= Solution
Below the critical photon energy, radiation is emitted over a wider angular cone. The usual low-frequency characteristic-angle estimate is
$$
\sigma_\theta(\varepsilon)\sim\frac1\gamma
\left(\frac{\varepsilon_c}{\varepsilon}\right)^{1/3},
$$
up to an order-one angular-width convention. At $\varepsilon=0.012\,\mathrm{eV}$ this gives
$$
\boxed{\sigma_\theta\sim1.49\times10^{-2}\,\mathrm{rad},\qquad
\sigma_y\sim0.149\,\mathrm m.}
$$
The aperture should therefore be enlarged \b[vertically]. A full width of order $0.30\,\mathrm m$ covers $\pm1$ characteristic width; approximately $0.90\,\mathrm m$ covers $\pm3$ and gives containment comparable with the preceding X-ray estimate in the same Gaussian model. The original $10\,\mathrm{mm}$ width would pass only roughly $\operatorname{erf}[0.005/(\sqrt2\,0.149)]\simeq2.7\%$ in that model. Equivalent optics could instead reduce the spot at a relocated aperture, at the expense of angular acceptance. These estimates concern geometric acceptance; THz coherence and bunch structure require further beam data.
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