Observation of 18 new microsecond isomers among fission products from in-flight fission

of 345 MeV/nucleon 238UDaisuke Kameda

BigRIPS team, RIKEN Nishina Center

The 159th RIBF Nuclear Physics SeminarRIKEN Nishina Center, February 26, 2013

1. Introduction2. Experiment3. Results and Discussion4. Summary

Introduction

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 deformation) 132Sn

78Ni

N=60 sudden onset of large deformation shape coexistence

Shape evolution shape coexistence

Shape transition ? where ? how ?

Large variety of nuclear isomers• Single-particle isomer

– Spin gap due to high-j orbits such as g9/2, h11/2

– Small transition energy• Seniority isomer (76mNi, 78mZn, 132mCd, 130mSn)

– Spherical core (g29/2)I=8+

or (h211/2)I=10+

• High-spin isomer – Coupling of high-j orbits, g9/2 and h11/2

• K isomer (99mY, 100mSr)– Large static deformation

• Shape isomer (98mSr, 100mZr, 98mY)– Shape coexistence

Paradise for various kinds of isomers

ng9/2

nh11/2

pg9/2

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

Comprehensive search for new isomers with T1/2 ~ 0.1 – 10 us over a wide range of neutron-rich exotic nuclei

Discovery of various kinds of isomers is golden 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).

In-flight fission of U beamEffective reaction to produce wide-range neutron-rich nuclei

Abrasion fission238U

9Be

Fission fragment

Fission fragment

Fissile nucleus Br = 7.249 Tm

DP/P = ±1 %

238U(345 MeV/u) + Be at RIBF

Coulomb fission238U

Pb

Fission fragment

Fission fragment

photon

Large kinematical cone (Momentum, Angle)

Superconducting in-flight RI beam separator “BigRIPS”

at RIKEN RI Beam Factory

U-beamLarge spread 345 MeV/u

Momentum ～ 10%Angle ~100 mr

New-generation fragment separator with large ion-optical acceptances

Fission fragments

First comprehensive search using the BigRIPS in-flight separator with a U beam at RIBF

compared to the case of projectile fragments

Experiment

BigRIPSSuperconducting in-flight separator

1. Superconducting14 STQ(superconducting quadrupole triplets) Large aperture f240 mm

2. Large ion-optical acceptancesMomentum 6 %, Angle Horizontal 80mr, Vertical 100 mr

3. Two-stage schemeSeparator-Spectrometer (Particle identification)Separator-Separator

Properties:Dq = 80 mrDf = 100 mrDp/p = 6 %Br = 9 TmL = 78.2 m

1st stage 2nd stage

F1～ F7

T. Kubo: NIMB204(2003)97.

D1

D2 D3D4 D5

D6

BigRIPS

ZeroDegree

Setting parameters • Target material and thickness• Magnetic rigidity• Achromatic energy degrader(s)• Slit widths

Conditions• Full momentum acceptance (+/- 3%)• Total rate < 1kcps (limit of detector system)• Good purity of new isotopes

Optimization of BigRIPS setting

ZN

Br

Rang

e

New

Known

Rang

e

Br

Experimental settings

U intensity (ave.)Target Br of D1 Degrader* at F1Degrader* at F5 　F1 slit F2 slit Central particleIrradiation timeTotal rate (ave.)

0.25 pnABe 3 mm7.990 Tm

2.2 mm(d/R=0.1)none

± 64.2 mm±15.5 mm

116Mo45.3 h

270 pps

0.22 pnAPb 1 mm(+Al 0.3mm)

7.706m2.6 mm(d/R=0.166)

1.8 mm± 64.2 mm

±15 mm140Sb

27.0 h870 pps

Setting 1 (Z~30) Setting 2 (Z~40) Setting 3 (Z~50)0.20 pnABe 5 mm7.902 Tm

1.3 mm (d/R=0.04)none

± 64.2 mm±13.5 mm

79Ni30.3 h

530 pps

*Achromatic energy degrader F1: wedge shape F5: curved profile

Total running time 4.3 days

(same as new-isotope search at RIBF in 2008)

Setup for particle identification (PID)

PPAC

Br with track reconstruction

TOF b Plastic scintillation

counter

DE

MUSIC g-ray

detector (next slide)

238U86+ 345MeV/u

degrader (degrader)

