electron hole pairs. The avalanche can be stopped by lowering the voltage below breakdown after each avalanche (quenching). APDs operated in Geiger mode are quenched passively or actively with integrated circuits. Passive quenching is achieved by setting a resistor in series with the APD. This forms a voltage divider. An avalanche lowers the almost infinite resistance of the APD to a very low value, causing the voltage to drop over the resistor instead of the APD. The APD recovers to its previous state and the voltage over the APD raises over breakdown voltage again. Every Geiger discharge depletes the whole APD and delivers always the same amount of charge no matter how many photons hit the APD at the same time. Due to their high quantum efficiency (QE) in visible light of over 90% silicon or gallium arsenide APDs operated in Geiger mode are widely used in single molecule spectroscopy, counting single photons released by a molecule one at a time.
0
Reverse bias voltage
log(amplification) breakdown
No Linear Geiger amplification mode mode
Figure 4.5: APD amplification versus voltage. Avalanches are generated from electron hole pairs over the generation threshold. In Geiger mode [45], each electron hole pair can generate a self-sustaining avalanche in the diode.
4.6 SiPM
Combining the single photon resolution of PMTs and the compact (and cheap) form factor of a silicon based integrated circuit, SiPMs have been developed. A Silicon photo multiplier (SiPM) is an array of passively quenched APDs in Geiger mode on the same silicon device.
Hundreds or thousands of APDs are connected in parallel (see figure 4.6) and reversely biased with the same voltage source. The single APDs are referred to as pixels. The pixels of a SiPM are uniform and their signals identical. If several pixels are hit by photons, the signals add up as can be seen in the left side of figure 4.7. On the right side the integration and the charge
28 4. Photon detection
Quenching ResistorAPD in Geiger mode
Bias Voltage
Figure 4.6: A SiPM is an array of passively quenched APDs connected in parallel adding up their signals. All are reversely biased in Geiger mode with the same bias voltage.
3 pixel
2 pixel 1 pixel
Time
Amplitude
Charge[QDC]
600 800 1000 1200
count
0 1000 2000
3000 0 1 2 3 pixel fired4 5
Figure 4.7: SiPM signal
Left: SiPM signal voltage versus time on a persistent oscilloscope screen. Each pixel fired provides the same amount of charge and the waveforms add to each other for several pixels fired at the same time
Right: The same signal integrated over time with a QDC in arbitrary units. In red the fired pixel scale. Taken with Tektronix 7540 on a Hamamatsu S10400 SiPM without light.
of the signal is shown. Each pixel discharges the same amount of charge. The gain of a SiPM is the charge of one pixel. Several pixels firing at the same time give the sum of multiple charges. The size of a pixel is in the µm range, enabling hundreds or thousands of pixels on a sub-millimeter scale. The quenching resistors are built-in on the silicon wafer, making it a very compact and easy to use device. Depending on the specific design of the SiPM, green light, and recently also blue light as well as UV light sensitive SiPM are produced by various manufacturers.
4.6.1 SiPM characterization parameters
Breakdown Voltage
The single pixels operated in Geiger mode for voltages over breakdown, see figure 4.5. The breakdown voltage is unique for each SiPM and depends on the temperature. All figures of
4.6 SiPM 29
merit are usually described in terms of excess bias voltage or over voltage,
∆V(T) =Vbias−V BD(T) (4.1)
. Gain
The charge generated from a single avalanche process of a single pixel (one APD) is the Gain G. In approximation the SiPM gain depends on the pixel capacity,
−G= Cpixel·∆V
e (4.2)
with e being the elementary charge. The gain thus depends on the temperature and the excess bias voltage which can be expressed as
1 G·
∂G
∂T
=− 1
∆V ·
∂VBD
∂T
(4.3) .
Dark Count Rate
Randomly discharging pixels leading to the dark count rate (DCR) of a SiPM are triggered by thermal excitation or tunneling of charge carriers through the band-gap. Only single pixels are discharged at a time. Typical values of DCR for a SiPM are 100 kHz- 1 MHz.
