Fast neutron-associated particle

Because the neutron direction is known, the FNAP approach does not require the use of coUimators to focus the incident beam and there is no need to pulse the source. However, as the neutrons are emitted in an essentially isotropic distribution, many neutrons still fail to impact the target bag and neutron shielding is needed in aU directions surrounding the source and bag regions. In addition, the scattering of the neutrons in the shielded material along with the resulting inelastic and capture 

The time resolution of the alpha and gamma correlated detection in FNAP is limited to about 1 ns. This results in a spatial depth resolution for the inelastic reaction of about 5 cm. The edge smearing from the deuteron spot size and neutron scattering within the bag similarly Hmit the resolution in the x and y directions. Thus, this approach typically has used a detection voxel of cm x 5 cm x 5 cm. 

Dead time considerations in the alpha particle detection limit the count rate, and hence limit the neutron flux that can be used with this approach. This means that large scan times will probably be required with most implementations of this approach. 

The incorporation of associated alpha particle detection in a sealed tube neutron generator (STNG) appears to severely aggravate the concerns over the limited neutron flux and tube lifetime previously detailed for STNG FNA approaches. A mean time to failure of some APSTNGs at a neutron flux of lO n/s is about 200 h [24]. Work is continuing to improve this mean time to failure. 

Signal-to-noise considerations make most neutron-based explosive detection approaches very difficult to implement. The basis for combining multiple detection approaches (FNA, along with thermal gamma detection and neutron transmission spectroscopy) in a FNAP application that preserves the small volume advantage of a APSTNG remains to be established. There are distinct advantages associated with the API approach, but the concomitant reductions in available neutron flux, issues of tube lifetime, and the intrinsic poor spatial resolution must be taken into consideration for potential applications. 

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FAST NEUTRON ACTIVATION ANALYSIS—John W. McKIveen SOLAR HEATING AND COOLING OF BUILDINGS—Richard S. Greeley ENVIRONMENTAL HEALTH CHEMISTRY—James D. McKinney ASBESTOS PARTICLE ATLAS—Walter C. McCrone CONTAMINANTS AND SEDIMENTS—Vol. 1 2—Robert A. Baker METHANE GENERATION RECOVERY FROM LANDFILLS—EMCON Associates... 

API, associated particle interogation FNA, fast neutron analysis PFNA, pulsed fast neutron analysis TNA, thermal neutron analysis. 

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). 

Frederick Joliot and Irene Curie discussed the "/-rays emitted in association with neutrons by berillium irradiated with a particles and reported to have observed under the same conditions also the emission of fast positrons. The origin of these particles was not yet clear at the time of the Solvay Conference. It was understood by the same authors a few months later when they discovered the artificial radioactivity induced by a-particle bombardment which normally takes place by emission of positrons. 

Atoms themselves are made up of even smaller particles. These subatomic particles are protons, neutrons, and electrons. Protons and neutrons cluster together to form the central core, or nucleus, of an atom. Fast-moving electrons occupy the space that surrounds the nucleus of the atom. As their names imply, subatomic particles are associated with electrical charges. Table 2.1 and Figure 2.2 summarize the general features and properties of an atom and its three subatomic particles. 

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