Neutrons are not emitted by most radionuclides that are used for medical or industrial purposes.
- Radiophamaceuticals and industrial radionuclides will set off gamma sensitive alarms. No legitimate reason to possess neutron source
- Detecting neutrons is usually unambiguous and indicates the presence of special nuclear materials.
VERY difficult to shield
- Shielding can even enhance the detection probability by increasing the thermalization and therefore the detection efficiency for thermal neutrons.
Neutron background is small and constant
- Neutron background rates throughout the world are remarkably constant.
- They vary from 0.007 n/cm2/sec at sea level to 0.014 n/cm2/sec near facilities like the Hanford Reservation in Washington State.
- These background rates are quite low compared to the expected count rates from small masses of metallic, oxide or weapons grade Pu.
Low false-alarm rate
- False positives are not acceptable when freight and people must be stopped and inspected when an alarm goes off.