Nuisances of SiPMs and how to deal with them in ... - KICP Workshops
Nuisances of SiPMs and how to deal with them in ... - KICP Workshops
Nuisances of SiPMs and how to deal with them in ... - KICP Workshops
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<strong>Nuisances</strong> <strong>of</strong> <strong>SiPMs</strong> <strong>and</strong> <strong>how</strong> <strong>to</strong><br />
<strong>deal</strong> <strong>with</strong> <strong>them</strong> <strong>in</strong> Cherenkov<br />
telescopes on the example <strong>of</strong> the<br />
Nepomuk Otte<br />
CTA SC-MST
Overview<br />
The Cherenkov imag<strong>in</strong>g technique<br />
Pho<strong>to</strong>n detec<strong>to</strong>r requirements<br />
SiPM nuisances: Effects, Impacts, Workarounds<br />
Slow output signals<br />
Temperature dependencies<br />
Optical crosstalk<br />
Bias
CTA-US SC telescope<br />
Schwarzschild Couder Optics<br />
Large FoV <strong>of</strong> 8 degrees<br />
Small plate scale -> small<br />
pho<strong>to</strong>n sensors<br />
Camera layout<br />
About 12000 pixel<br />
Grouped <strong>in</strong> modules <strong>of</strong> 64<br />
pixel Effective Mirror<br />
Area per Tel.<br />
Nepomuk Otte<br />
~100 m 2<br />
Field <strong>of</strong> View (FoV) 8 deg.<br />
Pixelation ~0.05 deg.<br />
Angular Resolution 0.02 – 0.05 deg.<br />
3
64 pixel each ~6x6 mm 2<br />
Uses 16 Hamamatsu S12545-<br />
3344M per module<br />
Monolithic SiPM array<br />
16 3x3 mm 2 <strong>SiPMs</strong><br />
4 <strong>SiPMs</strong> connected <strong>to</strong> form one<br />
pixel<br />
HV<br />
SiPM<br />
A Camera Module<br />
1k 100n<br />
Out<br />
Nepomuk Otte<br />
55 mm<br />
4
Module Conceptional Design<br />
Cold f<strong>in</strong>ger (Al) soldered <strong>to</strong> SiPM mount board<br />
Coax ribbon cable<br />
Heats<strong>in</strong>k-Fan combo<br />
<strong>SiPMs</strong> <strong>and</strong> SiPM Mount Board<br />
15x15 mm 2 TE<br />
Note, dimensions not <strong>to</strong> scale<br />
Insulation<br />
Delr<strong>in</strong><br />
Module frame
Gamma<br />
ray<br />
Particle<br />
s<strong>how</strong>er<br />
~ 120 m<br />
~ 1 o<br />
Imag<strong>in</strong>g Technique<br />
~ 10 km<br />
Nepomuk Otte<br />
Cherenkov radiation from e +/-<br />
~1° open<strong>in</strong>g angle<br />
5 pho<strong>to</strong>ns / m 2 for 100 GeV gamma<br />
ray arrive on ground<br />
Flash <strong>with</strong> 2-3 ns duration<br />
6
Night Sky Background Light<br />
From stars, zodiacal light,<br />
air glow, man made, ....<br />
Isotropic<br />
background aga<strong>in</strong>st which Cherenkov flash has <strong>to</strong> be discrim<strong>in</strong>ated<br />
10 8 pho<strong>to</strong>ns sec -1 cm -2<br />
Ways <strong>to</strong> reduce NSB<br />
● Small plate scale<br />
-> smaller pixel sizes<br />
● Filters<br />
● Tailored pho<strong>to</strong>n detec<strong>to</strong>r<br />
response<br />
Nepomuk Otte<br />
Benn, Ellison (1998)<br />
7
Pho<strong>to</strong>ndetec<strong>to</strong>r Requirements<br />
Highest possible pho<strong>to</strong>n detection efficiency (PDE)<br />
<strong>with</strong> peak response between 300 nm <strong>and</strong> 600 nm<br />
Lower energy threshold<br />
Better event reconstruction<br />
S<strong>in</strong>gle pe signal widths about 3 ns <strong>to</strong> 8 ns<br />