Pixel Crosstalk / After-pulse / Correlated Noise
After pulses are triggered by trapped charge carriers in the silicon lattice that are released after a certain amount of time. When the pixel recovered from the discharge, a new avalanche is triggered by those charge carriers, leading to correlated noise. Optical photons are released during the Geiger discharge inside a pixel due to coulomb interaction and Bremsstrahlung.
Those photons can trigger a discharge in neighboring pixels. Both pixel crosstalk and after-pulses are correlated noise and are almost indistinguishable if the waveform is not sampled.
For simplification in this workXTcorr combines after-pulses and crosstalk as correlated noise.
Photon Detection Efficiency
The fill-factor (FF) of a SiPM is the ratio of active (sensitive to light) versus the passive areas (resistors, lines). With the quantum efficiency (QE) defined as in section 4.5 and the probability of creating an avalanche depending on the excess bias voltage as AP, the photon
30 4. Photon detection
detection efficiency (PDE) is defined. The PDE determines the probability of one photon incident on the SiPM triggering a signal as
P DE=QE×AP ×F F (4.4)
.
Saturation response function
With higher light signals a majority of the pixels are already fired when they are hit with photons and do not discharge. This leads to a non linearity with higher light intensities and saturation for high light intensities. Figure 4.8 shows a detailed simulation developed by Chen Xu of a SiPM including all previously mentioned properties. Up to 10% fired pixels the deviation from linearity stays under 1%.
photo electrons
0 1000 2000 3000 4000
signal[pixel]
0 500 1000 1500 2000
Figure 4.8: Saturation of a SiPM with DCR = 200 kcps, XT=3%, 2304 pixel. A noticeable deviation from linearity over 1% occurs when more than 10% of the pixel are hit (green linear line for guidance).
ROOT SiPM simulation package from Chen Xu [46].
Chapter 5
Particle Flow
The hadronization of quarks and gluons following a hard scattering produce a narrow cone of boosted hadrons and other particles. These objects are called Jets. Many processes in ILC will include multi-jet final states. Physics analysis at the ILC could involve hadronic decays into 8 jets and more [47]. Of particular interest are the full hadronic decays ofW±(W±→qq)¯ and theZ0 (Z0 →qq) bosons. They have been set as a benchmark for detectors at the future¯ linear collider. Figure 5.1 shows possible production and decay. The bosons produce each jets with a multitude of hadrons. The goal is to measure the resulting jet energies as the sum off all contributing particles well enough to separate both processes. The natural width of the separation measured with a detector with perfect energy resolution is 3.1σ. A Jet energy resolution of Eσ = √ α
E( GeV) = 3.5 % gives a decent 2.6σ-2.3σ level and has been set as the goal for the jet energy resolution of an ILC detector. The energy of a typical jet is
Figure 5.1: Production and full hadronic decay into four jets fromW±andZ0at the ILC. The middle Figure shows the emission of the four back to back jets, for theW±W±in blue and theZ0 Z0in red. The rightmost Figure shows the resulting mass measurement. A resolution of 3.5 % on the energy is sufficient to separate both processes.
Picture by Mark Thomson
distributed as ≈60 % charged particles, ≈30 % photons, and ≈10 % neutral hadrons. At a 500 GeV ILC the primary interest is in 4-6 fermion final states e.g. e+e−→Z0H→qqb¯¯band e+e− → tt¯→ bqq¯¯bqq. This sets the typical energy scale for jets at ILC to be from 60 GeV¯ to 125 GeV (250 GeV in case of a 1 TeV ILC). Conventional Particle Physics approach to
32 5. Particle Flow
measure jet energies purely with the calorimeter systems cannot achieve this resolution since the typical HCAL resolutions are bigger than 60%√
E +c. New approaches like dual readout [48], totally active [49], and particle flow [50] are necessary. The two proposed detector concepts at ILC (ILD and SiD) are both detectors specifically designed for the particle flow approach.