# Applications of silicon strip and pixel-based particle tracking detectors

## Abstract

As the review of the European particle physics strategy is underway and planned future particle accelerators are being discussed worldwide, new, improved silicon tracking detector concepts are currently being studied for use at the proposed new facilities. This Technical Review outlines the current state of the art in silicon tracking detectors for particle physics along with examples of some recent applications in other areas. The various technologies currently available in particle physics are listed and their relative merits for different uses and environments are discussed. The silicon detector research programmes to allow optimal scientific returns from future particle physics experiments are described along with their current status.

## Key points

• Particle physics experiments depend on the ability to accurately track the paths of particles produced in high-energy collisions.

• Frontier experiments demand ever higher collision rates both to maximize sensitivity to rare processes and subtle effects, and to minimize the statistical errors on measurements.

• The high-rate capability, tracking precision, sensor granularity, readout speed and radiation resistance of the current generation of silicon detectors has led to their widespread adoption, particularly in collider experiments (which typically require the most demanding specifications).

• New technologies are emerging, both from within particle physics and through the design of specialist variants of commercial optical sensors, which offer significant improvements in terms of particle-track spatial resolution, fast-timing capability, detector radiation hardness, scattering material and cost per unit area.

• Other disciplines that already use particle physics silicon tracking technologies can only benefit from adopting these developments as they become available.

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## Acknowledgements

The author thanks his many colleagues in the international experimental particle physics community and especially those whose excellent presentations, articles, reviews and books are cited here. This Review could not have been completed without the encouragement of the Birmingham Particle Physics Group (whose recent research has relied to a significant degree on the UK’s Science and Technology Facilities Council 2015–2019 consolidated grant: ST/N000463/1) or without the wider support of the School of Physics and Astronomy at the University of Birmingham, UK. The author is also indebted to colleagues in the medical physics community and to the European Union’s Horizon 2020 Research and Innovation programme (under Grant Agreement no. 654168).

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Correspondence to Philip Allport.

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## Glossary

Bunch crossing

In a collider, particles are accelerated in a large number of bunches and they counter rotate in the accelerator and keep passing through each other to produce interactions.

Trigger

Only data from bunch crossings containing a collision of interest are kept. Very fast processing of a subset of the data provides the ‘trigger‘ to readout that data from temporary on-detector data storage.

Vertex

In a collider, the produced particle tracks spread out from the point where the collision occurred, the ‘primary vertex’. If one or more of these particles subsequently decay to produce more tracks, there are also ‘secondary vertices’ from which the additional tracks originate.

Beam pipe

Particle collisions take place in the vacuum pipe ‘beam pipe’ in which the particle bunches circulate and collide.

Vertexing

Extrapolating tracks back to where they meet (that is, to the vertex they originate from).

Granularity

The segmentation of the signal-collecting nodes defines the detector granularity and hence ability to cope with many simultaneous hits per unit area.

Calorimeter

A detector that measures charged or neutral particle energy through measuring the particle showers they produce when they interact in a dense absorber.

Bunch train

In some accelerators, bunches come grouped together in time (bunch trains) with relatively long periods of inactivity in between them.

The thickness of the material (X0) required to reduce the mean energy of a traversing electron by 1/e $$\left( \sim 1/2.718\right)$$.

Sensing matrix

The nodes within the sensor measuring the signal arranged in a matrix of pixels.

Zero suppression

Even after selection using the trigger, the number of electronics channels to readout is often too large, but many channels are empty. Zero suppression means only reading out channels with signals above a pre-set threshold.

Space points

With strip detectors, there is no information where the particle causing a ‘hit’ occurred along the strip direction. Adding orthogonal strips still only gives two projections, which leaves ambiguities if many particles pass through the detector. Pixels measure directly the hit coordinates (space point with third coordinate constrained by the detector plane thickness).

Lateral spread of the showers produced by electrons or photons (‘electromagnetic showers’) at depth through the material d = X0.

Distance (d  =  λINT) over which the probability of a particle not having interacted falls by 1/e for hadrons (particles that mostly interact via the strong nuclear force).

Pixelated fast-timing

Pixel detectors give true ‘space points’ (3D information), but if the timing of the hit is also recorded accurately (few tens of picoseconds) one can speak of 4D fast-timing pixels.

Flip-chip

To marry a pixel sensor with a readout chip requires a high-density interconnect (typically conducting ‘bumps’) patterned onto one surface and then the two connected face-to-face.

In astronomy, the response of an optical system to a source that itself approximates an infinitely narrow point source.

Relative stopping power

Different materials slow down protons to a different degree, giving differences in where protons stop for a given incident energy.

Multiplication layer

Detectors can be designed to create a high-field region just below the charge collecting node in which the collected charge is accelerated to sufficient energy to induce further electron–hole pair production ‘charge multiplication’.

Voxel

A pixel is a 2D area on a detector in which the third coordinate is usually only determined by the thickness of the silicon plane, which is typically much larger than the pixel dimensions. By making the sensing thickness the same dimensions as the pixel gives a volume ‘voxel’ within which the hit occurred.

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Allport, P. Applications of silicon strip and pixel-based particle tracking detectors. Nat Rev Phys 1, 567–576 (2019). https://doi.org/10.1038/s42254-019-0081-z

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