● wider signals -> contam<strong>in</strong>ation from NSB<br />
● narrower signals -> Cherenkov pho<strong>to</strong>ns do not pile up<br />
Jitter (e.g. TTS) < 1-2ns<br />
M<strong>in</strong>imized non-Poisson tails <strong>in</strong> pulse height distribution<br />
Afterpuls<strong>in</strong>g (PMTs) / Optical Crosstalk (<strong>SiPMs</strong>)<br />
Reduce accidental trigger<br />
Energy resolution<br />
Nepomuk Otte<br />
Improve SNR:<br />
Cherenkov Signal<br />
Night sky Background<br />
8
Intensities are not <strong>to</strong> scale<br />
Nepomuk Otte<br />
9
Pho<strong>to</strong>ndetec<strong>to</strong>r Requirements<br />
Highest possible pho<strong>to</strong>n detection efficiency (PDE)<br />
<strong>with</strong> peak response between 300 nm <strong>and</strong> 600 nm<br />
Lower energy threshold<br />
Better event reconstruction<br />
S<strong>in</strong>gle pe signal widths about 3 ns <strong>to</strong> 8 ns<br />
● wider signals -> contam<strong>in</strong>ation from NSB<br />
● narrower signals -> Cherenkov pho<strong>to</strong>ns do not pile up<br />
Jitter (e.g. TTS) < 1-2ns<br />
M<strong>in</strong>imized non-Poisson tails <strong>in</strong> pulse height distribution<br />
Afterpuls<strong>in</strong>g (PMTs) / Optical Crosstalk (<strong>SiPMs</strong>)<br />
Reduce accidental trigger<br />
Energy resolution<br />
Nepomuk Otte<br />
Improve SNR:<br />
Cherenkov Signal<br />
Night sky Background<br />
10
P(n) = λ n /n! exp(-λ)<br />
Non-Poisson tails<br />
NSB fluctuations are distributed follow<strong>in</strong>g a Poisson distribution<br />
Physical limit given by NSB<br />
below trigger<br />
Non-poisson tails<br />
Disc. Threshold<br />
Nepomuk Otte<br />
accidental triggers<br />
Otte, (2007)<br />
11
Pho<strong>to</strong>ndetec<strong>to</strong>r Requirements<br />
Highest possible pho<strong>to</strong>n detection efficiency (PDE)<br />
<strong>with</strong> peak response between 300 nm <strong>and</strong> 600 nm<br />
Lower energy threshold<br />
Better event reconstruction<br />
S<strong>in</strong>gle pe signal widths about 3 ns <strong>to</strong> 8 ns<br />
● wider signals -> contam<strong>in</strong>ation from NSB<br />
● narrower signals -> Cherenkov pho<strong>to</strong>ns do not pile up<br />
Jitter (e.g. TTS) < 1-2ns<br />
M<strong>in</strong>imized non-Poisson tails <strong>in</strong> pulse height distribution<br />
Afterpuls<strong>in</strong>g (PMTs) / Optical Crosstalk (<strong>SiPMs</strong>)<br />
Reduce accidental trigger<br />
Energy resolution<br />
Nepomuk Otte<br />
Improve SNR:<br />
Cherenkov Signal<br />
Night sky Background<br />
12
Operational Requirements<br />
Purpose <strong>of</strong> <strong>in</strong>strument is <strong>to</strong> detect air s<strong>how</strong>ers <strong>and</strong> reconstruct primary particle:<br />
Stable operation -> no drift <strong>of</strong> camera response (ga<strong>in</strong>, pulse shapes, PDE, ...)<br />
due <strong>to</strong><br />
Ambient environment (temperature, humidity, ...)<br />
Brightness <strong>of</strong> sky (NSB, stars)<br />
Ag<strong>in</strong>g<br />
...<br />
particle type, arrival direction, energy<br />
<strong>with</strong> as little uncerta<strong>in</strong>ties as possible or <strong>in</strong> other words<br />
the <strong>in</strong>strument should not be the limit<strong>in</strong>g fac<strong>to</strong>r <strong>in</strong> the reconstruction<br />
