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NOVA ScienceNOW

Island of Stability: Expert Q&A

  • Posted 10.10.06
  • NOVA scienceNOW

On October 10, 2006, nuclear chemist Ken Moody answered questions about the ongoing quest to forge new chemical elements and the search for a so-called Island of Stability.

Ken Moody

Ken Moody

Chemist Kenton Moody has worked at the Lawrence Livermore National Laboratory since 1985, making radiochemical diagnostic measurements in support of various national security programs. Full Bio

Photo credit: © NOVA/WGBH Educational Foundation

Ken Moody

Chemist Kenton Moody has worked at the Lawrence Livermore National Laboratory since 1985, making radiochemical diagnostic measurements in support of various national security programs. In addition to his classified assignments, he is the senior member of the Heavy Element group, performing basic research on the elements at the extreme limits of the periodic table, in collaboration with physicists at accelerator laboratories in the former Soviet Union. He is the codiscoverer of three dozen heavy isotopes and five chemical elements.

Q: What do you think are the most exciting new developments in your heavy-element research? Sally Kopman, St. Louis, Missouri

Ken Moody: Dear Ms. Kopman,

There are two areas that I find quite exciting. One is the continuation of the work that we have performed for about 15 years in collaboration with our colleagues at the Flerov Laboratory of Nuclear Reactions in Dubna, Russia, using an isotope separator connected to their cyclotron to collect rare isotopes and isolate them from the vast amount of unwanted reaction products. The second is a new project, again in collaboration with our Russian friends, in which we are performing chemistry experiments in hopes of determining the detailed chemical properties of the elements we are making.

Shortly after our work on element 114 was published, nuclear theorists started waffling on the position of the peak of the Island of Stability. After several decades when the predictions were static, suddenly we were presented with the opinion that 114 may not be the magic proton number, but that it might actually be higher, at element 118, 120, or even element 126, depending on the nuclear model used in the calculation. Since that time, we have performed separator experiments in which we have also produced isotopes of elements 113, 115, and 116. These are very difficult experiments, but none of them showed any conclusive proof of where the magic proton number lies, just that there was a significant lengthening of half-lives caused by "magicity." We have recently made an attempt to produce element 118; we hope that the results of this experiment will help lay the issue to rest.

On the chemistry front, most of the isotopes that we have produced decay by emission of a series of alpha particles; these decay chains lead to neutron-rich daughter products that often have half-lives that are very long. For instance, one of our element 115 isotopes decays by sequentially emitting five alpha particles, arriving at an element 105 isotope with a half-life of 20 hours! This allows us to perform chemistry experiments to find out how element 105 behaves compared with its other "homolog" elements, niobium and tantalum. Other chemistry experiments are planned for isotopes of elements 110 and 112, which have half-lives of many seconds to a few minutes.

Q: The synthesis of super-heavy elements requires huge energies to produce tiny amounts of these elements. Are there sufficient amounts of many of these elements to see what they look like or how they react, or do you simply rely on predictions due to their location on the periodic table? Neil Fiertel, Edmonton, Alberta, Canada

Moody: Dear Mr. Fiertel,

The issue of "visibility" is generally left behind when we leave experiments that can be done with nuclear reactors and start working with particle accelerators. The numbers of neutrons that are available as projectiles in reactors far outweighs the number of heavy ions that we can deliver with a particle accelerator, and the probability of having the heavy-element product survive is considerably greater with neutrons. Unfortunately, there are no neutron reactions that get us to the Island of Stability. In the high-flux reactor at Oak Ridge National Laboratory, they are able to make gram quantities of californium (element 98) isotopes, milligram quantities of einsteinium (element 99), and multimicrogram quantities of fermium (element 100). For elements heavier than fermium, made only with accelerators, quantities are best expressed as a number of atoms rather than in units of weight.

That being said, we can use the radioactive properties of the heavier elements to "see" how they behave in chemistry experiments. We accumulate a number of atoms, perform a chemical procedure (which must be quick compared to the lifetime of the isotope of interest), and look for the radioactive decays that signal the presence of the atoms. Our experiments are complicated by the fact that we produce atoms so infrequently that we rarely have more than a single atom in an experiment. This limits us to studying only certain types of chemical steps.

