microsecond isomers among fission products from inflight fission of 345 MeVnucleon 238 U Daisuke Kameda BigRIPS team RIKEN Nishina Center The 159 th RIBF Nuclear Physics Seminar RIKEN ID: 323285
Download Presentation The PPT/PDF document "Observation of 18 new" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
Observation of 18 new microsecond isomers among fission products from in-flight fission of 345 MeV/nucleon 238U
Daisuke KamedaBigRIPS team, RIKEN Nishina Center
The 159th RIBF Nuclear Physics SeminarRIKEN Nishina Center, February 26, 2013
Introduction
Experiment
Results and Discussion
SummarySlide2Slide3
IntroductionSlide4
Evolution of nuclear structures- between 78Ni and 132Sn-
Stable
New isotopes in RIBF 2008
Path of the r-process
Double closed-shells
(Spherical structure)
Double mid-shells
(Large d
eformation)
132
Sn
78
Ni
N
=60 sudden onset of
large deformation shape coexistence
Shape evolution shape coexistence
Shape transition
? where ? how ?Slide5
Large variety of
n
uclear isomers
Single-particle isomer
Spin gap due to high-
j
orbits such as
g
9/2
,
h
11/2
Small transition energy
Seniority isomer (
76
m
Ni,
78
m
Zn,
132
mCd,
130mSn)Spherical core (
g29/2)
I=8+ or (
h211/2)I
=10+High-spin isomer Coupling of high-j
orbits, g9/2 and
h11/2K isomer (99mY, 100mSr)Large static deformationShape isomer (98mSr, 100m
Zr, 98mY)Shape coexistence
P
aradise
for various
kinds of isomers
n
g
9/2
n
h
11
/2
p
g
9
/2Slide6
Search for new isomers at RIKEN
RIBF
in 2008D. Kameda et al., Phys. Rev. C 86, 054319 (2012)
Stable
New isotopes in RIBF 2008
Path of the r-process
Z~30
Z~40
Z~50
C
omprehensive search for new isomers
with
T
1/2
~ 0.1 – 10 us
over a wide range of neutron-rich exotic nuclei
Discovery of various
kinds of isomers is g
olden opportunity of study of
the evolution of nuclear structures
Experimental data
were recorded during the same runs as the search for new isotopes in Ref.
T. Ohnishi
et al
., J. Phys. Soc. Japan 79, 073201, (2010).Slide7
In-flight fission of U beamEffective reaction to produce wide-range
neutron-rich nucleiAbrasion fission
238U
9Be
Fission
fragment
Fission
fragment
Fissile nucleus
B
r
= 7.249 Tm
D
P/P = ±1 %
238
U(345 MeV/u)
+ Be
at RIBF
Coulomb
fission
238
U
Pb
Fission
fragment
Fission
fragment
photonSlide8
Large kinematical cone (Momentum, Angle)Superconducting in-flight RI beam separator “BigRIPS” at RIKEN RI Beam Factory
Large spread
345 MeV/u
Momentum
～
10%
Angle ~
100
mr
N
ew-generation fragment separator
with large ion-optical acceptances
Fission fragments
F
irst comprehensive search
using the
BigRIPS
in-flight separator with a U beam
at RIBF
c
ompared to the case of projectile fragmentsSlide9
ExperimentSlide10
BigRIPSSuperconducting in-flight separator
Superconducting14 STQ(superconducting quadrupole triplets) Large aperture f240 mm
Large ion-optical acceptancesMomentum 6 %, Angle Horizontal 80mr, Vertical 100 mrTwo-stage schemeSeparator-Spectrometer (Particle identification)Separator-Separator
Properties:
Dq
=
8
0
mr
Df
=
100
mr
D
p
/p =
6
%
B
r = 9 Tm
L = 78.2 m
1
st
stage
2
nd
stage
F1
～
F7
T. Kubo: NIMB204(2003)97.
D1
D2
D3
D4
D5
D6
BigRIPS
ZeroDegreeSlide11
Setting parameters Target material and thicknessMagnetic rigidityAchromatic energy degrader(s)Slit widths
ConditionsFull momentum acceptance (+/- 3%)Total rate < 1kcps (limit of detector system)
Good purity of new isotopesOptimization of BigRIPS setting
Z
N
B
r
R
ange
New
Known
R
ange
B
r
Slide12
Experimental settingsU intensity (ave.
