Heavy elements

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what is a heavy element?

• A heavy element is an element with an atomic number greater than 92. The first heavy element is neptunium (Np), which has an atomic number of 93. Some heavy elements are produced in reactors, and some are produced artificially in cyclotron experiments.

What is a superheavy element?

• The definition of superheavy elements (SHE) varies among different groups of people. We use the term term SHE to refer to those elements with an atomic number greater than or equal to 112. The first superheavy element is element 113, which has been recently discovered by a collaboration of scientists from the Lawrence Livermore National Laboratory and the Joint Institute of Nuclear Research in Russia. Like some of the heavy elements, superheavy elements are produced artificially in cyclotron experiments.

What is an atomic number?

• The atomic number refers to the number of protons in an element’s nucleus. Each element has a unique atomic number and is known by that number until it receives an official name. For example, the two new superheavyelements 113 and 115 have 113 and 115 protons, respectively, in their nuclei.

What are isotopes?

• Elements are defined by their atomic numbers or number of protons in the nucleus. Elements, however, have more than one isotope. An isotope contains varying numbers of neutrons in the nucleus. Gold, for example, has one stable isotope often denoted as

What are isotopes?

• . It has an atomic number of 79 (meaning 79 protons in the nucleus) and a mass number of 197, or the total number of neutrons and protons in the nucleus. Thus, there are 197 –79 = 118 neutrons in this isotope. However, more than 30 isotopes of gold are known. Each isotope has its own decay characteristics and half-life.

What are isotopes?

• For example, , an isotope with one more neutron (119) that the stable isotope , has a half-life of 2.7 days and decays by beta-decay. A very different gold isotope, , has a half-life of 5 ms and decays by alpha-decay.

How are new elements discovered?

• Several experimental techniques have been used to make new chemical elements. Some of these include heavy ion transfer reactions, cold or hot fusion evaporation reactions, neutron capture reactions, light-ion charged particle induced reactions, and even nuclear explosions. These techniques each have advantages and disadvantages making them suitable for studying nuclei in certain regions.

How are new elements discovered?

• The types of nuclear reactions that have been successfully used to produce new elements in the last decade are cold fusion reactions and hot fusion reactions. Cold fusion reactions use beam and target nuclei that are closer to each other in mass in order to produce a compound nucleus (the complete fusion of one target nucleus with one beam nucleus) with generally lower excitation energy that typically requires evaporation of one or no neutrons..

How are new elements discovered?

• This generates fewer neutron-rich isotopes of an element that have higher survival probabilities with respect to fission, but have lower fusion probabilities. An example of this type of reaction is 70Zn + 208Pb → 277112 + 1n with a cross-section of ~1 picobarn

How are new elements discovered?

• Because the 112 isotope ultimately decays by a emission to known nuclei [namely isotopes of elements 102 (No) and 104 (Rf)], identification of this element is straightforward. Hot fusion reactions use more asymmetric beam and target nuclei, produce a compound nucleus with generally higher excitation energy that typically requires evaporation of three to five neutrons, generate more neutron-rich isotopes of an element, have lower survival probabilities with respect to fission, but have higher fusion probabilities.

How are new elements discovered?

• An example of this type of reaction is 48Ca + 244Pu → 288114 + 4n with a cross-section of ~1 pb. Because of the neutron-richness of this isotope of element 114, it never subsequently decays to any known isotope, and thus its identification is more problematic. Cold fusion reactions have been successful in producing elements 104—112 and hot fusion reactions have recently provided evidence for elements 113—116 and 118.

What is a cyclotron?

• A cyclotron is a particle accelerator that boosts ions to very high velocities through a series of small kicks as the ions travel in a circular motion (or spiral). The cyclotron was invented at the University of California, Berkeley, by Ernest O. Lawrence, the namesake of the Lawrence Livermore National Laboratory.

What are some properties of artificially made isotopes?

• Isotopes of various elements that are created artificially in accelerator experiments are unstable and radioactive. Once produced, these new isotopes begin to decay; that is, change into another isotope. The time required for half of an isotope’s atoms to decay is called the isotope’s half-life. As the atomic number of each new heavy element increases, the half-life typically decreases, meaning that new elements tend to decay more quickly.

What are some properties of artificially made isotopes?

• However, physicists in the 1960s predicted that this trend toward shorter half-lives would change around element 114. They thought that some elements around element 114 would have longer half-lives, forming an “island of stability” in the midst of a “sea” of highly unstable elements.

What is the “island of stability”?

• The "island of stability" refers to a predicted region of superheavy elements on the chart of nuclides with half-lives that are longer by several orders of magnitude than the half-lives of other superheavy elements. Half-lives for elements in the island of stability may range from seconds to minutes, while half-lives for other superheavy elements may be measured in micro- or nanoseconds.

What is the “island of stability”?

• The existence of the island of stability was shown in 1998 with the discovery of the superheavy element 114. The island of stability is a specific subset of the superheavyelements, which is characterized by nuclei that have a spherical shape.

What is the “sea of instability”?

• The "sea of instability" refers to a region of elements on the periodic table that are highly unstable. These elements have extremely short half-lives that may be measured in micro- or nanoseconds. (A nanosecond is the time it takes for light to travel one foot.) This region of unstable elements surrounds the island of stability.

Why is discovering new superheavyelements important?

• Discovering new superheavy elements proves long-held nuclear theories regarding the existence of the “island of stability” and the ultimate limits of the periodic table of the elements. These discoveries also help scientists to better understand how nuclei are held together and how they resist the fission process. The skills that are acquired by conducting these heavy-element experiments can then be applied to solving national needs like stockpile stewardship and homeland security. For example, an improved understanding of the fission process will enable scientists to enhance the safety and reliability of the nation’s nuclear stockpile and nuclear reactors.

How can superheavy elements be used?

• Like most scientific discoveries, researchers do not yet know the immediate practical applications of the discovery of elements 113 and 115. Previously discovered heavy elements are used in smoke detectors (americium), neutron radiography and neutron interrogation (curium and californium), and nuclear weapons (plutonium). Scientists expect that practical applications of elements 113 and 115 also exist and will be discovered in the future.

How long did it take to discover elements 113, 114, 115, 116, and 118?

• Elements 113 and 115—The experiment began on July 14, 2003, and ended on August 10, 2003. In that time, four atoms of element 115 were produced that decayed after a given time, thereby producing element 113, which also decayed and so on. However, years of successful experiments, previous to the 115 and 113 discovery, were needed to show that the experiment could be successful. More than a year was then spent to clean the target material, ship it to Russia, make the target, and run the experiment.

• Element 114—The first element 114 experiment lasted about one year, and two atoms were discovered during that time.

• Element 116—The element 116 experiment also lasted about one year, and three atoms were discovered during that time.

• Element 118—Element 118 was produced during two separate experiments, each one lasting for several months. A total of three atoms were discovered in both experiements combined.

How long did it take to discover elements 113, 114, 115, 116, and

118?