Monochromatic methods for cargo interrogation
The overarching goal of this project is to develop a novel approach to the active detection of shielded nuclear materials that combines a high sensitivity with low radiation dose. The use of MeV, intrinsically monoenergetic γ-rays for transmission imaging provides a means for both high contrast imaging of shielded objects and the means to confirm the fissile nature of the material. This technique relies on the large difference between the pair production cross section of mid- and low-Z materials and SNM (U and Pu) and, subsequently, the large difference in absorption at high energies. The required monoenergetic γ’s are easily produced using small state-of-the-art proton or deuteron accelerators. The number of objects such as containers which contain no SNM is many orders of magnitude larger than the rare container which might have SNM and, as a result, systems in use are plagued by time-consuming false alarms. To mitigate this problem, after determining that a particular object cannot be cleared, the system can also be used to generate both neutrons and γ’s to provide positive identification. Unlike other approaches attempted to date, the source is in fact a multi-particle source in which both neutrons and γ’s are produced simultaneously. The result is that SNM may be detected in three ways: imaging through shielded material, detection of photofission neutrons and γ’s, and detection of neutron-induced fission products. The multi-particle approach is particularly flexible with respect to the types of shielding that can be penetrated.
A common method of inspecting commercial cargoes for the presence of fissile materials involves the use of 1-10MeV bremsstrahlung photon beams. While simple and reliable, this technique has many downsides, such as the large doses involved and its relative inefficiency at triggering NRF and photofission. Much progress in the field of active interrogation can be achieved by developing monochromatic, and possibly tunable gamma sources:
- monochromatic or quasi-monochromatic sources will allow for lower dose radiography and photofission based active interogation.
- tunable sources will increase the signal/dose for NRF applications, and will potentially reduce the required measurements times.
A collaboration lead by MIT is exploring the possibility of using proton and deuteron beams in 11B(d,nγ)12C and 12C(p,p’γ)12C reactions, which produce highly monochromatic photons.This program makes use of a 3MeV deuteron source at MIT-Bates linear accelerator to experiment with (d,nγ) reactions. This approaccan be used not only to achieve low dose radiography, but also low dose dual energy radiography and lower dose photofission for fissionable material detection.
To achieve the above mentioned reactions, an RF quadrupole (RFQ) accelerator is used to accelerate a deutron beam to 3MeV. A picture of the accelerator, with the overlayed schematics showing the beam and the target, can be seen to the right.
In this proof-of-concept configuration, the 3MeV impinges upon the 11B target, resulting in the emission of 4.4MeV and 15.1MeV gammas from a de-exciting 12C nucleus. These two energy photons are perfect for performing monochromatic dual-energy radiography, allowing to independently determine the atomic number (Z) and density of the cargo. The gammas traverse the cargo, and are then detected by a prototype array of NaI(Tl) detectors, as seen on the image to the right.
Data analysis and simulations
The observed photon spectra will contain information about the cargo material that has been traversed. The 15.1MeV gammas' attenuation is highly dependent on Z, due to the Z^2 dependence of the pair production cross section. The plot below shows the total scattering cross section for three nuclei of very different Z (carbon, iron and uranium), showing a strong differentiation at 15MeV. The transmission measurement at 4.4 MeV allows to determine the density, while at 15.1MeV the transmission allows to determine the Z.
The spectra below show the results of measurements for Al (Z=13), Fe (Z=26) and W(Z=72), showing a dramatic difference in the 15.1MeV transmission. At 15.1MeV the interaction is entirely dominated by pair production, while at lower energies Compton scattering is dominant. Since Compton scattering at ~4 MeV probes the electron density of the material, and since the electron density is approximately proportional to the mass density, transmission measurements at ~4 MeV are best for determining the areal (2D) density of the cargo.
The observed spectra are a convolution between in the incident photon energies as well as detector response functions. To better understand the later, Geant4 simulations have been performed. By switching various physics processes on and off, it is possible to infer the reasons behind various features in the response function. For example, the low energy tail from the 15.1MeV (also observed in the data) is due to the escape of the bremsstrahlung photons from the electrons and positrons which are pair produced by the 15.1 MeV gamma.