  A portable monoenergetic 24 keV neutron source based on the $^{124}$Sb-$^9$Be
photoneutron reaction and an iron filter has been constructed and
characterized. The coincidence of the neutron energy from SbBe and the low
interaction cross-section with iron (mean free path up to 29 cm) makes pure
iron specially suited to shield against gamma rays from $^{124}$Sb decays while
letting through the neutrons. To increase the $^{124}$Sb activity and thus the
neutron flux, a $>$1 GBq $^{124}$Sb source was produced by irradiating a
natural Sb metal pellet with a high flux of thermal neutrons in a nuclear
reactor. The design of the source shielding structure makes for easy
transportation and deployment. A hydrogen gas proportional counter is used to
characterize the neutrons emitted by the source and a NaI detector is used for
gamma background characterization. At the exit opening of the neutron beam, the
characterization determined the neutron flux in the energy range 20-25 keV to
be 5.36$\pm$0.20 neutrons per cm$^2$ per second and the total gamma flux to be
213$\pm$6 gammas per cm$^2$ per second (numbers scaled to 1 GBq activity of the
$^{124}$Sb source). A liquid scintillator detector is demonstrated to be
sensitive to neutrons with incident kinetic energies from 8 to 17 keV, so it
can be paired with the source as a backing detector for neutron scattering
calibration experiments. This photoneutron source provides a good tool for
in-situ low energy nuclear recoil calibration for dark matter experiments and
coherent elastic neutrino-nucleus scattering experiments.
