Islands of stability in transuranic elements

[Moontanman]

We hear about so called "islands of stability" in super heavy elements, what does this mean exactly? Can we really expect super heavy stable elements or are we talking about elements with half lives more like minutes or hours instead of nanoseconds? 

[swansont]

It’s based on the shell model of nuclear structure

A filled shell of neutrons or protons is a low-energy state, and isotopes tend to be stable against decay (especially beta decay), and you tend to have more stable isotopes with filled shells. (the atomic analogue is noble gases)

So e.g. Sn, with 50 protons, has 10 stable isotopes. Pb, which has 82 protons, has 4 stable isotopes. 126 is also a magic number (Pb-208 is doubly magic).  It’s thought that having 126 protons might at least have a longer half-life than nearby isotopes.

[Janus]

7 hours ago, Moontanman said:

We hear about so called "islands of stability" in super heavy elements, what does this mean exactly? Can we really expect super heavy stable elements or are we talking about elements with half lives more like minutes or hours instead of nanoseconds? 

It's the second case.  Elements that aren't stable, but relatively more stable than you would expect for such a heavy element.

[Curious layman]

12 minutes ago, Janus said:

It's the second case.  Elements that aren't stable, but relatively more stable than you would expect for such a heavy element.

But what can we do with them, what uses do they have?

[swansont]

7 hours ago, Curious layman said:

But what can we do with them, what uses do they have?

Why do they need to have a use?

[Danijel Gorupec]

7 hours ago, Curious layman said:

But what can we do with them, what uses do they have?

Are you asking if they can kaboom? I wonder. When I was a kid I read somewhere that super-heavy elements could have critical masses of several grams and thus be used for pocket-size 'gadgets'. I wonder if this is viable.

[swansont]

37 minutes ago, Danijel Gorupec said:

Are you asking if they can kaboom? I wonder. When I was a kid I read somewhere that super-heavy elements could have critical masses of several grams and thus be used for pocket-size 'gadgets'. I wonder if this is viable.

You’d need a significant half-life for that to work.

[John Cuthber]

4 hours ago, swansont said:

You’d need a significant half-life for that to work.

And significant luck. There's no guarantee that they would be fissionable  in a bomb.

We simply don't know.

It's possible that you could have a teeny little nuclear reactor- which would be cute, but utterly impractical.

[Curious layman]

9 hours ago, swansont said:

Why do they need to have a use?

It's their uses that excite me. The advances in tech.

So there's nothing we can do with them, well that's an anti climax. 

[Moontanman]

19 hours ago, John Cuthber said:

And significant luck. There's no guarantee that they would be fissionable  in a bomb.

We simply don't know.

It's possible that you could have a teeny little nuclear reactor- which would be cute, but utterly impractical.

If I remember correctly Americium can be used to make both a reactor and a bomb of sorts. It has been suggested for use in a direct fission reactor products spacecraft. I looked up the details, no bombs evidently but this was interesting.

https://en.wikipedia.org/wiki/Fission-fragment_rocket

Quote

Am 242m as nuclear fuel[edit]

In 1987 Ronen & Leibson [3][4] published a study on applications of 242mAm (one of the isotopes of americium) as nuclear fuel to space nuclear reactors, noting its extremely high thermal cross section and energy density. Nuclear systems powered by 242mAm require less fuel by a factor of 2 to 100 compared to conventional nuclear fuels.

Fission-fragment rocket using 242mAm was proposed by George Chapline[5] at LLNL in 1988, who suggested propulsion based on the direct heating of a propellant gas by fission fragments generated by a fissile material. Ronen et al.[6] demonstrate that 242mAm can maintain sustained nuclear fission as an extremely thin metallic film, less than 1/1000th of a millimeter thick. 242mAm requires only 1% of the mass of 235U or 239Pu to reach its critical state. Ronen's group at Ben-Gurion University of the Negev further showed that nuclear fuel based on 242mAm could speed space vehicles from Earth to Mars in as little as two weeks.[7]

242mAm as a nuclear fuel derive from the fact that it has the highest thermal fission cross section (thousands of barns), about 10x the next highest cross section across all known isotopes. 242mAm is fissile (because it has an odd number of neutrons) and has a low critical mass, comparable to that of 239Pu.[8] [9] It has a very high cross section for fission, and if in a nuclear reactor is destroyed relatively quickly. Another report claims that 242mAm can sustain a chain reaction even as a thin film, and could be used for a novel type of nuclear rocket.[6][10][11][12]

Since the thermal absorption cross section of 242mAm is very high, the best way to obtain 242mAm is by the capture of fast or epithermal neutrons in Americium-241 irradiated in fast reactor. However, fast spectrum reactors are not readily available. Detailed analysis of 242mAm production in existing PWRs was provided in.[13] Proliferation resistance of 242mAm was reported by Karlsruhe Institute of Technology 2008 study.[14]

In 2000 Carlo Rubbia at CERN further extended the work by Ronen [15] and Chapline[16] on fission-fragment rocket using 242mAm as a fuel.[17] Project 242[18] based on Rubbia design studied a concept of 242mAm based Thin-Film Fission Fragment Heated NTR[19] by using direct conversion of the kinetic energy of fission fragments into increasing of enthalpy of a propellant gas. Project 242 studied the application of this propulsion system to a manned mission to Mars.[20] Preliminary results were very satisfactory and it has been observed that a propulsion system with these characteristics could make the mission feasible. Another study focused on production of 242mAm in conventional thermal nuclear reactors.[21]

I'm not sure if the superheavy elements would have comparable half lives but they might be used in this fashion if they did. 

Edited February 16, 2021 by Moontanman

[swansont]

Americium isn't a fission fragment, so the identification as a fission fragment rocket is misleading.

The issue, I would think, is how much Am-242 you can make, and at what cost, considering that you need a conventional rector and multiple neutron absorptions to get you there.

[Moontanman]

1 hour ago, swansont said:

Americium isn't a fission fragment, so the identification as a fission fragment rocket is misleading.

The issue, I would think, is how much Am-242 you can make, and at what cost, considering that you need a conventional rector and multiple neutron absorptions to get you there.

I read it as the fission fragments coming from the fissioning of americurium not americurium as a fission fragment. 

[swansont]

3 minutes ago, Moontanman said:

I read it as the fission fragments coming from the fissioning of americurium not americurium as a fission fragment. 

Ah, yes. I only read the title initially. But that raises the question of why propose Am as a fuel. That suggests to me this is a bit of a pipe dream.