Nuclear physics: Magdalena Kowalska CERN, PH Dept.

Transcription

1 Nuclear physics: the ISOLDE facility Magdalena Kowalska CERN, PH Dept on behalf of the CERN ISOLDE team

2 Outline Forces inside atomic nuclei Nuclei and QCD, nuclear models Nuclear landscape Open questions in nuclear physics ISOLDE facility Radio isotope production Physics of ISOLDE Examples of experimental setups Physics case: island of inversion 2

3 Nuclear physics today 3

4 Forces acting in nuclei Coulomb force repels protons Strong interaction ("nuclear force") causes binding which is stronger for pn systems than nn systems Neutrons alone form no bound states (exception: neutron stars (gravitation!) Weak interaction causes β decay e n ν - 4 p

5 Nuclei and QCD Different energy scales In nuclei: non perturbative QCD, so no easy way of calculating Have to rely on nuclear models (shell model, mean field approaches) Recent progress: lattice QCD 5

6 Nuclear landscape stable + /EC decay - decay decay p decay spontaneous fission Proton drip line neutron drip line neutrons Magic numbers About 300 stable isotopes: nuclear models developed for these systems 3000 radioactive isotopes discovered up to now 6

7 Properties of radio nuclides Different neutron to proton ratio than stable nuclei leads to: New structure properties New decay modes => Nuclear models have problems predicting and even explaining the observations Example halo nucleus 11Li: Extended neutron wave functions make 11 Li the size of 208 Pb When taking away 1 neutron, the other is not bound any more (10Li is not bound) 7

8 Open questions in nuclear physics 2 kinds of interacting fermions Observables: Ground state properties: mass, radius, moments Half lives and decay modes Transition probabilities (NuPECC long range plan 2010) 8 Main models: Shell model (magic numbers) Mean field models (deformations) Ab initio approaches (light nuclei)

9 ISOLDE: CERN radio nuclide factory 9

10 Radioactive ion beam facilities Renaissance of nuclear physics: access to nuclei with new proton to neutron ratios testing ground for nuclear models allows many applications 10

11 Radioactive ion beam facilities 1967: ISOLDE CERN: first ISOL type facility Now: B. Sherill 11

12 ISOLDE short history First beam October 1967 Upgrades 1974 and 1988 New facility June

13 ISOLDE at CERN 13

14 ISOLDE within CERN accelerators PSB ISOLDE LINAC2 PS 14

15 ISOLDE elements Isotope production via reactions of light beam with thick and heavy target Production ionization separation 15

16 ISOTOPE production spallation + 1 GeV p 201 Fr fragmentation U 11 Li X p n fission Cs Y 16

17 Targets Over 20 target materials and ionizers, depending on beam of interest U, Ta, Zr, Y, Ti, Si, Target material and transfer tube heated to degrees Operated by robots due to radiation Target p+ p+ p+ Target p+ p+ Converter Converter Target 17 Standard

18 Ionization Surface Plasma Lasers 18

19 Production, ionization, extraction target Target+ vacuum+ extraction robots 19 Ion energy: 30 60keV

20 Separation Magnet separators (General Purpose and High Resolution) 20

21 Post acceleration REX accelerator Rf acceleration A/q selection Increase in charge state Ion trapping and cooling 3MeV*A beam to experiment 21

22 Production and selection exaple Facility 22

23 Experimental hall Decay spectroscopy Coulomb excitation Transfer reactions Laser spectroscopy Beta NMR Penning traps WITCH REX ISOLDE Mass sep. HRS ISCOOL RILIS Target stations HRS & GPS PS Booster 1.4 GeV protons ppp Travelling setups NICOLE Post accelerated beams MINIBALL and T REX Collection points Travelling setups COLLAPS CRIS ISOLTRAP TAS 23

24 Facility photos 24

25 Experimental beamlines 25

26 Produced nuclides More than 700 nuclides of over 70 chemical elements delivered to users by far largest choice among ISOL type facilities (experience gathered over 40 years) 26

