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Oh hey, since we've got (assumedly) a lot of physics nerds on this thread, and because my mind hasn't suddenly stopped being curious about random topics even though I grew old: here's one of my more recent things that left me with unanswered questions:

One of the commonly cited tritium-generating reactions is 7Li+n(>2.466 MeV) -> 4He + 3H. But is 7Li not also capable of transmutation to 8Li via slow neutron capture? If so would that not yield a 16.004 MeV beta to 8Be, and then immediately into 2 alphas with an additional energy of 0.092 MeV? If so, is there not potential for a future nuclear reactor? Spallation currently yields neutrons for about 25MeV each. If one could cut that in half or less - which I don't see any laws of physics in the way, just improvements in accelerator efficiencies and the spallation process - could this not yield a net positive, using direct deceleration/capture of the beta to generate power without having to suffer Carnot losses? And if so, would that not be a very desireable reactor - nonproliferative, abundant fuel, harmless waste, high ratio of fuel to energy conversion, direct spacecraft thrust possibilities, etc? Or am I totally off base here?

Cobalt-76 ? really? It seems to me that they should publish this in a peer-reviewed journal as it seems they have found a new Isotope.
At least according to these two nuclid charts cobalt-76 is unknown.

What I can't find, and might be somewhat useful for a debate on the matter, is a table of the various isotopes of the elements and the decay heat of each.

You may have heard of that newfangled thingee called "google". When I send the words "table of nuclides" into it and hit the button "I feel lucky", it ports me to http://atom.kaeri.re.kr/ , which appears to have all the data you're asking for.

This is because iron has the highest binding energy of any element.

Actually, the isotope with the highest binding energy per nucleon is nickel-62. You can look it up.

I'd paste in a nice table that I just made, except the lameness filter won't let me.

But anyway, the isotope of Nickel with the highest binding energy per nucleon, using figures from the linked table, is Ni-62 at (8.794497 +- 2.3e-05) MeV.

For Iron, it is Fe-58 at (8.792144 +- 2.4e-05) MeV.

By way of comparison, the most abundant isotope of Nickel is Ni-58, at 68% abundance according to Wikipedia.
Ni-58 has binding energy per nucleon of (8.731963 +- 2.5e-05) MeV.

As for Iron, viz. Fe-56 (at 92%), with (8.790248 +- 2.4e-05) MeV.

Anyway, binding energy is very important but it is certainly not the only thing which determines what isotopes get produced most often.

The problem with putting the fuel in the middle of the tokamak is that it's very dense in there. The toroidal and poloidal magnets, inner shielding, cooling units, and related cabling/equipment take up much if not all of the room in the inner ring so putting fuel and related cooling in the center is one of those things that was ruled out after some research. So the only place you can really put the fuel is in the outermost portion, within the magnets but outside the pressure vessel (the magnets being more D-shaped and the plasma chamber being a deformed oval leaves some room to the outermost). Lithium doesn't make a very good moderator though, better than most since it's low-Z but it would still take quite a few interactions to slow down a 14 MeV neutron down to the energy that U-238 would absorb. U-238 absorbs most in the 1/v range (low energy from about .1eV to .00253eV/thermal range), so you'd really have to slow down the neutrons to get to that range, and when slowing down neutrons, every collision matters to decrease the possibility of leakage. Hence it's much better to use fast spectrum actinides like Np, Pu, Am, and Cm for a fusion neutron source. As for the neutron flux distribution, it's pretty strong all over, except it's the strongest on the outside of the toroid since that's where the plasma density is the greatest so the majority of neutrons are slung outwards due to some of the more enjoyable aspects of plasma physics (drift motions and whatnot). So the best place to collect the neutrons is on the outside. Using different sizes of tokamaks is possible, but actually achieving fusion requires certain geometry constraints. Since the plasma is pulsed, the field is constantly changing and that relates directly with the plasma temperature. So you can't make it too small otherwise you can't reach ignition temperatures (~5keV for most). If you're still curious about some of the cross sections, there's an online plotting cgi gateway at the Korean Atomic Energy Research Institute (http://atom.kaeri.re.kr/). Cool stuff but not doppler broadened for the range used in fusion reactors.

Can't believe there aren't enough /.ers familiar with nuclear science to bring up the Chart Of The Nuclides....

