Prompt gamma ray neutron activation analysis

Glascock, M.D., Spalding, T. G., Biers, J. C., and Corman, M.F. (1984). Analysis of copper-based metallic artefacts by prompt gamma-ray neutron activation analysis. Archaeometry 26 96-103. 

The safe and verifiable disposition, either by incineration or chemical neutralization of chemical warfare (CW) agents requires correct a priori identification of each munition or container to be processed. A variety of NDE techniques have been used or tested for the examination and characterization of munitions.[l,2] In the U.S., three widely used techniques are X-ray radiography, acoustic resonance spectroscopy (ARS), and prompt gamma ray neutron activation analysis (PINS). The technical bases, instrumental implementations, and applications of the U.S. versions of these methods are briefly discussed. 

A conveiuent suite of NDE techniques is available for the characterization of muiution fill. Large numbers of similar stockpile munitions can be reliably and quickly classified using Acoustic Resonance Spectroscopy (ARS). Measurement times are about one minute per item. However, ARS measurements require a comparison template and are less usefiil for assay of old, rusted, and one of a kind munitions. For munitions of this type, x-ray radiography and assay by prompt gamma ray neutron activation analysis can reliably determine the munition fill, and ensure that the items are properly scheduled for destruction. 

With respect to the time of measurement, NAA falls into two categories (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where the measurements follow radioactive decay. The latter operational mode is more common. 

PGNAA Prompt gamma-ray neutron activation analysis... 

Prompt gamma ray neutron activation analysis (PGNAA) is a proven method for the identification of chemical warfare agents, which is widely used as a nondestructive technique. 

This can result in a radioactive product from the A(n, t)A reaction where A is the stable element, n is a thermal neutron, A is the radioactive product of one atomic mass unit greater than A, and y is the prompt gamma ray resulting from the reaction. A is usually a beta and/or gamma emitter of reasonably long half-life. Where access to a nuclear reactor has been convenient, thermal neutron activation analysis has proven to be an extremely valuable nondestructive analytical tool and in many cases, the only method for performing specific analyses at high sensitivities... 

The most utilized methods include X-ray fluorescence (XRF), atomic absorption spectroscopy (AAS), activation analysis (AA), optical emission spectroscopy (OES) and inductively coupled plasma (ICP), mass spectroscopy (MS). Less frequently used techniques include ion-selective electrode (ISE), proton induced X-ray emission (PIXE), and ion chromatography (IC). In different laboratories each of these methods may be practiced by using one of several optional approaches or techniques. For instance, activation analysis may involve conventional thermal neutron activation analyses, fast neutron activation analysis, photon activation analysis, prompt gamma activation analysis, or activation analysis with radio chemical separations. X-ray fluorescence options include both wave-length and/or energy dispersive techniques. Atomic absorption spectroscopy options include both conventional flame and flameless graphite tube techniques. 

Figure 1. Histogram of detection limits for neutron activation analysis (NAA) and neutron-capture prompt gamma-ray activation analysis (PGAA). Data from Reference 15.

Neutron-capture prompt-gamma ray activation analysis (PGAA) is a recent addition to the nuclear analytical arsenal. In this technique the instantaneous gamma ray emission from a sample is measured as it is irradiated in a flux of reactor neutrons (33,3, 35). Because the sample must be several meters from either the core of the reactor or (less commonly) from the detector, the sensitivity of this technique is generally poorer than in conventional NAA. However, it is possible to measure small quantities of many elements which do not give radioactive neutron-capture products, notably 0.01 mg of H, 50 ng B, and 1 mg P in an electronics context. 

Two elemental analyzer systems have been developed, the "Continuous On-line Nuclear Assay of Coal", CONAC, (Science Application, Inc., Palo Alto, CA) and "The Elemental Analyzer" (MDH Industries, Inc., Monrovia, CA). Both of these units are based upon the measurement of prompt gamma rays that are emitted from a nucleus following the capture of a neutron. This technique is commonly known as prompt neutron activation analysis, PNAA. 

Neutron activation analysis (NAA) is the general term used to describe a nuclear-based technique in which a solid or liquid sample is irradiated with neutrons. Capture or absorption of a neutron excites the nuclide that returns (promptly or after a delay) to ground state by emission of an energetic photon (gamma ray) and/or other particles from the nucleus (Figure 1.20). 

Prompt gamma neutron activation analysis is a process similar to INAA that was discussed in Section 3. While INAA uses the radioactivity emitted by the activation products for analysis, PGNAA uses the prompt gamma rays emitted during the neutron capture. 

Neutron activation analysis (NAA) can be accomplished by measurement of the prompt gamma rays during irradiation or by measurement of the delayed... 

Prompt activation analysis (Erdtmann and Petri, 1986 Alfassi, 1990) uses the prompt radiation accompanying a nuclear reaction for determining elemental or isotopic concentrations. The variety of prompt methods is large because a sample can be irradiated with various particles - neutrons, charged particles or gamma-rays. Prompt activation analysis permits the determination of several elements - about 17 elements in environmental matrices (Germani et al., 1980) - but most analysis are used for the determination of light elements (H, He, Li, B, C, N, Si, S, Cl) as well of Cd and Gd. 

The work of the author on the analysis of Mesoamerican bronze has already been mentioned, but in several other cases the non-destructive or small scale of sampling came into play. Gilmore and Ottaway discuss this point in the work of Thompson and Lutz larger artifacts of bronze were also studied Finally, it should be mentioned that other methods involving neutron reactions have also been used in the study of ancient bronze. For example, Li Hu Ho has published the analysis of a 4000-year old Qi culture bronze mirror Similarly, Gilmore has employed epithermal neutron activation. Glascock et al. have demonstrated the utility of prompt (capture) gamma rays in the non-destructive analysis of copper-based artifacts the method is not however sensitive to trace elements. ... 

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