Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 1/29

Particle detectorsfor low-energy Coulomb excitationRIB vs stable beams requirements

Magda ZielinskaIRFU/SPhN, CEA Saclay, France

• Coulomb excitation: Why do we need particle detectors?

• Coulex detectors: Stable beams: detectors in backward angles Stable and exotic beams: Detectors in forward angles – identification

problems

• What next? Future challenges What else has to be measured when we have our Coulex data?

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 2/29

Why do we like Coulomb excitation?

• it’s a very precise tool to measure the collectivity of nuclear excitations andin particular nuclear shapes• shape = fundamental property of a nucleus, "condensed" informationabout its structure• excitation mechanism purely electromagnetic, the only nuclear propertiesinvolved: matrix elements of electromagnetic multipole operators• nuclear structure information extracted in a model-independent way

238U

∆t=2 10 s.

-22

40Ar

~20 fm

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 3/29

Why do we like Coulomb excitation?

• studies no longer limited to stable or long-lived nuclei

• beam energies at exotic beam facilities perfect for Coulomb excitation (2-5MeV/A)

• high cross sections (excitation of 2+

1: barns)

• practical at the neutron-rich side

• direct measurement of nuclear shape (quadrupole moment including sign)– ideal tool to study shape coexistence

• B(E2) as a measure of collectivity - studies around magic numbers

• easy way to access non-yrast states and study their properties

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 4/29

Basic facts about Coulex

• Due to the purely electromagnetic interaction the nucleus undergoes atransition from state |i〉 to |f〉.

• Then it decays to the lower state, emitting a γ-ray (or a conversionelectron).

• The matrix elements 〈f ||M(E2)||i〉 describe the excitation and decaypattern → they are directly connected with γ-ray intensities observed in theexperiment.

• In the intrinsic frame of the nucleus they are related to the deformationparameters.

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 5/29

Basic facts about Coulex experiments

• Due to the purely electromagnetic interaction the nucleus undergoes atransition from state |i〉 to |f〉.

→ Cline’s "safe energy" criterion – if the distance between nuclear surfacesis greater than 5 fm, the nuclear interaction is negligible.

• Then it decays to the lower state, emitting a γ-ray (or a conversionelectron).

• The matrix elements 〈f ||M(E2)||i〉 describe the excitation and decaypattern → they are directly connected with γ-ray intensities observed in theexperiment.

• In the intrinsic frame of the nucleus they are related to the deformationparameters.

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 6/29

Basic facts about Coulex experiments

• Due to the purely electromagnetic interaction the nucleus undergoes atransition from state |i〉 to |f〉.

→ Cline’s "safe energy" criterion – if the distance between nuclear surfacesis greater than 5 fm, the nuclear interaction is negligible.

→ to properly describe the excitation process - particle detectors needed

• Then it decays to the lower state, emitting a γ-ray (or a conversionelectron).

• The matrix elements 〈f ||M(E2)||i〉 describe the excitation and decaypattern → they are directly connected with γ-ray intensities observed in theexperiment.

• In the intrinsic frame of the nucleus they are related to the deformationparameters.

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 7/29

Basic facts about Coulex experiments

• Due to the purely electromagnetic interaction the nucleus undergoes atransition from state |i〉 to |f〉.

→ Cline’s "safe energy" criterion – if the distance between nuclear surfacesis greater than 5 fm, the nuclear interaction is negligible.

→ to properly describe the excitation process - particle detectors needed

• Then it decays to the lower state, emitting a γ-ray (or a conversionelectron).→ gamma detectors needed

• The matrix elements 〈f ||M(E2)||i〉 describe the excitation and decaypattern → they are directly connected with γ-ray intensities observed in theexperiment.

• In the intrinsic frame of the nucleus they are related to the deformationparameters.

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 8/29

Why are particle detectors useful in Coulex experiments?

• to select Coulomb excitation events (intermediate-energy Coulex, exoticbeam experiments, experiments with oxide targets...)

• to describe the excitation process → information on θ scattering angleneeded

• to perform a Doppler correction of gamma rays → information on θ and φ

necessary• to measure particle-gamma angular correlations → information on φ

generally more useful

0 400 800 1200Eγ (keV)

1

10

100

1000

coun

ts /

kev

A

4 1+ → 2

1+21

+→ 0

1

+

6 1+ → 4

1+

2 2+ → 2

1+

0 2+ → 2

1+

2 2+ → 0

1+

8 1+ → 6

1+

4 2+ → 2

2+

2 3+ → 0

2+

4 2+ → 4

1+

400 800 1200Eγ (keV)

B

400 800 1200Eγ (keV)

C

400 800 1200Eγ (keV)

D

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 9/29

Reorientation effect

• influence of the quadrupole moment of the excited state on its excitationcross-section

• dependence on scattering angle and beam energy

• BE CAREFUL – influence of double-step excitation of higher states mayhave the same effect!

