toggle visibility Search & Display Options

Select All    Deselect All
 |   | 
Details
   print
  Records Links
Author (down) Studen, A.; Chesi, E.; Cindro, V.; Clinthorne, N.H.; Cochran, E.; Grosicar, B.; Honscheid, K.; Kagan, H.; Lacasta, C.; Llosa, G.; Linhart, V.; Mikuz, M.; Stankova, V.; Weilhammer, P.; Zontar, D. doi  openurl
  Title A silicon PET probe Type Journal Article
  Year 2011 Publication Nuclear Instruments & Methods in Physics Research A Abbreviated Journal Nucl. Instrum. Methods Phys. Res. A  
  Volume 648 Issue Pages S255-S258  
  Keywords PET; Silicon detectors  
  Abstract PET scanners with high spatial resolution offer a great potential in improving diagnosis, therapy monitoring and treatment validation for several severe diseases. One way to improve resolution of a PET scanner is to extend a conventional PET ring with a small probe with excellent spatial resolution. The probe is intended to be placed close to the area of interest. The coincidences of interactions within the probe and the external ring provide a subset of data which combined with data from external ring, greatly improve resolution in the area viewed by the probe. Our collaboration is developing a prototype of a PET probe, composed of high-resolution silicon pad detectors. The detectors are 1 mm thick, measuring 40 by 26 mm(2), and several such sensors are envisaged to either compensate for low stopping power of silicon or increase the area covered by the probe. The sensors are segmented into 1 mm(3) cubic voxels, giving 1040 readout pads per sensor. A module is composed of two sensors placed in a back-to-back configuration, allowing for stacking fraction of up to 70% within a module. The pads are coupled to a set of 16 ASICs (VaTaGP7.1 by IDEAS) per module and read out through a custom designed data acquisition board, allowing for trigger and data interfacing with the external ring. This paper presents an overview of probe requirements and expected performance parameters. It will focus on the characteristics of the silicon modules and their impact on overall probe performance, including spatial resolution, energy resolution and timing resolution. We will show that 1 mm(3) voxels will significantly extend the spatial resolution of conventional PET rings, and that broadening of timing resolution related to varying depth of photon interactions can be compensated to match the timing resolution of the external ring. The initial test results of the probe will also be presented.  
  Address [Studen, A.; Cindro, V.; Grosicar, B.; Mikuz, M.; Zontar, D.] Jozef Stefan Inst, Ljubljana, Slovenia, Email: andrej.studen@ijs.si  
  Corporate Author Thesis  
  Publisher Elsevier Science Bv Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0168-9002 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000305376900063 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 1070  
Permanent link to this record
 

 
Author (down) Studen, A.; Brzezinski, K.; Chesi, E.; Cindro, V.; Clinthorne, N.H.; Cochran, E.; Grosicar, B.; Grkovski, M.; Honscheid, K.; Kagan, H.; Lacasta, C.; Llosa, G.; Mikuz, M.; Stankova, V.; Weilhammer, P.; Zontar, D. doi  openurl
  Title Silicon detectors for combined MR-PET and MR-SPECT imaging Type Journal Article
  Year 2013 Publication Nuclear Instruments & Methods in Physics Research A Abbreviated Journal Nucl. Instrum. Methods Phys. Res. A  
  Volume 702 Issue Pages 88-90  
  Keywords PET; Silicon detectors; SPECT  
  Abstract Silicon based devices can extend PET-MR and SPECT-MR imaging to applications, where their advantages in performance outweigh benefits of high statistical counts. Silicon is in many ways an excellent detector material with numerous advantages, among others: excellent energy and spatial resolution, mature processing technology, large signal to noise ratio, relatively low price, availability, versatility and malleability. The signal in silicon is also immune to effects of magnetic field at the level normally used in MR devices. Tests in fields up to 7 T were performed in a study to determine effects of magnetic field on positron range in a silicon PET device. The curvature of positron tracks in direction perpendicular to the field's orientation shortens the distance between emission and annihilation point of the positron. The effect can be fully appreciated for a rotation of the sample for a fixed field direction, compressing range in all dimensions. A popular Ga-68 source was used showing a factor of 2 improvement in image noise compared to zero field operation. There was also a little increase in noise as the reconstructed resolution varied between 2.5 and 1.5 mm. A speculative applications can be recognized in both emission modalities, SPECT and PET. Compton camera is a subspecies of SPECT, where a silicon based scatter as a MR compatible part could inserted into the MR bore and the secondary detector could operate in less constrained environment away from the magnet. Introducing a Compton camera also relaxes requirements of the radiotracers used, extending the range of conceivable photon energies beyond 140.5 keV of the Tc-99m. In PET, one could exploit the compressed sub-millimeter range of positrons in the magnetic field. To exploit the advantage, detectors with spatial resolution commensurate to the effect must be used with silicon being an excellent candidate. Measurements performed outside of the MR achieving spatial resolution below 1 mm are reported.  
  Address [Studen, A.; Cindro, V.; Grosicar, B.; Grkovski, M.; Mikuz, M.; Zontar, D.] Jozef Stefan Inst, Ljubljana, Slovenia, Email: andrej.studen@ijs.si  
  Corporate Author Thesis  
  Publisher Elsevier Science Bv Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0168-9002 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000314682300026 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 1331  
Permanent link to this record
 

