In this work, integrated silicon-photonic waveguides are exposed to ionizing radiation. A microbeam X-ray source was utilized to deliver an isolated dose to a waveguide that behaved as one branch of an integrated Mach–Zehnder interferometer (MZI) structure, such that any radiation-induced phase shifts can be directly measured. It is found that the silicon waveguides used in the present study exhibit a nonmonotonic change in the effective index of refraction,

$n_{\text {eff}}$

, for both

$O$

- and

$C$

-band operation due to radiation exposure.

$n_{\text {eff}}$

first decreased with increasing dose, which is attributed to native SiO2 growth in the silicon core. At larger doses, this trend reverses and the observed net change in

$n_{\text {eff}}$

eventually becomes positive. This increase in

$n_{\text {eff}}$

can most likely be attributed to radiation-induced densification of SiO2. Overall, it was found that these commercially available silicon waveguides are sensitive to doses above

$1\times 10^{{9}}$

rad(Si), and the nonmonotonic trends suggest that competing simultaneous mechanisms affect the waveguides under high dose levels.

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