2014《核技术专业外语》教材

1、Lesson 1 The Basic Concepts for Nuclear Physics1.1 Atoms and NucleiThe atoms of all elements, which at one time were thought to be the fundamental particles of nature, consist of numbers of three more fundamental particles-protons, neutrons and electrons. The arrangement of these particles within th

2、e atom, and in particular the number of protons and electrons, determine the chemical identity of the element. The atom consists of nucleus in which all the positively charged protons and uncharged neutrons are closely grouped together, and a number of negatively charged electrons moving in orbital

3、paths around the nucleus. In an electrically neutral or unionized atom the number of protons is equal to the number of electrons, and this number, Z, is the atomic number of a particular element and identifies it. (This number corresponds to the position of the element in the Periodic Table.) The nu

4、mber of neutrons in the nucleus is denoted by N, and the sum of the number of neutrons and protons in the nucleus is called, for reason that will shortly be apparent, the mass number, A. N+Z=AThe term nucleon is applied to all particles, both protons and neutrons, in the nucleus. 1.2 Isotopes Atoms

5、having the same atomic number Z, but different numbers of neutrons N are called isotopes of the element identified by Z, and all elements have a number of isotopes，in some cases twenty or more. The naturally occurring elements each have one or more stable isotopes which exist naturally, and other is

6、otopes which are unstable or radioactive and can be produced by artificial means. Different isotopes of an element behave identically as far as their chemistry is concerned, which is not surprising as chemical bonds exist between electrons. Isotopes differ from one another physically in that the mas

7、ses and other characteristics of their nuclei are different, which is to be expected as it is in the nuclei that the difference between two isotopes lies. 1.3 The Units of Nuclear PhysicsThe properties of protons and electrons, and in particular their mass and charge, are important as they determine

8、 the way in which these particles behave. This is therefore a convenient point to introduce some of the units in common use in atomic and nuclear physics and nuclear engineering. The unit of mass is the unified atomic mass unit (u). It is defined as one twelfth of the mass of a neutral carbon 12 ato

9、m. Its value is 1u=1.6604 10-27 kgor alternatively 1kg=6.0231026 u 。The atomic mass of any isotope is equal to the mass of one atom of that isotope expressed in u. The atomic mass of an element is the weighted mean of the atomic masses of the naturally occurring isotopes of that element. The mol, or

10、 to be more precise if SI units are being used, the kilogram-mol of any isotope is that quantity whose mass expressed in kilograms is numerically equal to its atomic mass. From the definition of the unified atomic mass unit stated above, it is evident that the mass of one mol of any isotope may be e

11、xpressed in u as 6.0231026the atomic mass. 1.4 Mass DefectThe mass of atom is not equal to the sum of the masses of its constituent particles. For example the mass of the O-16 atom is obviously less than the sum of the masses of eight neutrons and eight hydrogen atoms. Somewhere in the process of bu

12、ilding the atom from its constituent particles the classical principle of conservation of mass appears to have been violated, and the difference between the mass of an atom and the sum of the masses of its constituent particles is known as the mass defect. The explanation is to be found in the princ

13、iple of the equivalence of mass and energy in which Einstein stated that mass and energy are different forms of the same fundamental quantity. In many reactions there is an interchange of mass and energy so that, particularly on an atomic scale, the laws of conservation of mass and conservation of e

14、nergy are not valid when applied separately to a reaction, and must be replaced by the low of conservation of mass plus energy. In any reaction in which mass changes, a decrease of mass is accompanied by the release of energy, and an increase of mass corresponds to the absorption of energy. 1.5 Bind

15、ing Energy The force of electrostatic repulsion between like charges, which varies inversely as the square of their separation, would be expected to be so large that nuclei could not be formed. The fact that they do exist is evidence that there is an even larger forced of attraction. This nuclear fo

16、rce acts only when the nucleons are very close to each other and binds them into a compact stable structure. Associated with the net force is a potential energy of binding. To disrupt a nucleus and separate it into its component nucleons, energy must be supplied from the outside. Recalling Einsteins

