Neutrons have been utilized for several decades to measure elements. In oil exploration, the carbon/oxygen ratio (C/O) is a measure of oil saturation ^[i]. In the coal industry, elements such as sulfur and chlorine are routinely measured with neutron interrogation ^{[ii] , [iii]} In the airline industry, the inspection of checked luggage for high explosives has been proposed through the use of neutrons for the identification of the nitrogen content within a piece of luggage ^[iv]. Neutron-based systems have also been proposed for the detection of narcotics and other contraband ^[v] , ^[vi] by the measurement of C/O.

The physical principles that all these methods are based upon have been established for a number of years, and have been extensively used by nuclear physicists and chemists for the investigation of nuclear structure.

• Figure 1. Nuclear reactions initiated by thermal neutrons (capture reactions) and fast neutrons (inelastic scattering).

In principle, a neutron impinging on an object can initiate one of several nuclear reactions with the chemical elements of which the object is composed (Figure 1). In most of these cases, as a result of these reactions, γ-rays are emitted with characteristic and distinct energies. These γ-rays are like the “fingerprints” of the elements contained in the object. By counting the number of γ-rays emitted with a specific energy (e.g. the γ-rays of sulfur), one can deduce the amount of the element contained within the object. In the case of an object that is hidden among other innocuous materials, the identification takes place through the correlation of various chemical elements observed, coupled to the information about the innocuous material itself.

Neutrons are highly penetrating particles. Their intensity is not diminished by the thickness of common containers. To a lesser extent, the outgoing γ-rays are also very penetrating, easily exiting the interrogated volume to be detected by an appropriate set of detectors placed outside the object. Thus, the method is non-intrusive (the interrogation can take place from a distance of several centimeters) and non-destructive because of the very small amount of radiation absorbed by the interrogated object.

Depending on the chemical elements that one wishes to measure, one might have to use neutrons of several energies. In many of the neutron-based applications currently in use, radioisotopic sources (Am-Be, ²⁵²Cf) are utilized for neutron production. These sources can excite a host of chemical elements (H, C, S, Fe, etc.) through neutron capture reactions. However, there are other elements such as C and O which need neutron energies several MeV higher than those available from the radioactive sources. To satisfy this, a neutron source is required that can produce the high energy neutrons for measurement of elements such as C and O, and low energy (0.025 eV) for elements such as H and Cl. It has been shown ^[vii], ^[viii] that such a task can be accomplished with the utilization of a pulsed neutron generator. This technique is called Pulsed Fast / Thermal Neutron Analysis (PFTNA).

The basis of PFTNA is a pulsed neutron generator utilizing the deuterium-tritium (d-T) reaction. The pulsed d-T neutron generator provides 14 MeV neutrons which in turn initiate several types of nuclear reactions ((n,n'γ), (n,pγ), (n,γ) etc.) on the object under scrutiny.The γ-rays from these reactions are detected by a suitable set of detectors (usually bismuth germanate (BGO) scintillators). During the neutron pulse, the γ-ray spectrum is primarily composed of γ-rays from the (n,n’γ) and (n,pγ) reactions on elements such as C and O, and is stored at a particular memory location within the data acquisition system. Between pulses, some of the fast neutrons that are still within the object lose energy by collisions with light elements composing the object. When the neutrons have an energy less than 1 eV, they are captured by such elements as H, N, and Fe through (n, γ) reactions. The γ-rays from this set of reactions are detected by the same set of detectors but stored at a different memory address within the data acquisition system. This procedure is repeated with a frequency of approximately 10 kHz. After a predetermined number of pulses, there is a longer pause that allows the detection of γ-rays emitted from elements such as Si and P that have been activated. Therefore, by utilizing fast neutron reactions, neutron capture reactions, and activation analysis, a large number of elements contained in an object can be identified in a continuous mode without sampling. Figure 2 shows the time sequence of the nuclear reactions taking place.

• Figure 2. Pulsed neutron generator time sequence.

PFTNA uses low resolution, high Z detectors such as bismuth germanate (BGO) or gadolinium ortho-silicate (GSO). Data analysis of the resulting γ-ray spectra is performed with the computer code SPIDER, a spectrum deconvolution code developed for the Windows 95/98/NT platforms ^[ix] . This code has been updated for use with the Windows 2000/XP platforms as well.

To use SPIDER, one must first measure the response of the detector in question to γ-rays from pure elements. For example, a block of pure graphite is used to determine the detector’s response to the C γ-rays. To determine the detector’s response to elemental H, a response is measured from a water sample.

In the absence of any sample placed in front of the detector, the detector records γ-rays emanating from the materials surrounding the detector, as well as from the materials inside and around the neutron generator. This spectrum is called the background spectrum. The counts in the i^th channel of the spectrum of a sample, S, can be represented by the equation:

    (1)

where B_i is the background spectrum at the i^th channel and k is its coefficient, E_i,j is the response of the j^th element at the i^th channel and c_j is its coefficient, and n is the total number of elements utilized to fit the spectrum. SPIDER employs a least-squares algorithm to fit Equation (1).

Primarily, identification of a substance by PFTNA is performed through examining the atomic ratios, e.g. the ratio of carbon atoms to oxygen atoms (C/O). The measurement of C/O is performed by taking the ratio of the intensities for carbon and oxygen γ-rays and then applying the ratio of the (n,n'γ) cross-sections for these elements.

In previous papers ^{[x] , [xi]}, the accuracy and precision of a measurement with PFTNA was examined. It was found that measurements could be performed with a PFTNA-based system as quickly as 30 s. However, more precise results are achieved at 300 s. The time of 300 s seems to be optimal since measuring for longer periods does not significantly reduce measurement uncertainties. Also, the measurement uncertainty is not affected greatly by the acquisition time of the background spectrum used in Equation 1.

The precision of the PFTNA-based system configured with a BGO detector was found to be about 5%. The accuracy with this detector ranged between 2 and 12%. This accuracy and precision is equivalent to those published in Reference 11 that utilized a GSO detector. The advantage in using BGO instead of a GSO detector is its lower price and higher efficiency.

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