1

Electron-Capture Delayed Fission (ECDF) in the Pb region

Andrei Andreyev

ISOLDE workshop 18th November 2008

178Tl150 ms

178,180,182Tl

Z=82

200,202Fr

192,194,196At

186,188Bi

2

Collaboration

Stanislav AntalicAndrei Andreyev

Nick BreeThomas Cocolios

Jan DirikenJytte Elseviers

Mark Huyse Paul Van den Bergh

Piet Van DuppenMartin Venhart

Robert PageKatsuhisa Nishio

U. Koster (ILL, Grenoble, France)S. Franchoo (IPN, Orsay, France)S. Vermote, C. Wagemans (University of Gent, Belgium)M. Veselský (Slovak Academy of Sciences, Bratislava, Slovakia)I. Tsekhanovich (Manchester University, UK)

Jarno Van De WalleRILIS & ISOLDE

3

• Electron-Capture Delayed Fission– what it is and why?

• Earlier ECDF studies in the U and Pb regions

• ECDF of 192,194At at SHIP: cold fission of 192,194Po

• ECDF of 180Tl at ISOLDE: first ever fission study at ISOLDE?

• Plans

Outlook

4

Electron-Capture Delayed Fission (ECDF, T1/2(ff)=T1/2(EC))

Discovery: parent isotopes 232,234Am(1966, Dubna)

PECDF depends strongly on:

• QEC of the parent: the higher QEC, the larger the PECDF

• Bfis of the daughter: the lower Bfis, the larger the PECDF

• Actually, QEC-Bfis is important

deformation

A,Z

QEC

Bf

A,Z-1

•2 step process: EC decay of a parent (A,Z) nucleus populates an excited state in the (A,Z-1) daughter, which then might fission (in competition with the decay to the g.s.)•Low-energy fission! (E*~5-10 MeV)•12 cases know so far (neutron-def. Uranium region)

EC

ECDF

NEC

NECDF

EC-delayed branch

PECDF= NECDF

NEC

5

ECDF Probability: Feeding Part & Decay Part

NECDF

NEC

PECDF= =

(QEC-E)2 – Phase factor for EC decay

S(E) – -strength function (nuclear matrix element)

QEC

(QEC-E)2S(E) dEf(E)tot(E)

(QEC-E)2S(E) dE

0

0

QEC

-ratio of the fission and total widths of excited levels in daughtern is not important for neutron-deficient nuclei)

f

f+

f

tot

=

f= {1+exp[ ]}-12(Bf-E) hf

12

-inverted parabola approximation D.L. Hill and J.A.Wheeler

=

9.710-7 T4 exp(E/) 2

, – level density, T - temperature

Measurement of PEDCF allows to deduce Fission Barrier Bf

6

-delayed Fission and r-process Nucleosynthesis

Trans-Uranium elements?Cowan et al, Phys. Rep. 208 (1991) 267

End point of r-process?I.Panov et al, NPA747 (2005) 633

Where the fission process will stop the r-process?

r-process

r-process to the region with A>250

Fission cycling: Fission products can serve as seed for the r-process! I. Panov et al, NPAA747 (2005) 633

Fission AA/2 ~125

Fission: dfn-inducedsf

r-process Fission: df n-induced, sf

Need fission data for very heavy exotic nuclei!

7

Why ECDF studies?

•Theoretical models for Bf have been ‘tuned’ by using these data• Large discrepancies between different models for n-def. and n-rich nuclei

Available data on fission barriers, Z ≥ 80 (RIPL-2 library http://www-nds.iaea.org/ripl-2/ )

N/Z~1.55-1.59

•ECDF gives access to nuclei with very exotic N/Z ratios: e.g. N/Z(178Hg)=1.225•Unexpected properties, e.g. mass-distributions, cold fission…?

