Wikipedia:Reference desk/Archives/Science/2014 August 29

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 29

Fission matrix

Can somebody add anything about fission matrix to Nuclear fission? --217.21.43.22 (talk) 07:37, 29 August 2014 (UTC)

This is really the wrong place to ask that. We're here to answer questions, not start working on something for you. A better place to ask would be Talk:Nuclear fission - and if you fail to get a response there, you could try Wikipedia talk:WikiProject Physics. SteveBaker (talk) 15:00, 29 August 2014 (UTC)

Fission matrix method is a computational algorithm used for undersampling diagnostics in Monte Carlo method calculations. It can be used to provide estimates of the fundamental mode fission distribution, the dominance ratio, the eigenvalue spectrum, and higher mode spatial eigenfunctions. It can also be used to accelerate the convergence of the power method iterations and to provide basis functions for higher-order perturbation theory.[1][2][3] 84.209.89.214 (talk) 19:46, 29 August 2014 (UTC)

"Shock" feeling in forearm while doing pushups?

Metadiscussion has devolved into trolling and personal attacks regarding a question that is against policy anyway. DMacks (talk) 15:25, 30 August 2014 (UTC)
The following discussion has been closed. Please do not modify it.
We do not answer requests for medical advice. See a qualified medical practitioner. AndyTheGrump (talk) The following discussion has been closed. Please do not modify it. I'm having some sort of nerve problem while working out despite stretching. When I do bodyweight workouts such as pushups and dips my forearms will produce a feeling very similar to a mild electrical shock while I am getting kind of close to failure but not quite at it, so it's particularly frustrating because I'll feel like I have a few good reps still in me but it doesn't feel right to keep going after these sensations. It will usually result in my arms feeling slightly tingly/numb immediately afterwards. I do the recommended amount and variety of arm stretches and even forearm stretches beforehand. Any idea what's causing this and how to prevent it? NIRVANA2764 (talk) 19:33, 29 August 2014 (UTC) ^^^^ Wow I thought this was a simple harmless question relating to exercise physiology. I hope one day people realize this is a lot less helpful than just giving general advice with a disclaimer, because I wasn't planning on seeing a medical professional anyway and I still won't now, so now I'll just seek advice from a different online resource without this restriction, despite the fact that loads of Reference Desk editors could almost certainly give me more helpful advice NIRVANA2764 (talk) 20:07, 29 August 2014 (UTC) "I'm having some sort of nerve problem..." = request for medical advice. AndyTheGrump (talk) 21:12, 29 August 2014 (UTC) You're right. I can think of any number of things that might be going on - which is precisely why he needs to see a doctor and get a face-to-face diagnosis. ←Baseball Bugs What's up, Doc? carrots→ 21:16, 29 August 2014 (UTC) Or, you know, ask this question at any number of other venues that do not have the strict policies that we do (e.g. [4] ), keeping in mind that we should not always trust advice from strangers on the internet. Not everyone can afford to go see a doctor whenever they want. SemanticMantis (talk) 21:26, 29 August 2014 (UTC) yeah I love that that's the reason. "Go see a doctor!" Yeah don't you think I would if I could? Why do you think I'm sitting here on Wikipedia instead? 66.87.144.86 (talk) 22:13, 29 August 2014 (UTC) Are you the OP logged out, or are you just trolling? Regardless, it is not possible for anyone to diagnose you over the internet. You need to see a doctor. If you can't afford a doctor's fee, tell them so, and see what they can do. ←Baseball Bugs What's up, Doc? carrots→ 22:28, 29 August 2014 (UTC) Man, it really sucks for people living in countries without socialised healthcare. I hope you guys at least feel bad about this, especially you, Grump. You probably enjoy it. 78.148.104.246 (talk) 14:46, 30 August 2014 (UTC)

The stability of thorium and uranium

What's the reason for the island of stability from around thorium to curium? AFAIK it's not near any closed nuclear shells: is this a semimagic number? Double sharp (talk) 21:05, 29 August 2014 (UTC)

Our article Island of Stability addresses this issue in great detail. The concept of an "island of stability" is really most meaningful beyond uranium, thorium and plutonium on the Periodic Table, for the reasons you mentioned. Uranium, thorium and cerium really exist on what's called the "Mountain of Stability" along with lead on the histogram of nuclear stability, not on an "island" of stability at all.