)Target Br of D1 Degrader* at F1
Degrader* at F5 F1 slit F2 slit Central particleIrradiation timeTotal rate (ave.)0.25
pnABe 3 mm7.990 Tm2.2 mm(d/R=0.1)
none
± 64.2
mm
±15.5 mm
116
Mo
45.3 h
270
pps
0.22
pn
A
Pb
1 mm(+Al 0.3mm)
7.706m2.6 mm(d/R=0.166)1.8 mm
± 64.2 mm±15 mm140Sb27.0 h
870 pps
Setting 1 (Z~30)
Setting 2 (Z~40)Setting 3 (Z~50)
0.20
pnA
Be 5 mm
7.902 Tm
1.3 mm (d/R=0.04)
none± 64.2 mm±13.5 mm79Ni30.3 h530 pps
*Achromatic energy degrader
F1: wedge shape F5: curved profileTotal running time 4.3 days
(same as new-isotope search at RIBF in 2008)Slide13
Setup for particle identification (PID)
PPAC
B
r
with track reconstruction
TOF
b
Plastic
scintillation counter
D
E
MUSIC
g
-ray detector (next slide)
238
U
86+
345MeV/u
degrader
(degrader)
BeamDump
Target
TOF
-
B
r
-
D
E
method
Δ
E
: Energy loss,
TOF
: Time of flight
B
r
: Magnetic rigidity
ZeroDegree
Z
D
E
=
f
(
Z
,
b
)
A/Q
=
B
r
/
gb
m
m
: nucleon mass
b
=
v
/
c
,
g
=1/(1-
b
2
)
0.5Slide14
Setup for isomer measurement
Al stopper t30mm for Z~30 t10mm for Z~40,50 Area 90x90 mm
2
Energy absorber
（
Al)
t15 mm for Z~30
t10 mm for Z~40
t8 mm for Z~50
F11 Ion
chamber
RI beam
TOF
from target
600-700 ns
Absolute photo-peak efficiency :
e
g
=8.4%
(
122keV),
2.3
%(1.4MeV
)
t30mm stop.e
g=11.9%(122keV), 2.7%(1.4MeV)
t10mm stop.
Off-line measurement with standard sourcesMonte Carlo Simulation with GEANT3Good reproducibility of off-line efficiencies as well as relative g-ray intensities of known isomers: 78mZn,95m
Kr, 100mSr, 127mCd, 128mCd,
129mIn, 131mSn, 132mSn, 134m
SnClover-type high-purity
Ge
detectors
Energy resolution:
2.1keV(FWHM)@1
MeV
gSlide15
Particle-g slow correlation techniqueDynamic range of E
g: 50-4000 keV ADC(Ortec, AD413)
Timing of ion implantation (PL) :Highly-sensitive detection of microsecond isomers
(after slew correction)
T
g
(ns)
E
g
(
keV
)
P
rompt
g
-rays:
~29 % / implant
delayed
g-rays of
Tg > 200 ns low background condition
T
g
: Time interval between g-ray and ion implant.Eg
: g-ray energy
t
T
g
Maximum time window :
20 us
TDC
(
Lecroy
3377):
t
g
-ray signal
(each crystal):
t
crystal ID1Slide16
High resolution and accuracy of A/QA/Q resolution: 0.035 ~ 0.04 % (s
) Clear separation of charge states (Q=Z-1,…)(thanks to track
reconstruction with 1st and 2nd order transfer matrixes)A/Q accuracy: |(A/Q)exp－(A/Q)
calc|< 0.1 % Clear event assignment
Q=Z
108
Zr
39+
111
Zr
40+
A/Q
Counts
Zr
(Z=40)
Q=Z-1
Q=Z-2
Z’=Z+1
For example, 0.2
% difference
of
A/Q
between
111
Zr
40+
and
108
Zr
39
+
T. Ohnishi
et al
., J. Phys. Soc. Japan 79, 073201, Slide17
ResultsSlide18
With delayed
g
gateWith delayedg gate
PID
plots
without/with delayed
g
-ray events
Z~30
Z~40
Z~50
Z
Z
Z
Time window:0.2-1.0 us
Time window:0.2-1.0 us
Time window:0.2-1.0 us
Z~30
w/o delayed
g
gate
With delayed
g
gate
A/Q
A/Q
A/Q
Z~40
Z~50
A/Q
Z~40
γ
ゲートあり
A/Q
Z~50
γ
ゲートあり
T
1/2
= 1.582(22)
m
s
Ref. 1.4(2)
m
s
*
e
-t/
t
+ a
(maximum likelihood)
）
E
g
(
keV
)
Counts/
keV
*J.