BeamDump

Target

TOF-Br-DE method ΔE: Energy loss, TOF: Time of flightBr: Magnetic rigidity

ZeroDegree

Z DE=f(Z,b)A/Q = Br /gbm

m: nucleon massb =v/c , g =1/(1-b2)0.5

Setup for isomer measurement

Al stopper t30mm for Z~30 t10mm for Z~40,50 Area 90x90 mm2

Energy absorber （ Al)• t15 mm for Z~30• t10 mm for Z~40• t8 mm for Z~50

F11 Ion chamber

RI beamTOF from target 600-700 ns

Absolute photo-peak efficiency :eg=8.4%(122keV), 2.3 %(1.4MeV) t30mm stop.eg=11.9%(122keV), 2.7%(1.4MeV) t10mm stop. Off-line measurement with

standard sources Monte Carlo Simulation with

GEANT3 Good reproducibility of off-

line efficiencies as well as relative g-ray intensities of known isomers: 78mZn,95mKr, 100mSr, 127mCd, 128mCd, 129mIn, 131mSn, 132mSn, 134mSn

Clover-type high-purity Ge detectors

Energy resolution: 2.1keV(FWHM)@1 MeVg

Particle-g slow correlation technique

Dynamic range of Eg: 50-4000 keV ADC(Ortec, AD413)

Timing of ion implantation (PL) :

Highly-sensitive detection of microsecond isomers

(after slew correction)

Tg (ns)

Eg (keV)

Prompt g-rays: ~29 % / implant

delayed g-rays of Tg > 200 ns low background condition

Tg : Time interval between g-ray and ion implant.Eg : g-ray energy

t

Tg

Maximum time window : 20 us

TDC (Lecroy 3377):

t

g-ray signal (each crystal):

t

crystal ID1

High resolution and accuracy of A/Q

• A/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

108Zr39+

111Zr40+

A/Q

Coun

ts

Zr (Z=40)

Q=Z-1

Q=Z-2Z’=Z+1

For example, 0.2% difference of A/Q between 111Zr40+ and 108Zr39+

T. Ohnishi et al., J. Phys. Soc. Japan 79, 073201,

Results

With delayedg gate

With delayedg gate

PID plots without/with delayed g-ray events

Z~30 Z~40 Z~50Z 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 delayedg gate

A/Q A/Q A/Q

Z~40 Z~50

A/Q

Z~40γゲートあり

A/Q

Z~50γゲートあり

T1/2= 1.582(22) ms Ref. 1.4(2) ms*

e-t/t + a (maximum likelihood)）

Eg (keV)

Coun

ts/k

eV

*J. Genevey et al., PRC73, 037308 (2006).

w/o delayed g gate

w/o delayed g gate

18 new isomers observedEnergy spectra Time spectra

A total of 54 microsecond isomers observed (T1/2= 0.1-10 ms) 18 new isomers identified: 59mTi, 90mAs, 92mSe, 93mSe, 94mBr, 95mBr, 96mBr, 97mRb,

108mNb,109mMo, 117mRu, 119mRu,120mRh, 122mRh,121mPd, 124mPd, 124mAg, 126mAg

A lot of spectroscopic information• g-ray energies• Half-lives of isomeric states • g-ray relative intensities• gg coincidence

Running time only 4.3 days!

Map of observed isomers

New level schemes for 12 new isomers: 59mTi, 94mBr, 95mBr, 97mRb, 108mNb, 109mMo, 117mRu, 119mRu, 120mRh, 122mRh, 121mPd, 124mAg

New level schemes for 3 known isomers: 82mGa, 92mBr, 98mRb Revised level schemes for 2 known isomers: 108mZr, 125mAg

17 proposed level schemes and isomerism

energy sum relation gg coincidence g-ray Relative intensity

Intensity balance with calculated total internal conversion coefficient Correspondence of decay curves and half-lives

Multi-polarities and Reduced transition probability Recommended upper limits (RUL) analysis Hindrance factor

Systematics in neighboring nuclei (if available) Nordheim rule for spherical odd-odd nuclei

Theoretical studies (if available)

Discussion

60

75

Discussion on the nature of nuclear isomerism

Large deformation and shape coexistence:• 95mBr, 97mRb, 98mRb N ~ 60 sudden onset of large

deformation and shape coexistence• 108mZr, 108mNb, 109mMo N ~ 68 shape evolution• 117mRu, 119mRu, 120mRh, 122mRh, 121mPd, 124mAg N ~ 75 onset of new deformation and shape coexistence

Evolution of shell structure in spherical nuclei • 59mTi Narrowing of N = 34 subshell-gap• 82mGa Lowering of ns1/2 in N = 51 isotones• 92mBr High-spin isomer• 94mBr, 125mAg E2 isomers with small transition energies

59Ti

82Ga90mAs, 92m,93mSe, 92mBr,

94m,95m,96mBr, 97mRb, 98mRb

108mZr, 108mNb, 109mNb,

109mMo, 112m,113mTc

117m,119mRu, 120m,122mRh, 121mPd, 124mAg,125mAg, 126mAg

N=34

59Ti

B(E2) = 3.68+0.37-0.34 W.u.

