Portable nuclear device missing from stolen vehicle in Pennsylvania

For anyone wondering exactly what it is and contains, the linked press release seems specific:

> The nuclear density gauge is a Troxler Model 3440, serial number 31109. The gauge is yellow in color and about the size of a shoe box, with an electronic keypad and a metal rod extending from the top surface. The Troxler gauge contains approximately 8 millicuries of Cesium-137 and 40 millicuries of Americium-241. The radioactive material is in a double encapsulated source capsule within the device to protect its integrity.

I'm still a little curious how these work, it sounds like it can measure density somehow. Wouldn't it need a detector on the other side of the material or something?

> High levels of radiation exposure over a short period of time can cause nausea and vomiting.

I wonder why they cut off their list of effects there.