Development of the Infrared Quantum Counter concept for low-threshold particle detection

An investigation on the feasibility of a new class of low-threshold, high efficiency particle detectors is carried out under the Axioma Project at the Laboratori Nazionali INFN in Legnaro. The present project is aimed at the detection of low-rate, low-energy deposition events, both for axionic dark matter searches and for coherent neutrino-nucleon scattering studies. The detection process is mainly based on the Infrared Quantum Counter concept applied to rare-earth (RE) doped crystals, as proposed by the Nobel prize Bloembergen in 1959. An energetic particle excites the rare-earth ions from the ground state to a low-lying (<$1$ eV), metastable (lifetime $ \gtrsim1\,\mathrm{ms}$) state. Simultaneously, a tuned pump laser promotes the excited ion to a higher lever (upconversion), that relaxes to the ground state or to some other lower lying state emitting a visible fluorescence photon. A very high detection efficiency can be reached depending on several factors, including the energy level scheme of the specific combination of RE ion and crystal matrix that may lead to excitation recycling induced light amplification processes. A first most important requirement is that the energy deposition by means of an energetic particle in the crystal leads to a high production of ions excited in the lowest metastable level above the ground state. A second important requisite is a high upconversion efficiency that depends on the laser light absorption cross section and lifetime of the excited ions. Finally, a third requisite is that the crystal is as transparent as possible to the laser if the ions are in the ground state. Owing to the scant number of studies on these subjects that can be found in literature, the best crystal-RE ion combination has to be sought by direct investigation in our lab. To this goal we have started a systematic investigation campaign of the properties of several combinations of single crystal hosts doped with different RE ions of selected concentrations. The efficiency of the production of excited ion levels is studied by exciting crystals by either electron impact or X-ray irradiation with a home-made electron gun. The emission spectrum can be recorded in the range $200\,\mathrm{nm}\le \lambda\le 5000\,\mathrm{nm} $ by suitably merging the spectra recorded with different spectrometers, including a FT-IR (Fourier Transform-InfraRed) one. The outcome of these measurements is the light yield in both the visible and infrared range. In particular, light yields as high as $10^5/\mathrm{MeV}$ can be obtained. From the spectra we are also able to identify the levels involved in the excitation by both the position of the emitted wavelengths and by their time evolution. We are also exciting the crystals with different lasers (diode, tunable dye and Ti:Sa laser, Nd:YAG higher harmonics) in order to shed light on possible different excitation mechanisms with respect to particle excitation. The upconversion efficiency and crystal transparency are studied by irradiating the crystals with finely tunable lasers at low temperature. The upconversion efficiency is measured by simulating the particle excitation with an infrared source of selected wavelength band and using a Ti:Sa or a dye laser tunable with picometer accuracy to promote the excited ions to the even higher excited levels whose decay produces the visible fluorescence light collected by a photomultiplier. In the future we will be also using a small $\gamma$-source to excite crystals. The crystal transparency is affected by non-resonant multiphonon-assisted absorption that results in a noise source that might degrade the overall detection efficiency. We are investigating this process by varying both crystal temperature and laser intensity. All these studies should allow us to define the optimum characteristics of the crystal-dopant system and to select the optimum actual crystal for low-threshold particle detection.