Gamma Detection for ISOL Beams

Outline:

RIBS: Experimental challengesExamples

• Fusion-evaporation• RIB Coulomb Excitation• Single-nucleon transfer reactions

Conclusions for RIA detector requirements

David RadfordORNL Physics Division

RIA Workshop March 2003

Physics OpportunitiesReaccelerated RIBs at RIA will provide an incredible opportunity

for whole classes of experiments:

Fusion-evaporation -- for example (among many others):• band structure in n-rich nuclei• new K-isomers in Hf region, etc.

Coulomb excitation• B(E2) values, transition moments• static quadrupole moments by reorientation

Single-nucleon transfer reactions• e.g. (d,p), (3He,d) in inverse kinematics• spectroscopic factors, shell-model wavefunctions

Experimental Challenges

Beams are radioactive• Stopped/scattered beam can give huge background• Good beam quality & careful tuning essential

Beams are generally isobar cocktails, i.e. contaminatede.g. HRIBF A=132 beam: 87%Te, 12% Sb, 1% Sn⇒ Need good γ-ray energy resolution

Beams are weak (or the interesting part is)• γ, γγ rates of interest generally ≤ 1 /s• Background rate from stopped beam may be ≥ ~ 104 /s (108 × 10-3)⇒ Need best possible efficiency⇒ Need clean trigger and good timing (to reduce randoms)

Usually require light targets, inverse kinematics• Large recoil velocity, Doppler broadening⇒ Need excellent angular resolution

The Holifield Radioactive Ion Beam Facility

25 MV tandemelectrostaticaccelerator

RIBinjectorplatform

RIB target/ion source

RIB injectorelectronics ORIC

K=105 cyclotron

Target/ion sourceremote handlingsystem

Recoil massspectrometerOn-line isotope

separator

Spinspectrometer

Nuclearreactions

Isobarseparator

Daresburyrecoilseparator

Stable beaminjector

Oak Ridge National LaboratoryPhysics Division

Available Neutron-rich Radioactive Ion Beams(over 100 beams with intensities ≥103 ions/sec)

E/A = 3 MeV/amuE/A = 3 MeV/amu

Setup for experiments with neutron-rich RIBS

Target

Recoil Detectors

CLARION

HyBall

11 segmented clover Ge detectors

95 CsI detectors with photodiodes

Foil plus multichannel plate

50 cm downstream from targetand at RMS achromat

Fusion-Evaporation Reactions

100 300 500 700 900E (keV)

50

150

250

350

450

550

650

0

10

20

Cou

nts

per

Cha

nnel

0

10

20

0

10

20

12C(118Ag,5n)125I γγ gates

~106 118Ag /s for 33 hours

Projection

125I, 126I

381 keV gate

580 keV gate

891 keV gate31/2- 27/2-

TACγγ -Recoil

5/2

9/2

11/2

15/2

19/2

23/2

27/2

31/2

35/2

7/2

11/2

15/2

9/2

13/2

17/2

21/2

13/2

7/2

11/2

15/2

19/2

704

381

580

614

506

891

779

648

114

655

786

948

482

608

684

850

596

435

333

173

489

801

536168

637

698

841

125I

D. Balabanski et al.Private comm.122Sn(7Li, 4n)

Example: A = 136 Coulex

396 MeV 136Te + 136Ba

on 0.83 mg/cm2 C target

200 400 600 800 1000 1200 1400

60

140

UngatedDoppler corrected

200 400 600 800 1000 1200 1400

150

350

UngatedNot Doppler corrected

200 400 600 800 1000 1200 1400

6

14

22Te2+ 0+

Ba2+ 0+

Prompt carbon gateDoppler corrected

Particle ID Spectra:

C

protons

C

protons

Ring 1 7-14

oRing 328-44

o

Coulomb Excitation of Pure 128Sn RIB

Pure Beam

100 300 500 700 900 1100E (keV)

0

20

40

60

Mixed Beam (~15% Sn)

0

400

800

1200

1600

70 74 78 82 86Neutron Number

0.1

0.3

B(E

2; 0

+

2+)

(e2 b

2 )

Sn

Te

Xe

128Sn 2+

1169 keV

128Te

3 106 Pure 128Sn s-1

91.5% g.s., 8.5% 7- isomer

B(E2; 0+ 2+) = 0.073(5) e2b2

(preliminary!)

BaF2 array (150 crystals) for gammas(2x37, 4x19)

132Sn Coulomb Excitation Measurement- Jim Beene, Robert Varner et al.

• BaF2 single crystals, 6.5cm dia., 20cm long

• ∆E = 10% at 1 MeV• ∆Ω=0.65(4π)• Detection eff. = 60%• Total eff. = 40%

BaF2 array (150 crystals) for gammas(2x37, 4x19)

“CD” detector for scattered Sn and Ti

132Sn Coulomb Excitation Measurement- Jim Beene, Robert Varner et al.

