Since the middle of the 20th century, nuclear physicists have developed the theory of the nuclear shell model, predicting that the atomic nucleus with a magic number of protons or neutrons has special stability. The theory predicts that the atomic nucleus near the "double magic number nucleus" (magic number nucleus) 298114 with a proton number of 114 and a neutron number of 184 will have high stability,Around it, there may be a "stability island" composed of hundreds of superheavy element nuclei, which may be synthesized. These nuclei are called superheavy nuclei.Elements with super heavy nuclei are called super heavy elements.
Super heavy elements The atomic weight of a hypothetical new element is greater than that of a discovered element.Once found, these elements will be listed in theperiodic tableAfter element.
According to the nuclear theory, it is predicted that there may be chemical elements with atomic numbers greater than 110.The corresponding nuclei are called superheavy nuclei.Since 1940, synthetic element 93NeptuniumAnd element 94plutoniumSince then, the periodic table has begun to "extend".With the successive synthesis of ultra plutonium elements, the question of where the periodic table can be extended naturally came up to people. In order to answer this question, around 1966, nuclear theorists based on the shell model theory of the atomic nucleus (seeNuclear model）Proposed aboutSuperheavy elementThe prophecy of existence.
Prophecy
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Developed in the late 1920sNucleusThe shell model theory explains the experimental results that the nuclei with magic numbers such as 2, 8, 20, 28, 50, 82 and 126 in the periodic table have large abundances and are particularly stable.Since 1948, American physicist MG. Mayer and others emphasizedProton numberZorNeutron numberNThe special stability of the nuclei of magic numbers.1959 Danish physicist SG. Nilsson et al. extrapolated Nilsson's single particle energy level diagram toZ=126, showingZ=114.Around 1960, Swedish physicist SA. E. Johansson, based on the general droplet model, made a shell correction by using the Nielsen orbit;His calculation proves thatN=184 There may be nuclei with long enough life nearby.From 1965 to 1966, Mayer et al. popularized the formula of the droplet model, and modified the shell when the nucleus is approximately spherical. Starting from the semi empirical mass formula, they predicted thatSuperheavy nuclear stable islandThe exists of.fissionThe barrier is as high as several megaelectron volts.Soviet physicist BM. Strubinsky developed the shell correction method and proposedZ=114 is the magic number of protons.In 1969, Nielsen and others carried out calculations and discussions comprehensively and systematically, and obtainedZ=114、N=The nucleus 298114 of 184 is a double magic number nucleus, around which there may be a stable island composed of hundreds of super heavy nuclei, of which the longest life can reach 108 years.In 1972, nuclear physicist E.O. Fissett and others predicted that the most stable half life of spontaneous fission of super heavy nuclei could reach 1015 years.In 1976, Danish nuclear physicist J. Landrup and others further revised the calculation model and predicted that the half life of the most stable overweight nuclear spontaneous fission or alpha decay was about one year.It can be seen that the half-life energies of superheavy nuclei calculated according to different models differ by dozens of orders of magnitude.1977 American nuclear physicist JR. Nicks et al estimated that the uncertainty factor of the prediction value of the half life of spontaneous fission of superheavy nuclei is 10 ± 7 ～ 10 ± 10 due to the single particle energy level diagram and calculation model.In 1986, the federal German nuclear chemist P. Moeller and others introduced the shell correction proposed by them, and calculated the half-life of the spontaneous fission or alpha decay of superheavy nuclei using a semi empirical formula, which is only 1~2 seconds. This result makes it very difficult to find the research of superheavy elements.
Find ways
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Since the theoretical prediction of super heavy elements was put forward in the 1960s, experimenters have searched for them through the following three ways:
New construction and reconstruction of heavy ion accelerator
① Some heavy ion accelerators are newly built or rebuilt. According to various conceivable heavy ion reaction mechanisms, heavier incident ions are used to bombard heavy element targets to synthesize superheavy elements.Designed and usednuclear reactionyes:
But the experimental results are all negative.
