover, we predict that the apparent constant ratio of CO2/H2O observed to date is probably not true for regions of low AV , and that the history of the evolution of clouds plays an essential role, even prior to their formation.Comment: Accepted for publication in A&

Fast-neutron induced background in LaBr3:Ce detectors

The response of a scintillation detector with a cylindrical 1.5-inch LaBr3:Ce crystal to incident neutrons has been measured in the energy range En = 2-12 MeV. Neutrons were produced by proton irradiation of a Li target at Ep = 5-14.6 MeV with pulsed proton beams. Using the time-of-flight information between target and detector, energy spectra of the LaBr3:Ce detector resulting from fast neutron interactions have been obtained at 4 different neutron energies. Neutron-induced gamma rays emitted by the LaBr3:Ce crystal were also measured in a nearby Ge detector at the lowest proton beam energy. In addition, we obtained data for neutron irradiation of a large-volume high-purity Ge detector and of a NE-213 liquid scintillator detector, both serving as monitor detectors in the experiment. Monte-Carlo type simulations for neutron interactions in the liquid scintillator, the Ge and LaBr3:Ce crystals have been performed and compared with measured data. Good agreement being obtained with the data, we present the results of simulations to predict the response of LaBr3:Ce detectors for a range of crystal sizes to neutron irradiation in the energy range En = 0.5-10 MeVComment: 28 pages, 10 figures, 4 Table

Impact of particles on the Planck HFI detectors: Ground-based measurements and physical interpretation

The Planck High Frequency Instrument (HFI) surveyed the sky continuously from August 2009 to January 2012. Its noise and sensitivity performance were excellent, but the rate of cosmic ray impacts on the HFI detectors was unexpectedly high. Furthermore, collisions of cosmic rays with the focal plane produced transient signals in the data (glitches) with a wide range of characteristics. A study of cosmic ray impacts on the HFI detector modules has been undertaken to categorize and characterize the glitches, to correct the HFI time-ordered data, and understand the residual effects on Planck maps and data products. This paper presents an evaluation of the physical origins of glitches observed by the HFI detectors. In order to better understand the glitches observed by HFI in flight, several ground-based experiments were conducted with flight-spare HFI bolometer modules. The experiments were conducted between 2010 and 2013 with HFI test bolometers in different configurations using varying particles and impact energies. The bolometer modules were exposed to 23 MeV protons from the Orsay IPN TANDEM accelerator, and to $^{241}$Am and $^{244}$Cm $\alpha$-particle and $^{55}$Fe radioactive X-ray sources. The calibration data from the HFI ground-based preflight tests were used to further characterize the glitches and compare glitch rates with statistical expectations under laboratory conditions. Test results provide strong evidence that the dominant family of glitches observed in flight are due to cosmic ray absorption by the silicon die substrate on which the HFI detectors reside. Glitch energy is propagated to the thermistor by ballistic phonons, while there is also a thermal diffusion contribution. The implications of these results for future satellite missions, especially those in the far-infrared to sub-millimetre and millimetre regions of the electromagnetic spectrum, are discussed.Comment: 11 pages, 13 figure

Heavy ion irradiation of crystalline water ice

Under cosmic irradiation, the interstellar water ice mantles evolve towards a compact amorphous state. Crystalline ice amorphisation was previously monitored mainly in the keV to hundreds of keV ion energies. We experimentally investigate heavy ion irradiation amorphisation of crystalline ice, at high energies closer to true cosmic rays, and explore the water-ice sputtering yield. We irradiated thin crystalline ice films with MeV to GeV swift ion beams, produced at the GANIL accelerator. The ice infrared spectral evolution as a function of fluence is monitored with in-situ infrared spectroscopy (induced amorphisation of the initial crystalline state into a compact amorphous phase). The crystalline ice amorphisation cross-section is measured in the high electronic stopping-power range for different temperatures. At large fluence, the ice sputtering is measured on the infrared spectra, and the fitted sputtering-yield dependence, combined with previous measurements, is quadratic over three decades of electronic stopping power. The final state of cosmic ray irradiation for porous amorphous and crystalline ice, as monitored by infrared spectroscopy, is the same, but with a large difference in cross-section, hence in time scale in an astrophysical context. The cosmic ray water-ice sputtering rates compete with the UV photodesorption yields reported in the literature. The prevalence of direct cosmic ray sputtering over cosmic-ray induced photons photodesorption may be particularly true for ices strongly bonded to the ice mantles surfaces, such as hydrogen-bonded ice structures or more generally the so-called polar ices.Comment: 22pages, 11 figures, accepted in A&

Charge and current-sensitive preamplifiers for pulse shape discrimination techniques with silicon detectors

New charge and current-sensitive preamplifiers coupled to silicon detectors and devoted to studies in nuclear structure and dynamics have been developed and tested. For the first time shapes of current pulses from light charged particles and carbon ions are presented. Capabilities for pulse shape discrimination techniques are demonstrated.Comment: 14 pages, 12 figures, to be published in Nucl. Inst. Meth.

Cross sections relevant to gamma-ray line emission in solar flares:3^3He-induced reactions on 16^{16}O nuclei

Gamma-ray production cross sections have been measured for gamma-ray lines copiously emitted in the $^3$He bombardment of $^{16}$O nuclei: the 937, 1042 and 1081 keV lines of $^{18}$F and the 1887 keV line of $^{18}$Ne. Four Ge detectors with BGO shielding for Compton suppression were used to measure the angular distributions of the gamma-rays. The excitation functions have been obtained for $^3$He bombarding energies from 3.7 to 36 MeV. Total cross sections are tabulated for calculations relevant to gamma-ray astronomy. The importance of these lines as diagnosis for the presence and properties of accelerated $^3$He in solar flares is discussed in light of the measured cross sections.Comment: Phys. Rev. C68 (2003) 0258XX, in pres