Region of our interest: A~180-200 N/Z~1.22-1.3

126

82

• Experimentally fission barrier are known only in the vicinity of the -stability line (e.g. N/Z(238U)=1.59)

• Must study isospin dependence of Bf values

Neutron Number

Bf [

Me

V]

Po

A. Mamdouh et al. NPA679 (2001), 337

LDM

TF

ETFSI

8

90 100 110 120 130 140 150Neutron Number N

Pro

ton

Num

ber

Z

208Pb80

90

100

Courtesy P. Möller

QEC values: P. Möller et al., ADNDT masses (1995), based on FRDM (1992)

Bf values: P. Möller et al., submitted to PRC (2008)

238U

Calculated Energy Window for EC-delayed Fission

9

ECDF in trans-U region

PD

F

• 12 known ECDF cases in trans-Uranium region (all odd-odd!)

• Relatively low QEC and Bf values (3-5 MeV)

PD

F

PD

F

180Tl ?180Tl ?

242Es (48)

248Es (4)

246Es (1)

QEC [MeV] QEC-Bf,shell [MeV]

PD

FWhy odd-odd EC-decaying parents?• Larger QEC in comparison with e-e and o-e neighbors (odd-even effect) • Even-even daughters are more fissile (specialization energy)

176 178 180 182 184 186 188 190 192456789

1011121314151617

Tl-Hg

Bf(Hg)

QEC(Tl)

10

ECDF possibleQEC > Bf

bEC>1%

QEC vs Bf values in Pb region (look for cases QEC > Bf)

Red lines: Thomas-Fermi Fission Barriers, W.D. Myers, W. Swiatecki Phys. Rev. C60 (1999) 014606

Black lines: QEC-values, P. Moller et al. At. Data and Nucl. Data table 59 (1995) 185

176 178 180 182 184 186 188 190 192456789

1011121314151617

184 186 188 190 192 194 196 198

4

6

8

10

12

14

16

18

20

186 188 190 192 194 196 198 200

3456789

101112131415161718

190 192 194 196 198 200 202 204 206

456789

10111213141516171819

192 194 196 198 200 202 204 206 20823456789

101112131415161718

200 202 204 206 208 210 212 214 216

0123456789

101112131415

206 208 210 212 214 216 218 220

2

4

6

8

10

12

14

198 200 202 204 206 208 210 212 214

2

4

6

8

10

12

14

16

18

180 182 184 186 188 190 192 1940

2

4

6

8

10

12

14

16

18

Tl-Hg

Po-Bi

E [

MeV

]

E [

MeV

]

E [

MeV

]

At-Po

Rn-At

Ra-Fr

Ac-Ra

Fr-Rn

Bf(Hg)

QEC(Tl)

Pb-Tl Bi-Pb

Mass Number

QEC(At)

Bf(Po)

QEC(Fr)

Bf(Rn)

Bf(Pb)

QEC(Bi)

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Velocity Filter SHIP (GSI, Darmstadt)

SHIP: Separation time: 1 – 2 μs

Transmission: 20 – 50 %

Background: 10 – 50 Hz

Det. E. resolution: 18 – 25 keV

Det. Pos. resolution: 150 μm

Dead time: 3 – 25 μs

31 cm

Rotating targets ~400-600 g/cm2

1 pA of 52Cr,58Ni~61012 pps

12

SHIP Detection System

A.N. Andreyev et al. ‘Conversion electron and decay spectroscopy at SHIP’, NIM A533, 416 (2004)

BOX of 6 Si detectors PSSD Ge Clover

, X-raysEsc. ff

Recoils

e

ff

Measure efficiently all possible decays:

• particle decay (, , protons, fission) E=0.1-250 MeV

• gamma decay E=10-4000 keV

• internal conversion electrons E=50-500 keV

•3 Time-Of-Flight detectors

•STOP detector – 16 position sensitive Si strips (35×80mm), pos. resolution FWHM=150 μm, energy resolution 14 keV

