Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

Investigation of radiative proton-capture reactions using high-resolution γ-ray spectroscopy

Philipp Scholz*, Felix Heim, Marvin Körschgen, Jan Mayer, Martin Müller, Julius Wilhelmy and Andreas Zilges

Institute for Nuclear Physics, University of Cologne

NUCLEAR PHYSICS IN STELLAR EXPLOSIONS WORKSHOP ´18

MTA Atomki, Debrecen, Hungary

*) pscholz@ikp.uni-koeln.de

Supported by the ULDETIS project within the UoC Excellence Initiative institutional strategy and by DFG (ZI 510/8-1, INST 216/544-1).

Supported by the Bonn-Cologne Graduate School of Physics and Astronomy.

γ-process nucleosynthesis

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

γ-process nucleosynthesis▪ Series of (γ,n), (γ,p), and (γ,α) reactions

▪ γ-ray induced reaction rates challenging to measure

▪ Measurement of (p,γ) and (α,γ) cross sections on heavy target nuclei

Hauser-Feshbach statistical model

▪ Optical-Model Potentials

▪ Nuclear Level Densities

▪ γ-strength functions

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10 MV FN-Tandem accelerator

▪ Up to 1.5 µA protons and 500 nA α-particles

HORUS γ-ray spectrometer

▪ 14 HPGe detectors

▪ Efficiency ~ 2% @ 1.3 MeV

▪ FWHM ~ 2 keV @ 1.3 MeV

▪ Five different angles with respect to the

beam axis

▪ γγ-coincidence measurements

Target chamber

▪ Beam current read-out

▪ Cooling trap around the target position

▪ Build-in silicon detector for Rutherford

Backscattering spectrometry

In-Beam γ-ray spectroscopy at HORUS

L. Netterdon et al., Nucl. Inst. Meth. A 754 (2014) 94; F. Heim et al., to be published

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

HPGe

HPGe

RBS detector

Beam

Target withcooling trap

In-beam determination of total cross sections

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

Ecm

+ Q

AX

A+1Y

Detection of ground-state transitions

▪ Determine the yield of as many g.s. transitions as possible from angular distributions

▪ Sum of single yields provides the total yield▪ Measured cross sections are “lower” limits

▪ Not as sensitive as activation or 4π-summing▪ Complex analysis with increasing number of g.s. transitions

▪ Possible measurement of reactions with unstable, long livedand stable products

▪ Detailed γ-ray decay information

Cross section measurements @ HORUS

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

110Cd 111Cd 112Cd 113Cd 114Cd

115In113In

116Sn115Sn114Sn112Sn

108Cd

116Te

116Sb

107Ag 109Ag

L. Netterdon et al., PLB 744 (2015) 358 J. Mayer et al., Phys. Rev. C 93 (2016) 045809

L. Netterdon et al., Phys. Rev. C 91 (2015) 035801 P. Scholz et al., PLB 761 (2016) 247

85Rb 87Rb

86Sr 87Sr 88Sr

89Y

90Zr 91Zr 92Zr

92Nb 93Nb

94Mo92Mo

91Nb

93Tcpublished

to be published

other groups

F. Heim, to be published

L. Netterdon, to be published

107Ag(p,γ)108Cd cross section measurement

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

107Ag(p,γ)108Cd

▪ Measurement at six proton energies between 2.0 MeV and 5.0 MeV

▪ Target thicknesses ~ 3×1018 atoms/cm²

▪ Beam current ~ 1µA

F. Heim et al., to be published

107Ag(p,γ)108Cd – total cross sections

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

Total cross sections

▪ Disagreement with data from Khaliel et al.

▪ Good reproduction via TALYS using microscopical density models (HFB + combinatorial models)

▪ Small adjustment of ctable parameter needed

➢ Impact of the γ-ray strength function?

F. Heim et al., to be published

The γ-ray strength function

Philipp Scholz 8

100

101

102

30 40 50 60 70 80 90 100 110

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v>

(G

LO

-up

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<s

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(G

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)

N

Fe

Mo

Cd

A.C. Larsen and S. Goriely, Phys. Rev. C 82 (2010) 014318

S. Goriely, Phys. Lett. B 436 (1998) 10

PDR

Upbend

▪ The γ-decay width depends on the nuclear level density and γ-ray strength functions

▪ Radiative capture reaction rates are very sensitive on the γ-decay width▪ Structures below particle separation thresholds enhance reaction rates

➢ How independent are these structures of excitation energy and mechanism?

Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

Partial cross sections and primary γ-ray transitions

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

Detection of primary γ-ray transitions

▪ Direct population of final states in the reaction product

▪ High energy γ-ray transitions in the single spectra of the HPGe detectors

▪ Intensity depends on the single γ-ray decay widths

▪ Can be used to study the γ-ray strength function in the reaction product

Ecm

+ Q

AX

A+1Y

107Ag(p,γ)108Cd – partial cross sections

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

F. Heim et al., to be published

Partial cross sections

▪ Measurement of eight partial cross sections at the highest beam energies

▪ Too low intensities at 2.0 MeV and 2.7 MeV

▪ Unsatisfying description within the statistical model

➢ Problem with γ-ray strength function in 108Cd?

▪ Assuming total decay widths, level densities, proton-OMP is well reproduced

➢ Real γ-strength can be obtained from cross section ratios:

𝑓𝑐𝑜𝑟𝑟 𝐸𝛾𝑖≈ 𝑓 𝐸𝛾𝑖

×𝜎𝑒𝑥𝑝

𝛾𝑖

𝜎𝑡ℎ𝑒𝑜

𝛾𝑖

γ-ray strength in Cd isotopes

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

F. Heim et al., to be published

▪ Determination of γ-ray strength values in 108Cd between ~ 10 MeV and 13.5 MeV

▪ Q-Value + low intensities in the single spectra deny access to lower γ-ray energies

▪ Visible fluctuations might be due to:▪ Influence of spin distribution?▪ Statistical fluctuations?▪ Imperfect description of other

nuclear physics input-parameter?

Primary γ-ray intensities from coincidence data

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

A+1Y

Ecm

+ Q

AX

Determining primary γ-ray intensities via coincidences

▪ Higher sensitivity via gates on secondary transitions

▪ Access to more transitions at one excitation energy

63,65Cu(p,γ) at 3.5 MeV/2.0 MeV

▪ One excitation energy below (p,n) thresholds➢ Constant total decay widths and level density

▪ Thick targets (1.5 and 2.5 mg/cm²)➢ Statistical approach to γ-decay widths

Two-step γ-ray cascades

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

▪ Sort coincident γ-rays according to their sum energy

▪ Gating in the sum-peak spectrum reveals TSC spectra ➢ Contains γ-rays participating in a TSC

population of level in the reaction product

Past: ▪ Statistical analysis of TSC spectra via, e.g.,

DICEBOX

New:▪ Extract primary intensities from the

depopulation of states via their absolute branching ratios

P. Scholz, PhD Thesis, to be published

Primary γ-ray intensities from radiative-proton captures

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

P. Scholz, PhD Thesis, to be published

Ratios of primary γ-ray intensities

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

▪ Ratios of primary γ-rays can be used to calculate relative strength▪ Focus on same spin and parities

➢ Reduce influence of spin distribution ➢ Strength function curves for different final states

▪ Different curves can be scaled towards each other at same γ-ray energies

𝒇(𝑬𝜸𝟏)

𝒇(𝑬𝜸𝟐)

≈𝑰𝜸𝟏

𝑬𝜸𝟏𝟑

𝑰𝜸𝟐𝑬𝜸𝟐

𝟑

P. Scholz, PhD Thesis, to be published

Results and conclusions

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

▪ Relative values need to be scaled/normalized

▪ Measurement of γ-strength values at energies below the separation thresholds possible

▪ Comparison to future Nuclear Resonance Fluorescence and Oslo-type experiments

▪ Testing the generalized Brink-Axel hypothesis

▪ Application to heavier target nuclei

P. Scholz, PhD Thesis, to be published

Summary & Outlook

Philipp Scholz, AG Zilges, IKP Investigation of radioactive proton-capture reactions using in-beam γ-ray spectroscopy

L. Netterdon et al., PLB 744 (2015) 358 L. Netterdon et al., Phys. Rev. C 91 (2015) 035801J. Mayer et al., Phys. Rev. C 93 (2016) 045809 P. Scholz et al., PLB 761 (2016) 247L. Netterdon, to be published F. Heim, to be published

▪ Combining cross section measurement with the investigation of nuclear physics input parameter

▪ Focus on the transition region between various processes for the nucleosynthesis of p nuclei

▪ N=50 isotones▪ Molybdenum and Cadmium

▪ Improving the predictions of reaction rates within the statistical model