核专业英语.pptx

PROFESSIONAL(SPECIAL)PROFESSIONAL(SPECIAL)ENGLISH COURSEENGLISH COURSENUCLEAR ENERGYNUCLEAR ENERGYChapter 8 Particle Accelerators(粒子加速器粒子加速器)A device that provides forces on charged particles by some combination of electric and magnetic fields and brings the ions to high speed and kinetic energy is called an accelerator.Many types have been developed for the study of nuclear reactions and basic nuclear structure,with an ever-increasing(持续增长持续增长)demand for higher particle energy.In this chapter we shall review the nature(性质性质)of the forces on charges and describe the arrangement(结构结构)and principle of operation of several important kinds of particle accelerators.8.1 ELECTRIC AND MAGNETICFORCES (电磁力电磁力)Let us recall how charged particles are influenced(感应感应)by electric and magnetic fields.First,visualize(想象想象)a pair of parallel metal plates separated by a distance d as in the sample capacitor(电容器电容器)shown in Fig.8.l.P73第二段第三行中A potential difference(势差势差)V and electric field E=V/d are provided to the region of low gas pressure by a direct-current(直流电直流电)voltage supply such as a battery(电池电池).If an electron of mass m and charge e is released at the negative plate(阴极板阴极板),it will experience a force Ee,and its acceleration will be Ee/m.It will gain speed,and on reaching the positive plate(阳极板阳极板)it will have reached a kinetic energy mv2/2=Ve.Next,let us introduce a charged particle of mass m,charge e,and speed v into a region with uniform magnetic field(均匀磁场均匀磁场)B,as in Fig.8.2.P73末行尾If the charge enters in the direction of the field lines(场力线场力线),it will not be affected,but if it enters perpendicularly(垂直地垂直地)to the field,it will move at constant speed on a circle.Its radius,called the radius of gyration(旋转旋转),is r=mv/eB,such that the stronger the field or the lower the speed,the smaller will be the radius of motion.Let the angular speed(角速度角速度)be (omega),equal to v/r.Using the formula for r,we find =eB/m.If the charge enters at some other angle,it will move in a path called a helix(螺旋状螺旋状),like a wire door spring(金属丝制的门弹簧金属丝制的门弹簧).P75首段Finally,let us release a charge in a region where the magnetic field B is changing with time.If the electron were inside the metal of a circular loop(圆环圆环)of wire of area A as in Fig.8.3,it would experience an electric force induced by the change in magnetic flux(磁通量磁通量)BA.The same effect would take place without the presence of the wire,of course.8.2 HIGH-VOLTAGE MACHINES(高压设备高压设备)One way to accelerate ions to high speed is to provide a large potential difference between a source of charges and a target.In effect,the phenomenon of lightning(闪电闪电),in which a discharge(放电放电)from charged clouds to the earth takes place,is produced in the laboratory.Two devices of this type are commonly used.The first is the voltage multiplier(电压倍增器电压倍增器)or Cockroft-Walton machine.The second is the electrostatic generator(静电发生器静电发生器)or Van de Graaff accelerator(Fig.8.5).An insulated(绝缘的绝缘的)metal shell is raised to high potential by bringing it charge on a moving belt(传动带传动带),permitting the acceleration of positive charges such as protons or deuterons.Particle energies of the order of 5MeV are possible,with a very small spread in energy.8.3 LINEAR ACCELERATOR(直线加速器直线加速器)Rather than(胜于胜于)giving a charge one large acceleration with a high voltage,it can be brought to high speed by a succession of(一连串一连串)accelerations through relatively small potential differences,as in the linear accelerator,sketched in Fig-8.6.P73第三行末It consists of a series of(一连串的一连串的)accelerating electrodes(电极电极)in the form of tubes with alternating electric potentials applied as shown.An electron or ion gains energy in the gaps between tubes and“drifts(漂流漂流)”without change of energy while inside the tube,where the field is nearly zero.By the time the charge reaches the next gap,the voltage is again correct for(做做的校正的校正)acceleration.8.4 CYCLOTRON AND BETATRON(一一)CYCLOTRON (回旋加速器回旋加速器)Successive electrical acceleration by electrodes(电极电极)and circular motion(圆周运动圆周运动)within a magnetic field are combined in the cyclotron.(Fig.8.7)P77第四行末Two hollow(空的空的)metal boxes called“dees(D字盒字盒)”(in the shape of the letter D)are supplied with alternating voltages(交流电压交流电压)in correct frequency and opposite polarity(极性极性).In the gap between dees,an ion gains energy as in the linear accelerator(直线加直线加速器速器),then moves on a circle while inside the field-free region(无电场区无电场区),guided by the magnetic field.Each crossing of the gap with potential difference V gives impetus(推动力推动力)to the ion with an energy gain Ve,and the radius of motion increases according to r=v/,where =eB/m is the angular speed(角速度角速度).