### The Use of Cosmic Ray Muon Tomography in the Detection of Concealed High-Z Materials

05:16 pm

Don't let the physics scare you away, I'm coming at the subject generally and mostly in plain English because that's all I had at the time (I do try to do that still, fyi, but I have a bit more knowledge to draw from now and can avoid some of the mistakes I've made here). As a note, completing this project did land me a gig that lasted from the summer of 2010 through the summer of 2013 on three projects related to cosmic ray muons (tomography and solar weather analysis), and formed the foundation for the work I've been doing since with upgrades to the ATLAS detector at the Large Hadron Collider (LHC) at CERN in Switzerland (I've never been there myself, but I've been to TRIUMF, Fermilab, and DESY as part of all of this... and maybe SLAC this coming spring or summer?). A very good friend once claimed that they saw me “living a life of small adventures”, and that does seem to be an ongoing thing.

The Use of Cosmic Ray Muon Tomography in the Detection of Concealed High-Z Materials

I. INTRODUCTION

A. The need for screening

It is becoming ever more important to monitor the flow of goods and people as a deterrent against state, criminal, or ideological organizations that may wish to wage war or cause serious disruption through the use of various asymmetric weapons systems within the territories we wish to consider secure. To that end, increasing surveillance and intrusive inspections have been implemented at points where the greatest risk exists, for instance at airports and border crossings. For an effective deterrent, all traffic through these key points of commerce and travel especially, as well as the appropriate measures for points between, require 100% screening to be maximally secure. For historic and economic reasons, this strategy of complete coverage presents an extreme challenge to even the most affluent and security conscious of societies. Furthermore, any onerous impediment to the efficient movement of goods and people elicits an economic cost of its own that can destroy the very prosperity that such security measures wish to protect.

While it can be argued that the smuggling of conventional weapons poses the greatest chance of occurring and resulting in harm being inflicted through their use, all but the largest of instances of such smuggling into otherwise stable countries are dwarfed by the already existent availability of these items within those countries. Where the national government of a country needs to protect its citizens against all forms of weapons smuggling, it has a special obligation to prevent the use of chemical, biological, radiological, and nuclear (CBRN) weapons against its population, infrastructure, services, and legitimate foreign interests: “Asymmetric CBR threats provide an adversary with significant political and force multiplier advantages, such as disruption of operational tempo, interruption/denial of access to critical infrastructure and the promulgation of fear and uncertainty in military and civilian populations. [...] Proliferation will continue to dramatically increase the threat from the use of CBR agents by states or terrorist organizations against unprotected civilian populations. Proliferation also poses an asymmetric threat against non-combatants outside the immediate theatre of conflict, including Canadians at home.”1 As such, most functional nations have embarked on integrated strategies to minimize the chances of CBRN related incidents. In general, those efforts can be categorized in five ways: supporting or directing the improvement of foreign CBRN control, detection, and enforcement; border CBRN detection equipment and domestic law enforcement training; the securing of legitimate CBRN materials within the country’s borders; improved intelligence operations to detect potential smuggling operations before they occur; and various domestic and international research and development projects to improve overall control and detection capabilities.2

Furthermore, of the CBRN threats, there are emergency measures and possible mitigations that can be taken to minimize the impact to the population and infrastructure of a successful attack with chemical, biological, or radiological weapons; however, the damage that would be inflicted should a nuclear device be detonated in a populated area would be devastating beyond measure to both the fabric and spirit of the country, its operation, and its people. Such results make special nuclear materials3 (as could be used in a nuclear bomb) particularly attractive targets for terrorists4 (“independent” or state sponsored): “Nuclear smuggling is an increasing concern for international security because creating a viable nuclear weapon only requires several kilos of plutonium or highly enriched uranium. The International Atomic Energy Agency has documented 18 cases of theft of nuclear [weapon grade] materials within the last decade, and probably more instances have occurred without report. This is especially prevalent within the former Soviet bloc, where large amounts of nuclear materials are insecurely guarded and inventories are often faultily kept.”5

Of particular concern is the realization that the view, held since World War II3, that the effort required to build a nuclear weapon was prohibitive, is no longer valid. This opinion had been based on the American experience of creating two small nuclear weapons, but it is now widely accepted that the expertise and technical capability to build a viable nuclear weapon is no longer the exclusive purview of large, economically advanced nation-states. In fact, the knowledge and infrastructure required is potentially within reach of any well-organized and funded group with sufficient long-term determination and resourcefulness: “The only real technological barrier to the clandestine construction of nuclear weapons is access to fissionable material itself. There is a growing black market for this material, and eventually demand will result in enough material reaching as-yet unidentified buyers to produce a nuclear weapon”3. In addition to the smuggling of processed special nuclear materials, given that uranium is roughly 40 times more prevalent in the Earth’s crust than is silver6, the smuggling of uranium ore or low quality extracted uranium from such ore is also a more likely possibility.

While it is widely acknowledged that “most known interdictions of weapons-useable nuclear materials have resulted from police investigations rather than by radiation detection equipment installed at border crossings”2, the asymmetric nature of the threat calls for exceptional measures in the effective detection of smuggled special nuclear and radiological materials that might make it past the intelligence operations to a port of entry into the country. Per the U.S. Container Security Initiative Strategic Plan: 2006-2011, “the cost to the U.S. Economy resulting from port closures due to the discovery or detonation of a weapon of mass destruction or effect (WMD/E) would be enormous. In October 2002, Booz, Allen and Hamilton reported that a 12-day closure required to locate an undetonated terrorist weapon at one U.S. seaport would cost approximately $58 billion. In May 2002, the Brookings Institution estimated that costs associated with U.S. port closures resulting from a detonated WMD/E could amount to$1 trillion, assuming a prolonged economic slump due to an enduring change in our ability to trade.”7 While this is a U.S. figure, it can be scaled appropriately to reflect the impact of such an event on any trading nation, or the domino effect such an act would have on global commerce if it happened anywhere.

