Concrete bollards on wide footpaths. Spacious front yards. Penthouse apartments.
The xrays must come from a truck parked on the street, so maximise the distance from any truck to a target using architecture.
Bollards and big yards around a spacious mansion should keep any xray truck hundreds of meters away from the target.
If in a big city - put the targets home in a penthouse on top of skyscraper. It's hard to get a truck up there, and theres a few hundred floors to absorb any angled shots. Even if they get it in the building you only need to secure the top few floors - the metal or concrete of the floors will absorb huge amounts of radiation.
Use helicopters, tunnels, or just a lift down to a subway platform in order to avoid walking a street level during commutes.
Employ short, petite people
Your government employs the smallest people they can find. The energy absorbed from an xray at a given distance is proportional to surface area of the target, and skinny and short people have less surface area to hit.
They're also harder to aim at, making more work for the bad guys and requiring them to be closer to the xray machine - more on that later.
Find them using an ir camera - or look for missing snow.
Xrays penetrate a wall imperfectly, and some energy goes into heating it up. This should show up on a thermal camera as an ellipse or circle of yellow or orange, depending on the impact angle. Highly visible on the otherwise cold black wall.
A truck or room running an xray machine at these power outputs is going to also heat up from all that power. To find an xray truck in the middle of an attack, just look for the red coloured truck on your thermal camera.
In snow itd also be the truck with no snow on it, or if launched from a house itd be the house with no snow on its roof. Apparently this is how cops find cannabis installations in cold climates.
Look for hospital admissions for radiation sickness for those operating the machine.
Notice how the dentist leaves the room when using the xray machine? Even though its aiming at your face, that little bit of reflected xray can hurt the machine operator over time.
The bad guys running the machine / guarding it / aiming it at the target are going to get tiny doses of xrays themselves, and theyll need treatment for that eventually.