Experimental Collision Physics

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Experimental Collision PhysicsSession IIIQuAMP II Summer School,Milton KeynesWednesday, 21st September, 2005.1

Lectures in this session arepresented bymembers of theAtomic and Molecular Interactions GroupInstitute of Physics.2


Table of Contents.Section 1.Introduction. Collision physics - an overviewPage 5Section 2.1Atomic & molecular Beam SourcesPage 8Section 2.2Atomic beam sources for unstable atomic speciesPage 20Section 3.1Ion sourcesPage 22Section 3.2Electron sourcesPage 26Section 3.3Electron guns & electrostatic lensesPage 29Section 4.Electron energy analysersPage 36Section 5.Detection of single particles & associated electronicsPage 41Section 6.Coincidence and counting techniquesPage 54Section 7.Data and signal analysisPage 61Appendix 1Data sheets for section 3.Appendix 2Data sheets for section 5.Appendix 3Data sheets for section 6.Appendix 4Data sheets for section 7.4

1. Collision Physics – An Overview.1.1 Outline.Collision physics includes ANY collision of a quantum particle with a target.Collision Particles may be: PHOTONS (eg from a Laser, Synchrotron source or FEL)ELECTRONS (usually of well defined momentum from an electron gun)IONS (usually from an ion source of well defined momentum)NEUTRAL PARTICLES (often in an excited state, eg metastable atoms)POSITRONS (although we are not talking about these today)TARGETS may include: ATOMS (usually in ground state, a laser excited state or a metastable state)MOLECULES (as with atoms)CLUSTERS (usually produced in an ultra-cold target preparation)IONS (from an ion source)TRAPPED ATOMS (eg in a Magneto-Optical Trap)TRAPPED IONS (eg in a Paul Trap, or an Ion Storage Ring)TARGETS almost always in GAS PHASE (but collisions with solids also carried out)Why study collisions in detail? Experimental test of fundamental interactions between quantum particles – eg long rangeCoulombic interaction between charged particles. Develop theoretical understanding of these interactions over a wide energy range (fromthreshold to very high impact energies) for all scattering geometries.Apply models & experimental results to real situations in eg: LasersAstrophysicsUpper & Lower Atmospheric physicsBio-physicsChemistrySurface & Materials ScienceLightingEngineering (ion sputtering, nanofabrication)To name a few.ANYWHERE that collisions between atoms, molecules, photons, electrons and ionsoccur can be studied experimentally & theoretically (in principle).5

1.2 Basic Experimental setup for study of collisions.InteractionRegionTargetIncident particleParticlesresultingfromInteractione.g. ions, electrons,photonsFIG 1. Collisional interaction between an incident particle & a target, resulting in scattered & ejected particles whosecharacteristics (momentum, spin, energy etc) are measured in detail.Incident particle of well controlled momentum reacts with target.INTERACTION occurs with target. This interaction may be: Elastic collisionInelastic collision leading to target excitationIonization of targetCapture of incident particle by targetFollowing the interaction several particles may appear in outgoing channel.Study details of these particles (in asymptotic limit) to ascertain what happened in the reaction zone.Details which are studied include: Final momentum of scattered, ejected particles.Fragmentation of target.Spin state of final particles.Ionization of target or incident projectile.It is the AIM of collision physics to ascertain what happened in the interaction zoneto produce these final products.ESSENTIAL that experiments are carried out to high precision.ESSENTIAL that theoretical models of the interaction are sophisticated enoughto accurately model the experimental results.NO MODEL AT PRESENT AGREES WITH EXPERIMENTOVER ALL ENERGIES & ALL GEOMETRIES.EXPERIMENTS ARE BECOMING INCREASINGLY SOPHISTICATED TODETAIL THESE INTERACTIONS MORE CLOSELY.It is the purpose of this session to outline how these experiments are carried out,& to provide insight into existing and new techniques being used.6

1.3 Session Timetable.9.00am - 10.00am1. Introduction. (5 minutes)2. Beam Sources – Preparation of the target for study. (20 minutes).3. Electron & ion sources – how to prepare & manipulate an incident beam. (20 minutes)4. Analysers –measuring the particles following the interaction. (15 minutes)10.00am - 10.30am Coffee & Tea break10.30am - 12.00 noon5. Detectors - how are single quantum particles detected? (20 minutes)6. Counting techniques – how to ascertain the reaction dynamics. (20 minutes)7. Data analysis – how to analyse different results & techniques. (20 minutes)8. General discussion - open forum on all topics discussed today. (30 minutes)7