Q: If element 114 is made with its magic number of neutrons, what is an estimate of its half-life time? John Thompson, New Castle, Indiana

Moody: Dear Mr. Thompson,

You have asked a very difficult question, probably more suited for a theorist than an experimental scientist like myself. However, let me make an estimate:

We have produced an isotope of element 114 with a mass number of 288, meaning it has 174 neutrons, leaving it 10 neutrons away from the magic number of 184. It has a half-life of 0.8 seconds. Its decay daughter is an isotope of element 112 which has seven more neutrons than the isotope of element 112 discovered at the GSI laboratory in Germany, which is so far away from the 184-neutron shell that its decay properties should exhibit no effect from extra stability due to neutrons. "Our" element 112 isotope is longer-lived than "their" element 112 isotope by a factor of 400. If adding seven neutrons results in a factor of 400 in lifetime, then adding 10 neutrons to an element 114 isotope with an 0.8-second half-life should give you a "doubly-magic" element 114 isotope with a half-life of about an hour. However, we do not know how the onset of "magicity" is affected by neutron number when you are a long way from the magic numbers, so I consider the one-hour estimate to be a lower limit.

I think you can see why chemists are very excited by our results. We just need to find a way to make nuclei closer to the center of the Island, and whole new prospects open up in the chemical sciences.

Q: Do you think that there are a finite number of elements to be found, or do you think that by synthesis or natural means there will always be another to discover? David Savage, Rochester, New York

Moody: Dear Mr. Savage,

I'm afraid that my opinion is that there is an end to the periodic table; however, I do not know where that extreme lies. Beyond element 105, nuclei exist only because of nuclear shell effects. These are quantum-mechanical rules like those that put order in the atomic electrons, imposing an order on the protons and neutrons so that the nucleus is not just an amorphous blob of nuclear particles. This ordering costs energy, which binds the nucleus so that it is more stable than would be the equivalent amorphous blob. However, as protons are added to the nucleus and its positive charge increases, the tendency for prompt disruption increases. Eventually, disruption has to win out over shell effects; when this happens, the nucleus cannot form and there will be no new elements. However, we're not there yet, and I fully expect there to be at least a couple more elements out there waiting to be discovered.

Q: Why are there 92 natural elements and not more or less? Is it possible we will discover more naturally occurring elements in the far reaches of space? David Winski, Chicago, Illinois

Moody: Dear Mr. Winski,

The term "naturally occurring elements" reflects our terrestrial bias—we think of the things we see here on Earth. Nucleosynthesis in our local area (mainly from the sun) is limited to the lighter elements. However, out in the universe, more extreme processes (like supernovae) produce elements up to at least fermium (element 100). Because of their short lifetimes, the more exotic elements tend to decay before they drift our way.

It is possible that you are asking me whether there is a change in the rules that control the lifetimes of exotic nuclei and which nuclei are stable if one were to visit a different region of space. If so, I'm afraid I have no answer—this is more of a metaphysical issue than a physical one.

Q: I've heard that some elements only exist because of having been created during a nova or supernova event. Is this true, and what precisely are those elements that are produced in that fashion? Bryan J. Lovejoy, Katy, Texas

Moody: Dear Mr. Lovejoy,

You are right. By modern thinking, the "big bang" that occurred at the start of things produced only the very lightest of the elements. Heavier elements, up to iron and nickel (elements 26 and 28) are produced in significant quantities in the various stages of stellar operation. The heaviest elements can only be produced in quantity in the high-intensity particle fluxes that are produced in the explosions of novas and supernovas.

Q: Can't you try "bowling" different elements together to reach the Island of Stability? Jacob, Mobridge Middle School, Mobridge, South Dakota

Moody: Dear Jacob,

You have pointed out the fundamental problem that we have in trying to reach the Island of Stability. Certainly there are other ways to make element 114: For instance, many years ago we considered bombarding a lanthanum target (element 57) with a lanthanum beam; this is a "symmetrical" reaction. Unfortunately, this would result in an element 114 isotope with only 164 neutrons, which is 20 neutrons away from the magic number. We also considered bombarding a target of lead (element 82) with germanium ions (element 32) to make element 114; this is an "asymmetric" reaction. However, the most neutron-rich stable isotopes of lead and germanium have 126 neutrons and 44 neutrons, respectively, making an element 114 isotope with only 170 neutrons, still far away from the magic neutron number of 184.

