# 5.1.1 Determine a Wire-Theta Correspondence

To associate the hits with the Moller scattering angle theta, the occupancy plots of the drift chamber hits by means of wire numbers and layer must be translated using the physical constraints of the detector. Using the data released for the DC:

DC: Drift Chambers(specs)

DC Geometry(geom)

Table 5.1 Constraints from DC Geometry File
Variable Measurement Definition
thstereo 6 degrees Tilt of wires normal to sector midpoint
thtilt 25 degrees Tilt relative to z
dist2tgt 228.078 cm Distance from the target to the first guard wire plane along the

normal of said plane

th_min 4.694 degrees Polar angle to the first guard wire’s (in the first guard wire plane) midpoint

where the wire mid-point is the intersection of the wire with the chamber mid-plane

wpdist 0.3861 cm Distance between the wire planes

At 25 degrees above the beam line direction, we can create a line that is perpendicular to the plane of the wires, which are at 65 degrees above the reverse beam line direction. This line will be a perpendicular line with respect to all parallel layers of wires with regards to the initial guard wire plane. We can use this geometry and dist2tgt, which is the distance from the target to the first guard wire in the first guard wire plane, to calculate other distances.

Figure 5.1.1: The distance and angles which are derived from the angle with respect to the beam line (thtilt) and the distance for the guard wire plane perpendicular bisector(dist2tgt). All distances and angles are for the midplane bisector.

Using trigonometry, we can find the distance along the beam line to the intersection with the initial guard wire plane.

The wire planes in the DC are parallel to each other and separated by a constant distance wpdist. The parallel planes of wires are shown in the Figure 5.1.2 below.

Figure 5.1.2: The placement of the different wire planes in the DC as seen along the perpendicular bisector plane.

Figure 5.1.3: The placement of sector midpoint line.

The angle th_min is defined as the polar angle to the first guard wire’s (in the first guard wire plane) midpoint where the wire mid-point is the intersection of the wire with the chamber mid-plane. The first guard wire can be found at 4.694 degrees above the beam line. This position can be taken as the starting position for the initial guard wire plane. The planes of each wire runs 65 degrees above the reverse beam direction.

Figure 5.1.4: The distance and angles based on the 1st location of the lowest guard wire from the beam line along the DC sector bisector.

Using the law of sines, we can find the distance along the x' axis the 1st guard wire is from the beam line.

Making a right triangle, with the x' component as the hypotenuse, we find the x and z components

This gives the location of the first guard wire, in the plane of the foremost guard wires, at the midplane position to be

Finding the separation distance between two adjacent sense wires

Figure 5.1.5: The separation between adjacent sense wires along the DC midplane bisector.

Figure 5.1.6: The wire cells as seen from the DC midplane bisector.

Figure 5.1.7: The distance and angles to the 1st sense based on the geometry along the DC midplane bisector.

The starting position for sense wires with respect to the first guard wire at midplane, can be found to be at:

Since the separation between adjacent sense wires is uniform and at a set angle of 25 degrees with respect to the beam line, we can use this fact to determine the angle theta each wire makes when measured from the vertex.

Figure 5.1.8: The distance and angles between the first layer of sense wires based on the 1st sense wire location along the DC midplane.

The new x and z coordinates for wire 2 can be found using the change in the components

This can be extended to any point along the same wire plane, starting from the coordinates for wire 1 minus the difference in position to the next wire crossing on the plane.

The angle theta that the wire makes with the vertex is given by

For the values found earlier for the starting position: