Home Page of the FIRST geometry

All the detectors CENTER positions are assumed to be referred to an ideal BOX that contains the whole detector and measure the position of the CENTER of the box in the general FIRST reference frame. This is not true for VTX (where the center is actually the BEGINNING of the detector and it is placed on the TARGET) and for KENTROS, where the center is the position of the edge of the barrel modules.

Tech drawings and survey report

Those info can be found on a dedicated wiki: click on this link: attention, some tech drawings can be quite outdated. If you have doubts ask.

Survey data interpretation

Start Counter is assumed to be in 0,0,0 (center of the FIRST reference frane). From our calculations this correspond to a point in the survey RF of coordinates 0.64,-3,3761.5 mm.

Matching the reference frames (SC position in FIRST and Survey RF).

Points 04 and 05 are 111.5 mm below the center of the SC [ASSY_1a]

• Y Start counter is -3.0 mm ABOVE the xy plane chosen in the survey (y axis first reference frame).

• X Start counter is +0.64 mm on the left with respect to xz plane chosen in the survey (x axis first reference frame)

• Z Start counter is +3761.5 mm along the beam with respect to yz plane chose in the survey (z axis first reference frame)

Beam Monitor.

Survey points 06-10 refer to the ENTRANCE window of the BM (face closer to the SC).

• Z position has not been measured. We use the position from Cerioni. z_sur = +160. mm (FIRST) 3921.5 (in survey RF)

• X position from mean of points 6-9: +0.1 mm (FIRST) 0.74mm (in survey RF)

• Y position from mean of points 6-9: -2.6 mm (FIRST) -5.6mm (in survey RF)

The center of the BM can be found @

• Z: 105mm AFTER the flange border (survey). Z_BM = 265 mm (FIRST) 4026.5 mm (in survey RF)

• X: same as survey point. X_BM = +0.1 mm (FIRST) 0.74 mm (in survey RF)

• Y: same as survey point. Y_BM = -2.6 mm (FIRST) -5.6 mm (in survey RF)

Survey points 11-12.

Check the updated tech drawings to fin the exact measured position: IR from survey

• X: the BOX is displaced +0.176 cm along x. The box is also tilted about 7mrad. (see plot attached). So that the real position of the TG (center of VTX detector) is +0.139 cm
• Y: is not measured. we assume 0.
• Z: a tilt in the x,z plane is measured. However this is a really LOW impact on z. The first mimo plane comes 61.7 cm after the SC (from cerioni + survey). The target is assumed to be 0.7 cm BEFORE the first mimo plane.

Kentros detector.

The position of the ring that holds Plexiglas light guides of the barrel has been measured as reported in the survey document where it is called Kentros_Kreis02:

• X : x_flaKE = -1.1 mm (FIRST) -0.5 mm (in survey RF)

• Y : Y_flaKE = 2.1 mm (FIRST) -0.9 mm (in survey RF)

• Z : Z_flaKE = 559 mm (FIRST) 4320 mm (in survey RF)

The lightguides are inserted inside holes in the ring. The tips of the lightguides go out of the holes 6.6 mm. Since they are 128 mm long the Z-coordinate of the scintillator end connected to the lightguides is 4320 mm + 128 mm – 6.6 mm = 4441 mm. The barrel scintillator modules are 500 mm long, therefore the Z-coordinate of the opposite end is 4941mm. This is also the beginning of Big Endcap, which is attached to the barrel scintillators. Since the thickness of the Big Endcap scintillator modules is 35 mm, the opposite side of the Big Endcap has Z = 4976 mm, which is also the beginning of Small Endcap. The thickness of the Small Endcap is also 35mm, so the opposite side has Z= 5011 mm.

In summary:

• Barrel scintillator modules: Z_min = 4441 mm and Z_max = 4941 mm
• Big Endcap scintillator modules: Z_min = 4941 mm and Z_max = 4976 mm
• Small Endcap scintillator modules: Z_min = 4976 mm and Z_max = 5011 mm

-- AlessioSarti - 15 Nov 2011