Fast-Timing with LaBr 3 :Ce Detectors and the Half-life of the I π = 4 Intruder State in 34 P

Transcription

1 Fast-Timing with LaBr 3 :Ce Detectors and the Half-life of the I π = 4 Intruder State in 34 P P.J.R. Mason University of Surrey Seminar, 11th October 2011

2 Outline Characteristics of LaBr 3 detectors Fast-timing techniques 34 P and M2 strengths approaching the island of inversion. More recent results and future measurements Summary and the future

3 Detector Performance Recently developed scintillator material. Excellent timing and reasonable energy resolution. Typical time resolution = ps (FWHM) Affected by the size of crystal: smaller crystal = better resolution Precision = FWHM / N 1/2 Measurements possible down to t(1/2) ~ 30 ps

4 Detector Performance

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

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

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

8 Fast-timing Techniques Gaussian-exponential convolution to account for timing resolution

9 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,

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

11 Outline Characteristics of LaBr 3 detectors Fast-timing techniques 34 P and M2 strengths approaching the island of inversion. More recent results and future measurements Summary and the Future

12 Motivation Nuclei with Z~10-12, N~20 observed to have unexpectedly high B.E. Linked to onset of deformation from filling of f 7/2 intruder orbital. N=20 shell gap diminished, allowing excitations from d 3/2 to f 7/2 to become favoured. Region of anomalous shell-structure is termed the island of inversion p 3/2 1f 7/2 1d 3/2 2s 1/2 1d 5/2 1p 1/2 1p 3/2 1s 1/2

13 Motivation Recent study of 34 P identified lowlying 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 < t 1/2 < 2.5ns New results by Bender et al. give =0 for mixing ratio but Chakrabarti et al. measured significant E3 mixing

14 Motivation Recent study of 34 P identified lowlying 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 < t 1/2 < 2.5ns New results by Bender et al. give =0 for mixing ratio but Chakrabarti et al. measured significant E3 mixing

15 Motivation Theoretical predictions suggest 2 + state based primarily on [ 2s 1/2 x ( 1d 3/2 ) -1 ] configuration and 4 - state based primarily on [ 2s 1/2 x 1f 7/2 ] configuration. Thus expect transition to go mainly via f 7/2 d 3/2, M2 transition. Different admixtures in 2 + and 4 - states allow mixed M2/E3 transition 20 1f 7/2 1d 3/2 2s 1/2 20 1f 7/2 1d 3/2 2s 1/2 1d 5/2 I = 2 + [ 2s 1/2 x ( 1d 3/2 ) -1 ] I = 4 - [ 2s 1/2 x 1f 7/2 ] 1d 5/2

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

17 Results

18 Results 429 Total in-beam Ge spectrum from LaBr 3 -Ge matrix 1876 Total in-beam LaBr 3 spectrum from LaBr 3 -Ge matrix

19 Results 429-keV gate 1048-keV gate 429-keV gate 1048-keV gate

20 Ge-Gated Time differences Gates in LaBr 3 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 - )

21 Ge-Gated Time differences Gate in Ge to create clean LaBr 3 -LaBr 3 -dt matrix Gates in LaBr 3 detectors to observe time difference and obtain lifetime for state Use a Ge gate to create clean LaBr 3 spectra with a gate on the 429-keV transition. But Statictics are a problem -triple coincidence -low LaBr 3 efficiency for 1876-keV

22 Ge-Gated Time differences Gate in Ge to create clean LaBr 3 -LaBr 3 -dt matrix Gates in LaBr 3 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 t 1/2 (2 + ) << t 1/2 (4 - ) (which is true, 2 + half-life was limited to <1ps by Bender et al.)