• 6 BOX Si detectors – for and escaping particles with a solid angle 80% of 2π

• GAMMA detectors – large-volume Clover detector for x rays or rays in coincidence with ’s

• VETO detector – reduces background

13

1

10

102

103

104

105

1

10

102

103

104

105

PSSD total

PSSD pause

EPSSD [MeV]0 45 90 135 180 225 270

56Fefull energy

Recoils

Fissions (66 events)

56Fe+141Pr194At+3nI(56Fe)~600 pnA

86103 nuclei66 fission events (pause)

Co

unt

s

First observed in the 52Cr+144Sm194At+pn reaction (SHIP,2006)16 fissions in pause

Identification based on:• Half-life T1/2,(194At)=280(20) ms T1/2,fiss(194At)=300(60) ms• Excitation functions• Cross-irradiation (56Fe,52Cr)• Double fold ff events (BOX)• Coincidences with gammas

Unambiguous identification of ECDF in 194At, SHIP(GSI)

A.Andreyev et al, paper in preparation

Scattered projectiles

On OFF5ms 15 ms

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.10.01

0.1

1

10

100

1000

Cro

ss-s

ec

tio

n [

nb

]

Beam energy [MeV/u]

195At()

194At ()

194At (ff)

193At ()

56Fe+141Pr197At*

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PECDF for 192,194At

192At

194At

-2 -1 0 1 2 3 4

1

10-1

10-2

10-3

10-6

10-4

10-5

LogPECDF=0.81475(QEC

-Bf)-4.2196

PE

CD

F

QEC

-Bf,Shells [MeV]

242Es

244Es246Es

248Es

250Md

232Am

234Am240Bk

238Bk

228Np

QEC and Bf values are from the TF model

Largest PECDF values ever obtained!

15

TKE(194At) ≈ 159 (7) MeV

(corrected for PHD)

Total Kinetic Energy in the EC-delayed fission of 194At

TKE [MeV]

TKE: Add up the energies of 2ff from the PSSD and BOX detectors

Co

unt

s

75 125 175 225 0

BOX of 6 Si detectors PSSD

ff1ff2

1200 1300 1400 1500 1600 1700

160

180

200

220

240

260Q-value (most probable mass split) TKE, Viola fit

194Po

RfNo

Md

CfFm

CmPuU

192Po

aver

age

or

mos

t p

roba

ble

TK

E [

MeV

]

Fissility Z2/A1/3

246Fm

252 No

Cm

252,256Cf

238U

246,250

236,240,244Pu

------ Q-value (most probable mass split)

Cold fission of 192,194Po!?

16

FIRST FISSION STUDY AT ISOLDE!?FIRST FISSION STUDY AT ISOLDE!?

December 1938: O. Hahn and F. Strassmann : discovery of nuclear fission

From 16th October 1967 on: ISOLDE operation

40 years later….June 2008: first ever fission study at ISOLDE!? : IS466 experiment

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IS466: ECDF ofIS466: ECDF of 180 180Tl isotope at ISOLDE (31 may-6 June Tl isotope at ISOLDE (31 may-6 June 2008)2008)

Annular Si Si

pure 30 keV Tl beam from RILIS+ISOLDE

Setup: Si detectors from both sides of the C-foil

• Simple setup & DAQ: 4 PIPS (1 of them – annular)• Large geometrical efficiency (up to 80%)• 2 fold fission fragment coincidences• ff-gamma coincidences• Digital electronics (5 DGF modules)

C-foils20 g/cm2 Si detectors

176 178 180 182 184 186 188 190 1924567

89

101112

1314

151617

Tl-Hg

Bf(Hg)

QEC(Tl)

Tl/Hg mass

E, MeV

30 keV beam from ISOLDE

SiAnnular Si

ff

ff

C-foil

MINIBALL Ge cluster

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IS466: EC and decay of 180Tl

5600 5800 6000 6200 6400 66000

10000

20000

30000

40000

50000

60000

180Tl

180Hg

176Pt

Alpha energy [keV]

