Precision Sub-Nanosecond Lifetime Measurements of Excited States for Some 'Interesting' Nuclei

1. Precision Sub-Nanosecond Lifetime Measurements of Excited States for Some 'Interesting' Nuclei Paddy Regan Department of Physics University of Surrey, & National Physical Laboratory, Teddington, UK p.regan@surrey.ac.uk ; paddy.regan@npl.co.uk

2. Some Nuclear Structure ‘Big’ Science Questions? • How do protons and neutrons interact with each other? • Can we write down a nuclear ‘force’ equation or approximation? • Evolution of nuclear single-particle structure in nuclei. • Why do nuclear excitations change from ‘single particle’ to ‘collective’ ? • Why do some nuclei exhibit significant deformation from sphericity? • How do we measure and identify nuclear ‘deformation’ ?

3. Excitation energy (keV) PHR, Physics World, Nov. 2011, p37 2+ 0+ Ground state Configuration. Spin/parity Ip=0+ ; Ex = 0 keV

4. Some nuclear observables? • Masses and energy differences • Energy levels • Level spins and parities • EM transition rates between states • Magnetic properties (g-factors) • Electric quadrupole moments? Essence of nuclear structure physics …….. How do these change as functions of N, Z, I, Ex ? What are the most useful ‘signatures’ of nuclear structural evolution?

5. Why measure gamma rays? • Characteristic, EM ‘fingerprint’ energies associated with de-excitations from excited nuclear states. • Can be used to identify the isotope and amount of given radionuclides present. • Can also give direct insight into internal nuclear structure by ordering of excited energy levels and rates of their decay.

6. Nuclear structure information. The ‘reduced matrix element’ , B(lL) tells us the overlap between the initial and final nuclear wavefunctions….. This contains the ‘Physics’. (trivial) gamma-ray energy dependence of transition rate, goes as. Eg2L+1 e.g., Eg5 for E2s for example. Transition probability (i.e., 1/mean lifetime as measured for state which decays by EM radiation) How is measuring excited state lifetimes useful?

8. Weisskopf ‘single particle’ estimates for lifetimes of excited nuclear states. Based on EM model transitions for protons in spherical orbits.

9. T (E2)= transition probability = 1/t (secs); Eg = transition energy in MeV 2+ B(E2: 0+1  2+1)   2+1 E20+12 0+ Qo = INTRINSIC (TRANSITION) ELECTRIC QUADRUPOLE MOMENT. This is intimately linked to the electrical charge (i.e. proton) distribution within the nucleus. Non-zero Qo means some deviation from spherical symmetry and thus some quadrupole ‘deformation’. In the nuclear totational model, B(E2: I→I-2) gives Qo by:

10. Idea ? Use LaBr3 (halide scintillation) detectors in coincidence to measure both discrete gamma-ray energies from decays from excited nuclear states and use the (measured) time difference between successive members of a gamma decay cascade to give direct measurement of the lifetimes of (i.e. transition rates from) the intermediate states.

11. Why LaBr3(Ce) • LaBr3(Ce) has good timing properties: Timing Resolution FWHM of 130-150 ps with 60Co for a Ø1”x1” crystal. • Acceptable energy resolution ~ 3% FWHM at 662 keV. • Peak emission wavelength in blue/UV part of EM spectrum (380 nm), compatible with PMTs

13. Expected, E1/2 dependence of FWHM on gamma-ray energy.

14. 5+ 138La ec (66%) b- (34%) 2+ 2+ 788.7 1435.8 0+ 138Ce80 0+ 138Ba82 138La, T1/2=1.02x1011 years A.A.Sonzogni, NDS 98 (2003) 515 Natural abundance of Lanthanum (Z=57): 99.91% 139La (stable) 0.09% 138La 138La (primordial NORM) gives rise to internal background activity within these detectors.

15. La chemical separation also can induce some (chemically similar) Ac into the detector material….specifically 227Ac (22 year half-life). (see talk later by Sean Collins on 223Ra decay).

