Nuclear physics worldwide is enjoying a kind of renaissance because of a new, extremely bright, gamma-ray light source that can be created with short-pulse lasers and energetic electron beams. These highly mono-energetic gamma-ray (MEGa-ray) sources produce narrow, laserlike beams of incoherent, tunable gamma rays. They make it possible to access and manipulate of the nucleus of the atom with photons (so-called nuclear photonics). In the early days of the laser, manipulation of the valence (outermost) electron structure of the atom became possible, leading to new applications and science. Similarly, nuclear photonics using laser-based gamma-ray sources promises both to open up wide areas of practical, isotope-related materials applications, such as detection for security and medical imaging, and to enable new discovery-class nuclear science.
In the US, at the Lawrence Livermore National Laboratory (LLNL) in Livermore, California, we are actively pursuing the development of high-brightness and high-flux MEGa-ray sources. Our work is focused on creating a machine that will advance peak source brightness by six orders of magnitude. This machine will create beams of 1–2.3MeV photons with color purity matching that of common lasers.1 In Europe, a similar but higher photon energy gamma source has been included as part of the core capability that will be established at the Extreme Light Infrastructure Nuclear Physics (ELI-NP) facility in Magurele, Romania, outside of Bucharest. This machine is expected to have an end-point gamma energy in the range of 13MeV. The machine will be co-located with two world-class, 10 petawatt laser systems, thus allowing combined intense-laser and gamma-ray interaction experiments.
Figure 1. Peak brilliance of a mono-energetic gamma-ray (MEGa-ray) source created from interaction of a 1nC (beam charge) electron beam and a 1J interaction laser compared with the Advanced Photon Source (APS) synchrotron. Above 2MeV, MEGa-ray peak brilliance exceeds that of synchrotrons by more than 15 orders of magnitude. ph: Photons. BW: Bandwidth.
Figure 2. Shown is the T-REX (Thomson-radiated extreme x-ray) 100–900keV light source at Lawrence Livermore National Laboratory (LLNL). T-REX was LLNL's first MEGa-ray machine.
The optimized interaction of short-duration, pulsed lasers with relativistic electron beams (inverse laser-Compton scattering) is the key to MeV-scale photon source monochromaticity, pulse brightness, and flux. In the MeV spectral range, such MEGa-ray sources can have many orders of magnitude higher peak brilliance than even the world's largest synchrotrons (see Figure 1). Consequently, they can efficiently perturb and excite the isotope-specific resonant structure of the nucleus.
This resonance structure depends on the number of neutrons and protons in the nucleus and is thus a unique signature of the isotope as opposed to the element. Because MEGa-ray photons are in the MeV spectral range, they are also highly penetrating and capable of seeing through dense objects. At LLNL, we constructed a proof-of-principle machine and used photons to detect the presence of lithium concealed behind aluminum and lead2, 3 (see Figure 2). The Thomson-radiated extreme x-ray (T-REX) MEGa-ray source created a record peak brilliance of 1.5×1015 photons/mm2/mrad2/s/0.1% bandwidth at 478keV. The T-REX uses an existing 120 MeV S-band linear accelerator and custom laser systems designed specifically for laser-based Compton scattering x-ray and gamma-ray sources. A detailed description is available elsewhere.2
Next-generation MEGa-ray machines will have beam fluxes that are three to five orders of magnitude higher than previous proof-of-principle demonstrations. Such capability should help to resolve a wide variety of nuclear problems and issues. Examples include rapid (milliseconds) detection of concealed nuclear material, high-precision (<100 parts per million) non-destructive assay of spent nuclear fuel assemblies, isotope-specific, high-resolution (∼10 micron) imaging of complex objects in 3D, simultaneous measurement of the magnitude and direction of moving isotopic material in dynamic, multi-component materials systems, and novel forms of medical radiography and radiotherapy. Recently, the ELI-NP project identified a wide range of potential scientific uses of next-generation machines as well, including generation of brilliant and intense positron beams and fundamental studies of nuclear structure and photofission.4 As a next step, we will focus our efforts on miniaturizing the technology to make it practical, for instance, for homeland security applications.
This work was performed under the auspices of the US Department of Energy by LLNL under contract DE-AC52-07NA27344.
Christopher P. J. Barty
Lawrence Livermore National Laboratory (LLNL)
Christopher Barty is the chief technology officer for the National Ignition Facility and Photon Science Directorate at the LLNL. He currently co-chairs the International Committee on Ultrahigh Intensity Lasers and is a fellow of the Optical Society of America.
C. P. J. Barty, MEGa-rays, petawatts, and nuclear photonics, SPIE Opt. Optoelectron., 2011. Paper 8080B-30.
F. Albert, Isotope-specific detection of low density materials with laser-based mono-energetic gamma-rays, Opt. Lett.
35, no. 3 pp. 354, 2010.
D. J. Gibson, Design and operation of a tunable MeV-level Compton-scattering-based (gamma-ray) source, Phys. Rev. ST Accel. Beams
13, no. 7 pp. 070703, 2010.