Si detector

Co

unt

s

• Very clean spectra: only 180Tl and its decay products! (first on-line use of SSL)

180Hg

EC

180Tl

176Pt

176Au

176Au

• Negligible direct 180Hg production (bEC(180Tl))

100 times statistics than in previous works

• New detailed decay scheme of 180Tl

• Detailed data (~10 new lines in 176Au)

• Correct low-energy level scheme of 176Au

Due to negligible direct production of 180Hg

PECDF(180Tl)=

PECDF(180Tl)=5(1)10-5 (PECDF=310-(7±1) )

Nfission(180Hg) N(180Hg)bEC(180Hg)

ECDF

19

IS466: ECDF of 180TlBefore the IS466 experiment: How 180Hg (Z=80, N=100, N/Z=1.25) fissions?

SYMMETRICAL mass split in two semi-magic 90Zr(Z=40,N=50, N/Z=1.25)?

IS466: NO! ASYMMETRICAL energy (thus, mass) split!

Eff1-Eff2 coincidences ~330 events

30 40 50 60 70 80 90 100

3040

5060

7080

90100

0

5

10

15

20

25

Eff1 [MeV]

Eff2 [MeV]

Singles 1300 ff

From comparison of TKE and Q-values Cold fission of 180Hg!

20

Future ECDF studies in the Pb regionFuture ECDF studies in the Pb region

178Tl150 ms

178,180,182Tl

Z=82

200,202Fr

192,194,196At

186,188Bi

Identification of new ECDF nuclei and detailed studies (e.g. Bf, TKE…)

ISOLDE:•180Tl – Beta strength function measurements with TAS•180Tl – HFS scan with RILIS : search for 2 isomeric states • 180Tl – mass measurement at ISOLTRAP•178,182Tl – ECDF experiments at ISOLDESHIP:•186,188Bi •200,202Fr, 192At

21

Thank you!Thank you!

22

TKE(194At) ≈ 159 (7) MeV

(corrected for PHD)

Total Kinetic Energy in the EC-delayed fission of 194At

TKE [MeV]

TKE: Add up the energies of 2ff from the PSSD and BOX detectors

Co

unt

s

75 125 175 225 0

BOX of 6 Si detectors PSSD

ff1ff2

1200 1300 1400 1500 1600 1700

160

180

200

220

240

260Q-value (most probable mass split) TKE, Viola fit

194Po

RfNo

Md

CfFm

CmPuU

192Po

aver

age

or

mos

t p

roba

ble

TK

E [

MeV

]

Fissility Z2/A1/3

246Fm

252 No

Cm

252,256Cf

238U

246,250

236,240,244Pu

------ Q-value (most probable mass split)

252Cf(sf): Q[252Cf-(142Ba+110Mo)]=219 MeV <TKE>=185 MeV(Q-TKE)=34 MeV

194Po(ECDF): Q[194Po – (98Mo+96Mo)]=166 MeV <TKE>=157(7) MeV(Q-TKE)=9(7) MeV (+E* after EC)

192Po(ECDF): Q[192Po – (96Mo+96Mo)]=165 MeV <TKE>=170(10) MeV(Q-TKE)= - 5(10) MeV (+E* after EC)

Cold fission of 192,194Po? (no neutron emission during fission!)

23

-3 -2 -1 0 1 2 3 41E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

1

Tl ???

250Md

228Np

232Am

234Am

238Bk

240Bk

246Es

242Es

244Es

PD

F

QEC-Bf,shell [MeV]

248Es

ECDF probability in the Pb region

190 192 194 196 198 200 202 204 206

4

6

8

10

12

14

16

18 At-Po

198 200 202 204 206 208 210 212 214

2

4

6

8

10

12

14

16

18

Fr-Rn

Mass Number

E [

MeV

]

E

[M

eV]

Large QEC and QEC-Bf=3-4 MeV Possibility to reach nuclei with 10-100% ECDF probability!

