Beta-decay studies of neutron-rich Fe, Co
and Ni isotopes at LISOL using the
MINIBALL detector
Dieter Pauwels, Oleg V. Ivanov, Maria Sawicka,
Mark Huyse, Piet Van Duppen,
Thomas E. Cocolios, Yuri Kudryavtsev,
Johnny Gentens, Paul Van den Bergh,
Irina Stefanescu, Jarno Van de Walle,
and MINIBALL Collaboration
Katholieke Universiteit Leuven, Instituut voor Kern- & Stralingsfysica
84
0
pp3/2 np1/2
-1
68Ni
ng9/2
( np1/2 ) -2
nf5/2 -1
np1/2 -1
To probe N=40 subshell gap we
can look for levels interpreted as
5/21214
pf5/2
68Ni
1/2-  1110
pp1/2
particle(hole)
1007
5/2-
694
1/2-
0
n-1
pp
3/2-
n+10
68Cu
69Cu
70Cu
3/2
 n-1
9/2+
2553
67Ni
65Co
68Ni
69Ni
p+1
Z=28
66Co
67Co
68Co
p-1
65Fe
66Fe
67Fe
68Fe
?
?
?
 (n+1 p+1)
pg9/2 ng9/2
1+
1980
pf5/2 ng9/2 2pp3/2 np1/2 -1
( ng9/2) +2 1+
pp3/2 ng9/2 6-
369
68Ni
242
0
 n+1
np3/2 -1
( ng9/2 ) +2
3/2-
1400
nf5/2 -1
( ng9/2 ) +2
5/2-
915
np1/2 -1
( ng9/2 ) +2
ng9/2
1/29/2+
321
0
Almost nothing is known about nuclear structure of 65,66,67,68Co
J.Van Roosbroeck, Phys.Rev.C 69 2004
2+
1+
9/2+
68Ni
W.F. Mueller, Phys.Rev.Lett. 83, 1999
842
pg9/2
 (p+1)
J.Van Roosbroeck, Phys.Rev.C 69 2004
pp1/2 np1/2-1 1+
68Ni
N=40
 (n-1 p+1)
L. Weissman, Phys.Rev.C 59 1999
68Ni
S. Franchoo Ph.D. Thesis, Leuven 1999
Nuclear Structure around semi-doubly magic 68Ni
Experimental set-up
Leuven Isotope Separator-On-Line
Cyclotron
30 MeV
Proton Beam
Mass Separator
Buffer
Gas
Ion Guide
(SPIG)
Gas Cell
Laser-ionized
reaction products
XIA
DGF-4C
238U
Targets
(10 mg/cm2)
Data-acquisition
System
A+ + e-
IGOR
A*
b-g Detection Set-up
Lasers
A
Detection set-up
6-fold segmented
MINIBALL Clusters
Beam
3 Plastic DE b-detectors
Analysis of b-gated g events (data 2005)
67Fe
189 keV
67Fe
2054 keV
67Co
694 keV
67Fe
1251 keV
Lasers ON Fe
Lasers OFF
67Fe
2080 keV
67Fe
2087 keV
67Fe
decay scheme after bg analysis
T1/2=411(32) ms
Lasers on Co
b-
b-
L. Weissman et al.
PRC59 (1999) 2004
67Co
(5/2-)
1251
67Co
694
189
572
(9/2+)
313
2155
(5/2-)
2054
2080
2087
67Fe
T1/2=425(20) ms
(1/2-)
67Ni
Single g spectrum: welcome 491 keV! (data 2005)
Lasers ON Fe
Lasers OFF
694 keV
491 keV
Similar half-lives
No b coincidences
No g coincidences
COMPLETE ANSWER: b-g-g CORRELATIONS
Cycle structure LISOL and Correlations (data 2006)
67Co
Activity@tape
491
b-
?
694
67Fe
189
b-
67Ni
…
-200 0
…
Beam ON
200
…
ttrig
1 macro-cycle
t (ms)
Beam OFF
t
Result: single g’s before (-200 – 0 ms) a b-694
keV trigger
491 keV
511 keV
Correlated events
694 keV
Random subtracted Correlations
Control random
events!
