Physics Primer

物理入门


Nuclear Reactions

核反应

Let's start with a very basic recap of chemistry and atomic physics: 

让我们从一个非常基础的化学和原子物理重述开始:

atoms are made out of a positively charged nucleus (protons + neutrons) and a negatively charged electron cloud (electrons) 原子由带正电的原子核(质子+中子)和带负电荷的电子云(电子)组成。

a chemical element is identified by its number of protons which we generally refer to as the atomic number (Z). E.g. Hydrogen has one proton, and ; Oxygen has eight protons, and ; Uranium has 92 protons, and

一个化学元素是由它的质子数来确定的,我们通常称之为原子序数(z)。例如,氢只有一个质子,和;氧有八个质子,和;铀有92个质子,和

the sum of the number of protons and neutrons in a nucleus is generally called nucleon number or atomic mass number (A). Because the proton and neutron masses are very close to each other (), and the electron mass is negligible compared to the proton mass (), the total mass of an atom can be approximated to the atomic mass number x the mass of one proton/neutron. 

核内质子和中子数之和通常称为核子数或原子质量数(A)。因为质子和中子的质量非常接近对方(),与电子的质量是质子的质量相比可以忽略不计的(),一个原子的总质量可近似为原子质量数x一个质子或中子的质量。

a useful quantity is the unified atomic mass unit (u) that is approximately equal to the mass of one proton/neutron: . Then the mass of some element X is going to be mX=AX×u. E.g. . As we will see, this is not quite true. The mass of an element is not just the mass of its components; instead one must also include the mass equivalent for the binding energy between the protons and neutrons. 

有用的量是统一的原子质量单位(U),约等于一个质子或中子的质量:。然后一些元素x是mX=AX×u。如。正如我们将看到的,这并不完全正确。一个元素的质量不仅是它的组成部分的质量,还必须包括质子和中子之间的结合能的质量当量。

two elements with the same number of protons, but different number of neutrons are called isotopes. E.g. U-238 and U-235 have 92 protons, but 146 and respectively 143 neutrons.

两个具有相同数量的质子,但不同数量的中子称为同位素。例如U-238和铀-235有92个质子,但中子的分别为146和143。

we are going to write when referring to an element X with atomic mass number A and atomic number Z. E.g. .

在引用一个元素x与原子质量数和原子序数Z如,铀235 = u92235时,我们要写

we are often going to write down nuclear reactions such as: 

我们经常写下核反应,例如:


For each nuclear reaction we can write down two equalities: 

每个核反应可以写下两个等式:

 , .

These two equalities impose the conservation of mass number () and conservation of charge (). 

这两个等式对质量数守恒()和电荷守恒()。

E.g.:

例如


In this nuclear reaction a neutron decays into a proton and two extra particles (an electron and a neutrino). The overall charge and atomic mass number are conserved. 

在这个核反应中,一个中子衰变成一个质子和两个额外的粒子(电子和中微子)。总电荷和原子质量数守恒。


Uranium-238 decays into Thorium plus an alpha-particle (2 protons and 2 electrons). 铀-238衰变为钍加一个α粒子(2个质子和2个电子)。

for a nuclear reaction to be correct, atomic mass number and charge must be conserved. However, not all correct nuclear reactions are possible or likely. Factors like binding energy, half-life, stability, kinetic energy etc. can play an essential role in favoring some reactions over others. 

为了使核反应正确,原子质量数和电荷必须守恒。然而,并非所有正确的核反应都是可能的或可能的。诸如结合能、半衰期、稳定性、动能等因素在某些反应中起着至关重要的作用。