180

24

Low-Energy ECDF data (232,234Am as examples)

H. L. Hall et al., PRC42 (1990) 1480• Fission barriers and their isospin dependence• Energy distributions of fission fragments, thus TKE• Mass distributions of fission fragments• Gamma multiplicities for fission fragments• Neutron multiplicities for fission fragments

K X rays in coincidence with ff

ALL THIS FOR VERY EXOTIC NUCLEI WHICH DO NOT FISSION SPONTANEOSLY!

25

QEC(At) and Bf(Po) in different models

190 192 194 196 198 2005

6

7

8

9

10

11

12

13

Q

EC o

r B

f [M

eV]

At/Po Mass Number

Qec_At_TF Qec_At_MN_FRDM Qec_At_MN_FRLDM Bf_Po_TF_FLDM Bf_Sierk_shells_FLDM Qec_DF Audi2003 Qec_AT_EFTSI

Bf

QEC

* exp QEC

26

-Delayed Fission (DF, T1/2(ff)=T1/2())

deformation

A,Z

Q

Bf

A,Z+1

•2 step process: decay of a parent (A,Z) nucleus populates an excited state in the (A,Z+1) daughter , which then might fission (in competition with the decay to the g.s. and/or neutron emission)

•Low-energy fission! (E*~5-10 MeV)

•Neutron-rich nuclei (only 5 candidates known so far)

ff DF

27

Example: Fission barriers – what do we know about them?

• Experimentally fission barriers Bf are known only in the vicinity of the beta stability line (e.g. N/Z(238U)=1.59)• Theoretical models for Bf have been ‘tuned’ by using these data Available data on fission barriers, Z ≥ 80 (RIPL-2 library http://www-nds.iaea.org/ripl-2/ )

N/Z~1.55-1.59

126

82

• For the r-process calculations we need fission data far away from stability: e.g. 260Po or 270U (N/Z>2!) – they might not be accessible in the Lab at all! – Use calculations?

N/Z>2

-2.2

28

Fission Barrier Calculations for the r-process nuclei

Neutron Number Neutron Number

Bf [

Me

V]

Full symbols – experimental data Lines – calculations (LDM,TF, ETFSI)

Po U

A. Mamdouh et al. NPA679 (2001), 337

LDM

TF

ETFSI

LDM

TF

ETFSI

• Good agreement between Bf,cal and Bf,exp for nuclei close to stability• Large disagreement far of stability (both on n-def. and n-rich sides)• Need measured fission data far of stability to ‘tune’ fission models

•Unfortunately, so exotic nuclei are not presently accessible by available techniques!

•That is why the underlying mechanisms and properties of beta-delayed fission (and of low-energy fission in general) have to be investigated by using alternative approaches and in other regions of the Nuclear Chart.

•According to semi-empirical estimates, the neutron-deficient nuclei in the U and Pb regions provide such a possibility via the ECDF decay

29

Fission Barrier Calculations for the r-process nuclei

Neutron Number

Full symbols – experimental data Lines – calculations (LDM,TF, ETFSI)

U

LDM

TF

ETFSI

• Good agreement between Bf,cal and Bf,exp for nuclei close to stability• Large disagreement far of stability (both on n-def. and n-rich sides)• Need measured fission data far of stability to ‘tune’ fission models

Bf [

Me

V]

Po

A. Mamdouh et al. NPA679 (2001), 337

LDM

TF

ETFSI

30

(Speculations?) Double-Magic Fission?

