Paddy Regan University of Surrey p.regan@surrey.ac.uk

1. Fast-Timing with LaBr3:Ce Detectors and the Half-life of the Iπ = 4– Intruder State in 34P(…and some other stuff maybe..) Paddy ReganUniversity of Surreyp.regan@surrey.ac.uk TRIUMF Seminar, 14th October 2011

2. Characteristics of LaBr3 detectors Fast-timing techniques 34P and M2 strengths approaching the island of inversion. More recent results: N=80 below (nh11/2)-2 isomers 188W (2+ lifetimes) Summary and the future Outline

3. Detector Performance • Recently developed scintillator material. Excellent timing and reasonable energy resolution. • Typical time resolution = 150 – 300 ps (FWHM) • Affected by the size of crystal: • smaller crystal = better resolution • Precision = FWHM / N1/2 • Measurements possible down to T1/2 ~ 30 ps

4. Detector Performance

6. Detector Performance Gain drift of detectors during 34P experiment Highly non-linear gains Substantial gain drift through-out experiment requires run-by-run gainmatching

7. Detector Performance Efficiency is ~1.3 times that of NaI(Tl) for the same volume Trade-off between efficiency and time resolution

8. Fast-timing Techniques Prompt response function from 152Eu source (gate on 152Gd peak) [J-M. Regis NIMA 662 (2010)] Time walk correction from 60Co source [N. Marginean, EPJA 46 (2010)]

10. Fast-timing Techniques Gaussian-exponential convolution to account for timing resolution

11. Fast-timing Techniques Centroid shift method for an analysis of short half-lives (Maximum likelihood method)  t=0 Difference between the centroid of observed time spectrum and the prompt response give lifetime, 

12. Fast-timing Techniques Mirror-symmetric centroid shift method. Using reversed gate order (e.g. start TAC on depopulating gamma, stop on feeding gamma) produces opposite shift 2 Removes the need to know where the prompt distribution is and other problems to do with the prompt response of the detectors

13. Characteristics of LaBr3 detectors Fast-timing techniques 34P and M2 strengths approaching the island of inversion. More recent results and future measurements Summary and the Future Outline

14. 2p3/2 28 1f7/2 20 1d3/2 2s1/2 1d5/2 8 1p1/2 1p3/2 2 1s1/2 Motivation • Nuclei with Z~10-12, N~20 observed to have unexpectedly high B.E. • Linked to onset of deformation from filling of f7/2 intruder orbital. • N=20 shell gap diminished, allowing excitations from d3/2 to f7/2 to become favoured. • Region of anomalous shell-structure is termed the “island of inversion”.

15. Motivation • Recent study of 34P identified low-lying I=4- state at E=2305 keV. • Spin and parity assigned on basis of DCO and polarization measurements. • I=4-→ 2+ transition can proceed by M2 and/or E3. • Aim of experiment is to measure precision lifetime for 2305 keV state and obtain B(M2) and B(E3) values. • Previous studies limit half-life to • 0.3 ns < t1/2 < 2.5ns • New results by Bender et al. give =0 for mixing ratio but Chakrabarti et al. measured significant E3 mixing

16. Motivation • Recent study of 34P identified low-lying I=4- state at E=2305 keV. • Spin and parity assigned on basis of DCO and polarization measurements. • I=4-→ 2+ transition can proceed by M2 and/or E3. • Aim of experiment is to measure precision lifetime for 2305 keV state and obtain B(M2) and B(E3) values. • Previous studies limit half-life to • 0.3 ns < t1/2 < 2.5ns • New results by Bender et al. give =0 for mixing ratio but Chakrabarti et al. measured significant E3 mixing

17. 1f7/2 1f7/2 20 20 1d3/2 1d3/2 2s1/2 2s1/2 1d5/2 1d5/2     Motivation • 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. • Thus expect transition to go mainly via f7/2→ d3/2, M2 transition. • Different admixtures in 2+ and 4- states allow mixed M2/E3 transition I = 2+ [2s1/2 x (1d3/2)-1] I = 4- [2s1/2 x 1f7/2]

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

20. Results

21. Results Total in-beam Ge spectrum from LaBr3-Ge matrix 429 1876 Total in-beam LaBr3 spectrum from LaBr3-Ge matrix

22. Results 429-keV gate 1048-keV gate 429-keV gate 1048-keV gate

23. Ge-Gated Time differences Gates in LaBr3 detectors to observe time difference and obtain lifetime for state Ideally, we want to measure the time difference between transitions directly feeding and depopulating the state of interest (4-)

24. Ge-Gated Time differences Gate in Ge to create clean LaBr3-LaBr3-dT matrix Gates in LaBr3 detectors to observe time difference and obtain lifetime for state Use a Ge gate to create clean LaBr3 spectra with a gate on the 429-keV transition. But… Statictics are a problem -triple coincidence -low LaBr3efficiency for 1876-keV

25. Ge-Gated Time differences Gate in Ge to create clean LaBr3-LaBr3-dT matrix Gates in LaBr3 detectors to observe time difference and obtain lifetime for state Set Ge gate on 1876-keV transition and look at the time difference between 1048-keV and 429-keV gammas. Assumes t1/2(2+) << t1/2 (4-) (which is true, 2+ half-life was limited to <1ps by Bender et al.)