Use of vehicles
② Use high-altitude balloons, spaceships and other vehicles with nuclear emulsion and other detectors to search for possible overweight nuclei in cosmic rays.Nuclear emulsion stack, hybrid stack composed of nuclear emulsion, polycarbonate and acetate fiber and other detection devices were used to rise to an altitude of about 40km for detection, but no positive results were obtained.
Using nuclear detection methods
③ In the earth, meteorite materials and moon samples, X-ray fluorescence spectrometry, mass spectrometry and various nuclear detection methods are used to find possible long-lived super heavy elements.The materials searched include hundreds of minerals from different sources, such as meteorite, moon rock, moon dust, gold, copper, platinum, lead, mercury, molybdenum, zinc, nickel, etc., flue ash, anode mud, seabed manganese nodules from metal smelters, and thousands of kilograms of pure platinum, mercury, lead, tungsten and other samples.But the results are negative.
Causes not found
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In order to overcome the coulomb barrier between heavy ions and target nucleus and generate full fusion reaction, it is necessary to increase the energy of heavy ions;The consequence of increasing energy often makes the residual nucleus in a highly excited state, which is easy to fission, thus affecting the formation of superheavy nuclei.Due to the low reaction cross section of heavy ions, it is necessary to increase the beam intensity of heavy ions by at least three or four orders of magnitude. General accelerators are still unable to meet this requirement.In order to realize the artificial synthesis of superheavy nuclei, nuclear chemists have proposed the idea of using cold fusion reaction to synthesize superheavy nuclei.In 1985, Moeller proposed the following reaction to synthesize 271110:64Ni+208Pb - → 271110+n
American nuclear chemistA. GiossoAnother cold fusion reaction is proposed:
59Co+209Bi─→266110+2n
In addition, there is also the idea of bombarding heavy transuranium targets with heavy ions with smaller atomic numbers, such as:
23Na+254Es─→273110+4n
26Mg+249Cf─→271110+4n
Whether the above assumptions can be realized remains to be tested by experiments.
The theoretical prediction error of the half-life of superheavy nuclei is very large, and the half-life of superheavy nuclei may be far less than 108 years.As a result, the possible superheavy nuclei that existed when the earth formed have all decayed, so they are not found in minerals, rocks and other samples.If the half-life is less than 105 years, the possible super heavy nuclei in cosmic rays will also decay before they reach the Earth, so they cannot be found in cosmic rays.If the half-life of superheavy nuclei is really about 108 years, the possible reasons why they have not been found so far are: ① The probability of natural synthesis of superheavy nuclei in the universe is very small. From the existing data, the synthesis of heavy elements in nature is mainly through the fast neutron absorption process, but this process may not reach the superheavy region;The cross section of heavy ion reaction synthesis may also be very small, so the number of superheavy nuclei synthesized in nature is very small, which is difficult to find. ②There is also some blindness in the search direction. In some work, the selection of samples mostly focuses on the homologous similarity of chemical properties (such as lead like in lead, platinum like in platinum, etc.), and less attention is paid to the application of existing isotopic geochemical laws. ③The superheavy nuclei are dispersed in a large number of matrices, and the superheavy nuclei cannot be concentrated or lost due to the problem of the concentration method. ④The content of superheavy nuclei in the sample is extremely small, and the detector sensitivity is not high enough to distinguish.
research and development
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In 2023,Researchers at Los Alamos National Laboratory in the United States revealed the phenomenon of nuclear fission in the universe for the first time. They found the potential characteristics of fission, which indicates that nature may produce super heavy nuclei that exceed the heaviest elements in the periodic table.[1]
significance
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Although the various experiments for searching for super heavy elements over the past decade have not been successful, it is possible to find the next double magic number nucleus after the lead 208 double magic number nucleus from the successful explanation of the existing experimental facts by the shell model theory.In other words, there may be an overweight nuclear stability island.Even if the life of the superheavy nuclei on the island is not as long as the predicted value - the stability island is not as high or large as the theoretical prediction.But the life of the atomic nucleus on the island must be longer than that of the atomic nucleus around it, which is enough to be measured with a nuclear detector.Optimistic nuclear chemists believe that there is no doubt about the existence of stability islands, and heavy ion synthesis reaction may be a more realistic way to synthesize superheavy nuclei.
