Rutherford made the famous α particle scattering experiment from 1909. The purpose of the experiment is to confirm the correctness of Thomson's atomic model, but the experimental results have become strong evidence to deny Thomson's atomic model. On this basis, Rutherford put forward the nuclear structure model.
In order to investigate the internal structure of atoms, it is necessary to find a probe particle that can shoot into atoms, which is the alpha particle emitted by natural radioactive substances. Rutherford and his assistant experimented by bombarding gold foil with alpha particles, as shown in the figure.
A small amount of radioactive element polonium (Po) is placed in a lead box, and its alpha rays are emitted from the small holes in the lead box, forming a beam of extremely fine rays and hitting the gold foil. When alpha particles pass through the gold foil, they hit the fluorescent screen to produce bright spots, which can be observed by microscope. In order to avoid the influence of alpha particles on the experimental results, the whole device is placed in a vacuum container, and the microscope with fluorescent screen can make a circle around the gold foil. At the beginning of nuclear physics research, we faced an important problem, that is, the nature of the interaction between nucleons. It is noted that most nuclei are stable. Through the study of γ decay, β decay and α decay of unstable nuclei, it is found that there must be a short-range saturated gravity between nuclei which is much stronger than electromagnetic interaction. In addition, a large number of experiments have proved that the interactions among proton-proton, proton-neutron and neutron-neutron are completely the same except electromagnetic force, which is the charge independence of nuclear force. 1935, Hideki Yukawa (1907 ~ 198 1) proposed that the interaction between nucleons is realized by exchanging a massless meson. 1947, π meson was discovered, and its properties coincided with Yukawa's theoretical prediction.
According to the theory of meson exchange, single π meson exchange produces long-range attraction between nucleons (≥3× 10- 13cm), and double π meson exchange produces saturated medium-range attraction [(1~ 3 )×10-3cm]. The study of the nature of nuclear force and the composition of the nucleus creates conditions for further revealing the structure of the nucleus.
In the early nuclear models, Bohr's droplet model, Fermi gas model, independent particle model of Butler and Elsas, and independent particle core-shell model of Meyer and Zhan Sen are influential. The most successful is the core-shell model of independent particles.
During the period of 1948 ~ 1949, Maria Goeppert1906 ~1972 redefined a set of magic numbers, namely 2, 8, 20, 28, 50 and 82. The basis of determining these magic numbers is that the chemical elements with these magic numbers as the core are relatively rich; The cross sections of fast neutrons and thermal neutrons in the magic nucleus are particularly small; The electric quadrupole moment of the phantom nucleus is particularly small; Fission products are mainly nuclei near the phantom nucleus; The binding energy of atoms can mutate near the phantom nucleus; Phantom nuclei are particularly stable relative to α decay; The energy released by β decay changes suddenly near the phantom nucleus. Inspired by Fermi, Meyer introduced a strong spin-orbit coupling force in the mean field, and successfully explained the existence of all magic numbers by using the energy level splitting caused by this force. Then, Zhan Sen (1907 ~ 1973) got the same result independently. In the book "Basic Principles of Core-Shell" co-authored by Meyer and Zhan Sen, they successfully explained the experimental facts such as nuclear magic number, spin, parity, magnetic moment, β decay and heterogeneous island by using the core-shell model. Due to the success of the core-shell structure model and its important role in nuclear physics research, Meyer and Zhan Sen jointly won the 1963 Nobel Prize in physics.
Core-shell model is put forward on the basis of comprehensive analysis of a large number of nuclear properties, nuclear spectra and experimental data of nuclear reactions, which gives a clear physical image of nuclear core movement. The core of this model is the idea of mean field. It holds that, just as electrons move in the average field of atoms, in the nucleus, each nucleus also moves independently in the average field of other nuclei, so the nucleus should also have a shell structure. This model is usually called the independent particle core-shell model.
The idea of mean field makes the core-shell model successful in many aspects, but it also has inevitable limitations, because the interaction between nucleons can not be completely replaced by mean field. In addition to the mean field, there are residual interactions between nucleons. With the development of nuclear physics research, some new experimental facts have been discovered since 1950s, such as large electric quadrupole moment, magnetic moment, electromagnetic transition probability, vibration spectrum, rotation spectrum of nuclear excitation spectrum, energy gap in heavy even nuclear energy spectrum, etc., which cannot be explained by the core-shell model of independent particles.
