Geoff FordenHe-3 and Homeland Security, Part 1

A recent news article reporting the significant shortfall of He-3 a rare isotope of helium—has received an unfortunately small amount of attention. Apparently, the Department of Homeland Security (DHS) has planned on putting 1,400 He-3 based neutron detectors in ports around the world to scan cargo containers for nuclear weapons. Since the primary source of He-3 in the US is from the decay of tritium used to boost nuclear weapons, these highly sensitive detectors have seemingly tied the prevention of nuclear terrorism to replenishing our nuclear stockpile. On the other hand, some back of the envelop calculations seem to show that natural helium can be enriched to the required purity of He-3 fairly cheaply using gas centrifuges normally processing uranium for either power plants for weapons.

The average wonk-reader—who is admittedly better informed on nuclear matters than the average person on the street—can still be forgiven for not immediately knowing how He-3 is related to detecting smuggled nukes. It has, of course, to do with He-3’s propensity for absorbing neutrons; those subatomic, electrically neutral particles given off by the spontaneous fission of fissile materials. Even thermal neutrons absorbed by helium-3 (whose nucleus consists of two protons and one neutron) cause the emission of a charged proton and a tritium nucleus. If the He-3 gas is in a proportional counter, these charged particles ionize the gas and result in a detectable signal. In essence, the He-3 has converted a neutral particle (the neutron originally released during the spontaneous fission inside a nuclear warhead) into a charged particle whose passage through the He-3 gas itself can be easily detected.


This progressions of images shows how a single pair of ionized charges gets amplified by the proportional tube’s electric field to a point where it can easily be detected.

Of course, He-3 is not the only material that does this. Other detectors include devices based on a boron-fluoride gas (BF, with the boron being the B10 isotope) to detect neutrons inside a proportional counter. But BF gas can only be run at relatively low pressures (and hence, low densities) and still function as the gas for a proportional counter. This in turns means that BF-based detectors, if they are going to have a large probability of detecting the one or two neutrons per second given off by a weapon’s pit in a reasonable amount of time, must be very thick. For instance, a high density He-3 detector has an absorption length of 0.9 cm while a typical BF-based detector’s absorption length is almost 20 times as long. That means that a He-3 detector could be 2.9 cm thick and have a very high detection probability—assuming the neutron goes in the direction of the detector in the first place. A BF-based detector with the same effective thickness (measured in absorption lenths) would be 54 cm thick.

Not only would such a thick detector be inconveniently thick in the hustle and bustle of a large port, it would be thick enough to absorb a significant additional amount of background neutrons from the top and sides. Consider a hypothetical detector with a 1m x 1m face. (I have no idea what the actual DHS detector looks like and, at this point, don’t really care. After all, these calculations are at the level of policy and not preamps.) About 200 background neutrons from cosmic rays cross through every square meter on the Earth’s surface every second. The front face of each detector therefore contributes this amount to the count; the detector must wait and count the signal until its additional number of counts amount to a statistically significant difference from the expected background count. The larger the background, the longer the detector must wait. The large BF detector sides contribute 108 additional counts for a total additional of 216, more than doubling the background. (The astute reader will notice I’ve only counted two sides, which is a “thin detector” approximation.)

A high density He-3 detector, on the other hand, has only an additional twelve counts each second. This difference has a significant effect on the time required to look at each shipping container, as we will see in the next section.

Battling the Background

All fissile material suffers spontaneous fission at some level. Of course, some fission much more often than others. The table below gives the number of fission neutrons emitted by three types of material: U238, U235, and Pu239 for a given amount of material. It also shows how long a single detector would have to sit and scan a typical 12 meter long shipping container. This could be significantly shortened by a more efficient setup with three detectors scanning a single cargo container (you win because of the R-squared effect of decreasing distance) but that might run into other difficulties I haven’t thought of yet.

Material Neutrons per gram per sec Material Mass Scan Time
U235 0.00001 23.5 kg ~7000 hours
U238 0.0136 2.5 kg 20 min.
Pu239 0.022 4 kg 3 min.

Clearly, U235 is essentially undetectable. But even weapons grade uranium still has enough U238 to be detectable, even if you have to wait 20 minutes for each scan. By way of comparison, if BF-based detector is used, its increased width more than doubles the background and more than quadruples the scan time. So a BF detector would take approximately 46 minutes to scan each container for a 25 kg uranium pit using 90% U235.

That’s a whole bunch of He-3!

