Moving the Tagger Microscope

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Moving the Tagger Microscope Along the Focal Plane

Tagger Microscope

Figure 1: CAD image of the upper and lower enclosures of the Tagger Microscope made during the design phase.

• Prior to Installation in the Tagger Hall
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  Figure 2: TAGM just prior to installation in the Tagger Hall. The side of the TAGM seen here will face towards the Tagger Magnet.

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  Figure 3: TAGM during reassembly just prior to installation in the Tagger Hall. The side of the TAGM seen here will face away from the Tagger Magnet when installed.

The tagger microscope (TAGM) consists of six major components: upper enclosure, lower enclosure, optical fibers, electronics, shielding, and darkening shroud. As seen from the CAD image above, Figure 1, the lower enclosure houses the microscope's electronics, while the upper enclosure contains the optical fiber bundle supports (a.k.a. popsicle sticks). The bundle supports are used to align the scintillating fiber (SciFi) longitudinal axis to the incoming electron's path angle as it passes through the tagger magnet's focal plane. The designed angular tolerance, e.g. fiber to beam angular error, is 0.5 degree. MC simulations were performed to show the effect of various amounts of misalignment between the electron angle and the SciFi axis. Figure 4 shows a plot of one of the simulations with the solid line showing the scintillation response of the central fiber, while the dashed lines are the response for the adjacent fibers.

Figure 4: Result of a MC simulation performed to predict the response of adjacent scintillating fiber columns due to fiber axis misalignment with respect to a post-bremsstrahlung electron's path. The solid line represents the response seen by the central fiber, while the dashed lines are the adjacent fibers' response.

    end of the optical fibers are affixed bundle supports  in the upper enclosure, and the light guide potion of the fiber extends down to the silicon photomultipliers (SiPMs) located on preamplifier boards in the lower enclosure.

Coordinate System

There are three coordinate systems often referred to inside the GlueX Tagger Hall. These coordinate systems have origins associated with the Goniometer center (room coordinates), Tagger Magnet center (map coordinates), and the focal plane (FP coordinates). The room coordinate system has the positive z-coordinate along the beam axis with the positive y direction pointing to the ceiling of the tagger hall from the coordinates origin placed at the Goniometer's center, where the beamline radiator is located. Figure 1 shown below, taken from a presentation by Dan Sober, shows the three coordinate systems relative to one another. The map coordinate system (magnetic mapping) places the y-coordinate along the long axis of the magnet with the positive x-axis parallel to the tagger hall floor in the direction of the focal plane. the positive z-axis of the map coordinate system points toward the tagger hall ceiling. When viewed from above the map y-coordinate is offset from the room coordinate z-axis by 6.5 degrees towards the focal plane (clockwise). The focal plane coordinate system places its x-axis along the electron focal plane with zero being at an electron energy equivalent to a photon energy around 11.7 GeV. The positive x-axis points toward increasing electron energy (decreasing photon energy). The positive y-axis of the focal plane coordinate system points towards the tagger magnet and is parallel to the tagger hall floor. This leaves the focal plane positive z-axis pointing towards the tagger hall ceiling.

Figure 1: The three coordinate systems often referred to in association with the Tagger Hall. (Image taken from a Dan Sober Presentation)

In Spring 2017 a survey of the various beamline components saw a small change in the Tagger Microscope position. The Microscope (upstream center) position derived from this survey is shown below and includes a magnet center shift from previous measurements.

Spring 2017 Survey of Tagger Microscope Position (Upstream Center)
X room =	- 1.16666 m
Z room =	7.41009 m
Angle room =	- 8.05220o

New Coordinates (with magnet center shift)
X map =	0.73488 m
Y map =	1.22645 m
X FP =	3.31245 m
Y FP =	-0.00932 m

Bundle Support (aka Popsicle Stick)

The Tagger Microscope (TAGM) contains 17 optical fiber bundle, each consists of a 5x6 array of 30 fibers for a total of 510 fibers (5 rows, 102 columns). Figure 2 below shows the fiber array as viewed by the electrons passing through the tagger magnet focal plane. Each optical fiber consists of a 2 mm² x 2 cm BCF-20 scintillating fiber (SciFi) thermally fused to 2 mm² x 165 cm BCF-98 light guide, see Figures 3 & 4. The SciFi end of each fiber is thermally bent into an "S" shape to remove the fiber from the electrons' path soon after passing through the SciFi, see Figure 5. The fiber begins its bend out of the electrons' path after only 4.5 cm. This length provides enough material past the fused joint to minimize strain on the joint resulting from the bend and fiber mounting straps, while also reducing the material in the line-of-fire that could result in backscatter.

Figure 2: Electron view of the TAGM scintillating fiber array.

• Transmission of scintillation light
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  Figure 3: An end view of a 2 mm² light guide fiber (BCF-98).

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  Figure 4: The mating of 2 cm. of scintillating fiber BCF-20 (left) joined to 165 cm. light guide fiber BCF-98 (right).

Figure 5: CAD image of a bundle support (blue) with a mounted 5x6 fiber bundle. The 'S' bend minimizes the amount of fiber in the path of the electrons.

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File:Magnetic Field Map.pdf

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Final

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