1953, Aage Niels Bohr (1922 ~), the son of Danish physicist and famous physicist n bohr, and his assistants martzen (1926 ~) and rainwater (196544). This model holds that in addition to the mean field, there are residual interactions between nucleons, which cause the correlation between nucleons, which is a supplement to the motion of independent particles, in which short-range correlation causes nucleon pairing. The nucleon pair model describing this correlation has been supported by a large number of experiments. The long-range correlation between nuclei will deform the nuclei and produce collective motion. The energy spectrum of nuclear rotation and vibration is the result of this collective movement, while the fission of heavy nuclei and the fusion reaction of heavy ions are the result of collective movement caused by large deformation of nuclear. According to the collective model of nuclei, each nucleus in the nucleus not only moves relative to other nuclei, but also vibrates and rotates as a whole. Nuclei in different states of motion not only have their own specific shapes, but also have different energies and angular momentum, which are separated, thus forming energy levels. For this reason, compared with the independent particle shell model only applicable to spherical nuclei, the collective model of nuclei has made great progress. It can be used to calculate the energy and angular momentum corresponding to various shapes of nuclear droplets. In addition, when the nucleus transitions from high energy level to low energy level, it can usually release energy in the form of gamma rays, which is consistent with the behavior of a large number of nuclei near the stability line. In addition, according to this model, when the shape of the nucleus is fixed, the moment of inertia remains unchanged. With the increase of angular momentum, the shape of the nucleus changes, and the moment of inertia also changes, resulting in the change of rotational energy level. Therefore, using this model to study the transition law of the rotational energy level of deformed nuclei has become the basis for studying singular nuclei. The collective model of nuclear solves the difficulty of independent particle core-shell model, successfully solves the experimental facts such as spherical nuclear vibration, deformed nuclear rotation and large quadrupole moment, and makes important contributions to the development of nuclear theory. For this reason, Bohr, Martzen and Rainwater won the 1975 Nobel Prize in Physics. The purpose of developing nuclear model is to describe the various forms of nuclear motion more accurately, so as to establish a more complete nuclear structure theory. Because the nature, law and mechanism of nucleon interaction are not completely clear, it is impossible to establish a nuclear structure and nuclear power theory through nucleon interaction like classical physics. We can only rely on the established model to theoretically calculate the nuclides or energy regions with experimental data, and then compare with the experimental results, adjust the model according to the comparison results, and then estimate the vacancy energy regions without experimental data through the model theory, develop experimental techniques, supplement vacancy data, and then compare with the experimental results. By the early 1970s, the progress of nuclear structure theory was mostly developed within the traditional scope.
The characteristics of traditional nuclear structure theory are:
(1) regardless of the nuclear structure itself;
(2) The nuclear force is mostly two-body interaction, and the interaction between nucleons is equivalent to the interaction between free nucleons;
(3) Nuclear matter is infinite;
④ Applying non-relativistic quantum mechanics;
⑤ The research object is natural nuclides under normal conditions (ground state or low excited state, low temperature, low pressure, constant density, etc.). ).
From the mid-1970s to the 1990s, the research of nuclear physics jumped out of the traditional scope and made great progress. Firstly, the development of experimental means, various middle and high energy accelerators and heavy ion accelerators have been put into operation one after another; Accordingly, the development of detection technology not only expands the scope of observable nuclear phenomena, but also improves the accuracy and analytical ability of observation; The transformation of nuclear data processing technology from manual to computer has accelerated the process of nuclear theory research. Influenced by the development of particle physics and astrophysics, the theory of nuclear physics has also begun to change from traditional non-relativistic quantum nuclear dynamics (QND) to relativistic quantum hadron dynamics (QHD) and quantum chromodynamics (QCD). A modern nuclear structure theory based on relativistic quantum field theory, unified theory of weak current and quantum chromodynamics is emerging. Although particle physics has become an independent discipline, nuclear physics is no longer the frontier of studying material structure, but the research of nuclear physics has entered a new stage of in-depth development.