Uniform camera response (ga<strong>in</strong>, PDE)<br />
Additional practical requirements<br />
Reliability, durability, low costs, ...<br />
Nepomuk Otte<br />
13
<strong>SiPMs</strong> the (almost) perfect Pho<strong>to</strong>n<br />
Detec<strong>to</strong>r for Cherenkov Telescopes<br />
Potential for very high PDE <strong>in</strong> the blue<br />
Robust<br />
Reliable<br />
Cheap<br />
....<br />
<strong>SiPMs</strong> beg<strong>in</strong> <strong>to</strong> outperform classical<br />
PMTs <strong>in</strong> astroparticle applications<br />
Biggest nuisances these days:<br />
Slow signals<br />
Temperature dependence <strong>of</strong> ga<strong>in</strong>, PDE, ...<br />
Optical Crosstalk<br />
All these nuisances can be elim<strong>in</strong>ated at device level<br />
but for the time be<strong>in</strong>g we have <strong>to</strong> <strong>deal</strong> <strong>with</strong> <strong>them</strong><br />
Nepomuk Otte<br />
MEPhI/Pulsar SiPM<br />
14
Output signals
Small signal model <strong>of</strong> an SiPM<br />
Diode<br />
Otte, PhD thesis<br />
Capacitances <strong>and</strong> resistances determ<strong>in</strong>e ga<strong>in</strong> <strong>and</strong> shape <strong>of</strong> output signal<br />
Signal shape not determ<strong>in</strong>ed by diode capacitance <strong>and</strong> resistance<br />
-> larger <strong>SiPMs</strong> -> larger capacitance -> slower signals<br />
Charge <strong>in</strong> output signal determ<strong>in</strong>ed by C_d <strong>and</strong> C_q
Faster output pulses by add<strong>in</strong>g one<br />
dedicated signal l<strong>in</strong>e<br />
SensL: get signal <strong>with</strong> extra l<strong>in</strong>e that is<br />
capacitive coupled between diode <strong>and</strong> resis<strong>to</strong>r<br />
pulse widths ~ ns<br />
from SensL
... or shape output signal <strong>with</strong> high pass<br />
3x3 mm 2 MPPC<br />
before<br />
Pole zero cancellation<br />
not really needed<br />
3 kΩ<br />
~24 pF<br />
Nepomuk Otte<br />
50 Ω<br />
after<br />
Use <strong>in</strong>put impedance<br />
<strong>of</strong> next stage<br />
18
Temperature Dependencies<br />
Critical E-field for breakdown depends on temperature<br />
Temperature dependent breakdown voltage<br />
Temperature dependent ga<strong>in</strong> <strong>and</strong> PDE<br />
Intr<strong>in</strong>sic dark rates are high but not an issue for Cherenkov telescopes<br />
NSB rates are generally higher -> no cool<strong>in</strong>g needed<br />
For example <strong>in</strong> the SCT we expect ~40 MHz NSB <strong>in</strong> a 6x6 mm 2 pixel<br />
-> about 1 MHz NSB per mm 2 sensor area<br />
Compare <strong>to</strong> typical <strong>in</strong>tr<strong>in</strong>sic dark rates <strong>of</strong> MPPCs <strong>of</strong> a few 100 kHz per mm 2<br />
Nepomuk Otte<br />
19
Diode capacitances<br />
G = ΔQ = C * ΔU<br />
Bias above breakdown<br />
Breakdown voltage<br />
<strong>in</strong>creases <strong>with</strong><br />
temperatures<br />
For fixed bias <strong>in</strong>crease<br />
<strong>of</strong> ΔU -> <strong>in</strong>crease <strong>of</strong><br />
ga<strong>in</strong><br />
Ga<strong>in</strong><br />
A typical value for ga<strong>in</strong> change is 2.5%/C (e.g. Hamamatsu MPPC)<br />
Nepomuk Otte<br />
