Ieee2014 seattle biteau_poster_v1.2
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Characterization of SiPM for CTA Medium-Sized Telescopes Jonathan Biteau 1,3 , A. Bouvier 1,2 , D. Chinn 1 , D. Dang 1 , K. Doyle 1 , C. Johnson 1 , A. Kuznetsov 1,2 , D. Williams 1 , W. Yeung 1 for the CTA Consortium 1 Santa Cruz Institute for Particle Physics and Department of Physics, University of California 2 Now at InVisage, Inc. 3 [email protected] Abstract We are part of an effort to build medium-sized Schwarzschild- Couder telescopes (SCT) exploiting silicon photomultipliers (SiPM) for the Cherenkov Telescope Array (CTA). We report on measurements of 1660 SiPM pixels of 6 x6 mm 2 that will populate the camera of a prototype SCT. For CTA, we aim to build many similar telescopes with >11,000 pixels per camera. We consider various competitive SiPM devices and compare their impact on the CTA-SCT capabilities. Outlook for the Prototype With a collected charge scaling as PDE x Gain x (1+Xtalk), the ~4% dispersion measured in the flash responses of the 1660 pixels is consistent with the relative dispersion measured in the PDE (~3%), gain (~2%), and crosstalk (~2%) of the sample. No faulty pixel has been identified among the 1660 tested ones and the assembly has started in Sep. 2014. All the components of the camera are expected to be complete by Apr. 2015, with a forecast commissioning on the SCT prototype in fall 2015. SiPM to Observe the Violent Universe Ground-based gamma-ray telescopes observe the Cherenkov emission generated by gamma-ray showers in the atmosphere. The past years have seen an increasing interest in SiPM to detect Cherenkov photons, an advantageous alternative to PMT [1]. Prototype SCT Camera The central sector of the prototype camera will be fully equipped with 400 SiPM tiles from Hamamatsu Photonics, model MPPC S12642-0404PA-050. As in [4], we have fully characterized the properties of a sample of these devices, by measuring their pulse shape, gain, dark rate, and optical crosstalk, for the sixteen 3 x3 mm 2 chips that compose a tile and for groups of 4 chips connected in parallel (6x6 mm 2 CTA pixels). We also measured the photodetection efficiency (PDE) of this sample and characterized the response to flashes of 415 tiles (1660 pixels), including 15 spare devices. Absolute Spectral Response of a Sample Our PDE measurements are performed at a temperature stable within ~0.1°, using a method based on zero-peak Poisson statistics [4]. Five pulsed LEDs probe the wavelength range 375-600 nm. The PDE of 14 tiles are shown as dashed lines in Fig. 1 at four different operating voltages. The averages and rms are shown as solid curves. A relative dispersion of ~3% is observed at all wavelengths and voltages. Better sensitivity below 400 nm would be desirable considering the Cherenkov spectrum. Relative Response of 1660 Pixels A full characterization of the 1660 pixels being impracticable, we have designed a setup to estimate their breakdown voltage from current-voltage characteristics, as well as their response to LED flashes at 365 nm. We chose an operating voltage of 3 V above breakdown voltage, to maximize the PDE while keeping the dark noise at a reasonable level. The charge distributions resulting from 90 p.e. flashes (per pixel) are stored simultaneously for the four pixels of a tile, as well as for a reference pixel. After spatial- and temperature- dependent calibration, based on repeated measurements of a single tile, the normalized response of the calibration pixels show an rms of 2.1% (see Fig. 1, dashed blue histogram), indicative of the accuracy of the devised test. The responses of the 1660 pixels show an rms of 4.4%, equivalent to a 3.8% intrinsic dispersion after correction for the accuracy of the test. Towards the SCT contribution to CTA The selection of Hamamatsu MPPC S12642-0404PA-050 for the prototype was motivated by a 30-35% PDE in the blue and a high packing fraction implied by the use of the Through Silicon Via technology. Despite being the best choice at the time, its level of crosstalk, ~50% at the operating point, and pulse rise time, ~20ns before shaping for a 6 x6mm 2 pixel, are not ideal, and newer devices are showing performance more favorable for the many SCT that will be built for CTA. With pulse widths < 10 ns, PDE in the blue reaching 40% and a crosstalk level below 20%, latest SiPM