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 Surrey
p.regan@surrey.ac.uk
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 (nh11/2)-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
Detector Performance
Detector Performance
Gain drift of detectors during 34P experiment
Highly non-linear gains
Substantial gain drift
through-out experiment
requires run-by-run
gainmatching
Detector Performance
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)]
Fast-timing Techniques
Gaussian-exponential convolution to account for timing resolution
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, 
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
Outline
• 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
Motivation
2p3/2
28
1f7/2
20
1d3/2
• 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”.
2s1/2
1d5/2
8
1p1/2
1p3/2
2
1s1/2
Motivation
• Recent study of 34P 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 < t1/2 < 2.5ns
• New results by Bender et al. give
=0 for mixing ratio but Chakrabarti
et al. measured significant E3
mixing
Motivation
• Recent study of 34P 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 < t1/2 < 2.5ns
• New results by Bender et al. give
=0 for mixing ratio but Chakrabarti
et al. measured significant E3
mixing
Motivation
• Theoretical predictions suggest 2+ state based primarily on
[2s1/2 x (n1d3/2)-1] configuration and 4- state based primarily on
[2s1/2 x n1f7/2] 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
1f7/2
1f7/2
20

20
n
1d3/2
1d3/2
2s1/2
2s1/2
1d5/2
1d5/2
I = 2+ [2s1/2 x (n1d3/2)-1]

n
I = 4- [2s1/2 x n1f7/2]
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
Results
Results
429
Total in-beam Ge spectrum
from LaBr3-Ge matrix
1876
Total in-beam LaBr3 spectrum
from LaBr3-Ge matrix
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-)
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
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.)
Ge-Gated Time differences
429
Total in-beam Ge spectrum
from LaBr3-Ge matrix
Total in-beam LaBr3 spectrum
from LaBr3-Ge matrix
Projection of LaBr3LaBr3 matrix gated by
1876 keV gamma in Ge
detectors
429
1048
Ungated LaBr3 Time difference
429-keV gate
1048-keV 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
The LaBr3-LaBr3
coincidences were
relatively clean where it
counts so try without
the Ge gate…
Results: T1/2 = 2.0(1)ns
429 / 1048
429 / 1876
(~prompt)
Results: T1/2 = 2.0(1)ns
429 / 1048
429 / 1876
(~prompt)
Results: Ge-gated Time Spectra
Results: Ge-gated Time Spectra
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
Discussion: Iπ = 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) = 0.0019(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 f7/2 → d3/2 singleparticle 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.
Discussion: M2 Strengths
• Experimental B(M2) and Mixing ratios from N=19 nuclei approaching the
island of inversion.
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
[1]
• SM calculations performed
with modified WBP
interaction [1].
• SM gives  = -0.023
disagreeing with the strong
E3 component suggested by
Chakrabarti et al.
Discussion: SM Calculations
Outline
• 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
N=80 Isotones
• N = 80 isotones above Z = 50 display
10+ seniority isomers from coupling of
(nh11/2)-2
• 6+ level weakly hindered in 136Ba,
(t(1/2) = 3.1(1)ns).
• Thought to be due to change in
configuration and seniority.
(nh11/2
only
)-2
Primarily
(g7/2)2
Primarily
(d5/2)2
isomer
10+
8+
6+
4+
2+
0+
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.
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”
S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)
“delayed”
“anticipated”
isomer
Will form part of thesis of T. Alharbi, University of Surrey
138Ce
– Lifetime of the 6+ State
• 0,2,4+ states thought to be based mainly on (nd5/2)-2
configuration. 6+ based on (ng7/2)-2.
• Change in configuration  hindrance (6+ state in
136Ba has t
1/2 = 3.1(1) ns.)
• Seniority may also play a role (6+ is maximum
coupling of (ng7/2)-2 hole pair).
preliminary
“anticipated”
HPGe gate
815keV gate
S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)
“anticipated”
HPGe gate
165keV gate
138Ce
– Lifetime of the 11+ State
Using “delayed”
HPGe gate
preliminary
S.-J. Zhu et al. Chin.Phys.Lett. 16, 635 (1999)
T1/2 ~ 170ps
138Ce
{815,165}
Lifetimes Summary
{77,390}
{418,403}
{815,467}
{254,338}
188W
– Lifetime of the 2+ State
T. Shizuma et al. Eur. Phys. J. A30, 391 (2006)
• 186W(7Li,p)188W, 33 MeV
• Reaction mechanism is a mix of
incomplete fusion and low-energy
transfer.
• ~54 hours beam time
296 keV gate (HPGe)
Contaminants
are 186Os
432 keV gate (HPGe)
189Ir
188W
– Lifetime of the 2+ State
Time difference 143-432 keV
Contaminated by
186Os [t (2+) = 875(15) ps]
1/2
• Estimate of 188W 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.
Summary and the Future
• LaBr3:Ce detectors have acceptable energy resolution and
excellent timing properties making them attractive for gammaray 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
Thank you