Transcription

1 LHCb Note A Possible Layout of an Inner Tracker Silicon Detector Olaf Steinkamp November 17, Introduction This note presents a possible layout for the sensitive area of an Inner Tracker Silicon detector. It is organized as follows: In section 2, the so-called Inner Tracker mother volume", the sensitive area that has to be covered by the Inner Tracker, is determined for each tracking station. A possible layout of silicon sensors and ladders is presented in section 3, and in section 4 a layout of the detection layers is proposed that uses these ladders to cover the mother volume" of each Inner Tracker station. In section 5, some component and read-out channel statistics are summarized for the proposed layout. The layout presented in this note has been used to implement an Inner Tracker Silicon detector in the LHCb simulation packages 1. However, the external constraints that define the size of the Inner Tracker mother volumes" (shape of beam pipe, z positions of tracking stations, maximum occupancies in Outer Tracker) are not yet well defined. Consequently, details of the layout are subject to change, and especially the numbers presented in section 4 have to be taken as preliminary estimates. 2 Sensitive Area Figure 1 shows a sketch of the mother volume", i.e. the sensitive area that has to be covered by the Inner Tracker. Its outer dimensions, x max and y max, are determined by the require- Xmax Xmin Xmin Xmax Ymax Ymin Ymin Ymax Figure 1: Definition of Inner Tracker mother volume". 1 F.Ronga, Inner Tracker Geometry Implementation in GEANT, LHCb note

2 November 17, 2000 Possible Layout of ITR Silicon Detector Station z min z max r beam d beam x min = y min x = y corner [mm] [mm] [mm] [mm] [mm] [mrad] [mrad] Table 1: Dimensions of beampipe hole; see text for details. ment oflow occupancies in the Outer Tracker. The dimensions of the central hole, x min and y min, are determined by the size of the beam pipe that passes through the detector. Inner Acceptance The current beam pipe follows a conical shape, opening up along the beam axis. From the request to approach the beam pipe as close as possible follows a different size of the beampipe hole for every station, depending on its z position. From the circular cross section of the beam pipe, x min = y min. Table 1 summarizes the relevant parameters for all stations. The z coordinates of the Inner Tracker stations were taken from LHCb note Relevant for the determination of the beam-pipe hole is z max, the downstream end of the Inner Tracker. Inner radius and thickness of the beam-pipe at these z positions were extracted from the geometry description file pipe3.0.cdf (SICB v232 and earlier). The conflict with beam-pipe elements that LHCb note reports for station 2 were ignored here, it was assumed that the beam pipe follows the standard 10 mrad cone. A 10mm clearance between beam pipe and detector elements was requested by the LHC group 3. An additional 7mm were added to take into account detector frames (thermal insulation) and dead area along the edge of silicon sensors (guard rings). This results in: x min = y min = r beam + d beam + 10mm + 7mm: For reference, table 1 also lists the corresponding polar angles along two lines from the interaction point at (0,0,0) to the center of a side and to a corner of the beam-pipe hole, respectively. The upstream end, z min, of the Inner Tracker was taken as reference for the calculation of these angles. Since the shape of the beam pipe and the z positions of some tracking stations are still the subject of optimization studies, all numbers presented in table 1 have to be taken as preliminary. 2 O.Steinkamp, Space Requirements and z Positions for Tracking Stations, LHCb note Georg v.holtey to O.S., August 15,

3 Possible Layout of ITR Silicon Detector November 17, 2000 Station x max y max x y corner [mm] [mm] [mrad] [mrad] [mrad] Table 2: Outer dimensions of Inner Tracker mother volumes". Outer Acceptance The outer dimensions of the mother volumes" were taken from the geometry description file wtrk.cdf (SICB v232 and earlier). Since studies of Outer Tracker occupancies, and their influence on tracking performance, are still under way, the values for stations 2 to 11 have to be taken as preliminary. Station 1 is an exception as no Outer Tracker is foreseen here, and the outer dimensions are determined by the overall acceptance of the experiment. The relevant values for all stations are summarized in table 2. Included are also the corresponding polar angles along three lines pointing from the interaction point at(0,0,0) to the centre of each side and to a corner of the sensitive area. The downstream end, z max, of the Inner Tracker was taken as reference for the calculation of these angles. The parameters x max and y max define the size of the sensitive area that the Inner Tracker has to cover at minimum. Due to the fixed granularity of silicon sensors, the actual sensitive area can extend beyond these values. 3 Silicon Sensors and Ladders In the silicon option of the Inner Tracker, the sensitive area will be covered by an array of rectangular, single-sided silicon sensors with read-out strips running parallel to one edge of the sensor. The size of a single sensor is limited by the size of the silicon wafer from whichitis produced. Near-vertical ladders" consisting of several sensors, with read-out strips running parallel to the long side of the ladder, are employed in order to cover the full height of the Inner Tracker sensitive area. The effective read-out strip length is determined by the number of sensors on a ladder that are connected in series ( ganged") to the same read-out channels. Several ladders are arranged side-by-side in order to cover the full width of the Inner Tracker sensitive area. Stereo views are obtained by rotating each ladder against the vertical by the appropriate stereo angle. From the point of view of mass production and logistics, it is highly desirable to limit the number of different sensor and ladder layouts to the minimum possible. For this study, it was assumed that the complete Inner Tracker is constructed using only one type of silicon 3