16. A ‘high(ish) background’ instrument… β-decay 0-255 keV 788-1000 keV 1.5-3 MeV EC α Activity: ~0.7counts/sec./cm3 ~0.1 counts/sec/cm3 J. McIntyre et al., NIM A 652, 1, 2011, 201-204

17. The ROSPHERE Gamma-ray Spectrometer array (at IFIN-HH Bucharest) • 14 HPGe detectors (AC) are used to detect coincident γ rays: • 7xHPGedets. @ 37o • 4xHPGedets. @ 64o • 3xHPGedets. @ 90o • 11 LaBr3(Ce:5%) detectors • 7x ø2”x2” and 4x ø1.5”x2” (Cylindrical) @ 37, 64 and 90ow.r.t. the beam axis.

18. Some physics examples…

19. Description of Doubly-Magic +1 Nuclei …….e.g. 209BiAssume inert (double magic) core and single, unpaired particle

20. Description of Doubly-Magic +1 NucleiAssume inert core and single, unpaired particle

21. Description of Doubly-Magic +1 NucleiAssume inert core and single, unpaired particle

22. Description of Doubly-Magic +1 NucleiAssume inert core and single, unpaired particle

23. Description of Doubly-Magic +1 NucleiAssume inert core and single, unpaired particle

24. Lifetimes in 209Bi The g.s of 209Bi 0+ h9/2 Z= 82, N=126 209Bi=208Pb+p The 1609 keV, 13/2+ level in 209Bi can be a mixture of the ground state (0+ ) in 208Pb coupled to the i13/2 single valence proton and the 3- octupole vibration coupled to the h9/2 single proton ground state proton configuration. 3- h9/2 O.J.Roberts, A.M.Bruce et al.,

25. Could also show some E3 vibrational mode? M2 E3

26. 209Bi • 209Bi was formed in the proton transfer reaction channel at IFIN-HH Bucharest. 208Pb(7Li,α2nγ,)209Bi • Target ̴ 20 mg/cm2 • Beam ̴ 4.5 pnA.

27. Analysis HPGe LaBr3(Ce) Use discrete HpGe gated energies to create symmetrised 2-D and 3-D coincidence arrays such as LaBr3(Eg1)-LaBr3(Eg2)-DT. Gate on gamma coincs to get DT values.

29. HPGe gates: To select the cascade • LaBr3(Ce:5%) gates: Above and below the level of interest.

30. Analysis HPGe LaBr3(Ce) START STOP O.J.Roberts, A.M.Bruce et al.,

31. Analysis

32. Analysis 992, 1609 1133, 1609 Background gate 1609, 1133 1609, 992 Select gates and background for subtraction. Create E1-E2-DT cube. Project out on DT axis. O.J.Roberts, A.M.Bruce et al.,

33. 2τ Half-life of the 13/2+ state in 209Bi established: T1/2 = 110 (10) ps

34. 34P19

35. ‘Fast-Timing’ in 34P • 34P19 has I=4- state at E=2305 keV. • Aim to measure a precision lifetime for 2305 keV state. WHY? • A I=4-→ 2+ EM transition is allowed to proceed by M2 or E3 multipolarity. • M2 and E3 decays can proceed by • nf7/2 →nd3/2 => M2 multipole • nf7/2 → ns1/2 => E3 multipole • Lifetime and mixing ratio information gives direct values of M2 and E3 transition strength • Direct test of shell model wfs… • (4-) = a1f1+ b1f2 + g1f3... • (2+) = a2f1’+ b2f2’ + g2f3’... Z=15 = N=19

37. 1f7/2 1f7/2 20 20 1d3/2 1d3/2 2s1/2 2s1/2 1d5/2 1d5/2     34P19 (Simple) Nuclear Shell Model Configurations • Theoretical predictions suggest 2+ state based primarily on [2s1/2 x (1d3/2)-1] configuration and 4- state based primarily on [2s1/2 x 1f7/2] configuration. • M2 decay can proceed via nf7/2→ nd3/2(Dj=Dl=2) transition. I = 2+ [2s1/2 x (1d3/2)-1] I = 4- [2s1/2 x 1f7/2] 15 protons19 neutrons 15 protons19 neutrons

