I've heard a couple of scary stories from experienced accellerator physiscists about something called neutron clouds. Apparently, if you have an experiment like a fixed-target experiment that produces a lot of neutrons with the correct energy, they don't just dissipate or get caught in surrounding matter. Instead, they hang around due to their large half-life (~15 minutes). The rumor goes that they actually form clouds, that wander around the facility, and that in the early days of some CERN experiment, people didn't think about the effect, and got a nasty (although not accute) dose when they entered the collision hall just after shutting down the beam.

The description of the behavior of these clouds varies in different accounts. Sometimes they just pass through everything, but sometimes they're supposed to behave like a real gas, being held back by walls (but creeping through small openings).

  • I can imagine this phenomenon is real, but how much of an issue is it in real experiments / nuclear facilities?
  • Do the clouds really behave like a gas (I'd think the n-n cross section is not big enough to create pressure)? How do they behave wrt. walls?
  • And in light of the recent nuclear waste transports in France and Germany: The waste emits a lot of gamma and neutron radiation, could it leave a temporary trail of low-energy neutron clouds behind?
  • $\begingroup$ ""The waste emits a lot of gamma and neutron radiation"" Who says so? Do You have some source? $\endgroup$
    – Georg
    Nov 28, 2011 at 13:35
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    $\begingroup$ First time I hear of this. It reminds me of an old joke in a tandem building :"there are neutrons outside the window" . PLease provide some links for these claims of such cold neutrons:A thermal neutron is a free neutron that is Boltzmann distributed with kT = 0.0253 eV (4.0×10−21 J) at room temperature. This gives characteristic (not average, or median) speed of 2.2 km/s.en.wikipedia.org/wiki/Neutron#Cold_neutrons . As only special material would contain them, they would disappear fast enough in an open environment. $\endgroup$
    – anna v
    Nov 28, 2011 at 14:09
  • $\begingroup$ @Georg, I've just read that on some news sites. The waste in question is from a reprocessing facility, so there are probably a lot of different isotopes - I haven't looked them up on a nuclide chart, though. It is insofar plausible, as beta and alpha radiation would be mostly shielded by the glass the waste is melted into. $\endgroup$
    – jdm
    Nov 28, 2011 at 16:11
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    $\begingroup$ ""Do the clouds really behave like a gas (I'd think the n-n cross section is not big enough to create pressure)? How do they behave wrt. walls? "" This is erronous. Pressure is a question of a wall, not of n-n cross section. Think of an ideal gas. $\endgroup$
    – Georg
    Nov 28, 2011 at 16:58
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    $\begingroup$ A cloud is possible like the cloud when some sodium or barium is vaporized outside of our atmosphere. But such a cloud is only temporal, vanishing soon. Prerequisite is thermal neutrons and absence of capturing nuclei. $\endgroup$
    – Georg
    Nov 28, 2011 at 19:20

3 Answers 3


Thermal neutrons capture on hydrogen and carbon with reasonable (i.e. not large, but significant) cross-sections (this is the delayed event detection methods of most organic liquid scintillator anti-neutrino detectors--i.e the one that don't dope their scintillator with Gadolinium).

So though a "cloud"--meaning a localized diffuse gas--of neutrons can develop in the neighborhood of a strong source (size of the cloud is driven by how far they go as they thermalise), their dissipation is driven by their mean capture time, not their half-life.

Confession: Here I am presuming that the mean capture time is significantly shorter than the half-life, but I haven't measured it in a "near the laboratory" setting. In organic liquid scintillator the capture time is on order of $200\text{ }\mu\text{s}$, but air has a lot less hydrogen and carbon in it. Note that the neutrons also go into the ground, the building, nearby vehicles and passers-by (if any) where they may find things to interact with.

At my grad-school we had a 2 Curie (i.e. huge) AmBe source. The source vault would register unusually high back-grounds on a survey meter for a few minutes after it was returned from the moderator tank to the shielded vessel, so that may be a rough measure of the time scale. It also says something about the strength of the radiation field: a few times the in-the-basement background level.

Shielding methodology for strong neutron sources generally incorporates a great deal of boron in various layers to help suck up the thermal neutron flux; not incidentally this means that most of the capture gammas are generated inside the shielding. Borated plastics are common as are borated concretes. These days Gadolinium is cheep enough that I imagine we'll start seeing it used in shielding design. The source vault in grad school was built of borated cinder block---two layers with a meter air-gap between.

Another not-very-quantitative story that might shed some light on this.

