Pique the Geek 20120506: Promethium, another odd Element

(9 pm. – promoted by ek hornbeck)

Last time we discussed technetium, and now we shall discuss the only other element with Z < 82 with no stable isotope, promethium (Z = 61).  But there is more business than just that, and it has to do with a suggestion that commenter Wreck Smurfy‘s suggestion that I use actual hyperlinks to key terms rather than just bolding them.  There shall be more about that later.

Promethium is actually not as interesting as technetium, but still has its moments.  It has a storied tale of claimed discoveries, and one of my personal interests is the history of chemistry, in particular infighting by contributors.  I got into one of those contests myself back in the day, when I supported a particular geometry for the lowest triplet excited state for cyclohexen-2-one, but that is another story altogether.

Promethium, chemical symbol Pm, is a member of the lanthanide series, and those are often called the rare earth elements.  They are not all that rare, at least several of them, but their chemistry is such that they were extremely difficult to separate and purify until modern ion exchange chromatographic methods were developed after World War II, many of those techniques outgrowths of classified work during the Manhattan Project.

Before we get started, let us do a little housekeeping.  Wreck last week asked why I use bold when I introduce key terms rather than hyperlink to them.  I said that I would ponder whether or not to use hyperlinks, and I did.  I was not able to come to a proper conclusion, so I ask my readers to join in the discussion.  Here is my reasoning for NOT using hyperlinks:

First, they can be distracting.  I know that when I flit from link to link in reading a piece I have lost the flow from the original piece.  Second, I happen to disagree with some of the definitions when I read them and linking to them would be misleading.  Third, it would greatly increase the time that I have to spend writing these pieces to check many sources to make sure that I link to ones that are accurate.

I offer this compromise:  I shall use them more frequently than I have in the past, but want to continue the general trend of using bold only for key terms.  I would very much appreciate my readers’ thoughts about this in the comments.

For many years it was claimed that the element with Z = 61 had been discovered, purified, refined, or otherwise found.  Every one of those claims were false, because it simply does not exist in quantities great enough to be found except by using advanced instrumentation with enormous sensitivity.  There are probably fewer than two pounds of natural promethium in the entire earth’s crust.  Most of that comes from spontaneous fission of ²³⁸U.  Some rich ores have been found to contain 4 parts in 10¹⁸ parts of rock.  That is why modern detection equipment is essential to detect it.

No isotope of promethium has a half life longer than 17.7 years.  Counting nuclear isomers, there are 55 promethium isotopes.  Thus, any promethium, either natural or artificial, has been produced recently.  It is of interest, though, that ¹⁴⁵Pm is stable to alpha decay, and actually decays by the capture of one of its inner electrons, a process aptly enough called electron capture.  When an nucleus captures an electron, its Z is decreased by 1 because the electron interacts with a proton to form a neutron.  The product is neodymium-145, Z = 60, and it is stable.  The interesting thing is that it is possible to keep ^145Pm around for a long time IF it were possible to strip it of all of its electrons.  Possible, but not feasible.

A commenter asked last week why technetium and, by extension, promethium have no stable isotopes.  I gave him a link, but thought that it would be useful to explain it here.  Here is another link, but it is not very clear.

The basic concept is that elements with odd Zs are less stable than ones with an even Z.  It has to do with nuclear quantum physics, and I am not even going to go there!  For very light elements this does not hold because of the mass defect, but for heavier elements it is pretty much universal.  It is also true that elements with odd numbers of neutrons are also less stable than elements with even numbers of neutrons in general.  When both are odd, the stability is even less (in most cases).  Here is where it gets tricky.  By the way, this applies only to beta decay, mediated by the weak nuclear force.  Beta decay can involve the emission of an electron (beta particle), a positron, or electron capture.  Those are just variations on a theme.  There is always a neutrino or an antineutrino involved to conserve angular momentum.

Think of energy surfaces that have a minimum for elements with an arbitrary total of protons and neutrons.  It does not matter if it is 20 each, 18 or one and 22 or the other, or so forth.  It turns out that there is some combination of nucleons that provides the most stable configuration.  As different combinations of nucleons are had, they are less stable than the lowest energy one.  Now it gets more complicated.  Since Z can be odd or even, there can be more than one energy surface occupying the same domain.

The bottom line is that there can be only ONE most stable nucleide occupying the intersection of the energy surfaces.  If there are existing stable nucleides in the domain with a given A, then here can not be a stable nucleide with a different configuration with respect to beta decay nearby.  For both technetium and promethium, there are stable isotopes of other elements that occupy lower energetic states, so there is “no room” on the energy surfaces for a stable isotope of either.

