The 94th Element

Jeremy Bernstein

Pluto photographed from the New Horizons spacecraft. © NASA/APL/SwRI

Glenn Seaborg, Joseph W. Kennedy, Edwin McMillan and Arthur Wahl discovered element 94 in Berkeley in 1941. McMillan and Philip Abelson had discovered element 93 the previous year. When Martin Heinrich Klaproth isolated element 92 in pitchblende in 1789, he called it uranium after the recently discovered planet Uranus. The scientists at Berkeley named elements 93 and 94 after the planets Neptune and Pluto. The discovery of plutonium was kept secret until after the war. At Los Alamos it was called ‘49’. This did not help much since Klaus Fuchs gave all the details about the bomb to the Russians. The Germans also realised the value of element 94 for making bombs but they never could make a reactor to produce the stuff.

Seaborg’s team bombarded uranium-238 with heavy hydrogen nuclei, or deuterons (a proton and a neutron; ordinary hydrogen has only a proton in the nucleus). This produces neptunium-238, which decays into the non-fissile isotope plutonium-238. Like other plutonium isotopes Pu-238 decays into an alpha particle (two protons and two neutrons, identical to a helium nucleus) and an isotope of uranium. The half life of this decay is a relatively short 87.7 years. (The fissile isotope Pu-239 has a half-life of more than 24,000 years.) It generates a good deal of heat: about half a watt per gram. The alpha particle is not a penetrating form of radiation, unlike beta particles (electrons or positrons) or gamma rays. It can be stopped by a sheet of paper. On a visit to the test site in Nevada in 1957, I was given the pit of a plutonium bomb to hold. It was warm to the touch because of the alpha decay but I was assured that I would not suffer any permanent damage. I was also told not to drop it. The pit was the size and weight of a bowling ball. This should have tipped me off that it was hollow since I never could have lifted a solid sphere of plutonium that size. The pits were being used to test 'boosted' weapons that use both fission and fusion.

Pu-238 can be used to generate electricity safely. One of the first applications designed at Los Alamos was for use in pacemakers. The latest application is in the batteries powering the New Horizons space probe which is now passing by Pluto. The batteries were designed at the Idaho National Laboratory and are said to have cost a hundred million dollars. They contain about 10.9 kilograms of Pu-238 oxide pellets. This is a large fraction of what was available. Pu-238 is a rare commodity. The United States stopped making it in 1988 and was buying it from the Russians until they ran out. There is now a project to make more Pu-238 in the US, at a cost of tens of millions of dollars, by the neutron irradiation of element 95, americium, which is produced in small amounts in nuclear reactors. Nasa does not have much left. A good deal of what they had is now heading for outer space. There is something fitting about this. Plutonium, which does not exist naturally, is created from uranium, which came to us in the first place from the collapsing of stars.


  • 28 July 2015 at 12:10pm
    Timothy Rogers says:
    Questions here rather than a comment. Once the Pluto fly-by has been completed, how long will this particular kind of power pack be capable of running the data collection and transmission system? And in which direction (e.g., towards which nearest star or star system) will the the probe be moving? Will we (or our descendants) still be receiving data from the probe 10, 25, or 50 years in the future?

    • 29 July 2015 at 1:09pm
      Rikkeh says: @ Timothy Rogers
      There is the suggestion that New Horizons will be retasked to a flyby of a Kuiper Belt object at some point in 2019.

      New Horizons currently has around 33kg of fuel left and so is able to make minor changes to its trajectory in order to fly past a suitable object (the current favoured target was discovered only in 2014). While it is going fast enough to escape the solar system(provided it doesn't hit anything), because it's still going to do a bit of steering, we don't know *where* it's going to end up yet.

      Onboard power is provided by an RTG, essentially a nuclear battery. It has a half life of 87 years and loses roughly 5% of power every 4 years. Its predicted "end of mission" based on the RTG's rate of decay is 2026.

      That being said, the Voyager probes (launched in the 70s) have managed to eek out their diminishing power for years through a series of power saving measures, which might be continued until the mid 2020s. This suggests cause for optimism that New Horizons might be sending back data for decades to come.

      I appreciate the above answers boil down to "dunno", but I hope they've been of interest.

  • 29 July 2015 at 3:21pm
    Marianne says:
    Pu-238 has been found on Cumbrian beaches, washed from back from the Sellafield waste pipe with the tides. Come along and build a sand sculpture of collapsing stars on the beautiful beach at St Bees ....but...whatever you do... make sure you do not inhale Pu.

  • 29 July 2015 at 6:50pm
    Terry Gulliver says:
    Probably many will assure you there is strong evidence of Pu (in daughter products) having been created in a Precambrian placer deposit by gravity concentration; it is not just a man-made thing.

  • 30 July 2015 at 1:27pm
    Rikkeh says:
    The thing about radiation is that you can measure it in extremely small amounts. For example, you can measure the dosage you get from eating a banana, or from sleeping next to someone.

    To say that something radioactive is around in measurable quantities somewhere means very little. The *level* of the dosage is what matters.

    • 30 July 2015 at 4:01pm
      nickpole says: @ Rikkeh
      But how do you put some in someone's tea without getting poisoned yourself?

    • 31 July 2015 at 11:15am
      Rikkeh says: @ nickpole
      Polonium-210 only emits alpha radiation, not gamma or beta. It's highly dangerous to your cells but it cannot penetrate the skin or even a sheet of paper. So you have to get the material inside someone for it to have any effect.

      A would-be assassin waiting to drop it into someone's food could put it behind his/her ear for safe keeping and be unlikely to suffer any ill effects. Handling it strikes me as being less dangerous than carrying around warfarin rat poison (which doesn't have a half life of only 5 months).

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