In addition to the mean field, the collective model of the nucleus also considers the residual interaction, thus increasing its prediction ability. However, it is very difficult to deal with nuclear multibody problems mathematically, which brings great difficulties to practical research. In recent ten years, some people have put forward various simplified nuclear structure models, among which the liquid point model is the main one, which is characterized by reflecting the overall behavior and collective motion of the nucleus and can better explain the integrity of the nucleus, such as binding energy formula, fission, collective vibration and rotation. In addition to the liquid point model, there is also the interacting boson model (IBM), which is also an attempt to study the nuclear structure by a simplified method. Because people don't know the nuclear force between nucleons, and because the nucleus is a multi-body system composed of multiple nucleons, considering the three-dimensional coordinate freedom, spin and homologous family freedom of each nucleon, the equation of motion can't be solved, and the interaction between multi-bodies is even more difficult. In the past, the independent core-shell model emphasized the motion characteristics of independent particles, while the collective model of nucleus emphasized the overall motion of nucleus. These two theories are not well combined. Although the many-body behavior of nucleons is very complicated, it is impossible to start with theoretical calculation. However, the experimental observation shows that the nucleus, a complex multi-Fermi subsystem, shows clear regularity and simplicity. This enlightens people whether we can first "freeze" some degrees of freedom and study the laws of nuclear motion and dynamics. From a simple point of view, this is the starting point of the interacting boson model.
1968, when Feshbach and his student F. lachllo studied the double-shell light nuclei, they regarded the particle-hole as a boson and put forward the concept of interacting boson. 1974, Laszlo applied this concept to the study of medium-heavy even-even nuclei. He cooperated with A. Arima and put forward the interacting boson model. According to this model, even-even nuclei include the verification part of double full shell and even nuclei outside double full shell. If the degree of freedom of verification is "frozen" and the valence nucleon is matched into a nucleon pair with angular momentum of 0 or 2, the fermion pair can be regarded as a boson, and the even-even nucleus can be described by the interaction between bosons, which can greatly simplify the problem. Their model has achieved great success in explaining the low-energy excited States of medium and heavy nuclei. The interacting boson model is more successful because it predicts the symmetry of the nucleus in hyperspace. It is pointed out that collective motion behaviors such as nuclear rotation and nuclear vibration reflect the symmetry of nuclear power. Because it reveals the symmetry of nuclear power, this model is more abstract, but more profound and essential. In the past, symmetry was often considered as a research topic in particle physics. In fact, nuclear physics is also a research field with extremely rich symmetry. Eugene Paul Wigner (1902 ~) was the first person to notice the nuclear symmetry. He was a Hungarian-born American physicist and Dirac's wife and brother. Wigner graduated from the Chemistry Department of Berlin University, and received his doctorate from 1925. 1930 was invited to the United States as a professor of mathematical physics at Princeton university together with John von neumann (1903 ~1957). 1936, both of them founded the neutron absorption theory, which made great contributions to the cause of nuclear energy. 1937, based on the spin and isospin of the nucleus, Wigner introduced the super multiple structure and established the law of parity conservation. Wigner won the 1963 Nobel Prize in Physics for his contribution to the theory of elementary particles in the nucleus, especially to the basic principle of symmetry. After Wigner, Eliot made a more in-depth study of the dynamic symmetry of the nucleus. 1958, Eliot studied the symmetry of the harmonic oscillator field and established the SU(3) dynamic symmetry theory of boson interaction. This theory is in good agreement with the nuclear theory with the mass number a of 16 ~ 24, but for the nuclear with larger a, this symmetry is destroyed and deviates greatly due to the spin-orbit coupling. In the interacting boson model proposed by Laszlo and Alimer in 1974, the boson with angular momentum of 0 is called the S boson, and the boson with angular momentum of 2 is called the D boson. The S and D bosons are extended to a 6-dimensional hyperspace, and any change in the state of the system can be realized by unitary transformation in the 6-dimensional space to form a U(6) group. The conservation of angular momentum of the nucleus is related to the invariance of space rotation, that is, the S and D systems have U(6) symmetry. They also found that there are three group chains in the S and D boson systems.
①U(6)U(5)SO(5)SU(3), referred to as U(5) limit.
②U(6)SU(3)SO(3), referred to as SU(3) limit.
③U(6)SO(6)SO(5)SO(3), SO(6) limit for short.
In the case of three group chains, the Hamiltonian related to the interaction between S and D bosons has an analytical solution, and the nucleus has the symmetry of the corresponding group. In the three limit cases, the energy eigenvalue has a definite dependence on angular momentum, and the dynamic symmetry is also different according to the performance of energy level order. In a word, this research result reveals the symmetry of nuclear structure and dynamics, which is very consistent with the experimental results, and IBM theory has achieved great success.