Tagger Microscope Placement in the Tagger Hall

Tagger Microscope Placement in the Tagger Hall

When discussing the location of the Tagger Microscope (TAGM) within the Tagger Hall it is often easiest to reference the starting point of the energy spectrum being tagged. For example, if the TAGM is said to be placed at 9.2 GeV this would indicate that the location of the most upstream column of fibers, with respect to the beamline, is such that a post-bremsstrahlung electron associated with a 9.2 GeV emitted photon will pass through the longitudinal axis of this column of scintillating fibers (SciFi). This column is referenced as column #1 in Figure 1 below. As the column numbers increase the electrons' energy increases, and as such the energy of the associated photon being tagged decreases.

Every bundle support (a.k.a. popsicle stick) supports a 5x6 array of 30 fibers. Each individual fiber is constructed from a 2.0 cm long section of BCF-20 SciFi thermally fused to 163.1 cm of BCF-98 lightguide. These fibers have a 2 x 2 mm² square transverse profile. The angle of each bundle support with respect to the focal plane (β angle) is selected to correspond to the average crossing angle as they pass through the focal plane of the electrons passing through these six columns of fibers. Therefore, for each bundle support there is a projection along this β angle that the support can be located and still remain within the design specifications of the TAGM. Since the β angle of the electrons only varies slightly over the entire bundle support's width, there is a fair amount of play with regard to how close the SciFi columns need to be to the focal plane of the Tagger Magnet, provided they remain along the electron of interest's path. To put this in prospective consider a bundle placed at around 9.2 GeV (E_γ). This 5x6 array of fibers will cover an energy range of 54 MeV and will see a change in β angle of only 0.11^o from its first column of fiber to its last (6^th column).

Figure 1: Electron view of the TAGM scintillating fiber array.

How to configure the scintillating fibers on the bundle support

The ideal configuration of the each fiber column would allow the center of the front face of the fibers to sit on the focal plane, as shown in Figure 2. Unfortunately, this configuration does not permit sufficient space to provide a method for holding the fibers in place other than gluing. During the prototyping phase testing showed that gluing the fibers to the bundle support was not a viable option due to unacceptable light loss, not to mention the difficult that would arise when trying to replace fibers. A 5x5 fiber configuration per bundle was used for the prototype, which was optically linked to a single preamplifier board. Unfortunately, having 25 silicon photomultipliers (SiPM) per preamplifier board led to too much electronic cross-talk and the likelihood of optical cross-talk between channels due to limited spacing between SiPM. Bundle supports with smaller fiber array configurations have too narrow of a base and could lead to improper alignment of the fibers due to the instability of the bundle support on the mounting rails. While using a bundle support with 6 or more columns side-by-side, as in Figure 3, extends the fibers too far past the focal plane in the y-direction. A compromise was reached by designing a bundle support with 30 fibers (5x6 array), which was split into two 5x3 fiber arrays (forward and rear bundle halves) offset to one another. This configuration shown in Figure 4 limits the fibers' intrusion past the focal plane (± Y_FP) and reduces the number of SiPM to 15 per preamplifier board by using one board per bundle half. The offset of the bundle halves is such that when placed at a β = 12^o the focal plane will pass through the center face of the center fiber in each bundle half.

Figure 2: CAD image showing each fiber column with the center of its front face on the focal plane. The fibers are at β = 20^o.

Figure 3: CAD image showing alignment with respect to the focal plane for a β angle of 9.5^o, which is similar to the angle required for tagging 6 GeV photons. This configuration places some of the fiber columns too far off the focal plane.

Figure 4: CAD image showing alignment with respect to the focal plane for a bundle support with a 5x6 fiber array split into two offset 5x3 bundle halves.

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