Active, nondestructive interrogation with neutrons and photons has seen a renaissance in recent years, owing to a broad spectrum of important applications in security, nuclear nonproliferation, contraband detection and materials analysis. Active methods are of high interest for such applications because they provide at least an order of magnitude greater sensitivity than passive methods. Accelerator-based neutron and photon active methods exploit two important factors to attain greater sensitivity: these are (i) the control of interrogating beam properties such as directionality, energy, intensity, polarization and the temporal distribution of radiation; (ii) well-founded, low energy nuclear physics that yields distinct "signatures" for elemental and isotopic content. This review addresses accelerator-based neutron and photon nondestructive testing methods and issues when applied to modern and emerging wide-ranging challenges in nondestructive detection.

The detection of high-energy prompt fission neutrons was investigated as a method of fissionable material detection. Neutron energy spectra of U238 and several nonfissionable materials were measured using a neutron time of flight spectrometer. The photonuclear reactions were induced in the targets using a pulsed bremsstrahlung beam for several endpoint energies between 8 and 15MeV. While fission neutrons can have energies in excess of 10MeV, neutrons emitted from nonfissionable materials have distinct energy limits that depend upon binding and incident particle energies. Hence the presence of high-energy neutrons can be used to discriminate fissionable from most nonfissionable materials.

At the Idaho Accelerator Center, we employed a pulsed bremsstrahlung photon beam generated from the 44-MeV, 1300MHz L-band electron Linac which was then directed onto a 99.9% natural magnesium target (isotopically 10% 25Mg). We investigated a method of measuring the 20.2ms half life of the photoproduced 24mNa isomer created in the process: γ+25Mg→p+24mNa. The telltale 472-keV gamma line was measured between the photon pulses with an HPGe detector shielded within a lead cave. We used an ion chamber for monitoring the photon beam and used a novel arrangement of Zr/Ni foils for ascertaining the angular distribution of the beam profile. We discuss our preliminary results and future plans for extracting photonuclear cross sections.

Radiation damage and other defect studies of materials are limited to thin samples because of inherent limitations of well-established techniques such as diffraction methods and traditional positron annihilation spectroscopy (PAS) [P. Hautojarvi, et al., Positrons in Solids, Springer, Berlin, 1979, K.G. Lynn, et al., Appl. Phys. Lett. 47 (1985) 239]. This limitation has greatly hampered industrial and in-situ applications. ISU has developed new methods that use pair-production to produce positrons throughout the volume of thick samples [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427, F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262]. Unlike prior work at other laboratories that use bremsstrahlung beams to create positron beams (via pair-production) that are then directed at a sample of interest, we produce electron–positron pairs directly in samples of interest, and eliminate the intermediate step of a positron beam and its attendant penetrability limitations. Our methods include accelerator-based bremsstrahlung-induced pair-production in the sample for positron annihilation energy spectroscopy measurements (PAES), coincident proton-capture gamma-rays (where one of the gammas is used for pair-production in the sample) for positron annihilation lifetime spectroscopy (PALS), or photo-nuclear activation of samples for either type of measurement. The positrons subsequently annihilate with sample electrons, emitting coincident 511keV gamma-rays [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427, F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262]. These gamma-ray photons are then either measured with a high-resolution germanium detector (PAES) or fast scintillators (PALS) and subsequently analyzed using standard positron data analysis methods. The high penetrability of few MeV photons allows one to study defects and characterize materials in thick samples up to hundreds of g/cm2 (approximately a meter in steel), a thickness that is completely inaccessible by any other non-destructive technique. We have demonstrated the proof-of-principle of these techniques to probe tensile strain in thick steel alloys and other metals, to measure positron lifetimes in bulk samples of lead, copper and aluminium with positron lifetime spectra that are free of the surface and source background lifetimes that complicate conventional positron lifetime measurements, and demonstrated the activation technique for damage studies of copper and single-crystal iron [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427]. We have also demonstrated the potential application of these techniques to 3-D imaging of defect density in thick structural materials [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262].

The Idaho National Laboratory, along with Los Alamos National Laboratory and the Idaho State University's Idaho Accelerator Center, are developing electron accelerator-based, photonuclear inspection technologies for the detection of shielded nuclear material within air-, rail-, and especially, maritime-cargo transportation containers. This paper describes a developing prototypical cargo container inspection system utilizing the Pulsed Photonuclear Assessment (PPA) technology, incorporates interchangeable, well-defined, contraband shielding structures (i.e., "calibration" pallets) providing realistic detection data for induced radiation signatures from smuggled nuclear material, and provides various shielded nuclear material detection results. Using a 4.8-kg quantity of depleted uranium, neutron and gamma-ray detection responses are presented for well-defined shielded and unshielded configurations evaluated in a selected cargo container inspection configuration.