17、 relation between mass and energy, this is the same as saying that mass must be supplied to the nucleus. A given nucleus is lighter than the sum of its separate nucleons, the difference being the binding mass-energy. 1.6 Nuclear Forces and Energy LevelsIt is appropriate at this stage to mention very

18、 briefly the forces that exist between the nucleons in an atomic nucleus. The Coulomb electrostatic force between charged particles is well known on a macroscopic scale, and exists on an atomic scale between protons in the nucleus, being a force of repulsion as they are all positively charged. The C

19、oulomb force is therefore a force which tends to disrupt or burst the atomic nucleus, and the fact that the nuclei of naturally occurring isotopes are stable and tightly bound indicates the existence of another force which binds the nucleus together and is stronger than the Coulomb force. This is th

20、e case, and experiments have shown the existence of a very powerful short range force of attraction that acts between particles that are close to each other, within about 310-15m.This short range nuclear force acts with nearly equal strength between two protons, two neutrons, or a proton and a neutr

21、on provided the separation is less than the distance quoted above, and it is this force which binds the atomic nucleus together. Normally atomic nuclei exist in an equilibrium or stable condition know as their ground state of energy. However, as a result of nuclear reactions (which might be caused b

22、y the bombardment of atoms by protons, neutrons or other light particles), nuclei can be produced in an excited or unstable condition in which one or a number of nucleons are raised to an excited state. The excited states or levels in a nucleus are similar to the excited state of atoms. In the case

23、of the latter, excitation results in an electron jumping from its normal orbit to another orbit further from the nucleus, and an atom may have a number of discrete excited states corresponding to an electron having made one or more such jumps. In the nucleus the situation is more complicated because

24、 excitation can result in several nucleons being raised to excited levels simultaneously, and some nuclei can have a very large number of closely spaced excited level. In general light nuclei have more widely spaced excited levels, and in all nuclei the spacing of the levels decreases as the excitat

25、ion energy increases. Most excited nuclei exist in this state for only a very short time, a typical average lifetime being about 10-14 seconds, and they decay, or become de-excited, by the emission of high energy electromagnetic radiation known as gamma radiation, or particles such as neutrons, or b

26、oth. In most reactions of interest to nuclear engineers involving the formation and decay of excited nuclei, the lifetime of the excited nucleus is so short that the process of formation and decay can be regarded as instantaneous. 1.7 FissionThe discovery of fission was mad in Germany in 1938 by Hah

27、n and Strassmann who were studying the radioactive isotopes formed as a results of the bombardment of uranium by neutrons in an effort to produce transuranium elements. One of the elements identified in the products of the reactions was radioactive barium 139, which indicated a hitherto unknown type

28、 of reaction in which the uranium nucleus split into fragments which were themselves nuclei of intermediated mass elements. Further work showed the presence of several other elements of medium mass number, and the existence of the fission process was definitely established. Shortly afterwards it was

29、 shown that neutrons were also emitted in the process and the possibility of a chain reaction was realized in which neutrons emitted in one fission event might be able to cause further fission, thus establishing a continuous reaction. The isotope of uranium that is principally responsible for fissio

30、n is uranium 235, which is present in naturally occurring uranium to be extent of 0.715 per cent. In this isotope fission can be caused by neutrons of any energy, low energy neutrons being the most effective. Fission in uranium 238, which comprises 99.285 percent of natural uranium, can only be caus

31、ed by neutrons of energy greater than 1 MeV. There three other isotopes of importance which can undergo fission. Thorium 232, the only naturally occurring isotope of that element, is fissionable with neutrons with energy greater than about 1.4MeV, and two isotopes, uranium 233 and plutonium 239, whi

32、ch do not occur naturally but can be produced artificially by nuclear reactions, undergo fission with neutrons of all energies, low energy neutrons being again the most effective. It is customary to refer to the five isotopes mentioned above (and any other isotopes which undergo fission with neutron

