核技術(shù)導(dǎo)論英文課件：Chapter 3 Basic Instrumentation for Nuclear Technology 1

1、Chapter 3. Basic Instrumentation for Nuclear TechnologyAccelerators DetectorsReactorsOutline of experiment: get particles (e.g. protons, ) accelerate them throw them against each other observe and record what happens analyse and interpret the dataHistory-WhyParticle SourcesAcceleration stageSpace ch

2、argeDiagnosticsApplication1.AcceleratorsNatures Particle AcceleratorsNaturally occurring radioactive sources:Up to 5 MeV Alphas (helium nuclei)Up to 3 MeV Beta particles (electrons)Natural sources are difficult and limited:Chemical processing: purity, messy, and expensive Low intensityPoor geometryU

3、ncontrolled energies, usually very broadExamples from the nature electrostatic discharge, - and -decays, cosmic rays. “Start the ball rolling”1927: Lord Rutherford requested a “copious supply” of projectiles more energetic than natural alpha and beta particles. At the opening of the resulting High T

4、ension Laboratory, Rutherford went on to reiterate the goal:What we require is an apparatus to give us a potentialof the order of 10 million volts which can be safely accommodated in a reasonably sized room and operated by a few kilowatts of power. We require too an exhausted tube capable of withsta

5、nding this voltage I see no reason why such a requirement cannot be made practical.5Why study.The construction, design and operation of particle accelerators uses knowledge from different branches of physics: electromagnetism, high frequency electronics, solid states physics, optics, vacuum technolo

6、gy, cryogenics, .Learning about particle accelerator is a good opportunity to learn about many different physical phenomenon.WhyThey have wide ranging applications well beyond physics: health, life science, materials and even archaeology!7Early accelerators1870: Discovery of the cathode rays by Will

7、iam Crookes- Charged rays - Propagation from the Cathode to the anodeA Crookes tube in which the Cathode rays are deflected by a magnetic field.1896: J.J. Thomson shows that the cathode rays are made of “particles” and measure the charge/mass ratio.These particles are called “electrons”Images source

8、: Wikipedia8BremsstrahlungWhen a charged beam hits an object, X-rays are emitted. This is used to produce X-rays in hospitals but it is also a source of hazardous radiations in accelerators.Bremsstrahlung is similar to synchrotron radiation.A charged particle emits radiation when it is accelerated.A

9、n electron that Coulomb scatters on a heavy nucleus will change direction = accelerationBremsstrahlung, braking radiation, is the name of the radiation emitted when a charged particle scatters on a heavy nucleus.Image source: /EducationResources/9Improved resolutionIn quantum mechanics the wavelengt

10、h of an object is related to its energy by The reach better resolutions, the energy of the probe must be increased.The energy of the electrons in Cathodic ray tubes is limited by the electrostatic generators available.In the 1930s several generators where invented to produce high electric fields.Ion

11、 sourceaccelerationsteeringanalyzervacuumHistory-WhyParticle SourcesAcceleration stageSpace chargeDiagnosticsApplication1.Accelerators12Particle sourcesHow particles are first produced?How to extract particles with the right properties?What are the limitations of the sources?The quality of the sourc

12、e is very important. If the particles emitted by the source do not have the right properties, it will be very difficult and/or expensive to rectify it later.Beams of nanoamperes to hundreds of amperesVery thin to very broad beams (m2 to m2) Negative to highly charged statee to protein molecule14Emis

13、sion of electron:Thermionic effect(image source: wikipedia)When a metal is heated more electrons can populate high energy levels.Above a certain threshold they electrons can break their bound and be emitted: This is thermionic emission.15Work functionTo escape from the metal the electrons must reach

14、 an energy greater than the edge of the potential well.The energy that must be gained above the Fermi energy is called the “work function” of the metal.The work function is a propertyspecific to a given metal. It canbe affected by many parameters(eg: doping, crystaline state,surface roughness,.)Exam

