No. MEMS means electromechanical, and scintillation needs solid state: a crystal and a photon detector of some kind. The crystal can't be miniaturized because there's a minimum thickness you need to capture the photon and Compton electrons*.

You're right, I stand corrected.

This page [0] has an image of a small scintillator plus photomultiplier, but probably too thick for a phone.

[0]https://scan-electronics.com/en/dosimeters/radiacode-101

Although some weird Japanese phone makers did have smartphones with radiation detector built-in (SHARP 107sh and 205sh), I don't think it would be a popular feature.

There isn't so many cases in daily life that one can encounter a radioactive source. Even if you are actively looking for one (don't do it), you will have a hard time if you don't work with radioactive sources occupationally.

Also, it's not physically possible to create a small and yet sensitive radiation detector.

There are radiation detection apps for cell phones that use the camera. Not very sensitive, but no new hardware is required.

If you're going to put increasingly obscure health sensors in your phone, I think air quality would come before radiation.