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A SiPM upgrade for VHE Astronomy and beyond

Periodic Reporting for period 1 - Light-Trap (A SiPM upgrade for VHE Astronomy and beyond)

Reporting period: 2015-04-01 to 2017-03-31

"Ground-based gamma-ray astronomy in the Very High Energy (VHE, E>100 GeV) regime has fast become one of the most interesting and productive sub-fields of astrophysics today. Also known as VHE Astronomy, it utilizes the Imaging Atmospheric Cherenkov Technique (IACT) to detect Cherenkov radiation (UV/Blue dominated) that has been emitted by Extensive Air Showers produced by gamma-ray photons interacting in the upper atmosphere. Research and development of new and improved technology for increasing the light-collection efficiency and field-of-view (FOV) of the camera systems used in VHE astronomy is always on going. These improvements can also allow for the consideration of an increase in the number of telescopes used in an array, if the cost of the cameras could be significantly reduced.
The primary path to these goals is to replace Photomultiplier Tubes (PMTs) with Silicon-PMs (SiPMs) that have substantially larger collection area than those currently on the market. These large-area SiPMs would also find multi-disciplinary uses e.g. in fluorescence telescopes for detection of Ultra High Energy Cosmic Rays and medical physics.
This project was targeted at increasing the physical area and sensitivity of SiPMs by attaching one to a “Light-Trap” disk. This Light-Trap disk will need to collect light over an area much larger than the SiPM itself, be sensitive to wavelengths where signal dominates over background and be much cheaper than the SiPM.
Here we propose a novel method to build relatively low-cost SiPM-based pixels utilizing wavelength-shifting (WLS) material (through a scintillating PMMA disk). We optimized the design of such a pixel, integrated them in an actual 7-pixel cluster that was installed into a camera on one of the MAGIC VHE telescopes and tested during real observations. The device boosted the sensitivity of a commercially available SiPM to UV light, while being essentially blind to longer wavelengths, and performed excellently under real-world conditions.
With some future improvements to the proof-of-concept design, it is feasible that large-scale cameras with UV-sensitive ""Light-Trap"" pixels could be produced, reducing camera costs while increasing FOV and maintaining scientific performance.
The research program was structured into two main phases, with several sub-projects as summarized in the following. The overall deliverables for the fellowship were to optimize and understand fully the performance of the Light-Trap SiPM design through GEANT4 simulations, and construct several prototype Light-Trap SiPM pixels to characterize their performance in the context of the detecting Cherenkov radiation from Extensive Air Showers (EAS).
The second phase was to develop a 7-pixel Light-Trap SiPM cluster through a combination of laboratory development and simulations. This cluster was to be installed on a camera of the MAGIC telescopes in La Palma for “in field” testing and performance evaluation for Cherenkov light produced by real EAS.
During the reporting period of 01/04/2015 – 31/03/2017, the first phase has been totally completed, while the second phase was partially completed (however, the second phase has been totally completed by 31/05/2017). The delay was the result of poor optical quality disks being delivered by industry, and the resulting efforts to understand why, what is the optimal production process, how the optical disk quality could be improved, and finally manufacturing and delivery of suitable-quality disks.

An overview of the results achieved is below:

• Built Geant4 simulations of the Light-Trap pixel
• Simulated two-disk and single-disk configurations with different wavelength shifters
• Estimated efficiencies for various optical quality scenarios, disk and reflector
• This study has not previously been conducted according to current technical literature
• Simulations provided useful cross-check points for the laboratory measurements
• Deliverable 1.1 (report on the design with simulation output results) was achieved
• The Light-Trap proof-of-concept was successfully tested in a laboratory. Coupling to a wavelength-shifting PMMA disk enhanced the UV performance of a commercially available SiPM. Furthermore, the Light-Trap was “blind” to longer wavelengths typically associated with Night Sky Background (Green, and Red). Enhancing the UV sensitivity of a commercially available SiPM while decreasing its sensitivity to longer wavelengths was the main objective of the fellowship.
• The construction of a stand-alone pixel which is a hybrid device combining a commercially available SiPM with a wavelength-shifting PMMA disk is innovative, and has been achieved for the first time in this fellowship.
• The performance of the Light-Trap pixel has matched expectations, and has potential to be used in future scientific or technological applications where a cheaper device with UV sensitivity and larger area is needed (e.g. Fluorescence detectors in Ultra-High Energy Cosmic Ray physics).
• Deliverable 1.2 (results and conclusions drawn from laboratory tests) was achieved.
• A Light-Trap cluster was installed on the MAGIC-I telescope.
• The cluster performed as expected, with no nominal currents and temperatures from all the devices. Pedestal, calibration, and Cherenkov shower events were recorded and our currently being analyzed.
• After very preliminary analysis, single photoelectron peaks have been observed.
This project with it’s focus on Monte Carlo simulations and development and testing of a new hybrid SiPM-based system has helped to drive European competitiveness in this highly active field of SiPM development. Ultimately, this was achieved through increasing the cost-effectiveness of SiPMs by expansion of their physical collection area, while enhancing their sensitivity to UV light without loss of robustness or increased cost. Although still being studied, it is expected that this will have several benefits to European industry and society through improvements in scientific productivity in the field of experimental astrophysics (large Field-of-View fluorescence detectors for example), and via potential next-generation medical imaging devices.
Light-Trap pixel with Sensl SiPM and PCB.
"First Light-Trap pixel attached to cluster ""flower"". The flower holds all seven pixels."
Early prototype Eljen PMMA scintillating disks (15 mm diameter).
Front view of the Light-Trap cluster located in the MAGIC-I camera.
A Light-Trap pixel under UV illumination. Note the ring of blue light surrounding the disk.
Fully populated Light-Trap cluster in the laboratory.
Light-Trap cluster being installed in the MAGIC-I camera in La Palma, Spain.