# GEn Geometry and Surveys

This page describes the BigBite and Neutron Arm positions, for GEn experiment.

### Definitions

#### BigBite

Several points are defined:
• Magnet origin: a point on the central plane between the poles, at the depth of the frontal face of the (blue) yoke, at the height of the waist in the yoke
• Detector origin: a point on the frontal face of the aluminum plate (3/8 inch thick) of the 1-st chamber, at the crossing of 2 diagonales

#### Coordinate Frames and Rotations

The survey results are presented in a form of the measured coordinates of certain points and of the measured angles of objects. The absolute coordinates are given in the "Master Reference System" (MRS) X, Y and Z (Z looks down the beam, toward beam dump, Y looks up, X looks left if one is standing looking down the beamdump). The shifts Δ-s absolute are given in the "Beam Following System" (BFS) which is tied to the ideal position of the object considered. The rotations to the ideal position are defined in the following sequence:
1. Rotate the object about Y. The angle yaw is positive if rotation is clockwise looking against Y. This defines a new reference system X1, Y1, Z1, where Y1≡Y.
2. Rotate the object about X1. The angle pitch is positive if rotation is clockwise looking against X1. This defines a new reference system X2, Y2, Z2, where X2≡X1.
3. Rotate the object about Z2. The angle roll is positive if rotation is anti-clockwise(!) looking against Z2. This defines the BFS reference system X3, Y3, Z3, where Z3≡Z2.

There is a way to see the drawings, provided by A.Gavalya, from a Linux computer:
```    metaframe        - login to Windows
JT2go            - Start ⇒ All Programs ⇒ JT2go
information bar  - Click on the information bar at the top and allow the access to files
open file        - Select file BB_DETECTOR_SHIELD
```

### Surveys

Summary of BB measurements
The absolute coordinates are given in the MRS. The ΔX-s are given in the BFS (the frame tied to the ideal position of the object). The Δα-s are given in the BFS, the sequence or rotations is defined above.
# Date case Big Bite Magnet Big Bite Detector
X,mm Y,mm Z,mm yaw, ° pitch, ° roll, ° X,mm Y,mm Z,mm yaw, ° pitch, ° roll, °
1 2006/02/08 ideal -917.3 0.0 607.1 56.50 0.00 0.00 -1892.8 151.6 1252.8 56.50 10.00 0.00
measured -913.5 -0.3 594.5 56.26 -0.35 0.06 -1872.6 164.8 1255.6 56.21 10.05 -0.09
ΔBFS -8.4 -0.3 -10.1 -0.24 -0.35 0.06 13.5 13.2 -15.3 -0.29 0.05 -0.09
1. 2006/02/01 BigHand survey pdf.
2. 2006/02/08 BigBite survey pdf. There is a correction to these numbers: BB yaw is 56.26° and detector yaw is 56.21°, Δ(yaw)BB=-0.24°, Δ(yaw)Det=-0.29°.
3. 2006/02/14 BigBite survey pdf. The BigBite external fiducials were measured on Feb 14 and the survey of the planes from Dec 22 were used. The coordinates of the corners of the aluminum detector frames are given. All three chambers are rolled differently - it has to be taken into account.
4. 2006/05/10 BigBite survey pdf.
5. 2006/05/16 BigBite survey pdf.

### Additional BB Measurements

Knowing the BB detector angle with an accuracy of about 1 mrad is important for the experiment. Therefore, additionally to the surveys performed by the survey group, simple independent measurements of the detector angle were made. These mesurements utilized the angle marks on the floor of the hall. The marks, made by the survey group, are normally used for positioning the HRS (spectrometers). They are located on two arcs with radii of about 10 m and 16 m. The accuracy of a mark is about 1 mm. The angle θ (around Y) of a line drawn through two marks, indicating the same angle, is known to an accuracy of about 1mm/6m = 0.2 mrad. We selected such a line, close to the center of the BB position, and marked a parallel line, at a distance of 43 inches = 1092.2 mm. Then, we measured the distance to the chambers aluminum frames from this line. Knowing the frames' widths (32 inches for DC1 and 39.75 inches for DC2-3) we can find out what is the θ angle of a line drawn through the chambers centers. The procedure is illustrated on this picture.

Our BB detector measurements
The coordinate system is the MRS, turned around Y, that Z axis looks along the BB axis. The angle of the detector is the angle between the projection of a line crossing the chamber centers, at a certain YBB (this is the coordinate along the chambers, in the dispersive plane, looking up), on the floor, and the median line on the floor. The median line is drawn at the assumed BB angle. ΔX is the measured distance from the median line to the detector center (positive, when the center is closer to the beam dump). ΔX does not take into account the tilt of the line of sight with respect to the median line ΔΘ (see below). Δθ is the derived detector angle with respect to the median line (positive if it goes away from the beam). The 'side' indicates from which side of BB, looking along the particles, the measurement was done. The line of sight could be tilted with respect to the median line, deliberately or not. The angle is indicated by ΔΘ. Measurement #9 was done with aluminum shielding on the BB left side, while measurement #10 was done with the shielding removed.
# Date θ° median side YBB,mm ΔΘ mrad ΔX, mm Δθ mrad
DC1 DC2 DC3 DC3-DC1 DC3-DC2 DC2-DC1
5 2006/02/23 56.5 right -880. 0.0 13.00 13.14 10.58 3.88     6.26     -0.70
6 2006/02/23 56.5 right -880. -2.9 62.27 61.81 57.94 4.03     6.51     -0.76
7 2006/02/23 56.5 right -880. 2.6 -35.51 -35.67 -37.11 3.58     6.13     -1.33
11 2006/02/23 56.5 right -130. 0.0 - 12.60 11.31 -     3.14     -
8 2006/05/02 52.0 right -880. 0.0 25.00 25.90 23.91 1.75     4.83     -4.20
9 2006/05/10 52.0 right -880. 0.0 25.40 25.60 24.21 1.91     3.38     -0.93
10 2006/05/10 52.0 right -880. 0.0 25.30 25.60 23.81 2.39     4.35     -1.40
The standard deviation for Δθ is about 0.7 mrad for DC3-DC1 and 1.4 mrad for DC3-DC2. These results are consistent with the surveyers results (see this plot). Here, we used the DC3-DC1 angle. At YBB=-880 mm we used the measurement #5. For YBB=-130 mm (measurement #11) we were not able to measure DC1. In order to convert the result to DC3-DC1 we took ΔθDC3-DC1(#11) ≈ ΔθDC3-DC1(#5) + ΔθDC3-DC2(#11) - ΔθDC3-DC2(#5) = 0.76 mrad.
E-Mail : gen@jlab.org
Last updated: 11 May 2006