Forward Time-of-Flight Geometry for CLAS12

Transcription

1 Forward Time-of-Flight Geometry for CLAS12 D.S. Carman, Jefferson Laboratory ftof geom.tex April 13, 2016 Abstract This document details the nominal geometry for the CLAS12 Forward Time-of- Flight System (FTOF). 1 FTOF Overview The Forward Time-of-Flight System (FTOF) is a major component of the CLAS12 forward detector used to measure the time-of-flight of charged particles emerging from interactions in the target. The average path length from the target to the FTOF counters is roughly 7 m. The requirements for the FTOF include excellent timing resolution for particle identification and good segmentation to minimize counting rates and to provide for flexible triggering options. The system specifications call for an average time resolution of σ T OF =80 ps at the more forward angles of CLAS12 and 150 ps at angles larger than 35. The system must also be capable of operating in a high-rate environment. The maximum counting rate occurs in the most forward direction where, at an operating luminosity of cm 2 s 1, the average rate per scintillator is approximately 250 khz. The CLAS12 detector is built around a six-coil toroidal magnet that divides the active detection area into six 60 -wide azimuthal regions called sectors. In each of the six sectors of CLAS12, the FTOF system is comprised of three arrays of counters, referred to as panels, named panel-1a, panel-1b, and panel-2. Each panel consists of a set of rectangular scintillators with a PMT on each end. Panel-1 refers to the counters located at forward angles (roughly 5 to 35 ) (where two panels are necessary to meet the 80 ps average time resolution requirement) and panel-2 refers to the sets of counters at larger angles (roughly 35 to 45 ). The positioning and attachment of the FTOF detector arrays to the Forward Carriage of CLAS12 are shown in Fig. 1. Each of the six panel-1a arrays contains 23 counters. The new highly segmented panel-1b arrays each contain 62 counters. Finally, each panel-2 array consists of 5 counters. The FTOF counter arrays in the angular range from 5 to 35 consist of two sets of six triangular arrays. Just upstream of the Preshower Calorimeter (PCAL) detectors, the panel-1a arrays are mounted. These detector sets were refurbished from the panel-1 TOF counters from the decommissioned CLAS spectrometer. Upstream of the panel-1a arrays the new panel-1b arrays are mounted. In the angular range from 35 to 45 the panel-2 arrays 1

2 Figure 1: View of the FTOF counters for CLAS12 highlighting the location of the panel-1 and panel-2 counters. The panel-1b counter arrays are shown in orange and the panel-2 counter arrays, mounted around the perimeter of the Forward Carriage, are shown in red. The panel-1a counter arrays mounted just downstream of the panel-1b arrays are not visible in this picture. The Forward Carriage is roughly 10 m in diameter. are mounted. These detectors were refurbished from the panel-2 arrays of the CLAS TOF system. A summary of the FTOF technical parameters is given in Table 1. 2 Nominal FTOF Geometry 2.1 Model Geometry The geometry for each FTOF array is specified by a limited set of parameters. Figure 2 shows a view of the scintillator arrays in a single representative sector. This view is in the sector mid-plane and the key parameters used to specify the geometry are indicated. These parameters include: R a - distance from the nominal CLAS12 center to the small angle upstream edge of panel-1a counter #1. R b - distance from the nominal CLAS12 center to the small angle upstream edge of panel-1b counter #1. th a min - polar angle of the ray marking R a. th b min - polar angle of the ray marking R b. th 1 tilt - tilt angle of the panel-1a and panel-1b arrays relative to a line perpendicular to the z-axis (or electron beamline). thick 1a - thickness of the panel-1a scintillators. 2

3 Parameter Design Value Panel-1a Angular Coverage θ = 5 35, φ : 50% at 5 85% at 35 Counter Dimensions L = 32.3 cm cm, w h = 15 cm 5 cm Scintillator Material BC-408 PMTs EMI 9954A, Philips XP2262 Design Resolution 90 ps 160 ps Panel-1b Angular Coverage θ = 5 35, φ : 50% at 5 85% at 35 Counter Dimensions L = 17.3 cm cm, w h = 6 cm 6 cm Scintillator Material BC-404 (#1 #31), BC-408 (#32 #62) PMTs Hamamatsu R9779 Design Resolution 60 ps 110 ps Panel-2 Angular Coverage θ = 35 45, φ : 85% at 35 95% at 45 Counter Dimensions L = cm cm, w h = 22 cm 5 cm Scintillator Material BC-408 PMTs Photonis XP4312B, EMI 4312KB Design Resolution 145 ps 160 ps Table 1: Table of parameters for the scintillators, PMTs, and counters for the FTOF panel- 1a, panel-1b, and panel-2 arrays. w 1a - width of the panel-1a scintillators. thick 1b - thickness of the panel-1b scintillators. w 1b - width of the panel-1b scintillators. gap 1a 1b - separation between panel-1a and panel-1b. R 2 - distance from the nominal CLAS12 center to the small angle upstream edge of panel-2 counter #1. th 2 min - polar angle of the ray marking R 2. th 2 tilt - tilt angle of the panel-2 arrays relative to a line perpendicular to the z-axis. thick 2 - thickness of the panel-2 scintillators. w 2 - width of the panel-2 scintillators. gap 1a - gap between neighboring panel-1a counters (not shown). gap 1b - gap between neighboring panel-1b counters (not shown). gap 2 - gap between neighboring panel-2 counters (not shown). 3