33、s of energy less than about 10 MeV) as fissionable, and to reserve the term fissile for the three isotopes 233U, 235U and 239Pu which undergo fission with low energy neutrons. The theory of fission is beyond the scope of this book, however a brief description of the generally accepted liquid drop mo

34、del will give a qualitative picture of the processes involved. The short range nuclear forces, which are analogous to the surface tension of a liquid drop, hold the nucleus in a more or less spherical shape in the same way that a liquid drop is spherical, however if the nucleus is excited, possibly

35、as a result of absorbing a neutron, its shape may be distorted. In the most cases the distortion is limited by the action of the nuclear forces and after de-excitation the spherical shape of the nucleus is restored, however it is possible that the distortion may lead to a dumbbell shape at which poi

36、nt the Coulomb force of repulsion between the two halves of the dumbbell exceed the nuclear force which is weakened by the distortion of the nucleus. Once this point is reached the nucleus splits into two fragments. The characteristics of fission will be described by considering uranium 235, however

37、 the fission of the other four isotopes is essentially the same in all respects. The first stage of the reaction is the absorption of a neutron in 235U to form 236U at an excited state. In some cases the 236U goes to its ground state of energy by the emission of gamma radiation, an example of an (n,

38、g) reaction, however in the majority of case the 236U nucleus splits as described above. The products of fission are two fission fragments whose mass numbers vary between about 70 to 160, a number of neutrons varying between none and five, beta particles, gamma radiation, neutrinos and energy. New W

39、ordsatom 5AtEm, 原子nucleus 5nju:klIEs, 原子核nuclei 5nju:kliai, nucleus 的复数形式nucleon 5nju:kliCn, 核子electron I5lektrRn, 电子proton 5prEJtRn, 质子neutron 5nju:trCn，中子neutrino nju:5tri:nEu, 中微子orbital 5C:bIt(E)l, 轨道的element 5elimEnt, 元素isotope 5aisEutEup，同位素hydrogen 5haidrEudVEn, 氢deuterium dju:5tiEriEm, 氘trit

40、ium 5tritiEm, 氚helium 5hi:ljEm, -liEm, 氦barium 5bZEriEm, 钡bismuth 5bizmEW, 铋boron 5bC:rEn, 硼lithium 5liWiEm，锂plutonium plu:5tEuniEm, 钚sodium 5sEudjEm, -diEm, 钠thorium 5WC:riEm，钍uranium juE5reiniEm, 铀transuranium 7trAnsju5reinjEm , 铀后元素radioactive 5reidiEu5Aktiv, adj.,放射性的fissile 5fisail, adj.,易裂变的em

41、ission i5miFEn, n.发射decay di5kei, v., n., 衰变electrostatic i5lektrEu5stAtik , adj., adj.,静电的momentum mEu5mentEm， n.,动量particle 5pB:tikl， n., 粒子radiation 7reidi5eiFEn， n., 放射性，放射，射线repulsion ri5pQlFEn， n., 排斥constituent kEn5stitjuEnt, adj., 组成的violate 5vaiEleit, vt., 违反valid 5vAlid, adj., 有效的，正确的insta

42、ntaneous 7instEn5teinjEs, adj., 瞬时的disrupt dis5rQpt, v., 使分离unionized Qn5aiEnaizd, adj., 未电离的as far as, adv., 远到，直到，至于，就more or less, adv., 或多或少， 有点tend to, v., 趋于in particular, adv., 特别to be more accurate/precise, adv., 更准确地说differ from, v., 不同于，differ from one anotherspeak of,讲起, 谈到, 特别推荐说,值得一提ass

43、ociated with, vt.与.常在一起, 和.联想在一起it is appropriate at this stage to, 在这个阶段合适 Lesson 2 RadiationThe word radiation will be taken to embrace all particles, whether they are of material or electromagnetic origin. We include those produced by both atomic and nuclear processes and those resulting from ele