However, a discussion of what's meant by "islands of stability" is probably in order.

from that article:

"The possibility of an "island of stability" was first proposed by Glenn T. Seaborg in the late 1960s. The hypothesis is based upon the nuclear shell model, which implies that the atomic nucleus is built up in "shells" in a manner similar to the structure of the much larger electron shells in atoms. In both cases, shells are just groups of quantum energy levels that are relatively close to each other. Energy levels from quantum states in two different shells will be separated by a relatively large energy gap. So when the number of neutrons and protons completely fills the energy levels of a given shell in the nucleus, the binding energy per nucleon will reach a local maximum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not possess filled shells."

"A filled shell would have "magic numbers" of neutrons and protons. One possible magic number of neutrons for spherical nuclei is 184, and some possible matching proton numbers are 114, 120 and 126 – which would mean that the most stable spherical isotopes would be flerovium-298, unbinilium-304 and unbihexium-310. Of particular note is Ubh-310, which would be "doubly magic" (both its proton number of 126 and neutron number of 184 are thought to be magic) and thus the most likely to have a very long half-life. (The next lighter doubly magic spherical nucleus is lead-208, the heaviest known stable nucleus and most stable heavy metal.)"

"Recent research indicates that large nuclei are deformed, causing magic numbers to shift. Hassium-270 is now believed to be a doubly magic deformed nucleus, with deformed magic numbers 108 and 162. It has a half-life of 3.6 seconds."

"Isotopes have been produced with enough protons to plant them upon an island of stability but with too few neutrons to even place them upon the island's outer "shores". It is possible that these elements possess unusual chemical properties and, if they have isotopes with adequate lifespans, would be available for various practical applications (such as particle accelerator targets and as neutron sources as well)."

"In particular, the very small critical masses of transplutonic elements (possibly as small as grams) implies that if stable elements could be found, they would enable small and compact nuclear bombs either directly or by serving as primaries to help ignite fission/fusion secondaries; this possibility motivated much of the early research and multiple nuclear tests by the United States (including Operation Plowshare) and the Soviet Union aimed at producing such elements."

(Note: In his 1960 book On Thermonuclear War, Herman Kahn predicted that by the mid-1970s, californium-252 would (be a logical candidate for the fissile in a "nuclear bullet," with a critical mass estimated at about 5 kilograms (and occupying the same space as a sidearm cartridge). Futurist and science-fiction author John Brunner described such "pocket nukes" in his 1969 science-fiction novel Stand on Zanzibar set in the year 2010.

However, californium-252 fissions spontaneously, releasing an average of 3.7 neutrons/fission and would not be safe for use as a munition; the shielding container for a gram of Cf-252 weighs fifty tons. Cf-252 is, however, in smaller quantities a very useful neutron source for starting nuclear reactors, scanning nuclear reactor fuel rods, and neutron activation analysis in the field.)

So, that's the story on "islands of stability" and the "magic numbers" connected with them. loupgarous (talk) 22:37, 29 August 2014 (UTC)

Five kilograms in a sidearm cartridge? I'm thinking grams?

I am reminded of an article in Nature that came out after the Indian government announced their first successful nuclear test. In the photo three of their researchers were holding up their hands in a gesture of victory: one in a V sign, one with five fingers spread, one with one finger held up... Wnt (talk) 01:46, 30 August 2014 (UTC)

And you'd be right. Californium below 900 Celsius is only 1.33 times as dense as lead (15.10 g/cc vs 11.342 g/cc).

The heaviest .50 caliber bullet listed in our .50_BMG article weighs 52 grams, so assuming (for convenience) that it's all lead, a californium bullet of that same weight would weigh close to 69 grams. That's assuming the Barrett "Light Fifty" qualifies as a "sidearm." The "sidearm" thing came up from a popular-audience article in Popular Science about "nuclear six-shooters".