LAB scattering angle [deg.]0 20 40 60 80 100 120 140

[b/s

r]Ω

/ d

σ d+

2

0

0.01

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0.03

<0prolate, ME

=0spherical, ME

>0oblate, ME

0.02

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0.045

0.05

110 115 120 125 130 135 140 145 150 155

Rel

ativ

e po

pula

tion

of th

e 0

sta

te

+ 2

laboratory theta angle

diagonal matrix element-0.9 eb0.0 eb

+0.9 eb

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 10/29

Gamma-particle angular correlations

• feasible at several thousands of counts in a given gamma line• determination of E2/M1 mixing ratios• determination of spin of a decaying level• distribution in phi usually more conclusive than in theta

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 11/29

Stable beam experiments

• usually multi-step excitation and complicated level schemes, search forsubtle effects• beam intensities of the order of pnA → 1010pps: particle detectors usuallyat backward angles• lifetimes of several states known: no need for other kind of normalisation• statistics enough for particle-gamma angular correlations

178Hf

A. Hayes et al., PRC 75 (2007) 034308

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 12/29

Exotic beam experiments

• usually one- or two-step excitation; level schemes not well known on theneutron-rich side

• beam intensities rather low: particle detectors at forward angles tomaximise the statistics

• normalisation to target excitation or Rutherford scattering needed

• low statistics, sometimes only one gamma line observed

• differential measurements at the limits of feasibility

• high background from β decay→ experiments without particle detection impossible

Energy [keV]0 500 1000 1500 2000 2500

Cou

nts

1

10

210

310

K44

Ca

44 K44

K44 Ca

44

1+ 0→ 1

+2

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 13/29

Simplest Coulex detector: no detector at all

Doppler correction impossible; how can we manage?

• traditional "thick target" measurements→ lifetimes should be long compared to stopping time

• strongly assymetric inverse kinematics, everything goes forward→ favours one-step excitation - suitable for example to search for mixed

symmetry 2+ states

128Xe on 12C

L. Coquard et al., PRC 80 (2009) 061304

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 14/29

Simplest Coulex detector: no detector at all

• possibility of collecting gamma singles in a particle-γ coincidencemeasurement:

independent data set (different ranges of incident energy and scatteringangles)

can help to disentangle various excitation patterns!

32S on 100MoK. Hadynska-Klek et al., to be published

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 15/29

"Standard" stable beam Coulex: detectors at backward angle s

• only scattered beam particles detected – in principle no need to know theirenergy (although it may help – makes possible to make cuts on incident

energy)• very compact geometry possible (chambers of 5 cm radius)• detectors used: Si (segmented/PIN diodes), plastic, solar cells, MCP,...

Munich Chamber, HIL Warsaw LUNA, JAEA Tokai

K. Wrzosek et al., Acta Phys. Pol. B39 (2008) 513 Y. Toh et al., Rev. Sci. Inst. 73 (2002)

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 16/29

"Standard" exotic beam Coulex: detectors at forward angles

• simultaneous detection of scattered projectiles and recoils• unambiguous identification necessary for excitation process description &

Doppler correction• detectors used: PPAC (stable beams), segmented Si / CsI(Tl) (exotic

beams)

CD detector

EXOGAM

Ar44

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 17/29

Identification ejectile-recoil: time

• CHICO: 4 π PPAC array designed for GAMMASPHERE• chamber diameter 36 cm (distance target-detector 15 cm)• timing resolution 500 ps• for 136Xe + 178Hf Coulex: 10 ns TOF difference, ejectile and recoil wellresolved

A. Hayes et al., PRC 75 (2007) 034308

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 18/29

Identification ejectile-recoil: energy

• for Si detectors and targets of 1-2 mg/cm2: ejectile and recoil should differin mass by roughly a factor of two

• this limits observed excitation for mass > 100 (heavy targets like Pt or Pbcannot be used)

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 19/29

Exotic beam experiments: future

• increase in RIB intensities

• multi-step excitation experiments will become common

• Coulex one of the most important methods to measure transitionprobabilities on the neutron-rich side

• need for novel particle detectors

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 20/29

Si detectors for high-intensity RIB Coulex?

