On this page of the Mr Prepper guide, you will learn where to find a Geiger counter necessary for completing one of the missions for the Minuteman.
You'll find the Geiger counter in Old Joe's Junkyard. You need the counter to complete one of the missions for Minuteman – he will ask you to bring it. This is on of the main mission in Mr Prepper.
To get to the Junkyard, you have to bring Bob his jacket from the mine. Joe doesn't show up at the location until after 5:00 p.m. You'll find the counter on his desk, next to the radio. If he is not there, come back here the next day (you can not wait for Joe to come - visit the location later, when Joe is already there).