1,037 research outputs found
Mean Transverse Energy of Ultrananocrystalline Diamond Photocathode
Nitrogen incorporated ultrananocrystalline diamond ((N)UNCD) could be an
enabling material platform for photocathode applications due to its high
emissivity. While the quantum efficiency (QE) of UNCD was reported by many
groups, no experimental measurements of the intrinsic emittance/mean transverse
energy (MTE) have been reported. Here, MTE measurement results for an (N)UNCD
photocathode in the photon energy range of 4.41 to 5.26 eV are described. The
MTE demonstrates no noticeable dependence on the photon energy, with an average
value of 266 meV. This spectral behavior is shown to not to be dependent upon
physical or chemical surface roughness and inconsistent with low electron
effective mass emission from graphitic grain boundaries, but may be associated
with emission from spatially-confined states in the graphite regions between
the diamond grains. The combined effect of fast-growing QE and constant MTE
with respect to the excess laser energy may pave the way to bright UNCD
photocathodes.Comment: 4 pages, 4 figure
Spectral Emission Properties of a Nitrogen-doped Diamond(001) Photocathode: Hot Electron Transport and Transverse Momentum Filtering
The electron emission properties of a single-crystal nitrogen-doped
diamond(001) photocathode inserted in a 10kV DC photoelectron gun are
determined using a tunable (235-410nm) ultraviolet laser radiation source for
photoemission from both the back nitrogen-doped substrate face and the front
homo-epitaxially grown and undoped diamond crystal face. The measured spectral
trends of the mean transverse energy (MTE) and quantum efficiency (QE) of the
emitted electrons are both anomalous and non-monotonic, but are shown to be
consistent with (i) the known physics of electron photoexcitation from the
nitrogen substitution states into the conduction bands of diamond, (ii) the
energy position and dispersion characteristics of the conduction bands of
diamond in the (001) emission direction, (iii) the effective electron affinity
of the crystal faces, (iv) the strong electron-(optical)phonon coupling in
diamond, and (v) the associated hot electron transport dynamics under energy
equipartition with the optical phonons. Notably, the observed hot electron
emission is shown to be restricted parallel to the photocathode surface by the
low transverse effective masses of the emitting band states - a transverse
momentum filtering effect.Comment: 14 pages, 4 figure
Ultracold electrons via Near-Threshold Photoemission from Single-Crystal Cu(100)
Achieving a low mean transverse energy or temperature of electrons emitted
from the photocathode-based electron sources is critical to the development of
next-generation and compact X-ray Free Electron Lasers and Ultrafast Electron
Diffraction, Spectroscopy and Microscopy experiments. In this paper, we
demonstrate a record low mean transverse energy of 5 meV from the cryo-cooled
(100) surface of copper using near-threshold photoemission. Further, we also
show that the electron energy spread obtained from such a surface is less than
11.5 meV, making it the smallest energy spread electron source known to date:
more than an order of magnitude smaller than any existing photoemission, field
emission or thermionic emission based electron source. Our measurements also
shed light on the physics of electron emission and show how the energy spread
at few meV scale energies is limited by both the temperature and the vacuum
density of states
Altered brain energetics induces mitochondrial fission arrest in Alzheimer's Disease.
Altered brain metabolism is associated with progression of Alzheimer's Disease (AD). Mitochondria respond to bioenergetic changes by continuous fission and fusion. To account for three dimensional architecture of the brain tissue and organelles, we applied 3-dimensional electron microscopy (3D EM) reconstruction to visualize mitochondrial structure in the brain tissue from patients and mouse models of AD. We identified a previously unknown mitochondrial fission arrest phenotype that results in elongated interconnected organelles, "mitochondria-on-a-string" (MOAS). Our data suggest that MOAS formation may occur at the final stages of fission process and was not associated with altered translocation of activated dynamin related protein 1 (Drp1) to mitochondria but with reduced GTPase activity. Since MOAS formation was also observed in the brain tissue of wild-type mice in response to hypoxia or during chronological aging, fission arrest may represent fundamental compensatory adaptation to bioenergetic stress providing protection against mitophagy that may preserve residual mitochondrial function. The discovery of novel mitochondrial phenotype that occurs in the brain tissue in response to energetic stress accurately detected only using 3D EM reconstruction argues for a major role of mitochondrial dynamics in regulating neuronal survival
Imaging Asteroid 4 Vesta Using the Framing Camera
