How to detect an atomic bomb (2)

Buried somewhere in the Nuclear Threat Initiative website is a readable summary called Detecting Nuclear Materials and Nuclear Weapons (see first 10 paragraphs or so). In particular, they discuss the tradeoffs of

1. active versus passive scanning
2. detecting highly enriched uranium (HEU) versus plutonium (Pu) and dirty bombs
3. Nuclear Emergency Search Teams (NEST)

What they say here is consistent with what I have found in other references.

Due to its low level of radioactivity, passive HEU detection appears to be significantly harder than detecting Pu or dirty bombs. The following references discuss specifically how HEU might be detected:

1. Uranium Attributes and Measurements by Lawrence Livermore National Lab
2. Emission and Absorption of Radiation in Science and Global Security.