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Spectroscopy

Spectroscopy. Nuclear Spectroscopy Survey. Chemical Evolution of our Universe. Chemical Evolution of our Universe. S. Mason. Chemical Evolution (Clarendon Press, 1992), pp. 40-45. Probes for Nuclear Structure.

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Spectroscopy

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  1. Spectroscopy NuclearSpectroscopy Survey Nuclear Spectroscopy

  2. Chemical Evolution of our Universe Nuclear Spectroscopy

  3. Chemical Evolution of our Universe S. Mason. Chemical Evolution (Clarendon Press, 1992), pp. 40-45. Nuclear Spectroscopy

  4. Probes for Nuclear Structure To “see” an object, the wavelength l of the light used must be shorter than the dimensions d of the object (l ≤ d). (DeBroglie: p=ħk=ħ2p/l) Rutherford’s scattering experiments dNucleus~ few 10-15 m Need light of wave length l<1 fm, equivalent energy E Not easily available as light Can be made with charged particle accelerators Nuclear Spectroscopy Scan energy states of nuclei. Bound systems have discrete energy states  unbound E continuum

  5. What Structure? - Spectroscopic Goals Bound systems have discrete energy states  unbound E continuum  Scan energy states of nuclei. • Desired information on nuclear states (“good quantum numbers”): • Energy relative to ground state (g.s.) (Ei,i=0,1,…; E*) • Stability, life time against various decay modes (a, b, g,…) • Electrostatic moments Qj see below • Magnetostaticmoments Mi see below • Angular momentum (nuclear spin) • Parity (=spatial symmetry of quantum y) • Nucleonic (neutron & proton) configuration (e.g., “isospin” quantum #) E* Nuclear Level Scheme + 0 - Nuclear Spectroscopy

  6. Particle and g Spectroscopy • Identify scattered/transmuted projectile & target nuclei, measure m, E, A, Z, I,.. of all ejectiles (particles and g-rays). Individual ormulti detector setups, spectrometers Off-line activation measurements Nuclear Experiment

  7. How to Excite Nuclei: Inelastic Nuclear Reactions Scan the bound nuclear energy level scheme Projectile q Coulomb excitation of rotational motion for deformed targets or vibrations. Target p, n,..transferrxns, e.g., (d, n), (3He,d). Final state is excited Nuclear Spectroscopy Fusion/compound nucleus reactions,Final state is highly excited, thermally metastable

  8. Torque exerted on deformed target  excitation of collective nuclear rotations Transfer of angular momentum from relative motion: 1800-q Coulomb Excitation (Semi-Classical) Angular dependence of excitation probability: For small energy losses (weak excitations: spherical Adiabaticity Condition: Fast “kick” will excite nucleus Otherwise adiabatic reorientation of deformed nucleus to minimize energy deformed Vcoul(t) tcoll t t(R0) Excitation probability in perturbation theory: !

  9. z b a Observation of Collective Nuclear Rotations Quadrupole moment (Deviation from spherical nucleus ) b:= quadrupole deformation parameter Deexcitation-Gamma Spectra Measured energies (keV) Mean Field Wood et al.,Heyde

  10. Method: Particle Spectroscopy Bound systems have discrete energy states  unbound E continuum (A+a) Excited states E Reaction A(a,b)B* Particle aBeam QuadrupoleMagnet Faraday Cup Populate (excite) the spectrum of nuclear states. Measure probabilities for excitation and de-excitation Reconstruct energy states {Ei,Ii,pi} from energy, linear and angular momentum balance. Obtain structure information from probabilities. Energy Spectrum of Products b Nuclear Experiment  Excited States g.s.  Counts  Particle Energy Eb

  11. Method: Simultaneous Absorption & Emission Spectroscopy pD Reaction 64Ni(p,p’)64Ni* :inelastic proton scattering, absorb E from beam pC 64Nigs+g1+g2+g3+… Scattered Proton Spectrum  p-energy loss pB pA Deexcitation g Spectrum Nuclear Spectroscopy g

  12. Example: Inelastic Particle Scattering 238U+d  238U*+d’ Scattered deuteron kinetic energy spectrum E’d = E’d (qd , Ed) Deexcitation-g Spectrum Coincident g cascades HPGe Nuclear Spectroscopy HPGe Beam HPGe Coincident g cascades indicate nuclear band structure (rotational, vibrational,...)

  13. Measuring Energy Transfer: The Q Value Equation How to interpret energies of scattered particles: W. Udo Schröder, 2008 Measure kinetic energy E3 vs. angle q3 to determine reaction Q value. Predict energies of particles at different angles as functions of Q.

