You're presumably thinking alpha particles if you're thinking of radiation which is stopped by, and therefore concentrates measured exposure at, your skin. I rather strongly doubt this machine uses alpha particles. Considerations which make me doubt this: they would get blocked by insulating materials such as e.g. the side of the surveillance van, the side of your vehicle, 3 cm of open air, etc.
No, X-rays have different penetrating power based on frequency. There using a back-scatter approach as it's just one van which means very low penetrating power. https://en.wikipedia.org/wiki/Backscatter_X-ray