 
Author (down) Sorel, M. url  doi
openurl 
  Title Expected performance of an ideal liquid argon neutrino detector with enhanced sensitivity to scintillation light Type Journal Article
  Year 2014 Publication Journal of Instrumentation Abbreviated Journal J. Instrum.  
  Volume 9 Issue Pages P10002 - 25pp  
  Keywords Noble liquid detectors (scintillation, ionization, double-phase); Neutrino detectors; Calorimeters; Time projection chambers  
  Abstract Scintillation light is used in liquid argon (LAr) neutrino detectors to provide a trigger signal, veto information against cosmic rays, and absolute event timing. In this work, we discuss additional opportunities offered by detectors with enhanced sensitivity to scintillation light, that is with light collection efficiencies of about 10(-3). We focus on two key detector performance indicators for neutrino oscillation physics: calorimetric neutrino energy reconstruction and neutrino/antineutrino separation in a non-magnetized detector. Our results are based on detailed simulations, with neutrino interactions modelled according to the GENIE event generator, while the charge and light responses of a large LAr ideal detector are described by the Geant4 and NEST simulation tools. A neutrino energy resolution as good as 3.3% RMS for 4 GeV electron neutrino charged-current interactions can in principle be obtained in a large detector of this type, by using both charge and light information. By exploiting muon capture in argon and scintillation light information to veto muon decay electrons, we also obtain muon neutrino identification efficiencies of about 50%, and muon antineutrino misidentification rates at the few percent level, for few-GeV neutrino interactions that are fully contained. We argue that the construction of large LAr detectors with sufficiently high light collection efficiencies is in principle possible.  
  Address [Sorel, M.] CSIC, Inst Fis Corpuscular IFIC, Valencia 46980, Spain, Email: sorel@ific.uv.es  
  Corporate Author Thesis  
  Publisher Iop Publishing Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 1748-0221 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000345858500045 Approved no  
  Is ISI yes International Collaboration no  
  Call Number IFIC @ pastor @ Serial 2056  
Permanent link to this record
 

 
Author (down) Sierra, D.A.; De Romeri, V.; Flores, L.J.; Papoulias, D.K. url  doi
openurl 
  Title Impact of COHERENT measurements, cross section uncertainties and new interactions on the neutrino floor Type Journal Article
  Year 2022 Publication Journal of Cosmology and Astroparticle Physics Abbreviated Journal J. Cosmol. Astropart. Phys.  
  Volume 01 Issue 1 Pages 055 - 26pp  
  Keywords dark matter detectors; dark matter experiments; neutrino properties; solar and atmospheric neutrinos  
  Abstract We reconsider the discovery limit of multi-ton direct detection dark matter experiments in the light of recent measurements of the coherent elastic neutrino-nucleus scattering process. Assuming the cross section to be a parameter entirely determined by data, rather than using its Standard Model prediction, we use the COHERENT CsI and LAr data sets to determine WIMP discovery limits. Being based on a data-driven approach, the results are thus free from theoretical assumptions and fall within the WIMP mass regions where XENONnT and DARWIN have best expected sensitivities. We further determine the impact of subleading nuclear form factor and weak mixing angle uncertainties effects on WIMP discovery limits. We point out that these effects, albeit small, should be taken into account. Moreover, to quantify the impact of new physics effects in the neutrino background, we revisit WIMP discovery limits assuming light vector and scalar mediators as well as neutrino magnetic moments/transitions. We stress that the presence of new interactions in the neutrino sector, in general, tend to worsen the WIMP discovery limit.  
  Address [Aristizabal Sierra, D.] Univ Tecn Federico Santa Maria, Dept Fis, Casilla 110-5,Avda Espana 1680, Valparaiso, Chile, Email: daristizabal@ulg.ac.be;  
  Corporate Author Thesis  
  Publisher IOP Publishing Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 1475-7516 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000751303400010 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 5123  
Permanent link to this record
 

