The advanced experimental fuel counter (AEFC) is used for characterization of spent fuel from research reactors stored under water. The AEFC can be operated in either passive neutron mode or active neutron mode, using an AmLi neutron source to generate fission in the fuel item. Radiation tolerant 3He tubes are embedded in the moderator, forming two different measurement sets. One set (the inner row) measures neutron coincidences to distinguish fission neutrons from background radiation. The second set (the outer row) is placed farther back within the polyethylene moderator, and its signal is approximately proportional to the fission rate in the fuel item.

(Source: IAEA Safeguards Techniques and Equipment)