27 Physics topics Applied Physics Nuclear Physics Nuclear Decay Spectroscopy and Reactions Structure of Nuclei Exotic Decay Modes Implanted Radioactive Probes, Tailored Isotopes for Diagnosis and Therapy Condensed matter physics and Life sciences Atomic Physics Laser Spectroscopy and Direct Mass Measurements Radii, Moments, Nuclear Binding Energies f(n,z) Nuclear Astrophysics Dedicated Nuclear Decay/Reaction Studies Element Synthesis, Solar Processes Fundamental Physics Direct Mass Measurements, Dedicated Decay Studies WI CKM unitarity tests, search for b n correlations, right handed currents 27

28 Laser and beta NMR spectroscopy COLLAPS setup Laser beam, Laser on fixed frequency Ion beam Collinear laser spectroscopy Neutralisation region Beta NMR Retardation zone Electrostatic deflection Excitation / Observation region Beam from ISOLDE Detection method depends on the case => optimised for best S/N ratio Observables: Hyperfine structure, isotope shifts, Nuclear Magnetic Resonance Information: ground state spin (+parity assignment), charge radius, moments => Probing single particle and collective properties 28

29 Laser and beta NMR spectroscopy

30 Penning trap mass spectrometry ISOLTRAP setup Observable: cyclotron frequency Information: mass => shell structure, deformations c 1 q 2 m B determination of cyclotron frequency (R = 10 7 ) removal of contaminant ions (R = 10 5 ) Precison Penning trap B = 6T MCP 2 To F MCP 3 / Preparation Penning trap B = 4.7T z 0 r0 reference ion source Bunching of the continuous beam MCP 1 RFQ cooler and buncher IS OLDE 60 kev ion beam 30 HV platform

31 Penning trap mass spectrometry 31

32 Coulomb excitation Miniball setup Ge detectors CD detector E E detector Double sided Si strip detector PPAC Beam dump detector REX ISOLDE E <3 MeV/u target Excitation of a projectile nucleus (radioactive) by the electromagnetic field of the target (made of stable nuclei) Beam Beam impurities Observables: Transition energies and intensities Information: reduced transition matrix elements => Study collectivity and deformations 32

33 Coulomb excitation 33

34 Transfer reations Miniball + T REX setup (Si detector barrel): gamma detectors and particle identification d p decay Typical reactions: one or two nucleon transfer (d,p), (t,p) Information: Observables energies of protons (+ E g ) (single particle) level energies angular distributions of protons (+ rays) spin/parity assignments (relative) spectroscopic factors particle configurations study single particle properties of nuclei = > Similar configurations = large overlap of wave functions = Large probability of transfer reaction 34

35 Island of inversion : discovery Nuclear shell model developed on stable nuclei works very well until 1975: mass measurements of neutron rich Ne, Na, Mg isotopes => trend in binding energies can be only explained by large deformation and thus no shell closure at N=20 => Collapse of the backbone 8 7 O 12 N Ne 16 F 9 15 O O 13 N 14 N Si Mg Na 19 Ne Ne F F O O 15 N N 15 Si 23 Al 22 Mg 21 Na 20 Ne 19 F 18 O 17 N Si 24 Al 23 Mg 22 Na 21 Ne 20 F 19 O 18 N 17 S 27 P 26 Si 25 Al 24 Mg 23 Na 22 Ne 21 F 20 O 19 N 18 S 28 P 27 Si 26 Al 25 Mg 24 Na 23 Ne 22 F 21 O 20 N 19 S 29 P 28 Si 27 Al 26 Mg 25 Na 24 Ne 23 F 22 O 21 N 20 S 30 P 29 Si 28 Al 27 Mg 26 Na 25 Ne 24 F 23 O 22 N 21 S 31 P 30 Si 29 Al 28 Mg 27 Na 26 Ne 25 F 24 O 23 N 22 S 32 P 31 Si 30 Al 29 Mg 28 Na 27 Ne 26 F 25 O 24 N 23 S 33 P 32 Si 31 Al 30 Mg 29 Na 28 Ne 27 F 26 S S S 34 P 35 P 36 P 33 Si 32 Al 34 Si 33 Al 35 Si 34 Al 31 Mg 32 Mg 33 Mg 30 Na 31 Na 32 Na 29 Ne 30 Ne 31 Ne 28 F F S 37 P 36 Si 35 Al 34 Mg 33 Na 32 S 38 P 37 Si 36 Al 35 Mg 34 Na 33 Ne S 39 P 38 Si 37 Al 36 Mg 35 Na 34 S 40 P 39 Si 38 Al 37 Mg 36 Na S 41 P 40 Si 39 Al 38 S 42 P 41 Si 40 Al S 43 P 42 Si 41 S 44 P 43 Si S 45 P 44 S 46 P 45 S 47 P 46 S Mass measurements at PS CERN C. Thibault et al., Phys. Rev. C 12, 644 (1975) 35

