What does it mean to synthesize elements
The creation of super heavy elements
With the help of particle accelerators, physicists produce super-heavy elements - these atoms with more than 104 protons in their nucleus do not occur naturally and usually decay in fractions of a second. Researchers like Christoph Düllmann fromInstitute for Nuclear Chemistry at the University of Mainz hope However, above a certain number of protons and neutrons, you will encounter more durable elements - the so-called island of stability.
It has been around four years since the number of elements in the periodic table continued to grow: At that time, elements 114 and 116 were officially recognized and were given the names Flerovium and Livermorium. Their atoms have a nucleus with 114 or 116 protons, which makes them one of the so-called super-heavy elements that do not occur naturally on Earth, but can only be generated with the help of particle accelerators - for example, the UNILAC at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt. With it, the existence of element 117 was confirmed in May 2014; the final proof for element 118 is still pending. The researchers are currently investigating the already known super-heavy cores in more detail.
Christoph Düllmann: “We are interested in their fundamental properties, be it the study of how best to manufacture them or the study of their nuclear properties. That is, how long do the isotopes of these elements live, how do they decay, and also their chemical properties. These are elements that have their place in the periodic table of the elements and we are interested in the question, do they fit there, do they behave like their next homologous element, which is directly above it in the periodic table. These similarities are ultimately the basis for the structure of the periodic table. "
The nuclear chemist and his colleagues primarily investigate various isotopes. A chemical element is characterized by the number of its protons in the nucleus, but the number of neutrons can vary, in which case one speaks of an isotope. Ordinary hydrogen, for example, only has one proton in the nucleus, but there are also hydrogen isotopes with one or even two neutrons in the nucleus.
“If you go to heavy elements, you come to atomic nuclei with more and more protons. These protons are positively charged and all repel each other. The more protons you put in these nuclei, the greater this repulsion will of course be. This leads to the nuclei becoming more and more unstable, that is, their half-lives decrease more and more. For the heaviest elements known today - they are in the range from 110 to 118 - the half-lives are usually very short, in the range of seconds and less. "
However, the stability of an atomic nucleus is not only influenced by the number of protons. There are other effects that play a role here.
“The atomic nucleus can also be understood in such a way that it contains shells that are gradually filled up. These hold a certain number of protons and neutrons. When such a shell is filled, it gives the atomic nucleus additional stability. "
The number of protons or neutrons it takes to complete the shells is what scientists call magic numbers. For example, tin with 50 and lead with 82 protons have magic numbers for the proton shells.
“The next magic number is currently open. There have been theoretical predictions since the 1940s and 1950s that place them in the area of element 114. More recent calculations still partly say element 114, partly element 120 or element 126. We do not know whether elements up to element 112 are recognized. You are officially discovered. The same goes for elements 114 and 116. And there are reports that I think are very credible that elements 117 and 118 have also been detected, but the status of the discovery is not yet official. "
UNILAC linear accelerator
For completely filled neutron shells, as with protons, the theory predicts that they contribute to a significantly higher stability of the respective isotope.
“This means that we are very interested in producing nuclei that are even more neutron-rich and that should have even longer half-lives in order to then reach the next proton shell as well as the next proton shell that has not yet been reached. The theory agrees where this shell closure should be: It is expected to be in the range of 184 neutrons. With the most neutron-rich nuclei we have at the moment, we are still several neutrons away from it. "
Christoph Düllmann and his colleagues are therefore working on isotopes of element 114, for which at least the proton shell could already be closed. The researchers have already produced various isotopes of 114 and found that the half-life increases noticeably with a few additional neutrons: from a tenth of a second to two seconds, that is, twenty times as much.
“We don't know whether this trend will continue linearly or whether it will still be evident when you get to the shell. In any case, this gives us clear indications that longer-lived isotopes are to be expected, but ultimately the question remains how these can be generated. "
The isotopes of superheavy elements are created from lighter elements by fusing their atomic nuclei.
“You think about what you have to merge with each other. It is clear that the sum of the protons of the starting nuclei must be 114. In practice, one type of atomic nucleus is applied to a thin film, i.e. a target is created. The other type of atomic nucleus must be accelerated to shoot at this film because the atomic nuclei, both of which are positively charged, repel each other first. This Coulomb repulsion has to be overcome. That is why we need the accelerator facility here at GSI: We make a beam out of one type and shoot at the target from the other type. "
Drift tubes in the accelerator
For element 114, a target made of neutron-rich plutonium isotopes is bombarded with a beam of calcium isotopes. On average, the researchers only receive a single atom of element 114 per day in this way, because the nuclei rarely fuse. Nevertheless, the low generation rate is sufficient to investigate essential properties of the new elements.
“In these experiments, we essentially measure the service life, which gives us a measure of the half-life. We measure which type of decay the element or isotope dominates and the probability with which the synthesis succeeds. We call this the cross section. In a first experiment of this kind, that's pretty much all the information you get. "
If a special device is added, the atomic mass can also be determined. This does not depend solely on the individual building blocks, i.e. protons, neutrons and electrons. Part of the total mass is converted into binding energy that holds the core together. If an atom is significantly lighter than the sum of its components, the binding energy is very large and the isotope is correspondingly stable.
“By weighing the atomic nuclei, you can learn something very directly about stability. Even if we have not yet done this with element 114 because we do not yet have the necessary sensitivity, our colleagues have succeeded in making these measurements with atomic nuclei of elements 102 and 103 in recent years. This is the first time they have carried out such direct mass measurements, beyond uranium, which has been the heaviest element for which these direct measurements have been made. "
Detector structure TASCA
Christoph Düllmann's team is also investigating whether element 114 belongs to the group of heavy metals - like lead, which is above it in the periodic table. However, other calculations may also apply, according to which element 114 is a noble gas, similar to radon. The solution lies in its chemistry, i.e. the reactions that element 114 enters into with other elements. For this reason, the researchers channel the isotopes of element 114 into a gold-coated channel. If it is a heavy metal, the atoms should bond to the gold atoms and disintegrate at the beginning of the detector. If, on the other hand, Elements 114 is a noble gas, Düllmann and his colleagues could prove that too.
“To do this, the end of the gold channel is simply cooled to very low temperatures, so that even very low binding forces, such as van der Waals bonds, which noble gases enter into, are sufficient to cause the radon or an element 114 in the form of noble gases to deposit on this gold surface allow. Then one would simply detect the decay of element 114 at the very end of this gold-coated channel. "
The experiments in the gold channel are still ongoing, Christoph Düllmann and his colleagues are hoping for the first results in autumn 2015. In addition to element 114, somewhat lighter atomic nuclei are also produced and investigated at the GSI. Just recently, a team of researchers announced the discovery of four new isotopes, one each from the elements berkelium and neptunium and two from americium. The proton numbers of these elements are between 93 and 97. The properties of the new isotopes are important for the further development of the theoretical models that describe the structures inside the atomic nucleus. Around 3000 different isotopes of the elements up to 114 are known to date. It can be assumed, however, that over 4,000 more exist and have not yet been discovered.
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