This device does not detect radiation outside the box. You have to put the object you want to measure radiation from inside the box, close to the sensor.
It's fine for what it is. But I think a particle detector that most people imagine is really something you want to be able to hold portably in the air and detect ambient/impinging radiation from unknown sources, rather than have to put samples in a closed container.
If there were some modification that made that possible (getting around the light-tight requirement), that would make this incredibly more useful.