Inner Detector

The inner detector consists of four regions, listed here in order from upstream to downstream: the nuclear target region, tracker, downstream ECAL, and downstream HCAL. Additionally, it

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contains the side ECAL, which is located between the edges of neighboring scintillator planes in the nuclear target and tracker regions. The inner detector is further subdivided along the z-axis into 120 units called modules. There are four types of modules: tracking modules, ECAL modules, HCAL modules, and passive nuclear targets. The tracker, downstream ECAL, and downstream HCAL are constructed exclusively from their respective module types and contain 62, 10, and 20 modules respectively. The nuclear target region contains 5 solid passive nuclear targets, a water target, and 22 tracking modules. The modules are installed orthogonal to the z-axis and there is a ∼2.5 mm air gap between each module.

3.1.1.1 Detector Modules A tracker module contains two hexagonal scintillator planes. Each scintillator plane is 1.7 cm thick and contains 127 triangular scintillator strips. Additionally, a hexagonal lead ring (otherwise known as the side ECAL) of thickness 2 mm, inner apothem 90 cm, and outer apothem 105 cm is located upstream of each scintillator plane. Each plane is rotated in one of three orientations, called views, with respect to the coordinate system. The orientation of the strips in each view are shown in Figure3.2. In the X view, the scintillator strips are parallel to the y- axis, thus providing position information along the x-axis. The U and V views are rotated in the x-y plane by 60◦clockwise and counterclockwise, respectively, relative to the X view. The downstream plane in each tracker module is in the X view, while the upstream plane can be either U or V. The modules are ordered such that the planes alternate throughout the tracker region according to the pattern U X V X. The use of three views enables MINERνA to reconstruct certain track configurations that would be ambiguous in two views: for example, two tracks that have the same projection along the z-axis or overlap in one view.

ECAL modules contain two scintillator planes in the same configuration as the tracker mod- ules, but missing a side ECAL collar. Instead each a 2 mm thick hexagonal lead plane with apothem identical to the scintillator planes is installed upstream of each scintillator plane in the ECAl. The full lead planes help contain electromagnetic showers that would otherwise escape the downstream end of the detector. The U X V X scintillator plane pattern is maintained in the ECAL.

HCAL modules contain an upstream hexagonal iron absorber of thickness 2.54 cm and a down- stream scintillator plane. The modules are arranged so that the scintillator planes are ordered in an X U X V pattern. In the low energy NuMI beam configuration, most hadrons that originate from

Figure 3.2: The X (left), U (center), and V (right) scintillator plane orientations as seen by an upstream observer. The inner lines show the direction of the scintillator strips. This figure is provided by Laura McCarthy.

neutrino interactions in the tracker region are contained by the HCAL.

The nuclear target region is unique in that it contains more than one type of detector module. Lead, iron, water, and carbon targets are separated by tracker modules as shown in Figure3.3. The analysis presented in this thesis does not use the nuclear target region and is negligibly affected by it, so it will not be described further. A more detailed description can be found in Ref. [54].

Each detector module includes six trapezoidal outer detector frames, to be described in more detail in Sec. 3.1.2. The frames are attached to the module in the pattern shown in figure 3.1, maintaining the hexagonal shape. With the exception of the thickness, the frames are identical across all regions of the detector. Downstream HCAL frames are slightly thicker than the others due to the thickness of the iron absorber.

3.1.1.2 Scintillator Planes Each scintillator plane is a 1.7 cm thick regular hexagon with a 107 cm apothem. The planes are composed of 127 scintillator strips glued together with 3M-DP190 translucent epoxy. Sheets of Lexan are attached to each plane with 3M-DP190 gray epoxy to in- crease rigidity and reduce exposure to ambient light. Black PVC electrical tape is used to seal joints in the Lexan and cover any remaining light leaks, allowing MINERνA to be sensitive to single photons generated in the scintillator. Figure 3.4 shows an edgewise view of a scintillator

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Figure 3.3: The MINERνA nuclear targets region. The beam direction is from left to right. Reprinted from [54], Copyright (2014) with permission from Elsevier.

Figure 3.4: Edgewise view of the strips in a scintillator plane. Each plane contains 127 triangular scintillator strips fitted side by side as shown. This figure is provided by Laura McCarthy.

plane and demonstrates how the strips are fitted together. The triangular shape ensures that every particle that crosses a scintillator plane will intersect at least two strips. This configuration im- proves position resolution relative to equally-sized rectangular strips by comparing the amount of scintillation light produced in each strip intersected by the particle.

The scintillator strips vary in length according to their position in the plane, and have a tri- angular cross section with a base of 3.3 cm and height of 1.7 cm. The scintillator is Dow Styron 663 W polystyrene ((C8H8)n) doped with 1% 2,5-diphenyloxazole (PPO) and 0.03% 1,4-bis(5-

phenyloxazole-2-yl) benzene (POPOP) by weight. The strips are co-extruded with a 0.25 mm reflective coating of polystyrene and 15% (by weight) TiO2. For readout, each strip contains a 2.6

mm diameter hole that runs the length of the strip. The hole is centered on the base and is located at a height of 0.85 cm above the base. A 1.2 mm diameter green wavelength shifting (WLS) fiber is put in the hole and fixed in place by optical epoxy (Epon Resin 815C and Epicure 3234).

The WLS fiber is a 175 ppm Y-11 doped, S-35, multiclad fiber made by the Kuraray Corpora- tion. It shifts the blue scintillation light to green in order to best match the photosensitivity of the MINERνA photomultiplier tubes (PMTs). Only one end of the fiber is read out. The other end is mirrored via an “ice-polishing” technique [56], followed by the deposition of aluminum and Red Spot UV Epoxy layers. The mirroring directs additional light to the readout end of the fiber.

The material composition of the planes must be well understood in order to interpret cross sec- tion results on scintillator. The fractional mass density of elements in each material is determined through direct measurement (pure scintillator and coated strips), assayed compositions (coated strips and epoxy), and manufacturer data sheets (Lexan and electrical tape). Table3.1summarizes

Table 3.1: Density and composition by mass percentage of scintillator plane materials [54]. Material Density (g/cm3) H C N O Al Si Cl Ti Scintillator 1.043 ± 0.002 7.6 92.2 0.06 0.07 - - - - Coating 1.52 6.5 78.5 - 6.0 - - - 9.0 Lexan 1.2 6.7 66.7 - 26.7 - - - - PVC tape 1.2 4.8 38.7 - - - - 56.5 - Transl. Epoxy 1.32 10.0 69.0 2.6 17.0 - - 0.5 - Gray Epoxy 1.70 5.0 47.0 1.7 27.0 6.0 6.0 0.05 -

each material’s fractional mass density and Table 3.2 summarizes the material assay for a single MINERνA scintillator plane.