Applications

The basis for elemental analysis using neutron generators is spectral analysis of gamma rays produced by nuclei which have been activated by absorption of neutrons.  There are three major types of nuclear reactions that are used in elemental analysis.  When induced by fast (14 MeV) neutrons, gamma rays from these three types of reactions have different temporal characteristics.  This fact may be exploited by using pulsed neutron generators and appropriate timing of detector circuitry.

1.  Prompt Inelastic scattering reactions (n, n’g) produce gamma rays when a neutron is scattered by a target nucleus.  There is no delay in the emission of the gamma ray.  The gamma ray energy is characteristic of the scattering nucleus.  These reactions are particularly useful for detection of C, N, O, Mg, Al, Si, S, and Ca.

Inelastic scattering reactions are the basis for many practical applications such as

• Bulk Materials Analysis               
• Detection of Explosives               
• Land Mines, Chemical Weapons, and Illicit Drugs               
• Oil Well and Minerals Logging
• In-Vivo Measurement of Body Composition

Pulsed neutron generators operating at approximately 10,000 Hz pulsing frequencies are suitable for these measurements.  The spectroscopy system is gated to accept only those gamma rays produced during the neutron pulse(s).  The detectors are gated off between neutron pulses.

Inelastic scattering reactions are also the basis for elemental imaging using the Associated Particle Imaging (API) technique.  When 14 MeV neutrons are produced by the D-T reaction, an associated 3.5 MeV alpha particle is emitted that is correlated in time and space with the neutron.  By detecting the correlated alpha particle, the direction of the associated neutron can be localized in a narrow “beam”.  Neutrons in the “beam” interact through prompt inelastic scattering (n,n´g reaction) with the elements in the material under investigation and emit gamma rays.  A time of flight system determines the time between the detection of the alpha particle and the recording of the gamma ray by a photon detector such as a NaI scintilator.  This time difference determines the distance traveled by the neutron before interacting with the element and producing the gamma ray.  Since the gamma rays are characteristic of individual elements, both the chemical composition and spatial distribution of the elements are simultaneously generated.  Since the correlated neutron “beam” can be tracked isotropicly, a three dimensional image can be generated.

Detection of the associated alpha particle requires that an alpha detector be built into the neutron tube.  This places stringent requirements on the alpha detector material in that it must withstand the temperatures used in processing the neutron tube without introducing contaminants into the tube.

2.  Gamma rays are produced by radiative capture reactions (n, g) when a fast neutron is thermalized and absorbed by the target nucleus.  The time characteristic of the appearance of these gamma rays is the time necessary to thermalize fast neutrons from the generator.  Radiative capture reactions are useful for detection of H, Cl, Al, P, S, Ca, Fe, Pb, B and many other elements.

Analyses based on gamma rays from radiative capture reactions are performed using a pulsed neutron source and gating the spectroscopy system to accept signals produced after the pulse(s) for a time period characteristic of the neutron thermalization time of the matrix of interest.  Pulse frequencies of ~1,000 Hz are typical.  Techniques based on radiative capture reactions are widely used in the oilfield service industry to measure the position of petroleum/water interfaces in producing wells.  This measurement depends upon the fact that a neutron burst decays much more rapidly in saline waters than in petroleum.  

3.  Reactions that produce long half life activation products (a few seconds or more) are the basis for fast neutron activation analysis (FNAA).  The most important application is the analysis of oxygen content in a wide variety of matrices including metals, geologic materials, coal, liquid fuels, and ceramic materials.  Determination of nitrogen content in biological materials, including nitrogen as a measure of protein content as well as nitrogen determination in fertilizers, explosives, and polymers are important applications.  Other elements that are routinely analyzed by FNAA include P, F, Cu, Ag, Mg, Al, Si, Mn, Fe, Zn As, Sn, and Pb.

Fast neutron activation analysis is normally done using high yield, continuous output neutron generators such as the A-711.  However, both short and long half life activation products may be analyzed using pulsed neutron generators and gating the spectroscopy system to reject signals from prompt inelastic scattering and radiative capture reactions.