models start to match the technical goals of ground-based gamma-ray astronomy. An improved spectral response below 400 nm, where the Cherenkov spectrum peaks, and a decreased sensitivity above 550nm, where the contribution of the NSB becomes significant, would further guide the selection of the SiPM model that will equip the SCT. We will continue our partnership with manufacturers to evaluate the improvements in the SiPM technology, that will be highly beneficial to gamma-ray astronomy and to our search for the most energetic phenomena in the Universe. Selecting the Photosensor for CTA We have characterized in detail a new set of SiPM devices. For the sake of brevity, we focus here on their spectral response. Figure 3 shows the PDE of seven devices developed by the Excelitas, Hamamatsu, and SensL companies, including the best devices studied in [4] (Hamamatsu LCT-3x3-100C, Excelitas C30742-33-050-C). Hamamatsu MPPC LCT4-3x3- 50C and SensL MicroFB-SMA-30035, with their 35-40% PDE in the blue, their maximum optical crosstalk of 10-15%, and their fast pulses (FWHM of 3-6 ns, for 3x3 mm 2 pixels), are particularly promising devices. Simulated Impact of Latest SiPM We processed simulated NSB photons and Corsika-generated gamma-ray showers through a detector model of CTA, accounting for the SiPM's PDE and crosstalk. Given a pixel threshold, corresponding to an array trigger rate on the night sky background (NSB) of 100 Hz, the image intensity at which half of the showers are detected is defined as the trigger threshold of the telescopes. Crosstalk and PDE are both increasing function of overvoltage, with PDE plateauing at large voltages. A trade off between these increases can be found for most devices, resulting in a minimum trigger threshold (see Fig. 4). The dark rate of the device, which at low overvoltage is an order of magnitude below the NSB, is not accounted for by the simulation at this stage, and it is expected to increase the threshold at the highest overvoltages. The most recent SensL and Hamamatsu devices improve the threshold by 20-30% with respect to the protoype device. Acknowledgments We gratefully acknowledge support from the agencies and organizations listed under Funding Agencies at this website: http://www.cta-observatory.org/, and in particular the U.S. National Science Foundation award PHY-1229792 and the University of California. We also acknowledge the generosity of the SiPM manufacturers Excelitas, Hamamatsu, and SensL, who provided samples tested in this work. Bibiliography [1] R. Mirzoyan & E. Popova, SPIE Conference Series, vol. 8621, Mar. 2013. [2] R. A. Cameron, SPIE Conference Series, vol. 8444, Sep. 2012. [3] J. Vanderbroucke for the CTA Consortium, sub. to Technology in Particle Physics, 2014 [4] A. Bouvier, et al., SPIE Conference Series, vol. 8852, Sep. 2013. IEEE NSS/MIC 2014 – ID N22-3 CTA is undergoing a prototyping phase, including construction of an SCT of medium size with a 9.7 m-diameter primary mirror [2]. This design provides a wide field of view of 8° and a 30-40% improved angular resolution with respect to single-mirror telescopes. This configuration also allows for smaller cameras. The optimal pixelation for the prototype is about 6x6 mm 2 , matched to the point spread function in the focal plane [3]. ~ 100 m ~ 10 m ~ 1 m ~ 1 cm ~ 100 μm Artist's concept of two gamma-ray showers observed by CTA. Structure of the SCT prototype. Exploded view of the prototype camera. SiPM tiles, Hamamatsu MPPC S12642-0404PA-050, chosen for the prototype. SiPM's APD cells. Prototype SCT camera. Only the central sector is equipped with SiPM (1600px). The camera is divided in 9 sectors of 13 to 25 modules, in turn composed of 64 pixels. 64px Fig. 1. Top: PDE of 14 prototype SiPM tiles as a function of wavelength at four operating voltages. Bottom: Typical spectra of the Cherenkov signal (blue) and irreducible noise (night sky background, red). Fig. 2. Distribution of the response of the 1660 pixels (solid green histogram). Multiple measurements of a single device, serving calibration purposes, are shown by the dashed blue histogram. Fig. 3. Top: PDE as a function of wavelength for 7 SiPM. The onset of the PDE plateau as a function of voltage is set as the operating point. Fig. 4. Trigger threshold of the SCT for 7 SiPM models, compared to the threshold of the prototype (at an overvoltage of 3-3.5 V).