4 The nominal values of the FTOF geometry parameters for each sector are listed in Table 2. Note that there are two values listed for gap 1b. The smaller value is the gap between scintillation bars mounted to a single backing structure and the larger value is the gap between scintillation bars on neighboring backing structures. Tables 10, 11, and 12 in the Appendix (Section 4) contain a listing of the coordinates of the center point on the upstream face of each scintillator in panel-1a, panel-1b, and panel-2 in Sector 1 (see Fig. 3 for the CLAS12 sector naming convention) calculated using these values. The coordinates are listed in the Hall B coordinate system with the z-axis along the electron beamline pointing downstream, the x-axis pointing toward beam left, and the y-axis pointing upward, thus defining a righthanded coordinate system. The origin is located at the center of the nominal CLAS12 target position. Figure 2: View of the FTOF scintillators for panel-1a, panel-1b, and panel-2 in the sector mid-plane with the key parameters indicated. There are five main users of the FTOF geometry information: 1. The designers working with the CAD program laying out the detectors on their support frames and their positions on the Forward Carriage; 2. The developers working on the event display program; 4

5 Parameter R a R b th a min th b min th 1 tilt thick 1a w 1a thick 1b w 1b gap 1a 1b R 2 th 2 min th 2 tilt thick 2 w 2 gap 1a gap 1b gap 2 Nominal Value cm cm cm cm 6.0 cm 6.0 cm cm cm cm 22.0 cm cm 0.04 cm, cm cm Table 2: Table of the nominal geometry parameters for the CLAS12 FTOF detector system. S2 S3 S1 S4 S6 S5 Figure 3: View of the upstream face of the Forward Carriage in Hall B showing the CLAS12 sector naming conversion for Sector 1 (S1) through Sector 6 (S6). The dashed lines denote the mid-planes for each sector. 5

6 3. The developers of the Monte Carlo program; 4. Those working on the event reconstruction and calibration software; 5. The survey and alignment team. It is essential that the underlying geometry be the same for all interested parties and that it be derived from a common source, namely the information contained in this document Scintillator Lengths The nominal scintillator layout and naming convention for panel-1a, panel-1b, and panel-2 are shown in Figs. 4, 5, and 6, respectively. The lengths of the individual bars are given in Tables 3, 4, and 5. In all cases, scintillator #1 in a given array is the one located closest to the beamline. Figure 4: View of the face of a generic FTOF panel-1a array showing the numbering scheme for the 23 scintillators that make up the counters in each of the six sectors of CLAS12. 6

7 Counter Length (cm) Counter Length (cm) Counter Length (cm) Table 3: FTOF panel-1a counter lengths (cm) for each scintillator. Each panel-1a scintillator is 15.0 cm wide. Figure 5: View of the face of a generic FTOF panel-1b array showing the numbering scheme for the 62 scintillators that make up the counters in each of the six sectors of CLAS12. 7

8 Counter Length (cm) Counter Length (cm) Counter Length (cm) Counter Length (cm) Table 4: FTOF panel-1b counter lengths (cm) for each scintillator. Each panel-1b scintillator is 6.0 cm wide. Figure 6: View of the face of a generic FTOF panel-2 array showing the numbering scheme for the 5 scintillators that make up the counters in each of the six sectors of CLAS12. Counter Length (cm) Table 5: FTOF panel-2 counter lengths (cm) for each scintillator. Each panel-2 scintillator is 22.0 cm wide. 8

