 Solution

Below the critical photon energy, radiation is emitted over a wider angular cone. The usual low-frequency characteristic-angle estimate isup to an order-one angular-width convention. At $\epsilon =0.012\mathrm{eV}$ this givesThe aperture should therefore be enlarged vertically. A full width of order $0.30\mathrm{m}$ covers $\pm 1$ characteristic width; approximately $0.90\mathrm{m}$ covers $\pm 3$ 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 $\mathrm{erf}[0.005/(\sqrt{2}0.149)]\simeq 2.7\%$ 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.