  The Jiangmen Underground Neutrino Observatory (JUNO) is a multipurpose
neutrino-oscillation experiment, with a 20 kiloton liquid scintillator detector
of unprecedented 3\% energy resolution (at 1 MeV) at 700-meter deep
underground. There are ~18,000 20-inch photomultiplier tubes (PMTs) in the
central detector with an optical coverage greater than 75%. Control of the
systematics of the energy response is crucial to archive the designed energy
resolution as well as to reach 1% precision of the absolute energy scale. The
detected number of photoelectrons in each PMT differs by two orders of
magnitude in the reactor antineutrino energy range in such a large detector,
which is a challenge to the single channel charge measurement. JUNO has
approved a new small-PMT system, including 25,000 3-inch PMTs, installed
alternately with 20-inch PMTs. The individual 3-inch PMT receives mostly single
photoelectrons, which provides a unique way to calibrate the energy response of
the 20-inch PMT system by a photon-counting technology. Besides, the small-PMT
system naturally extends the dynamic range of the energy measurement to help
the high-energy physics, such as cosmic muons and atmospheric neutrinos. We
will present the physics concept of this double calorimetry, the design and
implementation of the 3-inch PMT and its readout electronics system.