36 Island of inversion: explanation 36 S: 16p and 20n protons neutrons pf shell p 1/2 f 5/2 p 3/2 f 7/2 Intruder orbitals Decreased N=20 shell gap (or even dissapeared) Increased correlations pf shell 32 Mg: 12p and 20n protons p 1/2 f 5/2 p 3/2 f 7/2 neutrons Intruder orbitals sd shell d 3/2 s 1/2 d 5/2 sd shell normal inversion of the level filling In low lying excited states and even in the ground state neutrons occupy higher (intruder) orbitals before the usual shell is closed d 3/2 s 1/2 d 5/2 Inversion of the usual level filling But for nuclei with more protons N = 20 is again a magic number Name: island of inversion This hypothesis has to be tested/confirmed 36 experimentally

37 Mg spins and moments Spins, parities and magnetic moments => single particle nature Laser and beta NMR spectroscopy on 29 33Mg With COLLAPS setup 31Mg HFS p 1/2 f 5/2 p 3/2 f 7/2 N=20 pf shell p 1/2 f 5/2 p 3/2 f 7/2 p 1/2 f 5/2 p 3/2 f 7/2 N=20 d 3/2 s 1/2 d 5/2 neutrons G. Neyens, M. Kowalska et al, Phys. Rev. Lett. 94, (2005) D. Yordanov, M. Kowalska et al, Phys. Rev. Lett. 99 (2007) M. Kowalska, D. Yordanov et al Phys. Rev. C77 (2008) sd shell protons d 3/2 s 1/2 d 5/2 neutrons protons 29Mg 31Mg 33Mg 29Mg outside the island 31,33Mg inside; with 2 nucleons across N=20 d 3/2 s 1/2 d 5/2 neutrons

38 Mg Coulomb excitation Coulomb excitation on 30,32Mg Excitations across N=20 will increase collectivity, with MINIBALL setup due to more active nucleons and thus more correlations World results for Mg Coulomb excitation: only ISOLDE can provide pure e.m. excitation ( safe Coulex ) Increase in collectivity from 30Mg to 32Mg (N=20) 38 O. Niedermaier et al, Phys. Rev. Lett. 94 (2005) M. Seidlitz et al, Phys.Lett. B 700 (2011) 181

39 32Mg, transfer reaction Two neutron (t,p) transfer reactions on 30,32Mg Allow finding 1 st excited 0+ state and probing its nature: normal spherical structure or deformed structure based on 2 neutrons excited across N=20 proton angular distributions L=0 => 0+ > 0+ existence of excited 0+ state in 32Mg at 1058 kev cross section to populate 32Mg excited 0+ state points to its similarity to 30Mg ground state, i.e. spherical structure made of orbitals below N=20 39 L=0 => 0+ > 0+ K. Wimmer et al, Phys. Rev. Lett. 105 (2010)

40 32Mg, transfer reaction Coexistence of spherical and deformed states confirmed 0 + normal sd configurations 0 + E0 E2 inversion E2 30 Mg 32 Mg 31 Mg intruder fp configurations

