$Z$ dependence of thick-target external bremsstrahlung produced by $\beta$ particles of ${}_{}{}^{90}{}_{}{}^{}\mathrm{Sr}$-${}_{}{}^{90}{}_{}{}^{}\mathrm{Y}$

External bremsstrahlung spectra produced in thick targets of copper, molybdenum, cadmium, iodine, and lead by $\beta$ particles of ${}_{}{}^{90}{}_{}{}^{}\mathrm{Sr}$-${}_{}{}^{90}{}_{}{}^{}\mathrm{Y}$ were measured using a 4.55-cm × 5.08-cm NaI(T1) crystal. The conventional $\beta$-stopper method was followed. The measured spectra were unfolded according to the Liden and Starfelt procedure. The exponents of $Z$ for energy yield and photon yield were found to be 1.20±0.02 and 1.26±0.02, respectively.