You probably read about the
study cultivating Cladosporium sphaerospermum on the ISS to see how this radiation-hardy--nay, radiation-using--mold would handle radiation in space.
OK, backing up. You probably read the reports that a species of black mold was happily growing in a radiation area in the Chernobyl reactor building. It not only wasn't getting (obviously) killed, it seemed to thrive better. "Melanotic fungi migrate toward radioactive sources, which appear to enhance their growth."
That led to a lot of studies of melanin and radiation. In vitro studies suggest that melanin is capable of harvesting electromagnetic energy similarly to, but less efficiently than, chlorophyll--and apparently at higher energies than chlorophyll (which absobs in red and blue bands: roughly 1.8eV and 2.8eV).
Interesting. So the experimenters put together a sealed pair of test chambers (and a duplicate to run on Earth), with two scintillators, one for each chamber, to detect radiation. (Their sensitivity peaked at about 50KeV energy deposited in some unspecified time.) Above these were petri dishes, one of which had been innoculated with mold spores. They kept them cold so the mold wouldn't start growing until they got into space, and once in space every 35 seconds they measured the temperature and the amount of the surface that turned dark with mold. The mold grew just fine. They looked at the difference between the "counts" (number of scintillator signals) in the control side and the moldy side, and found that the difference started at about 0 and grew as the mold did.
Now the difference isn't huge: about 2.6%, which, since the "shielding" was only one-sided you could double to get what the effect would be if you were surrounded by this in your spacecraft. 5% reduction would be nice, but not really worth the glowing headlines. And you can see the error bars on this. But there does seem to be an effect. The dotted lines at about 20 and 200 hours represent times when they estimated that mold had achieved 50% and 100% surface coverage. They had a camera and an algorithm...
Now even medium energy particles are going to do some damage going through creatures. I don't have any idea how a chemical could harvest medium energy photons and electrons resulting from the initial particle going through at some random angle, and suspect it isn't possible.
Low energy photons and electrons would be another matter. We have, in chlorophyll, a proof of principle that if you go low enough in energy harvesting is quite feasible. Even electrons knocked loose with low energy won't go far. But how do we get from here (e.g. MeV protons) to there (10's to 100's of eV photons)?
Researching that was a bit frustrating. The concepts are easy enough, but illustrating with examples, not so much.
At high energies, a photon interacting with matter loses energy by kicking loose electrons, and pair-producing electrons and positrons. Each of these is typically high enough energy to do the same in turn, and you get an exponentially growing number of particles up until their energies drop below the threshold for such fun and games. (And yes, the positrons eventually annihilate and produce photons.) This is all well understood, and well modeled, and I'd hoped to show the rest of the story. Unfortunately, the old standbys of Geant and EGS don't try to follow the showers all the way down.
Once you get below about 1KeV, molecular differences have a very strong effect on the outcome, and just modeling a shower in a nice uniform material like iron gets very complicated. The difference between interacting with an inner shell and an outer shell electron isn't negligible anymore.
So while I could show the cascades that happen from high or medium energy to fairly low energy, I cannot illustrate how the rest of the shower goes, as a (e.g.) 10KeV photon produces weaker photons and electrons which in turn produce less energetic ones.
Near the end of the low energy shower, an electron or photon of a few eV can excite, or perhaps even ionize, a molecule of the scintillator. When the excited electron returns to its original state the molecule produces a photon in the visible spectrum. It'll go some random direction, but if you have enough excited molecules (meaning more energy dumped into the scintillator by the incoming particles), enough of them will head in the direction of the light-sensing part to produce a signal. In really sensitive systems, all you need is one, but your noise rate goes way up, so typically you'll set your signal threshold a bit higher.
Two or three of these visible-light photons hitting your phototube (or equivalent detector) at about the same time will make a little electrical signal that you can amplify, and count if it is bigger than your threshold setting.
FWIW, layering scintillator and stuff to stimulate showers (like iron, lead, what have you), produces showers that produce amounts of light roughly proportional to the energy of the initial particle--which is very handy. Calorimetry. Anyhow, this experiment was just using scintillator in counting mode.
I'd be interested in seeing what photon energies these molds are capable of harvesting. Experiments like this subject them to a broad spectrum of energies. It's probably pretty hard to do--the tunable x-ray systems I know about are designed for radiation doses that would probably toast the molds (you can give plants too much light too).
More as I learn more...
