Silicon Photomultipliers (SiPMs) are attractive candidates for light detectors for next generation liquid xenon double-beta decay experiments, like nEXO. In this paper we discuss the requirements that the SiPMs must satisfy in order to be suitable fo
r nEXO and similar experiments, describe the two test setups operated by the nEXO collaboration, and present the results of characterization of SiPMs from several vendors. In particular, we find that the photon detection efficiency at the peak of xenon scintillation light emission (175-178 nm) approaches the nEXO requirements for tested FBK and Hamamatsu devices. Additionally, the nEXO collaboration performed radioassay of several grams of bare FBK devices using neutron activation analysis, indicating levels of 40K, 232Th, and 238U of the order of <0.15, (6.9e10-4 - 1.3e10-2), and <0.11 mBq/kg, respectively.
Solid state breakdown counters combine threshold properties of nuclear track detectors with the convenience of electronic event registration in real time. Here we discuss the principle of operation of the breakdown counters and their potential applic
ability to the existing (MoEDAL) and foreseen (CosmicMoEDAL) searches of magnetic monopoles and other highly ionizing rare particles. Avenues of R&D that may lead to an increased appeal of the technology are also described
EXO-200 is a single phase liquid xenon detector designed to search for neutrinoless double-beta decay of $^{136}$Xe. Here we report on a search for various Majoron-emitting modes based on 100 kg$cdot$yr exposure of $^{136}$Xe. A lower limit of $T^{^{
136}Xe}_{1/2} >1.2 cdot 10^{24}$ yr at 90% C.L. on the half-life of the spectral index = 1 Majoron decay was obtained, corresponding to a constraint on the Majoron-neutrino coupling constant of $|< g^{M}_{ee} >|<$ (0.8-1.7)$cdot$10$^{-5}$.