All of a sudden, everything on the P/T seems so oversimplified

Here's a link for those who don't know how to use Google: http://atom.kaeri.re.kr/

Don't know where most of my last post went, so I'll submit it again

If that were the case, natural uranium ore would be a critical system with itself as a moderator, but that just isn't the case. If you look at the cross section graphs for Pu-239 as compared to U-238 for the fast energy range from greater than 1eV to ~10MeV, you'll see that the microscopic cross sections for absorption are relatively similar between the two isotopes which is ~1 barn, as to be expected since both are fertile isotopes that absorb to higher-Z elements, but that is for only for absorption which has mixed effects for a supercritical system. What you have to look at is what the isotope does with that absorbed neutron, which is referred to as the fission cross section. That for the Pu-239 is close to 1 barn for the entire fast spectrum including resonances, yet the fission cross section for U-238 is on the order of 10^-5 barns, which means that this is extremely, extremely unlikely to occur. So you must have read a typo or they meant to say U-235, because U-238 does not fast fission often as it is considered non-fissile, otherwise the Navy would be using natural uranium in their subs and carriers because it is loads cheaper than plutonium.

Former BNL site for table of the nuclides, now at KAERI if you want to compare cross sections.
http://atom.kaeri.re.kr/ton/index.html

Um, gammas are emitted as shown by this diagram of the alpha decay of Pu-238. I see about 29% of the time a ~43 keV gamma being emitted since the alpha doesn't always drop the U-234 to its ground state (only ~71% of the time). You might want to still make the sperm deposit.

Um, gammas are emitted as shown by this diagram of the alpha decay of Pu-238. I see about 29% of the time a ~43 keV gamma being emitted since the alpha doesn't always drop the U-234 to its ground state (only ~71% of the time). You might want to still make the sperm deposit.

Um, gammas are emitted as shown by this diagram of the alpha decay of Pu-238. I see about 29% of the time a ~43 keV gamma being emitted since the alpha doesn't always drop the U-234 to its ground state (only ~71% of the time). You might want to still make the sperm deposit.

My mistake was in remembering subcritical multiplication being the dominant process when in fact it seems not to be for the RTG's we all know and love (the ones on Cassini).

You can check the cross sections and decay rates at the online chart of the nuclides, hosted in South Korea. (Until recently, that was hosted at BNL; does anyone else find it ironic that the Koreans are now exporting nuclear information to Berkeley?

Po-210 has one decay product, whereas Pu-238 has at least 8 steps in some of its decay chains.

Oddly, there is a decay chain starting at Pu-238 that leads to Po-210!

Po-210 has one decay product, whereas Pu-238 has at least 8 steps in some of its decay chains.

Oddly, there is a decay chain starting at Pu-238 that leads to Po-210!

There's also an interactive one, color-coded for lifetimes, here. The half-life of these elements decreases from millenia to microseconds.

Cool link! As usual, since I'm not a physicist, the chart brings up more fun questions than it answers. Here's a question that I hope doesn't get me in trouble with Mr. Ashcroft & co!

According to the page I linked above, Uranium and Plutonium, the most well-known nu-cu-lar bomb materials, have isotopes with half-lives > 100,000 years. That explains how they can be stable enough to be worked into a sub-critical mass that can be compressed explosively into a critical mass.

But look up a couple of steps. Curium, element 96, has a couple of of isotopes with similar longevity. We know that after WWII, scientists studied the heck out of the trans-uranium elements... I wonder if anyone ever attempted to use Curium as a fissile material? Someone had to have the crazy idea to try Plutonium, so you have to figure someone tried it.

I did a quick Google, and didn't find much. But this article is pretty cool -- it turns out that Curium is patented! Glenn Seaborg (immortialized with his own element, #106 Seaborgium) patented it along with Americium -- the radioactive element in your home smoke detector. Does that mean that nobody can use Curium in their bombs without paying royalties to his estate?

So I'm wondering - what's the point ?

Elements 83 (bismuth) and under have one or more stable isotopes, and one or more unstable isotopes. So, for example, hydrogen (element 1) is stable, but deuterium (H-2) and tritium (H-3) are not. Nevertheless, these unstable isotopes are useful. Deuterium is used in nuclear medicine, in heavy water for nuclear reactors, and in fusion reactions. So...

Myth: Unstable isotopes are useless.
Myth Busted!

Past element 83, there are no stable isotopes. There's a pretty good chart showing the stable and unstable isotopes here. There's also an interactive one, color-coded for lifetimes, here. The half-life of these elements decreases from millenia to microseconds. However...

