Abstract :
“Tom Thumb” Neutron “Detective.” Splitting atoms of uranium, placed on a sensitive semiconductor no larger than a pinhead, make up the smallest known neutron detector. Recently developed by Westinghouse atomic scientists, it is designed to probe deep inside a nuclear reactor core and map the neutron flux — the most basic piece of information needed for proper reactor design. Its tiny size permits “looking” into tight, out-of-the-way spots, such as the spaces between close-fitting core fuel plates. Presented before a meeting of the ANS by Dr. K. H. Sun, advisory physicist in the materials engineering department in Pittsburgh, Pa., the detector was said to have “potential significance comparable to conventional gas or scintillation types of detectors” now in common use. It consists of a tiny slice or silicon of germanium having a sensitive junction or layer near the top surface of the slab and is known as a semiconductor diode. It is commonly used in such devices as rectifiers and solar batteries. The detector has a deposit of uranium on it about a millionth of an ounce. Hair-thin copper wires are soldered to it and then it is sealed inside one end of a thin aluminum tube about the size of a piece of spaghetti. The detector is wired to suitable electronic amplifyng and recording apparatus and is ready to be inserted into the reactor core. Slow-speed neutrons strike this uranium surface and cause U-235 atoms to split apart thus producing “fission fragments — fast charged nuclei of atoms that burst into being as the uranium nucleus falls apart.” The energized particles crash through the diode detector junction and upset its electrical balance. This results in an electrical pulse that can be detected and counted.
