Optical readout tracking detector concept using secondary scintillation from liquid argon generated by a thick gas electron multiplier

For the first time secondary scintillation, generated within the holes of a thick gas electron multiplier (THGEM) immersed in liquid argon, has been observed and measured using a silicon photomultiplier device (SiPM). 250 electron-ion pairs, generated in liquid argon via the interaction of a 5.9 keV Fe-55 gamma source, were drifted under the influence of a 2.5 kV/cm field towards a 1.5 mm thickness THGEM, the local field sufficiently high to generate secondary scintillation light within the liquid as the charge traversed the central region of the THGEM hole. The resulting VUV light was incident on an immersed SiPM device coated in the waveshifter tetraphenyl butadiene (TPB), the emission spectrum peaked at 460 nm in the high quantum efficiency region of the device. For a SiPM over-voltage of 1 V, a THGEM voltage of 9.91 kV, and a drift field of 2.5 kV/cm, a total of 62±20 photoelectrons were produced at the SiPM device per Fe-55 event, corresponding to an estimated gain of 150±66 photoelectrons per drifted electron.