Abstract. We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved -cosmology, astrophysics, nuclear, and particle physics -in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

Terbium offers 4 clinically interesting radioisotopes with complementary physical decay characteristics: 149 Tb, 152 Tb, 155 Tb, and 161 Tb. The identical chemical characteristics of these radioisotopes allow the preparation of radiopharmaceuticals with identical pharmacokinetics useful for PET ( 152 Tb) and SPECT diagnosis ( 155 Tb) and for a-( 149 Tb) and b 2 -particle ( 161 Tb) therapy. The goal of this proof-of-concept study was to produce all 4 terbium radioisotopes and assess their diagnostic and therapeutic features in vivo when labeled with a folate-based targeting agent. Methods: 161 Tb was produced by irradiation of 160 Gd targets with neutrons at Paul Scherrer Institute or Institut LaueLangevin. After neutron capture, the short-lived 161 Gd decays to 161 Tb. 149 Tb, 152 Tb, and 155 Tb were produced by proton-induced spallation of tantalum targets, followed by an online isotope separation process at ISOLDE/CERN. The isotopes were purified by means of cation exchange chromatography. For the in vivo studies, we used the DOTA-folate conjugate cm09, which binds to folate receptor (FR)-positive KB tumor cells. Therapy experiments with 149 Tb-cm09 and 161 Tb-cm09 were performed in KB tumor-bearing nude mice. Diagnostic PET/ CT ( 152 Tb-cm09) and SPECT/CT ( 155 Tb-cm09 and 161 Tbcm09) studies were performed in the same tumor mouse model. Results: Carrier-free terbium radioisotopes were obtained after purification, with activities ranging from approximately 6 MBq (for 149 Tb) to approximately 15 GBq (for 161 Tb). The radiolabeling of cm09 was achieved in a greater than 96% radiochemical yield for all terbium radioisotopes. Biodistribution studies showed high and specific uptake in FR-positive tumor xenografts (23.8% 6 2.5% at 4 h after injection, 22.0% 6 4.4% at 24 h after injection, and 18.4% 6 1.8% at 48 h after injection). Excellent tumor-to-background ratios at 24 h after injection (tumor to blood, ;15; tumor to liver, ;5.9; and tumor to kidney, ;0.8) allowed the visualization of tumors in mice using PET ( 152 Tb-cm09) and SPECT ( 155 Tb-cm09 and 161 Tb-cm09). Compared with no therapy, a-( 149 Tb-cm09) and b 2 -particle therapy ( 161 Tb-cm09) resulted in a marked delay in tumor growth or even complete remission (33% for 149 Tb-cm09 and 80% for 161 Tb-cm09) and a significantly increased survival. Conclusion: Because of its physical half-lives (T 1/2 ), decay properties, and energies, the lanthanide terbium is one of the few elements that features 4 clinically interesting radioisotopes (Table 1). 149 Tb has a half-life of 4.12 h and emits shortrange a-particles at an energy (E a ) of 3.967 MeV with an intensity of 17%. It is the only a-emitter among radiolanthanides with a suitable half-life for application in radionuclide therapy. 152 Tb (T 1/2 , 17.5 h) emits positrons of an average energy of 1.080 MeV with an intensity of 17%. The radionuclide would be useful for patient-specific dosimetry using PET before the application of therapeutic radiolanthanides. 155 Tb (T 1/2 , 5.32 d) decays by electron cap...

Abstract. Neutrinos, and in particular their tiny but non-vanishing masses, can be considered one of the doors towards physics beyond the Standard Model. Precision measurements of the kinematics of weak interactions, in particular of the 3 H β-decay and the 163 Ho electron capture (EC), represent the only model independent approach to determine the absolute scale of neutrino masses. The electron capture in 163 Ho experiment, ECHo, is designed to reach sub-eV sensitivity on the electron neutrino mass by means of the analysis of the calorimetrically measured electron capture spectrum of the nuclide 163 Ho. The maximum energy available for this decay, about 2.8 keV, constrains the type of detectors that can be used. Arrays of low temperature metallic magnetic calorimeters (MMCs) are being developed to measure the 163 Ho EC spectrum with energy resolution below 3 eV FWHM and with a time resolution below 1 μs. To achieve the sub-eV sensitivity on the electron neutrino mass, together with the detector optimization, the availability of large ultra-pure 163 Ho samples, the identification and suppression of background sources as well as the precise parametrization of the 163 Ho EC spectrum are of utmost importance. The high-energy resolution 163 Ho spectra measured with the first MMC prototypes with ion-implanted 163 Ho set the basis for the ECHo experiment. We describe the conceptual design of ECHo and motivate the strategies we have adopted to carry on the present medium scale experiment, ECHo-1K. In this experiment, the use of 1 kBq 163 Ho will allow to reach a neutrino mass sensitivity below 10 eV/c 2 . We then discuss how the results being achieved in ECHo-1k will guide the design of the next stage of the ECHo experiment, ECHo-1M, where a source of the order of 1 MBq 163 Ho embedded in large MMCs arrays will allow to reach sub-eV sensitivity on the electron neutrino mass.