9 Fig. 7 shows a plot of the FTOF panel-1a, panel-1b, and panel-2 counter lengths vs. counter number, demonstrating the linearity of the counter lengths in each array. Note that the only deviation from linearity vs. counter number occurs for panel-1a after counter #5. This behavior will have to be captured by the CLAS12 geometry service description. Fits to these data yield the counter length for a given counter number N counter as follows: Panel-1a: L(cm) = N counter (N counter = #1 #5), Panel-1a: L(cm) = N counter (N counter = #6 #23), Panel-1b: L(cm) = 6.40 N counter , Panel-2: L(cm) = N counter panel-2 panel-1a panel-1b Counter Length Counter Number Figure 7: Plot of the scintillator lengths (cm) vs. counter number for each of the FTOF arrays. 2.2 Nominal Geometry The positioning of the individual FTOF scintillators shown in Fig. 2 is based on the design model. The true positioning of the counters must account for the material layers that are wrapped around the bare scintillator material, as well as the nominal gaps between the bars as they are placed on their mounting frames. To determine the nominal scintillator positioning on the frames, we use the known thicknesses of the wrapping materials and the average counter-to-counter gaps determined from the assembled arrays. To determine the nominal geometry and positioning of the FTOF scintillators on the counter frames, the overall radial extent of each counter array must be known (i.e. the length along the sector mid-plane from the inside edge of counter #1 to the outside edge of the last counter). These details from direct measurements of the counters on their associated frames are shown in Table 6. The last two rows of Table 6 show the average overall radial width of each panel and the standard deviation σ of the sector-to-sector variations for each of the different FTOF arrays. 9

10 Array Radial Width (cm) Sector Panel-1a Panel-1b Panel Avg σ Table 6: FTOF overall sector radial width (cm) for panel-1a, panel-1b, and panel-2. From the information included in Table 6 on the overall radial extents of each counter array, along with knowledge of the wrapping materials and how the counters are stacked on the frames, the nominal gap between each of the counters on the frames can be computed and compared to a sampling of the measured values. The average gap between counters can be determined by the following formula for panel-1a and panel-2: W 1a,2 = (N c W c ) + (2 t material N c ) + (N c 1) gap. (1) For panel-1b the appropriate formula is given by: W 1b = (N c W c ) + (2 t material N c ) + (N c /2 1) gap. (2) In these expressions, the overall sector radial extents (W 1a,1b,2 ) come from the average values given in Table 6, N c is the number of counters in each sector (panel-1a = 23, panel-1b = 62, panel-2 = 5), W c is the nominal width of the bare scintillators in each array (panel-1a = 15.0 cm, panel-1b = 6.0 cm, panel-2 = 22.0 cm), t material is the material thickness on each side of the counters, and gap is the nominal counter-to-counter gap on each frame. Note that for panel-1b, the pairs of counters on a single backing structure have no gap between the scintillators except for the wrapping materials. The separation between counters on neighboring backing structures is given by the value of gap. To determine the nominal average value of gap for each FTOF panel, we need to know the details of the wrapping materials employed in the detector construction. For panel-1a and panel-2 the wrapping materials include: 1 layer of in ( cm) thick Kapton, 2 layers of in ( cm) thick aluminum foil, t material = cm (total thickness per side). For panel-1b, the wrapping materials include: 3 layers of in ( cm) thick Tedlar, 10

11 1 layer of in ( cm) thick aluminized Polyester film, t material = cm (total thickness per side). Using Eqs. (1) and (2) this gives the following nominal average counter-to-counter gap values: Panel-1a: gap = cm, Panel-1b: gap = cm (gap between counter pairs as noted above), Panel-2 : gap = cm. The computed gaps can be compared against a sampling of the measured counter-tocounter gaps. For panel-1a, the typical measured gaps fell into the range from cm to cm. For panel-1b, the typical measured gaps were in the range from cm to cm. For panel-2, the typical measured gaps fell into the range from cm to cm. The measured gaps are reasonably consistent with the average values computed for these gaps. 2.3 FTOF Relative Geometry To accommodate the size of the photomultiplier tubes on the side of the Preshower Calorimeter (PCAL) modules, a translational shift of all of the PCAL modules on the Forward Carriage was required. These shifts, which amounted to cm, are indicated in Fig. 8. Thus each PCAL module was shifted along the directions indicated in Fig. 8 by this distance. However, each of the FTOF arrays is mounted such that they are (roughly) symmetric about the sector mid-plane. Fig. 9 shows for a single representative sector the FTOF panel-1a array in relation to the active areas of the Region 2 drift chamber and the LTCC, projected downstream to the position of panel-1a. Also shown are the active areas of the PCAL and EC projected upstream to the panel-1a position. Fig. 10 shows the same projections but at the location of the FTOF panel-1b array. 3 Survey Plans The survey plans and requirements to locate the panel-1a, panel-1b, and panel-2 FTOF arrays after installation on the Forward Carriage in Hall B are detailed in the document: CLAS12 Forward Time-of-Flight Survey Requirements, carman/ftof/notes/ftof survey.pdf. The general approach to the FTOF survey was based on surveying the individual FTOF arrays after they were installed with respect to optical survey monuments positioned on the Forward Carriage. When the Forward Carriage is moved from its downstream installation position into its nominal upstream running position, the monument points will be surveyed again. These two sets of survey information will then provide the locations of the FTOF arrays at their nominal locations. 11