26. Ge-Gated Time differences 429 Total in-beam Ge spectrum from LaBr3-Ge matrix Projection of LaBr3-LaBr3 matrix gated by 1876 keV gamma in Ge detectors Total in-beam LaBr3 spectrum from LaBr3-Ge matrix 429 1048

27. Ungated LaBr3 Time difference 429-keV gate 1048-keV gate The LaBr3-LaBr3 coincidences were relatively clean where it counts so try without the Ge gate… e.g. The 1876-429-keV time difference is 34P. Should show prompt distribution as half-life of 2+ is short. FWHM = 470(10) ps

28. Results: T1/2 = 2.0(1)ns 429 / 1048 429 / 1876 (~prompt)

29. Results: T1/2 = 2.0(1)ns 429 / 1048 429 / 1876 (~prompt)

30. Results: Ge-gated Time Spectra

31. Results: Ge-gated Time Spectra

32. Discussion: B(M2), B(E3) values • Mixing ratio, E3/M2 limited to –1.03 to –0.27 by Chakrabarti et al. • Recent result by Bender et al. gives E3/M2 = 0. A B

33. Discussion: Iπ = 4– or4+? • Krishichayan et al. [1] suggested a 4+ spin-parity for the 2305-keV state based on polarisation measurements. • Ruled out by Chakrabarti et al. as their  implied unacceptable M3 strength (>200 W.u.). • However,  = 0 allows for a pure E2 transition and a 4+ assignment. • Upper limit of B(E2) = 0.0019(1) W.u. from present work. • With  = 0, B(M2) = 0.064(3) W.u. • Falls within the range of other transitions in this mass region assigned as f7/2→ d3/2 single-particle transitions. • Range from: 0.0330(10) W.u. (47Sc) to 0.63(6) W.u. (37Cl). • Notably, consistent with neighbouring N=19 nuclei, 33Si, 35S, 36Cl and 37Ar. • Arguments in [3] and [4] based on near degeneracy with 3- state and (t,3He) data. • Our measurement lends weight to 4- assignment, but we cannot rule completely out 4+ spin-parity. [1] [2] [3] [4]

34. Discussion: M2 Strengths • Experimental B(M2) and Mixing ratios from N=19 nuclei approaching the island of inversion.

35. Discussion: SM Calculations • Mixing ratio, E3/M2 limited to –1.03 to –0.27 by Chakrabarti et al. • Recent result by Bender et al. gives E3/M2 = 0. • SM calculations performed with modified WBP interaction [1]. • SM gives = -0.023 disagreeing with the strong E3 component suggested by Chakrabarti et al. A B [1]

36. Discussion: SM Calculations

37. Characteristics of LaBr3 detectors Fast-timing techniques 34P and M2 strengths approaching the island of inversion. More recent results and future measurements Summary and the Future Outline

38. N=80 Isotones • N = 80 isotones above Z = 50 display 10+ seniority isomers from coupling of (h11/2)-2 • 6+ level weakly hindered in 136Ba, (t(1/2) = 3.1(1)ns). • Thought to be due to change in configuration and seniority. isomer 10+ (h11/2)-2 only 8+ 6+ Primarily (g7/2)2 4+ 2+ Primarily (d5/2)2 0+

39. N=80 Isotones • Neighbouring N=80 nuclei, 138Ce and 140Nd expected to show similar hindrance (and are experimentally accessible at Bucharest.) • Competing transitions to negative parity states.

42. 138Ce – Lifetime of the 6+ State • 130Te(12C,4n)138Ce, 56 MeV • 84 ns Isomer allows HPGe gates “anticipated” or “delayed” relative to trigger. “anticipated” “delayed” isomer “delayed” “anticipated” S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999) Will form part of thesis of T. Alharbi, University of Surrey

44. 138Ce – Lifetime of the 6+ State • 0,2,4+ states thought to be based mainly on (d5/2)-2 configuration. 6+ based on (g7/2)-2. • Change in configuration  hindrance (6+state in 136Ba has t1/2 = 3.1(1) ns.) • Seniority may also play a role (6+ is maximum coupling of (g7/2)-2 hole pair). preliminary “anticipated” HPGe gate “anticipated” HPGe gate 815keV gate 165keV gate S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)

45. 138Ce – Lifetime of the 11+ State Using “delayed” HPGe gate preliminary T1/2 ~ 170ps S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)

50. 138Ce Lifetimes Summary {815,165} {77,390} {418,403} {815,467} {254,338}