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Appendix A â Underground Nuclear Test Diagnostics This section briefly summarizes the diagnostic methods used for underground nuclear tests that produced the bulk of the archival data 7 . Two classes of diagnostics are covered: prompt diagnostics and radiochemistry. Prompt diagnostics as the name implies measure the immediate behavior of the nuclear weapons at the time of the explosion. They monitor the gamma ray, neutron, and X-ray flux produced by the nuclear reactions within the primary and secondary of the tested weapon. Three different measurements are considered here: reaction history, neutron flux (NUEX and PINEX), and neutron output from the DT fusion reactions (THREX). ï· Reaction history diagnostics monitor the gamma ray flux arising from fission or the interaction of neutrons with other elements. The time history of these data provides a measure of alpha, the quantity that characterizes the criticality of the weapon and other important aspects of weapon performance. ï· NUEX (neutron experiment) measures neutron output over time, which provides a time-of-flight inference as to the energy of the escaping neutrons in the field of view. A variation of this diagnostic is PINEX (pinhole camera experiment) which produces an image of neutron or gamma ray flux from a specific region of the device. PINEX can also be gated to provide a measure of neutron flux at particular energy levels. Usually the gates are set to observe 14 MeV neutrons so that the location and intensity of the DT reactions in the device can be determined. ï· THREX (threshold experiment) measures neutron output from DT reactions over time. The DT reactions produce 14 MeV neutrons and some of these escape the device and are detected by the THREX sensors. The neutron production rate is temperature dependent so this measurement can be used to infer the temperature of the reacting DT source. Radiochemistry measures the products of nuclear fission and reactions of the neutrons emitted from the device. lt differs from prompt diagnostics in that radiochemical data are not collected at the time of the explosion. Radiochemical tracers are placed at various locations in the device and recovered, along with samples of actinides and fission products, from core samples from the bomb residue after the test. Radiochemistry provides the most accurate means of measuring weapon yield. ï· The neutron flux from the explosion causes different radioisotopes to form from the original tracers. These radioisotopes have long decay times compared to the time needed to recover the tracers after the explosion. The recovered tracers then undergo chemical analysis to determine the relative abundance of the radioisotopes. Changes in this quantity, as a result of the explosion, give a measure of the time-integrated neutron flux at the original position of the 7 Â F.N. Mortensen, J.M. Scott, and S.A. Colgate, How Archival Test Data Contribute to Certification, Los Alamos Science 2 8(2003): 38-46. 11
radiochemical tracer. ï· A related diagnostic is the change in the ratio of plutonium isotopes as a result of the explosion. This ratio depends on the number of fission, capture, and (n, 2n) reactions that take place. lt informs inferences of efficiency (mass fission/total mass) and boost performance. 12
