随着放射线的不断放出，放射性的能级呈指数式衰减，原子数衰减到原子原始数值的一半时所经历的时间称为同位素的半衰期。半衰期从零点几秒到几百万年各有不同。
Isotopes with long half-lives have many uses in medicine and industry, but they must be handled and disposed of with great care, in case they cause radiation damage.
中子被放射出时以高速运动。正如所知，这种高速运动的中子几乎不可能被可裂变铀俘获。
However, if slowed down to thermal speeds, their probability of capture is greatly increased.
然而，如能使中子减速到热运动速度，热中子被俘获的可能性就会大大增加。
有一些裂变产物，例如铯-137,就是一种很有用的辐射源；目前已经能够从废弃的燃料中分离出我们所期望的同位素。
14．Chain Reaction连锁反应
Chain Reaction
连锁反应
When fission occurs, an average of 2-5 neutrons are emitted from the nucleus.
在使用石墨缓和剂的情况下，铀通常以铀棒形式插进石墨切槽内。
These channels areso arranged as toform a lattice structure, the object of which is to reduce neutron escape to a minimum.
如果原子核是重核，也就是说原子核如含有大量质子和中子，就可能更加趋于不稳定。
Unstable nuclei attempt to achieve stability by emitting some form of radiation, until they transform themselves into stable isotopes.
由于中子不带电荷，所以不受带电粒子和靶原子的质子的影响。而且，如果中子同核的能级协调一致，就会更易于被其他任何粒子俘获。
In the event of neutron capture, the mass number of the nucleus will be raised, and it will thus become unstable and radioactive.
13．Radioactivity放射性
Radioactivity
放射性
Atomic nuclei consist of combinations of protons, or positively-charged particles, and neutrons, or uncharged particles.
分裂的裂片有着很强的放射性，而且如不定期清除，就会污染裂变的铀。
A number of these fission products, such as Caesium-137, are very useful as irradiation sources, and it is now possible to separate out the desired isotope from the spent fuel.
不稳定的原子核总是要通过放射出某种放射线的方式达到稳定，也就是说直到原子本身衰变为稳定的同位素才能稳定。
There are radioactive isotopes of every element, either those existing in nature or else those activated artificially by bombardment of stable nuclei with nuclear particles such as protons, alpha-particles or neutrons.
每种元素都有放射性同位素。有的放射性同位素是自然界中存在的，有的利用诸如质子、α粒子或中子一类的核子，对稳定的原子核进行轰击而用人工方法制造的。
However, a particle will not be absorbed by the target nucleusunless its velocity correspondswith one of the energy levels of the nucleus.
核发生裂变时，原子核平均会放射出2至5个中子。
If the fission process can beso arranged thatone of these liberated neutrons is captured by another U-235 nucleus to produce another fission, then the reaction will become self-sustaining.
For example, calcium-48, having 20 protons and 28 neutrons, is a stable isotope of calcium.
例如钙-48含有20个质子和28个中子，是钙的稳定同位素。
Butif there isan excess or deficiency of e will be unstable.
Both these substances are of low atomic weight and have low neutron absorption cross-sections.
这两种物质的原子量都低，而且中子吸收截面也低。
With the graphite moderator, the uranium which is generally in the form of rods is inserted into channels cut out of the graphite.
半衰期长的同位素在医学和工业方面有许多用途，但必须极其慎重而严格地控制和处理，以免招致辐射的伤害。
Neutron bombardment of the very heavy uranium atoms may have a quite different result.
用中子对非常重的铀原子进行轰击，可能会产生全然不同的结果。
原子核是由质子与中子相结合而构成;质子也叫带正电荷的粒子，中子也叫不带电荷的粒子。
The number of protons and neutrons in each element can vary, but only certain combinations are stable.
每一种元素中质子和中子的数目可能有所不同，而只有一些元素的组合比较稳定。
一旦俘获了中子，原子核的质量数便会增加，因而也就会变得不稳定并且具有放射性。
As radiation continues, the level of radioactivity falls exponentially, and the time taken for it to reach half its original value is known as the half-life of the isotope, which may vary from a fraction of a second to millions of years.
可是在任何一种组合中，如果中子数目不足或者过剩，同位素就不稳定。
A nucleus is more likely to be unstableif it is aheavy one—that is,if it containsa large number of protons and neutrons.
It may cause the nucleus of the fissile U-235 atom to split into two parts.
轰击可以使可裂变的铀-235原子核裂成两部分。
This nuclear fission releases large quantities of energy which finally takes the form of the great energy, and at the same time other neutrons are ejected from the nucleus.
In the normal thermal reactor, the uranium is surrounded by a large mass of moderating material.
在普通的热核反应堆中，铀被围在大量的减速材料当中。
The liberated neutronscolliderepeatedly with the light atoms of the moderatorin such a way thatthey lose much of their energy and eventually become thermalized.
这种核裂变能释放出大量最终以热能形式出现的能量。与此同时，从核子里还放出另外一些中子。
The fission fragments are highly radioactive, and will contaminate the fissile uraniumif they are not removedperiodically.
倘若所设计的裂变过程能使得所放出的一个中子被铀-235的另一个核子所俘获并进而产生另一裂变，那么随后反应就会自动持续进行。
When emitted neutrons travel at a high velocity, and it is known that such fast neutrons have little chance of being captured by the fissile uranium.