Pulsed elemental analysis with neutrons

The nanosecond pulsed beam with time gating at the detector and the associated particle method (APM) render the three-dimensional (3D) elemental analysis of solids possible (Overley 1987 Rynes et al. 1999). The APM is based mainly on the D-D and D-T reactions by the detection of He and He particles, respectively, emitted at 180° to the neutron direction. The 4-5 cm/ns travel time of the neutrons allows the imaging of the interrogated volume along the direction of the ns pulsed neutrons with a spatial resolution of 5 cm. Some 2D-3D fast neutron imaging principles and techniques are summarized by Gozani (1994), Mikerov et al. (1998, 2001), and Chen and Lanza (2001), while typical thermal neutron radiography systems are demonstrated by Balasko et al. 1998, 2001) and Shaikh et al. (1998, 2001). 

This technique is extremely sensitive for many elements. But for lead, it is not as sensitive as more conventional chemical (instrumental) methods. The interference free detection limit using irradiation with a neutron flux of 10 neutrons/cm -sec for 1 hour is only as low as 2 /ig (G9). By reactor pulse analysis, as little as 0.5 ng can be detected. These sensitivities are satisfactory if sufficient sample is available. This technique appears to offer little advantage, and, in view of the relative unavailability and the expense of facihties, it will not find much use for clinical lead analysis. 

Analysis of the 90-element fuel storage container In use at Sandia Laboratories annular core pulse reactor (ACPR) was performed to characterize its subcritical multiplication properties under full and partial loading conditions. Neutron transport calculations were performed with the DTF-IV code (Sjf transport theory) and 16 energy-group cross sections. Experimental verification exists for some of the calculated arrays but the limited number of ftiel elements available (40) prevented a fill loading of the storage container. 

In recent years a number of other non-intrusive techniques for determining the contents and status of unopened munitions have been developed. These include neutron interrogation techniques such as neutron-induced prompt photon spectroscopy and hydrogen concentration measurement, and ultrasonic pulse echo and acoustic resonance techniques. Equipment based on the use of neutron-induced prompt photon spectroscopy has been fielded and successfully used by US EOD teams on many occasions over the last two years. This equipment, which is based on the analysis of photons emitted as the result of the interaction of neutrons with the chemical elements present in the munition and its contents, makes it possible to identify the presence or absence of key elements such as phosphorus, sulphur, chlorine, fluorine and arsenic within the munition. Clearly this information not only helps to identify the presence of a chemical fill, but in many circumstances also means that it is possible to identify the fill. 

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