Download Camera Conceptual Design Report - GAE

A. PHOTON DETECTOR
I. Introduction
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II. Pixel size
Coordinator: Razmick Mirzoyan
Authors: …
PMT 1,5 inch (38mm/photocathode 34mm) (hemispherical window option from ETE)
Pixel size
Large 0,11 deg
Mid-size 0,18 deg
Small : 0,25 deg
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III. Light guides/Shape of the entrance window
Coordinator: Pierre Espigat
Experimental tests: Maxim Shayduk
Authors: …
1. Simulations (3 options, flat, curved and hemispherical): Pierre Espigat
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IV. High Voltage Control and Distribution.
Coordinator: Karl-Heinz Sulanke
Authors: …
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To reduce the heat production in the camera, one should use two power supplies. One, with -1500V /
100mA, feeding the resistor divider chain, the cathode and dynodes 1..5. The other, with -300V / 500mA,
feeding dynodes 6..8.
The DESY proposal of the CTA HV-distribution / control is based on the combination of a very low ripple
commercial power supply with a self designed HV control circuitry.
The latter is optimized for high accuracy and extra low power dissipation. Special features, needed by
CTA, like anode current limitation and shutdown input, have been worked in.
A single PCB the HVCDB (High Voltage Control and Distribution Board), houses all electronic components
and the PMTs. It is arranged rectangular to the Readout Board.
The High Voltage Control and Distribution Board within the cluster
HV-Control and
Backplan
e
RJ45 conn.
Distribution Board
Analog Frontend
(ethernet)
Analog Pipeline, FPGA, ..
PCIe
PCIe or
con
n.
other HF-conn.
Hamamatsu
PMT R9420
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Features of the HVCDB (High Voltage Control and Distribution Board):
- distribution of the HV within in a cluster of 7...16 PMTs
- individual HV set and read back (FPGA + software)
- anode current readback
- anode-current limitation / HV-turn down in case of bright stars etc.
- HV-autorecovery
- individual shutdown input (FPGA + software)
- power dissipation per pixel at max. anode current (100µA): < 70 mW
- estimated price per pixel (> 1000 pcs.): 30 €
HV-control circuit, principle
-HV_in
Cathode
DY1
HV_set
divider
DY7
1:1000
HV_read_back
V. Photon sensor possible upgrade
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DY8
Coordinator: Razmick Mirzoyan
Authors: Razmick Mirzoyan
SiPMs (known also as MPPC, GAPD, Micro-channel APD) are novel light sensors that are rapidly
maturing. Until now, the prime choice for a low noise, single photon sensitive light detector has been the
classical photo multiplier tube (PMT). Along with many advantages offered by the PMT it has also several
disadvantages: fragile vacuum tube, manual assembly, high voltage bias requirements, magnetic field
sensitivity, and cannot be exposed to ambient light. These disadvantages can be overcome by compact
solid-state detectors. To date, the main alternative has been the avalanche photodiode (APD). The APD can
be operating in a classical linear mode with a relatively high noise level, or in the Geiger mode as a highspeed photon counter, over modest photon-count range. The more-recent Silicon Photomultipliers (SiPM)
addresses this point, by using a single chip containing several hundred to thousands micro-APD cells (µcells) coupled to a common signal output terminal. Each micro-APD is operated in the Geiger mode
whereupon an arriving photon can trigger the cell, leaving the surrounding cells un-triggered ready to
collect other arriving photons. The photon-counting dynamic range is comparable to the number of microcells. In the design of the SiPM, a key factor is to have the chosen number of µ-cells each truly independent
of its neighbors, i.e. having as small “crosstalk” as possible. Crosstalk between pixels would increase excess
noise factor, distort linearity, limits the dynamic range and reduces time and amplitude resolutions.
A “typical” SiPM consists of 100 to 5000 avalanche photodiode µ-cells located on a common substrate
having an area of 1 to 25 mm² with common anode (see Fig. 2). The front surface of each µ-cell is connected
to the power supply that creates the electrical field inside the active pn-junction. The µ-cells are electrically
decoupled from each other through individual (usually poly-Si) resistors. The applied voltage is adjusted to
some special value that allows operating each µ-cell in the Geiger discharge mode when it is fired by an
incoming photon. In practice, for providing high Geiger efficiency and correspondingly high PDE, the
applied voltage shall be 15-20% higher than the breakdown voltage of the APD, which is typically in the
range of several tens Volts at room temperature.
Typical gain of a SiPM is 106, providing for a single photo-electron (ph.e.) a few mV signals over a 50  load
resistor. One may consider single µ-cells as tiny charged capacitors (with a typical capacitance of ~0.3 pF
for a micro-cell size of 100µm x 100µm), which at the beginning are charged to the level of the power
supply via the above-mentioned resistors. An incident photon initiates a Geiger avalanche process that
starts to discharge the capacitor until it arrives at the breakdown voltage. Because of the identical topology
of all µ-cells, they have the same capacitance and therefore for a given over-voltage they provide the same,
fixed gain. This implies that, once fired, independent on the number of incident photons, any given µ-cell
will produce the same charge at the anode. The produced charges from individual cells will instantly sum
up in the common anode. Obviously the sum charge would be proportional to the number of fired cells. The
typical dark rate of an SiPM at room temperature is 1 MHz/mm2, i.e. on the level of a single µ-cell
amplitude. See the references (Renker, Lorenz 2009) and (Dolgoshein et al, 2006) details for details.