I know that this was a pitiful attempt at an explanation, but this is not really well worked out either theoretically and certainly not empirically.  If anyone out there reading is versed in nuclear quantum mechanics and can explain it better, please add a comment, or if you are not registered, send me the explanation on Facebook and I will make sure that it gets added.  You will recognize the proper David Smith (near Richmond) by the picture of me in the scarf.

A simpler way to grasp it, but less technical, is The Isobar Rule, postulated in 1934 by Josef Mattauch.  It states simply that if two elements with their Zs differing by one have isotopes with the same A, one of those isotopes must be unstable.  That one is much easier to understand, but does not give much reason for why it is so.

Anyhow, there are reasons that promethium was “discovered” over and over.  Since it is a lanthanide, its chemistry, particularly separating it from other lanthanides, was extraordinarily difficult prior to the development of modern ion exchange techniques.  Many claims were made because mixtures of other lanthanides were mistaken for promethium.

With the acceptance of The Isobar Rule, everyone knew that promethium would be hard to find, but might be able to be made.  Law and his group at Ohio State probably produced promethium in 1938, but not much and not enough to do any chemistry on it.  However, I believe that most scientists agree that it was first produced there and then.

The element was produced in macroscopic amounts in 1945 at what is now called Oak Ridge, as a product of fission of uranium-238 induced by intense slow neutron bombardment.  There was no doubt that element 61 was known then, but was not revealed until almost two years later.  Classified research is a quirky thing.  We know now that is was probably the fission of ²³⁵U by thermal neutrons that produced the promethium, but fission of ²³⁸U with fast neutrons is a viable process for producing it.

The metal itself was not produced until 1963, when enough promethium salts were available to make reduction practical.  A few of its properties were determined, and it behaves like other rare earth elements.  The metal itself is of little use.

The US used to make around a pound and a half of ¹⁴⁷Pm annually by thermal neutron irradiation of highly enriched ²³⁵U in reactors designed for such work.  That production was ended years ago, and Russia is the main provider.  There may be plans for the US to reenter the production activity, but with the budget being controlled by elements hostile to science it is not clear if that will happen in the near future.

As I said earlier, promethium does not have nearly the utility that technetium has, but id does have uses.  The only isotope used commercially is ¹⁴⁷Pm.  This is because it is a pure beta emitter, so penetrating gammas are out of sight.  (Subtle reference to my last Popular Culture installment.)  Most of the uses for it is because of those betas.

It can be used as a battery, and is on some guided missiles.  Since it has a relatively low radiological hazard potential it is also used in those anti-static brushes that used to discharge delicate objects such as analytical balances.  It is also used for light sources like signals that are remote and hard to service.  It is not good for watch hands because of the short half life, but is better than tritium with only a 12.3 year half life.  Additionally, tritium has higher energy beta, so everything equal a promethium lamp is brighter.

There is some speculation that it might be used for batteries for long distance space probes, but the relatively short half life is a disadvantage.  ²³⁸Pu is better because of its almost 100 year half life.  However, those batteries are not as efficient as beta cells, so we are likely to see more of them.

The Wikipedia article mentions possible future uses as a portable X-ray source, but I believe that they are probably referring to ¹⁴⁶Pm is the isotope intended.  The reason that I say this is that the low energy betas ¹⁴⁷Pm emits do not produce X-rays very efficiently.  ¹⁴⁶Pm produces X-rays by twp processes:  electron capture which causes X-rays to be produced when an electron “falls” from an higher energy electron shell to the K shell that is missing the electron.  Another mechanism that this isotope employs to produce X-rays is called by the German term Bremsstrahlung and is associated with the rapid deceleration of the highly energetic electrons that are emitted during its beta decay.

¹⁴⁷Pm is not a very serious radiation risk when handled with proper precautions, but the isotopes with hard beta radiation and the ones that decay by electron capture have to be handled carefully.

Well, you have done it again!  You have wasted many more eisteins of perfectly good photons reading this odd piece.  And even though Liz Cheney admits to herself that she is always saying stupid sounding stuff on the Fox “News” Network when she reads me say it, I always learn much more than I could possibly hope to teach writing this series.  Therefore, please keep those comments, questions, corrections, and other feedback coming.  Tips and recs are also always welcome.  Please put your two cents’ worth in about bold versus hyperlinks for key terms.  I shall be away for a while during Comment Time (The Girl is going to cut my hair tonight), but will be here before the night is done.  I shall return tomorrow at around 9:00 Eastern for Review Time.  Remember, no science or technology item is off topic in the comments.

Finally, my wrist is improving very rapidly now.  I am going to sleep without the splint tonight so the The Girl and I can wash it and then I shall let it dry overnight.  I am back to around 50% dexterity and strength now, and that is light years better than last week.

Warmest regards,

Doc, aka Dr. David W. Smith

Crossposted at

The Stars Hollow Gazette

Daily Kos, and

firefly-dreaming