2008年雷射電漿物理暑期學校 【電漿實驗專題】 近代核子物理實驗 中央研究院 物理所 章文箴 2008/07/11（五） 台大物理系111教室
Outline • Discovery of nuclear structure • Development of “modern” nuclear physics • Nuclear structure at extreme and excited states • Hadron physics with laser-electron photon • Laser Nuclear Physics
Models of Atom Rutherford model of atom Thomson model of atom
Rutherford experiment • From angular distribution of rescattered -particles Rutherford concluded existence of positively charged core of atom then called nucleus • The size of the nucleus was much smaller(10-14m) than size of the atom (10-10m)
All observed nuclei: • stable nuclei • b decaying nuclei • particle emitting nuclei Stable and decaying nuclei Stable nuclei neutron- rich proton- rich Off the valley of stability...
beta + proton g neutron + light particles fissionbreak-up of a nucleus g two light nuclei beta - neutron g proton + light particles alpha emission of 4He 2 protons + 2 neutrons "classical" radioactivities
Nuclear Stability Proton unstable Stablenuclei Neutron unstable
Designer Nuclei in Nuclear Landscape superheavy nuclei 225Ra 62Ga 78Ni 134Sn 11Li 283112 45Fe 68Se 101Sn 149Tb 82 126 protons terra incognita 50 82 stable nuclei 28 • How do protons and neutrons make stable nuclei and rare isotopes? • What are properties of neutron matter? • What are the heaviest nuclei that can exist? • What is the origin of simple patterns in complex nuclei? 20 50 8 28 known nuclei 2 20 8 2 neutrons
Radiation Detectors • Photographic Film • To detect , and radiations • Spark counter • To detect -particles • Ionization Chamber • To detect -particles • Cloud Chamber • To detect and particles • Geiger-Müller Tube • To detect , and radiations
Isomer ? What is an isomer ? Metastable (long-lived) nuclear excited state. ‘Long-lived’ could mean ~10-19 seconds, shape isomers in alpha-clusters or ~1015 years 180Ta 9-->1+ decay. Why/when do you get isomers? If there is (i) large change in spin (‘spin-trap’) (ii) small energy change (iii) dramatic change in structure (shape, K-value)
Superdeformed Nuclei • A band terminates when all valence particles outside a doubly magic (spherical)core are aligned
Astrophysical Consequences of Isomers 180Ta is ‘stable’ in its isomeric state, but its ground state decays in hours! Longstanding problem as to how the isomeric state is created in nature (via eg. S-process). Possible mechanism via heavier nuclei spallation or K-mixing of higher states in 180Ta.
Some nuclei are more important than others - + - + - + + - + - 149Tb 18F,22Na 225Ra Over the last decade, tremendous progress has been made in techniques to produce designer nuclei, rare atomic nuclei with characteristics adjusted to specific research needs nuclear structure tests of fundamental laws of nature 45Fe applications astrophysics
Medical Imaging - 99mTc Technetium-99m a metastable isomer that decays into 99Tc by g emission with a half-life of 6h. Then decays into ruthenium by b emission, but with a half-life of 2.1 x 105 years. N.B. As a gamma emitter, 99mTc remains the same element during its residence in the body so it doesn’t change its chemistry when it decays. Highly penetrating g radiation. Long half-life = low activity.
Gamma camera CT scanner E.g. g-camera image of 131I (from NaI solution) uptake in a normal and diseased thyroid gland, showing localisation of iodine. E.g. tomographic image of a single anatomical level of the brain using 18F-labelled glucose.
