In wars between states, the perpetrator of any nuclear attack is clear; however, in the case of nuclear detonation by non-state actors, the origin of a bomb or the identity of its makers is not necessarily evident. Therefore, robust nuclear forensics capabilities become all the more crucial, both for attribution in a post-attack setting and for tracking material as a means of deterrence. The following paper will highlight the some of the processes involved in nuclear attribution.
Attribution is the "end-product of nuclear forensic analysis", requiring the integration of forensic sample analysis, investigation, and intelligence. [1] Since terrorist groups do not have the resources to fabricate weapons-grade fissile material, source and route attribution help reveal the uranium/plutonium supply line. [2] Sample analysis is comprised of acquisition, analysis, and comparison to known examples. After an explosion, a Constant Phoenix CW-135 aircraft, the only aerial platform designed to collect debris from radioactive fallout, picks up particulates and gaseous emissions. [3] These samples typically include fission products, unexploded fissile material, and irradiated non-nuclear components.
One of the most useful radiochemical lab tests is gamma-ray spectroscopy. In radioactive decay, daughter isotopes often release γ-rays after α-decay in order to bring them to their ground state. The emitted γ-ray energy spectrum is unique for each nuclide, so spectroscopy can be used to determine the bulk composition and relative abundances of radioactive species. Elements can also be detected by measuring the time of emission, since each isotope has a characteristic half-life. [1,4] Gamma-ray spectroscopy is advantageous because it is non-destructive - emitted photons have energies from 10 to 100's keV, so they are not attenuated well by the packing material of the weapon. [5]
Further analysis is done to obtain a precise profile of the nuclides present. Optical microcopy with an energy dispersive x-ray detector can give a resolution up to 100 nm, while an electron microprobe gives compositions of most elements down to 100 μg/g. Ultimately, detection limits reach close to 10 fg/g for the full suite of elements from hydrogen to plutonium. [1]
Knowing the composition of samples reveals much that is relevant to a weapon's origin and history. The enrichment level and purity of uranium depends on where it was mined and how it was processed. Plutonium is exposed to various neutron fluxes and energies depending on the reactor and how long it is left in. Other indicators point to the device's sophistication: medium mass elements (Ga and Nb) indicate alloys were adding to make more shapeable weapons components, while fission products help determine the efficiency of the fuel. [2]
In the case of a nuclear attack, attribution will be an urgent priority; however, it is a complicated and time-consuming process, and there are many obstacles to amassing and sustaining the requisite capabilities. Today only one Constant Phoenix plane is in operation, and radiochemistry and nuclear chemistry appear to be losing popularity as fields of work. [6] Finally, effective comparison of seized samples to known signatures necessitates a complete international database of nuclear explosives, which does not yet exist. [2]
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[1] K. J. Moody, I. D. Hutcheon and P. M. Grant, Nuclear Forensic Analysis (CRC Press, 2005).
[2] M. May, J. Davis and R. Jeanloz, "Preparing for the Worst," Nature 443, 907 (2006).
[3] S. D. Drell and C. W. Stubbs, "Realizing the Full Potential of the Open Skies Treaty," Arms Control Today 41,No. 4 (2006).
[4] W. Dunlop and H. Smith, "Who Did it? Using International Forensics to Detect and Deter Nuclear Terrorism," Arms Control Today 36, No. 8 (2006).
[5] K. Mayer, M. Wallenius and I. Ray, "Nuclear Forensics - A Methodology Providing Clues on the Origin of Illicitly Trafficked Nuclear Materials," Analyst 130, 433 (2005).
[6] D. Wiles, "The Demise of Radiochemistry: Moving beyond Nuclear Science to More Relevant Fields within Environmental and Organic Chemistry," Canadian Chemical News, September 2009, p. 26.