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Synopsis
Synopsis
In order to provide greatly increased precision, leading to higher efficiency for detecting charm and bottom decays, and to accommodate much higher collision rates, the ALICE Collaboration replaced the inner tracking system (ITS) in 2020. The new ITS improved impact paramter resolution by a factor of 3(5) in rø(z) at low transverse momentum and increase the tracking capability. This was accomplished by getting closer to the interaction point, from 39 mm to 23 mm for the first layer, and reducing spatial resolution down to 5 µm x 5 µm. In addition, the new ITS will have the ability to readout at a rate of 100 kHz, a factor of 100 faster than the current ITS. Read more at ITS Detector pages.
Design & Construction at LBL
Design & Construction at LBL
The new ITS consists of seven layers of innovative Monolithic Active Pixel Sensors (MAPS). With readout electronics directly on the silicon, a thinner wafer is produced resulting in a reduction in overall radiation length (0.3% and 1% X0 for the inner and outer barrel respectively). A simplified diagram can be seen in Figure 1.
Figure 1. Left - a hybrid pixel sensor consisting of a silicon sensor (thickness varies, but is usually >300 µm) bump-bonded to a readout chip. Right - a MAPS sensor with the electronics directly on the silicon, giving rise to a thinner overall detector.
The ITS upgrade consists of an inner barrel, with three layers very close to the beam pipe, and an outer barrel with two middle layers and two outer layers as shown in Figure 2.
Figure 2. The ALICE ITS upgrade, a 7-layer, barrel geometry, pixel detector. It is comprised of an inner barrel and outer barrel with radii from 23 – 400 mm. It will have 10 m2 of silicon and 12.5 billion pixels.
The basic unit of the ITS outer barrel is a stave of MAPS sensors. Each stave is composed of two half-staves glued together onto one carbon fiber space frame support structure. The LBL RNC group was the project leader for the ALICE USA ITS upgrade and was solely responsible for building the middle layers, which are a total of 54 staves, each about a meter long.
Figure 3. The ALICE ITS ML staves in various stages of construction on a Coordinate Measuring Machine. 1: Half-stave assembly on a carbon fiber cold plate. 2: Half-stave gluing to carbon fiber space frame. 3: Final metrology of a stave before power bus soldering.
Figure shows some of the assembly steps. Using a Coordinate Measuring Machine (CMM), modules were place to within 50 µm of nominal position on a carbon fiber cold plate. Modules then were soldered together and tested before being glued onto a space frame. Staves were then soldered to power and bias buses and folded to complete the process.
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