What is passive gamma ray emission? I was trying to find the meaning of passive gamma-ray emission through the internet. I haven't found any helpful article except some research paper just denoting the word passive ray emission. They also didn't define the meaning. When I searched for the picture it showed a graph with the number of counts vs energy that didn't dictate any information why it was passive. And what were the criteria for being an active gamma-ray emission? so it would be helpful if someone can explain it.
 A: I found this:

The use of passive gamma-ray and neutron detection and imaging technologies in security-sensitive areas and ports has had more impact than most other techniques in detecting and deterring illicit transportation and trafficking of illegal radioactive materials.

So it is a term used in detectors

Passive Gamma-Ray and Neutron Imaging Systems for National Security and Nuclear Non-Proliferation in Controlled and Uncontrolled Detection Areas: Review of Past and Current Status

From glancing through  the article I conclude that the term "passive radiation" and "active radiation" comes from the filters used for detecting the radiation, which are called "passive" and "active"

There are generally two types of coded apertures: passive masks and active masks. In the case of passive masks, a highly absorbing material is used to stop and eliminate non-normally directed radiation quanta from reaching the detector.

These masks are used so that the detectors work efficiently to detect ambient radiation.

Passive physical collimation shows noticeable drawbacks over a considerable range of the energy spectrum, especially at high energies where radiation fields have enough energy to penetrate the opaque pattern of the mask . On the other hand, active coded aperture designs use radiation sensitive materials, such as B- and Gd-doped glass plates for detecting low energy neutrons, as part of the collimation and detection process, which allows the detection of radiation quanta with a wider energy range.