15、ple values:(image source: wikipedia)16Summary: electrons in solidsAt low temperature all electrons are in the lowest possible energy level, below the Fermi level.As the temperature increase some electrons will go above the Fermi level.But only those with an energy above the Fermi level greater than

16、the work function are “free”.(image source: /content/m13458/latest/17Thermionic emissionThe Richardson-Dushman equation gives the electronic current density J (A/m-2) emitted by a material as a function of the temperature:With A the Richardson constant:(image source:Masao Kuriki, ILC school)18Thermi

17、onic cathode materialTwo parameters are important when considering a thermionic cathode material:W=Work function (as low as possible)Te=Operation Temperature (preferably high)Cesium has a low work function (W2eV) but a low operation temperature (Te=320K) = not good for high currentMetals: Ta (4.1eV,

18、 2680K), W(4.5eV, 2860K)BaO has good properties (1eV; 1000K) but can oxidize by exposure to air = sinter of BaO+WBaO provided slowly to the surface.19field enhanced thermionic emission(image source: )Under a very intense electric field some electrons will be able to tunnel across the potential barri

19、er and become free.This is known as field effect emission.20Electric field biasOnce the electrons are free they may fall back on the cathode.To avoid this an electric field needs to be applied.If a negative potential is applied to the cathode the electrons will be attracted away from the cathode aft

20、er being emitted.However this field affects the work function.22Photon-enhanced thermionic emission(image source: wikipedia)A photon incident on a piece of metal can transfer its energy to an electronIf the photon transfers enough energy the electron can be emitted.By using powerful lasers the photo

21、electric effect can be used to produce electron beams.This is known as the photo-electric emission.23Photo-electric emission (2)A UV photon at 200nm carries an energy of about 6 eV, this is enough to “jump” over the work function of most metals.As seen in electromagnetism, electromagnetic waves (pho

22、tons) can penetrate inside a metal. The photo-electricemission may thustake place away from the surface.(image source: Dowell et al., Photoinjectors lectures)24The 3 steps of photo-electric emissionPhoto-electric emission takes place in 3 steps:1) Absorption of a photon by an electron inside the met

23、al. The energy transferred is proportional to the photon energy.2) Transport of the photo to the physical surface of the metal. The electron may loose energy by scattering during this process.3) Electron emission (ifthe remaining energy isabove the work function;including Schottky effect)25Quantum e

24、fficiency (QE)For photo-electric emission, it is useful to define the “quantum efficiency”:Typical QE for a photo-cathode is only a few percent or less!The quantum efficiency will decrease during the life of the cathode: it may get damaged or contaminated.26Thermionic emittance (1)Velocity distribut

25、ion of thermionic electrons:The higher the temperature, the wider the transverse energy (momentum) spread.300K = 0.049eV spread2500K = 0.41eV spreadThe transverse momentum spread determines the beam divergence.(image source: Dowell et al., Photoinjectors lectures)27Ion (and proton) sourcesAn electri

26、c discharge creates a plasma in which positively and negatively charged ions are present (as well as neutrals).If such plasma experiences an intense electric field ions will separate in opposite directions.This is a rather crude and inefficient (but very simple) way of producing any sort of ions.In

27、a Penning ion source a magnetic field is used to increase the probability the free electron ionize extra neutrals.(images source: CERN)Ion source SINCSSource of Negative Ions by Cesium Sputtering - SNICS II Principle of Operation liquid metal ion source (LMIS), Focused Ion Beam Electrospray ionisati

28、on Charge Residue Model electrospray droplets undergo evaporation and fission cycles, eventually leading progeny droplets that contain on average one analyte ion or less. The gas-phase ions form after the remaining solvent molecules evaporate, leaving the analyte with the charges that the droplet ca

