Neutron Radiography and Neutron Activation Analysis Source

An RFQ linac structure coupled with either the RFD or RFI linac structure and integrated with an appropriate target and neutron moderator could form the basis for a neutron source for the Neutron Radiography non-destructive imaging technique or the Neutron Activation Analysis non-destructive testing technique.

Neutron radiography is a powerful non-destructive imaging technique for the internal evaluation of materials or components. It involves attenuation of a neutron beam by an object to be radiographed, and registration of the attenuation process (as an image) on film or video. Neutron radiography is similar in principle to X-ray radiography, and is complimentary in the nature of information supplied. The interactions of X-rays and neutrons with matter are fundamentally different, however, forming the basis of many unique applications using neutrons. While X-rays interact with the electron cloud surrounding the nucleus of an atom, neutrons interact with the nucleus itself. In general, as the atomic number increases, X-ray attenuation increases for lower X-ray energies and/or higher density materials. There is no such rule, however, for the attenuation of thermal neutrons. Some light elements (e.g. hydrogen and boron) have high thermal neutron attenuation coefficients, while some heavier elements (e.g. lead, iron, chromium, etc.) have relatively smaller attenuation coefficients.

The material or object to be radiographed is placed in a collimated beam of thermal neutrons. The neutrons are attenuated by elements in the object with high thermal neutron attenuation coefficients as the beam passes through. The remaining neutrons are then collected at the imaging plane. The result is a negative image of the object, with the light areas indicating the greatest neutron attenuation by the object, and the dark areas indicating the least neutron attenuation. The image may be created either through exposing high resolution photographic film (14" x 17") by means of a gadolinium converter screen, or through using a real time Thomson tube video camera.

Neutron Activation Analysis (NAA) is a sensitive analytical technique useful for performing both qualitative and quantitative multi-element analysis of major, minor, and trace elements in samples from almost every conceivable field of scientific or technical interest. For many elements and applications, NAA offers sensitivities that are superior to those attainable by other methods, on the order of parts per billion or better. In addition, because of its accuracy and reliability, NAA is generally recognized as the "referee method" of choice when new procedures are being developed or when other methods yield results that do not agree. Worldwide application of NAA is so widespread it is estimated that approximately 100,000 samples undergo analysis each year.

The basic essentials required to carry out an analysis of samples by NAA are a source of neutrons and instrumentation suitable for detecting the unique gamma ray signals emitted by the target material's irradiated nuclei.

There are a variety of NAA techniques, some of which depend on the energy of the source neutrons. Thermal Neutron Analysis (TNA) is a form of NAA utilizing low-energy neutrons (up to 0.5 eV). Epithermal Neutron Activation Analysis (ENAA) is a form of NAA utilizing medium-energy neutrons (between 0.5 eV and 0.5 MeV). Fast Neutron Activation Analysis (FNAA) and Pulsed Fast Neutron Analysis (PFNA) are forms of NAA utilizing high-energy neutrons (above 0.5 MeV).