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

Fast-Timing with LaBr3:Ce Detectors and the Half-life of the Iπ = 4– Intruder State in 34P

(…and some other stuff maybe..) Paddy Regan

University of [email protected]

TRIUMF Seminar, 14th October 2011

Outline

• Characteristics of LaBr3 detectors

• Fast-timing techniques

• 34P and M2 strengths approaching the island of inversion.

• More recent results:– N=80 below (h11/2)-2 isomers

– 188W (2+ lifetimes)

• Summary and the future

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


Highly non-linear gains

Substantial gain drift through-out experiment requires run-by-run gainmatching

Gain drift of detectors during 34P experiment


Efficiency is ~1.3 times that of NaI(Tl) for the same volume

Trade-off between efficiency and time resolution

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)]


Gaussian-exponential convolution to account for timing resolution


Centroid shift method for an analysis of short half-lives

(Maximum likelihood method)

Difference between the centroid of observed time spectrum and the prompt response give lifetime,

t=0


2

Mirror-symmetric centroid shift method.

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

Outline




• More recent results and future measurements

• Summary and the Future

Motivation

2

8

20

28

1s1/2

1p3/2

1p1/2

1d5/2

2s1/2

1d3/2

1f7/2

2p3/2

• 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”.

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

Motivation

20

1d5/2

2s1/2

1d3/2

1f7/2

20

1d5/2

2s1/2

1d3/2

1f7/2

I = 2+ [2s1/2 x (1d3/2)-1] I = 4- [2s1/2 x 1f7/2]

• 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

Experiment18O(18O,pn)34P fusion-evaporation at 36 MeV ~ 5 – 10 mb50mg/cm2 Ta2

18O Enriched foil18O 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

Results

Results

Total in-beam Ge spectrum from LaBr3-Ge matrix

Total in-beam LaBr3 spectrum from LaBr3-Ge matrix

429

1876

Results

429-keV gate

429-keV gate

1048-keV gate

1048-keV gate

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-)


Gate in Ge to create clean LaBr3-LaBr3-dT

matrix


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


Gate in Ge to create clean LaBr3-LaBr3-dT

matrix


Assumes t1/2(2+) << t1/2 (4-)(which is true, 2+ half-life was limited to <1ps by Bender et al.)

Set Ge gate on 1876-keV transition and look at the time difference between 1048-keV and 429-keV gammas.

Total in-beam Ge spectrum from LaBr3-Ge matrix

Total in-beam LaBr3 spectrum from LaBr3-Ge matrix

429

429

1048


Projection of LaBr3-LaBr3 matrix gated by 1876 keV gamma in Ge detectors

Ungated LaBr3 Time difference

429-keV gate 1048-keV gateThe 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

Results: T1/2 = 2.0(1)ns

429 / 1048

429 / 1876(~prompt)

Results: Ge-gated Time Spectra

Discussion: B(M2), B(E3) values

A

B

• 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.

Discussion: Iπ = 4– or 4+?

[1]

[3]

• 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.

[2]

[4]

Discussion: M2 Strengths

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

Discussion: SM Calculations

A

B

• 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.

[1]

Discussion: SM Calculations

Outline




• More recent results and future measurements

• Summary and the Future

(h11/2)-2 only

N=80 Isotones

0+

2+

4+

6+

8+

10+isomer

Primarily(d5/2)2

Primarily(g7/2)2

• 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.

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.

• 130Te(12C,4n)138Ce, 56 MeV• 84 ns Isomer allows HPGe gates

“anticipated” or “delayed” relative to trigger.

Will form part of thesis of T. Alharbi, University of SurreyS.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)

“anticipated”“delayed”

“anticipated”“delayed”

isomer

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).

S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)

“anticipated”HPGe gate

815keV gate 165keV gate

“anticipated”HPGe gate

preliminary


S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)

preliminary T1/2 ~ 170ps

Using “delayed” HPGe gate


{815,165}

{815,467}

{77,390}

{418,403}

{254,338}

138Ce Lifetimes Summary

• 186W(7Li,p)188W, 33 MeV• Reaction mechanism is a mix of

incomplete fusion and low-energy transfer.

• ~54 hours beam time

T. Shizuma et al. Eur. Phys. J. A30, 391 (2006)

296 keV gate (HPGe) 432 keV gate (HPGe)

Contaminants are 186Os

189Ir

188W – Lifetime of the 2+ State

• Estimate of 188W 2+ half-life from this short run gives unusual behaviour in B(E2).

• BUT… measurement is unreliable at this stage.

• Precision measurement to be made soon.

Time difference 143-432 keVContaminated by 186Os [t1/2(2+) = 875(15) ps]

188W – Lifetime of the 2+ State

Summary and the Future

• LaBr3:Ce detectors have acceptable energy resolution and excellent timing properties making them attractive for gamma-ray spectroscopy.

• 34P 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.

Thank you

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

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