440 research outputs found
On the Relativistic Description of the Nucleus
We present here a formalism able to generalise to a relativistically
covariant scheme the standard nuclear shell model. We show that, using some
generalised nuclear Green's functions and their Lehmann representation we can
define the relativistic equivalent of the non relativistic single particle wave
function (not loosing, however, the physical contribution of other degrees of
freedom, like mesons and antinucleons). It is shown that the mass operator
associated to the nuclear Green's function can be approximated with the
equivalent of a shell-model potential and that the corresponding ``single
particle wave functions'' can be easily derived in a specified frame of
reference and then boosted to any other system, thus fully restoring the
Lorentz covarianc
Static properties of nuclear matter within the Boson Loop Expansion
The use of the Boson Loop Expansion is proposed for investigating the static
properties of nuclear matter. We explicitly consider a schematic dynamical
model in which nucleons interact with the scalar-isoscalar sigma meson. The
suggested approximation scheme is examined in detail at the mean field level
and at the one- and two-loop orders. The relevant formulas are provided to
derive the binding energy per nucleon, the pressure and the compressibility of
nuclear matter. Numerical results of the binding energy at the one-loop order
are presented for Walecka's sigma-omega model in order to discuss the degree of
convergence of the Boson Loop Expansion.Comment: 40 pages, 13 figure
Functional approach to the non-mesonic decay of Lambda-hypernuclei
We present an evaluation of the non-mesonic decay widths for
Lambda-hypernuclei (Lambda N --> NN, Lambda NN --> NNN) within the framework of
the polarization propagator method. The full Lambda self-energy is evaluated
microscopically in nuclear matter by using the functional approach, which
supplies a theoretically well grounded approximation scheme for the
classification of the relevant diagrams, according to the prescriptions of the
bosonic loop expansion. We employ average Fermi momenta, suitably adapted to
different mass number regions (medium-light, medium and heavy hypernuclei).
Moreover, we study the dependence of the decay rates on the NN and Lambda-N
short range correlations. With a proper choice of the parameters which control
these correlations in the new approximation scheme, it is possible to reproduce
the experimental decay widths for A > 10 hypernuclei.Comment: 25 pages, 8 figure
Functional approach to the electromagnetic response function: the Longitudinal Channel
In this paper we address the (charge) longitudinal electromagnetic response
for a homogeneous system of nucleons interacting via meson exchanges in the
functional framework. This approach warrants consistency if the calculation is
carried on order-by-order in the mesonic loop expansion with RPA-dressed
mesonic propagators. At the 1-loop order and considering pion, rho and omega
exchanges we obtain a quenching of the response, in line with the experimental
results.Comment: RevTeX, 18 figures available upon request - to be published in
Physical Review
Connecting scaling with short-range correlations
We reexamine several issues related to the physics of scaling in electron
scattering from nuclei. A basic model is presented in which an assumed form for
the momentum distribution having both long- and short-range contributions is
incorporated in the single-particle Green function. From this one can obtain
saturation of nuclear matter for an NN interaction with medium-range attraction
and short-range repulsion, and can obtain the density-density polarization
propagator and hence the electromagnetic response and scaling function. For the
latter, the shape of the scaling function and how it approaches scaling as a
function of momentum transfer are both explored.Comment: 24 pages, 15 figures. A reference has been corrected and update
INFESTATION RATE OF BRANCHIOBDELLIDS IN AUSTROPOTAMOBIUS PALLIPES ITALICUS FROM A STREAM OF CENTRAL ITALY: PRELIMINARY RESULTS
Species identity, infestation rate and distribution over the host body of branchiobdellids (Annelida, Clitellata) have been analysed in an Austropotamobius pallipes italicus population from the National Park of the Foreste Casentinesi, Monte Falterona and Campigna (central Italy). Branchiobdella italica - the nearly unique species present in our samples - did not differ in its occurrence among the size classes of its host, while female crayfish showed a significantly higher infestation rate than the other sex. Branchiobdellids were mostly found on gill chambers and cephalothorax base, and only in the case of larger crayfish and males did they occupy other body areas
Analyzing Virtual Reference Station for GPS surveying: experiments and applications in a test site of the northern Apennine (Italy).