V.1. Linear Dynamic Range
As long as the number of instantly impinging photons N-photon is less than half of the total number of µ-cells
M-µ-cell (N-photon ≤ M-µ-cell/2), at first approximation, a given µ-cell will be hit only by one photon. Under this
condition the sensor output will show a linear dependence on the input light flux.
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V.2. µ-cell Recovery Time
The µ-cell recovery time is proportional to the product of the cell capacitance and the value of the
individual decoupling resistor and is typically in the range of 0.1-1 µs. While the cell is recovering
(recharging), its Geiger efficiency increases towards the preset value.
V.3. Photon Detection Efficiency
The maximum achievable photon detection efficiency (PDE) of an SiPM depends on the following factors:
 Geometrical efficiency: ratio of the light sensitive area of a single cell to its total area: Geomeff.
 Geiger efficiency: this is a direct function of the applied over-voltage: 15-20 % over-voltage can
provide efficiencies in the range close to 100 %: Geigereff.
 Wavelength-dependent transport of impinging photons into the sensitive volume of the SiPM:
Transmiteff().
 Intrinsic quantum efficiency (QE) of Si: QEintrinsic.
So one can write:
PDE() = Geomeff. x Geigereff. x Transmiteff.() x QEintrinsic
Although it has been shown in Monte Carlo simulations, and
verified through tests, that SiPMs with a peak PDE of 5060%, for both, the yellow–green and the blue–near-UV range,
are practicably achievable, nevertheless the commercially
available devices show a peak PDE in the range of 30-40%
only (see the Fig.XYZ below).
Fig.xyz. Measuerd PDE for few light sensors. D-SiPM stands
for an experimental production of 3mm x 3mm size SiPMs by
Prof. Dolgoshein and his team, MPPC- stands dor 3mm x
3mm MPPC from Hamamatsu, UBA-MAPMT stands for a
multi-anode PMT form Hamamatsu with Ultra-bialkali anode
and SBA-PMT stands for PMT with Super-bialkali photocathode from Hamamtsu. Figure taken from the presentation
of H. Miyamoto at TIPP-09 conference (March 2009) in
Japan.
V.4. Cross-Talk
A critical parameter in the performance of an SiPM, that makes it less than an ideal light sensor, is the socalled cross-talk. This effect is similar to afterpulsing in a photo multiplier tube, except that for currently
available devices without special cross-talk suppression it has ~ 30 times higher level than in PMTs
(usually afterpulsing in a PMT is ~1% effect measured on single ph.e. level) and thus can inhibit achieving
high sensitivity in self-trigger configurations. During the avalanche process in silicon, light is produced
with a probability around 10-5 photons/electron (see, for example, Mirzoyan, et al., 2009). In spite of the
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typical very low number of generated secondary photons (several tens), these can be captured by neighbor
cells, which are then triggered to fire as well. The triggering of neighbors may even spread in a chain
reaction. Obviously this effect violates the one-to-one accordance between the incidence light and
produced output charge, creating an excess noise factor. The cross-talk (cf. Fig.GZM) can happen in two
ways:
a) direct capture of secondary photons in the neighbor active regions (instantaneous cross-talk) and
b) creation of a charge photo-carrier in its bulk that subsequently may migrate towards the active
region (this process may take up to several ten nano seconds).
Obviously this negative effect degrades the amplitude resolution, heavily contributing into the noise figure
F of a SiPM (typically F is around 1.6…1.7). Since the cross-talk is proportional to the applied gain, one
immediate remedy would be operating the device under relatively low gains. But this may necessitate
using special low noise, wide-band amplifiers of large dynamic range, thus complicating the setup.
Figure GZM
A schematic view on the cross-talk and the chosen double remedies: The triangular trench optically isolates
the micro-cells and the second p-n junction stops the charge carrier from the volume penetrating into the
active junction. The about 2 orders of magnitude reduced cross-talk improves such important parameters of
the SiPM as its linearity, the dynamic range as well as event-by-event sharp timing.
There are experimental samples from a few producers where effective measures (trenches that optically
isolate the µ-cells from each other or even more sophisticated, applying a 2nd p-n junction below the active
avalanche region, that stops induced by the cross-talk charge carriers in neighbouring cells from migrating
inside their active avalanche zone) for suppressing the cross-talk are taken (see for details (Buzhan et al.,
2009). This important problem has been understood by the interested community and we believe that the
next generation of SiPMs may have much lower level of cross-talk.
Besides an anticipated in future cost advantage of SiPM over PMTs (normalized to 1mm² sensitive detector
area for the time being the SiPM are by one order of magnitude more expensive) , the governing factors are
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the 2-3 times higher photon detection efficiency, better time and amplitude resolutions, its compact size
and overall reduced system complexity.
