Tuesday, March 16, 2010

Hall D Picture

Hall D is a new Experimental Hall/Detector package being built at Jefferson Lab. Previously I reported on one of my projects, ced, the event display for the CLAS 12 GeV detector in Hall B.

I was asked to develop a prototype, sharing as much software as possible, for the detector being designed and built for Hall D. This prototype (called ded)  is marching along nicely and producing some pretty 2D pictures of simulated particles traversing and depositing energy in the various detector packages. Here I provide a sample.

The scale of the picture below is about 7 meters (23 feet) wide and 3 meters (10 ft) tall. Everything in this detector is cylindrical and surrounds the beam line (the thick horizontal line through the center from left to right). This is a plane slice through the cylinders at constant azimuthal angle1. The small blue "rectangle" is the target. The table enumerates the particles that were generated in this event by modeling the physics process. The colored lines are the result of my software taking the particles in the table and "swimming" them through the detector (numerically solving the differential equation of the relativistic motion in a magnetic field.) The swum particles should match the "X's", which indicate interactions with the detector. For the most part they do--except sometimes it doesn't look quite right because of the 2D projection. Also, the swimming continues even after the particle, in the simulation, was "lost".

The incident beam (from the left) in this instance is a tagged photon beam. Tagged means we know the energy. In JLab we do not produce a photon beam directly--we produce an electron beam. Here is how we convert it to a tagged photon beam: Upstream (to the left) of this picture a magnet bends the electron beam and shakes off a photon. The photon continues down to the target. By knowing the energy of the beam, and by measuring the energy of the bent electron, and by having good timing resolution, you can tag the photon with the electron from whence it came. You can get the energy from conservation of energy. CoolnessNth.

Click to enlarge.

1 Remember 3D cylindrical coordinates? The azimuthal angle, φ, is the angle in the xy plane. In 3D polar (spherical)  coordinates physicists also call the angle in the xy plane φ, and the angle with respect to the z axis, the polar angle, we call θ. Mathematicians, confused as they often are, and still smarting because a physicist (Newton) invented calculus, screw this up. For reasons unfathomable the dear souls call the polar angle φ and the azimuthal angle θ.

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