APPEC | Astroparticle Physics European Consortium |www.appec.org Physics European APPEC |Astroparticle LOW LIGHT-LEVEL DETECTION LIGHT-LEVEL LOW In Astroparticle PhysicsandinMedicalApplication In Astroparticle APPEC | Astroparticle Physics European Consortium |www.appec.org Physics European APPEC |Astroparticle Forum2015 APPEC Technology 22–23 April 2015 Munich Nymphenburg Castle Low light-level detection: Key technologies developed in astro-particle physics Detection of light is one of the major, basic prin- sitivity to magnetic fields, low bias, high photo ciples of measurements and diagnostics in sci- detection efficiency. Nonetheless, they are still ence and many applications. Medical diagnostic so compact that instrumenting large surface applications (e.g., X-raying) provide an example area requires complicated readout electronics. A with an enormous societal impact and any im- breakthrough in the technology also addressing provement in detection efficiency of light sensors the SiPM noise issue and the relatively long dead will turn into a straightforward gain for health time is going to take place in the next years. In and life quality. addition to these two technologies a complete proof-of-the-concept exists for a novel design Nowadays most of the light detectors employ the of low light-level detectors, called Abalone, photoelectric effect, which occurs when mat- which has to be evaluated for its commercializa- ter releases/emits electrons upon exposure to tion. Other novel ideas like the superconducting light; for the discovery/explanation of the photo transition edge sensors (TES), the Neganov-Luke effect Albert Einstein received the Nobel Prize of light devices, or new generation organic image the year 1921. Many applications require single sensors require further R&D before becoming a photon detection in various energy domains. The commercial product. low light-level detection in the coming years has the promise of a technology revolution. Compa- Currently, with about 600000 PMTs/year med- rable to the change from the electron valve to ical diagnostic instrumentation is the largest the semiconductor transistor technology and its consumer of PMTs; PMTs are used in Positron subsequent tremendous innovation boost and Emission Tomography (PET) scanners, Gamma applications both in science and in everyday life, cameras, and many applications in Life Sciences. significant improvements of currently existing Besides specific applications of PMTs, e.g., in oil products can be expected when the Photo Multi- drilling industry, large scale experiments in basic plier Tubes (PMTs) will be largely replaced by the research are consumers of up to several 100000 emerging technology of Silicon Photo Multipliers low light-level sensors. The demand of reaching (SiPMs). lower and lower levels in light detection efficien- cy with highest precision in astroparticle, particle, SiPMs are robust and feature numerous advan- and nuclear physics experiments is one of the tages compared to PMTs, such as compactness, main R&D drivers in the domain of low light-level lightweight, automated mass production, insen- detection. Any improvement in the PMT technology evolv- demonstrated that the topic of this Technology ing from science projects has allowed medical di- Forum addresses a rather challenging and timely agnostics industry to immediately come up with R&D field. advanced products. Together with this immedi- ate market effect there is a clear positive societal The clear intention of APPEC was to take a de- impact: a doubling of a light sensor’s efficiency tailed look at the developments during these 5 in a medical diagnostic instrument will allow to years and reveal alternative detector technolo- half the radiation doses for patients and thus gies that may find application in astro-particle reduces the possible negative consequences of physics experiments. Generally speaking, “tradi- irradiation. PET scanners allow one studying the tional” photomultiplier technology has con- functional activity in vivo, which is important for stantly and substantially been advanced during cancer research, Alzheimer studies as well as drug the last decade. Figure 1 demonstrates these ad- tests. When replacing PMTs with SiPM devices it vancements by plotting the signal-to-noise ratio might be possible, for instance, to combine PET for a single impinging photon as a function of the with the very strong magnetic field of MRI coils. quantum efficiency (QE); the signal-to noise ratio Also, one will profit from ultra-fast time resolution (SNR) of a given photocathode with QE = P can be of SiPM, for the so-called Time-of-Flight reduction calculated as SNR = √ [N x P/(1 - P)]. of the background. Figure 1 demonstrates that the long existing PMT The APPEC