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High-precision frequencies of acoustic modes in red giant stars are nowavailable thanks to the long observing length and high-quality of the lightcurves provided by the NASA Kepler mission, thus allowing to probe the interiorof evolved cool low-mass stars with unprecedented level of detail. Wecharacterize the acoustic signature of the helium second ionization zone in asample of 18 low-mass low-luminosity red giants by exploiting new modefrequency measurements derived from more than four years of Keplerobservations. We analyze the second frequency differences of radial acousticmodes in all the stars of the sample by using the Bayesian code Diamonds. Wefind clear acoustic glitches due to the signature of helium second ionizationin all the stars of the sample. We measure the acoustic depth and thecharacteristic width of the acoustic glitches with a precision level on averagearound $\sim$2% and $\sim$8%, respectively. We find good agreement withtheoretical predictions and existing measurements from the literature. Lastly,we derive the amplitude of the glitch signal at $\nu_\mathrm{max}$ for thesecond differences and for the frequencies with an average precision of$\sim$6%, obtaining values in the range 0.14-0.24 $\mu$Hz, and 0.08-0.33$\mu$Hz, respectively, which can be used to investigate the helium abundance inthe stars.

High-precision acoustic helium signatures in 18 low-mass low-luminosity red giants. Analysis from more than four years of Kepler observations