L. Lagamba

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.

RICH DETECTOR AT JEFFERSON LAB, DESIGN, PERFORMANCE AND PHYSICS RESULTS

Since 2004 the hadron spectrometer of Hall A at Jefferson Lab is equipped with a proximity focusing RICH. This detector is capable of identify kaon from pion and proton with an angular separation starting from 6 sigma at 2 GeV/c. The RICH design is conceptually similar to the ALICE HMPID RICH; it uses a C6F14 liquid radiator and a 300 nm layer of CsI deposited on the cathode pad plane of an asymmetric MWPC. The RICH has operated for the Hypernuclear Spectroscopy Experiment E94-107, which took data in the last two years. Design details and performance along with first physics results from the hypernuclear experiment are shortly presented.

High-resolution hypernuclear spectroscopy electron scattering at JLab, Hall A

The characteristics of the Jefferson LAB electron beam, together with those of the experimental equipments, offer a unique opportunity to study hypernuclear spectroscopy via electromagnetic induced reactions. Experiment 94-107 started a systematic study on 1p-shell targets, 12C, 9Be and 16O. We present the results from 12C, 16O and very preliminary results from 9Be. For 12C for the first time measurable strength in the core-excited part of the spectrum between the ground state and the pΛ state was shown in for the first time. A high-quality 16ΛN spectrum was produced for the first time with sub-MeV Energy resolution. A very precise BΛ value for 16ΛN, calibrated against the elementary (e, e′K+) reaction on hydrogen, has also been obtained. Final data on 9Be will be available soon. The missing energy resolution is the best ever obtained in hypernuclear production experiments.

Electron Induced Hypernuclear Spectroscopy. Results in Hall A at Jefferson Lab

The characteristics of the Jefferson LAB electron beam, together with those of the experimental equipments, offer a unique opportunity to study hypernuclear spectroscopy via electromagnetic induced reactions. Results are presented on 12C, 16O and 9Be targets.In the hypernuclear missing‐mass spectrum the experiment achieves sub‐MeV energy resolution by exploiting the characteristics of the High Resolution Spectrometer pair and the beam quality available at JLab.These studies have provided, among other things, the first quantitative information on core‐excited states in hypernuclei, as well as the precise determination of Lambda binding energy in the Λ16N hypernucleus.For the experiment on oxygen, a waterfall target has been employed, allowing simultaneous measurement of the elementary reaction on protons: a crucial measurement to disentangle the contribution of the elementary reaction from the measured hypernuclear production cross section, yielding direct access to the nucleus‐hypernucleus transition stru...

Λ9Li and Λ16N high resolution spectroscopy by electron scattering at Jefferson Lab in Hall A

The preliminary results of the Λ 9 Li and Λ 16 N spectra obtained by electron scattering experiment in Hall A at Jefferson Lab in the framework of the experiment E94-107 are reported. These hypernuclei are produced bombarding respectively a 9Be and a waterfall target. After a short description of the experimental equipment and of the data analysis procedures, the main characteristics of the measured spectra, as excited levels, energy resolution, and signal to noise ratio are evidenced.

High resolution hypernuclear spectroscopy at Jefferson Lab, Hall A: The experimental challenge

The E94-107 experiment in Hall A at Jefferson Lab has started a systematic study of 1p-shell hypernuclei. Data have been taken on C-12, Be-9 and O-16 targets. The counting rate for hypernuclear electroproduction decreases dramatically as the scattering angle increases. Therefore, the electron scattering angle has to be as forward as possible to get high virtual photon flux and kaon angle has to be as close as possible to the virtual photon direction to minimize momentum transfer. In order to allow experiments at very forward angle in Hall A, two superconducting septum magnets were added to the High Resolution Spectrometers (HRS). The two magnets bend particles scattered at 6° into each HRS, introducing only a small perturbation on the HRS optics thus preserving the excellent momentum resolution of the HRS. With the new setup a momentum resolution of 10−4 FWHM on both HRS arms was obtained. One of the challenges of the experiment at very forward angle is the identification of very small peaks in the missingenergy spectrum; this requires a powerful Particle Identification (PID) system that provides unambiguous kaon selection.

Hypernuclear spectroscopy in JLab’s Hall A

Results are presented from a new experiment (E94-107) in Hall A of the Thomas Jefferson National Accelerator Facility (JLab) producing Λ 12 B using electroproduction, (e,e’K+). In the hypernuclear missing-mass spectrum the experiment achieves very good energy resolution (640 keV FWHM) by exploiting the characteristics of the High Resolution spectrometer pair and the exceptional beam quality available at JLab. The spectrometers were used with the addition an INFN provided pair of septum magnets to reach the desired small angles. Also, the Hall A standard complement of equipment was further augmented by the addition of a Ring Imaging Cherenkov detector (RICH) to achieve the best possible kaon identification.

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.