Some humans have been genetically engineered to be able to detect ionizing radiation to better survive the harsh environments found outside of earth's protective magnetosphere. What could be the physiological mechanism for such a biological Geiger counter, and would they be able to tell what direction the radiation is coming from?
Editted to take account of the fact that cosmic radiation *is* important for this question
Important thing to note: what benefit do you expect to gain from your enhancements? In space, and on the surface of airless worlds, cosmic radiation is basically omnipresent. Neutron radiation is only really a risk around badly shielded nuclear reactors and engines (so don't hang around those). Alpha and beta radiation is mostly a risk from nuclear waste. Most people simply won't benefit from being able to detect radiation, and those people whose job involves radioactive hazards can just carry a regular geiger counter.
Colonisation of space isn't a good justification. Recolonisation after a nuclear war might well be.
You should also consider how your new senses interact with medical technology. If you need macroscopic metal deposits in your body, you might find yourself having problems if you ever need an MRI, and they'll obscure x-rays, too. If you're sensitive to x-rays, then medical x-ray imaging may become quite unpleasant and require sedation instead of being a simple and painless procedure. If you're sensitive to beta radiation, branchytherapy becomes more difficult... radiotherapy in general is may be problematic and unpleasant. Better hope you have other good cancer treatment options!
With that out of the way:
You can roughly divide ionising radiation into three categories:
- minimally penetrating particles, such as alpha or beta radiation
- highly penetrating particles, such as neutrons or cosmic radiation
- short wavelength EM, such as x-rays or gamma rays
You can't trivially detect all of them with the same sort of device... even "geiger counters" need special materials for their windows to allow alpha and beta particles in, and carefully chosen materials that will convert EM radiation to charged particle radiation (ionisation via the photoelectric effect) to make it easier to detect these wavelengths that don't interact readily with the gas inside the tube. Neutrons are also inconvenient and various strategies are needed to reliably detect them.
Each of these probably needs a separate sensing mechanism; there's unlikely to be one single organ that will do it all.
Alpha and beta
The principle source of these is from nuclear reactions and radioactive decay. Outside of earth's magnetosphere, a good source of these is of course the sun, plus any nuclear powerplant (fission or fusion) you might have brought with you.
Alpha particles won't penetrate the skin, and beta particles wont readily penetrate the skin unless the source is close and the flux is high. You can't easily have a sense for these that which uses sensory neurons in the skin, because the epidermis (outermost layer of the skin) will block too much of the radiation to be useful.
Sensors for these, then, will have to be inside the body. I'm not totally certain how you'd make an alpha or beta sensitive nerve, unfortunately, but the particle do very readily give up their energy so artificially evolving or maybe even designing a suitable mechanism seems like it might be possible. L'Dutch's suggestion of an ion channel is part of a solution, but is unlikely to be a solution in itself
In Ken MacLeod's book The Stone Canal, one of the characters has modified olfactory receptors in her nose which tickle when she inhales radioactive gas or dust. When the stimulus is strong enough she sneezes, which isn't a bad reflex to have if you've just snorted a noseful of radioactive materials. This same approach might be extended to the lining of the lungs and maybe the eye mucosa... exposure to radioactive materials might then cause hayfever-like symptoms which might be a good way to clear the material out of the lungs and breathing passages even if it isn't big enough or irritating enough to trigger the normal coughing and sneezing reflexes.
Making the lining of the disgestive tract more irritable, resulting in vomiting and diarrhoea on exposure to radiation (but crucially, before that radiation has had much time to actually kill enough cells to cause sickness) will help speed the offending material out of the body before it can do too much damage.
Shielding against these kinds of radiation is straightfoward and lightweight. Furthermore, there's not a lot of air in most places in space, so being sensitive to it isn't particularly useful, as your spacesuit or clothing will block external sources and a portable air supply will prevent you inhaling any sources. This is only really a useful trait if you work around badly made fission reactors or have to deal with fallout from nuclear weapons.