29、rried. 1 mbar10-3 mbar10-5 mbar10-6 mbarRotarypumpTurbopumpTurbopumpHeatedCapillary(180C)ESI needle4kVFused silicacapillaryTube lensSkimmerOctapoleLensesAcceleration tubeHistory-WhyParticle SourcesAcceleration stageSpace chargeDiagnosticsApplication1.AcceleratorsAcceleration stageOnly works on charg

30、ed particlesElectric Fields for AccelerationMagnetic Fields for Steering Magnetic fields act perpendicular to the direction of motion.For a relativistic particle, the force from a 1 Tessla magnetic field corresponds to an Electric field of 300 MV/mLorentz Forcetypes of accelerators: electrostatic (D

31、C) accelerators Cockcroft-Walton accelerator (protons up to 2 MeV) Van de Graaff accelerator (protons up to 10 MeV) Tandem Van de Graaff accelerator (protons up to 20 MeV) resonance accelerators cyclotron (protons up to 25 MeV) linear accelerators: electron linac: 100 MeV to 50 GeV proton linac: up

32、to 70 MeV synchronous accelerators synchrocyclotron (protons up to 750 MeV) proton synchrotron (protons up to 900 GeV) electron synchrotron (electrons from 50 MeV to 90 GeV)Induction: Induction linac, betatronelectrostatic accelerators:generate high voltage between two electrodes charged particles m

33、ove in electric field,energy gain = charge times voltage drop;Cockcroft-Walton and Van de Graaff accelerators differ in method to achieve high voltage.Cockcroft-Walton High voltage source using rectifier units Voltage multiplier ladder (made of diodes and capacitors) allows reaching up to 1MeV (spar

34、king).First nuclear transmutation reaction achieved in 1932: p + 7Li 24HeCW was widely used as injector until the invention of RFQFermilab 750 kV C-W preacceleratorVan de Graaff Voltage buildup by mechanical transport of charge using a conveyor belt. up to 20MVThe charged particles are extracted fro

35、m an ion source housed inside the high-voltage terminal and accelerated down an evacuated tube to ground potential.Tandem Van de GraaffNegative ions accelerated towards a positive HV terminal, then stripped of electrons and accelerated again away from it, doubling the energy.Negative ion source requ

36、ired!The Million Volt BarrierSummary of Problems in getting HV 1929Voltage GeneratorsInsulators 750 kV max holding !PowerSafety in using HV FundingImagination RF AcceleratorsHigh voltage gaps are very difficult to maintainSolution: Make the particles pass through the voltage gap many times!First pro

37、posed by G. Ising in 1925First realization by R. Wiedere in 1928 to produce 50 kV potassium ionsMany different typesRadiofrequency oscillating voltageRF LINAC basic ideaParticles accelerated between the cavitiesCavity length increases to match the increasing speed of the particlesEM radiation power

38、P = rfCVrf2 the drift tube placed in a cavity so that the EM energy is stored.Resonant frequency of the cavity tuned to that of the accelerating fieldRF LINAC phase focusingE. M. McMillan V. Veksler 1945The field is synchronized so that the slower particles get more accelerationHistory-WhyParticle S

39、ourcesAcceleration stageSpace chargeDiagnosticsApplication1.AcceleratorsHistory-WhyParticle SourcesAcceleration stageSpace chargeDiagnosticsApplication1.AcceleratorsWhat do you want to know about the beam?Intensity (charge) (I,Q)Position (x,y,z)Size/shape (transverse and longitudinal)Emittance (tran

40、sverse and longitudinal)EnergyParticle lossesProperties of a charged beamAlmost all accelerators accelerate charged particles which interact with matter.Thats almost all what you need to use to build diagnostics (together with some clever tricks).Faraday cup (1)Lets send the beam on a piece of coppe

41、r.What information can be measured after the beam has hit the copper?Faraday cup (2)Two properties can be measured:Beam total energyBeam total chargeBy inserting an ammeter between the copper and the ground it is possible to measure the total charge of the beam.At high energy Faraday cups can be lar

