The IBL is located at a nominal distance of 33.5mm from the beam axis, where this dis-tance refers to the sensors position and it is the closest layer to the interaction point of the ATLAS detector as shown in Figure 3.6. Given the small sensor distance from the beam axis (compared to 50.5mm for the Pixel B-Layer), the sensors and front-end electronics must cope with a much higher hit rate and radiation with respect to the other layers. To address these requirements, a new front-end read-out chip has been developed, the FE-I4 [38] and two silicon sensor technologies were developed.
The most important layout parameters of the IBL detector are listed in Table 3.2 and a picture of the layout is shown in Figure 3.7. The detector consists of 14 mechanical sup-port (staves) equipped with pixel silicon modules surrounding the beam pipe, providing full azimuthal coverage. Each stave hosts the electrical and cooling services to 20 pixel modules that are attached to the stave and provide a |⌘| coverage up to 3. Two module types are installed on each stave based on two different silicon sensor technologies: Planar and 3D.
Value
Number of staves 14
Number of physical modules per stave 20 (12 planar, 8 3D)
Number of FEs per stave 32
Coverage in ⌘, no vertex spread |⌘| < 3.0 Coverage in ⌘, 2 (122mm) vertex spread |⌘| < 2.58
Active|z| stave length 330.15mm
Geometrical acceptance inz min, max 97.4%, 98.8%
Stave tilt in 14
Overlap in 1.82
Center of the sensor radius 33.5mm
Radiation length at z = 0 1.9%X0
Table 3.2: Main layout parameters for the IBL detector. The quoted radiation length is averaged over the stave width and includes the IBL support and positioning tubes.
Figure 3.6: The layout of the ATLAS tracking system with the additional layer of IBL.
Side A Z = 0 Side C
3D Planar Planar 3D
(a)$
(b)$ (c)$
R29.0/R29.3 - IPT R23.5 - Inner beam-pipe R31.0 - IBL inner envelope R40.0 - IBL outer envelope
R33.5 - Module radius Stave
FE-I4B chip 3D sensor Module flex Flex pigtail Stave flex
3D - HV TAB EXTENSION R42.5/R43.0 - IST
Figure 3.7: IBL layout: (a) Stave layout showing the position of Planar and 3D modules.
(b) Anr section showing the beam pipe, the Inner Position Tube (IPT), the staves of the IBL detector and the Inner Support Tube (IST). (c) An exploded r view of the corner of a 3D module fixed to the stave.
Pixel IBL
Active Surface [m2] 1.73 0.15
Number of channels [⇥ 106] 80.36 12.04
Pixel size [µm2] 50⇥400 50⇥250
Pixel array [pixels] 18⇥160 80⇥336
Chip size [mm2] 7.6⇥10.8 20.2⇥19.0
Active Fraction [%] 74 89
Analog current [µA/pixel) 26 10
Digital current [µA/pixel) 17 10
Analog voltage [V) 1.6 1.4
Digital voltage [V) 2.0 1.2
Data out transmission [MBit/s) 40 160
Sensor type Planar Planar / 3D
Sensor thickness [µm] 250 200 / 230
Layer thickness [%X0] 2.8 1.9
Cooling fluid C3 F8 CO2
Table 3.3: Comparison of the main parameters of the Pixel detector and the IBL detector.
Planar sensors populate the central stave region, while 3D sensors the side region, as shown in Figure 3.7(a). The forward region of the staves are populated by 3D sensors where due to the electrode orientation they can guarantee a betterz-resolution in the track reconstruction after heavy irradiation. The details about the sensors are given in Section 3.4.2. The staves are inclined by 14° with respect to the radial direction in order to achieve an overlap of the active area between staves. This tilt also compensates for the Lorentz angle of drifting charges in the case of Planar sensors, and the effect of partial column inefficiency for normal incidence tracks in the case of 3D sensors. Due to space constraints, the sensors are not overlapped along the stave (in z). However to minimize the dead region, modules are glued on the stave with a physical gap of 200µm.
A comparison between the IBL and the 3 layer Pixel Detector technical parameters is re-ported in Table 3.3.
The IBL volume, which contains the staves and the services, is the space between an external Inner Support Tube (IST) fixed on the Pixel structure and a precision mechanical support called the Inner Positioning Tube (IPT). The central part of the ATLAS beam pipe is slid into the IPT tube and attached to it. A key feature of this design approach is that independent volumes are generated and the fast removal of either the beam pipe with respect to the IBL package, or of the IBL and beam pipe with respect to the Pixel package, are allowed. As the Pixel Detector was extracted in 2013 for repair and service refurbishment, the IST was inserted. The IBL package, including the beam pipe, was then slid inside the IST once the Pixel detector was re-installed and fully re-connected.
The reduction of the material budget leads to an optimization of the tracking performance.
The average IBL radiation length is 1.88 X0 for normal incidence tracks at z = 0 and it corresponds to⇠ 70% of that for the Pixel B-Layer1. A lower radiation length with respect
1The estimated IBL radiation length has been confirmed in collision data. The difference with respect to the estimate reported in the IBL TDR [36] is mainly due to an initial underestimated of the module material and to the addition of the IPT.
Item Value [%X0] Beam pipe 0.32
IPT 0.12
Module 0.76
Stave 0.60
Services 0.19
IST 0.21
IBL total 1.88
Table 3.4: Averaged IBL material budget over the azimuthal angle for normal incident tracks at z = 0. The beam pipe material is excluded from the IBL total.
to the Pixel has been achieved by: a new low-mass module design; local support structures (staves) made of low density carbon foam; the use of CO2 evaporative cooling that optimizes the mass flow and the pipe size; the use of aluminium conductors for the electrical power services. Table 3.4 reports the main contributions to the IBL material budget. Figure 3.8 shows the material traversed by a track of origin z = 0 as a function of ⌘, smeared over the azimuthal angle.