geometry is a distinct measurable human characteristic which is unique to
Materials Access Control. Control of materials entering or exiting
a building includes special nuclear materials (SNM), metal detectors,
explosives, and packages. Detailed information for the following can be
found in SAND 87-1927, Entry Control Systems Technology Transfer Manual.
Special Nuclear Materials. Monitors are utilized to detect
concealed SNM on persons, in packages, or in vehicles exiting a controlled
area. An SNM monitor generally consists of a Thermal Neutron Activation
detector unit, signal processing electronics, and alarm logic. Thermal
neutrons are used to irradiate containers when a search is being made for
concealed SNM. The emitted neutrons and gamma rays are coincidentally
detected by scintillator sensors to discriminate against source neutrons and
gamma radiation. Detection is based on the high nitrogen contents of
explosives. There are a number or SNM monitors available. Examples include:
a) Doorway monitors. Most commercially available SNM
monitors are doorway monitors. While they provide high throughput,
sensitivity is reduced.
b) Portal monitors. Portal monitors detect the presence
of SNM within a detection volume that is usually much larger than that of
doorway monitors. As a result, throughput times are generally longer than
for doorway monitors.
c) Hand-held monitors. Los Alamos Scientific Laboratory
(LASL) has developed a hand-held SNM monitor for manual and vehicle search.
This monitor has the advantage of significantly lower cost compared to
doorway monitors; however, it has the disadvantage of long search times.
d) Vehicle monitors. The primary difficulty in detecting
SNM at vehicle entry control portals is the ease with which SNM can be
shielded within a vehicle. At present, the most practical method of
monitoring vehicles for SNM is a search conducted by guards equipped with
e) Sewer monitors. Lawrence Livermore Laboratory has
developed a sewage monitor system that detects the deposition of radioactive
material into a sewage system. The systems are not limited to sewage, but
also can be used with other liquid effluents.
Metal Detectors. Metal detectors are used to detect weapons
and hand tools intended for sabotage, or for the detection of metal used to
shield SNM. Active metal detectors generate a time-varying electromagnetic
field and respond to changes in the field caused by the introduction of
metallic objects. Such changes are used to generate alarms.