Understanding the Nature of the Low-energy Enhancement in...

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6th Workshop on Nuclear Level Density and Gamma StrengthMay 8th – 12th, 2017

Understanding the Nature of the Low-energy Enhancement in the Photon Strength Function of

56Fe.

Michael David JonesNuclear Science DivisionLawrence Berkeley National Laboratory

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Contract No. DE-AC02-05CH11231.

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Motivation

• A broad range of applications:

• Reaction networks, R-process

• Reactor design, Fuel cycles

• The PSF is a measure of the average 𝛾-decay properties:

f (Eγ)≡ f J π(E γ)=

ΓJ (E x , Eγ)ρJ (E x)

Eγ

2λ+1

𝜎

𝐸𝛾

GDR

Pygmy

???

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Motivation

• Recent discovery of low-energy enhancement of PSF has generated great interest.

What is the mechanism behind the enhancement?

A.V. Voinov et. al. PRC 74, 014314 (2006)

S. Frauendorf et. Al. AIP Conference Proceedings 1619, 81 (2014)

A.C. Larsen et. al. PRC 82, 014318 (2010)



56Fe

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Motivation• Generated by large number of weak low-energy dipole transitions (high-lying) of

all considered spins well above yrast.

• Possibly M1 although some mechanisms consider E1.

A measurement of the polarization in the enhancement region is needed to determine its nature.

A.C. Larsen et. al. PRL 111, 242504 (2013)S. Frauendorf et. Al. AIP Conference Proceedings 1619, 81 (2014)

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Experimental Method• Use a direct reaction to populate

states in quasi-continuum.

p’

p

56Fe

γ1γ2

• Extract PSF using a new model independent method [1].

• Identify events in the “up-bend”

• Use GRETINA as a polarimeter to measure the M1 or E1 character.

Figure courtesy of L. Kirsch

A.C. Larsen et. al. PRL 111, 242504 (2013) [1] M. Wiedeking et. al. PRL 108, 162503 (2012)

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Experiment at ANL

• 56Fe(p,p’) at 16 MeV

Phoswich Wall (WashU)D.G. Sarantites et. al. NIMA Vol. 790, pg 42-56 (2015)

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Extraction of PSF

• Assume dipole radiation [1]:

• Model independent

f (Eγ)≡ f J π(E γ)=

ΓJ (E x , Eγ)ρJ (E x)

Eγ

2λ+1

N L(E x)∝ f (Eγ)Eγ3ΣσJ (E x)

Unambiguous entry and exit channel!

Accept Reject

1 2 3

1

2

3

R=f (E x−E L1)

f (E x−E L2)=

N L1(E x)(E x−E L2)3

N L2(E x)(E x−E L1)3

𝑅 =𝑓(𝐸γ)

𝑓(𝐸γ′)=𝑁𝐿 ∗ (𝐸γ

′)3

𝑁𝐿′ ∗ (𝐸γ)

3

[1] M. Wiedeking et. al. PRL 108, 162503 (2012)

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Ratio Method

1

2

3

𝛿𝐸 ~ 2

𝛿𝐸 ~ 0.5𝐸1

𝐸2𝛿𝐸

Can extract shape but not absolute magnitude.

R=f (E

γ1)

f (Eγ2)

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Particle ID

• Triple coincidence (p + γ + γ) and tagging final states in 56Fe eliminates other reaction channels:

• (p,α)53Mn

• (p,pn)55Fe

• (p,n)56Co

• (p,pα)52Cr

228Th56Fe(p,p’)

(p,α)

(p,pn)

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Calorimeter Gating

• Measure intensity of feeding to first four 2+ states.

• Correct for efficiency, branching, and background.

ΣEγ=E x±500 keV

56Fe

E Li

Accept Reject

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Ratios

• Crossing unity implies a local minimum

R=f (E

γ1)

f (Eγ2)

Eγ1>Eγ2

𝛿𝐸 ~ 2

𝛿𝐸 ~ 0.5𝛿𝐸 ~ 0.3 𝛿𝐸 ~ 1.2

𝛿𝐸 ~ 2.5𝛿𝐸 ~ 1.5

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Photon Strength Function (PSF)

• Confirm Oslo Method with independent technique.

• New data for Eγ < 1 MeV (600 keV)

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4+ Ratios

• Agreement between PSF built with different spins (Jπ)

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• Use Z-axis for reaction plane.

• 1st and 2nd interaction point give Compton plane.

Possible thanks to tracking!

