An Increasingly Rare Isotope

Andrew J. Keller
February 18, 2011

Submitted as coursework for Physics 241, Stanford University, Winter 2011

Helium is an element familiar to most everyone, finding applications in industry, medicine, and the levitation of celebratory balloons. Fewer people realize that helium has two isotopes, He-4 and He-3, with the latter having been the subject of several recent government hearings. He-3 is a rare but stable isotope of helium with a natural abundance of a scant 0.00014%. [1] It is produced via the radioactive decay of tritium, a radioisotope of hydrogen with a half-life of 12.32 years that is produced in nuclear reactors and used primarily in hydrogen bombs. [1] At present the most significant usage of He-3 is in the detection of illicit nuclear materials via their neutron emissions, and demand for the isotope far exceeds the available production. Given that replenishment of the supply would entail further production of tritium, a process inextricably linked with domestic and international concerns over nuclear proliferation, nature would be hard pressed to show us a greater irony. To understand why it is fair to call the present He-3 shortage a crisis, one must understand first why He-3 is so special and then understand just how scarce the resource is becoming.

Properties of Interest

From a scientific standpoint, He-3 is a great neutron detector, which is important, given that weapons grade plutonium has a neutron activity of order 105 counts per kilogram per second. [2] Neutrons are hard to detect because they carry no charge, and so most neutron detectors rely on nuclear reactions to capture the neutrons and emit charged particles, which can then be detected electrically. [3] He-3 has an unusually large neutron absorption cross-section, 5333(7) barns for thermal neutrons, and in the nuclear reaction that occurs, He-3 captures a neutron and emits a proton, becoming tritium in the process. [1] The tritium eventually decays back into helium-3.

Of three popular isotopes incorporated in neutron detectors, He-3, Li-6, and B-10, all have neutron absorption cross sections that scale with the inverse square root of neutron energy up to about 10 keV. [3] What is important to note is that the cross-section is higher for lower energies, with He-3 having the largest cross section everywhere in this low-energy range. The neutron absorption cross-section is not the only measure of how good of a neutron detector a given isotope is. Cd-113, with a natural abundance of 12.22%, might seem like a great choice given its thermal neutron capture cross section of 20600(400) barns, but one must keep in mind that cadmium is a metal, and it would be expected to screen any free charges. [1] Of course, He-3 is not perfect either: the Q-value of the nuclear reaction is low compared to processes involving Li-6 and B-10, leading to lower discrimination against gamma-ray events. [3]

He-3 possesses other useful properties unrelated to the detection of illicit nuclear material. It is irreplaceable in cryogenics, where it is used in mixtures with He-4 in dilution refrigerators to obtain temperatures in the millikelvin range. No other refrigeration technology can achieve temperatures below 300 mK indefinitely, and so it is critical for low-temperature physics research, if not as important politically. [4]

Scarcity of Helium-3

Given the important and unique properties of He-3 and its politically inconvenient means of production, one might guess that it would be hard to obtain. In recent years, even the U.S. government's own demand has outstripped its supply. In the opening remarks of a House of Representatives subcommittee hearing about the "helium-3 supply crisis," subcommittee chairman Brad Miller claimed that in the span of one year, the cost per liter of He-3 rose tenfold, and that supply was low enough to force a halt to a Department of Homeland Security plan to deploy next-generation neutron detectors. [5] He also noted that He-3 shortages "prevented many firms and researchers from acquiring He-3 at all, at any price." Perhaps the numbers speak for themselves: the U.S. now produces only 8000 liters per year, but Homeland Security sought to purchase detectors from Raytheon that would have consumed 200,000 liters alone, largely from the preexisting stockpile. [5] Neglecting any prior surplus, one can take the ratio to find that to fulfill the demand for one particular usage of He-3 would have required a quarter century of production, a stupefying number that really quantifies the problem.

© Andrew J. Keller. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] V. F. Sears, "Neutron Scattering Lengths and Cross Sections," Neutron News 3, No. 3, 26 (1992).

[2] G. W. Phillips, D. J. Nagel and T. Coffey, " A Primer on the Detection of Nuclear and Radiological Weapons," Center for Technology and National Security Policy, National Defense University, 2005.

[3] G. F. Knoll, Radiation Detection and Measurement, 2nd Ed. (Wiley, 1989).

[4] F. Pobell, Matter and Methods at Low Temperatures, 3rd Ed. (Springer, 2007).

[5] "Caught by Surprise: Causes and Consequences of the Helium-3 Supply Crisis," opening statements by Chairman Brad Miller, Hearing of the Subcommittee on Investigations and Oversight, Committee on Science, Space, and Technology, U.S. House of Representatives, April 22, 2010.