Clean Beams at ISOL Clean Beams at ISOL FacilitiesFacilities

GSI Workshop on Astrophysics and Nuclear Structure, January 15-21, 2006 in Hirschegg, Austria

O.Arndt, H. Frånberg, C.Jost, K.-L. Kratz, U. Köster

altogether 52 r-process nuclei have been measured (at least) via their T1/2, which lie in the process path at freeze-out (nn1020 cm-3).

These r-process isotopes range from 68Fe to 139Sb.

Where, Why, What - MotivationsWhere, Why, What - Motivations

The large majority of these exotic nuclei was identified at ISOL facilities,in particular at CERN/ISOLDE.

R-abundance peaks and neutron-shell numbers

already B²FH (Revs. Mod. Phys. 29; 1957) C.D. Coryell (J. Chem. Educ. 38; 1961)

...still today important r-process properties to be studied experimentallyand theoretically.

K.-L. Kratz (Revs. Mod. Astr. 1; 1988)

climb up the N= 82 ladder ...A 130 “bottle neck“

“climb up the staircase“ at N=82;major waiting point nuclei;“break-through pair“ 131In, 133In;

“association with the rising side of majorpeaks in the abundance curve“

132Sn50

131In8249

133In8449

129Ag8247

128Pd8246

127Rh8245

126

127

128

129

130

131

132

133

Pn~85%

165ms278m

s

46ms(g)

r-processpath

(n,)

(n,)

(n,)135 136 137

134 135

131 132 133

130

134

158ms(m)

130Cd8248

162ms

Where, Why, What - MotivationsWhere, Why, What - Motivations

What we knew already in 1986 ...

K.-L. Kratz et al (Z. Physik A325; 1986)

Exp. at old SC-ISOLDEwith plasma ion-sourceand dn counting

Problems:high background from

-surface ionized 130In, 130Cs-molecular ions [40Ca90Br]+

Request: SELECTIVITY !

Shell-model (QRPA; Nilsson/BCS) prediction

1.0

T1/2(GT) = 0.3 s

4.11+

2.0

g7/2, g9/2

Q = 8.0 MeV

1+

1+

1+

1+

1+

1+

1+

0

1.0

3.0

4.0

5.0

6.0

1-

IKM

z –

15

5R

(19

86

)

T1/2 = 230 ms

T1/2 = (195 ± 35) ms

Ag Cd In CsSn Sb Te I Xe

at an ISOL facility • Fast UCx target• Neutron converter• Laser ion-source• Hyperfine splitting• Isobar separation• Repeller• Chemical separation• Multi-coincidence setup

Request: Request: Selectivity !Selectivity !

50 800 >105

the Ag “needle” in the Cs “haystack”

Why ?

How?

Request: Request: Selectivity !Selectivity !

Proton-beam on neutron converter ↷ only fission, avoids p-rich isobars

UCx target and neutron converter

A. Nolen et al.

HRS design≥ 1/104

Request: Request: Selectivity !Selectivity !Isobar separation

in reality, „on a good day…“M/M ≈ 1/4000

Mass scan at HRS (ISOLDE) in 2002;efficiency corrected

In

Cs Cd

Cd 2.000

In 17.000

Request: Request: Selectivity !Selectivity !Laser ion-source (RILIS)

Chemically selective,three-step laser ionizationof Ag into continuum

130Cd1669 keV

130Cd 1732 keV

Laser ON

Laser OFF

130Sb1749 keV

Energy [keV]

-singles spectrum

Laser ON

Laser OFF

Comparison of Laser ON to Laser OFF spectra

Properties of the laser system:Efficiency ≈ 10%Selectivity ≈ 103

Request: Request: Selectivity !Selectivity !Molecular sidebands

Separation on higher mass ↷ Suppression of isobaric background

SSn SSn 3232

Chemical yield ~ 30%

• RILIS with standard MK3 target unit used

• chemical treatment is performed by adding 32S to the target

• other possibilities of chemical treatments for further elements have to be investigated.

Request: Request: Selectivity !Selectivity !Molecular sidebands

Separation on higher mass

100

101

102

103

131mSn

131Sb933.1 und 943.4 keV

304.3 keV

450.0 keV

1226.0 keV131Sn798.5 keV

20 ms collection time per p-pulse

↷ Suppression of isobaric background

in the transfer line between target and ion-source ↷ thermochromatography

0

10

20

30

40

50

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53

distance / cm

acti

vit

y / %

Zn Rb Ag In Cd Cs

Request: Request: Selectivity !Selectivity !

here: deposition of Zn, Rb, Ag, In, Cd and Cs a quartz tube with a temperature gradient

↷ separation Cd, from Cs, In

Prototype UCx target at CERN/ISOLDE with

temperature-controlledquartz transfer-line

was tested in Oct. 2005

Diploma thesis C. Jost (2005)

Surface chemistry

Request: Request: Selectivity !Selectivity !Surface chemistry

Thermochromatographytarget set-up at ISOLDE

Request: Request: Selectivity !Selectivity !LASER-off gamma-spectrum of mass 131

3 C protons onto converter, 12 seconds collection,quartz transfer line at 600°C

Total suppression of all surface-ionized species (in particular 131In)!

All gamma lines are due to background on the tape!

U. Koester, H. FrånbergC. Jost, O.Arndt

Surface chemistry

laser ions

surface ions

Comparison of

spatial

beam profiles

28 Volt repeller:

selection between

laser ions (laser & repeller on)

and

surface ions (laser &

repeller off)K. Wendt et al.

Request: Request: Selectivity !Selectivity !

“Skimmer” with negative voltage retains positive, surface-ionized species↷

Repeller

middle ring(20 3He counters)

paraffine matrixwith Cd shielding

outer ring(22 3He counters)

inner ring(22 3He counters)

central hole for tape stationand - or - detector

Request: Request: Selectivity !Selectivity !Detector

Selectivity through -delayed neutron counting

-n multifold coincidences are the future detection systems for extremeneutron-rich nuclei.

Proton-rich isobars are excluded by the detector itself.

ConclusionConclusion

• High resolution mass separation combined with laser ion sources is not sufficient to produce isobaric clean beam for nuclear astrophysics. Additional selectivity is needed for further progress in r-process and nuclear structure investigations far from stability.

• Repeller can clean RIBs from unwanted surface-ionized species.

• Molecular sidebands are powerful tools to clean up RIBs in the 132Sn region. But molecular sidebands must be investigated separately for each element.

• Thermo chromatography in the transfer line is a good possibility to produce pure beams. But still most absorption enthalpies on different surfaces are unknown.

• Further investigations on chemical treatments of ion beams are urgently needed.

• Additional selectivity can be reached by “intelligent” detector systems like multifold coincidence set-ups.

127Ag

p1/2

g9/2

T1/2(m)=(15860) ms

T1/2(g)=(46 ) ms-9+5

129mAg 82g9/2p1/2129gAg 82

Request: Request: Selectivity !Selectivity !Hyperfine splitting

Separation of isomersby fine-tuning of laser frequency