23 Ge-Gated Time differences 429 Total in-beam Ge spectrum from LaBr 3 -Ge matrix Total in-beam LaBr 3 spectrum from LaBr 3 -Ge matrix Projection of LaBr 3 - LaBr 3 matrix gated by 1876 kev gamma in Ge detectors

24 Ungated LaBr 3 Time difference 429-keV gate 1048-keV gate The LaBr 3 -LaBr 3 coincidences were relatively clean where it counts so try without the Ge gate e.g. The keV time difference is 34 P. Should show prompt distribution as half-life of 2 + is short. FWHM = 470(10) ps

25 Results: T 1/2 = 2.0(1)ns 429 / / 1876 (~prompt)

26 Results: T 1/2 = 2.0(1)ns 429 / / 1876 (~prompt)

27 Results: Ge-gated Time Spectra

28 Results: Ge-gated Time Spectra

29 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

30 Discussion: I π = 4 or 4 +? 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) = (1) W.u. from present work. [1] [2] [3] [4] With = 0, B(M2) = 0.064(3) W.u. Falls within the range of other transitions in this mass region assigned as f 7/2 d 3/2 singleparticle transitions. Range from: (10) W.u. ( 47 Sc) to 0.63(6) W.u. ( 37 Cl). Notably, consistent with neighbouring N=19 nuclei, 33 Si, 35 S, 36 Cl and 37 Ar. Arguments in [3] and [4] based on near degeneracy with 3 - state and (t, 3 He) data. Our measurement lends weight to 4 - assignment, but we cannot rule completely out 4 + spin-parity.

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

32 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. A B SM calculations performed with modified WBP interaction [1]. SM gives = disagreeing with the strong E3 component suggested by Chakrabarti et al. [1]

33 Discussion: SM Calculations

34 Outline Characteristics of LaBr 3 detectors Fast-timing techniques 34 P and M2 strengths approaching the island of inversion. More recent results and future measurements Summary and the Future

35 N=80 Isotones N = 80 isotones above Z = 50 display 10 + seniority isomers from coupling of ( h 11/2 ) level weakly hindered in 136 Ba, (t(1/2) = 3.1(1)ns). Thought to be due to change in configuration and seniority. ( h 11/2 ) -2 only Primarily ( g 7/2 ) 2 isomer Primarily ( d 5/2 )

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

37 138 Ce Lifetime of the 6 + State 130 Te( 12 C,4n) 138 Ce, 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

38 138 Ce Lifetime of the 6 + State 0,2,4 + states thought to be based mainly on ( d 5/2 ) -2 configuration. 6 + based on ( g 7/2 ) -2. Change in configuration hindrance (6 + state in 136 Ba has t 1/2 = 3.1(1) ns.) Seniority may also play a role (6 + is maximum coupling of ( g 7/2 ) -2 hole pair). anticipated HPGe gate preliminary anticipated HPGe gate 815keV gate 165keV gate S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)

39 138 Ce Lifetime of the 11 + State Using delayed HPGe gate preliminary T 1/2 ~ 170ps S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)

40 188 W Lifetime of the 2 + State T. Shizuma et al. Eur. Phys. J. A30, 391 (2006) 186 W( 7 Li, p) 188 W, 33 MeV Reaction mechanism is a mix of incomplete fusion and low-energy transfer. ~54 hours beam time 296 kev gate (HPGe) 432 kev gate (HPGe) Contaminants are 186 Os 189 Ir

41 188 W Lifetime of the 2 + State Time difference kev Contaminated by 186 Os [t 1/2 (2 + ) = 875(15) ps] Estimate of 188 W 2 + halflife from this short run gives unusual behaviour in B(E2). BUT measurement is unreliable at this stage. Precision measurement to be made soon.

42 Summary and the Future LaBr 3 :Ce detectors have acceptable energy resolution and excellent timing properties making them attractive for gammaray spectroscopy. 34 P 2305-keV state half-life measurement appears to confirm negative parity assignment and support a weakening of the N=20 shell closure Current and future experiments with low-energy stable beams at Bucharest provide opportunity to make measurements close to stability. The FATIMA array, part of DESPEC@FAIR will use an array of LaBr3:Ce detectors after in-flight separation for decay spectroscopy experiments. Allows lifetime measurements but also ordering of transitions.

43 Thank you

44 Thank you