Of course, the 5 kilo figure for californium-252's critical mass was probably a "bare crit," and didn't reflect how much less californium might be required for use in a nuclear weapon applying tampers (to reflect some of those 3.7 neutrons/fission back toward a hypothetical californium "pit"). Bare crits for conventional fissiles (U-235 and Pu-239) are significantly heavier than actual critical masses in modern nuclear weapons, or even the two weapons used in Japan in World War Two. How much heavier is "born secret" classified information under the Atomic Energy Act.

Good catch, Wnt. Thanks. loupgarous (talk) 01:26, 31 August 2014 (UTC)

I was actually talking about the "island of relative stability", also discussed in the same article as a section Island of stability#Islands of relative stability. You can see there the islands I was mentioning (significantly at ²³²Th, ²³⁵U, and ²³⁸U, with a few isolated ones at ²⁴⁴Pu and ²⁴⁷Cm). My question is, why do these islands exist? (I think it's fine to call them islands, as they're separated from the main H–Bi continent by a strait around Rn.) Double sharp (talk) 08:09, 30 August 2014 (UTC)

I'd think of them as "mesas" or "plateaux" on the "mountain of stability." And no, I can't find explanations for why the conventional fissile elements behave as they do. I guess this re-opens the discussion, because all I can find on "islands of stability" concerns Seaborg's and other researchers' conjectures and experimental observations on the behavior of transplutonium elements. Sorry. loupgarous (talk) 01:26, 31 August 2014 (UTC)

OK, I found this post (#5). It seems that Th to Cm are in the "happy zone" past bismuth, because (1) they're light enough that spontaneous fission hasn't become a significant decay mode yet, as it will shortly become at increasing Z, and (2) the binding energy trend here ensures that the alpha decay energy is low and hence the half-life is very long. Also, ²³²Th, ²³⁸U, ²⁴⁴Pu, and ²⁴⁷Cm continue the mountain range from ²⁰⁸Pb onward, being peaks that break the surface: they're on the trend of stability. The moment you move off that thin line, beta decay becomes the dominant decay mode again. Double sharp (talk) 15:56, 31 August 2014 (UTC)

Importance of great white sharks to the marine ecology ?

I saw a TV show about several fatal shark attacks in Western Australia, in 2005, I believe. After that, they were culled, but many people protested, saying they were vital to the ecosystem. So, are they ? I imagine the concern is that, without them, seal populations will rise, and fish stocks will be depleted. I can think of at least 3 ways around this, though:

1) Orca take over the role of seal predators (possibly after having been introduced to the area by humans).

2) Other sharks take over this role.

3) People do, by hunting seals.

So, has this been studied ? Do we actually know the effect removing great white sharks from the environment will have (either with or without remediation efforts on our part) ?

StuRat (talk) 23:38, 29 August 2014 (UTC)

I think that, a predatory fish are brutalized, because they are hunted in shallow water, which they had never done, because the habitat of predatory fish is always a deep water, it need to think about this problem.--Alex Sazonov (talk) 00:42, 30 August 2014 (UTC)

In the 1975 movie Jaws, they said that most shark attacks on humans occur in shallow water a few feet from shore. I suppose they made that statement based on some fact somewhere. ←Baseball Bugs ^{What's up, Doc?} carrots→ 01:01, 30 August 2014 (UTC)

That's one of those bullshit statistics. Most shark attacks on people happen in shallow water a few feet from shore because there aren't that many people splashing around in 50 feet of water a half a mile out. Of course you have more shark attacks there; that's where all the people are... --Jayron32 01:33, 30 August 2014 (UTC)

Well, surfers can be found a half mile out, where the water is 50 feet deep, and sure enough, many of the shark attacks are on them. StuRat (talk) 05:20, 30 August 2014 (UTC)

Yes, of course, but the number of surfers are MUCH MUCH less than the number of waders. The fact remains that the only reason there are more total shark attacks near shore is because there are more total people near shore. That a surfer can be attacked out to sea does not much effect the total statistics any. You're confusing the meanings of the words "most" with "all". --Jayron32 19:52, 30 August 2014 (UTC)

