There's a medical test called a PET scan, with PET standing for Positron Emission Tomography. Yes, positron as in antimatter. How it works is the subject is injected with a radioactive substance that decays into antimatter. That antimatter reacts with the normal matter in the subject's body releasing photons. Those photons are then observed by the scanner which builds up a useful picture from them.

Now, I'm not sure how much antimatter is generated in the subject's body, but it's a hell of a lot more than a single positron. And it all annihilates within the subject who doesn't notice a thing. So a single positron is not going to generate much of a bang.

you'd need several dozen, maybe a hundred uranium nuclei doing fission at once to knock over a coarse grain of sand.