A detection apparatus is usable to detect the neutron absorption capability of a control element of a nuclear installation and includes a neutron radiograph apparatus and a robot apparatus. The neutron radiograph apparatus includes a neutron emission source of variable strength, a detector array, a mask apparatus and a positioning robot all under the control of a central processor and data acquisition unit. The neutron emission source is advantageously switchable between an ON state and OFF state with variable source strength in the ON state, which avoids any need for shielding beyond placing the neutron emission source in an inspection pool at the nuclear plant site including but not limited to the spent fuel or shipping cask laydown pools. The neutron emission source is situated at one side of a wing of the control element and generates a neutron stream, the detector array is situated on an opposite side of a wing, and the neutron emission source and detector array are robotically advanced along the wing. The detector array is monitored in real time, and various masks of the mask apparatus can be positioned between the neutron emission source and the detector array to more specifically identify the position on the blade where the neutrons are passing through.