2.1 Atomic & Molecular Beam SourcesDr Andrew Murray,University of Manchester, Manchester M13 9PL, UK.Email: Andrew.Murray@manchester.ac.uk.2.1 Introduction.Time constraints mean can only discuss small subset of all different sources.Going to select the following types (with examples from work being carried out in Manchester): Atomic beam sourcesMolecular beam sourcesCold atomic beam sources & targetsEach has different configurations & requirements.Basic aim of all beam sources:To provide a bright target beam which is collimated, and has high density.These criteria can be very difficult to achieve in practice!Most experiments require high target density in interaction region as phenomena to be studiedoften have very small cross section (measured by probability of detection).SOMETIMES beam density must be limited due to collisions.e.g. RADIATION TRAPPING: Experiments involving radiation from ground state targets needto limit target beam density in interaction region.Trapping occurs since photons emitted by an excited target may be absorbed by another target,photons then being re-released in random direction (multiple scattering).Reduces information which can be obtained from system under study.ATOM 1ATOM EmissionNo Correlation in Direction, PolarizationPhotonDetectorATOM 3FIG 1. Radiation trapping in target beams of high density. The excited atom radiates a photon which is absorbed and reradiated by a second & third atom. Information on the direction & polarization of the initial photon may then be lost.Hence density of the source must be chosen carefully.8

2.1.1 The Interaction Region.Interaction region is defined as volume of space where reaction occurs – this may be with anelectron, an ion, a neutral excited particle or a laser beam.Interaction region must be WELL DEFINED IN SPACE (typical size 1mm3), so momenta ofthe reaction products are accurately known.For collision experiments, momenta and energy of reaction products often measured INCOINCIDENCE –products must come from same target.Interaction region usually governed by target beam profile, incident electron/ion/laser beamprofile, or using lenses (electrostatic or optical) in experiment. Gun, Analysers must be positionedaccurately (see eg Murray et al, Meas Sci Tech 16, N19 ering Planek1IncidentElectronξ1k0 E inck2ξ2E1detectedelectronsE2CapillaryNozzleFIG 2. The interaction region defined by the target beam, electron/ion beam and the optics used in the experiment. Inthis case, an (e,2e) experiment is shown. The target is ionised, leaving two electrons to be detected in coincidence.2.2 Atomic beams.Atomic beams mostly come in two different types:2.2.1 Effusive gas sources.Produced from a capillary (nozzle) which directs atoms into interaction region.This is the simplest beam delivery system possible.RedirectionchamberCapillary Atomic BeamInteractionRegionGas FeedFrom ExternalGas BottleFIG 3. Simple capillary (nozzle) used to deliver atoms/molecules in the gas phase to the interaction region.9

Capillary may include array of micro-capillaries –improves target density BUT they areexpensive [see eg Buckman S J et al 1993 Meas. Sci. Tech. 4 1143].Micro-arrayCapillaryGas FeedAtomic BeamFIG 4. Micro-capillary array to deliver high density of atoms/molecules in the gas phase to the interaction region.Capillary almost always constructed of NON-MAGNETIC materialUse materials such as: Molybdenum (excellent for low energy electron scattering - expensive)Platinum-Iridium (good tensile strength - expensive)Stainless steel nozzles NOT generally used, as manufacture usually makes these slightlyMAGNETIC. – NOT GOOD for low energy electron collisions!Capillary arrays usually GOLD COATED to reduce charge build up on insulating surfaces – careis therefore required when using these. Quality of target beam from capillaries.Delivery from a capillary is reasonably good but angular spread is relatively high (depends ondriving pressure, type of gas used, quality of output orifice, length of tube).Intensity (Normalised to unity)1.2Single Capillary10.841 FWHM0.60.422 FWHM0.20Micro-CapillaryArray-40-20020Beam Angular Divergence (deg)40FIG 5. Angular spread of target beam delivered from single capillary & micro-capillary arrays, showing the improvement possible with a mutliple array of nozzles (from Buckman et al MST 4, 1143 (1993)).Capillary delivery systems usually used for atoms & molecules in gas phase, eg: Helium, Argon, Neon, Krypton, XenonMolecules: eg H2, CO2, N2 etc.Advantages of capillaries: Cheap & easy to constructReliableHigh density easily achieved10