4 November 17, 2000 Possible Layout of ITR Silicon Detector Physical length 102 mm Physical width mm Length sensitive area 100 mm Width sensitive area mm Strip pitch 235 μm Number of strips 384 (= 3 128) Table 3: Proposed geometry of silicon sensor. 6" wafer. On the right, a sketch of the sensor on a sensor. Five different types of ladders, with one, two, three, four and six sensors, had to be employed. Sensor Layout The proposed sensor geometry is summarized and depicted in table 3. It was assumed that the sensors will be produced on 6" wafers with a usable diameter of about mm. This value takes into account that an approximately 7.5mm wide ring around the edge of the wafer is not usable. A strip length (i.e. length of the sensitive surface of the sensor) of 100 mm was chosen because it fits well to the geometry of a 6" wafer and allows to cover a large part of the Inner Tracker sensitive area with ladders of four sensors. Guard ring structures cause an approximately 1mm wide insensitive band" around the edge of the sensor, such that the overall physical length of the sensor is 102mm. The usable surface of a 6" wafer is fully exploited if the physical width of the sensor is about 92mm, corresponding to a width of the sensitive area of about 90mm. Expected strip occupancies, required spatial resolution and operational constraints (signal collection, high-voltage performance and strip capacitance) suggest a strip pitch of about 2μm. A value of 235μm was chosen here, because it allows to divide the sensitive width of the sensor over 384 strips, corresponding to 3 read-out chips of 128 input channels each. With this choice, the precise width of the sensitive area is 90.24mm and its overall width 92.24mm. Ladder Layout For stations 2 to 11, the full height of the sensitive area to the left and to the right of the beam-pipe hole can be covered by ladders of four sensors. Asketch of such a ladder is shown in figure 2. The sensors are mounted against each other edge-to-edge with no overlap. This is the mechanically simplest solution, but guard ring structures on the sensors cause a 2 mm wide 4

5 Possible Layout of ITR Silicon Detector November 17, 2000 Sensor Support Readout PCB FrontEnd Chip Figure 2: Four-sensor ladder with read out at both ends. insensitive area in between two consecutive sensors. Simulation studies will have to show if this is acceptable from the point of view of physics performance. In order to keep strip occupancy and the effective strip length low 4, these ladders will be equipped with read-out electronics at both ends. Two sensors each will be connected in series ( ganged") to the front-end chips at each end. The sensitive area above and below the beam pipe can be covered by shorter ladders of one or two sensors, depending on the station. These ladders will be read out from one end only. Station 1 requires longer ladders, of three sensors above and below the beam-pipe hole, and of six sensors to its left and its right. As in the other stations, the six-sensor ladders will be read out from both ends, the three-sensor ladders from one end only. The effective strip length of 30cm from three ganged sensors causes a large input capacitance to the read-out chips. If this turns out to be a problem for signal shape or noise performance of the chip, a layout may be employed as depicted in figure 3 for a six-sensor ladder: the number of read-out chips at each end of the ladder is doubled, the two outermost sensors are ganged to one set of chips, whereas the innermost sensor is connected to the second set of read-out chips via a 20cm long Kapton cable. The total strip capacitance of the ensemble of one sensor and the Sensor Support Readout PCB FrontEnd Chip Kapton cable Figure 3: Possible layout of a six-sensor ladder with double" read out at both ends. Kapton cable is expected to be comparable to that of two ganged sensors. 4 The effective strip length determines the input capacitance seen by the read-out chip and is limited by signal shape and noise performance of the chip 5