It's been known for decades that certain numbers of protons are "magic" in that they "pack together" in a very stable manner. Same thing with neutrons. As we approach the next "magic" numbers, the half-lives of the elements should start going back up. And they do.

In this latest experiment, the particular isotope of element 113 *may* have lasted for as long as 1.2 seconds. That's a long time for such a heavy element. Elements under 113 last for much less time, so that shows that we may be reaching the region of stability.

The region of stability is apparently close by, and *stable* superheavy elements will assuredly have useful properties.

And that's why nuclear chemists continue to search for heavier and heavier artificial elements. Because one day one of them will last for more than a few seconds. And then one day, one of them will last forever. Instant revolution in materials science.

Myth: There's no point searching for superheavy elements.
Myth Busted!

--Rob

Alpha emitters are great for this kind of work, because alpha particles have a high interaction cross section once they're inside the body. That concentrates their damage in a small space. (You can handle blocks of alpha-decay material without hazard, because the alpha particles plough into your epidermis and stop there, wreaking terrible damage on ... tissue that's already dead.)

I bopped on over to one of the online charts of the nuclides to check out the decay chain of Ac-225. Indeed, the next two daughters are alpha-emitters, but the first one, Fr-221, has a 5-minute half-life. That ought to give it plenty of time to get ducted around into your bloodstream and into the rest of your body before emitting the next two alphas and a couple of beta particles, eventually transmuting to stable Bismuth.

So the developers aren't being quite candid when they say that the daugter alpha particles could inflict additional damage on the tumor. Sure, they could -- but (with the antibody bonds long since broken by the recoil from the initial decay) that atom could end up anywhere in your body before decaying again.

This stuff is interesting -- I used to make radioactive saline at the Reed Reactor Facility for medical uses, so I poked around the chart of the nuclides to see how one would make Ac-225. Ideally, you want to start with a nice, stable (or at least long-lived) element, kick a neutron into it (by lowering the ore into a nuclear reactor), and let it turn into what you want via a series of rapid decays. (That's one way to make the Americium 241 in smoke detectors; I'll leave the source element as an exercise for the reader). But Ac-225 doesn't seem to have any such nice precursor decay paths with short half-lives. The half-life is short enough that you wouldn't want to get it from spent fuel (too `hot' until after the Ac-225 is gone!), so I'm not entirely sure how you'd make it.

Alpha emitters are great for this kind of work, because alpha particles have a high interaction cross section once they're inside the body. That concentrates their damage in a small space. (You can handle blocks of alpha-decay material without hazard, because the alpha particles plough into your epidermis and stop there, wreaking terrible damage on ... tissue that's already dead.)

I bopped on over to one of the online charts of the nuclides to check out the decay chain of Ac-225. Indeed, the next two daughters are alpha-emitters, but the first one, Fr-221, has a 5-minute half-life. That ought to give it plenty of time to get ducted around into your bloodstream and into the rest of your body before emitting the next two alphas and a couple of beta particles, eventually transmuting to stable Bismuth.

So the developers aren't being quite candid when they say that the daugter alpha particles could inflict additional damage on the tumor. Sure, they could -- but (with the antibody bonds long since broken by the recoil from the initial decay) that atom could end up anywhere in your body before decaying again.

This stuff is interesting -- I used to make radioactive saline at the Reed Reactor Facility for medical uses, so I poked around the chart of the nuclides to see how one would make Ac-225. Ideally, you want to start with a nice, stable (or at least long-lived) element, kick a neutron into it (by lowering the ore into a nuclear reactor), and let it turn into what you want via a series of rapid decays. (That's one way to make the Americium 241 in smoke detectors; I'll leave the source element as an exercise for the reader). But Ac-225 doesn't seem to have any such nice precursor decay paths with short half-lives. The half-life is short enough that you wouldn't want to get it from spent fuel (too `hot' until after the Ac-225 is gone!), so I'm not entirely sure how you'd make it.

The RTG can't shield the neutrons... the sheilding would weigh more than the rest of the spacecraft.. Beyond Jupiter RTG radiation is that largest non-gravitational effect, larger than solar pressure even

Sorry, but effectively there aren't any neutrons. The P238 isotope used is used because it decays to U234 with the emission of an alpha particle and releases this energy fairly quickly, but not too fast - a half-life of 87.7 years. The alternative spontaneous fission only occurs .00000019% of the time. Effectively zilch.