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