GLUEX

Cables affecting BCAL efficiency

A simple GEANT simulation was done in order to calculate the influence of the material in front of BCAL on the photon detection efficiency. The main concern at the moment is the influence of the FDC cables, located in a narrow slit between the FDC and BCAL. The setup is shown on this picture. The photon was considered to be detected if it released more than 90% of its energy in BCAL.

Poisson calculated field map

The results are shown here. The DC cables kill about 20% of low 20 MeV photons at small angles.

I simulated some hypothetical meson decays in order to find out the net loss inflicted by the cables on the detection efficiency. Two sets of data were simulated - without the cables and with the cables. The requirements for detection were:

No requirements for the charged particles.
Each photon in the event should be detected either in BCAL or FCAL and the energy left in the calorimeter should be at least 90% of the photon energy. The energy in the calorimeter was calculated adding the energy of the particles, produced by the photon considered, when they hit the calorimeter surface. Two extreme cases were considered:
1. Strong requirement - only one photon with the highest energy was taken. This is basically a no-conversion requirement.
2. Weak requirement - all the particles originated from the photon were added, independently on their location in the calorimeter. It seems that this condition is appropriate for BCAL, since all the particles come close to each other - within 2-3 cm.
3. A more realistic combination is to treat BCAL as case 2) and FCAL as case 1).

The beam energy was simulated between 8 and 9 GeV. All the decays were simulated accordingly to their phase space. The following processes were tried (a binary reaction with dσ/dt=A*exp(5*t) assumed):

γp → X⁺(2000)n, X⁺→b1(1229)⁺π^o, b1(1229)⁺→&omega(782)π⁺, &omega(782)→π⁺π^-π^o.
There are 4 photons and 3 charged pions in the final state. The cables would kill about 32% of events in case 1), about 10% in case 2) and about 12% in case 3). The next plot shows the radius distribution of the photons directions to a plane Z=400cm (the exit of the barrel). It indicates that a ring of R=60-65cm at the exit of the barrel may absorb a few percent of the photons.

About 45% of charged pions in the final state exit the barrel. About 7-20% of them have momenta larger than 3 GeV/c (the gas Cherenkov pion threshold).
γp → X⁺(2000)n, X⁺→η(548)π⁺, η(548)→γγ
There are 2 photons and one charged pion in the final state. The cables would kill about 4% of events in case 3).
About 30% of charged pions in the final state exit the barrel. About 75% of them have momenta larger than 3 GeV/c (the gas Cherenkov pion threshold).

E-Mail : gen@jlab.org
Last updated: Jan 31, 2007