-0.8% / C<br />
But values <strong>of</strong> 0.5% /C are possible if cell capacitances are reduced<br />
20
PDE<br />
Different effects contribute <strong>to</strong> the PDE:<br />
● Reflection <strong>of</strong>f the surface<br />
● Deadlayer<br />
● Geometrical efficiency<br />
● QE (location beneath surface <strong>and</strong><br />
thickness <strong>of</strong> depleted region)<br />
● Breakdown probability (E-field strength<br />
<strong>and</strong> geometry)<br />
Breakdown probability depends on overvoltage above<br />
breakdown (rule <strong>of</strong> thumb bias ~20% above breakdown<br />
for ~100% breakdown probability)<br />
-> temperature dependent breakdown voltage<br />
-> temperature dependent PDE<br />
Nepomuk Otte<br />
21
Slope = capacitance<br />
-> can change <strong>with</strong><br />
temperature <strong>in</strong> some<br />
devices<br />
Does keep<strong>in</strong>g ga<strong>in</strong> stable help?<br />
Operate at same ga<strong>in</strong> => not the same E-field<br />
=> not the same breakdown probability<br />
=> not the same PDE<br />
Alternate methode: operate at stable temperature +/- 0.5 C or better<br />
Nepomuk Otte<br />
U<br />
Q<br />
22
Conceptual Design<br />
Primary goal for cool<strong>in</strong>g is temperature stabilization elim<strong>in</strong>at<strong>in</strong>g need for ga<strong>in</strong> stabilization<br />
Ma<strong>in</strong> concerns waste<br />
power<br />
Pro<strong>to</strong>type tests <strong>of</strong> one module<br />
ambient temperature: 24 C<br />
Cold f<strong>in</strong>ger temp. : 1 C<br />
Waste power: 2 W<br />
~250W heat for entire camera<br />
meets predicted values<br />
Aim for operat<strong>in</strong>g temperature<br />
between 10C <strong>and</strong> 20C -> lower<br />
waste heat<br />
Cold f<strong>in</strong>ger (Al)<br />
Cold Side<br />
Warm side<br />
Heats<strong>in</strong>k-Fan combo<br />
Entrance w<strong>in</strong>dow<br />
15x15 mm 2 TE<br />
Insulation<br />
(Solimide)<br />
Delr<strong>in</strong><br />
Module frame
Optical Crosstalk<br />
Nepomuk Otte<br />
24
Pho<strong>to</strong>n Emission dur<strong>in</strong>g Breakdown<br />
Avalanches produce a lot <strong>of</strong> pho<strong>to</strong>ns,<br />
emission processes are be<strong>in</strong>g debated<br />
Pho<strong>to</strong>ns <strong>in</strong> a very narrow energy<br />
range propagate out <strong>of</strong> their<br />
orig<strong>in</strong>at<strong>in</strong>g cell <strong>and</strong> absorb <strong>in</strong><br />
neighbor<strong>in</strong>g if<br />
Pho<strong>to</strong>n energy is between 1.1 eV <strong>and</strong> 1.4 eV<br />
Pho<strong>to</strong>n <strong>in</strong>tensity: 3x10 -5 pho<strong>to</strong>ns per<br />
avalanche electron<br />
-> Intensity is direct proportional <strong>to</strong> ga<strong>in</strong><br />
ANO NIM A (610) 2009, 105–109<br />
Nepomuk Otte<br />
Picture by C. Merck<br />
25
Outer reflective surface<br />
Optical Crosstalk (OC)<br />
OC is determ<strong>in</strong>ed by geometry <strong>and</strong> ga<strong>in</strong><br />
Direct OC<br />
Indirect OC<br />
Hamamatsu 3x3mm 2 MPPC, shaped signal<br />
Trenches <strong>and</strong> lower cell capacitance help <strong>to</strong> reduce/elim<strong>in</strong>ate OC<br />
Ketek, Excelitas, Hamamatsu, SensL, ST Microelectronics, ....