Doppler broadening of positron annihilation spectroscopy was performed during high-power short-pulsed laser irradiation to investigate the effects of the induced shock waves on the measured positron annihilation parameters of Cu and W. The measurements were performed using the recently developed technique of accelerator-based γ-ray induced positron annihilation spectroscopy. The Doppler broadening was measured in coincidence with nanosecond laser pulses. Preliminary results show no significant effects in these measurements. These experiments have, however, demonstrated the potential of measuring positron annihilation spectra in coincidence with shock waves.

Two techniques have recently been developed to quickly and easily apply positron annihilation spectroscopies to large structural components found in civil engineering, aviation, etc. In this paper, the authors discuss how to extend imaging capabilities to these new techniques, which will enable defect imaging similar to that obtained with positron micro-beams but at much larger sample sizes. Preliminary two-dimensional defect imaging results are presented from a highly damaged 30.5×30.5cm copper plate.

Accelerator-based γ-ray-induced positron annihilation spectroscopy performs positron annihilation spectroscopy by utilizing MeV bremsstrahlung radiation generated from an accelerator (We have named the technique “accelerator-based γ-ray-induced PAS,” even though “bremsstrahlung” is more correct here than “γ rays”. The reason for that is to make the name of the technique more general, since PAS may be performed by utilizing MeV γ rays emitted from nuclei through the use of accelerators as described later in this article and as in the case of positron lifetime spectroscopy [F.A. Selim, D.P. Wells, and J.F. Harmon, Rev. Sci. Instrum. 76, 033905 (2005)].) instead of using positrons from radioactive sources or positron beams. MeV γ rays create positrons inside the materials by pair production. The induced positrons annihilate with the material electrons emitting a 511-keV annihilation radiation. Doppler broadening spectroscopy of the 511-keV radiation provides information about open-volume defects and plastic deformation in solids. The high penetration of MeV γ rays allows probing of defects at high depths in thick materials up to several centimeters, which is not possible with most of the current nondestructive techniques. In this article, a detailed description of the technique will be presented, including its benefits and limitations relative to the other nondestructive methods. Its application on the investigation of plastic deformation in thick steel alloys will be shown.

A positron lifetime spectroscopy (PLS) technique was developed using coincident γ rays induced by proton capture. Proton capture in some light elements induce coincident MeV γ rays, allowing positron lifetime to be measured. One γ quantum provides a start signal for the positron lifetime spectrometer, whereas the other γ quantum bombards the sample under investigation, generating a positron inside it through pair production. The stop signal is obtained from the detection of one of the two 511keV photons emitted from positron annihilation with the sample electrons. This new technique can extend PLS, which is a powerful tool to identify the size and concentration of defects, to thick materials and a broad range of applications. It also eliminates the source contribution from the measured spectra, which may lead to the identification of more defect types in a sample.

Stress measurements were performed using accelerator-based γ-ray induced positron annihilation spectroscopy technique, which allows probing of defects at high depths in thick materials up to several centimeters. Induced stresses due to tensile, fatigue, cold work, and bending tests were investigated in steel alloys of about 1-cm thickness. The measurements showed the dependence of the line-shape parameter of the annihilation peak S on the induced deformation in the four tests. They also revealed an interesting behavior for the change of S parameter with tensile deformation, related to the engineering stress-strain curve of the material. Transmission electron microscopy measurements of dislocation density in cold work deformation suggested that the saturation of positron annihilation parameters often observed in cold work data is not due to compete positron trapping at defects. It was also shown that the S parameter has a weak sensitivity and quickly saturates in fatigue test when compared with the other mechanical tests, which was interpreted as being due to the brittle nature of fatigue failure and the very little plastic deformation associated with it.

Standard shock wave measurements provide no information about the structural changes in the lattice during the generation and propagation of shock waves. It is important to acquire such information to understand the mechanisms of defect formation in laser processing of materials. Positron annihilation spectroscopy has been shown to be a valuable tool to study electron states and open volume defects in solids for more than 40 years. We report the first dynamic measurements of positron annihilation during short-pulsed laser irradiation of materials. Nanosecond high-power laser pulses were synchronized with nanosecond bremsstrahlung pulses from a 30 MeV electron linac to irradiate the materials. Bremsstrahlung photons generate positrons inside the material. These positrons, in turn, annihilate with the shocked material's electrons emitting 511 keV radiation, which is influenced by the momentum distributions of the electrons. Due to positron trapping at defects, structural changes of the lattice are reflected in the Doppler broadening of the 511 keV annihilation peak. Measurements were conducted on crystalline silicon during laser pulses and after laser irradiation.

