Pulsed Neutron Activation Technique

The PNA technique is not commonly used, because it involves radioactivity. In essence, the PNA technique is an on-line tagging method. Short bursts of 

The overall scheme of the ANL PNA system (Porges et al., 1984) is shown in Fig. 6.25. An RF ion source is used to generate a 150-keV deuteron beam as a burst of deuterons strikes a tritiated foil, 14-MeV neutrons are produced and fan out into a 4n solid angle. A copper/lead collimator concentrates forward neutrons onto the pipe that passes directly in front of the target snout. Two downstream y detectors (Nal/Tl crystal scintillators) are used to register the counts. 

Typical records obtained from slurries are shown in Figs. 6.26 and 6.27. The variation of the signal profile can be explained by different flow models (Porges, 1984). The averaged flow velocity can be derived from the PNA signal recorded by the detector at a known distance downstream from the PNA source. The PNA velocity results agreed with timed-diversion measurements to within 0.5% and no systematic deviation was found. We believe that the accuracy achieved by the PNA technique can be better than that obtained by other techniques because the PNA system irradiates the entire duct, and the shape of the readout is directly related to the motion of the tags. 

A solid/gas flow instrument seldom measures mass flow rate directly instead, it generally measures the volumetric flow rate of solids, which includes measurements of solids velocity and concentration. Every sensing technique basically responds to variations in solids concentration measuring solids velocity again requires two sensors separated by a known distance so 

FIGURE 6-28 Typical solids distribution and velocity profiles of roping type flow over the pipe cross section. (Source Yan, 1996) 

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A laboratory technique that is commonly used to calibrate particle velocity is the tracer technique. The tracers can be optically illuminating particles or short-lived radioactive tracers. We will briefly discuss two techniques that can be used to produce radioactive tracers on-line pulsed neutron activation (PNA) and off-line irradiation. 

Other techniques that have been used include subtractive differential pulse voltammetry at twin gold electrodes [492], anodic stripping voltammetry using glassy-carbon electrodes [495,496], X-ray fluorescence analysis [493], and neutron activation analysis [494],... 

Coincidence techniques have also been used for Compton interference reduction in the use of large volume Ge(Li) detectors together with plastic scintillator anticoincidence shields 70), In some cases it might be desirable to use the coincidence electronics to gate the multichannel analyzer to accept only non-coincident pulses. In 14 MeV neutron activation procedures the annihilation radiation resulting from the decay of 13N produced indirectly from the carbon in the plastic irradiation unit may be discriminated against by gating the analyzer to accept only non-coincident events. 

As mentioned before, two interlaboratory studies were organised prior to certification, involving ca. 15 laboratories using techniques such as cold vapour atomic absorption spectrometry, direct current plasma atomic emission spectrometry (DCP-AES), differential pulse anodic stripping voltammetry (DPASV), microwave plasma atomic emission spectrometry (MIP-AES), electrothermal atomic absorption spectrometry (ETAAS) and neutron activation analysis with radiochemical separation (RNAA). 

Electrothermal vaporization isotopa dilution inductively coupled plasma mass spectrometry (ETV-ID-ICP-MS) has been utilized for the analysis of cadmium in fish samples. Radiochemical neutron activation analysis (RNAA), differential pulse anodic stripping voltametry (ASV) and the calorimetric dithizone method may also be employed. The AAS techniques appear to be most sensitive, with cadmium recoveries ranging from 94 to 109% (Koplan, 1999). 

Modem methods for study of metal-activated enzymes include NMR and ESR spectroscopy, water relaxation rates by pulsed NMR (PRR), atomic absorption, Mbssbauer, X-ray and neutron diffraction, high-resolution electron microscopy, UV/visible/IR spectroscopy, laser lanthanide pertubation methods, fluorescence, and equilibrium and kinetic binding techniques. Studies with Mg(II)-activated enzymes have been hampered by the lack of paramagnetic or optical properties that can be used to probe its environment, and the relative lack of sensitivity of other available methods initial velocity kinetics, changes in ORD/CD, fluorescence, or UV properties of the protein, atomic absorption assays for equilibrium binding, or competition with bound Mn(II) °. Recent developments in Mg and 0-NMR methodology have shown some promise to provide new insights . ... 

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