Neutrons and cosmic rays
For people on earth, the easy option here is to just not bother. If you're not in space and not hanging around things undergoing fission or fusion you just won't encounter them (neutron emission is rare except in very short lived isotopes being formed as part of an ongoing nuclear reaction). They're also very highly penetrating, which would make them very difficult to localise, but at least means you could have your sensory organs for them almost anywhere.
One possible solution is to have some kind of moderating material that converts them into another kind of radiation that you are sensitive to... charged particles hitting metal will release bremmstrahlung radiation and if you're sensitive to x- and gamma rays you could feel that. Large natural deposits of metal in your body might be awkward to form, however, but carrying around a bit of metal sheet as an external moderator might work. If you're in a metal-hulled spacecraft without adequate particle shielding, some of the incoming charged particle radiation will have already generated x-rays via this mechanism.
Other approaches exist to convert neutron radiation into more easily detectable forms. Natural internal deposits of suitable boron compounds might be practical, or again, you can carry around a piece of suitable material with you and wait til your
spidey radiation senses near it start tingling.
Trying to localise the source of cosmic radiation will be largely pointless. In space, it comes from everywhere, more or less. On a world without a decent atmosphere and magnetosphere, it will come from the sky. because it is so highly penetrating, your sensory bits will probably be triggered regardless of their orientation or meagre shielding; you'll simply be able to feel that you're exposed to a hostile environment and maybe you'll be able to tell how hostile it is.
Localising sources of neutron radiation will be quite difficult, again because they are highly penetrating (moreso than charged particles, for the same energy levels) and so shielding your sensory gear so it can detect neutrons from a specific direction is probably impractical. You'd probably just get a sense of impending doom, and maybe a rough idea of the strength of the neutron flux.
Gamma rays and x-rays
Another kind of highly penetrating radiation. In space, a major source of these will be charged particle radiation hitting metal objects and releasing bremmstrahlung radiation. Your nuclear reactors will also be a good source of x- and gamma rays.
Interestingly, there already exist some naturally (ish) evolved radiotrophic fungi which have been found in the Chernobyl sarcophagus and use a form of photosynthesis using melanin that reacts to short wavelength light like gamma radiation.
Humans can't be photosynthetic, but they do have melanin so copying the same approach could potentially be used... specialist melanosomes could be formed in some skin sensory neurons that could provide whole-body sensitivity to short wavelength radiation. It might feel like someone is shining a heat lamp at you, for example.
This would also improve UV sensitivity, so you'd be less likely to hang around in the sun and get sunburnt, though increased levels of melanin in the skin would make you less prone to sunburn anyway.
Highly penetrating radiation might be difficult to sense with directionality (because it might trigger the sensory nerves when it enters and leaves the body, and if it passes through your arms and torso you'll get multiple entrace an exit points, etc) but if you bring a handy metal sheet as a partial shield and you still feel the radiation with no change in intensity, then it is probably coming from the unshielded direction. If you can get your body to naturally grow highly x-ray opaque materials around sensory clusters, that would probably help too.
You might be able to modify retinal cells to respond to x-rays and gamma rays using the same sort of techniques, but because your eyes will not be able to focus or block much short wavelength radiation you won't be able to get a very good idea of what is emitting the xrays becuase your whole retina will be stimulated. The sensation may be fainter or absent for sources behind you where your skull is providing shielding. You might just end up with foggy vision or a sort of optical glare which might be irritating or extremely debilitating, and not what you want when you're in a highly radioactive environment!
Actually the human body already has an organ that comes close to this; the eye.
Astronauts since the 1960s have reported seeing flashes in their eyes, even when closed while in space. It turns out the flashes are the retina being impacted with cosmic rays outside the Earth's magnetosphere. In effect this means that astronauts that received more rapid flashes would be experiencing higher doses of cosmic rays than those with only an occasional flash.
Directionality is difficult because these things go so fast and radiation would be in a similar vein. Besides, we know the source of cosmic rays in our solar system; it's the sun. Something similar would be happening in other harsh environments and to put it bluntly, people would likely just head in the direction that leads to a lower rate of flashes occurring.