接上页The unique feature(特色特色,特点特点)of the cyclotron is that the time required for one complete revolution(周转周转,周期周期),T=2/,is independent of the radius of motion of the ion.Thus it is possible to use a synchronized(同步的同步的)alternating potential of constant frequency,angular frequency =2,to provide acceleration at the right instant.(二二)BETATRON (电子感应加速器电子感应加速器)(P78第三行第三行)Electrons are brought to high speeds in the induction accelerator(感应加速感应加速器器)or betatron.A changing magnetic flux(磁磁通量通量)provides an electric field(电场电场)and a force on the charges,while they are guided in a path of constant radius.Figure 8.8 shows the vacuum chamber(真空室真空室)in the form of a doughnut(圆环圈圆环圈)placed between specially shaped magnetic poles(磁极磁极).The force on electrons of charge e is in the direction tangential(切线的切线的)to the orbit of radius r.The rate at which the average magnetic field(磁场磁场)within the loop changes is B/t,provided by varying the current in the coils of the electromagnet(电磁石卷电磁石卷).The magnitude(大小大小,数量数量)of the force is8.5 SYNCHROTRON (同步加速器同步加速器)A major step toward higher particle energies resulted from the invention of the synchrotron.It consists of the periodic acceleration of the particles by radio-frequency electric fields,but with a time-varying magnetic field that keeps the charges on a circular path.Ions(that are out of step)are brought back into step,i.e.,they are synchronized.P80首段By the use of accelerators of greater sophistication and higher particle energy,many new subnuclear particles(亚核粒子亚核粒子)such as mesons(介子介子)and xi,sigma,and lambda particles have been discovered and the internal structure of nuclei has become better understood.8.6 PRODUCTION OF NUCLEAR FUELS(核燃料生产核燃料生产)It is possible that accelerators can be used directly to help solve the energy problem.Experiments at California radiation laboratories showed that large neutron yields were achieved in targets bombarded by charged particles such as deuterons or protons of several hundreds MeV energy.P80第三段 接上页New dramatic(戏剧性的戏剧性的,生动的生动的)nuclear reactions are involved.One is the stripping reaction(剥裂反应剥裂反应),Fig.8.9(a),in which a deuteron is broken into a proton and a neutron by the impact(碰撞碰撞,冲击冲击)on a target nucleus.Another is the process of spallation(裂变裂变,蜕变蜕变)in which a nucleus is broken into pieces by an energetic projectile.Figure 8.9(b)shows how a cascade(喷流喷流)of nucleons is produced by spallation.A third is“evaporation(蒸发蒸发)”in which neutrons fly out(冲出冲出)of a nucleus with some 100 MeV of internal excitation energy,see Fig.8.9(c).The average energy of evaporation of neutrons is about 3 MeV.The excited nucleus may undergo(经历经历)fission,which releases neutrons,and further evaporation from the fission fragments can occur.P80第三段 接上页It has been predicted(预言预言)that as many as 50 neutrons can be produced by a single high-energy(500 MeV)deuteron.The neutrons could be captured in isotopes such as uranium-238 or thorium-232 to produce new nuclear fuels as discussed in Section 6.3.It has also been suggested that partially(部分地部分地)burned nuclear fuel can be exposed to neutrons from an accelerator target to bring the fissile isotope(裂变同位素裂变同位素)content(满足满足)up to operable level.P80第四段8.7 SUMMARYCharged particles such as electrons and ions of light elements are brought to high speed and energy by particle accelerators,which employ electric and magnetic fields in various ways.In the high-voltage machines a beam of ions is accelerated directly through a large potential difference(势差势差),produced by special voltage multiplier circuits(电压倍增电路电压倍增电路)or by carrying charge to a positive electrode;in the linear accelerator(直线加速器直线加速器),ions are given successive accelerations in gaps between tubes lined up in a row;in the cyclotron(回旋加速器回旋加速器),the ions are similarly accelerated but move in circular orbits because of the applied magnetic field;in the betatron(电子感应加速器电子感应加速器),a changing magnetic field produces an electric field that accelerates electrons to relativistic speeds;in the synchrotron(同步加速器同步加速器),both radio-frequency and time-varying magnetic field are used.High-energy nuclear physics research is carried out through the use of such accelerators.Through several nuclear processes,high energy charged particles can produce large numbers of neutrons which can create new fissile materials for use as fuels.