41 Mg: laser spectroscopy Laser spectroscopy with COLLAPS Charge radius (fm) Uncertainty of the slope due to atomic F factor uncertainty not included N=14 N=20 Atomic number, A HFS structure of 21Mg observed in decay asymmetry Smallest radius at N=14, not N=20: Migration of the shell closure 41 D. Yordanov et al, in preparation

42 What did we learn? Normal sd configuration 2 neutrons in pf shell mixed configuration Already at N=19 neutrons occupy higher orbits Transition to island of inversion is sudden Spherical and deformed shapes coexist at low energies Deformed ground states show mainly 2 neutrons across N=20 (in pf shell) New magic number (N=14 or N=16) appears Theories start agreeing with experiment => Mechanism driving the island connected to tensor part of nuclear interaction => There can be other islands like this one (but interaction details need to be worked out based on more experiments) 42

43 Summary and outlook Nuclear physics describes the atomic nucleus Since several decades radioactive beam facilities have given access to many short lived nuclei These nuclei have different properties than stable nuclei on which nuclear models were based Open questions include the description and prediction all observed properties and theoretical connection to QCD ISOLDE facility at CERN provides many raio nuclides Various setups look at the nuclear physics, fundamental studies and applications Complementarily of methods is well seen in the famouv island of inversion region One answer brings new questions 43

44 Thank you for your attention 44

45 Example island of inversion 36 S: 16p and 20n protons neutrons pf shell p 1/2 f 5/2 p 3/2 f 7/2 Intruder orbitals Decreased N=20 shell gap (or even dissapeared) Increased correlations pf shell 32 Mg: 12p and 20n protons p 1/2 f 5/2 p 3/2 f 7/2 neutrons Intruder orbitals sd shell d 3/2 s 1/2 d 5/2 normal inversion of the level filling In low lying excited states and even in the ground state neutrons occupy higher (intruder) orbitals before the usual shell is closed sd shell d 3/2 s 1/2 d 5/2 Inversion of the usual level filling But for nuclei with more protons N = 20 is again a closed shell Name: island of inversion This hypothesis has to be investigated experimentally 45

46 Results island of inversion Laser and NMR spectroscopy Transfer reactions Beta decay asymmetry (%) Doppler tuning voltage L=0 => 0+ > 0+ Laser spectroscopy Laser spectroscopy Charge radius (fm) N=14 decay N=20 Atomic number, A 46

47 Results island of inversion Normal configuration 2 neutrons in next shell mixed configuration => Theories can now reproduce most results => Mechanism driving the island connected to tensor part of nuclear interaction => There can be other islands like this one (but interaction details need to be worked out based on more experiments) 47

48 Spin (orbital+intrinsic angular momentum), parity (I ) Nuclear g factor and magnetic dipole moment (g I and I ) Electric quadrupole moment (Q) Charge radius ( r 2 ) Give information on: Configuration of neutrons and protons in the nucleus Size and form of the nucleus 1/2 + I 0d 3/2 g I and I I p =2 + = Q r 2 volume 0d 3/2 1s 1/2 3/2 + 0d 5/2 0d 3/2 1s 1/2 0d 5/2 1s 1/2 0d 5/2 I p =2 + = d 3/2 1s 1/2 0d 5/2 Q=0 spherical 48 Q>0 prolate Q<0 oblate deformation pairing

49 Production process 49

50 Fast timing gamma spectroscopy Gamma spectroscopy with BaF2 crystals (very fast response, <ps lifetime studies) => Transition energies and probabilities, deformations 50

51 Mg: fast timing fast timing t) using BaF 2 detectors =>level lifetimes in 30,31,32Mg (transition probabilities) 2+ state lifetime: Spherical ground state Deformed ground state 1789 kev level established : candidate for deformed 0+ configuration in 30Mg 51H. Mach et al., Eur. Phys. J. A 25, s01, 105 (2005)