The statement was only "most shark attacks on humans occur in shallow water a few feet from shore". You seem to have inferred that they are saying that most sharks live there, and then refuted that assumption. However, nothing like that was stated. That statistic is entirely valid, say, for deciding which areas need to be monitored for sharks. Yes, there may well be far more sharks offshore, but they aren't the most dangerous ones, those in shallow water are. StuRat (talk) 18:58, 31 August 2014 (UTC)

For reference sake, perhaps assisting with further research, see mesopredator release hypothesis in general (not without controversy) or Patterns and ecosystem consequences of shark declines in the ocean in particular, for example. ---Sluzzelin talk 01:30, 30 August 2014 (UTC)

The questioner might find "It's Okay to Be Smart's" video, "What If There Were No Sharks?", of interest. Click here for you tube link - Nunh-huh 01:39, 30 August 2014 (UTC)

Tiger sharks are almost similar in size and eat seals too. In ecology all predators form a parasitic (non-mutual) Symbiosis with theyr prey. So Predators are generally not very important for the ecology. Usually no other species relies on them. A common argument is that predators keep their prey from overpopulating. But Humanity almost extinct all Whales without anyone noticing for example an overpopulation of Krill. --Kharon (talk) 01:56, 30 August 2014 (UTC)

Who says so? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:34, 30 August 2014 (UTC)

People fear sharks on an irrational level (and a rational one too, don't get me wrong); culling a species that chomped a few bathers is equivalent to derailing all the trains in your town because a kid got run over by one in another country. In the face of that, it might make the most sense to offer a similar appeal to emotion ("We're breaking nature again!") to get people to hesitate before fully committing to intentional extinction. Consider this, though: what if great whites are helping to keep down the numbers of other shark species that really have a taste for human meat, like the tiger and bull? Matt Deres (talk) 03:14, 30 August 2014 (UTC)

When does the sharks back into their natural habitat? As I know, all sharks is predators, and all the predators always is hunting in packs.--Alex Sazonov (talk) 03:26, 30 August 2014 (UTC)

The Whale shark arguably is not a predator. And there are plenty of predators that hunt solitary, e.g. Cheetahs or Golden eagles. --Stephan Schulz (talk) 12:51, 30 August 2014 (UTC)

I believe most sharks are solitary hunters, but their are exceptions, like hammerhead sharks. StuRat (talk) 23:59, 30 August 2014 (UTC)

All sharks always had a fear of enclosed spaces. I think that, the sharks can even tag their territory (aquatory).--Alex Sazonov (talk) 03:37, 30 August 2014 (UTC)

Who says sharks have a fear of enclosed spaces? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:34, 30 August 2014 (UTC)

I don't know if they have a fear per se, but sharks have to keep moving in order to breath or having moving water flow through them in the form of ocean currents. If they sit still, they drown. ScienceApe (talk) 19:24, 30 August 2014 (UTC)

And what is determines the migration of sharks?--Alex Sazonov (talk) 20:03, 30 August 2014 (UTC)

Some of us think that those who hunt sharks when the sharks do what comes naturally should be hunted to extinction themselves. HiLo48 (talk)

We're certainly working on that, little by little. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:59, 31 August 2014 (UTC)

ping StuRat I have a question or two, if you don't mind.

(1) Is there a factual basis for your statement "I imagine the concern is that, without them (sharks, from the context of the article), seal populations will rise, and fish stocks will be depleted"?

I can't imagine seals moving far enough from their habitat to seriously deplete fish stocks. I had the impression that man, in industrial fishing boats, is the apex predator of fish stocks.

(2) Are sharks even close to being the apex predator of seals? I thought polar bears and man were in that niche. loupgarous (talk) 13:06, 31 August 2014 (UTC)

Well, the TV show didn't state why they thought hunting sharks might cause a problem, and this was my best guess at what they imagined. This is in Australia, so no polar bears there. I'm not sure if people there hunt seals much, either. I don't understand what you mean about seals having to leave their habitat to deplete fish stocks. They can deplete fish stocks wherever they naturally hunt, if their population gets high enough. They might only deplete a few species, but that might be enough to cause panic among fishermen. As for fishermen depleting fish stocks, yes, that certainly can happen, but we can switch to other species until one rebounds, etc. Seals presumably don't know to do that until the first species is wiped out. StuRat (talk) 18:49, 31 August 2014 (UTC)