  According to many dark matter models, a potential signal registered in a
detector would feature a single-scattering nuclear recoil (NR). So, it is
crucial to calibrate the detector's response to NR events. The conventional
calibrations implement $\sim$ keV to MeV neutrons, which can be produced by an
accelerator, a neutron generator, or a radioactive source. Although the
calibrating methods have been widely employed, they could be improved in
several ways: (a) the incident neutron energy should be more monoenergetic, (b)
the calibrating NR energy should line up with the region of interest (ROI) of
the experiment, and (c) the intensity of the beam should be appropriate. In the
paper, we introduce a novel NR calibration method for liquid helium detectors,
in which a helium beam ($\alpha$ particles) will be implemented to calibrate
the detectors. The helium beam can (i) be tuned precisely to have a jitter of
$\lesssim $ 4\% (the $\alpha$ beam's kinetic energy is equivalent to the recoil
energy in the conventional calibrations with fast neutrons); (ii) have an
energy between $\sim$ 100 eV and tens of keV; and (iii) provide a tunable flux
from nA to 100 $\mu$A, which presents convenience in beam pipe configuration to
obtain a $\sim$ 100 Hz events rate so that the events pileup would be
ignorable.