52 30Mg: E0 transition E0 decay of 30Mg electron spectrometer Identification of 0+ state at 1789 kev ; small mixing amplitude with spherical ground state => deformed state 30Mg: spherical 0+ground state, deformed 1 st 0+ state (2 neutrons across N=20) => shape coexistence deformation W. Schwerdtfeger et al., Phys. Rev. Lett. 103, (2009) 52

53 Results decay of light nuclei / Li T 1/2 = 8 ms - Even a neutron rich-nucleus emits charged particles Li+t Li+d 9 Li Be+2n n d Energy (MeV) Be Be+n 53

54 Atomic vs nuclear structure Atoms shell model: e - fill quantized energy levels Description Nuclei shell model (but not only): p and n separately fill quantized energy levels n, l, m l, s, parity (-1) l n, l, m l, s, parity (-1) l Quantum numbers max. S possible Lowest en. levels (due to Coulomb force): J= L+S= l i + s i or J= j i l i +s i ) min. S possible (due to strong force pairing): J = j i l i +s i ) weak Spin-orbit coupling strong for 3 electrons in a d orbital for 3 nucleons in a d orbital d 3/2 d 5/2 calculated by solving Energy levels Schrödinger equation with central potential dominated by nuclear Coulomb field 54 not easily calculated; nucleons move and interact within a selfcreated potential

55 55

56 Nuclear potential 56

57 Nucleon nucleon interaction distance > 1 fm 1 fm π-meson distance < 0.5 fm? 57 three-body forces?

58 Nuclei and QCD Scales: 1 GeV In nuclei: non perturbative QCD, so no easy way of calculating kev

59 Binding energy per nucleon sun energy B 8 H He fusion reactor energy fission U (MeV) Fe particle number A 59

60 Valey of stability β + β - β + decay N-Z 60 β - decay

61 Physics with radioactive ion beams Nuclear physics Strong interaction in many nucleon systems Nuclear driplines? = Mean field Astrophysics Nucleo synthesis, star evolution Abundances of elements 34Ne: 10 protons + 24 neutrons Does it exist? fp shell sd shell p shell s shell relative abundance Fundamental studies Beyond standard model (neutrino mass, ) Applications, e.g. Solid state physics, life sciences 61 mass

62 Modelling nuclear interaction 62

63 REX post accelerator Nier-spectrometer Select the correct A/q and separate the radioactive ions from the residual gases. A/q resolution ~150 Optional stripper EBIS Super conducting solenoid, 2 T Electron beam < 0.4A 3-6 kev Breeding time 3 to >200 ms Total capacity charges A/q < 4.5 MASS SEPARATOR REXEBIS ISOLDE 9-GAP RESONATOR 7-GAP RESONATORS IH RFQ Rebuncher ISOLDE beam Primary target Linac Experiments 18 May MeV/u 2.2 MeV/u 1.2 MeV/u Length 11 m Freq. 101MHz (202MHz for the 9GP) Duty cycle 1ms 100Hz (10%) Energy 300keV/u, 1.2-3MeV/u A/q max. 4.5 (2.2MeV/u), 3.5 (3MeV/u) 0.3 MeV/u Total efficiency : 1-10 % New opportunities in the physics landscape at CERN 63 REXTRAP 60 kev REX-trap Cooling (10-20 ms) Buffer gas + RF (He), Li,...,U 10 8 ions/pulse (Space charge effects >10 5 )

64 Nuclear shell model Developped about 50 years ago for stable nuclei: Some nuclides more stable and more difficult to excite than others Backbone of nuclear physics: with closed shell assumption Based on analogies to atomic shells, but differences: No central potential but a self created potential Nucleon nucleon interaction has tensor (noncentral) components Two kinds of nucleons In ground state: all odd number of protons or neutrons couple to spin 0 Strong spin orbit coupling changes magic numbers: 8,20,28,50, No analitic form of nucleon nucleon interaction in nuclear medium Challenges: dissapearing and migrating magic numbers => The backbone is not as well understood 64 as we thought Describes singleparticle properties