901 research outputs found
The role of -induced reactions on lead and iron in neutrino detectors
We have calculated cross sections and branching ratios for neutrino induced
reactions on ^{208}Pb and ^{56}Fe for various supernova and
accelerator-relevant neutrino spectra. This was motivated by the facts that
lead and iron will be used on one hand as target materials in future neutrino
detectors, on the other hand have been and are still used as shielding
materials in accelerator-based experiments. In particular we study the
inclusive ^{56}^{56}Co and ^{208}^{208}Bi cross
sections and calculate the neutron energy spectra following the decay of the
daughter nuclei. These reactions give a potential background signal in the
KARMEN and LSND experiment and are discussed as a detection scheme for
supernova neutrinos in the proposed OMNIS and LAND detectors. We also study the
neutron-emission following the neutrino-induced neutral-current excitation of
^{56}Fe and ^{208}Pb.Comment: 23 pages (including 7 figures
Muon capture on nuclei with N > Z, random phase approximation, and in-medium renormalization of the axial-vector coupling constant
We use the random phase approximation to describe the muon capture rate on
Ca,Ca, Fe, Zr, and Pb. With
Ca as a test case, we show that the Continuum Random Phase
Approximation (CRPA) and the standard RPA give essentially equivalent
descriptions of the muon capture process. Using the standard RPA with the free
nucleon weak form factors we reproduce the experimental total capture rates on
these nuclei quite well. Confirming our previous CRPA result for the
nuclei, we find that the calculated rates would be significantly lower than the
data if the in-medium quenching of the axial-vector coupling constant were
employed.Comment: submitted to Phys. Rev.
Neutrino induced transitions between the ground states of the A=12 triad
Neutrino induced reactions on C, an ingredient of liquid
scintillators, have been studied in several experiments. We show that for
currently available neutrino energies, 300 MeV, calculated
exclusive cross sections CN for both muon
and electron neutrinos are essentially model independent, provided the
calculations simultaneously describe the rates of several other reactions
involving the same states or their isobar analogs. The calculations agree well
with the measured cross sections, which can be therefore used to check the
normalization of the incident neutrino spectrum and the efficiency of the
detector.Comment: 9 pages REVTEX, 2 postscript figures, text and figures available at
http://www.krl.caltech.edu/preprints/MAP.htm
DEFINITION OF A TRANSITION SURFACE WITH THE PURPOSE OF INTEGRATION BETWEEN A LASER SCANNER 3D MODEL AND A LOW RESOLUTION DTM
ABSTRACT: Thanks to quickly spreading technologies like laser scanning, which are becoming a quite common means of data acquisition for architectural objects or cultural heritage sites (but not only!), integration between datasets of different origin and resolution is still an open problem. This paper describes an approach whose goal is to define a surface which models a proper transition between a high resolution, laser-scanner-acquired model and a low resolution digital terrain model (DTM), by means of some "extra" information around the high resolution object as sort of "collar". This information is generally present in laser scanner models and instead of pruning it during point cloud editing, we use it for our modelling purposes. We present a (so far) deterministic approach, some initial results and discuss still unresolved issues and future improvements. KURZFASSUNG: Dank der schnellen Verbreitung von Technologien wie z.B. Laser-Scannen, die zur Vermessung architektonischer Objekte oder KulturgĂŒter immer hĂ€ufiger Anwendung finden, bleibt die Integration von DatensĂ€tzen unterschiedlichen Ursprungs und verschiedener Auflösung ein noch ungelöstes Problem. In diesem Artikel wird ein Ansatz beschrieben, dessen Ziel die Definition einer ĂbergangsflĂ€che zwischen einem hoch aufgelösten, durch Laser-Scanner aufgenommenen Model und einem gröberen digitalen GelĂ€ndemodell (DGM) ist, indem einige zusĂ€tzliche Informationen um das hoch aufgelöste Objekt so wie ein "Kragen" verwendet werden. Diese weiteren Daten sind im Allgemeinen in den Laser-Scanner-DatensĂ€tzen bereits vorhanden und werden hiermit zum Zweck der OberflĂ€chenmodellierung genutzt, statt -wie sonst ĂŒblich -in der Editierungsphase eliminiert zu werden. Ein deterministischer Ansatz wird, zusammen mit den ersten Test-Ergebnissen, vorgestellt. Offene Probleme und kĂŒnftige Aufbesserungen werden angesprochen
Monitoring surface resonances on Co2MnSi(100) by spin-resolved photoelectron spectroscopy
The magnitude of the spin polarization at the Fermi level of ferromagnetic
materials at room temperature is a key property for spintronics. Investigating
the Heusler compound CoMnSi a value of 93 for the spin polarization has
been observed at room temperature, where the high spin polarization is related
to a stable surface resonance in the majority band extending deep into the
bulk. In particular, we identified in our spectroscopical analysis that this
surface resonance is embedded in the bulk continuum with a strong coupling to
the majority bulk states. The resonance behaves very bulk-like, as it extends
over the first six atomic layers of the corresponding (001)-surface. Our study
includes experimental investigations, where the bulk electronic structure as
well as surface-related features have been investigated using spin-resolved
photoelectron spectroscopy (SR-UPS) and for a larger probing depth
spin-integrated high energy x-ray photoemission spectroscopy (HAXPES). The
results are interpreted in comparison with first-principles band structure and
photoemission calculations which consider all relativistic, surface and
high-energy effects properly.Comment: 9 pages, 8 figures, Heusler alloy, electronic structure and
photoemissio
Signatures of Nucleon Disappearance in Large Underground Detectors
For neutrons bound inside nuclei, baryon instability can manifest itself as a
decay into undetectable particles (e.g., ), i.e.,
as a disappearance of a neutron from its nuclear state. If electric charge is
conserved, a similar disappearance is impossible for a proton. The existing
experimental lifetime limit for neutron disappearance is 4-7 orders of
magnitude lower than the lifetime limits with detectable nucleon decay products
in the final state [PDG2000]. In this paper we calculated the spectrum of
nuclear de-excitations that would result from the disappearance of a neutron or
two neutrons from C. We found that some de-excitation modes have
signatures that are advantageous for detection in the modern high-mass,
low-background, and low-threshold underground detectors, where neutron
disappearance would result in a characteristic sequence of time- and
space-correlated events. Thus, in the KamLAND detector [Kamland], a
time-correlated triple coincidence of a prompt signal, a captured neutron, and
a decay of the residual nucleus, all originating from the same
point in the detector, will be a unique signal of neutron disappearance
allowing searches for baryon instability with sensitivity 3-4 orders of
magnitude beyond the present experimental limits.Comment: 13 pages including 6 figures, revised version, to be published in
Phys.Rev.