 
Author (down) Schreeck, H.; Paschen, B.; Wieduwilt, P.; Ahlburg, P.; Andricek, L.; Dingfelder, J.; Frey, A.; Lutticke, F.; Marinas, C.; Richter, R.; Schwenker, B. doi  openurl
  Title Effects of gamma irradiation on DEPFET pixel sensors for the Belle II experiment Type Journal Article
  Year 2020 Publication Nuclear Instruments & Methods in Physics Research A Abbreviated Journal Nucl. Instrum. Methods Phys. Res. A  
  Volume 959 Issue Pages 163522 - 9pp  
  Keywords DEPFET; Radiation damage; Particle tracking detectors; Belle II  
  Abstract For the Belle II experiment at KEK (Tsukuba, Japan) the KEKB accelerator was upgraded to deliver a 40 times larger instantaneous luminosity than before, which requires an increased radiation hardness of the detector components. As the innermost part of the Belle II detector, the pixel detector (PXD), based on DEPFET (DEpleted P-channel Field Effect Transistor) technology, is most exposed to radiation from the accelerator. An irradiation campaign was performed to verify that the PXD can cope with the expected amount of radiation. We present the results of this measurement campaign in which an X-ray machine was used to irradiate a single PXD half-ladder to a total dose of 266 kGy. The half-ladder is from the same batch as the half-ladders used for Belle II. According to simulations, the total accumulated dose corresponds to 7-10 years of Belle II operation. While individual components have been irradiated before, this campaign is the first full system irradiation. We discuss the effects on the DEPFET sensors, as well as the performance of the front-end electronics. In addition, we present efficiency studies of the half-ladder from beam tests performed before and after the irradiation.  
  Address [Schreeck, Harrison; Wieduwilt, Philipp; Frey, Ariane; Schwenker, Benjamin] Georg August Univ Gottingen, Phys Inst 2, Friedrich Hund Pl 1, D-37077 Gottingen, Germany, Email: harrison.schreeck@phys.uni-goettingen.de  
  Corporate Author Thesis  
  Publisher Elsevier Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0168-9002 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000518368800016 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 4316  
Permanent link to this record
 

 
Author (down) Sahin, E. et al; Gadea, A.; Algora, A. doi  openurl
  Title Structure of the N=50 As, Ge, Ga nuclei Type Journal Article
  Year 2012 Publication Nuclear Physics A Abbreviated Journal Nucl. Phys. A  
  Volume 893 Issue Pages 1-12  
  Keywords NUCLEAR REACTIONS U-238(Se-82, Ga-81), (Se-82, Ge-82), (Se-82, As-83), E=515 MeV; measured E-gamma, I-gamma (theta), gamma gamma-coin, reaction fragments, (fragment)gamma-coin using PRISMA magnetic spectrometer, gamma after deexcitation using Ge Compton-suppressed detectors of CLARA array, thin and thick target; deduced sigma(theta), levels, J, pi; calculated levels, J, pi using shell model  
  Abstract The level structures of the N = 50 As-83, Ge-82, and Ga-81 isotones have been investigated by means of multi-nucleon transfer reactions. A first experiment was performed with the CLARA PRISMA setup to identify these nuclei. A second experiment was carried out with the GASP array in order to deduce the gamma-ray coincidence information. The results obtained on the high-spin states of such nuclei are used to test the stability of the N = 50 shell closure in the region of Ni-78 (Z = 28). The comparison of the experimental level schemes with the shell-model calculations yields an N = 50 energy gap value of 4.7(3) MeV at Z = 28. This value, in a good agreement with the prediction of the finite-range liquid-drop model as well as with the recent large-scale shell model calculations, does not support a weakening of the N = 50 shell gap down to Z = 28.  
  Address [Sahin, E.; de Angelis, G.; Gadea, A.; Corradi, L.; Fioretto, E.; Gottardo, A.; Guiot, B.; Modamio, V.; Napoli, D. R.; Silvestri, R.; Stefanini, A. M.; Valiente-Dobon, J. J.] Ist Nazl Fis Nucl, Lab Nazl Legnaro, I-35020 Legnaro, Italy, Email: eda.sahin@lnl.infn.it  
  Corporate Author Thesis  
  Publisher Elsevier Science Bv Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0375-9474 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000310091000001 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 1191  
Permanent link to this record
 