44、ctrical acceleration, noting that there is no essential difference between X-rays from atomic collisions and gamma rays from nuclear decay; protons can come from a particle accelerator, from cosmic rays, or from a nuclear reaction in a reactor. The word materials will refer to bulk matter, whether o

45、f mineral or biological origin, as well as the of which the matter is composed, including molecules, atoms, electrons, and nuclei.When we put radiation and materials together, a great variety of possible situations must be considered. Bombarding particles may have low or high energy; they may be cha

46、rged, uncharged, or photons; they may be heavy or light in the scale of masses. The targets may be similarly distinguished, but also exhibit degrees of binding that range from none (free particles), to weak(atoms in molecules and electrons in atoms) , to strong(nuc1eons in nuclei). In most interacti

47、ons, the higher the projectile energy in comparison with the energy of binding of the structure, the greater is the effect. Out of the broad subject we shall select for review some of the reactions that are important in the nuclear energy field. Looking ahead, we shall need to understand the effects

48、 produced by the particles and rays from radioactivity and other nuclear reactions. Materials affected may be in or around a nuclear reactor, as part of its construction or inserted to be irradiated. Materials may be of biological form, including the human body, or they may be inert substances used

49、for protective shielding against radiation. We shall not attempt to explain the processes rigorously, but be content with qualitative descriptions based on analogy with collisions viewed on an elementary physics level.2.1 Excitation and Ionization by ElectronsThese processes occur in the familiar fl

50、uorescent lightbulb, or in a vacuum tube used in electrical devices, in an X-ray machine, or in matter exposed to beta particles. If an electron that enters a material has a very low energy, it will merely migrate without affecting the molecules significantly. If its energy is larger, it may impart

51、energy to atomic electrons as described by the Bohr theory, causing excitation of electrons to higher energy states or producing ionization, with subsequent emission of light. When electrons of inner orbits in heavy elements are displaced, the resultant high energy radiation on is classed as X-rays.

52、 These rays, which are so useful for interna1 examination of the human body, are produced by accelerating electrons in a vacuum chamber to energies in the kilovolt range and allowing them to strike a heavy element target. In addition to the X-rays due to transitions in the electronic orbits, a simil

53、ar radiation called bremsstrahlung ( German : braking radiation ) is produced. It arises from the deflection and resulting acceleration of electrons as they encounter nuclei or atomic electrons.Beta particles as electrons from nuclear reactions have energies in the range 0.01 lMeV, and thus are capa

54、ble of producing large amounts of ionization as they penetrate a substance. As a rough rule of thumb, about 32eV of energy is required to produce one ion pair. The beta particles lose energy with each event, and eventually are stopped. For electrons of 1MeV energy, the range, as the typical distance

55、 of penetration, is no more than a few millimeters in liquids and solids or a few meters in air.2.2 Heavy Charged Particle Slowing by AtomsFig. 2.1 Interaction of heavy ion with electronCharged particles such as protons, alpha particles, or ions such as the fragments of fission are classed as heavy

56、particles, being much more massive than the electron. For the same particle energy they have far less speed than an electron, but they are less readily deflected in their motion than electrons because of their inertia. The mechanism by which heavy ions slow down in matter is primarily electrostatic

57、interaction with atomic electrons. As the positively charged projectile approaches and passes, with the attraction to electrons varying with distance of separation as l / r2,the electron is displaced and gains energy, while the heavy particle loses energy. Figure 2.1 shows conditions before and afte

58、r the collision schematically. It is found that the kinetic energy lost in one collision is proportional to the square of Z, the number of external electrons in the target atom, and inversely proportional to the kinetic energy of the projectile. A great deal of ionization is produced by the heavy io

59、n as it moves through matter. Although the projectile of energy in the million-electron-volt range loses only a small fraction of its energy in one collision, the amount of energy imparted to, the electron can be large compared, with its binding to the atom or molecule, and the electron is completely removed. As the re