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Eur. Phys. J. Plus (2014) 129: 236
DOI 10.1140/epjp/i2014-14236-6
Why new neutron detector materials must replace helium-3 Alan J. Hurd and Richard T. Kouzes
Eur. Phys. J. Plus (2014) 129: 236 DOI 10.1140/epjp/i2014-14236-6
THE EUROPEAN PHYSICAL JOURNAL PLUS
Regular Article
Why new neutron detector materials must replace helium-3⋆ Alan J. Hurd1,a and Richard T. Kouzes2 1 2
Los Alamos National Laboratory, Los Alamos, New Mexico, USA Pacific Northwest National Laboratory, Richland, Washington, USA Received: 29 July 2014 / Revised: 15 September 2014 c Societ` Published online: 27 October 2014 – a Italiana di Fisica / Springer-Verlag 2014 Abstract. Helium-3 has such unique physical and nuclear properties that to a physicist it seems appalling the isotope was once indiscriminately released to the atmosphere as a waste gas. Not gravitationally bound to our planet, a He-3 atom is effectively lost to the human race once released. Consequently, when a confluence of independent factors in national security and research in the last decade created a “custody battle” over this scarce isotope, an intense search for substitutes and alternative technologies ensued for various applications. This Focus Point of EPJ Plus is dedicated to neutron detector alternatives.
The 2008–2009 helium-3 reserve “cliff” (fig. 1) deeply alarmed users in physics, geophysics, chemistry, medicine, and other fields when the price shot up to $1200 per liter from $80 per liter just two years before. Research in alternatives for neutron detection —the major application for He-3— expanded around the world. By way of motivation to this issue, a brief backstory and prospectus for He-3 supply is in order, which leads us to the United States and its national security sector. While physicists are thankful for He-3’s amazing and useful thermodynamic properties in cryogenic refrigerators, the gas is equally admired for its very high capture cross section for thermal neutrons: A 1 cm diameter tube of He-3 at 10 atm is essentially neutron-opaque. The quantum efficiency of He-3-filled proportional counters set the gold standard while also scoring high in pulse resolution, gamma rejection, physical robustness, and low toxicity. Remarkably, spin-polarized He-3 nuclei are an effective neutron spin filter in a variety of unique nuclear physics and materials experiments as well as an effective contrast agent for magnetic imaging of lungs. In the 1970s the more versatile He-3 rapidly replaced B-10 enriched, toxic BF3 gas in detectors whence He-3 demand zoomed to 70000 liters per year by 2005. However, supply and demand, normally exhibiting lock-step in steady markets, were moving in opposite directions in the U.S. (Russia and the U.S. are effectively the only sources of He-3, as will be clear soon.) The inevitable collision was noticed very late because key supply information was confined to the national security sector by a “need-to-know” policy for sensitive nuclear information. Throughout the Cold War and after, the U.S. and Russia accumulated kilogram quantities of He-3 as the decay product of manmade tritium, which has a 12.4 y half-life. Nuclear arsenal readiness was maintained by replenishing certain components that use tritium; meanwhile dedicated nuclear reactors actively produced fresh tritium. Following the START Treaty in 1991, weapons disassembly along with recycling allowed reserves of He-3 to build up eventually reaching 235000 liters in the U.S. by 2000 (ref. [1]) even though U.S. production of tritium had halted in 1988. Through annual public auction the gas was available to commercial suppliers, but the total supply was classified because one could calculate the number of weapons in the arsenal from the reserve (ref. [2]) (The U.S. arsenal size was unclassified briefly in 2010.) While the national security community knew He-3 reserve numbers well, those responsible for disbursements did not, and the two could not communicate owing to security. The next factor in this perfect storm constrained the future of He-3 drastically. Following the terrorist attacks in the U.S. in 2001, the Department of Homeland Security made plans to deploy a large number of He-3–based monitors to detect movement of special nuclear materials around the world. International treaty verification needs called for additional, similar monitors. The large U.S. reserve could cover these needs but little else as reflected in comparing the reserve and disbursement curves of fig. 1. Contribution to the Focus Point on “3 He replacement in neutron detection: Current status and perspective” edited by N. Colonna, A. Pietropaolo, F. Sacchetti. a e-mail:
[email protected] ⋆
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Eur. Phys. J. Plus (2014) 129: 236
Fig. 1. Helium-3 reserve and disbursements under high- and low-demand scenarios. The precipitous “cliff” in He-3 stockpile in 2008–2009 alerted officials to an impending crisis. Historical data were taken from ref. [1], which attributed them to Steve Fetter, U.S. Office of Science and Technology Policy, “Overview of helium-3 Supply and Demand,” presentation at the American Association for the Advancement of Science Workshop on helium-3, April 6, 2010. Projections were provided by Jehanne Gillo, U.S. Department of Energy (personal communication).
