Authority on Radiation needed

[Jpax2003]

Ok, while watching BattleStar Galactica last night with a friend he commented on how cool it was that the BSG withstood a nuke (opening credits). I mentioned that most the energy might have been released as X-rays with some neutron activation on the hull. Then he says that neurons wouldn't interact with the hull, but only with water or biological material in any people. Read that again, he says neutrons don't interact with materials other than water!

I gave him a funny look and asked him how a nuclear chain reaction works if the material doesn't interact with neutrons, and he said it was due to alpha and beta radiation. Incredulous, I slap myself on the forehead and said "what?" He then tells me that he knows this because he took a class on nuclear materials to be certified to use certain tools that require the use of radiation sources (americium or cesium) for his job as a geologist. That was his appeal to authority and no matter how I tried to explain, he wouldn't budge. And this guy really is trusted with nuclear materials. I've seen the box, chained up in his work pickup's bed with a big nuclear materials sign on it. (No, it was not hidden, just chained and locked up but otherwise left in the open in his apartment complex's public parking lot when he's off duty.) I've known this guy for over 6 years and never did I realize how ignorant he was of something so dangerous about which he claimed and should have been an expert. And I thought he was smarter than the average person! These are the people between you and unintentional radiation exposure. Be afraid, be very afraid.

But I'd just check with all of you experts out there, incase my understanding is wrong. #-o Do neutrons only interact with water? Maybe he was thinking of water as a mediator in general. He said that neutrons would go right through the other materials like metal. Maybe he was confused with neutrinos. While alpha and beta radiation may be released during a fission chain reaction, I think my question was straightforward and unambiguous: "If uranium doesn't absorb a neutron then how does it maintain a chain reaction?"

[azazul]

Now I am not really an expert so to speak, but I just took a class in radiation physics. You are correct in that the neutrons would interact with any material. The reason water is used as a mediator is because it contains lots of hydrogen. The majority of interactions will be between the neutron and the nuclei of atoms(The electrons won't be affected much since neutrons lack charge and the electrons are so small). Assuming that the majority of the neutrons hit a hydrogen nucleus at some point, then the neutron will lose much of its energy. As opposed to being knocked directly back without losing much energy if it runs into a larger atom nucleus. Now all this does not mean that it does not interact with anything else. It will indeed interact with any nucleus that is in its path. So to sum up, your friend is very much mistaken, but then they probably have not had that much in the way of physics classes.

[Jpax2003]

I'm not sure what classes he had, but he has a 4 year degree, probably a BS, from a major state college as well as whatever was required for certification. Of course he has a habit of thinking he is right and I am wrong and rarely admits he made a mistake. I have developed a habit of ignoring him after he appeals to his own authority. :D

[azazul]

he has a 4 year degree
Do you know what in? Hopefully not physics.

[The Bad Astronomer]

Send a PM to JayUtah and ask him to post here. He's your best bet on this board.

[Jpax2003]

he has a 4 year degree
Do you know what in? Hopefully not physics.

Geology. IIRC, the university has a decent geology program. I almost went into meteorology which is the same or a related department, I think. Then again a nearby nuclear plant was built on top of a fault that the geologists didn't notice until well into construction. I guess you can't trust geologists with anything radioactive. :D

[azazul]

Did your friend happen to say how neutrons only affect water? The principles involved?

[W.F. Tomba]

Invite him to the BABB and we'll see how long his nuclear notions hold up . . . :lol:

[Donnie B.]

Your friend is dead wrong, and the counterexample you give is a perfectly valid one. Neutrons do interact with water, but also with virtually any other material.

I bet you're right in suggesting he's got them confused with neutrinos. If he's heard about Kamiokande or one of the other neutrino detectors, he'll have noticed that they use large volumes of water (or light hydrocarbons) to snag the occasional neutrino, with photomultiplier tubes to detect the resulting flashes of brehmstralung (sp.?) light emission.

Another possible point of confusion is that neutrons of various energy will interact in different ways with a given material. The effect is called "cross section". Neutrons having a certain range of energy are far more likely to be absorbed (or cause fission, or whatever) than others. One would say, for example, that 235U has a cross section for fission that peaks at [some particular neutron energy].