Neutron diffraction and scattering provide valuable tools for probing the structure of bulk materials. Neutron scattering facilities throughout the world generate neutrons either with a nuclear reactor or with high energy particle accelerators through (p,n) spallation reactions. We suggest a photonuclear-based neutron source, using an electron linear accelerator, which may provide an inexpensive method to perform neutron scattering and diffraction experiments. This paper reports the results of calculations using Monte Carlo computer code (MCNPX) to optimize the neutron conversion targets and to study the properties of the obtained neutron source. Measurements and calculations for the neutron yield and the energy distributions of the neutrons are presented. As a preliminary experiment we have used a 20 MeV electron linear accelerator at the Idaho Accelerator Center (IAC) to generate neutrons using lead and tungsten targets.

Neutron diffraction and scattering provide valuable tools for probing the structure of bulk materials. Neutron scattering facilities throughout the world generate neutrons either with a nuclear reactor or with high energy particle accelerators through (p,n) spallation reactions. We suggest a photonuclear-based neutron source, using an electron linear accelerator, which may provide an inexpensive method to perform neutron scattering and diffraction experiments. This paper reports the results of calculations using Monte Carlo computer code (MCNPX) to optimize the neutron conversion targets and to study the properties of the obtained neutron source. Measurements and calculations for the neutron yield and the energy distributions of the neutrons are presented. As a preliminary experiment we have used a 20 MeV electron linear accelerator at the Idaho Accelerator Center (IAC) to generate neutrons using lead and tungsten targets.

The recent development of Accelerator-based Gamma-induced Positron Annihilation Spectroscopy (AGPAS) has shown the possibility of probing residual stress in thick materials by Doppler broadening measurements (1). In those measurements, the residual stresses were reflected in the line shape parameters of the 511 keV annihilation peak. However, since positron lifetime spectroscopy is a powerful tool to distinguish between different types of defects, such as dislocations and vacancy clusters (2), it is crucial to enable positron lifetime measurements in AGPAS. In this work, a new method is developed to conduct positron lifetime measurements on thick engineering materials using accelerators. By focusing 1.5 MeV protons from a Van de Graff accelerator on a thin Al window, coincident γ-rays of 2.8 MeV and 1.78 MeV are induced through (p,γ) reactions. The 1.78 MeV quantum provides a start signal for the positron lifetime spectrometer, where as the 2.8 MeV quantum bombards the material under investigation. This, in turn, creates a positron, which annihilates with one of the material electrons emitting two 511 keV photons. The stop signal for the positron lifetime is provided by the detection of one of the two 511 keV photons. The measured positron lifetime spectrum depends on the electron densities and hence provides information about the size of open volume defects. The method enables positron lifetime measurements in thick engineering materials up to tens of gm/cm2, a thickness not accessible by conventional positron

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Diffraction methods are the most reliable nondestructive methods of measuring residual stress. X-ray diffraction is restricted to penetration depths of some 10 µm, whereas neutron diffraction can provide information at significantly higher depths, but the cost is very high and the availability of facilities is much lower. A sensitive nondestructive probe for detecting defects and measuring stresses in thick materials (tens of cm) does not exist. The first highly penetrating system to measure stress/strain in thick materials based on using bremsstrahlung beams from small electron accelerators (3-6 MeV Linacs), is presented in this paper. These bremsstrahlung beams, which exhibit excellent penetration, create positrons inside the materials via pair production. The positrons annihilate with the material electrons emitting 511 keV radiation, which is influenced by the momentum distributions of the atomic electrons. Because of the fact of positron trapping at defects, positron annihilation is very sensitive to any change in the microstructure and can probe nano-void defects. Open volume defects, voids or dislocations are reflected in the line width of the 511 keV peak. Tensile stresses and resultant strains have been measured in an engineering alloy using this technique. This technique can be used to infer residual stress and to detect defects in crystals, polymers, metals and alloys up to tens of gm/cm2 for material science and engineering applications. The low cost and compact size of small electron accelerators, and the high penetrability of MeV bremsstrahlung beams allow the development of portable systems for industrial applications.

Experiments to benchmark photo-neutron production calculations for an Accelerator Driven Sub-Critical System (ADS) are described. A photo-nuclear based neutron source with output > 10(13) n/sec has been proposed as a driver for a program using the sub-critical assembly at Idaho State University. The program is intended to study ADS control issues arising from coupling an accelerator neutron source with a sub-critical assembly. The experiments were performed using the 20 MeV electron linear accelerator at the Idaho Accelerator Center (IAC). Results of calculations, that were made using ACCEPT, PINP, MCNP, and MCNPX codes to optimize photo-nuclear based neutron conversion targets, are compared to experimental data for a single energy measurement.