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This review is dedicated to the 90th anniversary of the outstanding experimental physicist Boris Anatolyevich Dolgoshein, in whose laboratory exactly 50 years ago the first two-phase emission detector has been created. Today two-phase emission detectors found the best application in the most sensitive at the moment experiments searching for cold dark matter in the form of weakly interacting massive particles (WIMPs). Multi-ton active mass WIMP detectors of the upcoming G3 generation shall become sensitive to solar neutrinos interactions, to double-beta decay of isotopes containing in the working media with naturally occurring isotope abundances. The RED-100 detector constructed at NRNU MEPhI will be used for investigation of the reactor neutrino elastic coherent scattering off xenon nuclei at the Kalinin NPP in 2021. ProtoDUNE-DP prototype emission detector (6x6x6 m3 300 tons LAr) is currently being assembled at the CERN Neutrino Platform in support of the DUNE project focused on investigation of high energy neutrino oscillations. Thus the detector technology invented at MEPhI 50 years has demonstrated a great potential to be used in a variety of fundamental research programs.

This paper reviews applications of two-phase emission detectors using xenon as working media. This kind of detectors invented at MEPhI is extremely sensitive to ionization (down to single electrons) and can be very massive (in ton scale) in order to provide high count rate for quite rare events and organize an active shielding from natural radioactivity in the wallles configuration of readout system. The emission detectors found their unique application in the most sensitive at the moment experiments searching for cold dark matter in the form of weakly interacting massive particles (WIMPs). The RED-100 detector recently constructed at NRNU MEPhI can be used for the first observation of the elastic coherent neutrino scattering off xenon nuclei when the detector is installed practically on the Earth's surface.

— A standard diaphragm pump has been upgraded for continuous circulation cleaning of noble gases, the liquid phase of which is used as the working medium of emission detectors, in order to ensure stepless regulation of the flow rate of the purified gas in the range of 8–15 L/min.

The cryogenic system of the RED-100 two-phase emission detector has been modified to enable operation with liquid argon as a working medium with the aim of searching for the effect of coherent elastic scattering of reactor electron antineutrinos by argon nuclei.

— It is shown that the working volume of the RED-100 two-phase emission detector, which operates in a ground-based laboratory with liquid xenon used as a working substance, is a source of single-electron noise signals with a characteristic frequency of ~200 kHz. Possible mechanisms of generation of these noises and methods for their suppression are discussed.

A method for lateral localization of a gamma source in a medium with a miniature single channel scintillation detector is developed and experimentally tested. Method is based on simultaneous registration of two (or more) gamma lines emitted from an area of increased uptake of a radiopharmaceutical in the patients body. The proposed method is tested with the most widely used medical radioisotope technetium-99m which has a 140 keV gamma emission line and a 18 keV characteristic x-ray line. The results of experimental studies show that the lateral coordinate of a point-like 99mTc source can be determined with an accuracy of ± 4 mm at the depth of soft tissues equivalent phantom up to 30 mm.

The system for forming a trigger for the RED-100 liquid xenon detector has been developed. The trigger can be generated for all types of events required to calibrate the detector and data acquisition, including events with one ionization electron. The system has an event detection mechanism where each event is assigned with the timestamp and event type. The trigger system is required in the systems searching for rare events to keep only the necessary information from the ADC array. The characteristics and implementation of the trigger system that provides high efficiency operation even at low-energy events have been described.

The RED-100 (Russian Emission Detector) is being constructed for the experiment to search for elastic coherent neutrino scattering off atomic nuclei. This fundamental process was predicted several decades ago by the Standard Model of electroweak interactions but has not been discovered yet. The RED-100 is a two-phase emission xenon detector containing ∼200 kg of the liquid Xe (∼ 100 kg of that is in a fiducial volume). One of the possible sites to carry out the experiment is the SNS (Spallation Neutron Source) facility at Oak Ridge National Laboratory, USA. SNS is the world's most intense pulsed source of neutrinos and unique place to study neutrino properties. The energy spectrum of neutrinos produced at the SNS extends up to ∼ 50 MeV and satisfies coherence condition. These neutrinos give kinetic energies of Xe recoils up to a few tens of keV where the response of nuclear recoils is well-known from neutron calibrations of dark matter detectors. The detector will be deployed in the basement under the experimental hall at a distance of ∼30 meters from the SNS target. The expected signal and background (neutron and gamma) are estimated for this specific location. The detector details, current status and future plans are provided.

To determine the depth of the area of radiopharmaceutical accumulation a method of simultaneous recording of two lines of gamma rays of different energies and quantitative comparison of the intensity of these lines on the surface of the patient's body is provided. Since the coefficient of linear absorption of gamma radiation in the medium depends not only on the characteristics of the medium, but also on the gamma radiation energy, the intensity of gammas of different energies is attenuated differently after passing through the same absorber layer (soft tissues). Thus, the quantitative comparison of the relative intensities of gamma lines on the surface of the patient's body allows to determine the depth of area of the accumulation of the radiopharmaceutical. The result is achieved by analyzing the energy spectrum of the source, obtained with a semiconductor spectrometer, by measuring the ratio of areas of the absorption peaks of the radioisotope and defining the depth of gamma source using the calibration dependence between the areas ratio and the medium layer thickness. The most widely used medical radioisotope technetium-99m has two gamma-lines - 140 keV and 18.5keV, which allows one to apply the proposed method to search for the sentinel lymph nodes and non-palpable malignant tumors in the soft tissues.

The RED100 two-phase liquid xenon emission detector for neutrino coherent scattering experiments is equipped with 38 Hamamatsu R11410-20 photomultiplier tubes capable to operate at cryogenic temperatures and made of low background materials. A dedicated characterization procedure has been carried out for each PMT unit to be installed into the detector. The results presented here include single photoelectron analysis, gain curves for a wide range of the bias voltage values, data on dark count rate for 34 PMT samples. Peculiar noise characteristics of selected PMT units are analysed and discussed.