ATLAS silicon strip detectors and charge sensing

Some time ago, scientists at the Large Hadron Collider (LHC) at CERN reported the potential discovery of a new fundamental particle, which does not fit anywhere in the standard model of physics. According to “the news”, the latest data from ATLAS and CMS (LHC’s two largest detectors) shows two unexpected data “bumps” from the usual gamma-ray flashes, which are correlated and acquired from two separate detectors. According to physicists, this may point to the existence of a particle that dwarfs by light-years even the recent discovery of gravitational waves.

It is not yet sure if this measurement data would get confirmed or rejected, but the latest news point that the significance of the results is fairly low, owning a sigma of 1.6 approximately. That fact inspired me to write a bit about the basics of the basics in silicon strip detector charge sensing, which is a stone age technology in commercial light sensing CMOS image sensors nowadays.

So what are strip detectors and how are they used? These are basically PN-junctions with an extremely wide aspect-ratio and, as their name suggests, look like strips. Here’s a sketch:

A bird's eye view of silicon strip detectors

A bird’s eye view of silicon strip detectors

These strips usually share an N-type substrate while each is P+ doped, covered by aluminium with some extra insulation layers in between. The LHC scientists are interested in observing interference patterns in X and gamma rays caused by the decay of the sought after particles. Apart of their intensity, what also interests them is the spatial trajectory of the high-energy rays. In order to detect the 2D-position of the gamma rays, they have invented a very clever strip array configuration. Let me explain, here’s another sketch:

Particle incidence angle detection using parallel strip configuration

Particle incidence angle detection using parallel strip configuration

A falling particle would have a higher probability of generating electron-hole pairs in the strip which is crossed by the X-ray photon, which already creates a kind of a 1-dimensional readout. To obtain the angular information, the adjacent strips could also be read-out and a particle correlation can be reconstructed. In other words, if the gamma ray happens to fall with some angle of e.g. 45 degrees, it will thus generate electron-hole pairs in two or three adjacent silicon strips. This gives us already almost 2D particle trajectory information. However, CERN engineers have decided to expand the technique even further, by adding another cross-pair of detectors underneath the upper set:

Hybrid X- and Y- direction parallel strip sensor configuration

Hybrid X- and Y- direction parallel strip sensor configuration

That way not only they can extract position and angular information in the x-direction, but also the y-direction, which, by using some post-processing provides accurate particle intensities and trajectories. But how can these PN silicon strips be read out?

The simplest method in reading out thousands of strips, is the use of an integrated charge amplifier and digitization electronics per each channel. Charge sensitive amplifiers have not been very “widely” used in the past with passive pixel CMOS image sensors, and have proven to be very suitable for single detector readout. These are still used in single-line CMOS line scan sensors due to their low-noise capabilities for low detector capacitance.

Typically, operational amplifier-based integrators using an integrating capacitor in the feedback are a commonly used scheme which is sketched below:
A basic charge amplifier topology for strip sensor readout

A basic charge amplifier topology for strip sensor readout

These amplifiers have high input impedance, they integrate weak charge pulses and convert them into voltage pulses for amplification and then buffer the output for readout from the next block in the chain. Because of that operation, this type of amplifier is called a “charge amplifier”. The first stage of a charge amplifier is usually a low-noise differential pair and its open-loop gain is set sufficiently high so that its amplification is not influenced by the detector capacitance which reduces the gain in the feedback. The output stage is a low-impedance buffer so it could drive the next circuits in the chain, typically an S/H stage of an ADC.

When particle decay rays strike the silicon strips, signal charge pulses Qs are generated, with an amplitude proportional to the particle energy. Due to this charge generation, the input potential of the charge amplifier lifts up and during the same time, a potential with reverse polarity appears at the output, due to the negative feedback amplifier. However, because the amplifier’s open-loop gain is sufficiently large, its output potential works through the feedback loop so that it causes the input terminal’s potential drop to zero, after some settling time dependent on the unity-gain bandwidth of the opamp itself. As a result, the signal charge pulses Qs are integrated to the feedback capacitance Cf and the output’s voltage changes according to the integrated charge. At that moment, since the feedback resistor Rf for DC is connected in parallel to the feedback capacitor Cf, the output voltage slowly discharges with the time constant determined by τ=Cf · Rf. The output voltage of such a charge amplifier scheme is dampened by the size of the feedback capacitor Cf, thus Qs and Cf must be chosen wisely to fulfill the specifically desired dynamic range. As a result it can be observed that the noise performance and dynamic range of this readout scheme is of highest trade-off. Increasing the dynamic range, leads to a lower swing on the capacitor and hence increases noise, the reverse is also applicable.

Note that the ATLAS detector has a total of over 200 m2 (square meters!!!) of pure detector strips! With a strip size of 0.01mm by 40cm we get a pretty decent number of about 50 000 strips and readout channels respectively. With such a huge set of sensors both ATLAS and CMS rely on the statistical significance of their measurements and the weird correlation in the slight gamma peaks, might truly be caused by a completely new fundamental particle. However, the readout complexity of such an enormous set of sensors is colossal, which makes induction of errors a plausible explanation as well.

Fingers crossed that all the sensing electronics works flawlessly and that all abnormal peaks detected are due to a newly detected particle.