  A prototype quasi-parasitic thermal neutron beam monitor based on isotropic
neutron scattering from a thin natural vanadium foil and standard $^3$He
proportional counters is conceptualized, designed, simulated, calibrated, and
commissioned. The European Spallation Source designed to deliver the highest
integrated neutron flux originating from a pulsed source is currently under
construction in Lund, Sweden. The effort to investigate a vanadium-based
neutron beam monitor was triggered by a list of requirements for Beam Monitors
permanently placed in the ESS neutron beams in order to provide reliable
monitoring at complex beamlines: low attenuation, linear response over a wide
range of neutron fluxes, near to constant efficiency for neutron wavelengths in
a range of 0.6-10 \r{A}, calibration stability and the possibility to place the
system in vacuum are all desirable characteristics. The scattering-based
prototype, employing a natural vanadium foil and standard $^3$He proportional
counters, was investigated at the V17 and V20 neutron beamlines of the
Helmholtz-Zentrum in Berlin, Germany, in several different geometrical
configurations of the $^3$He proportional counters around the foil. Response
linearity is successfully demonstrated for foil thicknesses ranging from 0.04
mm to 3.15 mm. Attenuation lower than 1% for thermal neutrons is demonstrated
for the 0.04 mm and 0.125 mm foils. The geometries used for the experiment were
simulated allowing for absolute flux calibration and establishing the possible
range of efficiencies for various designs of the prototype. The operational
flux limits for the beam monitor prototype were established as a dependency of
the background radiation and prototype geometry. The herein demonstrated
prototype monitors can be employed for neutron fluxes ranging from
$10^3-10^{10}$ n/s/cm$^2$.
