  Boron vacancy centre ($V_{\rm B}^-$) ensembles in hexagonal boron nitride
(hBN) have attracted recent interest for their potential as two-dimensional
solid-state quantum sensors. Irradiation is necessary for $V_{\rm B}^-$
creation, however, to date only limited attention has been given to optimising
the defect production process, especially in the case of bulk irradiation with
high-energy particles, which offers scalability through the potential for
creating ensembles in large volumes of material. Here we systematically
investigate the effect of electron irradiation by varying the dose delivered to
a range of hBN samples, which differ in their purity, and search for an optimum
in measurement sensitivity. We find that moderate electron irradiation doses
($\approx 5\times 10^{18}$~cm$^{-2}$) appear to offer the best sensitivity, and
also observe a dependence on the initial crystal purity. These results pave the
way for the scalable and cost-effective production of hBN quantum sensors, and
provide insight into the mechanisms limiting $V_{\rm B}^-$ spin properties.