P. Moller (private communication): Most probably – NOT as a main channel• Most probable mass splits ~90/~90 (90Zr/90Zr N/Z=1.25)

• but what about very asymmetrical split at the wings? :

Schmidt et al., NPA 665 (2000) 221

How its daughter 178Hg(N/Z=1.225) would fission? Would it give ‘double-magic’ fission – two double-magic ff’s? very neutron-rich 78Ni (Z=28,N=50 N/Z=1.79) very neutron-deficient 100Sn (Z=50,N=50 N/Z=1)

One of the goals of ISOLDE proposal is ECDF of 178Tl

31

ECDF Probability: Fission/gamma competition

Measurement of PEDCF allows to deduce Fission Barrier Bf

e.g . H.V. Klapdor et al., Z.Phys.A292, 1979,249; D. Habs et. al. Z.Phys. A285 (1978), 53

-ratio of the fission and total widths of excited levels in daughtern is not important for neutron-deficient nuclei)

f

f+

f

tot

=

f= {1+exp[ ]}-12(Bf-E) hf

12

-inverted parabola approximation D.L. Hill and J.A.Wheeler

= 9.710-7 T4 exp(E/) 2

, – level density, T - temperature

QEC

PECDF=

(QEC-E)2S(E) dEf(E,Bf)tot(E)

(QEC-E)2S(E) dE

0

0

QEC

NECDF

NEC

~

32

Previous Studies in the Pb region (Dubna)Yu. A. Lazarev et al. Europhys. Lett. 4 (1987) 893; and Inst. Phys. Conf. Ser. No132 (1992) 739

“Most probable” candidates: 180Tl (PECDF=310-(7±1)), 188Bi,196At (no PECDF data) • Irradiations inside the cyclotron (no A,Z selection for products) • Rotating wheel system, thick effective targets (2 mg/cm2)• Cross-irradiations, apparent fis~15-50 pb• Mica detectors (fission tracks only)• Never confirmed (not in the Tables) • No continuation with these studies so far

PD

F

180Tl ?180Tl ?

QEC [MeV] QEC-Bf,shell [MeV]P

DF

176 178 180 182 184 186 188 190 192

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Tl-Hg

Bf(Hg)

QEC(Tl)

33

An example: Fission Barrier of 232Pu D. Habs et. al. Z.Phys. A285 (1978), 53

QEC

PECDF~(QEC-E)2S(E) dEf(E)

tot(E)

(QEC-E)2S(E) dE

0

0

QEC

f= {1+exp[ ]}-12(Bf-E) hf

12

Measured PECDf (232Am) =(1.3 )10-2 Bfis(232Pu)=5.3(4) MeV Assuming uncertainty of: QEC=±200 keV hf =±100 keV a factor of 3 in PECDF

+4

-0.8

Bf precision of ~7.5% - well comparable to direct methods!

34

Experimental information on fission - Low Experimental information on fission - Low energyenergy

- particle induced x - e.m. –induced E*~11 MeV

Region of ourinterest: A~186-200 N/Z~1.26-1.3

N/Z~1.55-1.59

35

Electromagnetically-Induced Fission In-flight (FRS, GSI)

A. Grewe et al. NPA614 (1997), 400

Sierk (FRLDM)

-EM fission

• Primary beam 238U at 1 AGeV• 1 g/cm2 Cu primary target • Separated RIBs from FRS• Pb secondary target• (El.fission)~2.1 b (234U)

Pashkevich

ActiveTarget

Fission Fragments

Identified SecondaryBeam E~620 AMeV

- particle induced+SF x -e.m. -induced

Fission from E*~12 MeV (E1 GDR)

For comparison: Bf(234U) ~ 6 MeV Bf(220Th) ~ 7.5 MeV Bf(218Ac) ~ 7.5 MeV

Thus, still fission from quite above the barrier!

In contrast, df is near (or sub)-barrier effect!

36

246Fm

252 No

Cm

252,256Cf

238U

246,250

236,240,244Pu

1200 1300 1400 1500 1600 1700

160

180

200

220

240

260Q-value (most probable mass split) TKE, Viola fit

194Po

RfNo

Md

CfFm

CmPuU

192Po

aver

age

or

mos

t p

roba

ble

TK

E [

MeV

]

Fissility Z2/A1/3