  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. 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. We analyze the second frequency differences of radial acoustic
modes in all the stars of the sample by using the Bayesian code Diamonds. 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.