Genevey
et al., PRC73, 037308 (2006).
w/o delayed
g
gate
w/o delayed
g
gateSlide19
18 new isomers observed
Energy spectra
Time spectraSlide20
A total of 54 microsecond isomers observed (T
1/2= 0.1-10 ms)
18 new isomers identified: 59mTi, 90mAs, 92m
Se, 93mSe, 94mBr,
95m
Br,
96m
Br,
97m
Rb,
108m
Nb,
109m
Mo,
117m
Ru,
119m
Ru,
120mRh,
122mRh,121m
Pd, 124mPd,
124mAg, 126m
Ag
A lot of spectroscopic informationg-ray energies
Half-lives of isomeric states
g-ray relative intensitiesgg
coincidence
R
unning time only 4.3
days!
M
ap of observed isomersSlide21
New level schemes for 12 new isomers: 59mTi, 94m
Br, 95mBr, 97mRb,
108mNb, 109mMo, 117mRu, 119m
Ru, 120mRh, 122m
Rh,
121m
Pd,
124m
Ag
N
ew level schemes for 3 known isomers
:
82m
Ga
,
92m
Br,
98m
Rb
R
evised level schemes for 2 known isomers: 108mZr
, 125m
Ag
17 proposed level schemes and isomerism
energy sum relation
gg coincidence g
-ray Relative intensityIntensity
balance with calculated total internal conversion coefficient
Correspondence of decay curves and half-livesMulti-polarities and Reduced transition probabilityRecommended upper limits (RUL)
analysisHindrance factor
Systematics in neighboring nuclei (if available
)Nordheim
rule
for spherical odd-odd nuclei
Theoretical
studies (if available
)Slide22
DiscussionSlide23
60
75
Discussion on the nature of nuclear isomerism
Large deformation and shape coexistence:
95m
Br,
97m
Rb,
98m
Rb
N
~
60 sudden onset of large deformation and shape coexistence
108m
Zr,
108m
Nb,
109m
Mo
N
~ 68 shape
evolution
117m
Ru
,
119mRu, 120m
Rh,
122m
Rh,
121m
Pd,
124m
Ag
N
~ 75 onset of new deformation
and shape coexistence
Evolution of
shell structure in spherical nuclei
59m
Ti
N
arrowing of
N
=
34
subshell-gap
82m
Ga
Lowering of
n
s
1/2
in
N
= 51
isotones
92m
Br
High-spin isomer
94m
Br,
125m
Ag
E
2 isomers with small transition energies
59
Ti
82
Ga
90m
As,
92m,93m
Se
,
92m
Br,
94m,95m,96m
Br,
97m
Rb,
98m
Rb
108m
Zr,
108m
Nb
,
109m
Nb,
109m
Mo,
112m,113m
Tc
117m,119m
Ru,
120m,122m
Rh
,
121m
Pd,
124m
Ag,
125m
Ag,
126m
AgSlide24
N
=34
59
Ti
B
(
E
2) = 3.68
+0.37
-0.34
W.u
.