38. 1f7/2 1f7/2 20 20 1d3/2 1d3/2 2s1/2 2s1/2 1d5/2 1d5/2     34P19 (Simple) Nuclear Shell Model Configurations • Theoretical predictions suggest 2+ state based primarily on [2s1/2 x (1d3/2)-1] configuration and 4- state based primarily on [2s1/2 x 1f7/2] configuration. • M2 decay can go via nf7/2→ nd3/2(Dj=Dl=2) transition. I = 2+ [2s1/2 x (1d3/2)-1] I = 4- [2s1/2 x 1f7/2] 15 protons19 neutrons 15 protons19 neutrons

39. 1f7/2 20 1d3/2 2s1/2 1d5/2   34P19 (Simple) Nuclear Shell Model Configurations • Theoretical predictions suggest 2+ state based primarily on [2s1/2 x (1d3/2)-1] configuration and 4- state based primarily on [2s1/2 x 1f7/2] configuration. • M2 decay can go via nf7/2→ nd3/2(Dj=Dl=2) transition. M2 s.p. transition I = 2+ [2s1/2 x (1d3/2)-1]

40. Experiment details 18O(18O,pn)34P fusion-evaporation @36 MeV. 34P cross-section,  ~ 5 – 10 mb Target, 50mg/cm2 Ta218O enriched foil 18O. Beam from Bucharest Tandem (~20pnA). Array 8 HPGe and 7 LaBr3(Ce) detectors • 3 (2”x2”) cylindrical • 2 (1”x1.5”) conical • 2 (1.5”x1.5”) cylindrical

41. 4-

42. 4- {429,1048} {429,1876} T1/2(4-) = 2.0(2) ns ; 4-→ 2+ = M2 decay. Consistent with ‘pure’ pf7/2→pd3/2 transition. Precision test of nuclear shell model at N=20

43. 188WMeasuring nuclear (quadrupole) deformation.

44. Half-life of the yrast 2+ state in 188W • Neutron-rich A ~ 190 nuclei, a long predicted prolate – oblate shape transition region. e.g. Bengtsson et al. Phys. Lett. B190 (1987) 1 • Unusual (energy) deviation at 190W compared to trend of other nuclides. • Measurement of B(E2;2+→0+) gives best measure of (evolution of) low-lying collectivity

45. 110 111 112 113 114 115 N 2 neutrons more than heaviest stable Tungsten (Z=74) isotope (186W). Populate 188W using 186W(7Li,ap)188W ‘incomplete fusion’ reaction.

46. Sum of time differences between • 143-keV (2+ → 0+) transition and any • higher lying feeding transition.

47. Time difference between 143 keV 2+→0+ and feeding transitions. T1/2=0.87(12) ns

48. B(E2) gives value for deformation for 188W of b2=0.18(1) PES calculations based on a deformed Woods-Saxon potential predict b2=0.19.

49. Facility for Antiproton and Ion Research (FAIR) NUSTAR: SuperFRS and experiments on three (energy) branches…. > 800 collaborators 350m ~1GeV/u fragmentation/fission fragment separator Low-energy branch / DESPEC

50. FATIMA for DESPEC • FATIMA = FAst TIMing Array = A high efficiency, gamma-ray detection array for precision measurements of nuclear structure in the most exotic and rare nuclei. • Specs. • Good energy resolution. • Good detection efficiency • Excellent timing qualities (~100 picoseconds). • Purchased 31 x LaBr3 1.5” x 2” crystals for array (expect 36 in total). • Fully digital, time stamped DAQ in final array. • Can use to measure lifetimes of excited nuclear states; provide precision tests of shell model theories of nuclear structure. • UK contribution to DESPEC (Decay Spectroscopy) project within NUSTAR. • Part of ~ £8M UK STFC NUSTAR project grant.