Coulomb excitation of 44Ar

0 20 40 60 80 100 120 140

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410

15

Direct beam of intensity 103 pps hitting 5-10% of detector area

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 21/29

Si detectors for high-intensity RIB Coulex?

Coulomb excitation of 44Ar

0 20 40 60 80 100 120 140

1

10

210

0

0 20 40 60 80 100 120 140

1

10

210

310

8

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7

0 20 40 60 80 100 120 140

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410

15

Direct beam of intensity 103 pps hitting 5-10% of detector area

Rate equivalent to Rutherford scattering of 108 pps beam at 15 < θ < 25

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 22/29

Diamond CVD detectors

• promising detector material for harsh environment: used for beammonitoring in high-energy physics (CERN, HADES...)

• high Eg → negligible leakage current at the room temperature

• low dielectric constant → low capacitance → low noise

• high carrier mobility → fast signal collection → excellent timing properties

• but... contradictory infomation on radiation hardness of SC diamonddetectors

I. Zamboni et al., submitted to Diamond and Related Materials

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 23/29

Additional measurements needed for Coulex data analysis...

• lifetime measurements necessary for integral cross-section measurements

-80 -60 -40 -20 0 20 40 60 80

Qs(2

1

+) (efm

2)

200

300

400

500

600

700

800

900

B(E

2; 2

1+→

01+)

(e2 fm

4 )

A.M. Hurst et al.,

Phys. Rev. Lett. 98, 072501 (2007)

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 24/29

Additional measurements needed for Coulex data analysis...

• lifetime measurements necessary for integral cross-section measurements

-80 -60 -40 -20 0 20 40 60 80

Qs(2

1

+) (efm

2)

200

300

400

500

600

700

800

900

B(E

2; 2

1+→

01+)

(e2 fm

4 )

J. Ljungvall et al.,

Phys. Rev. Lett. 100, 102502 (2008)

increase precision of quadrupole moments/intra-band matrix elementsfor differential measurements

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 25/29

Additional measurements needed for Coulex data analysis...

• lifetime measurements necessary for integral cross-section measurements

-80 -60 -40 -20 0 20 40 60 80

Qs(2

1

+) (efm

2)

200

300

400

500

600

700

800

900

B(E

2; 2

1+→

01+)

(e2 fm

4 )

J. Ljungvall et al.,

Phys. Rev. Lett. 100, 102502 (2008)

increase precision of quadrupole moments/intra-band matrix elementsfor differential measurements

• beam composition (isobaric contamination/isomeric ratio)• beam energy• conversion coefficients/E0 branchings

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 26/29

Beam purity, cocktail beams

Especially for exotic nuclei, what we want to study may be a small fraction ofthe beam... What can we do to determine the beam composition?

• Measure gamma-ray spectra of decay products – and what if we havestable contaminants?

• Ion chambers at the beam dump, energy measured by particle detectors –if difference in mass is big enough

• Laser ionisation with periodically blocked laser – excellent, not possible forall elements

• Mass separator for scattered ejectiles – limitation to forward scatteringangles

• Brute force approach: if a dominant contamination is much heavier (e.g.Xe in Ar beam) – choosing a target giving normal kinematics for Ar, inversefor Xe and measuring in backward angles only

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 27/29

Incident energy

• Strong dependence of multi-step excitation andreorientation effect on beam energy

• Correct beam energy required!

-1.5

-1

-0.5

0

0.5

1

68 69 70 71 72 73 74 75 76

Ecorr

Ebeam〈2+ 1‖E2‖2+ 1〉[eb]

Energy [MeV] 0

20

40

60

80

100

120

140

68 70 72 74 76 78 80 82

χ2

Beam energy [MeV]

K. Wrzosek-Lipska, PhD thesis (2011)

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 28/29

E0 strengths

• decay branch unvisible for Ge detectors• important for 0+ states (74Kr, 100Mo,...) and heavy nuclei

α (2+

2→ 2+

1) in 184Hg: 23(5)

E. Rapisarda et al., to be published

• electron spectroscopy measurements for strongly converted transitions?

Magda Zielinska, CEA Saclay SPES One-Day Workshop, Florence, 27 September 2012 - p. 29/29

Summary

• low-energy Coulomb excitation has a long tradition and a great future

• high intensity RIBs bring both opportunities and challenges

• progress in gamma-ray detection should be accompanied by developmentof new particle detectors for Coulex