6 November 17, 2000 Possible Layout of ITR Silicon Detector 4 Station Layout 0-Degree" Layers Figures 4 to 14 show the proposed layouts for the 0-degree layers of each Inner Tracker station. The stations were assembled from two L"-shaped half stations, with the region of overlap between the two half stations shown in the upper right and lower left quadrants Figure 4: Layout for station 1. Black lines and measures on top and left indicate the mother volume", measures on right and bottom indicate dimensions of the actual sensitive area. On both sides of the beam pipe, the sensitive area was filled with four-sensor ladders (sixsensor ladders for station 1), starting from the edges of the beam-pipe hole. Neighbouring ladders overlap by 2.47mm, in order to avoid dead space caused by the guard ring structures on the sensors and to provide overlapping read-out strips for the relative alignment of the ladders. The number of ladders was chosen such that the mother volumes" defined in section 1 are fully covered. The actual sensitive area can extend beyond y max. In station 1, the six-sensor ladders do not fully cover the mother volume" vertically. Simulation studies will have to show if this small reduction in the outer acceptance has an effect on physics performance. The sensitive area above and below the beam pipe was then filled with shorter ladders starting, again with an overlap of 2.47mm, from the first long ladder next to the beam-pipe 5 The material budget of a station is doubled in these overlap regions and in order to avoid hot spots" in the overall material distribution of the tracking system, the actual arrangement should be such that the overlap is in the upper left and lower right quadrants for every second station. 6

7 Possible Layout of ITR Silicon Detector November 17, 2000 hole. Vertically, these ladders where aligned with respect to the edge of the beam-pipe hole. The type of ladders (one- or two-sensor) was chosen such that the mother volume" defined in section 1 is fully covered. The actual sensitive area can extend beyond x max. The number of ladders was chosen such that there is no acceptance gap in between the two half stations. The actual overlap between left and right half-stations can be significantly larger than needed for full acceptance coverage. Stereo Views The layouts for +/ stereo views can be similar to those shown in figures 4 to 14, as long as the stereo angles are small. An example is shown in figure 15, for station 5 and the currently foreseen stereo angle of 5 ffi. Each ladder was rotated by the stereo angle. y" Views No proposal has been prepared yet for the so-called y" views with horizontal read-out strips, that are currently foreseen for stations 1, 2, 10 and Conclusions Ignoring the y views and assuming four detection layers for each station, the presented layout results in the numbers of sensors, ladders and read-out channels given in table 4. The total number silicon sensors 1456 ladders sensor ladders sensor ladders sensor ladders 8 4-sensor ladders sensor ladders 32 read-out channels Table 4: Components and channel statistics for the proposed layout, ignoring y"-layers with horizontal read-out strips. quoted number of 286k read-out channels assumes standard" read-out of six- and threesensor ladders in station 1, i.e. three sensors ganged to one read-out channel. Assuming double" read-out of these ladders, as shown in figure 3, the number of read-out channels increases to 313k. For comparison, a purely geometric estimate of the minimum number of read-out channels that would be required to cover the mother volumes" of the Inner Tracker with a strip pitch of 235 μm gives: 286k 4layers 2top/bot (10 stations cm + 1 station 79 cm) 235μm = 231k; assuming standard" read-out in station 1, or 258k channels assuming double" read out for station 1. 7

8 November 17, 2000 Possible Layout of ITR Silicon Detector Figure 5: Station 2 The difference between this estimate and the number of read-out channels quoted for the proposed layout is mainly due to the, for some stations significant, overlaps between left and right half-stations, resp. the extention of the sensitive area beyond x max. These are consequences of the use of one standard sensor for the complete Inner Tracker. Future studies will have to show if possible cost savings could justify the introduction of a second sensor type of reduced width and number of read-out strips. On the other hand, slight adjustments of mother volumes" for some stations could also help to decrease the number of read-out channels. Another issue for optimization studies concerns the question of station mechanics, that was not taken into account here. Slight modifications of the layout for some stations may be desirable in order to avoid the need to design and produce mechanics individually for every station. However, such optimization studies make sense only when the external constraints that define the Inner Tracker mother volumes" are well understood. 8

9 Possible Layout of ITR Silicon Detector November 17, Figure 6: Station Figure 7: Station 4 9

10 November 17, 2000 Possible Layout of ITR Silicon Detector Figure 8: Station Figure 9: Station 6 10

11 Possible Layout of ITR Silicon Detector November 17, Figure 10: Station Figure 11: Station 8 11

12 November 17, 2000 Possible Layout of ITR Silicon Detector Figure 12: Station Figure 13: Station 10 12

13 Possible Layout of ITR Silicon Detector November 17, Figure 14: Station Figure 15: Stereo view in station 5. 13