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Transcript of Ieee2014 seattle biteau_poster_v1.2
Characterization of SiPMfor CTA Medium-Sized Telescopes
Jonathan Biteau1,3, A. Bouvier1,2, D. Chinn1, D. Dang1, K. Doyle1, C. Johnson1, A. Kuznetsov1,2, D. Williams1, W. Yeung1 for the CTA Consortium1 Santa Cruz Institute for Particle Physics and Department of Physics, University of California2 Now at InVisage, Inc.3 [email protected]
AbstractWe are part of an effort to build medium-sized Schwarzschild-Couder telescopes (SCT) exploiting silicon photomultipliers (SiPM) for the Cherenkov Telescope Array (CTA).
We report on measurements of 1660 SiPM pixels of 6x6 mm2 that will populate the camera of a prototype SCT. For CTA, we aim to build many similar telescopes with >11,000 pixels per camera. We consider various competitive SiPM devices and compare their impact on the CTA-SCT capabilities.
Outlook for the PrototypeWith a collected charge scaling as PDE x Gain x (1+Xtalk), the ~4% dispersion measured in the flash responses of the 1660 pixels is consistent with the relative dispersion measured in the PDE (~3%), gain (~2%), and crosstalk (~2%) of the sample.
No faulty pixel has been identified among the 1660 tested ones and the assembly has started in Sep. 2014. All the components of the camera are expected to be complete by Apr. 2015, with a forecast commissioning on the SCT prototype in fall 2015.
SiPM to Observe the Violent UniverseGround-based gamma-ray telescopes observe the Cherenkov emission generated by gamma-ray showers in the atmosphere.
The past years have seen an increasing interest in SiPM to detect Cherenkov photons, an advantageous alternative to PMT [1].
Prototype SCT CameraThe central sector of the prototype camera will be fully equipped with 400 SiPM tiles from Hamamatsu Photonics, model MPPC S12642-0404PA-050.
As in [4], we have fully characterized the properties of a sample of these devices, by measuring their pulse shape, gain, dark rate, and optical crosstalk, for the sixteen 3x3 mm2 chips that compose a tile and for groups of 4 chips connected in parallel (6x6 mm2 CTA pixels). We also measured the photodetection efficiency (PDE) of this sample and characterized the response to flashes of 415 tiles (1660 pixels), including 15 spare devices.
Absolute Spectral Response of a SampleOur PDE measurements are performed at a temperature stable within ~0.1°, using a method based on zero-peak Poisson statistics [4]. Five pulsed LEDs probe the wavelength range 375-600 nm.
The PDE of 14 tiles are shown as dashed lines in Fig. 1 at four different operating voltages. The averages and rms are shown as solid curves. A relative dispersion of ~3% is observed at all wavelengths and voltages. Better sensitivity below 400 nm would be desirable considering the Cherenkov spectrum.
Relative Response of 1660 PixelsA full characterization of the 1660 pixels being impracticable, we have designed a setup to estimate their breakdown voltage from current-voltage characteristics, as well as their response to LED flashes at 365 nm. We chose an operating voltage of 3 V above breakdown voltage, to maximize the PDE while keeping the dark noise at a reasonable level.
The charge distributions resulting from 90 p.e. flashes (per pixel) are stored simultaneously for the four pixels of a tile, as well as for a reference pixel. After spatial- and temperature-dependent calibration, based on repeated measurements of a single tile, the normalized response of the calibration pixels show an rms of 2.1% (see Fig. 1, dashed blue histogram), indicative of the accuracy of the devised test.
The responses of the 1660 pixels show an rms of 4.4%, equivalent to a 3.8% intrinsic dispersion after correction for the accuracy of the test.
Towards the SCT contribution to CTAThe selection of Hamamatsu MPPC S12642-0404PA-050 for the prototype was motivated by a 30-35% PDE in the blue and a high packing fraction implied by the use of the Through Silicon Via technology. Despite being the best choice at the time, its level of crosstalk, ~50% at the operating point, and pulse rise time, ~20ns before shaping for a 6x6mm2 pixel, are not ideal, and newer devices are showing performance more favorable for the many SCT that will be built for CTA.