I was friends with one of the Radiation Safety guys at JLAB. Part of his job was monitoring the radiation level at the fence around the secure area with the accelerators, experimental halls, etc. Mostly they just put out general purposes detectors and compared the results with background reading from nearby, but early on they built a more sophisticated detector out there to understand the various contributions to the dose (probably trying to tune their Monte Carlos, those guys are really big into modelling). He told me two interesting things

  • If they ran the accelerator at high current and high duty cycle they could about double the dose at the fence (i.e. the accelerator related dose was as big as the background at the fence).
  • Neutron sky-shine was the single biggest contributor. Sky-shine means that the neutrons got out through the lightly shielded roofs of the halls (only 50 cm of concrete and 2 meters of packed earth), and their detectors saw radiation coming from the captures/decays that occurred above them.

The fence was about 40 meters from the beam dumps.

  • $\begingroup$ Are You shure about capture by carbon? In certain reactors (eg Tschernobyl type) graphite is used as moderator. $\endgroup$
    – Georg
    Nov 28, 2011 at 16:54
  • $\begingroup$ The C-12 capture cross-section is about 1% that on H-1, so it is not at all large, but it is big enough to show up in (e.g.) KamLAND's data set. But those are thermal cross-sections, and in the reactor core the neutrons are generally pretty hot. There is also $n + ^{12}\text{C} \to n + ^{12}\text{C} + \gamma \text{ (4.9 MeV)}$ for more energetic neutrons, but it's cross-section is also pretty energy dependent. $\endgroup$ Nov 28, 2011 at 17:10

If there existed some material which could confine a thermal neutron gas for a substantial fraction of the neutron lifetime, you could build "bottles" of this material, fill them with neutrons, and monitor the decays to measure the neutron lifetime.

And in fact is is possible --- but only for so-called "ultra-cold neutrons," which have kinetic energies below 100 nano-eV, not for thermal neutrons with milli-eV kinetic energies. The state of the art for UCN storage bottles is a combined lifetime due to neutron decays and wall losses of about 400 seconds. These UCN bottles tend to be hand-sized to person-sized. In fact there is no point to making them much larger: you can show by unit conversion that 100 neV is the kinetic energy lost by a neutron climbing about a meter above the surface of the Earth ($U=mgh$), so a person-sized UCN bottle doesn't need any lid. (The open-topped neutron bottle built by Serebrov and collaborators at PNPI was called the "Gravitrap.")

For free thermal neutrons in air, moving a few meters per millisecond, the mean free path between scatters (mostly from water vapor) is many meters. The idea of a cloud of neutrons lingering in a room or hallway or along a railway track for minutes after their source is removed, like smoke clouds, is not really credible.


When visiting a research nuclear reactor, I have seen some tubes for neutrons, they are called neutron channels. They were some meters long. So yes, the neutron gas can be contained to some extent.

  • $\begingroup$ Are you referring to guides, or beamlines, for carrying neutrons from the reactor core to an experiment? $\endgroup$
    – rob
    Aug 4, 2016 at 15:18
  • $\begingroup$ @rob: Yes, he is. With the right materials if the neutrons hit at a very shallow angle you actually can reflect them. frm2.tum.de/en/the-neutron-source/reactor/guiding-the-beams years ago, about 2 weeks before FRM2 went operational I made a photo (predates digital cameras) of these guides. $\endgroup$
    – datenwolf
    Sep 9, 2016 at 20:12
  • $\begingroup$ @datenwolf by the way, are they reflected by electron clouds? $\endgroup$
    – Anixx
    Sep 9, 2016 at 20:23
  • $\begingroup$ @Anixx: I have no idea. I just stumbled across this Q&A, saw your reply and was immediately reminded of our tour of the FRM2 facility, just weeks before the reactor went critical for the first time. And the whole neutron guide facility and some of the explanation got "stuck" in my head. This was one of the most impressive tours I participated: We still could go everywhere (only the reactor access pool was closed of, as some fuel cores, which use HEU, were already in storage there), because nothing was neutron-activated and all of the experiments of the first run were already in place. $\endgroup$
    – datenwolf
    Sep 9, 2016 at 20:39
  • $\begingroup$ @datenwolf I have been on a tour (organized by my school) to Moscow Physico-Technological Institute where there was a research reactor, used by students. We were allowed to access the reactor pool. We only were said not to touch things because they could be "dirty". We also were said it would be better if we visited the reactor when it were working because we would see Cherenkov radiation just a meter or so under our feet. $\endgroup$
    – Anixx
    Sep 9, 2016 at 20:44

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