Strangeness in the nucleon and the ratio of proton-to-neutron neutrino-induced quasi-elastic yield
The electroweak form factors of the nucleon as obtained within a three flavor
pseudoscalar vector meson soliton model are employed to predict the ratio of
the proton and neutron yields from , which are induced by quasi-elastic
neutrino reactions. These predictions are found to vary only moderately in the
parameter space allowed by the model. The antineutrino flux of the up-coming
experiment determining this ratio was previously overestimated. The
corresponding correction is shown to have only a small effect on the predicted
ratio. However, it is found that the experimental result for the ratio
crucially depends on an accurate measurement of the energy of the knocked out
nucleon.Comment: 17 pages, LaTeX, 2 tables, 4 figures, Discussion on shape of strange
form factors added, Z. Phys. A, to be publishe
Neutrino-induced neutron spallation and supernova r-process nucleosynthesis
In order to explore the consequences of the neutrino irradiation for the
supernova r-process nucleosynthesis, we calculate the rates of charged-current
and neutral-current neutrino reactions on neutron-rich heavy nuclei, and
estimate the average number of neutrons emitted in the resulting spallation.
Our results suggest that charged-current captures can be important in
breaking through the waiting-point nuclei at N=50 and 82, while still allowing
the formation of abundance peaks. Furthermore, after the r-process freezes out,
there appear to be distinctive neutral-current and charged-current
postprocessing effects. A subtraction of the neutrino postprocessing effects
from the observed solar r-process abundance distribution shows that two mass
regions, A=124-126 and 183-187, are inordinately sensitive to neutrino
postprocessing effects. This imposes very stringent bounds on the freeze-out
radii and dynamic timescales governing the r-process. Moreover, we find that
the abundance patterns within these mass windows are entirely consistent with
synthesis by neutrino interactions. This provides a strong argument that the
r-process must occur in the intense neutrino flux provided by a core-collapse
supernova.Comment: 34 pages, 4 PostScript figures, RevTe
Microscopic theories of neutrino-^{12}C reactions
In view of the recent experiments on neutrino oscillations performed by the
LSND and KARMEN collaborations as well as of future experiments, we present new
theoretical results of the flux averaged and
cross sections. The approaches used are
charge-exchange RPA, charge-exchange RPA among quasi-particles (QRPA) and the
Shell Model. With a large-scale shell model calculation the exclusive cross
sections are in nice agreement with the experimental values for both reactions.
The inclusive cross section for coming from the decay-in-flight of
is to be compared to the experimental value
of , while the one due to
coming from the decay-at-rest of is which
agrees within experimental error bars with the measured values. The shell model
prediction for the decay-in-flight neutrino cross section is reduced compared
to the RPA one. This is mainly due to the different kind of correlations taken
into account in the calculation of the spin modes and partially due to the
shell-model configuration basis which is not large enough, as we show using
arguments based on sum-rules.Comment: 17 pages, latex, 5 figure
Polarization transfer in the O reaction at forward angles and structure of the spin-dipole resonances
Cross sections and polarization transfer observables in the O
reactions at 392 MeV were measured at several angles between
0 and 14. The non-spin-flip () and spin-flip
() strengths in transitions to several discrete states and broad
resonances in O were extracted using a model-independent method. The
giant resonances in the energy region of 27 MeV were found to be
predominantly excited by transitions. The strength distribution
of spin-dipole transitions with and were deduced.
The obtained distribution was compared with a recent shell model calculation.
Experimental results are reasonably explained by distorted-wave impulse
approximation calculations with the shell model wave functions.Comment: 28 pages RevTex, including 9 figures, to be published in Phys. Rev.
C.; a typo in Eq. (3b) was correcte
- âŠ