Technology Forum 2015 technology has not yet reached any limit and can substantially be further advanced. Photocathode 2015 2010 The APPEC Technology Forum 2015 (ATF2015) on material cleanness and its depositing technol- 2005 low light-level Detection in astroparticle physics ogy may improve the quantum efficiency even Signal-to-Noise Ratio and in medical applications was organized five further that future PMTs will be capable to detect Quantum Efficiency years after a similar Technology Forum organized each single impinging photon. Figure 1: Development of in the frame of the EU funded ASPERA project. the quantum efficiency About 80 technology developers from academia PMTs are produced by companies in all sizes, from and resulting signal-to- and industry attended the ATF2015, which was very small ( ~1cm in size) to very large ( 50cm). noise ratio of PMTs. Note again organized at the premises of the Carl-Frie- Test samples of Hamamatsu PMTs for CTA are that in the time between drich von Siemens Foundation at the Nymphen- confirming the currently available high quality, the 1960s until 2005 it burg Castle in Munich. Compared to the ear- substantially better than the requirements on QE, was believed that this lier event in 2010 the presence of many young after-pulse rates and Peak/Valley ratio of single technology cannot be fur- scientists at ATF2015, male and female, clearly photon counting set by the CTA collaboration. ther advanced. 3 SiPM technology finds first application in astro- Cryogenic PMTs are currently the standard light particle physics experiments (e.g., in gamma sensors applied in dark matter searches using LAr cameras and dark matter experiments). Examples and LXe. In addition to standard requirements are the FACT camera and a SiPM-based sensor such as high QE, low dark count rate (DCR) and cluster for MAGIC, which is under tests and evalu- stable performance these PMTs need to be opti- ation. mized concerning a very low radioactive con- tamination, especially a low U and Th content is Currently, a large variety of SiPM matrixes are necessary to suppress any neutron background. available in the sensor market, also alternative readout solutions are there. SiPM matrixes with Gaseous PMTs (GPM) are explored as alternative improved filling factor are currently being devel- in low light-level detection in cryogenic applica- oped to overcome the low geometrical effi- tions. This technology may provide a high filling ciency of these devices. However, for fast timing factor and may allow to fully surround an experi- applications the size of SiPMs is limited to several ment and to detect light in all directions. First mm, because of the charge collection time; also, measurements with 4” GPMs demonstrate a large increasing the cell size to higher values will unfor- dynamic range, good stability, energy and time tunately increase its gain and the much unwant- resolution as well as a low DCR. ed crosstalk. SiPM-based matrixes with complete readout, like in a CMOS (or in a CCD) camera would allow 2010 2015 a simple assemby in arbitrary shape, arriving at PMT Peak QE 28–34% 36–43% large coordinate-sensitive imaging camera. How- Photo Electron Collection Ef- 60–80% 94–98% ever, stray heat might cause a problem in fast on- ficiency on the 1st Dynode chip digital readout solutions. Afterpulse rate (for a set thresh- 0.5% < 0.01% old ≥ 4 ph.e.) Inter-calibration of PMT and SiPM solutions still SiPM Peak Photon Detection Efficiency 20–30 50–60% indicate higher photon detection efficiencies for DCR 1–3 MHz/mm² 50–100 kHz/mm² PMT, however, more measurements are needed. Light crosstalk > 40–60% 5–10% Table 1 provides a comparison of basic param- eters for PMTs and SiPMs available in 2010 and Table 1: Comparison of basic parameters characterizing 2015. The improvements of both technologies PMTs and SiPMs available in 2010 and 2015 are clearly demonstrated. 4 The GERDA experiment uses a combination of ising low background single photon detection conventional and novel light detectors and is technology to find wider application in research. thus the first experiment with a large SiPM array operated at cryogenic temperature. Several developments of optical modules for high-energy neutrino experiments have been The successful application of a tungsten transi- presented, single and multi-PMT designs. Pro- tion-edge sensor (TES) operated below 100 mK in totyping for a completely new design, a wave- the ALPS-II experiment to detect single photons length-shifting optical module (WOM), has been in the near-infrared demonstrates that this tech- presented. Further R6D is required to demon- nology is entering astro-particle physics. One can strate the performance of the WOMs design. speculate that further R&D may help this prom- APPEC