Gamma spectroscopy • Radiation interaction in scintillator produces light (may be in visible range) • Quantification of output requires light amplification and detection device(s) • This is accomplished with the: • Photocathode • Photomultiplier tube • Both components are • placed together as one unit • optically coupled to the scintillator
Basic Physics Processes in a Sodium Iodide (NaI) Calorimeter The amount of light given off by NaI is proportional to the amount energy absorbed. The light yield is ~ 1 photon per 25 eV deposited in NaI, lmax=415 nm, decay time ~250nsec NaI is often used to measure the energy low gamma rays Compton Scattering ge-→ge- “elastic scattering” Photoelectric Effect g absorbed by material, electron ejected Pair Production g→e+e- creates anti-matter g g g e- e- e+ e- g NaI 0.05 < hv < 10 MeV hv > 10 MeV g-ray must have E>2me hv < 0.05 MeV Attenuation of the gamma rays is energy dependent radiation length of NaI ~2.5 cm but only useful for E > few MeV Richard Kass
Scintillation detector Photocathode Scintillation event Photomultiplier tube Gamma ray Dynodes Photoelectrons Fluor crystal NaI (Tl) Reflector housing
Major components of PM Tube • Photocathode material • Dynodes • electrodes which eject additional electrons after being struck by an electron • Multiple dynodes result in 106 or more signal enhancement • Collector • accumulates all electrons produced from final dynode • Resistor • collected current passed through resistor to generate voltage pulse
Generalized Detection System using a Scintillator (Crystal & Photomultiplier) Scaler Detector Amplifier Pre- Amp Discriminator Multi- Channel Analyzer High Voltage Oscilloscope
Commonly Used Scintillators (1) Effective average decay time For g-rays.(2) At the wavelength of the emission maximum.(3) Relative scintillation signal at room temperature for g-rays when coupled to a photomultiplier tube with a Bi-Alkalai photocathode.
Commonly Used Scintillators (1) Effective agerage decay time For g-rays.(2) At the wavelength of the emission maximum.(3) Relative scintillation signal at room temperature for g-rays when coupled to a photomultiplier tube with a Bi-Alkalai photocathode.
Coincidence helps! Amplification Amplification Timing Multi-channel Analyzer Detector Coincidence Unit Source Gate Detector Scaler Timing After Tsoulfanidis, 1995
Inner BGO ball at G.A.S.P. ACS BGO BALL
Projectile Fragmentation Reactions projectile Final fragment Excited pre-fragment target hotspot Energy (velocity) of beam > Fermi velocity inside nucleus ~30 MeV/u Can ‘shear off’ different combinations of protons and neutrons. Large variety of exotic nuclear species created, all at forward angles with ~beam velocity. Some of these final fragments can get trapped in isomeric states. Problem 1: Isotopic identification. Problem 2: Isomeric identification.
Hadronic Properties of GeV Photon Vector-Meson Dominance Model
Bremsstrahlung Energy loss by Bremsstrahlung (charge particles) Effect plays a role only for e± and ultra-relativistic µ (>1000 GeV) Ec(e-) in Cu(Z=29) = 20 MeV, Ec(µ) in Cu ≈ 1 TeV. muons in multi-GeV range can traverse thick layers of dense matter.
Laser-Electron-PhotonS facility 3.3 GeV electron Backward-Compton scattering Collision Recoil electron Tagging counter a) Storage Ring Laser light Energy spectrum of BCS photons Bremsstrahlung Compton g-ray b) Laser hutch c) Experimental hutch
Linearly Polarized Photon • Backward Compton scattering with UV laser light • Intensity (typ.) : 2.5 * 106 cps • Tagging region : 1.5 GeV< Eg < 2.4 GeV • Linear polarization : 95 % at 2.4 GeV Counts Linear polarization Eg(GeV) Eg(Tagger) (GeV)
Super Photon Ring 8 GeV (SPring-8) Harima Science Garden City
Laser System Ar ion laser (MLUV,CW 5.5W) Polarization rotator Focusing lens
e Collision in Storage Ring Straight section e- (8GeV) Laser Bending magnet g Tagging counter e’
LEPS Detector System g Dipole Magnet (0.7 T) TOF wall Start counter Aerogel Cherenkov (n=1.03) MWDC 3 Silicon Vertex Detector MWDC 2 MWDC 1 1m
Elementary Observables • Momentum • Time-of-Flight • Energy Loss • Particle Identification • Invariant Mass Reconstruction
Momentum Measurements • Definition • Newtonian Mechanics • Special Relativity • But how does one measure “p”?