In the meantime, the projected need for He-3 grew steadily in medicine, physics, and (especially) materials research sectors (ref. [3]) In some fields of use there is simply no substitute for the unique physical characteristics of He-3: clearly dilution refrigeration and quantum fluids research are two such fields. For neutron detection, however, alternatives do exist at a cost in efficiency. The cost-benefit balance is well illustrated by the detector needs for neutron scattering, a technique that is inherently and chronically flux limited. The cost to equip instruments with detectors of any type is far lower than the cost to increase source flux for given science metric. To put it another way, having spent $1B to build a high-flux neutron source, it is important to utilize every neutron possible. In the late 90s and early 2000s, the U.S. and Japan built new megawatt-class accelerators for neutron scattering and Europe planned a third one. Along with many existing facilities, the attending instrument suites were to be heavily reliant on He-3 because a penalty in quantum efficiency is tantamount to impaired accelerator or reactor power. This decisive cost-benefit ratio strongly prodded instrument designers to plan for 4π detection in new detector systems. Moreover, popular inelastic scattering instrument types require detection at a far enough distance from the sample to use time-of-flight energy discrimination resulting in very large detector arrays. Finally, because U.S. supply typically had covered the needs of Europe and Japan as well as the U.S., the projected He-3 reserve was doomed in the face of ballooning need for neutron facilities. The elements of a train wreck were in place by 2005 but the full impact was not felt until 2008 when gas price exceeded $1200 per liter —to as high as $2000— following the cessation of Russian exports. By some estimates (ref. [1]) the true price would be as high as $18000 without subsidy from weapons programs in the U.S. and Russia. Actions by governments and scientists have mitigated the impact of the He-3 scarcity. Tritium production in the U.S. was restarted in 2003 at one commercial reactor for weapons refurbishment, and reportedly several Russian reactors are producing tritium as well. Most importantly, international research and development began before 2009 on substitute technologies, including a consortium organized by large neutron facilities. In an essential step, the White House formed an interagency policy committee (IPC) by which priorities could be established between national security and scientific uses of U.S. gas as discussed below. One particular policy of the IPC was of major impact: no He-3 was allowed to be used in further deployments of radiation portal monitor systems. In addition, exploratory discussions began on new sources of He-3.
Eur. Phys. J. Plus (2014) 129: 236
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At this writing in 2014, requests for He-3 have dropped to a surprisingly low level, enough so that usage asprioritized should be reasonably well covered beyond 2040 without new sources of He-3 (ref. [4] and fig. 1). This drop reflects lowered design expectations for detector instrumentation, adoption of substitute detector technologies in several sectors, and increased availability of Russian gas. The drop also reflects caution in the community during uncertain times. The prioritization policy toward non-U.S. users may leave some less than satisfied. As of 2013, requests from non-U.S. neutron scattering facility are not being met by the IPC, who give first consideration to U.S. programs in the annual 10000 liter “federal allocation”. First priority is given to programs requiring the unique physical properties of He-3, second to national security, and third to programs for which substantial costs have been incurred, such as the Spallation Neutron Source. Even though U.S. requests have dropped (the “lowdemand” scenario in fig. 1 reflects recent history), there are no plans to meet non-U.S. requests outside the public auction unless the request involves direct U.S. collaborations, research complementary to the U.S., or advocacy by U.S. scientists to receive second consideration. The outlook for helium-3 has improved enormously since 2008 when it appeared that reserves had been fully committed. Even assuming a worst-case consumption of 14000 liters per year and no new sources, the U.S. reserve should last through 2024 (fig. 1) assuming that new technologies are actually employed. By eliminating the auction and assuming continued low demand, the reserve should last beyond 2040. In the meantime, alternative neutron detector technologies are being developed at a rapid pace, which will stretch gas supply for the most demanding detector applications. Some of those technologies are discussed in this Focus Point. The authors thank J. Gillo of the US Department of Energy for helpful suggestions and data. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396. Pacific Northwest National Laboratory is operated by Battelle under DOE Contract DE-AC05-76RL01830. This work was reviewed for classification as document LA-UR-14-26292
References 1. D.A. Shea, D. Morgan, The Helium-3 Shortage: Supply, Demand, and Options for Congress (Congressional Research Service, Library of Congress, 2010). 2. Weaknesses in DOE’s Management of Helium-3 Delayed the Federal Response to a Critical Supply Shortage, Report to Congressional Requesters (United States Government Accountability Office, Library of Congress, 2011). 3. T. Feder, Phys. Today 62, 21 (2009). 4. J. Gillo, The Isotope Program fills critical needs for 3He in the United States. An earlier shortage has been mitigated, National Isotope Development Center Newsletter, March 2014.