This is not a perfect system; these cosmic rays are doing damage to the eye and to the body every time they pass through you, so eventually you're going to be blind if this is a regular occurrence. Also, this is not a finely tuned mechanism for detecting different kinds of radiation, but then neither is a Geiger Counter. If all you're looking for is a reading on the radiation levels in a given location, then the best approach may well be to modify the eye to 'see' radiation a little better (after all, light is just radiation of a specific wavelength, and different colours are just minor differences in that wavelength) and toughen the eye a little more biologically to be able to withstand long term exposure to low levels of radiation over a longer period.
Ultimately, nature is far more likely to adapt an existing system to a new purpose than it is to generate a new organ from scratch, and we already have organs that detect very specific wavelengths of radiation called light. It just seems far more likely that humans in the conditions you describe would have their eyes enhanced over time by evolution to detect more bands of radiation as visible light to meet the need you describe.
Ionizing radiation, as the name says, induce ionization in the matter with which it interacts.
Cells are able to interact and control ionized charges (see for instance the $Na^+$/$K^+$ pump), so it is plausible that a mechanism able to detect charge formation can be obtained by the cells: as soon as a meaningful charge is balanced by the cells, this trigger a signal.
In order to detect the direction of the impinging radiation, considering that the probability of interaction is proportional to the depth of penetration, one can imagine 3 orthogonal tubes as a mean for directionality discrimination.
A scintillator is a material that gives off visible light when struck by ionizing radiation. Engineer your modified humans to accumulate suitable compounds in the retina, and you've got someone who can see radiation. The classic scintillator is zinc sulfide, which is sensitive to X-rays and beta radiation, but there are a wide range of other organic and inorganic substances (usually crystals) that you can use, with different sensitivities. The vitreous humor of the eye, for example, appears to be sensitive to high-energy nuclei.
Keep in mind that most radiation passes through the human body without trouble. Except for low-energy beta radiation, your modified humans won't be able to tell where the radiation is coming from without assistance. But give them radiation-blocking helmets with eyeholes to provide a pinhole lens effect, and they'll be able to look around for the source, rather than just seeing a general radiation glow.
You're probably not going to be able to engineer someone to see alpha radiation: although there are substances that glow in response to the radiation, it's too easy to block. Suitable substances for alpha shielding include "clothes", "skin", and "air".
The body is able to detect radiation. When there is enough radiation in the enviroment to make you sick, the telltale sign is that you get sick. Just the same, when the level of radiation is lethal, the body signals that by dying.
That is a characteristic shared with most animals. Humans, however, have evolved beyond that and gained a skill that allows us to detect radiation without suffering any ill effects. It's called taming. Send a dog into the cave first, and if he comes back alive after an hour, you're probably good. Usually small vertebrates are affected faster and more strongly than humans.
In the absence of animals, you can use the skills of language and bullying to send others to test radiation levels for you.
To find the direction of the source, have three sacrificial aninals or people stand in different places and triangulate based on how long they take to get sick. Make sure all test subjects belong to the same species and have approximately the same body mass and age.
I asked myself the same question (except for evolution instead of genetical engineering). The answer I found back then was more basic but if you go for full genetical engineering, here are my thoughts:
Unfortunately, I could not find a reference but I know that there are some bacteria that benefit from radiation by using it for their energy conversion. They basically use the radiation energy to create some ATP. Based on that, we could engineer an organ that produces heat or fires pain signals only if this form of energy is available.
For natural evolution this probably would be a single organ that changes its purpose (like a Lymph node that becomes a sense organ) but if we get to jump to the most desirable form I would spread this across the skin so the subject could detect the source of alpha and probably beta radiation (alpha radiation does not penetrate and beta radiation gets severely reduced). That way one would feel radiation as a form of heat/pain (under the skin/ only at certain locations/ unlike the not-modified companions). This way one might even be able to tell the types of radiation apart, considering how alpha radiation only affects one side and gamma radiation merely gets reduced at all.
