A. Leone

Empirical tests and model of a silica aerogel Cherenkov detector for CEBAF

Abstract A prototype of a silica aerogel Cherenkov detector to be used for pion-proton discrimination in one of the HRS spectrometers of CEBAF Hall A has been built and tested. Each detector housing is made of carbon fiber composites and incorporates planar parabolic mirrors. The prototype was tested in electron and pion beams. The detection efficiency for particles well above the Cherenkov emission threshold has been measured to be greater than 99%, and the false trigger rate was measured to be less than 3 × 10 −2 . The results are consistent with a Monte Carlo simulation of the prototype detector, indicating that the performance of the detector is well understood.

Silica aerogel threshold Cherenkov counters for the JLab Hall A spectrometers: improvements and proposed modifications

Recently approved experiments at Jefferson Lab Hall A require a clean kaon identification in a large electron, pion, and proton background environment. To this end, improved performance is required of the silica aerogel threshold Cherenkov counters installed in the focal plane of the two Hall A spectrometers. In this paper we propose two strategies to improve the performance of the Cherenkov counters which presently use a hydrophilic aerogel radiator, and convey Cherenkov photons towards the photomultipliers by means of mirrors with a parabolic shape in one direction and flat in the other. The first strategy is aerogel baking. In the second strategy we propose a modification of the counter geometry by replacing the mirrors with a planar diffusing surface and by displacing in a different way the photomultipliers. Tests at CERN with a 5GeV/c multiparticle beam revealed that both the strategies are able to increase significantly the number of the detected Cherenkov photons and, therefore, the detector performance.

Probing theΔNNcomponent of3He

The 3He(gamma,pi^+/- p) reactions were measured simultaneously over a tagged photon energy range of 800<E_gamma<1120 MeV, well above the Delta resonance region. An analysis was performed to kinematically isolate Delta knockout events from conventional Delta photoproduction events, and a statistically significant excess of pi+p events was identified, consistent with Delta++ knockout. Two methods were used to estimate the DeltaNN probability in the 3He ground state, corresponding to the observed knockout cross section. The first gave a lower probability limit of 1.5+/-0.6+/-0.5%; the second yielded an upper limit of about 2.6%.

PERFORMANCE AND DESIGN CHARACTERISTICS FOR THE HALL A AEROGEL CHERENKOV COUNTERS

Abstract We report the design characteristics and the performance obtained with the silica aerogel Cherenkov counter recently completed for the HRS-H spectrometer of Hall A at CEBAF. Several modifications have been incorporated as a result of the prototype performance, which was reported in an earlier article. The counter meets or exceeds the original design specifications and will be used to separate pions and protons in the HRS-H spectrometer in Hall A.

Subthreshold rho^0 photoproduction on 3He

A large reduction of the rho^0 mass in the nuclear medium is reported, inferred from dipion photoproduction spectra in the 1 GeV region, for the reaction 3He(gamma,pi+ pi-)X with a 10% duty factor tagged-photon beam and the TAGX multi-particle spectrometer. The energy range covered (800 < E(gamma) < 1120 MeV) lies mostly below the free rho^0 production threshold, a region which is believed sensitive to modifications of light vector-meson properties at nuclear-matter densities. The rho^0 masses extracted from the MC fitting of the data, m*(rho^0) = 642 +/- 40, 669 +/- 32, and 682 +/- 56 MeV/c^2 for E(gamma) in the 800-880, 880-960, and 960-1040 MeV regions respectively, are independently corroborated by a measured, assumption-free, kinematical observable. This mass shift, far exceeding current mean-field driven theoretical predictions, may be suggestive of rho^0 decay within the range of the nucleonic field.

Test of a Silica Aerogel Threshold Cherenkov Counter for the TJNAF Hall A Spectrometers with 1-GeC/c - 4-GeV/c Particles

The silica aerogel threshold Cherenkov counter for the Jefferson Lab Hall A spectrometers has been tested at CERN with a beam of positrons, pions and protons in order to characterize its efficiencies to particle detection in the 1 to 4 GeV/c momentum range. A detection efficiency to particles above the Cherenkov threshold (e{sup +} and {pi}{sup +}) greater than 95% and to particles below the threshold (p) of about 5% has been quoted in the whole momentum range explored. The detection efficiency to protons is due both to induced {delta}-rays production (3.5%), and to accidentals due to the high counting rate.