The profile shows the mass distribution curve of uranium -235 fission fragments. It can be clearly seen from the figure that the distribution curve has two peaks, one near the mass number 95 and the other near the mass number 138. The bimodal curve shows that most of uranium nuclear fission is asymmetric fission, and the probability of symmetric fission is very small (close to the mass number 1 18). This asymmetric fission was confirmed by various experimental methods shortly after the discovery of fission phenomenon, but today, with great progress in nuclear theory, the reason for this asymmetric fission is still a mystery. When uranium nuclear fission, it is the most common situation to split into two fragments, and it has also been observed to split into three (or even four) fragments, but the probability is very small, only a few thousandths. This so-called "triple fission" phenomenon was first discovered by Qian Sanqiang and He Yu 1946, a famous nuclear physicist in China. Although the probability of triple fission is very small, it is still being studied because it can explain the fission mechanism more clearly.
Fragments produced by nuclear fission are generally neutron surplus. They gradually convert the surplus neutrons into protons by emitting electrons (beta decay), that is, they reach a stable state through a series of beta decay. Therefore, most fission products are usually beta radioisotopes. Why are fragments produced by nuclear fission usually neutron excess? Why not lack neutrons or the number of neutrons is just the same as the number of protons?
As we know, the nucleus is composed of protons and neutrons (collectively referred to as nucleons). There is a strong force between nucleons, called nuclear force, which is short-range gravity. Inside the nucleus, this force is very strong, outside the nucleus, it quickly drops to zero, and the nucleus is kept inside the nucleus by this force. In addition, there is electrostatic repulsion between protons, which increases with the increase of atomic number, that is, with the increase of the number of protons in the nucleus. Therefore, in order to maintain the nuclear stability, more nuclear forces generated by excess neutrons are needed to balance this repulsive force. Therefore, the ratio of neutron number to proton number in stable nuclei increases with the increase of atomic number. For example, the ratio of neutron number to proton number of light elements such as carbon and oxygen is 1, that of medium elements such as bromine and barium is 1.3, and that of heavy elements such as uranium and thorium is 1.6. If the ratio of the number of neutrons to the number of protons in the nucleus is less than or greater than the corresponding appropriate ratio, it is unstable.
In the case of uranium nuclear fission, the ratio of neutron number to proton number of uranium is about 1.6, so the ratio of neutron number to proton number of fragments is of course about 1.6. But fission produces medium-mass elements, which are stable when the ratio of neutron number to proton number is about 1.3. Obviously, these fragments are neutron surplus, which will inevitably reduce the ratio of neutron number to proton number to about 1.3 in the form of β decay, thus reaching a stable state. However, this will naturally lead to a question: in the process of uranium nuclear fission, will some redundant neutrons be directly emitted in the form of free neutrons without being left in the debris? This important problem has been studied by many scientists, and the results show that some free neutrons, usually called secondary neutrons, are indeed released when uranium fission. Before telling the significance of this fact, let's take a look at another important fact: uranium nuclear fission will release huge energy while releasing secondary neutrons. Please look at the following calculation:
Suppose that uranium 235 absorbs one neutron, splits into bromine 85 nucleus and lanthanum 148 nucleus, and releases three neutrons at the same time. The mass of uranium 235 is 235. 124, bromine 85 is 84.938, lanthanum 148 is 147.96, and neutron is 1.009.
So the total mass before fission is: 235.438+024+1.009 = 236.438+033;
The total mass after fission is:147.96+84.938+3×1.009 = 235.925;
The mass loss during fission is: 236.438+033-235.925 = 0.208.
Where is the lost quality? According to Einstein's theory of relativity, they become energy. Einstein deduced a famous formula of mass-energy conversion: where C is the speed of light (about 300,000 kilometers per second), M is the mass of a stationary object, and E is the energy contained in a stationary object. From this formula, it can be easily calculated that the energy released by uranium nuclear fission is about 194 MeV. Roughly speaking, each fission releases about 200 mev of energy.
This value is very huge. For example, the energy released by complete fission of 1g uranium -235 is equivalent to the energy released by complete combustion of 2 million g (2 tons) of high-quality coal. In other words, fission energy is about two million times that of chemical energy!
When uranium fission occurs, one releases neutrons and the other releases enormous energy. These two valuable attributes have attracted people's attention. People are particularly interested in how many neutrons can be released in each fission, because it is related to whether the chain reaction can be realized, that is, whether a road can be opened up in the practical utilization of atomic energy.
After the efforts of many scientists, it was quickly determined that each uranium -235 nucleus released an average of about 2.5 neutrons when it split. Nature has made this special arrangement for us: the number of secondary neutrons is greater than 1! Thus, the discovery of uranium nuclear fission became an extraordinary discovery. If the average number of secondary neutrons produced by each fission is less than 1, then the value of this discovery is completely different from our interest in it.
Nuclear fission of uranium occurs under the action of neutrons. If two secondary neutrons are released during fission, these two secondary neutrons will cause two uranium nuclear fission, and four secondary neutrons will cause four uranium nuclear fission. In this way, the scale of the reaction will automatically become larger and larger, and a picture of uranium nuclear chain reaction will appear in front of us immediately. How many scientists it attracted! Conditions of chain reaction: the volume of uranium block must be greater than the critical volume.
Indeed, scientists have made great efforts to realize the nuclear fission chain reaction and make it benefit mankind. Now let's analyze the conditions for realizing the chain reaction theoretically. Neutron is the medium to realize the chain reaction of nuclear fission, so if the chain reaction of a system can continue, the number of neutrons must at least not decrease with time.
We usually refer to the ratio of the number of neutrons in a certain generation to the number of neutrons in the previous generation in the system as neutron multiplication coefficient, which is expressed by K. When k= 1, the number of neutrons in the system remains unchanged and the chain reaction continues at a constant rate. This state is called critical state. K>l, the number of neutrons will be more and more, and the scale of chain reaction will be larger and larger, which is called supercritical. And k