Sturm und Drang
Hey Doc – I got one for you! I read this article saying that thunderstorms can cause radiation levels to go up – if I read it right, they can make beta, gamma, and maybe even neutron radiation. Is this for real? I mean, I know the internet’s not always infallible. But – hey – it still sounds cool enough to maybe try to chase down, doesn’t it?
So I have to admit to being a bit dubious about this one – I’ve read anecdotal accounts about thunderstorms maybe triggering gamma radiation, but the purported mechanism was due to the high temperatures generated by lightning bolts (higher temperatures produce higher-energy photons, but it takes several million degrees to produce gamma radiation). But even requiring those high temperatures…I can maybe see thunderstorms producing gamma radiation. Beta radiation…that’s actually easier for me to accept – beta particles are simply very high-energy electrons, and I can accept that the high electrical fields in a thunderstorm not only give rise to lightning, but could also be able to accelerate electrons to such high energies. But what really gave me pause was the neutrons – neutrons come from the atomic nucleus and it takes a lot of energy to knock a neutron out of an atom. But after taking a look at the paper and some of their data, it turns out that I was likely looking at this the wrong way. And being published in Physical Review Letters D, one of the top scientific journals for physics, lends even crazy ideas a lot more credibility.
What I was missing is that there are other ways to produce gamma and neutron radiation than simply heating atoms to sky-high temperatures. What the authors propose is that thunderstorms produce very high electrical field gradients and that, in addition to producing very big sparks (i.e. lightning), these electrical fields also accelerate electrons to very high energies – on the order of millions of electron volts, which is in the range of lower-energy linear accelerators. These high-energy electrons are (according to the authors) what produce the gamma radiation as well as knocking neutrons out of atomic nuclei. So let’s take a quick look at each of these in turn.
I wrote about how particle accelerators work – it turns out that nature can do the same thing without the machinery. Any electrical field will cause electrons to move, and the stronger the electrical field, the faster they’ll go – if they increase their speed by a factor of ten their kinetic energy will bump up 100-fold. One electron volt (a measure of energy) is the energy a single electron has when it’s accelerated by an electrical field of 1 volt; the electrical gradients in a thunderstorm are high enough to boost electrons up to energies in the millions of electron volts (MeV) – and at these energies, a lot of things become possible. Oh – and most beta radiation has energies in the vicinity of a few to several tenths of an MeV; a few to several hundred thousand electron volts (keV). This is some serious energy.
So that explains the beta radiation, but there’s still the gamma and neutron.
When charged particles pass through matter, such as air, they pass by atoms and, when that happens, they change direction. As an electron, for example, approaches an atom it will first approach the negatively charged electron cloud, which will push the electron away – which deflects the electron into a different direction. Or if it penetrates the electron cloud the electron will be deflected by the positive charge from the protons in the atom’s nucleus. Whenever a charged particle changes direction like this it emits an x-ray or a gamma ray – this process is called bremsstrahlung (German for “braking radiation”) and we see and use it all the time; it’s where x-rays come from in medical radiology machines.
To get into the math a little bit, the probability that a bremsstrahlung x-ray will be produced and the energy of the radiation is proportional to the atomic mass of the atom the electron is encountering. Since most of the atoms in air are relatively low-mass this means that there aren’t many bremsstrahlung x-rays produced when these atoms pass through the air – but if there are enough of the high-energy electrons produced in a thunderstorm then even a low probability interaction can produce a LOT of x-ray photons.
This is the place where I should explain that, to a physicist, the difference between x-rays and gamma rays has to do with how much energy the individual photons have. The highest-energy photons are called gamma rays, next-highest are called x-rays. To a radiation safety person like me, what matters is not the energy so much as the origin – photons that come from an atom’s nucleus comprise gamma radiation and x-rays come from the electron cloud. So a physicist will note that x-rays are always lower-energy than gamma rays; a health physicist, however, will note that it’s possible to have a photon emitted from an atom’s nucleus to have less energy than one emitted from the electron cloud. For the purposes of this answer, even though I’m a radiation safety guy who thinks that gamma rays come from an atomic nucleus, let’s agree to call the bremsstrahlung radiation from the high-energy electrons accelerated by the electrical fields within a thunderstorm “gamma” radiation. But we still have to explain the neutrons, don’t we?
It turns out that if you slam a high-energy photon into an atomic nucleus it can transfer enough energy to the neutron to eject it from the nucleus – who knew? So if we can accelerate electrons to energies of several MeV, those electrons can produce bremsstrahlung photons with enough energy to knock a neutron out of a nucleus – or the electrons can do the same directly.
What this all means is that there’s a plausible physical mechanism behind thunderstorms producing beta, gamma, and neutron radiation.
Of course, this could all just be a bed-time story for physicists to tell each other; what makes it potentially real (or not) is what we actually see in the real world. And it turns out that the folks writing this paper actually saw what they postulated, they include data to support their claim, and their paper was published in a prestigious and influential journal. That gives them an awful lot of credibility.
So…thunderstorms that produce beta, gamma, and neutron radiation? Apparently so – and how cool is that!