Laboratory Profile: Jefferson Lab The Initial Complement of Experimental Equipment (Nuclear Physics News)

The Initial Compliment of Experimental Equipment

Hall C: The first experimental facility to come into operation at Jefferson Lab is Hall C. The hall first received five-pass 4 GeV beam in May 1995. After optics setup improvements, the first experiment began on November 15 that year. The hall's initial complement of equipment includes two general-purpose, moderate resolution, magnetic spectrometers: the High-Momentum Spectrometer (HMS), shown in Figure 8, has a large solid angle, 10-3 resolution, and a maximum momentum of 7 GeV/c, while the complementary Short-Orbit Spectrometer (SOS), shown in Figure 9, has a large (+-20%) momentum acceptance and a very short (7.4 m) optical path to facilitate detection of particles having short lifetimes, such as low-momentum p's and K's.

The first experiments in Hall C addressed proton propagation in the nuclear medium and the validity of quark counting rules in the photodisintegration of the deuteron. The near-term experimental program includes the separation of the monopole and quadrupole charge form factors of the deuteron, and measurements of the neutron electric form factor and kaon electroproduction.

Hall C is also designed to accommodate specialized detectors built to solve specific problems. For example, the G0 spectrometer now under construction is an eight-sector, focusing, toroidal spectrometer that will be used for precise measurements of parity violation in the scattering of polarized electrons from protons aimed at investigating the weak neutral current structure of the proton and possible contributions from strange quarks.

Hall A: The second experimental facility to come on line will be the pair of optically identical High-Resolution (10-4) Spectrometers (HRS) in Hall A (see Figure 10). Each has a relatively large solid angle and a maximum momentum of 4 GeV/c. The detector packages have been optimized differently--one for electrons, and the other for hadrons. The hadron spectrometer detector includes a focal plane polarimeter.

These spectrometers will be used for high-precision studies, mainly involving the (e, e' p) and (e->, e' p->) reactions. The first experiments in Hall A will include measurements of elastic scattering from the deuteron and of the electric form factor of the proton (both free and in the nuclear medium), an investigation of nucleon structure through virtual Compton scattering, and a study of high-momentum components in the 16O wavefunction.

Hall B: The final hall to begin physics operations will be Hall B. It will be equipped with a large acceptance (nearly 4p) detector, the CEBAF Large Acceptance Spectrometer (CLAS). Its main missions are to carry out experiments that require the simultaneous detection of several loosely correlated particles in the hadronic final state, and to permit measurements at limited luminosity.

An exploded view of the CLAS is shown in Figure11. The magnetic field in the detector has a toroidal configuration generated by six iron-free superconducting coils (shown in an early stage of the detector installation in Figure 12). The particle detection consists of drift chambers to determine the trajectories of charged particles, Cherenkov counters for the identification of electrons, scintillation counters for the trigger and for time-of-flight measurements, and electromagnetic calorimeters to identify electrons and to detect photons and neutrons. The superconducting coil for the CLAS has successfully completed full- current acceptance tests. Most of the detectors for the spectrometer have been assembled and tested, and the initial complement are being installed in preparation for first delivery of beam to the hall in December 1996. The remaining detectors and ancillary equipment should be completed in time for full commissioning to begin in spring 1997.

The continuous nature of the electron beam is critical to the functioning of such a multiparticle coincident detector. Hall B also includes a bremsstrahlung photon tagging facility so that the CLAS can investigate real as well as virtual photon processes. The CLAS acts in many ways as a large electronic bubble chamber; data on many different reactions may be obtained simultaneously. The initial run period for this detector will include both electro- and photo-production data on nucleon resonance excitation, vector meson production, and strangeness production. The CLAS spectrometer will also be used in a variety of other investigations requiring data on multiparticle final states, including short-range correlations between nucleons in nuclei, the importance of three-body forces in nuclei, and the modification of the nucleon's properties in the nuclear medium.