Academic paper on nuclear physics-laser nuclear physics
In recent ten years, laser technology has made great progress, the laser intensity has exceeded 1022W/cm2, and the laser electric field has reached ~ 4 & times; 10 12V/cm。 When this kind of high-intensity laser irradiates the target, many nuclear reactions caused by laser can occur. In this paper, the research progress in this field is reviewed, and some suggestions for laser-generated electrons in the future are put forward. Proton? Neutron? The development potential of X-ray and positron is discussed.
Keywords chirped pulse amplification, particle cloud, positron emission tomography, Coulomb explosion
1 What is it?
In recent ten years, the laser technology has made remarkable progress, the laser intensity has exceeded 1022W/cm2, and the laser electric field intensity has reached 3.8 times. 10 12V/cm, which is 759 times larger than the Coulomb field on the Bohr orbit of electrons in hydrogen atoms, which is equivalent to adding a voltage of about 40kV to the atomic size and a voltage of about 0.38V to the nuclear size. Under this strong electric field, all atoms will ionize in a very short time, producing protons of several million electron volts to hundreds of million electron volts and dozens of million electron volts.
In laser plasma, when I= 10? 20? W/cm2, the energy of accelerating protons can be as high as 58MeV, and the acceleration gradient is about1mv/&; Mu; Protons are only accelerated by 60&; Mu; M, how to increase the acceleration distance becomes a very important research content. The mechanism of accelerating protons is quite complicated, and some ideas of accelerating models have been put forward. The experimental results show that the system has a good application prospect. This is manifested in:
(1) laser energy is converted into proton beam energy with high efficiency, which is related to laser energy. When the laser pulse energy is 10J? When the width is 100fs, the conversion efficiency is 1%, while when it is 500J? At 500fs, the conversion efficiency is 10%, and people get 10? 13? Proton/pulse, the proton pulse width is about 1ps, which is equivalent to 10? 25? Protons per second, right? 1.6。 Times; ? 106A pulse proton flow.
From theory to experiment, it is necessary to study how to further improve the energy conversion efficiency, especially whether the conversion efficiency will continue to rise when the laser energy is further improved.
(2) The divergence angle of proton beam is small, and the observed transverse divergence angle is 0.5 mm & middotMrad is smaller than the divergence angle of proton beam accelerated on accelerator.
(3) The acquisition of high-energy proton beams may be realized in the next decade. According to the calculation results of Buranov et al., when I= 10? 23? At W/cm2, the proton can accelerate above 1GeV, and at I= 1026W/cm2 and 1028W/cm2, the proton energy can reach 100GeV and 10TeV.
(4) At present, tens of mev proton beams have been obtained, which have been used to produce PET? 18? F and other short-lived positron sources, 109Bq? 18? F source can already be used in PET.
(5) generate 200MeV protons and use them to treat cancer. Because of its superior performance in energy deposition, small size and low cost, it has a good prospect in cancer treatment and can be used for neutron photography. At present, the proton energy dispersion produced by laser acceleration is 17%. The application of cancer treatment needs divergence &; Music; About 3%, so the work of reducing energy divergence is being carried out in some laboratories.
3.3 Laser generates neutrons [10, 1 1]
The ultra-short ultra-intense laser heats deuterium clusters to produce nuclear fusion, which has produced 104 neutrons/pulse or 105 neutrons/joule. From the point of view of the efficiency of converting laser energy into neutrons, it is equivalent to the neutron yield per joule laser on the large-scale laser NOVA on LLNL in the United States, which is one order of magnitude higher than that on the large-scale laser device Gekko 12 of Osaka University in Japan. Therefore, it is a promising desktop neutron generator, because the time width of this neutron source is only 1ps, and it is a neutron source with high neutron flux, which can be used in material science and neutron photography.
Coulomb explosion will occur when deuterium clusters absorb laser energy. It should be said that the mechanism of Coulomb explosion is still unclear, especially how deuterium molecules and small clusters of deuterium produced after cluster explosion produce deuterium-deuterium fusion reaction. There is still room for further improvement, such as how to use multiple ultra-short and ultra-strong lasers to irradiate clusters at the same time, or use a pulsed magnetic field greater than 50T to delay the decay time of thermal plasma, thus improving neutron yield.
The interaction between ultra-short and ultra-intense laser and deuterated polyethylene produces neutrons. Hilsher et al. used a Ti: sapphire laser (300mJ, 50fs, 10Hz, 10? 18? W/cm2) also produces 104 neutrons/pulse, which is about 3.3× 104 neutrons/pulse. 104 neutron. Disdier et al. used 20J, 400 fs, 5&; Times; 10 14W laser irradiation CD? 2 target, obtained 107 neutrons, 3.5&; Times; 105 neutron, which is a very high neutron yield. They also need to irradiate the CD with 500J, 500FS, 1PW laser? 2. In order to get more neutrons.
Laser irradiation CD? Plane target, besides studying the laser energy in CD? Besides the energy distribution on the target, how to make full use of the deposition energy is a very important issue. A large part of deposition energy is converted into kinetic energy of plasma. In the case of plane target, how to design the target surface shape to make the kinetic energy of plasma contribute the most to D-D reaction?
3.4 Laser generates hard ultra-short (~ 100fs)X-rays [12]
Ultra-short intense laser (50mJ, 0.5TW, 100fs) and 50MeV electron beam scattering can generate hard X-rays with a frequency of 4 nm, 4 nm 4nm 300fs. Although the conversion efficiency is not high, the generated X-ray intensity can produce diffraction peaks on the surface of Si, which can be used to study the surface transformation process of Si (from solid phase &: rarr; Protein folding dynamics can also be studied. The folding time of protein is 1ns, and its state in the folding process can be understood by 300fs hard X-ray.
3.5 Laser generates positrons [13, 14]
Electrons of several trillion electron volts are well collimated, and then projected onto a high Z target, which passes through the Trident process (Z+E-&; Rarz & prime number; +2e-+e+) and Bethe-HEitler process (z+r&; Rarz & prime number; +e-+e ++ r & amp； Prime number; ) generates positrons, and each pulse can generate 2&; Times; 107 positrons are stored in a magnetic field after being slowed down, which is very useful for the research of basic science and materials science.
4 main problems and analysis
This new interdisciplinary subject has only a history of more than ten years in the world, but it has developed very rapidly. Scientists engaged in laser technology and nuclear physics began to hold academic seminars together and participate in some experiments. Because it is a new growth point, it is developing rapidly, and it is easier to find some new phenomena, so the enthusiasm for cooperation is growing. With the development of ultra-short and ultra-intense laser technology, particles are accelerating. Nuclear physics? Even in particle physics, some useful work can be done. China's development lags behind, the intersection and cooperation between disciplines have not really formed, and the understanding and communication between disciplines are not enough. So we only did some interdisciplinary work. According to China's strength in laser technology and nuclear physics, we should be able to do more and better work. At present, there are many research units with ultra-short and ultra-strong laser devices, but there are not many achievements in running them well and doing physical work well.
The situation in China is similar to that in the world. One problem is the communication between experts engaged in intense laser technology and those engaged in nuclear physics and particle physics? Insufficient discussion will affect the development of this interdisciplinary subject.
From strong field physics to ultra-short and ultra-strong laser technology, and then to its application in various fields, it is an example of mutual promotion and interaction between basic science and technological progress all over the world. The demand of basic research and the foundation of optical science and nonlinear science promote the development of ultra-short and ultra-intense laser technology, and the development of high-intensity laser provides a brand-new world for the development of physics.
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