The Framing Camera (FC) onboard the Dawn spacecraft serves a dual purpose. Next to its central role as a prime science instrument it is also used for the complex navigation of the ion drive spacecraft. The CCD detector with 1024 by 1024 pixels provides the stability for a multiyear mission and its high requirements of photometric accuracy over the wavelength band from 400 to 1000 nm covered by 7 band-pass filters. Vesta will be observed from 3 orbit stages with image scales of 227, 63, and 17 m/px, respectively. The mapping of Vesta s surface with medium resolution will be only completed during the exit phase when the north pole will be illuminated. A detailed pointing strategy will cover the surface at least twice at similar phase angles to provide stereo views for reconstruction of the topography. During approach the phase function of Vesta was determined over a range of angles not accessible from earth. This is the first step in deriving the photometric function of the surface. Combining the topography based on stereo tie points with the photometry in an iterative procedure will disclose details of the surface morphology at considerably smaller scales than the pixel scale. The 7 color filters are well positioned to provide information on the spectral slope in the visible, the depth of the strong pyroxene absorption band, and their variability over the surface. Cross calibration with the VIR spectrometer that extends into the near IR will provide detailed maps of Vesta s surface mineralogy and physical properties. Georeferencing all these observation will result in a coherent and unique data set. During Dawn s approach and capture FC has already demonstrated its performance. The strong variation observed by the Hubble Space Telescope can now be correlated with surface units and features. We will report on results obtained from images taken during survey mode covering the whole illuminated surface. Vesta is a planet-like differentiated body, but its surface gravity and escape velocity are comparable to those of other asteroids and hence much smaller than those of the inner planets o
Dynamical ionization ignition of clusters in intense and short laser pulses
The electron dynamics of rare gas clusters in laser fields is investigated
quantum mechanically by means of time-dependent density functional theory. The
mechanism of early inner and outer ionization is revealed. The formation of an
electron wave packet inside the cluster shortly after the first removal of a
small amount of electron density is observed. By collisions with the cluster
boundary the wave packet oscillation is driven into resonance with the laser
field, hence leading to higher absorption of laser energy. Inner ionization is
increased because the electric field of the bouncing electron wave packet adds
up constructively to the laser field. The fastest electrons in the wave packet
escape from the cluster as a whole so that outer ionization is increased as
well.Comment: 8 pages, revtex4, PDF-file with high resolution figures is available
from http://mitarbeiter.mbi-berlin.de/bauer/publist.html, publication no. 24.
Accepted for publication in Phys. Rev.
Surgical treatment of symptomatic pineal cysts without hydrocephalus-meta-analysis of the published literature
Background To examine published data and assess evidence relating to safety and efficacy of surgical management of symptomatic pineal cysts without hydrocephalus (nhSPC), we performed a systematic review of the literature and meta-analysis. Methods Following the PRISMA guidelines, we searched Pubmed and SCOPUS for all reports with the query 'Pineal Cyst' AND 'Surgery' as of March 2021, without constraints on study design, publication year or status (PROSPERO_CRD:42,021,242,517). Assessment of 1537 hits identified 26 reports that met inclusion and exclusion criteria. Results All 26 input studies were either case reports or single-centre retrospective cohorts. The majority of outcome data were derived from routine physician-recorded notes. A total of 294 patients with surgically managed nhSPC were identified. Demographics: Mean age was 29 (range: 4-63) with 77% females. Mean cyst size was 15 mm (5-35). Supracerebellar-infratentorial approach was adopted in 90% of cases, occipital-transtentorial in 9%, and was not reported in 1%. Most patients were managed by cyst resection (96%), and the remainder by fenestration. Mean post-operative follow-up was 35 months (0-228). Presentation: Headache was the commonest symptom (87%), followed by visual (54%), nausea/vomit (34%) and vertigo/dizziness (31%). Other symptoms included focal neurology (25%), sleep disturbance (17%), cognitive impairment (16%), loss of consciousness (11%), gait disturbance (11%), fatigue (10%), 'psychiatric' (2%) and seizures (1%). Mean number of symptoms reported at presentation was 3 (0-9). Outcomes: Improvement rate was 93% (to minimise reporting bias only consecutive cases from cohort studies were considered, N= 280) and was independent of presentation. Predictors of better outcomes were large cyst size (OR= 5.76; 95% CI: 1.74-19.02) and resection over fenestration (OR= 12.64; 3.07-52.01). Age predicted worse outcomes (OR= 0.95; 0.91-0.99). Overall complication rate was 17% and this was independent of any patient characteristics. Complications with long-term consequences occurred in 10 cases (3.6%): visual disturbance (3), chronic incisional pain (2), sensory disturbance (1), fatigue (1), cervicalgia (1), cerebellar stroke (1) and mortality due to myocardial infarction (1). Conclusions Although the results support the role of surgery in the management of nhSPCs, they have to be interpreted with a great deal of caution as the current evidence is limited, consisting only of case reports and retrospective surgical series. Inherent to such studies are inhomogeneity and incompleteness of data, selection bias and bias related to assessment of outcome carried out by the treating surgeon in the majority of cases. Prospective studies with patient-reported and objective outcome assessment are needed to provide higher level of evidence.Peer reviewe
- …