Non-Poisson tails due <strong>to</strong> optical<br />
crosstalk<br />
For us relevant is only direct OC:<br />
It causes r<strong>and</strong>om large pulses lead<strong>in</strong>g <strong>to</strong> an <strong>in</strong>crease <strong>in</strong><br />
accidental trigger rates <strong>and</strong> worse event reconstruction<br />
P(n) = λ n /n! exp(-λ)<br />
Physical limit<br />
below trigger<br />
Non-poisson tails<br />
Nepomuk Otte<br />
Disc. Threshold<br />
accidental triggers<br />
Otte, (2007)<br />
27
PMT<br />
signals<br />
Concept <strong>of</strong> the SumTrigger <strong>with</strong> Clipp<strong>in</strong>g<br />
Signal<br />
Clipper<br />
Signal<br />
Clipper<br />
Signal<br />
Clipper<br />
Signal<br />
Clipper<br />
.<br />
.<br />
.<br />
Clipp<strong>in</strong>g is an effective way <strong>of</strong> reduc<strong>in</strong>g OC effects at trigger<br />
(orig<strong>in</strong>al idea E. Lorentz)<br />
Σ<br />
Concept proven <strong>in</strong> MAGIC <strong>and</strong> also applied <strong>in</strong> FACT<br />
(see next talk)<br />
For the US SCT we do not plan a clipp<strong>in</strong>g stage <strong>in</strong> the trigger<br />
Because<br />
several <strong>SiPMs</strong> <strong>with</strong> OC elim<strong>in</strong>at<strong>in</strong>g trenches become available<br />
Ketek, ST Microelectronics, Hamamatsu, ....<br />
Optimization <strong>of</strong> po<strong>in</strong>t <strong>of</strong> operation PDE vs. optical crosstalk<br />
See talk by D. Williams
Dependency <strong>of</strong> ga<strong>in</strong> on NSB rate<br />
Larger NSB -> larger current<br />
-> larger voltage drop<br />
over R1 (~10k)<br />
-> lower bias on SiPM<br />
-> lower ga<strong>in</strong> / PDE<br />
Example Hamamatsu:<br />
Change NSB rate by 100 MHz<br />
-> 25% change <strong>in</strong> ga<strong>in</strong><br />
Aga<strong>in</strong>, lower capacitance would help<br />
Replace <strong>with</strong> <strong>in</strong>duc<strong>to</strong>r<br />
Possible solution use <strong>in</strong>duc<strong>to</strong>r <strong>in</strong>stead <strong>of</strong> resis<strong>to</strong>r<br />
Nepomuk Otte<br />
29
Conclusions<br />
● <strong>SiPMs</strong> are great devices but we are still wait<strong>in</strong>g for the perfect<br />
version<br />
● PDE <strong>of</strong> ~60% <strong>in</strong> the blue (the <strong>to</strong>ughest one)<br />
● No optical crosstalk (trenches)<br />
● Fast output signals (tap<strong>in</strong>g at diode)<br />
● Temperature dependence <strong>of</strong> PDE <strong>and</strong> ga<strong>in</strong> ~0.5%/C (small capacitances)<br />
● Cheap (ma<strong>in</strong> costs are lithographic masks)<br />
● Exist<strong>in</strong>g devices are equal <strong>to</strong> or outperform PMTs which is why we<br />
build IACTs <strong>with</strong> <strong>SiPMs</strong> (see next talk)<br />
● Ma<strong>in</strong> nuisances can be worked around but compromises have <strong>to</strong> be<br />
made