Dark Matter and Double Beta Decay experiments require extremely low radioactivity within the detector materials. For this purpose, the University of California, Los Angeles and Hamamatsu Photonics have developed the QUartz Photon Intensifying Detector (Qupid), an ultra-low background photodetector based on the Hybrid Avalanche Photo Diode (HAPD) and entirely made of ultraclean synthetic fused silica. In this work we present the basic concept of the Qupid and the testing measurements on Qupids from the first production line.Screening of radioactivity at the Gator facility in the Laboratori Nazionali del Gran Sasso has shown that the Qupids safely fulfill the low radioactive contamination requirements for the next generation zero background experiments set by Monte Carlo simulations.The quantum efficiency of the Qupid at room temperature is > 30% at the xenon scintillation wavelength. At -100 • C, the Qupid shows a leakage current smaller than 1 nA and a global gain of 10 5 . In these conditions, the photocathode and the anode show > 95% linearity up to 1 µA for the cathode and 3 mA for the anode. The photocathode and collection efficiency are uniform to 80% over the entire surface. In parallel with single photon counting capabilities, the Qupids have a good timing response: 1.8 ± 0.1 ns rise time, 2.5 ± 0.2 ns fall time, 4.20 ± 0.05 ns (FWHM) pulse width, and 160 ± 30 ps (FWHM) transit time spread.The Qupids have also been tested in a liquid xenon environment, and scintillation light from 57 Co and 210 Po radioactive sources were observed.
A multi-pixel photon sensor with single-photon sensitivity has been developed. Based on a hybrid photo-detector (HPD) technology, it consists of a photocathode and a multi-pixel avalanche diode (MP-AD). The developed HPD has a proximity-focused structure, where the photocathode and MP-AD face each other with a small gap of 2.5 mm. The MP-AD, which has an effective area of 16mm×16mm, is composed of 8×8 pixel and has been specially designed for the HPD. The maximum gain of the HPD is 5×10 4 , sufficiently high to detect single photons with a timing resolution better than 100 ps. Up to four photoelectrons can be clearly identified as distinct peaks in a pulse-height spectrum, thanks to the low noise characteristics of the HPD. It is also 1 demonstrated that the HPD can be operated with good performance in a magnetic field as high as 1.5T.
Over the years, fluorescence microscopy has evolved and has become a necessary element of life science studies. Microscopy has elucidated biological processes in live cells and organisms, and also enabled tracking of biomolecules in real time. Development of highly sensitive photodetectors and light sources, in addition to the evolution of various illumination methods and fluorophores, has helped microscopy acquire single-molecule fluorescence sensitivity, enabling single-molecule fluorescence imaging and detection. Low-light photodetectors used in microscopy are classified into two categories: point photodetectors and wide-field photodetectors. Although point photodetectors, notably photomultiplier tubes (PMTs), have been commonly used in laser scanning microscopy (LSM) with a confocal illumination setup, wide-field photodetectors, such as electron-multiplying charge-coupled devices (EMCCDs) and scientific complementary metal-oxide-semiconductor (sCMOS) cameras have been used in fluorescence imaging. This review focuses on the former low-light point photodetectors and presents their fluorescence microscopy applications and recent progress. These photodetectors include conventional PMTs, single photon avalanche diodes (SPADs), hybrid photodetectors (HPDs), in addition to newly emerging photodetectors, such as silicon photomultipliers (SiPMs) (also known as multi-pixel photon counters (MPPCs)) and superconducting nanowire single photon detectors (SSPDs). In particular, this review shows distinctive features of HPD and application of HPD to wide-field single-molecule fluorescence detection.
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