They are looking for gamma radiation which is a result of certain decay chains. So it is a passive detection device, a gamma ray is very energetic photon.
That is what a rational person would use, but a rational person wouldn't be checking for radiation in electronics from Fukushima. Any radiation emission was extremely confined, there were no plants in the areas that possibly could have been exposed (The closest center of semiconductors to that locale is at Tskuba), even if there where production that was exposed they wouldn't be operational. Inputs into semiconductor plants are extremely pure. etc. etc.
Because you're dealing with an irrational actor, there is no possible way that we could know what type of detector they might decide is appropriate. And indeed they may take the shipment apart and ESD everything or they might be using some "special" machine that some charlatan has sold them, which might damage the chips.
It's best you ask them what they are using. It would be nice to know what country this is.