E
2 isomer with small transition energy
59
m
Ti(Z=22,N=37): narrowing of the
N
=34 subshell gap
(ns)
(
keV
)
n
f
5/2
n
p
1/2
p
f
7
/2
59
m
Ti
28
n
p
3/2
34
n
f
7/2
n
f
5/2
n
p
-1
1
/2
Narrowing of
the
N
=34 subshell gap
59m
Ti
40
n
g
9
/2Slide25
N
=51 systematics of
nd5/2 and vs1/2O. Perru et al., EPJA28(2006)307.
Systematics of
p
f
5/2
(
81
Ga
g.s.
)
D.
Verney
Perru
et al.,
PRC76(2007)054312.
(p
f
5/2
n
d
5/2
)
I
p
=0-
(p
f
5/2
n
s
1/2
)
I
p
=2-
82
Ga(
Z
=31,
N
=51): Lowering of
n
s
1/2
orbit in
N
=51 isotones
82
Ga
E
2 isomer with small transition energy
Nordheim
rule
Odd-mass
N
=51 isotones
1031
532
462
260
1/2+
5/2
+
(1/2+)
(1/2+)
(1/2+)
(5/2
+)
(5/2
+)
(5/2
+)
Z = 38
36
34
32
b.g
.
0
0
0
0
30
?
n
s
1/2
n
d
5/2Slide26
60
50
97
Rb
95
Br
new
new
new
new
new
new
new
N
=60
N
=60
Energy spectra of new isomers in the
N
~60 region
N
=61
N
=59
N
=58
N
=57
N
=60 sudden onset of large
prolate
deformation
large
prolate
deformation
spherical
shape
What is the nuclear isomerism?
double
mid-shellsSlide27
60
Se
BrKrRbSr
YZr
As
97
Rb
95
Br
Spherical
Prolate
Shape isomer
Shape isomerism proposed
Shape isomer
Shape isomer
Prolate
Spherical
[431]3/2
+
Prolate
Spherical
Prolate
H
indered nature
H
indered
nature of 178-keV transition
H
indered
E1:
B
(E1)=9.37
+0.61
-0.56
x 10
-8
W.u
.
(RUL limits up to
M2
)
Spherical
98
Rb
E
1,
M
1,
E
2Slide28
96
Kr: S
.
Naimi et al., PRL105, 032502 (
2010) and M. Albers et al., PRL108, 062701 (2012)
0
698
331
215
0
0
102
Mo
100
Zr
98
Sr
0+
0+
0+
0
2
+
0
2
+
0
2+
96KrProlate-deformed 0+
Spherical 0
+
0
0+
Reversed
(our interpretation)
(
97
Rb)
?
96
Kr
(g.s
.,0
+
) :
not well deformed
599
77
0
0
99
39
Y
97
37
Rb
[422]5/2+
[431]3/2+
(5/2-)
95
35
Br
0
(Spherical)
(5/2-)
538
deformed
spherical
deformed
Evolution of shape coexistence in the
N
=60 even-even
nuclei
Evolution of shape coexistence in the
N
=60
odd-mass nuclei
This work
Reversed
This work
R.
Petry
et al., PRC31, 621 (1985
)
98
Sr,
100
Zr,
102
Mo (review paper) :
K
.
Heyde
et al., Rev. Mod. Phys. 83, 1501 (
2011)
spherical
deformedSlide29
Se
Br
KrRb
SrY
Zr
As
92
Br
Spherical
Prolate
92m
Br,
94m
Br
:
Isomers in spherical shell structure
94
Br
60
B
(
E
2
)= 2.5(3)
W.u
.