E2 isomer with small transition energy

59mTi(Z=22,N=37): narrowing of the N=34 subshell gap

(ns)

(keV)

nf5/2

np1/2

pf7/2

59mTi

28

np3/2

34

nf7/2

nf5/2

np-11/2

Narrowing of the N=34 subshell gap

59mTi

40ng9/2

N=51 systematics of nd5/2 and vs1/2O. Perru et al., EPJA28(2006)307.

Systematics of pf5/2 (81Gag.s.) D. Verney Perru et al., PRC76(2007)054312.

(pf5/2nd5/2)Ip=0-

(pf5/2ns1/2)Ip=2-

82Ga(Z=31,N=51): Lowering of ns1/2 orbit in N=51 isotones

82Ga

E2 isomer with small transition energy

Nordheim rule

Odd-mass N=51 isotones1031

532462

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?

ns1/2

nd5/2

60

50

97Rb95Br

new

new

new

new newnew

new

N=60

N=60

Energy spectra of new isomers in the N~60 region

N=61N=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-shells

60

SeBrKrRbSrYZr

As 97Rb95Br

Spherical ProlateShape isomer

Shape isomerism proposed

Shape isomer

Shape isomer

Prolate

Spherical

[431]3/2+

Prolate

Spherical

Prolate

Hindered nature

Hindered nature of 178-keV transition

Hindered E1: B(E1)=9.37+0.61

-0.56 x 10-8 W.u.

(RUL limits up to M2)

Spherical

98Rb

E1,M1,E2

96Kr: S. Naimi et al., PRL105, 032502 (2010) and M. Albers et al., PRL108, 062701 (2012)

0

698

331215

00

102Mo100Zr98Sr

0+0+0+

02+

02+02+

96KrProlate-deformed 0+

Spherical 0+

00+

Reversed (our interpretation)

(97Rb)

?

96Kr (g.s.,0+) : not well deformed

599

7700

9939Y97

37Rb

[422]5/2+[431]3/2+(5/2-)

9535Br

0

(Spherical)

(5/2-)

538deformed

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)

98Sr,100Zr, 102Mo (review paper) : K. Heyde et al., Rev. Mod. Phys. 83, 1501 (2011)

spherical

deformed

SeBrKrRbSrYZr

As

92Br

Spherical Prolate

92mBr, 94mBr: Isomers in spherical shell structure

94Br60

B(E2)= 2.5(3) W.u.

Spherical E2 isomer

(pg9/2ng7/2)8+

(pg9/2nh11/2)10-

High-spin isomer

Analogy of known high-spin isomers of 94mRb

Systematics of low-lying spherical E2 isomers of N=59 isotones

Shape evolution around the double mid-shell region- Variety of shapes: prolate, triaxial, oblate, tetrahedral -

Deformed E2 isomer

triaxial

triaxial

6050

109Mo

108Nb

108Zr

Deformed E2 isomer or shaper isomer

Prolate

Prolate or Oblate

Observed known isomers112m,113mTc: Triaxial shape A.M. Bruce et al., PRC82, 044311(2010)109mNb: Oblate shape H. Watanabe et al., PLB696, 186(2011)108mZr: 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.

60119Ru117Ru

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?

60119Ru117Ru

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

E1, M1: hindered natureE2: not hindered value

We propose shape coexistence in a new deformation region

E1, M1

Hindered nature

Extended Thomas-Fermi plus Strutinsky Integral (ETFSI-Q) model J.M. Pearson et al., PLB 387, 455 (1996)

Experimental systematics at N~60S. Naimi et al., PRL105, 032502 (2010)

N=60 N=75N=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

50 55

Exp.

Cal.

Unknown 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

65

60

125mAg(Z=47,N=78) : Spherical E2 isomer

new

new

new

B(E2)=1.08(12) W.u.75

Revised level scheme670, 684, 715, 728-keV g-rays were previously reported in I. Stefanescu et al., Eur. Phys. J. A 42, 407 (2009).

Spherical structure appears at N=78 closeness of 132Sn

• We performed a comprehensive search for new isomers among fission fragments from 345 MeV/u 238U using the in-flight separator

• We observed in total 54 isomeric decays including 18 new isomers

• The present results allow systematic study of nuclear structures– N=34 region: Isomeric E2 decay in 59mTi due to the narrowing of the N=34

subshell – N=51 region: Isomeric E2 decay in 82mGa due to the shell evolution of s1/2 orbit– N=60 region: Shape isomerism for 97mRb, 95mBr, 98mRb– N=68 region: K-isomerism for 108mZr, Isomeric transition between deformed

states in different bands for 108mNb, 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

Summary

What’s next?• Opportunity of detailed isomer spectroscopy

– More efficient g-ray detector such as EURICA– Low-energy g-ray detector (LEPS)

• Opportunity of systematic measurement of nuclear moments of isomeric states– TDPAD– Spin-controlled RI beam

• Opportunity of efficient isomer tagging in the RI-beam production

Thank you very much