 
Author (down) Ruhr, F. et al; Escobar, C.; Miñano, M. doi  openurl
  Title Testbeam studies of barrel and end-cap modules for the ATLAS ITk strip detector before and after irradiation Type Journal Article
  Year 2020 Publication Nuclear Instruments & Methods in Physics Research A Abbreviated Journal Nucl. Instrum. Methods Phys. Res. A  
  Volume 979 Issue Pages 164430 - 6pp  
  Keywords Particle physics; Tracking detectors; ATLAS; HL-LHC; Test beam  
  Abstract In order to cope with the occupancy and radiation doses expected at the High-Luminosity LHC, the ATLAS experiment will replace its Inner Detector with an all-silicon Inner Tracker (ITk), consisting of pixel and strip subsystems. In the last two years, several prototype ITk strip modules have been tested using beams of high energy electrons produced at the DESY-II testbeam facility. Tracking was provided by EUDET telescopes. The modules tested are built from two sensor types: the rectangular ATLAS17LS, which will be used in the outer layers of the central barrel region of the detector, and the annular ATLAS12EC, which will be used in the innermost ring (R0) of the forward region. Additionally, a structure with two RO modules positioned back-to-back has been measured, demonstrating space point reconstruction using the stereo angle of the strips. Finally, one barrel and one RO module have been measured after irradiation to 40% beyond the expected end-of-lifetime fluence. The data obtained allow for thorough tests of the module performance, including charge collection, noise occupancy, detection efficiency, and tracking performance. The results give confidence that the ITk strip detector will meet the requirements of the ATLAS experiment.  
  Address [Ruehr, F.; Argos, C. Garcia; Hauser, M.; Moos, F.; Rodriguez, A. Rodriguez; Sperlich, D.; Wiik-Fuchs, L.] Albert Ludwigs Univ Freiburg, Phys Inst, Freiburg, Germany, Email: frederik.ruehr@cern.ch  
  Corporate Author Thesis  
  Publisher Elsevier Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 0168-9002 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000573295200013 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 4548  
Permanent link to this record
 

 
Author (down) Ros, A.; Lerche, C.W.; Sebastia, A.; Sanchez, F.; Benlloch, J.M. doi  openurl
  Title Retroreflector arrays for better light collection efficiency of gamma-ray imaging detectors with continuous scintillation crystals without DOI misestimation Type Journal Article
  Year 2014 Publication Journal of Instrumentation Abbreviated Journal J. Instrum.  
  Volume 9 Issue Pages P04009 - 14pp  
  Keywords Gamma detectors (scintillators, CZT, HPG, HgI etc); Interaction of radiation with matter; Gamma camera, SPECT, PET PET/CT, coronary CT angiography (CTA); Detector design and construction technologies and materials  
  Abstract A method to improve light collection efficiency of gamma-ray imaging detectors by using retroreflector arrays has been tested, simulations of the behaviour of the scintillation light illuminating the retroreflector surface have been made. Measurements including retroreflector arrays in the setup have also been taken. For the measurements, positron emission tomography (PET) detectors with continuous scintillation crystals have been used. Each detector module consists of a continuous LSO-scintillator of dimensions 49x49x10 mm(3) and a H8500 position-sensitive photo-multiplier (PSPMT) from Hamamatsu. By using a continuous scintillation crystal, the scintillation light distribution has not been destroyed and the energy, the centroids along the x- and y-direction and the depth of interaction (DOI) can be estimated. Simulations have also been run taking into account the use of continuous scintillation crystals. Due to the geometry of the continuous scintillation crystals in comparison with pixelated crystals, a good light collection efficiency is necessary to correctly reconstruct the impact point of the gamma-ray. The aim of this study is to investigate whether micro-machine retro-reflectors improve light yield without misestimation of the impact point. The results shows an improvement on the energy and centroid resolutions without worsening the depth of interaction resolution. Therefore it can be concluded that using retroreflector arrays at the entrance side of the scintillation crystal improves light collection efficiency without worsening the impact point estimation.  
  Address [Ros, A.] Inst Fis Corpuscular IFIC, Valencia 46980, Spain, Email: anrogar2@i3m.upv.es  
  Corporate Author Thesis  
  Publisher Iop Publishing Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 1748-0221 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000336123800049 Approved no  
  Is ISI yes International Collaboration no  
  Call Number IFIC @ pastor @ Serial 1798  
Permanent link to this record
 

