Patent Number: 055725597
Section: summary

FIELD OF THE INVENTION This invention relates to a method and apparatus for performing radiography with high energy photons generated by activating water with 14-MeV deuterium-tritium (D-T) fusion neutrons via the .sup.16 O(n,p).sup.16 N reaction followed by the decay of .sup.16 N. More specifically, this invention involves a method and apparatus for studying thick dense objects which are not easily studied with lower energy X-rays or neutrons and which is capable of providing detailed information regarding the structure and composition of the object including the identification of such features as hidden holes and discontinuities in atomic number. BACKGROUND OF THE INVENTION The concept of using penetrating photons to examine the interior regions of objects that cannot be observed directly is about 100 years old. The revolutionary discovery of X-rays by Roentgen in 1895 led promptly to the development of non-destructive, non-invasive interrogation techniques applicable to various objects including the human body. Since the time of Roentgen, this method has developed enormously and now finds routine application in practically every aspect of modern life, e.g., manufacturing, construction, quality control, medicine, defense, transportation, security and basic and applied research. The fundamental principles of photon radiography are well known and widely described in the literature. The most widely used approach involves X-rays in the range of a few keV to several hundred keV that are produced at relatively low cost by electron bombardment of medium to high atomic number metals in sealed, evacuated X-ray tubes. While this approach is extremely versatile, there are limits based on the penetrating capacity of these photons and on attainable source intensities. Photons with higher energies and source intensities can be obtained from radioactive gamma-ray sources, e.g., .sup.60 Co (or .sup.137 Cs) and from electron accelerators such as linacs and synchrotons. Radioactive sources are difficult to handle and store safely. Also, the range of geometric configurations that are possible with these materials is somewhat limited, mainly due to safety considerations. Accelerator sources are capable of producing very high radiation intensities and relatively high photon energies, but like X-ray tubes, they involve continuous energy photon spectra. These machines are also generally rather costly to build and operate. Because photon transmission through matter is highly energy dependent, radiography with continuous energy sources generally suffers from lack of adequate contrast and the inability to select proper exposure. The present invention addresses the aforementioned limitations of the prior art by providing a radiographic method and apparatus which provides essentially monoenergetic, variable intensity, highly penetrating photons in an arrangement which is relatively inexpensive, safe and flexible in configuration for various applications. OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide one or more monoenergetic photon beams for use in the non-destructive, non-invasive analysis and testing of thick dense materials and objects. It is another object of the present invention to provide a photon source which is monoenergetic, of variable intensity, highly penetrating and is relatively safe and inexpensive to operate. Yet another object of the present invention is to provide a high energy photon source which employs the deuterium-tritium fusion reactor cooling process and does not present either chemical or radioactivity hazards. A further object of the present invention is to provide apparatus and method for determining the composition and structure of a solid object requiring only modest resolution, but substantial photon penetrating power and has the capability to contrast varying thicknesses of materials and elemental compositions, particularly for metals and higher atomic number materials. The present invention contemplates a method and apparatus for performing radiography with the high energy photons generated by activating water with 14-MeV D-T fusion neutrons via the .sup.16 O(n,p).sup.16 N reaction followed by the decay of .sup.16 N. More specifically, this invention involves a method and apparatus for performing scans of thick dense objects using highly monoenergetic photons produced by activating water with energetic neutrons. The apparatus thus includes a neutron source (normally a 14-MeV neutron generator), a sealed tube of rubber or flexible material in the form of a continuous loop, pure water which is placed inside the sealed tube for receiving the neutron radiation; a water pump; a water flow rate meter; a shielding and collimator system for forming the photon beam and a sodium iodide photon detector and associated electronics for detecting photons transmitted through the material or object being investigated; and for subsequently recording the signals. The water is continuously circulated between the region where it is bombarded with neutrons and becomes radioactive and the radiography portion of the system. The specific activity of the water (Curies per milliliter) depends upon the strength of the neutron field, the time the water spends in this field, and the transport time between the field region and the radiography portion of the system. In general, the intensity of the photon emission at the position of the radiography portion of the system depends on the water flow rate, the volume of water, the intensity of the neutron field and various geometrical factors. A portion of the water line is heavily shield, except for a collimator arrangement for forming the photon beam. The sodium iodide detector is also shielded and views the photon source through a similar collimator arrangement. The object or material to be studied by radiography is transported step-by-step through the gap between the photon source and the detector. The data recorded are photon transmissions, i.e., the ratio of incident photons per unit time and transmitted photons per unit time.