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Start of funding 01.07.2015
Development of a single VUV photon sensitive detector system based on Silicon Photo Multipliers
Dr. Thilo Michel
Friedrich-Alexander-University of Erlangen-Nuremberg
Lehrstuhl für Teilchen- und Astroteilchenphysik
Prof. Dr. Giorgio Gratta
Stanford University
Department of Physics
Commercially available Silicon Photomultipliers (SiPM) are not sensitive to VUV (vacuum ultraviolet)
photons, which is necessary for future large scale experiments in Particle and Astroparticle Physics, such
as nEXO. The experiment nEXO will try to detect the neutrinoless double beta decay of 136Xe in a large
chamber filled with liquid xenon. Since the detection of this very rare process can only tolerate an
extremely low level of radioactive background, exceptionally pure materials have to be used.
One possible way to achieve the desired photon detection efficiency of at least 15% could be to coat
SiPMs with a wavelength shifter, encapsulated in a quartz housing due to the solubility in liquid xenon.
This aim of this project is to determine the feasibility of such a detector system and to test it under real
conditions in a liquid noble gas chamber. One needs to characterize the new detector with respect to
photon detection efficiency, crosstalk probability, reflectivity and other parameters.
Project website:
http://ecap.physik.uni-erlangen.de/novel/research/nEXO.shtml.
Final report:
In our project we studied Silicon Photomultipliers (SiPMs) as potential candidates to detect the VUV-scintillation light of xenon in the future tonne-scale double-beta decay experiment nEXO. A SiPM manufacturer provided us with a SiPM coated with the wavelength shifting substance Lumogen, which absorbs UV-photons and emits longer-wavelength photons in the green. We incorporated this sensor in the Erlangen cryogenic SiPM characterization setup and measured the photon detection efficiency (PDE) to be about 12% with a cross-talk of about 35%. Unfortunately, these values were smaller than expected and especially were smaller than the PDEs obtained with VUV-sensitive SiPMs without any coating developed for nEXO. We investigated the potential of Lumogen coated screens (thin layer of Lumogen between two sheets of fused silica) for nEXO in a bachelor but found that they cannot compete with a system using VUV-sensitive SiPMs. Therefore, we decided to focus on SiPMs which are sensitive to VUV-light without wavelength shifters and characterized them at cryogenic temperatures in our setups in Erlangen and in Stanford. The combined results from Erlangen and Stanford indicated that the new VUV-HD SiPMs meet the nEXO requirements in terms of PDE and cross-talk and are a potential photosensor candidate for the nEXO experiment. After finding a suitable candidate, the integration of a large area SiPM assembly into the liquid xenon test setup at Stanford was planned whereby special attention had to be paid to the mounting system of the SiPMs. An optical simulation with the Monte-Carlo-based Chroma software was developed to quantify the improvement in light collection by installing the SiPMs in the liquid xenon setup. The results of this project went into 3 peer-reviewed publications.