With pulse widths < 10 ns, PDE in the blue reaching 40% and a crosstalk level below 20%, latest SiPM models start to match the technical goals of ground-based gamma-ray astronomy. An improved spectral response below 400 nm, where the Cherenkov spectrum peaks, and a decreased sensitivity above 550nm, where the contribution of the NSB becomes significant, would further guide the selection of the SiPM model that will equip the SCT.
We will continue our partnership with manufacturers to evaluate the improvements in the SiPM technology, that will be highly beneficial to gamma-ray astronomy and to our search for the most energetic phenomena in the Universe.
Selecting the Photosensor for CTAWe have characterized in detail a new set of SiPM devices. For the sake of brevity, we focus here on their spectral response.
Figure 3 shows the PDE of seven devices developed by the Excelitas, Hamamatsu, and SensL companies, including the best devices studied in [4] (Hamamatsu LCT-3x3-100C, Excelitas C30742-33-050-C). Hamamatsu MPPC LCT4-3x3-50C and SensL MicroFB-SMA-30035, with their 35-40% PDE in the blue, their maximum optical crosstalk of 10-15%, and their fast pulses (FWHM of 3-6 ns, for 3x3 mm2 pixels), are particularly promising devices.
Simulated Impact of Latest SiPMWe processed simulated NSB photons and Corsika-generated gamma-ray showers through a detector model of CTA, accounting for the SiPM's PDE and crosstalk. Given a pixel threshold, corresponding to an array trigger rate on the night sky background (NSB) of 100 Hz, the image intensity at which half of the showers are detected is defined as the trigger threshold of the telescopes.
Crosstalk and PDE are both increasing function of overvoltage, with PDE plateauing at large voltages. A trade off between these increases can be found for most devices, resulting in a minimum trigger threshold (see Fig. 4). The dark rate of the device, which at low overvoltage is an order of magnitude below the NSB, is not accounted for by the simulation at this stage, and it is expected to increase the threshold at the highest overvoltages.
The most recent SensL and Hamamatsu devices improve the threshold by 20-30% with respect to the protoype device.
AcknowledgmentsWe gratefully acknowledge support from the agencies and organizations listed under Funding Agencies at this website: http://www.cta-observatory.org/, and in particular the U.S. National Science Foundation award PHY-1229792 and the University of California. We also acknowledge the generosity of the SiPM manufacturers Excelitas, Hamamatsu, and SensL, who provided samples tested in this work.
Bibiliography[1] R. Mirzoyan & E. Popova, SPIE Conference Series, vol. 8621, Mar. 2013.[2] R. A. Cameron, SPIE Conference Series, vol. 8444, Sep. 2012.[3] J. Vanderbroucke for the CTA Consortium, sub. to Technology in Particle Physics, 2014 [4] A. Bouvier, et al., SPIE Conference Series, vol. 8852, Sep. 2013.
IEEE NSS/MIC 2014 – ID N22-3
CTA is undergoing a prototyping phase, including construction of an SCT of medium size with a 9.7 m-diameter primary mirror [2]. This design provides a wide field of view of 8° and a 30-40% improved angular resolution with respect to single-mirror telescopes. This configuration also allows for smaller cameras. The optimal pixelation for the prototype is about 6x6 mm2, matched to the point spread function in the focal plane [3].
~ 100 m
~ 10 m
~ 1 m
~ 1 cm
~ 100 μm
Artist's concept of two gamma-ray showers observed by CTA.
Structure of the SCT prototype.
Exploded view of the prototype camera.
SiPM tiles, Hamamatsu MPPC S12642-0404PA-050, chosen for the prototype.
SiPM's APD cells.
Prototype SCT camera. Only the central sector is equipped with SiPM (1600px). The camera is divided in 9 sectors of 13 to 25 modules, in turn composed of 64 pixels.
64px
Fig. 1. Top: PDE of 14 prototype SiPM tiles as a function of wavelength at four operating voltages. Bottom: Typical spectra of the Cherenkov signal (blue) and irreducible noise (night sky background, red).
Fig. 2. Distribution of the response of the 1660 pixels (solid green histogram). Multiple measurements of a single device, serving calibration purposes, are shown by the dashed blue histogram.
Fig. 3. Top: PDE as a function of wavelength for 7 SiPM. The onset of the PDE plateau as a function of voltage is set as the operating point.
Fig. 4. Trigger threshold of the SCT for 7 SiPM models, comparedto the threshold of the prototype (at an overvoltage of 3-3.5 V).