132Sn γ-yields

470 MeV495 MeV

• BaF2• γ-particle Coincidence gate• γ-identification in BaF2• pack multiplicity • Array multiplicity• Calibrations approximate

• CD• Good strip• Good wedge E• Good wedge T

E(2+) E(2+)

134Sn at 400 MeV

• Success with 132Sn ->– Measured 134Sn rate of

2000/s (out of 10k/s total)• E(2+)=0.725, near 48Ti

E(2+)• Choose 90Zr target

E(2+)=2.186• Same detector array• Bragg counter

134Te134Sn134Ba

Our results in context

BaF2

Transfer Reactions with Neutron-Rich RIBS

Be

α

2α

200 600 1000 1400 1800

E (keV)

5

15

25

Cou

nts

per

chan

nel

424

(1/2

- 3

/2- )

p1/

2

657

(3/2

- 7

/2- )

p3/

2

1126

(5/2

- 7

/2- )

f 5/2?

1180

11/2

- 7

/2-

1400

(5/2

- 7

/2- )

f 5/2?

1830

9Be(134Te,8Be)135Te

~4 MeV per nucleon

2α

Gammas gated by 2α clusters

7/2

3/2

1/2(5/2 )

11/2

(5/2 )

657

424

11261180

1400

f7/2

p3/2

p1/2 f5/2?

πg7/2 2νf7/2

f5/2?

135Te

Neutron transfer with 134Te and 138Ba

200 600 1000 1400 1800E (keV)

010203040

0

4

8

12

0

400

800

1200

16000

1020304050

9Be(134Te, 8Be)135Te

13C(134Te, C)

9Be(138Ba, 8Be)139Ba

13C(138Ba, C)

424p1/2 p3/2

657p3/2

1126f5/2?

118011/2- 1400

2+

133Te

231i13/2 11/2-

455p1/2 p3/2

627p3/2

1283h9/2

1421f5/21308

11/2-

2+

137Ba

(d,p) Reactions - e.g. d(132Sn, p)133Sn

Inverse kinematics leads to large kinematic broadening of proton energy

Even with ~mm proton position resolution, beam spot gives large Ep spread

Energy loss of heavy beam in target also spreads proton energy

These effects lead to poor resolution in Q-value

Cannot resolve states that are closer than ~300 keV

70 80 90 100 110 120θlab (degrees)

−300

−250

−200

−150

−100

−50

0

dEp/d

θ (

keV

/deg

ree)

g.s.Ex = 0.5 MeV

70 80 90 100 110 120θlab (degrees)

0

4

8

12

Ep

(M

eV)

d(94Sr,p) at 4 MeV/u

g.s.Ex = 0.5 MeVEx = 1.0 MeVEx = 1.5 MeVEx = 2.0 MeV

New Idea: Use Gamma-ray Calorimetry

Have seen that gamma detection can be used to identify excited states

CLARION efficiency is too small for angular correlation measurements

10% energy resolution would be sufficient to resolve most levels

For moderate resolution but high efficiency, can select levels using the sum energy as well as individual cascades

Spin Spectrometer: a good gamma calorimeter for these experiments?

70 large NaI crystals

Energy resolution 10-12%, efficiency 85%

Plan to try using the Spin Spectrometer to select levels of interest, and measuring proton cross-sections and angular correlations in coincidence.

The ultimate tool will be GRETA

Conclusions -- Gamma detection for ISOL⇒ Gamma detection will be used in almost all experiments with

reaccelerated beams at RIA⇒ Need best possible efficiency, even at high energies (~ 4 MeV)

• The weakest beams will be the most interesting• Many experiments will require γγcoincidences (or higher)• May want to use array as a calorimeter and/or gamma-veto

⇒ Need clean trigger and good timing• Real-to-random ratio can be crucial• Gammas of interest may also be generated by beam decay

⇒ Need good energy resolution• Helps in bypassing problems from isobaric contamination• Improves signal-to-noise

⇒ Need good angular resolution• Avoids Doppler broadening at high recoil velocity

⇒ Need as much space as possible inside the array• Will need to be be used with a wide variety of auxiliary detectors

Conclusions -- Gamma detection for ISOL

⇒ GRETA: the ideal detector for ISOL experiments?• Excellent efficiency, ~ 50% for low multiplicity• Good efficiency even at high energies• Best possible energy and angular resolution• Good calorimeter

⇒ I predict that GRETA will be requested for the vast majority ofexperiments using reaccelerated (& fragmentation?) beams at RIA!

• RIB experiments will be expensive, beam time in great demand• Will always want as complete information as possible – e.g transfer⇒ Must be “portable”⇒ Must be kept as flexible as possible

→ inner space / diameter→ trigger and readout

Conclusions -- Gamma detection for ISOL⇒ Will also want a few other smaller systems for dedicated applications

• Close-packed clover array for decay studies etc.• BaF2 array or similar for very high energy gammas• Perhaps another ball with even better calorimetric properties

This page intentionally left blank

Lead B(E2) Systematics

208Pb B(E2) = 8.5 W.u.; 19% of Isoscalar E2 EWSR

204 206 208 210Mass Number

0.1

0.2

0.3

B(E

2; 0

+

2+)

(e2 b

2 )

2

4

6

8

10

W.u

.