That's what the moderator in a nuclear reactor does: it converts high-energy "fast" neutrons (produced by a fission event) into lower-energy neutrons that have a much higher probablilty of producing another fission (assuming they happen to hit a 235U nucleus). In a weapon core, no moderator is required simply because the density of fissionable nuclei is so high that even fast neutrons will do the job well enough.

[swansont]

DonnieB pretty much nailed it. Having water in a reactor (depending on the design) is usually to transport the heat and to slow down (moderate) the neutrons because the fission cross-section is higher, and the capture/fission rate is smaller. In fact, the ideal moderator doesn't capture neutrons, it only scatters them - the deuterium in heavy water is better at this than the Hydrogen in light water - to maximize the fraction of neutrons available to be absorbed in the fuel.

Technically the photons released in the fissions and nuclear decays are gammas, as they come from nuclear reactions (physics definition rather than the astronomy definition). Most of the energy immediately released is kinetic energy of the fission fragments. Subsequent photons from ionization/recombination and Bremsstrahlung would be X-rays.

(And not to use argument from authority, but I was an instructor of physics and reactor principles for the US Navy's nuclear propulsion program.)

[jrkeller]

While I do not considered myself an expert in this field, I took several classes on nuclear physics and nuclear power plants during my years at college. That's enough to tell you that your friend is wrong.

Breeder reactors (http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fasbre.html) use the principle of neutron absorption to create new fuel.

[Glom]

If only water absorbed neutrons, we wouldn't have a problem.

[Eta C]

Your friend is dead wrong, and the counterexample you give is a perfectly valid one. Neutrons do interact with water, but also with virtually any other material.

To add to the argument from authority, I'm a particle physicist. Neutrons don't primarily interact via EM forces (they are neutral after all) but via the strong nuclear force. As a result, they penetrate materials more than charged particles (or photons) do. Hydrocarbons provide the best shielding as they slow down neutrons to energies where their interaction cross sections increase (see below). Water or graphite will also do.

I bet you're right in suggesting he's got them confused with neutrinos. If he's heard about Kamiokande or one of the other neutrino detectors, he'll have noticed that they use large volumes of water (or light hydrocarbons) to snag the occasional neutrino, with photomultiplier tubes to detect the resulting flashes of brehmstralung (sp.?) light emission.

The primary reason superK and Sudbury use water is for the Cerenkov detection mechanism (not bremstrallung). When a neutrino interacts the particles produced are detected via the Cerenkov light they give off. A pile of steel or lead would also make a good interaction mass, but would be harder to instrument.

Another possible point of confusion is that neutrons of various energy will interact in different ways with a given material. The effect is called "cross section". Neutrons having a certain range of energy are far more likely to be absorbed (or cause fission, or whatever) than others. One would say, for example, that 235U has a cross section for fission that peaks at [some particular neutron energy].

That's what the moderator in a nuclear reactor does: it converts high-energy "fast" neutrons (produced by a fission event) into lower-energy neutrons that have a much higher probablilty of producing another fission (assuming they happen to hit a 235U nucleus). In a weapon core, no moderator is required simply because the density of fissionable nuclei is so high that even fast neutrons will do the job well enough.

Quite right. Your standard nuclear reaction occurs when a uranium atom absorbs a neutron and splits into two smaller atoms plus three more neutrons. These neutrons split other atoms and your chain reaction proceeds. The cross section for this absorbtion goes up as the neutron energy goes down, so a moderator such as water or graphite is necessary to get a working reactor. Most modern power reactors use water, but the first Chicago pile used graphite (as did the more infamous reactor at Chernobyl).

[Glom]

[Most modern power reactors use water, but the first Chicago pile used graphite (as did the more infamous reactor at Chernobyl).

So do the British AGRs and the new very high temperature reactors.

[Hutch]

Send a PM to JayUtah and ask him to post here. He's your best bet on this board.

I would seldom have reason to correct the BA on anything, but given that:

Swansont--I was an instructor of physics and reactor principles for the US Navy's nuclear propulsion program.

jrkeller--I took several classes on nuclear physics and nuclear power plants during my years at college.

EtaC--To add to the argument from authority, I'm a particle physicist.

I would note that perhaps even the BA underestimated the level of brainpower on this site!!

I've learned more from you guys here than I have in the past 30 years on a wide variety of Science-not just astronomy. Just a quick thanks... =D> =D> =D>

[Glom]

Don't forget my signature.