42、ge: More than 1m at Diamond.Image source: PBeam current monitorRemember: as the charge travelling in the beam pipe is constant the current induced on the walls (of the beam pipe) will be independent of the beam position. By inserting a ceramic gap and an ammeter the total charge travelling in a beam

43、 pipe can be measured.Beam current monitor vs Faraday cupBoth devices have pros and cons.A Faraday cup destroys the beam but it gives a very accurate charge measurementsA Beam current monitor does not affect the beam but must be calibrated.Both tend to be used at different locations.Screen (1)If a t

44、hin screen is inserted in the path of the particles, they will deposit energy in the screen.If this screen contains elements that emit light when energy is deposited then the screen will emit light.Example of such elements; Phosphorus, Gadolinium, Cesium,.Screen (2)It is not possible to stay in the

45、accelerator while the beam is on so the screen must be monitored by a camera.To avoid damaging the camera the screen is at 45 degrees.On this screen you can see both the position of the beam and its shape.Note the snow on the image.Wire-scannerBy inserting a thin wire in the beam trajectory (instead

46、 of a full screen) it is possible to sample parts of the beam.By moving the wire in the transverse direction one can get a profile of the beam.It is possible to use wire diameters of just a few micrometres.Longitudinal propertiesIt is not possible to directly image the longitudinal profile of a bunc

47、h.By giving longitudinal impulsion to the beam it is possible to make it rotate and observe its longitudinal profile.Beam lossesIt is important to monitor the beam losses directly:Small beam losses may not be detected by other systemsBeam losses are a source of radiation and activationMost beam loss

48、es indicate that there is a problem somewhere.Limitation of these monitorsMonitors in which the matter interacts are prone to damage.With high energy high intensity colliders such damages are more likely to occur.To the left: hole punched by a 30 GeV beam into a scintillating screen.Laser-wireTo mit

49、igate the problem of broken wires in wire-scanners it is possible to replace the wire by a laser.This technique called “l(fā)aser-wire” also allow to reach better resolutions.High power lasers (or long integration times) are needed.Synchrotron radiationSynchrotron radiation carries information about the

50、 beam which emitted it.It is commonly used to study the beam shape.Energy measurementsTo measure (or select) the energy of the particles a bending magnet is often the best solution.Diagnostics overviewSummaryThere are two ways of measuring the properties of a beam:By forcing it to interact with matt

51、erBy looking at the EM radiation emitted.How to build the best diagnostic is then a matter of imagination!History-WhyParticle SourcesAcceleration stageSpace chargeDiagnosticsApplication1.AcceleratorsSeveral accelerator based methods can be used to date old artefacts. Hospitals use accelerators every

52、day to treat some forms of Cancer. The data storage capacity of electronic devices has been improved. The structure of molecules, including drugs, can be studied with intense sources of X-rays. Material hardness can be studied with neutrons Intense flux of neutrons can burn unwanted nuclear material

53、sThe Shroud of TurinThe shroud of Turin is a piece of cloth which was first mentioned in the middle age.On it the face of a man can be seen.Some claim that it is the shroud that was used after the Christs crucifixion.In the 1980s 3 AMS laboratory independently dated the sample they were provided to

54、1260-1390.Dating old artefactsTherapyComparison of the physical dose distribution (upper diagram) and the survival rate of cells (lower diagram) as a function of penetration depth for ion and photon beams. The enhanced energy deposition at the end of the particle range and the corresponding dramatic

55、 decrease of cell survival show that heavy ion beams are excellent tools for the treatment of deep seated tumours. therapySub-micron micromachining interactionsMasked processes (electromagnetic)LightX-raysDirect write processesElectronsLow energy heavy ions (eg gallium)High energy light ions (protons)FDSPMProton Beam Micro-machining Examples of structures in PMGI and PMMA(2 MeV proton beam)Structures produced in a 12 mmthick PMGI resist layer.Map of Singapore (60 mm high) produced in bulk PMMA.Cogs (60 mm high) p