This is a game that you can play yourself if you have a list of isotopes like that in the Handbook of Chemistry and Physics: Add together nuclei that exist in nature whose protons add to 114, and pick the pair that give you the largest number of neutrons. You will find that the most favorable situation is the most "asymmetric" one, like germanium and lead.

Now, you have to cheat and pick targets that don't exist in nature but that can be artificially produced in the necessary quantities; this includes elements up to number 98 (californium). You will find that the most advantageous case is that involving Ca-48 and Pu-244, producing an element-114 isotope with 178 neutrons. There are some more subtle considerations (like barriers to fusion and the extra energy produced by the colliding nuclei), but the main issue just comes down to a simple numbers game that anyone can play.

Q: What about bombarding Pb-212 with Ge-74 (both in the stable column)? I know the lead only has a 10-hour half-life, but surely your experiment is shorter than that. Ken, Vestavia Hills, Alabama

Moody: Dear Ken,

It's an interesting suggestion, but there are technical difficulties in irradiating a Pb-212 target. It is very difficult to obtain quantities of Pb-212 above the nanogram range (we need milligrams), and the radiation field associated with it is very high. The other problem is the length of the experiment: This is something that didn't come out in the NOVA broadcast, but our experiments take several months each. This would mean replacing a highly dangerous, short-lived target at least once a day, while a large group of chemists worked around the clock to separate the next batch of Pb-212 and fabricate it into a target.

This is feasible, but it makes working with plutonium targets look very attractive by comparison. Also, your Pb + Ge experiment combines to give only 172 neutrons, compared with 178 neutrons in the Pu + Ca case, and we expect the most interesting physics the closer we get to 184 neutrons, which should be the center of the Island of Stability.

Q: What model is used to predict the configuration of protons and neutrons in the nucleus (i.e. the "magic island")? Is Schroedinger's Wave Equation used, or is it something else entirely? Dara Stevens, Smyrna, Tennessee

Moody: Dear Ms. Stevens,

Certainly the Schroedinger Wave Equation is applicable, but just like in atomic physics, its use is limited when you start talking about problems involving many particles. There are two issues that complicate the nuclear picture even more than the atomic picture: 1) The nuclear problem is a relativistic one; and 2) We do not know the mathematical form of the strong nuclear force (the bungee cords in the NOVA segment).

The prediction of the Island of Stability comes about in the Nuclear Shell Model. In this model, order over the configuration of protons and neutrons is imposed by quantum mechanics, as is the nuclear shape (magic nuclei tend to be spherical, but other nuclei tend to be either squeezed or flattened to some extent). Disagreements between theoretical predictions generally arise based on which model is used to estimate the functional form of the nuclear potential.

Q: What is the difference between artificial and natural elements? Lewiston High School 11, Lewiston, Maine

Moody: Hey Lewiston High School,

The superficial answer (which I'm guessing is not what you want) is that natural elements occur in nature, and artificial elements do not. This implies that natural elements are those that are stable or are long-lived compared with the age of the Earth. By this definition, the elements from hydrogen (element 1) to bismuth (element 83) are stable [except for technetium (element 43) and promethium (element 61)], as are the radioactive elements thorium (element 90) and uranium (92). Hah! If it were only so easy!

The uranium and thorium isotopes undergo radioactive decay, which produces radioactive isotopes of the elements from mercury (element 80) to protactinium (element 91). While these radioisotopes are short-lived compared with the age of the Earth, the radioactive decay processes replenish them as quickly as they decay. The radon (element 86) isotope that is found in the air in basements has a half-life of only about four days, but it is a "natural" element. Additionally, in uranium ore bodies, a variety of processes result in the fission of some very minor fraction of the uranium, and some of these fissions produce the missing technetium and promethium, thus filling in all the atomic numbers from 1 to 92. Additionally, the radioactivity of uranium ore bodies produces a very small flux of neutrons; these neutrons are captured and produce an exquisitely small amount of plutonium, again "all natural."

Our viewpoint has been terrestrial and not universal. Supernovas are thought to produce all the elements up to fermium (the limit accessible by neutron capture reactions). So strictly speaking, the realm of the artificial elements really only begins with mendelevium (element 101), and with some thought I might have to back off on that.

