conclusions of the chapter, and a reference list. This system of organization will allow the reader to select, within some limits, those parts of the text which are of specific interest to him without his having to read the entire book. The first buds of this book appeared several years ago; however, the support given in the last few years by the NRC Office of Standard Development through Mound Laboratory of Monsanto Research Corporation was essential to its relatively early conclusion. I would therefore like to express my thanks to Messrs. P. Ting, E. Selleck, and R. J. Jones of NRC. The patience and indulgence of Dr. W. Strohm, who monitored the contract for Mound Laboratory are deeply appreciated as well as his critique and comments. I would also like to acknowledge the help given to me in preparing this manuscript through discussion, written material, and photographs: the safeguards group (Ql) at Los Alamos Scientific Laboratories was very helpful, especially Drs. R. Walton and H. Menlove.

Detection and interdiction of nuclear materials in all forms of transport is one of the most critical security issues facing the United States and the rest of the civilized world. Naturally emitted gamma rays by these materials, while abundant and detectable when unshielded, are low in energy and readily shielded. X-ray radiography is useful in detecting the possible presence of shielding material.Positive detection of concealed nuclear materials requires methods which unequivocally detect specific attributes of the materials. These methods typically involve active interrogation by penetrating radiation of neutrons, photons or other particles. Fortunately, nuclear materials, probed by various types of radiation, yield very unique and often strong signatures. Paramount among them are the detectable fission signatures, namely prompt neutrons and gamma rays, and delayed neutrons gamma rays. Other useful signatures are the nuclear states excited by neutrons, via inelastic scattering, or photons, via nuclear resonance fluorescence and absorption.The signatures are very different in magnitude, level of specificity, ease of excitation and detection, signal to background ratios, etc. For example, delayed neutrons are very unique to the fission process, but are scarce, have low energy, and hence are easily absorbed. Delayed gamma rays are more abundant but "featureless," and have a higher background from natural sources and more importantly, from activation due to the interrogation sources.The prompt fission signatures need to be measured in the presence of the much higher levels of probing radiation. This requires taking special measures to look for the signatures, sometimes leading to a significant sensitivity loss or a complete inability to detect them.Characteristic gamma rays induced in nuclear materials reflecting their nuclear structure, while rather unique, require very high intensity of interrogation radiation and very high resolution in energy and/or time.The trade off of signatures, their means of stimulation, and methods of detection, will be reviewed.Index Terms-Delayed neutrons and gamma rays, neutron fission and photofission, nuclear fission detection, prompt neutron and gamma rays, SNM detection.

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