Disadvantages of capillaries: Poor beam qualityMay produce radiation trapping problemsOnly useful for targets in gas phase at room temperature & in vacuum2.2.2 Target Beam Ovens.Target Beam Ovens used when atoms/molecules are in solid or liquid phase (eg Hg) in vacuum atroom temperature.Oven heats sample to enter gas phase – this may occur through a liquid transition or directly throughsublimation.Heated targets in gas phase usually emitted into interaction region through capillary nozzle –oftenuse Molybdenum as it has very high melting temperature.Many different designs of oven are possible.For an excellent review see Ross & Sonntag, Rev. Sci. lnst. 66 (1995) p4409.Design depends on whether maximum density is priority, or whether collimation is a priority.Design types include: Directly heated ovens (often using thermo-coax) – Temp up to 2000K possibleIndirectly heated ovens (eg by electron bombardment, RF heating) – Temp 2500K possibleRe-circulating ovens (excellent beam quality, reduced density) – only used for atoms whichhave a liquid phase, eg Na, K, Rb, Li, Cs, Mg.Second type not often used as it is difficult to eliminate RF fields & electrons emitted fromheating process. Directly heated ovens.Example: Calcium oven used at Manchester (Cvejanovic & Murray, M.S.Tech. 13, p1482 (2002)).Heat ShieldsSealCrucible Heater JacketHeat ShieldsCalcium pelletsHeat ShieldsNozzle Heater JacketFIG 6. The directly heated calcium oven used in (e,2e) coincidence studies at Manchester. The crucible and nozzleheaters are copper jackets wound with Thermo-coax. The seal is a stainless steel plug close fitted onto astainless steeledge. The capillary nozzle is molybdenum, internal diameter 1.4mm.11

T 1000K in this design.Use of heat shields to reduce energy needed to heat oven.Use of two heaters (body nozzle). – Ensures output capillary is hotter than main crucible.Use of high stability constant current supplies for heaters – ensures constant temperatureof all oven parts.Use of monitoring thermo-couples for accurate temperature assessment.Calcium charge loaded in argon – important for volatile substances (explosive in air).Oven seal is S/S to S/S – need to test design for reaction at high temperatures! (eg Ca Cuseal rapidly destroys Cu seal, & may destroy oven).IMPORTANT:Stainless Steel for oven is 310 grade – do NOT use 316, 304 grades due to poor magneticproperties of these grades.Output capillary is Molybdenum – non magnetic, high temperature.Output beam characteristics from Calcium oven is poor, due to re-scattering inside capillary.Necessary as high densities in interaction region are essential for these experiments.Optimise target beam density in interaction region by monitoring experiment while trying toeliminate deposition of calcium throughout vacuum chamber.Deposition causes major difficulties for low energy electron experiments.Advantages of directly heated ovens: Relatively simple to construct.Relatively low cost.Easy to operate.Produces target beam from atoms/molecules in solid, liquid phase at room temperature.Disadvantages of directly heated ovens: Difficult to control beam quality in interaction region.Low yield & low beam density.Heating may cause problems in vacuum chamber.Deposition of target onto sensitive components (eg lens elements) can be major problem.May require frequent intervention & cleaning.Some targets may explode on contact with air – can be quite exciting when opening vacuumsystem. Re-circulating Ovens. Complex & involved oven design which may have significant advantages. Used for targets which pass through liquid phase prior to gas phase. Re-circulating oven designs use condenser to re-liquify unused atoms/molecules, redirectingthese back to crucible.12

ONLY atoms/molecules which pass through an output aperture in condenser become targetbeam – produces well collimated beam at interaction region.Advantages of re-circulating ovens: Charge in crucible lasts much longerTarget Beam very well collimatedContamination of vacuum chamber markedly reduced.Disadvantages of re-circulating ovens: Control of oven working parameters very difficult – temperatures need to be within closetolerances throughout oven.Procedure for heating and cooling of different oven components must be carefully controlled.Beam delivery must be horizontal – due to drops falling onto nozzle & subsequent ovenvomiting!Only limited number of atoms can be used.Example: The re-circulating potassium oven at Manchester.Copper Heater Jacket(top plate not Shown)CondensationChamberPotassium SprayCollimated OutputBeamNozzleLiquid potassiumcatchment funnelT 70 CCopper Jacket(oil Temperature)Crucible Thermo-coaxPotassiumHeater CurrentPipe Temperature GradientHeatersCollimatingApertureSilicon OilFeed/ReturnT 400 CrecirculatingPipeMountingPointFIG 7. The re-circulating potassium oven at Manchester, which uses a thermal bath of liquid silicon oil passing aroundthe condenser to ensure excellent thermal control of the re-liquification process of the effusing atoms. Note use of individual heaters with associated heat shields around different oven parts.Note use of liquid oil cooled condenser – achieves close temperature control even whenatomic beam liquefies onto condenser (problem with alkali’s which have high heat conduction).Collimation achieved by nozzle condenser aperture sizes spacing – beam angle is only3 for this K oven.Used in new atom beam source of cold atoms (see below).13

