Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
A. PHOTON DETECTOR I. Introduction … 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 … 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 … IV. High Voltage Control and Distribution. Coordinator: Karl-Heinz Sulanke Authors: … … 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 CTA-FPI &ELEC WP Revision Reference Created Revised 1 CTA-FPI&ELEC CDR 09 January 6, 2010 January 6, 2010 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 3 August 9, 2017 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. CTA-FPI &ELEC WP Revision Reference Created Revised 1 CTA-FPI&ELEC CDR 09 January 6, 2010 January 6, 2010 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 5 August 9, 2017 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 CTA-FPI &ELEC WP Revision Reference Created Revised 1 CTA-FPI&ELEC CDR 09 January 6, 2010 January 6, 2010 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 7 August 9, 2017 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. CTA-FPI &ELEC WP Revision Reference Created Revised 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. … VII. References. 9 August 9, 2017 1 CTA-FPI&ELEC CDR 09 January 6, 2010 January 6, 2010