Spherical
E
2 isomer
(
p
g
9/2
n
g
7/2
)
8+
(
p
g
9/2
n
h
11/2
)
10
-
High-spin isomer
Analogy of known high-spin
isomers of
94m
Rb
Systematics of low-lying spherical
E
2 isomers of
N
=59 isotonesSlide30
Shape evolution around the double mid-shell region
- Variety of shapes:
prolate
, triaxial, oblate, tetrahedral -
Deformed
E
2
isomer
triaxial
triaxial
60
50
109
Mo
108
Nb
108
Zr
Deformed E2 isomer
or shaper
isomer
Prolate
P
rolate
or Oblate
Observed known
isomers
112m,113m
Tc
:
Triaxial
shape
A.M
. Bruce et al.,
PRC82
, 044311(2010
)
109m
Nb
: Oblate shape
H
. Watanabe et al.,
PLB696
,
186(2011)
108m
Zr: Tetrahedral
shape
T
.
Sumikama
et al., PRC82,
202501(2011)
K
-isomer
Prolate
Five isomeric
g
-rays at 174, 278, 347, 478, 604-keV were previously reported.Slide31
60
119
Ru
117
Ru
new
N
=75
N
=75
N
=75
new
new
new
new
new
Energy spectra of new isomers in the
N
~75 region
- Unexplored region so far -
N
=77
N
=77
N
=73
N
=78
N
=79
new
new
What happens here ?
What is the isomerism? Slide32
60
119
Ru
117
Ru
Our proposed level
schemes and isomerism
Shape isomer
Shape isomer
(Shape isomer)
(Shape isomer)
(Shape isomer)
(Shape isomer)
Hindered
nature of 185-keV transition
E1, M1
E
1,
M
1: hindered nature
E
2: not hindered value
We propose shape coexistence in a n
ew deformation region
E1, M1
Hindered
natureSlide33
Extended Thomas-Fermi plus
Strutinsky
Integral (ETFSI-Q) model J.M. Pearson et al., PLB 387, 455 (1996)
E
xperimental
systematics at
N
~60
S.
Naimi
et al., PRL105, 032502 (2010
)
N
=60
N
=75
N
=60
Theoretical indication of large deformation at
N
~75
- Mass systematics -
Well-known humps at
N
~60
sudden onset of
large static
deformation at
N
=60
5
0
55
Exp.
Cal.
U
nknown
onset of large static deformation at
N
~75, similarly to the case at
N
~60
onset of static oblate deformation?
Predicted humps at
N
~75 as well as
N
~60
6
5Slide34
60
125m
Ag(Z=47,N=78) : Spherical E2 isomer
new
new
new
B
(E2)=1.08(12)
W.u
.
75
Revised level scheme
670, 684, 715, 728-keV
g
-rays
were p
reviously reported
in
I.
Stefanescu
et al., Eur. Phys. J. A 42, 407 (2009).
Spherical structure appears at
N
=78
closeness of
132
SnSlide35
We performed a comprehensive search for new isomers among fission fragments from 345 MeV/u
238
U using the in-flight separator
We observed in total 54 isomeric decays including 18 new isomersThe present results allow systematic study of nuclear structuresN=34 region: Isomeric E2 decay in 59mTi due to the narrowing of the N
=34 subshell
N
=51 region: Isomeric E2 decay in
82m
Ga due to the shell evolution of
s
1/2
orbit
N
=60 region: Shape isomerism for
97m
Rb,
95mBr, 98mRb
N=68 region: K-isomerism for 108mZr, Isomeric transition between deformed states in different bands for 108m
Nb, 109mMo, (shape isomerism for 108mNb)N=75 region: Shape isomerism for
117mRu, 119mRu. The origin is shape coexistence in a new large deformation region at N~75
SummarySlide36
What’s next?Opportunity of detailed isomer spectroscopyMore efficient g-ray detector such as EURICALow-energy g
-ray detector (LEPS) Opportunity of systematic measurement of nuclear moments of isomeric statesTDPADSpin-controlled RI beam Opportunity of efficient isomer tagging in the RI-beam production
Thank you very much