B. Alikhani et al. NIM A 675 (2012) 144-154

Angular Distributions & Polarization

Z-axis

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Angular Distributions & Polarization56Fe, 847 keV, E2

55Fe, 476 keV, M1

N ~ 5MTracked

Tracked

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• Can improve asymmetry at cost of statistics (1/3)• Ѳ𝑙𝑎𝑏 ∈ 70 − 110 [deg]

• 𝑟12 > 10 [mm]56Fe, 847, E2 55Fe, 931, M1+E2

Polarization

Cuts double asymmetry from ~3% to 6%

𝑅 ~ 𝐴0[1 + 𝐴1cos 2ξ ] 𝐴1 = 𝑃𝑄

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Sensitivity in Low Statistics

• 56Fe, 847 keV, E2N = 325

A1 ~ 20%

• 55Fe, 931 keV, M1

N= 325 N= 55

A1 ~ 40%

Flat distribution is outside 1 sigma

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Distortion from Coincidence

• Calorimeter gating requires gamma-gamma coincidences.

• GRETINA is asymmetric in φ.

• Empirically determined with 2+ (847 keV), assumed to be general.

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• Angular distribution agrees with L=1, but may have quadrupole.

Angular Distributions

• Polarization is flat.

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Enhancement Region [0.5 – 1.5]

• Angular distribution agrees with L=1, but may be isotropic.

• Polarization has small excess.

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Three-Step Cascades

• Spectrum of primaries contain some known low-lying states in 56Fe.

• These originate from 3+ step cascades which leak through due to wide Phoswich gate.

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[0 – 1.5] Enhancement Region

• Removing triples cleans up the angular distribution:

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• Slight excess at 90 [deg], but consistent with Uniform Dist.

Enhancement Region [0.5 – 1.5]

𝑅 ~ 𝐴0[1 + 𝐴1cos 2ξ ]

𝐴1 = 0.26 ± 0.11 (𝑠𝑡𝑎𝑡)

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Energy Comparison

[0.5 – 1.5] MeV [1.5 – 5.0] MeV [6.0 – 8.0] MeV

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Conclusions

• Ratio method agrees with Oslo data.

• 4+ and 2+ PSF identical.

• Angular distributions agree with L=1, but contain some L=2.

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Next Steps

• Estimate alignment in (p,p’).

Need 3x more counts

• Enhancement Region shows excess at 90o in polarization.• M1 + E1? • Pure E1? • Pure M1 not likely?

N ~ 250

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LBNL + UC BerkeleyL.A. Bernstein, C.M. Campbell, R.M. Clark, H.L. Crawford,

M. Cromaz, P. Fallon, L. Kirsch, I.Y. Lee, A.O. Macchiavelli, L.W. Phair, A. WiensiThemba LabsM. Wiedeking

ANLS. Zhu, A.D. Ayangaekaa, S. Bottoni, M.P. Carpenter, H.M. Davids, R.V.F. Janssens,

T. Lauristen. Washington University

W. Reviol, D.G. SarantitesUniversity of Oslo

A. Gorgen, M. Guttormsen, A.C. Larsen, S. SiemLLNL

D.L. BleuelOhio University

A.V. Voinov

Acknowledgements

Thank you!

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Photon Strength Function (PSF)

• Strong low-energy enhancement

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2+ Ratios

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Reaction Plane• Asymmetry of GRETINA and Phoswich requires normalization with in-

beam photopeak.

• Distribution must be weighted by theoretical 𝑤(Ѳ)• Double-folding

𝐴2, 𝐴4

Unfolded Folded

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Phoswich Wall Performance

Energy [MeV] Width [MeV] R = FWHM/E [%]

15.47 0.778 12

10.87 0.647 14

5.47 0.472 20

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Motivation• Generated by large number of weak low-energy M1 transitions (high-lying, sum

coherently)

• E1 transitions from thermally unblocked states to the continuum (TCQRPA)

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846 (2+)

• Test sensitivity with first excited state of 56Fe• Compton angle and relative distance (r12)

56Fe

𝑟12

𝑧 − 𝑎𝑥𝑖𝑠θ𝑐

θ𝑙𝑎𝑏γ1

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Polarization

B. Alikhani et. al. NIM A 675 (2012) 144-145

• Compton Scattering:

𝑀𝜆

60Ni: 1173 keV (E2)

𝐸𝜆

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Angular Distributions

• Angular distribution of 846 (E2) relative to 2598 (M1 + E2)

• Can normalize with source data as well (152Eu, 60Co).

56FeMultipole character accessible!

𝑅 =𝐼(847, θ)

𝐼(2598, θ)

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