We present an investigation on the feasibility of a new class of low-threshold particle detectors for low-rate event.
The detection process is based on the Infrared Quantum Counter concept applied to rare-earth doped crystals, as proposed in literature (1). An energetic particle excites the rare-earth ions from the ground state to a low energy, long lived state. Then, a suitably tuned pump laser promotes the excited ions to a higher state, that quickly and non radiatively relax towards a nearby Stark level. The radiative transition between this level and the lower lying, intermediate one provides the signal of the particle passage.
The advantage of using a laser consists of the conversion of the infrared photon, that would normally be emitted, into one or more visible photons that can be detected with higher efficiency and signal to noise ratio. Actually, the ions returning to the intermediate excited state can again be upconverted, thereby accomplishing a loop process which results in an amplification of the emitted light.
The most important requirements are an efficient excitation of the low lying level and a high upconversion efficiency. Moreover, the crystal has to be as trasparent as possible to the laser when the ions are in the ground state.
We have carried out measurements of cathodo- and radioluminescence from 200 nm to 5000 nm of two singlecrystals (Nd:YAG 1.1% and Tm:YAG 4.4%) thereby proving that the low lying levels are efficiently populated.
The upconversion efficiency and the trasparency in Er:YLF samples are studied at low temperature to limit the phonon influence on such properties. A SiC infrared lamp is shone on to the crystal exciting the low lying intermediate state and simulating in this way the particle passage. A tunable Ti:Saffire laser with picometer accuracy is used to selectively promote the ions to the highest state. The visible luminescence due to the de-excitation of the latter state is detected with a photomultiplier tube.
We have achieved a high upconversion efficiency, but the signal to noise ratio is still to be improved because the multiphonon assisted side band absorption give rise to a double resonance process, that is responsible for a large fluorescence background. At present, we are investigating several combination of crystal matrices and dopants in different concentrations in order to reduce this noise.
(1) N. Blombergen, Phys. Rev. Lett. 1 (1959) 84-85