Sensors are often utilized within the field of bomb disposal as a means to locate explosives such as C4. The sensors come in a variety of functions, such as detecting ionized leaks, decorative substances, explosive charges via neutron emission, or, albeit most unreliable, metal.
Because explosives of any type often release their own vapor particles via leaks in the sealment of the explosives, a special type of bomb-detecting sensor, called an Ion Mobility Spectrometer or more informally "Sensor A," was developed to "sniff out" leaked particle clusters of C4 and other explosives via a method based on ion mobility spectrometry technology. Because of this, their detection is often marked as a cloud on radar to depict the general area of the explosives.
Sensor B is a type of bomb-detecting sensor developed and utilized in the event that the explosives were securely sealed to prevent leaks, utilizing the combined use of a neutron scintillator and a hydrogen bomb detector. Instead of detecting any ionized particles released from the explosives, Sensor B detects the explosives themselves via neutron emission and picking up rear diffusion, due to neutrons interacting with the hydrogen material inside the explosive. When picking up one of the rear diffusions, the sensor will make a ping sound alerting that an explosive is nearby, to which the pings will arrive in shorter intervals the closer it is to the explosive's location.
During the Big Shell Incident in 2009, Peter Stillman had brought with him some Sensor A units that he intended to supply with SEAL Team 10's Bravo Squad to track down C4 explosives set up by Dead Cell member and former protege Fatman. However, due to SEAL Team 10's demise at the hands of Fortune, he ended up supplying Raiden and Iroquois Pliskin with two Sensor A devices, along with two coolant sprays as compensation for the recent lack of manpower in the disposal effort. In addition to the ionized leaks prevalent in the C4 bombs, it was also set up to also detect cologne used by Fatman on his C4 explosives as his modus operandum. Because they were set up to specifically locate Fatman's explosives, they would not detect any C4 explosives planted by either Pliskin or Raiden.
Stillman also spent some time building two Sensor B units in Strut C's pantry, while Pliskin and Raiden were busy disarming Fatman's explosives, in case it turned out Fatman had something extra planned. His suspicions were later confirmed by Pliskin when he stumbled upon the C4 charges in Strut H that were both securely sealed and lacked cologne. Raiden later used it in an attempt to locate the C4 explosive at the Deep Sea Dock in Strut A, after the sealed C4 had been activated due to the decoy C4's deactivation. It only picked up Fatman's C4, namely because it was slightly different from regular C4 due to Fatman adding a large amount of wax.
Raiden later utilized Sensor A again after Fatman began setting up explosives on Strut E again, and continued to use it when disarming one of his bombs as well as during his fight with Fatman until the latter ended up killed and his ultimate C4 package disarmed.
Behind the scenes
Sensor A and Sensor B are items that appear in Metal Gear Solid 2: Sons of Liberty. Sensor A displays the scented C4's general location as a yellow cloud on the Soliton Radar, while Sensor B emits a beeping that increases in frequency as the player gets closer to the odorless C4.
- "Ion mobility spectrometer explosive detector. Activates when equipped. Displays Fatman's scented C4 on radar."
- ―Sensor A description in Metal Gear Solid 2: Sons of Liberty
- "Neutron scintillator explosive detector. Beeps when detecting Fatman's odorless C4."
- ―Sensor B description in Metal Gear Solid 2: Sons of Liberty