Apr 30 2012

Pique the Geek 20120429: Technetium, An odd Element

Last week a commenter suggested this topic, and I am always happy to get reader feedback and try to honor requests.  Technetium is one of only two elements with an atomic number (Z) less than 82 (Z =  43) without a stable isotope, the other one being promethium, with Z = 61.  Dimitri Mendeleev predicted this element after he had perfected the Periodic Table of the elements in 1871.  He called it ekamanganese since it occupies the place in the table one row under manganese.

Technetium was claimed to have been discovered over and over, and credit to its discovery goes to Emilio Segre and Carlo Perrier in 1936.  It was discovered in a foil that Ernest Lawrence had given Segre that was composed of molybdenum.  Some of the molybdenum had been transmuted into technetium, and the Italian team confirmed this.

However, this may not be the first confirmation of technetium.  There is good evidence that a report made by Otto Berg, Walter Noddack, and Ida Tacke from 1925 confirming technetium by its X-ray spectrum.  Later workers were unable to confirm this, so the name that Noddack gave it, masurium, was dropped.  Very recent work, some of it reexamination of their original X-ray plates and some of it independent, indicates that the German team actually did discover this element.

As an aside, Ida Tacke married Walter Noddack.  She was also nominated for the Nobel Prize three times.  Of special note is that she was the first person to postulate the idea of nuclear fission.  She and Walter do have one element with which they are credited as discovering, rhenium, Z = 75.

The problem is that technetium occurs in exceedingly small amounts naturally on earth.  The reason for this is that there is no stable isotope of technetium, and the half life of the longest lived one is only 4.2 million years, a blink of the eye in geological terms.  The one that does occur in nature has even a shorter half life, so all natural technetium has an atomic mass (A) of 99, and the isotope used in medicine is produced by one of the processes that produces natural technetium.

When 235U undergoes fission, around 6% of the fission products is 99Mo.  This decays to 99mTc with a half life of just under three days.  Almost all the technetium produced is made in special reactors designed to increase the rate of fission of the uranium.  It is also formed in fuel rods in power reactors and is thus a significant contributor the waste.  The “m” means that this nuclear isomer  is metastable, decaying to 99Tc by emission of a gamma photon.  The half life for this process is a little over six hours.

This photon as an ideal energy to be detected and is the basis for the use of this isotope in medical imaging.  A 99mTc “generator” consisting of 99Mo.  As it decays to 99mTc, that isotope can be stripped from a chromatographic column and either used as is or tagged with other pharmaceutical agents for specific targeting of various systems and organs.

After then gamma activity is gone (almost all is gone in a day because of the short half life), 99Tc remains.  It has a half life of a little over 200,000 years and is a pure beta emitter.  It decays to the stable ruthenium-99 with no other products.  The electrons emitted are in a tight band are are of relatively low energy, making this isotope ideal for use of calibration standards for beta counting equipment.

A very interesting property of technetium is its anticorrosive effect on steel.  When bare steel is placed in water containing a minute amount of technetium in the form of pertechnate ion ( TcO4-), it just will not rust, even at elevated temperature.  Its radioactivity certainly limits its use for this purpose, but in some closed systems it can be used.

By far the greatest use of technetium is for medical imaging.  Very little is used for other purposes because of its radioactivity and expense.  Fortunately, for medical imaging purposes only tiny amounts are needed.

How much technetium is there?  Lots!  Estimates are that the nuclear power industry has produced several hundred tons of the element, most all of it still in spent nuclear fuel rods sitting in cooling ponds or dry storage.  Technetium is more mobile than many other radioisotopes, so is of special concern in the environment.  Since it is produced in high quantities from power reactors, it is certainly a concer,

Extraterrestial technetium is of theoretical interest.  It was once thought that all nuclei heavier than nickel were produced only in high energy processes such as supernovae.  Spectral lines originating from technetium have been observed coming from some red giant (old) stars, and there is no way that primordial technetium could account for that because of its short half life.  Thus, it must be produced contemporaneously inside those stars, or else it would have decayed away billions of years ago.  This is pretty good evidence that these endoergic processes do indeed occur.

Finally, I have a personal connexion to technetium.  One of my professors in graduate school at The University of Arkansas, the late Professor Paul K. Kuroda, predicted many decades ago that there was a good possibility that a natural nuclear reactor could have formed billions of years ago before the 235U concentration became too low because of nuclear decay.  Sure enough, in 1972 such an ancient natural reactor was found at Oklo, in Gabon.  Not just one, but several or these reactors were discovered.  Part of the evidence for them is the presence of very high concentrations of 99Ru, the stable decay product from 99Tc.

Well, you have done it again!  You have wasted many more einsteins of perfectly good photons reading this metastable piece.  And even though Newt Gingrich again realizes that he will never be President when he ready me say it, I always learn much more than I could possibly hope to teach in writing this series.  So please keep those comments, questions, correction, and other feedback coming!  Tips and recs are also welcome.  I shall hang around as long as comments warrant tonight and shall return for Review Time around 9:00 PM Eastern tomorrow.

My wrist continues to improve, and the recovery seems to be accelerating.  I am far from 100%, but have much more movement.  I am still typing with only the index finger on my right hand, so if there are lots of typos you know why.

Warmest regards,

Doc, aka Dr. David W. Smith  

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