 
Author (down) Renner, J.; Cervera-Villanueva, A.; Hernando, J.A.; Izmaylov, A.; Monrabal, F.; Muñoz, J.; Nygren, D.; Gomez-Cadenas, J.J. url  doi
openurl 
  Title Improved background rejection in neutrinoless double beta decay experiments using a magnetic field in a high pressure xenon TPC Type Journal Article
  Year 2015 Publication Journal of Instrumentation Abbreviated Journal J. Instrum.  
  Volume 10 Issue Pages P12020 - 19pp  
  Keywords Pattern recognition, cluster finding, calibration and fitting methods; Double-beta decay detectors; Time projection chambers; Particle tracking detectors (Gaseous detectors)  
  Abstract We demonstrate that the application of an external magnetic field could lead to an improved background rejection in neutrinoless double-beta (0 nu beta beta) decay experiments using a high-pressure xenon (HPXe) TPC. HPXe chambers are capable of imaging electron tracks, a feature that enhances the separation between signal events (the two electrons emitted in the 0 nu beta beta decay of Xe-136) and background events, arising chiefly from single electrons of kinetic energy compatible with the end-point of the 0 nu beta beta decay (Q(beta beta)). Applying an external magnetic field of sufficiently high intensity (in the range of 0.5-1 Tesla for operating pressures in the range of 5-15 atmospheres) causes the electrons to produce helical tracks. Assuming the tracks can be properly reconstructed, the sign of the curvature can be determined at several points along these tracks, and such information can be used to separate signal (0 nu beta beta) events containing two electrons producing a track with two different directions of curvature from background (single-electron) events producing a track that should spiral in a single direction. Due to electron multiple scattering, this strategy is not perfectly efficient on an event-by-event basis, but a statistical estimator can be constructed which can be used to reject background events by one order of magnitude at a moderate cost (about 30%) in signal efficiency. Combining this estimator with the excellent energy resolution and topological signature identification characteristic of the HPXe TPC, it is possible to reach a background rate of less than one count per ton-year of exposure. Such a low background rate is an essential feature of the next generation of 0 nu beta beta experiments, aiming to fully explore the inverse hierarchy of neutrino masses.  
  Address [Renner, J.; Imzaylov, A.; Monrabal, F.; Munoz, J.; Gomez-Cadenas, J. J.] CSIC, Inst Fis Corpuscular IFIC, Calle Catedrat Jose Beltran 2, Valencia 46980, Spain, Email: jrenner@ific.uv.es  
  Corporate Author Thesis  
  Publisher Iop Publishing Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 1748-0221 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000369998500053 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 2549  
Permanent link to this record
 

 
Author (down) Renner, J. et al; Romo-Luque, C.; Carrion, J.V.; Diaz, J.; Martinez, A.; Querol, M.; Rodriguez-Ponce, J.; Teruel-Pardo, S. url  doi
openurl 
  Title Monte Carlo characterization of PETALO, a full-body liquid xenon-based PET detector Type Journal Article
  Year 2022 Publication Journal of Instrumentation Abbreviated Journal J. Instrum.  
  Volume 17 Issue 5 Pages P05044 - 17pp  
  Keywords Cryogenic detectors; Gamma camera; SPECT; PET PET; CT; coronary CT angiography (CTA); Liquid detectors  
  Abstract New detector approaches in Positron Emission Tomography imaging will play an important role in reducing costs, lowering administered radiation doses, and improving overall performance. PETALO employs liquid xenon as the active scintillating medium and UV-sensitive silicon photomultipliers for scintillation readout. The scintillation time in liquid xenon is fast enough to register time-of-flight information for each detected coincidence, and sufficient scintillation is produced with low enough fluctuations to obtain good energy resolution. The present simulation study examines a full-body-sized PETALO detector and evaluates its potential performance in PET image reconstruction.  
  Address [Romo-Luque, C.; Carrion, J. V.; Diaz, J.; Martinez, A.; Querol, M.; Rodriguez-Ponce, J.; Teruel-Pardo, S.] CSIC, Inst Fis Corpuscular IFIC, Calle Catedrat Jose Beltran 2, E-46980 Valencia, Spain, Email: paola.ferrario@dipc.org  
  Corporate Author Thesis  
  Publisher IOP Publishing Ltd Place of Publication Editor  
  Language English Summary Language Original Title  
  Series Editor Series Title Abbreviated Series Title  
  Series Volume Series Issue Edition  
  ISSN 1748-0221 ISBN Medium  
  Area Expedition Conference  
  Notes WOS:000811102400010 Approved no  
  Is ISI yes International Collaboration yes  
  Call Number IFIC @ pastor @ Serial 5264  
Permanent link to this record
Select All    Deselect All
 |   | 
Details
   print

Save Citations:
Export Records:
ific federMinisterio de Ciencia e InnovaciĆ³nAgencia Estatal de Investigaciongva