Soft X-ray reflectivity: from quasi-perfect mirrors to accelerator walls

Reflection of light from surfaces is a very common, but complex phenomenon not only in science and technology, but in every day life. The underlying basic optical principles have been developed within the last five centuries using visible light available from the sun or other laboratory light sources. X-rays were detected in 1895, and the full potential of soft- and hard-x ray radiation as a probe for the electronic and geometric properties of matter, for material analysis and its characterisation is available only since the advent of synchrotron radiation sources some 50 years ago. On the other hand high-brilliance and high power synchrotron radiation of present-days 3rd and 4th generation light sources is not always beneficial. Highenergy machines and accelerator-based light sources can suffer from a serious performance drop or limitations due to interaction of the synchrotron radiation with the accelerator walls, thus producing clouds of photoelectrons (e-cloud) which in turn interact with the accelerated beam. Thus the suitable choice of accelerator materials and their surface coating, which determines the x-ray optical behaviour is of utmost importance to achieve ultimate emittance. Basic optical principles and examples on reflectivity for selected materials are given here.Comment: 11 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba, Italy; CERN Yellow Report CERN-2013-002, pp.105-11

Polarization measurements and their perspectives: PVLAS Phase II

We sketch the proposal for a "PVLAS-Phase II" experiment. The main physics goal is to achieve the first direct observation of non-linear effects in electromagnetism predicted by QED and the measurement of the photon-photon scattering cross section at low energies (1-2 eV). Physical processes such as ALP and MCP production in a magnetic field could also be accessible if sensitive enough operation is reached. The short term experimental strategy is to compact as much as possible the dimensions of the apparatus in order to bring noise sources under control and to attain a sufficient sensitivity. We will also briefly mention future pespectives, such as a scheme to implement the resonant regeneration principle for the detection of ALPs.Comment: Paper submitted to the proceedings of the "4th Patras Workshop on Axions, WIMPs and WISPs", DESY, Hamburg Site /Germany, 18-21 June 200

Maps for Electron Clouds: Application to LHC Conditioning

In this communication we present a generalization of the map formalism, introduced in [1] and [2], to the analysis of electron flux at the chamber wall with particular reference to the exploration of LHC conditioning scenarios.Comment: 3 pages, 4 figure

VUV photoemission studies of candidate Large Hadron Collider vacuum chamber materials

In the context of future accelerators and, in particular, the beam vacuum of the Large Hadron Collider (LHC), a 27 km circumference proton collider to be built at CERN, VUV synchrotron radiation (SR) has been used to study both qualitatively and quantitatively candidate vacuum chamber materials. Emphasis is given to show that angle and energy resolved photoemission is an extremely powerful tool to address important issues relevant to the LHC, such as the emission of electrons that contributes to the creation of an electron cloud which may cause serious beam instabilities and unmanageable heat loads on the cryogenic system. Here we present not only the measured photoelectron yields from the proposed materials, prepared on an industrial scale, but also the energy and in some cases the angular dependence of the emitted electrons when excited with either a white light (WL) spectrum, simulating that in the arcs of the LHC, or monochromatic light in the photon energy range of interest. The effects on the materials examined of WL irradiation and /or ion sputtering, simulating the SR and ion bombardment expected in the LHC, were investigated. The studied samples exhibited significant modifications, in terms of electron emission, when exposed to the WL spectrum from the BESSY Toroidal Grating Monochromator beam line. Moreover, annealing and ion bombardment also induce substantial changes to the surface thereby indicating that such surfaces would not have a constant electron emission during machine operation. Such characteristics may be an important issue to define the surface properties of the LHC vacuum chamber material and are presented in detail for the various samples analyzed. It should be noted that all the measurements presented here were recorded at room temperature, whereas the majority of the LHC vacuum system will be maintained at temperatures below 20 K. The results cannot therefore be directly applied to these sections of the machine until measurements at cryogenic temperatures, i.e., in the presence of cryosorbed gas layers, are obtained. However, these results are directly relevant to all the warm regions of the LHC vacuum system, such as the experimental vacuum chambers and warm element vacuum chambers in the insertion regions

Does alexithymia have a mediating effect between impulsivity and emotional-behavioral functioning in adolescents with binge eating disorder?

Objective: Binge eating Disorder (BeD) has recently been included in the Dsm-5. like many other eating disorder symptoms, BED is often present in adolescence. No studies have specifically investigated the influence of impulsivity and alexithymia on the emotional-behavioural functioning of adolescents diagnosed with BeD. Method: in this study, we recruited n = 162 adolescents (age range: 14-18) and divided them into two groups: 78 adolescents diagnosed with BeD, according to the Dsm-5 criteria (group A), and 84 healthy controls (group B). participants completed the youth self-report (ysr/11-18), the toronto Alexithymia scale (tAs-20) and the Barratt impulsiveness scale (Bis-11). Results: the results showed that group A had higher scores of alexithymia, impulsivity and maladaptive emotionalbehavioural functioning than Group B. Furthermore, alexithymia had a mediating effect on the relationship between impulsivity and emotional-behavioural functioning. Conclusions: These results allow us to hypothesise that alexithymia is a key variable influencing the emotional and behavioural problems of adolescents affected by BED. Our data confirms the previous studies underlining the association of impulsivity and alexithymia with the diagnosis of BeD in adolescents, and our study contributes to the previous literature, emphasising the central role of alexithymia in the mediating effect between impulsivity and emotional and behavioural problems. these results suggest the importance of promoting prevention and treatment polices focused on alexithymia

