Apollo and radiation

[Waspie_Dwarf]

Having a hangover would explain why every electron I have ever met is just so negative.

[Kelfazin]

When someone says you need lead in order to shield against Van Allen belt radiation or solar radiation, that's a pretty good indicator that he hasn't studied the problem and is probably just repeating what someone else told him.

Lead, concrete, rock, and other dense materials are sometimes required to shield against strong electromagnetic wave radiation. Where the hazard is gamma rays or x-rays you would choose that kind of material.

The Van Allen belts are composed of charged particles -- protons, electrons, and helium nucleii. So is the dangerous component of solar and cosmic radiation.

Protection by shielding involves putting something between you and the particles that absorbs them. Dense materials will do that, of course, but at a price. When a charged particle is absorbed in the shielding, the absorption triggers the release of secondary radiation in the form of braking radiation. An atom's behavior when it absorbs one of these wayward particles causes its electrons to get happy briefly, and then drop back down to their ground states. That drop releases a photon. The wavelength of the photon depends on the atomic mass of the atom that did the absorbing. Lead, steel, beryllium, and other heavy metals (heavy in the sense of physical density as well as atomic mass) happen to emit secondary radiation in the x-ray band, which isn't a good thing.

Now two feet of lead will indeed shield you from Van Allen belt radiation. Also from microwaves, gamma rays, cell phone reception, your in-laws, and the effects of a modest tactical nuclear device. That's substantial shielding.

It would work in this case because the incoming charged particles would be absorbed in the outermost layer of lead while the x-rays induced there would be absorbed by the inner layers. In fact, all that would probably happen within the first few millimeters of the shielding, leaving the remaining several centimeters as dead mass.

Many materials are capable of absorbing charged particles effectively and are thus useful as shielding. If the secondary radiation shortens in wavelength as atomic mass increases, then what you need is something that has a very light atomic mass, but can occur in physical densities sufficient to put enough practical mass behind each unit area of incidence. Shielding is thus often specified as grams per square centimeter (of incident area). Hydrogen atoms are very light, but free hydrogen in gaseous form isn't dense enough to work. Its liquid and solid forms are impractical for engineering.

So we compromise by hooking that hydrogen up with other atoms into chemical compounds that can provide higher physical densities with better handling and manufacturing properties. Water will work, but it too has handling issues. We then turn to the various polymer substances that hook hydrogen onto carbon and sometimes oxygen in creative ways. One of the best candidates these days is high-density polyethylene (HDPE), the stuff they make hard-hats out of. The carbons and oxygens in these various compounds don't have as good absorption properties as the hydrogen, but they serve to hold lots of hydrogen in place and keep it physically dense and manufacturable without incurring too great a braking radiation penalty.

Many natural products such as wood and felt would be suitable shielding, if provided in enough thickness.

Aluminum also works, being an atomically light metal as well as having mechanical and thermal properties that make it a good aerospace material for other reasons. Because its braking radiation edges into dangerous territory, you need a substantial thickness of it. For missions lasting around ten years, suitable shielding for electronics such as to attenuate it to ground levels against solar flares and Van Allen belt encounters would amount to a thickness in single-digit centimeters.

Lead foil would work too, but you're going to build the spacecraft out of aluminum anyway. Why not just attenuate the incident radiation with your pressure- and load-bearing structure too, if it's sufficient? Even when proper thicknesses are contemplated (and two feet is overkill by several orders of magnitude), lead is qualitatively not the best choice.

The odd behavior of braking radiation results in some interesting behavior. If shielding is too thin it will actually increase the radiation load on the payload. As particles are trapped in the outer layers the absorption creates x-rays, which are more penetrating than the original particle. So you have a minimum shield thickness that does no more than attenuate the total radiation picture down to the unshielded level. Then you add additional shielding to attenuate the secondary.

I've been thinking about this a little more in relation to the EMU Extravehicular Visor Assembly (EVVA). Is the gold visor used for light filtering and radiation protection? Would gold be a poor choice because of its density? Is the clear visor made from HDPE?

[JayUtah]

The clear visor is made of Lexan, a polycarbonate. In the fully deployed configuration the astronaut has the Lexan fishbowl helmet, a transparent Lexan visor, and the gold-coated Lexan visor. Lexan is naturally opaque to ultraviolet. The gold coating is for EM attenuation in optical wavelengths. The EMU and LEVA are designed for protection against ambient radiation, but not against any substantially elevated activity.

[JeDi]

When a charged particle is absorbed in the shielding, the absorption triggers the release of secondary radiation in the form of braking radiation. An atom's behavior when it absorbs one of these wayward particles causes its electrons to get happy briefly, and then drop back down to their ground states. That drop releases a photon. The wavelength of the photon depends on the atomic mass of the atom that did the absorbing. Lead, steel, beryllium, and other heavy metals (heavy in the sense of physical density as well as atomic mass) happen to emit secondary radiation in the x-ray band, which isn't a good thing.

I'd like to put some corrections onto this. First, braking radiation (continuous spectrum) and relaxation of electronic (core hole, in the case of x-rays) states (discrete spectrum) are two completely different, actually unrelated phenomena. With respect to x-ray emission from metal surfaces they just happen to occur in roughly the same energy band, but this is mostly by pure coincidence.

The high energy edge of the continuous part (braking) is solely determined by the energy of the impacting particle, its tailing function, i.e. the intensity distribution towards lower photon energies, is mainly governed by the average electron density in the material. The comparison of the spectra of aluminium (low e^--density, rather smooth hill-like distribution) and tungsten (high e^--density, strong peak at the edge, steep decrease towards low energies) at the same influx energy is a prime textbook example for this. The effective cross section of atomic cores may play a similar role, but for electrons at least it seems of less importance. Atomic mass plays no role of its own here.

The energy of core hole relaxation is (sort of) a function of atomic number, but even quite light elements reach the x-ray band. Aluminium anodes are in fact used for normal (i.e. non-soft) x-ray spectroscopy. It appears quite difficult to even reach the soft x-ray band at all, using anode emission. Before the broad availability of suitable synchrotron radiation, tricky thingies like yttrium anodes had to be used for this.

What may be of some relevance here is the competition of x-ray and Auger emission, where instead of a photon yet another electron is emitted. The ratio depends on the atomic number, the Auger effect is most prominent for light elements (carbon is a standard subject of Auger spectroscopy) while heavier elements yield more x-ray emission. This may result in a lower yield of secondary radiation seen in (pseudo) transmission at a given thickness.

So the picture with respect to particle shielding is that materials with moderate electron density, by braking the incoming particles gently, spread the energy over the lower energy range, thus favouring conversion into heat by reabsorption - which is what you finally want. Furthermore light elements tend to emit less x-rays by specific emission (core holes) than heavy elements.

Beryllium, by the way, isn't a heavy metal by atomic mass (9), number (4) or mass density (1.9g/cm^-3 at RT), rather one of the lightest.

And as a last nitpick: atomic mass is quite irrelevant here, it is the atomic number that counts, i.e. nuclear charge. Yes, for your explanation they correlate well and atomic mass has made its way as a dirty idiom, but next time you do mass spectroscopy, you will see the difference. Don't lose your way in the isotopic forrest!

[Donnie B.]

Don't lose your way in the isotopic forrest!

I, for one, will be sure to drop some baryonic breadcrumbs.

[JayUtah]

I'd like to put some corrections onto this.

Thanks; you're hired.