D. Outline of Thesis

Because of the sensitivity of Passive Muon Tomography (PμT) systems to high-Z materials (versus lighter elements) they are a much more targeted solution than more indiscriminate imaging systems, and the lack of an active radiation source eliminates the potential health concerns associated with x-ray and gamma ray imaging systems. While PμT systems only address a particular class of risk, specifically the threat posed by the trafficking of special nuclear materials that could form the basis for a bomb or large well-shielded shipments of radionuclides that could be used in a “dispersal” device, the asymmetric nature of the threat justifies the commercialization of this technology to compensate for the serious limitations of existing technologies in this area of detection. Carleton University’s proposal to use large-area drift chambers for muon detection will result in a device that will provide excellent spacial and temporal resolution with very cost effective readout electronics and data processing requirements; however, the initial requirement for a flowing gas in the first generation solution presents a negative offset through higher infrastructure and ongoing maintenance costs that would need to be mitigated as part of a widespread deployment of this particular solution.

II. COSMIC RAYS

A. Overview

Primary cosmic rays are very high energy charged particles (into the range of many TeV24) that originate mostly outside of the solar system, from astrophysical sources, and are comprised primarily of protons (~80%) and helium nuclei (~14%), with the remaining being heavier nuclei such as carbon, oxygen, and iron. These can also interact with interstellar gasses to create a much lower flux of secondary cosmic rays comprised mostly of anti-protons and lithium, beryllium, and boron nuclei23. When cosmic rays interact with the Earth’s atmosphere at high altitudes, they produce showers of thousands of “secondary” particles, usually also called “secondary cosmic rays”. Most of the particles so generated decay or interact with atmospheric atoms before they can reach the surface of the Earth; however, a shower of gamma rays, electrons, neutrons, and muons24 (due to relativistic time dilation) do reach the lower altitudes of the atmosphere and the surface itself. Of these, the cosmic ray muons are of primary interest in this application due to their high energy, penetrating power, and the relative ease that their path and momentum can be precisely determined.

III. Detectors

A. Overview )

IV. Implementation

V. Further exploration

In addition to the use of the proposed muon tomography systems in border security and container/vehicle inspection, the basic technology can be useful in other applications as well. Furthermore, with appropriate research and development, enhancements to the basic technology are possible that will reduce the total cost of ownership and operation.

A. Use as a scientific instrument

With the possibility of large area muon detectors being deployed along borders and in key strategic locations, it should be noted that each one of these devices can be used as an element in a larger cosmic ray observatory. The information on incident angle and momentum of incoming cosmic ray muons could provide a wealth of data to astrophysicists and particle physicists alike (who can analyze the data against various models developed for subatomic phenomena to support or discard various hypotheses). One major issue is that data on the contents of scanned targets cannot be shared with the general public due to security concerns. This can be addressed by sending data only when a scan is not in progress. Alternatively, if the initial momentum (before interaction with cargo) is reconstructed by projecting the final momentum backwards through the gathered tomographic data when cargo is present, there will be no way to determine anything about the contents of the scanned cargo from the data. In any case, the angular information from the top pair of detectors is gathered before any interaction with cargo and should not present any security risk as it is a purely astronomical data source at that point.

B. Developing a sealed chamber (no gas flow)

The major disadvantage of the drift chamber solution proposed by Carleton University is the need for a flowing gas mixture. If it were possible to seal the chamber and operate it for long periods without needing service, then it would be both cost effective from a readout electronics perspective and from the longer term operational cost and complexity perspective through the elimination of the need to manage gas supplies and disposal. Much work has been done over the years on sealed gas ionization based detectors, and research and development in this area could have a large impact on the cost of muon tomography systems in the field.

C. Use of active muon source system

One of the issues with using cosmic ray muons as a source of radiation for tomographic purposes is their relatively low flux (1 muon (cm2 min)-1). This low flux means that it takes at roughly a minute for a basic scan to determine whether there is any high-Z material of concern. By using an artificial source for a higher muon flux, it could be possible to do the scans faster or to build a more complete tomographic image of the contents of a shipping container or other target of interest. The issue is, of course, that this introduces a vary dangerous ionizing radiation source to the situation and the lack of any additional radiation is one of the attractive elements to using cosmic rays muons as the probe.

D. Use in sealed-container inventory determination and management

There are many installations, for instance Chalk River in Ontario, where there are sealed containers with unknown quantities of potentially dangerous materials in them. There are also situations where contents of containers are claimed to contain certain materials, but need to be verified as part of nuclear control treaties. In those cases, cosmic ray muon tomography could provide an excellent tool for cataloguing and monitoring the contents of these containers. Since this is more of an audit application, the lower flux and time to acquire the necessary level of data are not as much of an issue as for applications that impinge on commerce.

VI. Conclusion

Passive cosmic ray muon tomography systems present an excellent solution to the issue of deterring and detecting the trafficking in nuclear and radiological materials – in the first case through direct detection of high-Z materials, and in the second case, being able to detect high-Z shielding that might be hiding lower-Z radiological materials. The system further distinguishes itself by not introducing any new sources of radiation, thus sidestepping any potential health or safety concerns from the public or business. Carleton University’s proposed drift chamber muon detectors build upon decades of experience in implementing high resolution muon analysis systems, and can be used to determine to a high degree of accuracy both angular and momentum data on the muons passing through a detector system for analysis by the tomographic software. The low cost of readout electronics compensates for the higher cost due to the requirement for gas-filled chambers, and will result in a competitive solution for field-deployable systems.

VII. References )

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