VUV photoemission studies of candidate LHC vacuum chamber materials

In the context of future accelerators and, in particular, the beam vacuum of the LargeHadron Collider (LHC), a 27 km circumference proton collider to be built at CERN, VUVsynchrotron radiation (SR) has been used to study both qualitatively and quantitatively candidatevacuum chamber materials. Emphasis is given to show that angle and energy resolvedphotoemission is an extremely powerful tool to address important issues relevant to the LHC, suchas the emission of electrons that contribute to the creation of an electron cloud which may causeserious beam instabilities. Here we present not only the measured photoelectron yields (PY)from the proposed materials, prepared on an industrial scale, but also the energy and, in some cases,the angular dependence of the emitted electrons when excited with either a white light (WL)spectrum, simulating that in the arcs of the LHC or monochromatic light in the photon energy rangeof interest. The effects on the materials examined of WL irradiation and/or ion sputtering,simulating the SR and ion bombardment expected in the LHC, were investigated. The studiedsamples exhibited significant modifications, in terms of electron emission, when exposed to the WLspectrum from the BESSY TGM7 beamline. Moreover, annealing and ion bombardment alsoinduce substantial changes to the surface thereby indicating that such surfaces would not have aconstant electron emission during machine operation. Such characteristics may be an importantissue to define the surface properties of the LHC vacuum chamber material and are presented indetail for the various samples analysed

Surface photovoltage effects in photoemission from metal-GaP(110) interfaces: Importance for band bending evaluation

Photoelectron spectra from metal overlayers on GaP(110) show that the photoionization light source may induce a surface photovoltage, causing an energy shift of valence- and core-level peaks. We analyze the dependence of this surface photovoltage on metal coverage, substrate doping, and temperature. The presence of a surface photovoltage seriously affects the determination of surface band bending by photoelectron spectroscopy, a technique which is generally thought to reflect the equilibrium electronic structure of metal-semiconductor interfaces

Surface photovoltage effects in photoemission from metal/GaP(110) interfaces: Temperature‐dependent Fermi level movement

In recent experiments of metal deposition onto cleaved GaP(110) surfaces we have shown that light sources used in photoelectron spectroscopy may induce a surface photovoltage (SPV), which causes a substantial deviation from the ground state potential distribution, and may induce errors in the determination of band bending by photoemission. Here we analyze the temperature‐dependent movement of the surface Fermi level in n‐ and p‐type GaP(110) surfaces as a function of indium and silver deposition, taking into account the presence of the SPV. It is found that changes in the substrate temperature not only modify the adlayer morphology and metallicity, but also the surface electron‐hole recombination rate. We observe that the temperature‐dependent shift of the semiconductor core levels is always accompanied by a similar shift of the metal core level and Fermi edge, suggesting that the reversible temperature‐dependent band bending recently reported for metal/III–V semiconductor interfaces is related to the SPV, and does not represent a ground state property of the interfacial electronic structure. Implications of these results on current models concerning Schottky barrier formation are discussed

The Chemical Origin of SEY at Technical Surfaces

The secondary emission yield (SEY) properties of colaminated Cu samples for LHC beam screens are correlated to the surface chemical composition determined by X-ray photoelectron spectroscopy. The surface of the "as received" samples is characterized by the presence of significant quantities of contaminating adsorbates and by the maximum of the SEY curve (dmax) being as high as 2.2. After extended electron scrubbing at kinetic energy of 10 and 500 eV, the dmax value drops to the ultimate values of 1.35 and 1.1, respectively. In both cases the surface oxidized phases are significantly reduced, whereas only in the sample scrubbed at 500 eV the formation of a graphitic-like C layer is observed. We find that the electron scrubbing of technical Cu surfaces can be described as occurring in two steps, where the first step consists in the electron induced desorption of weakly bound contaminants that occurs indifferently at 10 and at 500 eV and corresponds to a partial decrease of dmax, and the second step, activated by more energetic electrons and becoming evident at high doses, which increases the number of graphitic-like C-C bonds via the dissociation of adsorbates already contaminating the "as received" surface or accumulating on this surface during irradiation. Our results demonstrate how the kinetic energy of impinging electrons is a crucial parameter when conditioning technical surfaces of Cu and other metals by means of electron induced chemical processing.Comment: 6 pages, contribution to the Joint INFN-CERN-EuCARD-AccNet Workshop on Electron-Cloud Effects: ECLOUD'12; 5-9 Jun 2012, La Biodola, Isola d'Elba, Italy; CERN Yellow Report CERN-2013-002, pp.99-10