A gauge is provided for measuring one or more characteristics of a construction material such as a road surface. The gauge includes a detector, a base that carries the detector, and a source housing carried by the base and defining a shield material circumferentially extending inwards. A source rod is positioned within the housing and carries a source that is translatable between a shielded position within the housing and a measuring position external of the housing. The source rod has a source shield on the top thereof and a shield material spaced-downwardly from the source such that the source is completely enclosed when contained within the base.