  Low-Gain Avalanche Diodes are a type of silicon Avalanche Photo-Diodes
originally developed for the fast detection of minimum ionizing particles in
high-energy physics experiments. Thanks to their fast timing performance, the
Low-Gain Avalanche Diode paradigm enables detectors to accurately measure
minimum ionizing particles with a timing resolution of a few tens of
picoseconds. Such a performance is due to a thin substrate and the presence of
a moderate signal gain. This internal gain of a few tens is enough to
compensate for the reduced charge deposition in the thinner substrate and the
noise of fast read-out systems. While Low-Gain Avalanche Diodes are optimized
for the detection of minimum ionizing particles for high-energy particle
detectors, it is critical to study their performance for the detection of
different types of particle, such as X-rays, gamma-rays, or alphas. In this
paper, we evaluate the gain of three types of Low-Gain Avalanche Diodes: two
devices with different geometries and doping profiles fabricated by Brookhaven
National Laboratory, and one fabricated by Hamamatsu Photonics with a different
process.
  Since the gain in LGADs depends on the bias voltage applied to the sensor,
pulse-height spectra have been acquired for bias voltages spanning from the
depletion voltage up to breakdown voltage. The signal-to-noise ratio of the
generated signals and the shape of their spectra allow us to probe the
underlying physics of the multiplication process.
