The most stable atomic and molecular processes are often the hardest to measure and investigate. Such low-threshold energy measurements can be probed effectively, however, by trapping and isolating “guest” particles in the “host” matrix of an unreactive and stable solid. A method called matrix isolation spectroscopy employs laser excitations to detect guest atoms or molecules and offer discernable measurements of their dynamics.

A new apparatus, presented in Review of Scientific Instruments, combines matrix isolation spectroscopy methods, using noble gas crystals, with efficient electron extraction in vacuum to demonstrate ultralow threshold particle detection. The apparatus features a gas purification system and chamber cleaning process to facilitate high purity crystal formation.

For the un-doped matrix, cryogenic noble gas crystals had fewer than parts-per-billion impurities, particularly for high-electronegativity atoms that are detrimental to the method’s tiny electronic signals. UV laser pulses drove photo-assisted electron emission in a gold foil, adjacent to the crystal, to inject electrons into the matrix for crystal purity tests. A static electric field and microchannel plate collected and measured charges.

Dopants added to the matrix offer energy-level architectures that can further lower the detection threshold. The authors propose a hybrid method that exploits laser-induced ionization of the embedded dopant of the inert-gas matrices. With precise measurements of emitted electrons, triggered by the transition between the ground and first excited state of the dopant, capturing additional deposited energy of this amount would arise in the form of additional measured charge.

The potential sensitivity of the apparatus is such that it could be adapted to probe low-energy particle events. According to Marco Guarise, an author of the work, the final purpose of the project is to develop a novel detector for “particles with feeble interaction with ordinary matter” like cosmological axions.

Source: “Experimental set-up for the growth of solid crystals of inert gases for particle detection,” by M. Guarise, C. Braggio, R. Calabrese, G. Carugno, A. Dainelli, A. Khanbekyan, E. Luppi, E. Mariotti, M. Poggi, and L. Tomassetti, Review of Scientific Instruments (2017). The article can be accessed at