12 S S S S S S Figure 8: View of the face of the Forward Carriage indicating the translational shift directions of the PCAL modules relative to the sector mid-planes. Panel 1a Projections DC Region 2 LTCC EC PCAL Figure 9: View of the active area outlines of the Forward Carriage detectors in a single sector projected to the location of FTOF panel-1a. 12

13 Panel 1b Projections DC Region 2 LTCC EC PCAL Figure 10: View of the active area outlines of the Forward Carriage detectors in a single sector projected to the location of FTOF panel-1b. The survey information will then be analyzed to determine the X, Y, and Z translational shifts of each array in their local sector-based coordinate system, as well as the associated pitch, roll, and yaw angles about these different coordinate axes. These translational and rotational offsets compared to the ideal model geometry will need to be included in the FTOF geometry service. For the nominal geometry, all FTOF arrays are assumed to be positioned symmetry about the sector mid-plane. Note that the survey information does not locate each of the counters in the different arrays. The survey data is most useful to define the as installed plane of each of the counter arrays. The locations of the individual counters within each array will be determined using the nominal geometry information from Section FTOF Materials The FTOF system includes material layers and supports that lie within the active area of the detector. These materials will need to be properly included within the Monte Carlo detector description. The material layers include the reflective and light-tight layers that are used to wrap the individual scintillator bars and the supports include the composite backing structures that support the bars from flexing and are used to attach them to the sector frames that lie in the detector shadow region. Note that the tape used to attach the scintillator bars to the backing structures are not included in this specification. The specification of the FTOF material layers and supports for panel-1a, panel-1b, and panel-2 are included in Tables 7, 8, and 9, respectively. 13

14 Material Layer Thickness Comment Scintillator Layers Al foil t=0.001 in 2 layers per side Kapton t= in 1 layer per side Pb sheet t=0.005 in upstream face only Backing Structure St.St. skin (304SS) t= in 1 layer on either side of foam Polyurethane foam (FR-3705) t=1.00 in ρ=5 lb/ft 3 Table 7: FTOF layers and support specifications for panel-1a. The 5.0-cm thick scintillator bars of panel-1a are assumed. Material Layer Thickness Comment Scintillator Layers Aluminized Polyester film t= in 1 layer per side Kapton t= in 3 layers per side Backing Structure St.St. skin (304SS) t=0.029 in 1 layer on either side of foam Polyurethane foam (FR-3705) t=1.971 in ρ=5 lb/ft 3 Table 8: FTOF layers and support specifications for panel-1b. The 6.0-cm thick scintillator bars of panel-1b are assumed. Material Layer Thickness Comment Scintillator Layers Al foil t=0.001 in 2 layers per side Kapton t= in 1 layer per side Pb sheet t=0.005 in upstream face only Backing Structure St.St. skin (304SS) t= in 1 layer on either side of foam Polyurethane foam (FR-3705) t=3.00 in ρ=5 lb/ft 3 Table 9: FTOF layers and support specifications for panel-2. The 5.0-cm thick scintillator bars of panel-2 are assumed. 14

15 Appendix Using the values in Table 2 the x and z coordinates at the center of the upstream face of each bar can be computed for counter N using: Panel-1a & Panel-2 Panel-1b x = R sin(th min) + [(N 1)(t + gap) + 0.5t] cos(th tilt) (3) z = R cos(th min) [(N 1)(t + gap) + 0.5t] sin(th tilt) (4) N pair = (mod(n, 2) + N)/2, N member = mod(n + 1, 2) (5) x = R sin(th min) + [(2t + gap 1 + gap 2 )(N pair 1) + (t + gap 1 )N member + 0.5t] cos(th tilt) (6) z = R cos(th min) [(2t + gap 1 + gap 2 )(N pair 1) + (t + gap 1 )N member + 0.5t] sin(th tilt) (7) Counter x (cm) y (cm) z (cm) R (cm) Table 10: Table of the nominal coordinates in the Hall B coordinate system locating the center point on the upstream face of the scintillators for each FTOF counter in panel-1a for Sector 1. 15

16 Counter x (cm) y (cm) z (cm) R (cm) Counter x (cm) y (cm) z (cm) R (cm) Table 11: Table of the nominal coordinates in the Hall B coordinate system locating the center point on the upstream face of the scintillators for each FTOF counter in panel-1b for Sector 1. Counter x (cm) y (cm) z (cm) R (cm) Table 12: Table of the nominal coordinates in the Hall B coordinate system locating the center point on the upstream face of the scintillators for each FTOF counter in panel-2 for Sector 1. 16