Periodic Reporting for period 1 - Light-Trap (A SiPM upgrade for VHE Astronomy and beyond)
Reporting period: 2015-04-01 to 2017-03-31
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 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.