Context : High-precision frequencies of acoustic modes in red giant stars are now available thanks to the long observing length and high-quality of the light curves provided by the NASA Kepler mission , thus allowing to probe the interior of evolved cool low-mass stars with unprecedented level of detail .

Aims : We characterize the acoustic signature of the helium second ionization zone in a sample of 18 low-mass low-luminosity red giants by exploiting new mode frequency measurements derived from more than four years of Kepler observations .

Methods : We analyze the second frequency differences of radial acoustic modes in all the stars of the sample by using the Bayesian code D iamonds .

Results : We find clear acoustic glitches due to the signature of helium second ionization in all the stars of the sample .
We measure the acoustic depth and the characteristic width of the acoustic glitches with a precision level on average around \sim 2 % and \sim 8 % , respectively .
We find good agreement with theoretical predictions and existing measurements from the literature .
Lastly , we derive the amplitude of the glitch signal at \nu _ { \mathrm { max } } for the second 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 in the stars .

Conclusions :