V.5. Dark Noise of SiPMs
The noise from SiPMs of 1mm² area at room temperature is in the range of ~1MHz/mm² . s. For the time
being there is no consensus among researchers about this rate. Some report values few times less than the
above rate. One possible reason for that is the fact that some products, specifically from Hamamatsu,
because of their design, cannot be operated at high enough overvoltage for saturating their PDE (typically
Hamamatsu devices are operated at ~70 V and their operational range is less than 2 V, i.e. maximum
applied overvoltage < 3%). As a result one is operating these devices at a relatively low Geiger efficiency
and PDE values and thus reporting low values of dark rates. For providing close to 100 % Geiger efficiency
and saturating the PDE the applied voltage shall be on 15-20 % higher than the breakdown one.
Extrapolating from the noise of ~1MHz/mm² . s one should expect a noise of 9 MHz from a sensor of 3x3
mm² and ~25 MHz for the 5x5 mm² sensor. By cooling these sensors from 25°C down to 1°C one will
render the above rates to ~1MHz and ~3MHz correspondingly. Please note that the equivalent rate of
photons from the Light Of the Night Sky (LONS) will be ~3MHz and ~20MHz correspondingly, i.e. one
magnitude more that the internal noise.
V.6. Temperature Sensitivity
One of the main drawbacks of SiPMs is their relatively high gain sensitivity on temperature, which is in the
range of 4-10 %/°C for devices of different manufacturers. In fact, with lowering the temperature the
breakdown voltage decreases. As a result the applied effective overvoltage and the gain are increasing. For
a stable operation of SiPMs one needs to either to stabilize the temperature to the order of better than 1°C
or to organize a temperature sensitive feedback through the applied voltage for counteracting the gain
change. There is some reported experience in both above mentioned methods that can be critically
reviewed and applied for the use in CTA imaging cameras. If the active cooling will be used as an option for
stabilizing the temperature of a SiPM camera then it is desirable to possibly work at positive temperatures,
even if it is only marginally above the 0°C. In this way one will experience only minor problems with
possible humidity condensation. Otherwise one will need to use an evacuated chamber around the camera
matrix of SiPMs, which means a more complex mechanical/optical construction will be needed. In such
case one will need to reduce the losses of light reflected from the chamber input window as well as to
provide effective transport of heat from inside the camera body.
V.7. Newer Products
The recent IEEE conference in Orlando has shown that new companies are emerging in the market
with newer devices that could be of interest for the CTA consortium. According to preliminary
information from Philips their new SiPMs are CMOS compatible, include active quenching and
local digitizer (in fact a simple inverter because the SiPM by its principle of operation delivers
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discrete “digitised” amplitudes) and a memory for every cell that are ~ 30µm x 50µm large. In this
way their devices are allowing a sub-mm coordinate resolution (this feature shall be interesting
for other than CTA applications). For the time being they reported a peak PDE of ~30 % (although
at the conference it was stated that the PDE measurement did not exclude yet the contributions
from the cross-talk and the after-pulsing). Also the Radiation Monitoring Devices (RMD) from
USA, well-known for their large size avalanche diodes, came with newer SiPM design, that still
shows a rather low PDE. Perkin-Elmer from Canada, who is alos known for their APD production,
is another newcomer who starts producing blue-sensitive advanced SiPMs with double cross-talk
suppression topology. According their claim the cross-talk level shall be well-below 1% for the
sensor gain of 106. The expected time-line for their commercial products is late spring 2010. This
company ensures their readiness for taking input from us for producing appropriate SiPMs for the
needs of the CTA project. Hamamatsu is strongly improving the parameters of their MPPCs, as
well as the integration level of electronics on their sensors. Relatively large matrixes of MPPCs
became recently available from
Hamamatsu (see the Fig.ZGT below).
Fig.ZGT. Photo of a matrix of MPPCs
from Hamamatsu. Every single
element of the above matrix consists
of 4 sensors of 3mm x 3mm size.
The photo above includes 64 such
matrixes.
The company will work on
enhancing the integration level of SiPM matrixes pursuing a program on implementation of
electronics (amplifier, power distribution) on the back side of matrixes.
V.8. Conclusions
One may anticipate that in a time span of 1-2 years from now competitive sensors will become
available from several companies. These, compared to superbialkali PMTs, may show almost
double as high PDE, a comparable cost for the sensor area and in fact may become interesting for
using in CTA.
Keeping the above in mind we shall foresee a modular construction of the CTA cameras that
would allow one to exchange the front photo sensor part without any big change of the following
trigger and readout electronic chain.
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V.9. References.
Buzhan, P., Dolgoshein, B., et al., NIM A 567 (2006) p.78
Renker, D., Lorenz, E., JINST 4 (2009) P04004
Buzhan, P., Dolgoshein, B., et al., NIM A 610 (2009) p.131
Mirzoyan, R., Kosyra, R., Moser, H.-G., NIM A 610 (2009) p.98
VI. Conclusions.
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VII. References.
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