Radiative B-decay as a test of CKM unitarity

We point out that $R \equiv Br(b\rightarrow d \gamma)/Br(b\rightarrow s \gamma)$ is a sensitive probe of possible violation of CKM unitarity. We compute $R$ in a minimal extension of the Standard Model containing an additional isosinglet charge ($-1/3$) quark, which leads to a deviation from CKM unitarity.