DHS probably selected He3 detectors because they figured it would be very hard to shield a bomb so that it didn’t emit neutrons. I hope they are right, though I can think of several things to do that might be efficient ways of shielding the neutrons. Assuming the type of detectors I’ve been talking about (a square meter face and three absorption lengths thick), each detector needs 93 grams of He-3. And 1,400 such detectors means 130 kg of the stuff. That’s a whole bunch of He-3, especially if you wait around and collect He-3 from the tritium decays! Not to mention the political implications of starting up tritium production again. ( The US has only produced a total of about 225 kg of tritium between 1955 and 1996.) There has to be a better way of producing He-3 for these detectors and I think there is. But for that, you will have to read tomorrow’s post: He-3 & DHS: A Modest Proposal. (Hint: it has to do with the fact that He-3 diffuses through He-4 more than 13,000 times faster than U(235)F6 diffuses through natural UF6.)

Note added: Perhaps a future post will consider the consequences of perfect detectors detecting only all the bombs in cargo containers. Thats not to say that you wonk-readers cannot comment on that of course.

Comments

  1. Azr@el (History)

    “Not only would such a thick detector be inconveniently <<think>> in the hustle and bustle of a large port,” The indicated word should be <<thick>>.

    Low temperature rectification would be a far cheaper means of Helium isotope separation than centrifuges or gas diffusion. But seriously, the DHS requirement for neutron detectors is pork; smuggling nukes thru ports is simply James Bond^3.

  2. Geoff Forden (History)

    Thanks! Typo corrected.

  3. yousaf

    I agree w/ Azr@el: this is DHS pork, and will only cause huge port inefficiencies due to false positives.

    On this subject, I recommend John Mueller’s new book “Atomic Obsession” — even though I do not agree 100% with all its arguments.

    If terrorists want to explode a nuclear bomb in the U.S., they most certainly will not try to smuggle their “crown jewel” in a container. It is, e.g., far easier to charter or buy a 50-foot yacht and bring it to dock at South Street Seaport in NYC, and then proceed. (I think a nice 50-foot Hinckley with deep-blue Awlgrip favored by the our Northeast nobility would particularly evade coast guard suspicions).

    But — as argued in painful detail by Mueller — it is highly unlikely that terrorists will even be able to manufacture such a bomb from scratch in the first place. (Jeffrey and colleague’s study from a few years back, notwithstanding).

    And it is also highly unlikely that a state would pass terrorists a nuke.

    This is confirmed by an NDU study.

    The NDU study concluded that Iran desires nuclear weapons mainly because it feels strategically isolated and that “possession of such weapons would give the regime legitimacy, respectability, and protection.”

    In other words, Iran desires nuclear weapons for the purpose of deterrence, just like every other nuclear-armed nation. The NDU study continued, “[W]e judge, and nearly all experts consulted agree, that Iran would not, as a matter of state policy, give up its control of such weapons to terrorist organizations and risk direct U.S. or Israeli retribution.” And it said the “United States has options short of war that it could employ to deter a nuclear-armed Iran and dissuade further proliferation.”

    And as you say, “these highly sensitive detectors have seemingly tied the prevention of nuclear terrorism to replenishing our nuclear stockpile”.

    This is a dumb, expensive, inefficient and, possibly, dangerous idea. It will preoccupy policy makers and scientists and engineers in a non-solution to the wrong problem. It must have been dreamed up by a committee.

  4. thermopile

    All Pu-239 is contaminated with some level of Pu-240, which is a strong neutron emitter (about 1,000 neutrons per gram). This site claims WGPu is about 7% or less Pu-240.

    He-3 is an inert neutron detector with good sensitivity and good gamma rejection capabilities. The current perceived replacement is boron trifluoride (BF3), and is readily available today. There are others, such as boron-10 lined proportional counters and lithium-6 doped glass.

    See also Glenn Knolls excellent book, “Radiation Detection and Measurement,” Chapter 14 (Slow Neutron Detection Methods), for good descriptions of each detector type.

    The testimony that started this whole story can be found here . The helium-3 discussion begins at about the 52:00 mark, if you watch the webcast.

  5. Tim (History)

    Assuming I’m a nuclear capable nation state and not a terrorist (which seems more likely), isn’t it fairly easy for me to simply increase my enrichment percent in order to be able to produce a “stealth bomb” with high purity U-235?

  6. Murray Anderson (History)

    Urenco has a table at Urenco enriched isotopes marking elements with isotopes enriched by Urenco centrifuges, suitable for enrichment, and not suitable for enrichment. Helium is not suitable.

    Murray Anderson

  7. Geoff Forden (History)

    Murray Anderson,
    Any idea why they say it is unsuitable? My calculations, which I will post later today, show that it should be very good for centrifuge enrichment.

  8. Murray Anderson (History)

    Geoff,
    I don’t know, but posted because Urenco should be the authority on this subject. I do notice that the process gas molecules, in cases where they do enrich, are much heavier than helium. There could be serious leakage problems with helium molecules.