You could, for example, mix in plant DNA that encodes the structure of the lower epidermis of leaves modifying the stoma to heat up/ firing pain signals when there is enough radiation to use it for cellular respiration.
It is worth mentioning though, that genetical engineering is neither our only nor our best option. Cyborgs are. For example, the colorblind Neil Harbisson got an artificial sensor implanted that allows him to sense colors (ranging from ultraviolet to infrared) as vibrations in his skull. It took time for his brain to adjust but now it can process the information and he has a natural feeling for it (he can even stream a camera's point of view allowing him to take "VR" tours through space and so on). In an interview, he mentioned that he considers adding more artificial senses and named radiation as one.
Creating a cyborg takes way less time and its outcome is way easier to predict than genetical engineering. Given enough knowledge and simulation, you could engineer someone who has the character traits and physiology you need but building an artificial sensor and implanting it to someone with the required character traits is way easier and cheaper. Given the level of technology needed to make the genetical engineering work at all, we could easily build an [Edit: conveniently small] implant with little to no maintenance and little to no risk of rejection.
Also, note that plot tools like blackmailing these people with implanted bombs or artificial needs that only you can cover or addressing moral questions are possible for both scenarios though I'd expect a genetically engineered person to be programmed to be loyal, fearless and so on. If this is what you want to play with you can use brain implants and/or hormone-based brainwashing for the cyborg but if you don't want obedient soldiers, going for genetical engineering would rise a minor plot whole since loyalty is much easier to genetically engineer than new organs. [Eidt: that is, as long as it's a rare and expensive procedure available only to the government. If this is available for civilians as well, they would make their children smart rather than oediant.]
As said elsewhere, the body already detects radiation in the form of radiation sickness. The root cause of this is the death of cells, especially rapidly dividing ones. The cause of the cell death is DNA damage - radiation breaks the chemical structure of DNA.
Because DNA damage can lead to mutations and cancer, many organisms actually terminate cells when it occurs. Eukaryotic DNA is linear, and the ends (telomeres) are capped with special structures and proteins. When there are uncapped free ends, these almost certainly come from some sort of DNA damage event, and can be recognized by proteins. The famous p53 protein is present in all human cells, and when it detects DNA damage it triggers a special mechanism (apoptosis) for the cell to kill itself. In the majority of cancers, p53 must first be impaired before the carcinogen can produce a cancer cell.
You can imagine a specialized group of cells that have extremely fragile DNA (not all base sequences are equally strong, so this is quite easy). They arise from specialized stem cells, but then terminally differentiate (ie. cannot divide anymore). Their purpose is to be a radiation detector - whenever there is radiation, their DNA breaks first. Modified versions of proteins like p53 detect this, but instead of triggering apoptosis they effect some other signal. This could even be directly stimulating neurons for instant radiation sensation, or it could be slow release of some chemicals for delayed sensing. These cells are very differentiated, like red blood cells, and are wholly incapable of replicating, so despite their lack of anti-DNA damage machinery they are not likely to become cancerous (or maybe their stem cells are likely to become cancerous, and that is the drawback?).
This is not feasible with today's biotechnology (we don't understand differentiation enough to create new organs and we don't understand DNA damage that well) but certainly plausible as a molecular mechanism. Creating a whole new category of sensation in the brain seems like a hopelessly complex task, so the neural feedback of such an organ would probably hijack an existing one - so radiation might feel "hot", "wet", "sweet", "fuzzy" depending on which existing sense you pick. Keep in mind that there are actually not just 5 senses as per the colloquialism - there's at least 12 or 13.
If the organ is internal, it can detect only gamma rays, which penetrate many materials. Beta and alpha radiation cannot pass through the outer layer of dead skin. The exception is that beta-emitting radioactive particles might be inhaled and there get very close to cells, so perhaps your organ should be in the airways to detect beta-emitting airborne particles. However, you could also have small organs (the "rad eyes") on some part of the body that people scan over an object like you would with a Geiger counter.