Random events
Random substracted results (I)
491 keV events coming before b-694 keV 491 keV events coming after b-189 keV
491 keV in 67Co!
491 keV after b-189 keV!
T1/2: 329 (28) ms
T1/2: 483 (56) ms
?
67Ni
329 (28) ms
189
b-
67Co
(56) ms
499 (24) ms 483
512 (43) ms
b-
425 (20) ms
694
491
67Co
67Fe
694
189
67Fe
b491
b-
67Ni
PRC 59 2004
Random substracted results (II)
• Placing isomer
• T1/2 isomeric and ground state
from direct correlations
491
• Branching ratios (preliminary)
0.98 (5)
491
189
67Co
0.95 (15)
67Co
0.05 (15)
• Grand-daughter correlations!!!
Speculative interpretation
[440]1/2+ ??
411 (32) ms
b-
Fe41
50%
28
499 (24) ms
[303]7/2- 329 (28) ms
[321]1/2-
67
27
20
1g9/2
2p1/2
1f5/2
2p3/2
1f7/2
1d3/2
2s1/2
1d5/2
[321]1/2- ??
7/2- p-1
Co40
b-
9 +
2
5 2
1 2
694
50%
491 189
2054
2080
2087
67
26
50
67
28
Ni39
Conclusions and outlook
• 67Co level scheme fully analyzed
• Isomerism in 67Co
• Isomer @ 491 keV and g.s. 67Co fully characterized thanks to bgg correlations
 Placement, T1/2, Branching
• Correlations in seconds domain thanks to
 Pure beams from laser ionization
 Read-out on a event-on-event basis
 Segmentation of MINIBALL clusters: Multiplicity 1 491 keV g events
• need for SM calculations with 48Ca core:
 Allow for proton excitations across the Z=28 gap
 Interpretation of the isomer
• New beam time @ LISOL is granted for a 64-66Fe b-decay campaign in July 2007:
onset of deformation?
Limitations correlations
corr
Nexp
  acc .Nthcorr
rc
Aind
+ Abkg
Simulation
 5
tr
tr
  2 - 1 N true

Atrue

 I br c
tr
 tr 
 cT1/ 2 N
Random subtracted results
694 keV events coming before b-694 keV 491 keV events coming before b-694 keV
No correlation!
Correlation!
Randoms: strategy
Step 1: A dataset of single g, b and b-g histograms Hi in steps of 5 ms.
Step 2: Scan the trigger distribution in steps of 5 ms to know
the number of triggers Ntr.
Step 3: Build the random (b-)g’s from the dataset and normalize to Ntr.
0
ttrig
5 ms
200 tcorr (ms)
tcycle
H inorm 
Hi
trandom
tcorr
tcorr=Ntr*5 ms
trandom=Dtmeas/tcycle*5 ms
Neutron-induced background (data 2006)
Macro-cycle: 10s/0s/1 Micro-cycle: 100ms/100ms
Neutron-induced events during the proton beam ON!
Single g events
during and 15 ms
after proton beam
ON are rejected.
115 ms
85 ms
Result: e.g. single g’s before (-200 – 0 ms) or after
(0 - 200 ms) a b-694 keV trigger
67Fe
694 keV
b-
67Co
694
511 keV
BEFORE
189
491 keV
b-
Count-rate g’s: ~35 Hz
DT/histogram/trigger:85 ms
AFTER
# randoms/histogram: ~19000
491
# triggers:6353
67Ni
?
In 67Co!!
67Fe
situated on the chart of nuclides
Experiments: 1. Nov. 2005
50
50
1g9/2
40
2p1/
21f5/2
2p3/2
28
p
2. June 2006
40
28
1f7/2
n
Z
n-1
N=40
E
n+1
67Cu 68Cu 69Cu 70Cu 71Cu 72Cu
p+1
66Ni 67Ni 68Ni 69Ni 70Ni 71Ni
Z=28
65Co 66Co 67Co 68Co 69Co 70Co
p-1
64Fe 65Fe 66Fe 67Fe 68Fe 69Fe
N