    Murray Anderson

  9. tbaum (History)

    We at AAAS will be hosting a workshop Feb 11 2010 to dig into He-3 supply and demand issues further, including alternative technologies for detecting neutrons. Please join us if you can. Details at http://helium3.aaas.org/.

    I’d also note that there are several other very important application for He-3, including lots of basic science, medical imaging, oil & gas searches, and gyroscopes. And while DHS wants lots of the stuff, the Spallation Neutron Source at Oak Ridge National Lab has bought it by the bucket-loads in the past year or so.

  10. yousaf

    Benn,
    Unfortunately I will not be able to attend. Your event asks, “The AAAS Center for Science, Technology and Security Policy invites you to participate in a one-day workshop to gather input from the scientific, industrial and medical communities on how to decrease overall demand for helium-3?”

    In abstentia, I would vote for suspending this DHS pork project. That will reduce the He3 demand somewhat.

  11. tang (History)

    What is missing here is that DOE/NNSA is deploying far more He-3 than DHS ever will.

  12. Pete (History)

    A homemade neutron absorber can be made using a simple over-the-counter combination of paraffin and (20 Mule Team) borax. It can be created by alternating layers. The paraffin slows down the neutrons which are then captured by the boron in the borax. 10 cm will probably do the trick. A few years ago we needed to build one that surrounded the end of our beam-line during a nuclear physics experiment where one of the side reactions created > 4,000,000 neutrons/s with energies > 1 MeV. 25 cm of the combination above brought the radiation levels down to the point that we could work beside the beamline during the experiment.

    The good news is that any BF filled detector could be cheaply and easily shielded on one (or a few) side(s) virtually eliminating the background problem. The bad news is that anyone with half a brain would shield the source in such a way thus eliminating any real hope to detect the neutrons.

  13. jfassett (History)

    You have jumped to the conclusion that the only source of helium-3 is from nuclear weapons production. However, He-3 is a trace isotope in helium (at levels of 50 ppb to ppm). Can it be economically separated? The discussion about centrifuge separation is misleading; I believe fractional distillation will work.

    Of course, this topic is intermeshed with the government’s role of getting out of subsidizing helium production all together. Maybe a strategic alliance could be proposed with the party balloon industry to solve this problem.

  14. Mark Gubrud

    As Azr@el, He-3 and He-4 spontaneously separate below the tricritical point at .86K. So if you have a mixture of the two, probably the most efficient way to separate them would be with a dilution refrigerator. However, where do you find this mixture?

  15. Hairs (History)

    Moon dust is comparatively rich in He-3 (“comparative” as in compared to the miniscule amounts occurring naturally on Earth) so maybe we should re-start the Apollo programme and go get us some!

    More seriously, it’s hard to believe that a group or nation sophisticated enough to build a nuclear weapon won’t be able to shield the hidden weapon’s neutron emissions. Boron-10 and samarium-149 both spring to mind, with the samarium-149 conveniently being a reactor poison that is created in the fissioning of nuclear fuel. Recovery wouldn’t necessarily be easy, but then if you can recover the Pu-239 presumably a bit of samarium is no greater challenge. Other absorbers include cadmium and gadolinium (especially Gd-157, which has the gimungous cross-section of 0.25 megabarns – yes, MEGAbarns!).

    Of course, if you integrate the detector long enough you’ll always be able to pick up some signal above “background”; but what exactly IS background? Hide the weapon in a cargo of granite table tops and the natural emissions of the granite will vastly outweigh whatever the “usual” background is. Similarly, if you can’t wait long enough with a boron-based detector to distinguish a weak source from background then you’re hardly going to waiting long enough to pick up a well-shielded weapon.

    The USA handles something around 5 million “twenty foot equivalent” containers per year, so assuming the smugglers try to bring in one weapon every 10 years, then with no other intelligence to guide you, that’s a 1 in 50 million random event you’ve got to detect. To distinguish a shielded weapon, or one “hidden” in a naturally high background activity, to that accuracy just doesn’t seem likely. To avoid one false negative you’d have a false positive rate so high that you’d end up searching more containers than are currently opened. Therefore to make the detector practical you’d have to do targeted searches based on other intelligence; in which why not do that anyway and save on the detector??

    I’m tempted to file the idea in the “Plausible in theory, (uselessly?) difficult in practice” bin.

  16. Robin Franke (History)

    Wouldn’t SILEX technology — if proven beyond the prototype scale — be available for He-3 production?

  17. Chad (History)

    Even if these detectors do work and are one day effective, all it would take is a group to smuggle their device onto a beach far away from the port. Given you could prob fit the bomb in a relatively small landing craft this would be fairly easy to do!

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