500

1500

2500

3500

4500

E2+

(ke

V)

208Pb

Neutron Transfer: different targets

νp1/2

νp3/2

j = l - 1/2

j = l + 1/2

Te C

Are j = l - 1/2 j = l + 1/2 transitions preferred?

RIB Coulex Analysis

Measure =-HyBall coincidences

HyBall singlesC

R

Calculate for Coulex and Rutherford as a function of B(E2)dd

Integrate over the first three rings of HyBall

Compare calculated with observed to get B(E2)C

R

HH

Correct for isobaric content of the beam

- determined from Coulex of stable contaminants, decay counting and X-ray spectra

Beam Composition from Bragg-Curve Detector

Bragg detector delta-E spectrumA = 132 cocktail beam

Te

Sb

Sn

780 800 820 840 860

1500

3500

06-Oct-02 16:03:10 a132dect

Bragg Detector

Intensity and Purity of 132,133,134Sn Beams from SnS

A = 132

A = 133

A = 134

Range in Bragg Detector

101

102

103

104

105

Rel

ativ

e In

tens

ity

SnSbTe

B(E2) Results with QRPA Calculations (Terasaki et al.)

70 74 78 82 86 90Neutron Number

0.1

0.3

0.5

B(E

2; 0

+

2+)

(e2 b

2 )

Xe

Sn

Te

Ba

Ce

New Results QRPA

B(E2) Results with new 132Sn and QRPA Calculations

70 74 78 82 86 90Neutron Number

0.1

0.3

0.5

B(E

2; 0

+

2+)

(e2 b

2 )

Xe

Te

Sn

Ba

Ce

New Results QRPA

Coulomb Excitation of Pure 130Sn RIB

200 600 1000 1400E (keV)

0

4

8

12

Cou

nts

70 74 78 82 86Neutron Number

0.1

0.3

B(E

2; 0

+

2+)

(e2 b

2 )

Sn

Te

Xe

130Sn 2+

1221 keV

5 105 Pure 130Sn s-1

89% g.s., 11% 7- isomer

B(E2; 0+ 2+) = 0.023(4) e2b2

= 1.2 s.p.u.

C = 1 mb

132,134Sn Coulomb Excitation

• Collectivity of doubly magic 132Sn

• Systematics of collectivity in nearby nuclei - 134Sn

• Unique opportunity – pure Sn beams

(104-105/s)– BaF2 array (large ε)

• 132Sn + 48Ti 495 MeV• 132Sn + 48TI 470 MeV • 134Sn + 90Zr 400 MeV

132Sn Results (Preliminary)

Sample gamma-ray spectrum:

48Ti 2+→0+

132Sn 2+→0+

• 132Sn beam, doubly stripped

~96% pure

1.3 x 105 ions/s!

3.56 MeV/u

• 48Ti target

• High γ efficiency (~ 40%)

• Two-week experiment

• Fast γ–ion coincidences

to suppress background

B(E2; 0+→2+) ~ 0.14(6) e2b2

983 keV; 1.2 barns

4041 keV

Present Status of the Spin Spectrometer

Spin Spectrometer has not been used for about 10 years

New electronics and data acquisition system being assembled

Washington University (St. Louis) helping with support and electronics

HV has been applied to all 70 detectors; all detectors are giving output pulses

Next step: resolution and efficiency tests for all detectors

Tests of subset gave promising results; 8-14% resolution for 137Cs

Spectrometer will need to be moved to a new beam line

Present target room needed for second RIB platform

Some HRIBF beam currents

Singly stripped - suitable for Coulex For doubly stripped, divide by five

104

105

106

107

108

Ions

/s o

n ta

rget

113Ag

118AgN = 82126Sn

132Sn

127Sb

134Sb

132Te

136Te

Proton-induced fission of uraniumUC target

HRIBF will be the only facility that can accelerate thesebeams above the Coulomb barrier for at least 4 - 5 years.

Total of over 100 beams with at least 103 ions/s.

Coulex Results: Sn & Te Spectra

200 600 1000 1400E (keV)

200

600

1000

1400

50

150

250

350

450

200 600 1000 1400E (keV)

0

10

20

30

0

10

20

30

40

0

20

40

60

A = 136

A = 134

A = 132

A = 128

A = 126

126Sn

1141 keV

128Sn

1169 keV

132Te974 keV

134Te1279

keV

136Te606 keV

126Te666 keV

128Te743 keV

134Ba605 keV

136Ba819 keV

Note multiple beam components

Sn

Pb

C

Ge

Si

6

14

32

50

82

Carbon

Silicon

Germanium

Tin

Lead

Pure Ge and Sn Beams

• Neutron-rich RIBs are “cocktail” beams;Sn can be a small componentA=132 beam: 87%Te, 12% Sb, 1% Sn

• Solution: Extract from ion source as SnS+

and mass analyze for molecular ion• Sulfur is added to the UC target via H2S• Convert SnS+ to Sn- in a Cs-vapor cell

and mass analyze again for Sn mass• Selection process against Te, Sb is unknown

- TeS+ SbS+ unstable?• Similar purification for Ge beams