  14. Transmutation in Compound Nucleus Reactions Use mass tables to calculate Q Excited levels of ER EER* g.s. Decay of CN populates states of the “evaporation residue” nucleus Nuclear Spectroscopy Q

  15. Technology: Typical Setups Particle Experiments DE-E/TOF Setup Simultaneous measurement of time of flight, specific energy loss and residual energy of particles  E, Z, A Particle Particle Total Energy Time of Flight = Total Energy = ToF DE-E Setup Simultaneous measurement of specific energy loss DE and residual energy E of particles  E, Z, (A) Nuclear Spectroscopy E Residual Energy Modified, after K.S. Krane, Introduction to Nuclear Physics, Wiley&Sons, 1988

  16. Particle ID with Detector Telescopes 20Ne + 12C @ 20.5 MeV/u - qlab = 12° Na Ne F O N C B Be Li He Particle ID (Z , A, E) Specific energy loss, spatial ionization density, TOF SiSiCsI Telescope (Light Particles) Si TelescopeMassive Reaction Products DE-E Telescope DE E-DE Nuclear Spectroscopy

  17. THE CHIMERA DETECTOR Laboratori del Sud, Catania/Italy CHIMERA characteristic features REVERSE EXPERIMENTAL APPARATUS BEAM 688 telescopes TARGET 30° Nuclear Spectroscopy 1° 1m Chimeramechanical structure

  18. Technology in g spectroscopy Greta HPGe Array Realistic coverage: Ω/4p=0.8 Spatial resolution: ∆x=2 mm GRETA Detector Configuration ƒTwo types of irregular hexagons, 60 each, 3 crystals/cryostat= Œ 40 modules Detector – target distance = 15 cm $750k  GRETINA Nuclear Spectroscopy

  19. g-Ray Tracking with HPGe Detector Array Use Compton scattering laws to identify & track interactions of all g’s. Conventional method requires many detectors to retain high resolution and avoid summing. Nuclear Spectroscopy I.-Y. Lee, LBNL, 2011

  20. Characteristic Examples of Level Schemes Collective vibrations: O+g.s., even spins & parity, regular sequence with bunching of (0+,2+,4+) triplet Collective rotations: Regular quadratic sequence 0+,2+,4+,…. Only even or only odd spins, DI=2, uniform parity. Single-particle spectra: Irregular sequence, half-integer spins, various parities. Nuclear Spectroscopy

  21. Characteristic Level Schemes-Light Nuclei Different densities of states. Similarities in energies, spin, parity sequence for mirror nuclei: (N, Z)=(a, b)=(b,a)mirror Nuclear Spectroscopy

  22. Gamma Decay of Isobaric Analog States For same T, wfs for protons and neutrons are similar  “Mirror Nuclei” 19Ne and 19F W.u. Gamma Decay

  23. Examples of Level Schemes: SM vs. Collective Nuclear Spectroscopy

  24. Example of g –particle Spectroscopy Nuclear Spectroscopy

  25. 241Am 5.378 MeV 5.433 MeV 159 5.476 MeV 120 103 5.503 MeV 76 110 5.546 MeV 60 100 33 90 0 g Energy (keV) 80 237Np 70 60 50 40 30 a Energy (keV) - 5 MeV 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 Example a-Decay of 241Am, subsequent g emission from daughter Find coincidences (Ea, Eg) 9 g-rays, 5 a Principles Meas

  26. 241Am 5.378 MeV 5.433 MeV 159 5.476 MeV 120 103 5.503 MeV 76 110 5.546 MeV 60 100 33 90 0 g Energy (keV) 80 237Np 70 60 50 40 30 a Energy (keV) - 5 MeV 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 Example a-Decay of 241Am, subsequent g emission from daughter Find coincidences (Ea, Eg) 9 g-rays, 5 a Principles Meas

  27. 241Am 5.378 MeV 5.433 MeV 159 5.476 MeV 120 103 5.503 MeV 76 110 5.546 MeV 60 100 33 90 0 g Energy (keV) 80 237Np 70 60 50 40 30 a Energy (keV) - 5 MeV 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 Example Principles Meas

  28. 241Am 5.378 MeV 5.433 MeV 159 5.476 MeV 120 103 5.503 MeV 76 110 5.546 MeV 60 100 33 90 0 g Energy (keV) 80 237Np 70 60 50 40 30 a Energy (keV) - 5 MeV 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 Example Principles Meas

  29. 241Am 5.378 MeV 5.433 MeV 159 5.476 MeV 120 103 5.503 MeV 76 110 5.546 MeV 60 100 33 90 0 g Energy (keV) 80 237Np 70 60 50 40 30 a Energy (keV) - 5 MeV 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 Example Principles Meas

  30. 241Am 5.378 MeV 5.433 MeV 159 5.476 MeV 120 103 5.503 MeV 76 110 5.546 MeV 60 100 33 90 0 g Energy (keV) 80 237Np 70 60 50 40 30 a Energy (keV) - 5 MeV 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 Example Principles Meas