2.3 Molecular beam Sources.Two main types used – Capillary sources (as above) & Supersonic sources.For easy delivery, use a simple capillary source as it is cheap.For COLD molecular beams, generally use supersonic sources.2.3.1 Supersonic Molecular Beams.Molecules can have extremely complex energy levels due to: Electronic levelsVibrational levelsRotational levelsTo simplify an experiment and for comparison to theory, advantageous to reduce number of energylevels in target as much as possible.Can be achieved by COOLING the molecules (reduces their internal energy)Cooling usually achieved in a supersonic expansion of the target gas.Supersonic expansion occurs when high pressure gas (1 – 2 atm) is released through small apertureinto high vacuum (typically 10-8 torr background pressure).Multiple collisions occur in expansion phase – leads to high translational velocity with low internalenergy of individual molecules.With correct operation, only the lowest (v, J) (0, 0) state is populated.Supersonic expansion usually occurs in source chamber separated from interaction chamber.Source chamber requires a high speed pump to quickly remove unused gas.Supersonic nozzles are usually PULSED to reduce the load on the pumpsSupersonic ExpansionOutputApertureNozzle SealNozzleValve FaceValve BodyHT InputGas InletFIG 8. Supersonic nozzle as used in pulsed laser/electron experiments at Manchester. The nozzle uses a peizo-electricflexure assembly to rapidly pulse the nozzle at up to 200Hz.14

Supersonically cooled molecular beam is usually skimmed prior to interaction region – this processcollimates molecular beam at interaction region.Skimmer often used to separate interaction chamber from source chamber.Target SupersonicBeamInteraction ChamberBottom FlangeO Ring Face1mm ApertureCoupling Flange(310 S/S)SkimmerSupersonicExpansionTop of NozzleFIG 9. Exponentially flared skimmer used to collimate the supersonic molecular beam prior to passing to the interaction chamber. The skimmer orifice is 1mm, the thickness of the skimmer metal being 5µm at the orifice.Typical speeds of supersonic beam depends on target: He 1800m/s H2 2000m/sN2 800m/sTranslational speed distribution also well defined, typically 5% – 10% of translation speed.Beam Intensity (Arb Units)1.2Peak Velocity 1750m/s10.8Speed Mach 200.6FWHM 100m/s0.40.20100015002000Beam Velocity (m/s)2500FIG 10. Speed distribution from supersonic expansion of He, fitted to experiment. The expansion produces a beam ofspeed 1750m/s with a width around 6% of the average.Example: Supersonic beam delivery system at Manchester used for laser experiments.1mmNozzleLocate skimmerSkimmerSupersonicExpansionFlange of Interaction ChamberPulsedNozzleSource Chamber250mmDiffusion PumpFIG 11. The source chamber used in Manchester for supersonic expansion. The skimmer is located 150mm from thenozzle to ensure collimation of the beam into the interaction chamber.15

TurboPumpA Source ChamberB Laser FlangeC Intteraction ChamberInteractionVacuumChamberµ-metal Internal LiningCLaser Alignment ottom flangeANozzle ghs2500 l/sDiffusion PumpPulsedNozzleFIG 12. Source & interaction chambers used for combined laser & electron collision experiments at Manchester. Thewell defined momentum of the target allows target deflection following electron collision to be determined by themetastable detector on the vertical rotating table. (Murray & Hammond, Adv. At. Mol. Opt. Phys. 47, p163 (2001)). Note use of large 250mm diffusion pump in source chamber (much faster recovery to highpressure loads than turbo pumps)Note use of separate source chamber from interaction chamber – ensures differentialpumping & high vacuum in interaction region.Nozzle is pulsed up to 200Hz – timing of pulse accurately controlled to ensure overlap ofelectron beam, laser beams, detectors in interaction region.By careful design of nozzle & pressure, clusters can be formed for experiments involvingvan der Waals interactions.Details on different designs can be found in the literature.2.4 Laser Cooled & Trapped Targets.Laser cooling & trapping techniques now being used to produce ultra-cold atomic targets (lowtranslational energy).Technique can produce either a slow beam of atoms or targets can be confined in a Magneto-OpticalTrap (MOT).Advantages of cold targets: Well collimated beams possible with very high density.Virtually no Doppler profile associated with beam source.16

Trapped atoms have well defined momentum for deflection experiments.Very high densities can be created in Magneto-Optical Traps.Rapid re-accumulation of trapped atoms possible with long trapping lifetimes.Future use of Bose Einstein Condensates for entirely new quantum experiments.Disadvantages of cold targets: Complex beam source to design, build &

5 1. Collision Physics – An Overview. 1.1 Outline. Collision physics includes ANY collision of a quantum particle with a target. Collision Particles may be: PHOTONS (eg from a Laser, Synchrotron source or FEL) ELECTRONS (usually of well defined momentum from an electron gun) IONS (usually from an ion source of well defin

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