The availability of a GPS network of 10-20 km mean size, provides good topographical support for the measurement of ground displacements, even at a local scale such as a landslide. In particular, a series of multitemporal kinematic or rapid-static GPS acquisitions of a landslide allows a good characterization of its displacements if the measurements are referred to a GPS reference network. Nevertheless, a wider network formed by stations located at long distances, for example at several tens of kilometers, characterized by large spacing, can lead to results affected by high noise, degrading the accuracy of final point positions. In order to obtain an adequate GPS reference network, some virtual reference stations (VRSs) can be introduced, even if a network refinement based on VRS cannot reach the same accuracy of a real local network. Some experiments, including measurements on a real landslide, have been performed in order to evaluate the performance of this technique. The results point out that the standard deviation of the obtained solutions is about two or three times larger than those which can be reached using a real local network
SOLUZIONE DINAMICA DELLA RETE ASSOGEO E INQUADRAMENTO EUREF-RDN
L’importante sviluppo avuto recentemente dai servizi GPS di posizionamento differenziale in tempo
reale (RTK e VRS), ha permesso di diminuire in modo considerevole i tempi e quindi i costi per un rilievo
topografico, questo senza incidere in modo significativo sulla precisione delle misure eseguite [Pesci et al.,
2008]. L’infrastruttura che permette la realizzazione di questi servizi è costituita da una rete GPS di stazioni
permanenti (SP) distribuite più o meno regolarmente su di una griglia la cui maglia, cioè la distanza tra le
stazioni, è di qualche decina di chilometri.
Assogeo S.p.a. a partire dal 2006 ha sviluppato nell’Italia centro-settentrionale una rete di stazioni
permanenti (Fig. 1a) in grado di supportare i diversi servizi di posizionamento in tempo reale.Attualmente questa rete è costituita da 32 stazioni equipaggiate con ricevitore ed antenne geodetiche a
doppia frequenza (Tab. 1) in continua espansione. Un singolo operatore, anche se equipaggiato di un solo
ricevitore GPS, ha la possibilitĂ di connettersi al centro operativo Assogeo ed eseguire il proprio rilievo
topografico, in tempo reale. Il sistema di riferimento in cui vengono forniti i risultati del rilievo è quello su
cui vengono calcolate le posizioni delle diverse SP, per questo motivo è importante che tale sistema di
riferimento sia compatibile con quello utilizzato in ambito cartografico e topografico definito dall’Istituto
Geografico Militare Italiano (IGMI). Inoltre, è necessario che la stima della posizione delle diverse SP sia la
piĂą precisa possibile e che quindi venga calcolata utilizzando tutte le informazioni acquisite da tali stazioni.
A questo proposito IGMI ha istituito la Rete Dinamica Nazionale [RDN, Baroni et al. 2009] formata da 99
SP giĂ operanti sul territorio e omogeneamente distribuite, offrendo un riferimento per allineare le diverse
reti GPS per il servizio di posizionamento in tempo reale sorte sul territorio italiano.
La rete RDN è a sua volta allineata ad un sistema di riferimento convenzionale ufficializzato in
Europa, cioè il sistema ETRF2000 [Bruyninx et al., 2009], e riferito all’epoca 2005.0, secondo le più recenti
direttive EUREF [Bruyninx, 2004, Kenyeres and Bruyninx, 2004].
Per uniformare le soluzioni allo stesso sistema di riferimento utilizzato dall’IGMI, è necessario quindi
che anche le posizioni delle SP della rete Assogeo siano calcolate in ETRF2000 all’epoca 2005.0 e poiriportate al 2008.0. In questo lavoro vengono proposte e confrontate due strategie di calcolo per ottenere, in
un modo semplice e intuitivo, il sistema di riferimento desiderato. Si tratta di procedure differenti che
analizzano il medesimo insieme di osservazioni utilizzando lo stesso software, ma utilizzando strategie
diverse, questo per verificare la reale precisione con cui può essere stimata la posizione di una stazione GPS
permanente
GPS-based monitoring of land subsidence in the Po Plain (Northern Italy)
We use regional and local networks of continuously-operating GPS stations (CGPS) distributed in the northern–central part of the Italian peninsula to investigate the subsidence phenomenon of the Po plain sedimentary basin and vertical movements of the surrounding areas. The observations of 146 scientific and commercial stations are analyzed and compared, adopting analytical techniques widely used to study GPS coordinate time series. The use of simple antenna supports in commercial installations, instead of a more rigorous geodetic monument, does not seem to induce significant differences in the noise characteristics and in the amplitudes of annual and semi-annual periodic signals.
The vertical velocity field deduced from 129 sites with observation time spans greater than one year, located in the Central–Northern Italian Peninsula, indicates the presence of two mainly subsidence areas: the Po Plain and the Arno Plain, while the sites located in the Alps and Apennine domains show relatively low uplift.
The areas of the Po Plain monitored by GPS seem to indicate that the subsidence rate is constant or, in some cases, decreasing with respect to the values obtained from the last measurements, performed up to 2006 by means of both SAR and levelling techniques. Only the central part of the eastern Po Plain close to the Apennine border (Modena city area) is characterized by a peak in subsidence consisting in a velocity of about 15 mm/yr
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