  31. 241Am 5.378 MeV 5.433 MeV 159 5.476 MeV 120 103 5.503 MeV 76 110 5.546 MeV 60 100 33 90 0 g Energy (keV) 80 237Np 70 60 50 40 30 a Energy (keV) - 5 MeV 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 Example No a-g coincidences !  must be g.s. transition Principles Meas

  32. 241Am 5.378 MeV 5.433 MeV 159 5.476 MeV 120 103 5.503 MeV 76 110 5.546 MeV 60 100 33 90 0 g Energy (keV) 80 237Np 70 60 50 40 30 a Energy (keV) - 5 MeV 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 Example Principles Meas

  33. Exercises Nuclear Spectroscopy

  34. Exercises: Nuclear Electrostatic Moments Consider a nucleus with a homogeneous, axially symmetric charge distribution • Show that its electric dipole moment Q0 is zero. • Assume for the charge distribution a homogenously charged rotational ellipsoid with semi axes a and b, given by . Show that 3. From the measured 242Pu and 244Pu g spectra determine the deformation parameters b of these two isotopes. Nuclear Deformations

  35. Alpha-Gamma Spectroscopy • 251Fm247Cf Nuclear Deformations

  36. Spectroscopy • Theoretical Tools Nuclear Spectroscopy

  37. z Spectroscopic vs. Intrinsic Quadrupole Moment I≠0: Transformation body-fixed to lab system mI=I q Any orientation quadratic dependence of Qz on mI Gamma Decay “The” quadrupole moment

  38. Electromagnetic Radiation Protons in nuclei = moving charges  emits electromagnetic radiation, except if nucleus is in its ground state! Monopole ℓ= 0 Heinrich Hertz Propagating ElectricDipole Field Dipole ℓ = 1 Gamma-Gamma Correlations Quadrupole ℓ =2 Point Charges NuclearCharge Distribution E. Segré: Nuclei and Particles, Benjamin&Cummins, 2nd ed. 1977

  39. Nuclear Electromagnetic Transitions Conserved: Total energy (E), total angular momentum (I) and total parity (p): Illustration conserved spatial symmetry Ip2+ Ip2+ g Ei 1- 1- Ef 0+ 0+ Gamma-Gamma Correlations Initial Ni spatial symmetry  retained in overall combination Nf+g Consider often only electric multipole transitions.Neglect weaker magnetic transitions due to changes in current distributions.

  40. z mi mf Selection Rules for Electromagnetic Transitions Conserved: Total energy (E), total angular momentum (I, Iz), total parity (p): Quantization axis : z direction.Physical alignment of I possible (B field, angular correlation) Coupling of Angular Momenta mg Gamma-Gamma Correlations

  41. Transition Probabilities: Weisskopf’ss.p. Estimates Consider single nucleon in circular orbit (extreme SM) Gamma Decay WeisskopfEstimates

  42. Weisskopf’s Eℓ Estimates Experimental E1:Factor 103-107 slower than s.p. WE Configurations more complicated than s.p. model, time required for rearrangement Experimental E2:Factor 102 faster than s.p. WE Collective states, more than 1 nucleon. Experimental Eℓ (ℓ>2):App. correctly predicted. Gamma Decay T1/2 for solid lines have been corrected for internal conversion.

  43. Weisskopf’s Mℓ Estimates Experimental Mℓ:Several orders of magnitude weaker than Eℓtransitions. Gamma Decay T1/2 for solid lines have been corrected for internal conversion.

  44. Isospin Selection Rules Reason: n/p rearrangements  multipole charge distributions photon interacts only with 1 nucleon (t = 1/2) DT = 0, 1 Example E1 transitions: Enhanced (collective) E2, E3 transitions DT = 0 Collective (rot or vib) WF does not change in transition. Gamma Decay

  45. Gamma Decay of Isobaric Analog States For same T, wfs for protons and neutrons are similar  “Mirror Nuclei” 19Ne and 19F W.u. Gamma Decay

  46. z e |e|Z q Monopoleℓ = 0 Dipole ℓ = 1 Quadrupole ℓ =2 Electrostatic Multipole (Coulomb) Interaction symmetry axis QuadrupoleQ ≠0, indicates deviation from spherical shape Different multipole shapes/ distributions have different spatial symmetries Point Charges Nuclear Deformations Nuclear charge distribution

  47. e,m Magnetization: Magnetic Dipole Moments Moving charge e current density j  vector potential , influences particles at via magnetic field =0 Nuclear Spins mLoop = I x A= current x Area(inside) current loop:

  48. End Spectroscopy Nuclear Spectroscopy

  49. Lecture Plan Intro to Nuclear Structure Nuclear Spectroscopy

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