The Accelerator

The Laboratory hosts CEBAF (Continuous Electron Beams Accelerator Facility) a 4 GeV electron beam accelerator, with a maximum current of 200$\mu$A. This accelerator delivers it's beam into 3 experimental Halls, as shown in Figures 2.1 and 2.2.

  

Figure 2.1: Upper view of the accelerator and Halls

  

Figure 2.2: Schematics 3D view of CEBAF

The unique capability of CEBAF, is it's ability to deliver a quasi-continuous electron beam. At similar luminosities, a continuous beam gives better results than a pulsed beam, because as the events are more spaced temporally, the detector is less affected by intrinsic dead times.

Another interesting feature is that the three Halls can get a different beam energy at the same time, by selecting electrons that have made one, two, or up to five revolutions in the accelerator (Figure 2.2), gaining 800 MeV per revolution.

The energy beam that CEBAF is providing (up to 6 GeV) is called 'intermediate' because it stands between low energy machines (like the heavy ions accelerator, GANIL in France 20 to 100 MeV/nucleon) and high energy machines (LEP 200 delivering hundreds of GeV).

This level of energy is particularly interesting because neither the low energy models, that assume nucleons as being elementary particles (quarks are strictly confined inside the nucleons), nor the high energy models (free quarks soup) are valid. We are between these two regimes and the theories that try to modelize this world (non perturbative QCD) are still under heavy development.

The electron, a point-like electro-magnetic probe, is very well adapted to make accurate measurements inside the nucleus at this intermediate energy.

The Halls

The three different Halls of Jefferson Lab contain detectors with different characteristics, allowing a vast range of experiments.

Hall A (Figure 2.3) hosts two high-resolution magnetic spectrometers, one more specifically designed for electrons, the other one for hadrons. These detectors are made to make very precise measurements (momentum resolution 10^-4) but this good resolution is achieved by restricting the solid angle coverage. The acceptance is within a window of 7.8 msr and 10% in $\Delta p / p$ [11]. This apparatus is well adapted for (e,e') and (e,e'p) experiments on light nuclei, and also for nucleon form factor experiments.

  

Figure 2.3: Exploded view of the Hall A

Hall C (Figure 2.4) hosts the High Momentum Spectrometer (HMS) and the Short Orbit Spectrometer (SOS). The resolution in momentum is lower than in Hall A (5.10^-4 for HMS and 2.10^-3 for the SOS) but the acceptance higher (respectively 18% and 40% in $\Delta p / p$). The HMS spectrometer has been made to detect high momentum particles (up to 6 GeV/c protons). It also detects scattered electrons, fixing the kinematic in (e,e'X) experiments. The SOS is shorter and limited at 1 GeV/c, it has been designed to detect low energy secondary particles as $\pi$ and K, that have a short life time and could decay before being detected by a longer arm [11]. Some other dedicated devices are installed in the Hall. The HNSS (Hypernuclear Spectrometer System) designed for hypernuclei studies, will detect small angles electrons and kaons. Finally, the G0 spectrometer is used to measure the weak form factor of polarized proton.

  

Figure 2.4: Exploded view of the Hall C

We will talk more about the CLAS detector in Hall B (Figure 2.5) in 2.2.²

  

Figure 2.5: Exploded view of the Hall B

Footnotes

...HREF="node7.html#sec:CLAS">2.2.²

Figures are not to scale, Hall B is the smallest Hall, with a diameter of about 30 m compared to a diameter of roughly 50 m of Hall A or C.