[Paper Review] SciFi: A large Scintillating Fibre Tracker for LHCb

P. Hopchev|arXiv (Cornell University)|Oct 23, 2017

Particle Detector Development and Performance3 citations

TL;DR

The LHCb SciFi tracker replaces the existing tracking system with a large, homogeneous scintillating fibre detector using 250-µm diameter fibres read out by cooled silicon photomultipliers (SiPMs) and custom ASICs. It achieves 70 µm spatial resolution, 99% hit efficiency, and 16 photo-electrons of light yield in testbeams, with radiation-hardened design for 35 kGy dose and 10¹² neq/cm² neutron fluence over its lifetime.

ABSTRACT

The LHCb detector will be upgraded during the Long Shutdown 2 (LS2) of the LHC in order to cope with higher instantaneous luminosities and to read out the data at 40 MHz using a trigger-less readout system. The current LHCb main tracking system will be replaced by a single homogeneous detector based on scintillating fibres. This contribution gives an overview of the scintillating fibre tracker concept and presents the experience from the series production complemented by most recent test-beam and laboratory results.

Motivation & Objective

• Replace the LHCb main tracking system with a single, homogeneous scintillating fibre tracker to handle increased luminosity and 40 MHz trigger-less readout.
• Achieve sub-100 µm spatial resolution in the bending direction with a low material budget of 1% per layer.
• Ensure long-term performance under high radiation, including 35 kGy ionizing dose and 10¹² neq/cm² neutron fluence.
• Develop radiation-hardened components, including cooled SiPMs and fibre mats with reduced cross-talk.
• Enable full serial production and integration of 590,000 channels for the 2019 detector upgrade.

Proposed method

• Use 2.4 m long scintillating fibre mats composed of six layers of 250 µm diameter SCSF-78MJ fibres with TiO₂-doped epoxy glue to suppress optical crosstalk.
• Read out signals at one end of each mat using 128-channel SiPM arrays bonded to flex-cables, with light collected via a mirror at the opposite end.
• Cool SiPMs to -40°C using 3D-printed titanium cold-bars to suppress dark count rate after neutron irradiation.
• Process signals via the PACIFIC ASIC, which performs pre-amplification, fast shaping, and dual-gated integration with near-zero dead time.
• Digitize signals using triple-threshold tunable comparators and clusterize data in FPGAs on clusterization boards before optical transmission to DAQ.
• Design detector modules from aligned fibre mats bonded to honeycomb/carbon-fibre panels, with reference alignment pins for precision assembly.

Experimental results

Research questions

• RQ1 Can a large-scale scintillating fibre tracker achieve sub-100 µm spatial resolution under high radiation and particle multiplicity?
• RQ2 How do radiation-induced degradation effects (ionizing dose and neutron fluence) impact fibre attenuation and SiPM dark count rate over the detector lifetime?
• RQ3 Can cooled SiPMs maintain sufficient signal-to-noise ratio under 10¹² neq/cm² neutron fluence, enabling reliable 40 MHz readout with 4.5 photo-electron threshold?
• RQ4 What is the achievable light yield and position resolution in a full-scale prototype under beam test conditions?
• RQ5 Can the modular, serial production approach ensure consistent performance across 590,000 channels with 1% material budget per layer?

Key findings

• The SciFi tracker achieves a spatial resolution of 70 µm in testbeam campaigns using proton/pion and electron beams at CERN and DESY.
• A nominal light yield of 16 photo-electrons per 1 MeV deposited energy was measured, meeting design goals.
• Hit efficiency of 99% was confirmed in testbeam and laboratory measurements, even after irradiation.
• The SiPM dark count rate reaches 14 MHz per channel at -40°C after 6×10¹¹ neq/cm² neutron fluence, but remains manageable due to a 4.5 photo-electron cluster threshold.
• Fibre attenuation length degrades by up to 40% at the innermost region after 35 kGy ionizing dose, with minimal effects at 50 cm from beam pipe.
• The full detector system, comprising 590,000 channels, is being serially produced and will be installed in 2019 for the LHCb upgrade.