Great question!

Q: Where can I get an illustration of Seaborg's islands to use in teaching my high school science class? Checked Google/Google images and various Seaborg Web sites. It was cool. Gary Ruderman, Illinois

Moody: Dear Mr. Ruderman,

There is a Web site devoted to the Livermore Heavy Element Group:

http://https://www-pls.llnl.gov/?url=science_and_technology-chemistry-he_publications

It includes links to other laboratories around the world that are involved in heavy-element research, including our collaborators at the Flerov Laboratory of Nuclear Reactions, Joint Institute of Nuclear Research, in Dubna, Russia, which is where we do our experiments. There is at least one figure showing the Island of Stability. There are also some very cool photos of chemistry being performed on quantities of radionuclides that glow by their own induced fluorescence or by Cherenkov light.

Q: If all elements originated from the big bang and evolved through some cosmic process, is it possible for the deconstruction of elements to take place, to revert back to the state found at the time of the big bang? How reversible are the construction/deconstruction processes of elements? R.C. Poirier, Fort Lauderdale, Florida

Moody: Dear Mr. Poirier,

Wow, reverse engineering the universe!

I won't say that it can't be done; however, following the big bang, the formation of galaxies, the condensation of individual stars, and the explosive dispersal of the heaviest elements all involved processes that can best be described as chaotic. Also, it is just now being appreciated how dark matter is distributed in the universe, and its role in shaping the part of the universe that we can see. I will defer to the astrophysicists on this one, but my own opinion is that this is a problem that won't be solved in detail anytime soon.

Great question!

Q: I always enjoyed chemistry from high school through grad school (ms in chemical engineering). I always was curious about the element technetium. Why is it not found naturally? In the periodic table, it is surrounded by other stable, naturally occurring elements, but for some reason atomic number 43 is not stable. How come? I would always look it up in the indexes of my textbooks, but there would be very little about that element. Christina Sposato, Scottsdale, Arizona

Moody: Dear Ms. Sposato,

Technetium and promethium are two "holes" in the stable elements that comprise the periodic table below bismuth. Neither one has a stable isotope. This is a hard one to explain in a few sentences, but I will give it a try:

In the elements below lead (82), radioactive processes are dominated by positive and negative beta decay. At each atomic mass number, there are one, two, or sometimes three stable isotopes. Any other nuclei with that mass number are radioactive, and neutrons in the nucleus decay into protons or protons into neutrons, until one of the stable isotopes is achieved. The atomic mass number is not changed by these decay processes. It is mass that is conserved and not proton number, and it just happens that there are no atomic mass numbers where one of the stable isotopes has either 43 or 61 protons. As a result, all isotopes of technetium and promethium are radioactive.

If you want a more complete explanation, I would refer you to the section on the "liquid drop model" in any nuclear science text.

Q: Why can't we use Mendeleev's table? Vlad, Canada

Moody: Dear Vlad,

I'm sorry if we gave you the impression that there is something wrong with Mendeleev's table. Quite the opposite! It is still our best tool for predicting the chemical properties of new elements based on their chemical homologs, which occur just above them in the table. We recently designed an experiment to chemically isolate element 105 based on the chemical properties of its homologs, niobium and tantalum, and were successful.

However, there is a potential problem. As the nucleus gets more and more positively charged, the innermost electrons move more quickly in their orbitals. By the time you reach the tranactinides, these speeds are quite relativistic, which causes the mass of the electron to increase and the electron orbital to contract. The effect of this redistribution of charge propagates throughout the electrons of the atom, eventually reaching the valence electrons that control its chemical properties. At this time, we have not seen any evidence of gross changes in the chemical properties of the heaviest elements over what we would expect from periodicity, but there are some predictions that at element 114 periodicity breaks down and instead of having chemistry like that of lead, it will behave more like a noble gas. Clearly, we are very interested in this and would like to study it.

The onset of relativitistic effects in the innermost electrons is not abrupt, and we see evidence for it in several places: The reason that thallium (element 81) has a prominent +1 oxidation state is caused by relativity, as is the characteristic color of metallic gold—thank Einstein for your jewelry, folks.

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