arXiv subjects

T. K. Eriksen

Publications and source records attributed to T. K. Eriksen.

At least 19 recordsLinked to original sources

arXiv2026

Statistical and non-statistical $γ$-decay properties of $^{64}$Zn

We present a study on the $γ$-decay properties of $^{64}$Zn using the Oslo method on $^{64}$Zn($p,p^\prime γ$) data combined with $^{64}$Zn$(γ,n)$ cross-section measurements at the NewSUBARU facility. With the Oslo method, we have measured the $γ$-ray strength function ($γ$SF) and the nuclear level density (NLD) below the neutron threshold. We observe that the NLD trend in the quasi-continuum region of $^{64}$Zn is best characterized by a constant-temperature-like model. %with temperature parameter $T_{\rm CT}=1.21(5)$ MeV. Surprisingly, we find that $γ$-ray transitions from the quasi-continuum decaying directly to the $0^+$ ground state seem to be strongly hindered with a hindrance factor of $κ\approx 0.5$, which could be an indication of non-statistical effects in the ground-state decay due to, \textit{e.g.}, differences in nuclear shapes. For $γ$ energies above the neutron separation energy, the NewSUBARU ($γ, n$) data set probes a significant part of the giant dipole resonance. Furthermore, we find that the Oslo-method $γ$SF shows a rather smooth behavior, with a clear low-energy enhancement (LEE) for $E_γ < 4$ MeV.

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arXiv2026

Exploring the statistical properties of the neutron-deficient $^{109}$In isotope with the Oslo method

The nuclear level density (NLD) and the $γ$-ray strength function (GSF) of the neutron-deficient $^{109}$In isotope were extracted for the first time with data from the $^{106}$Cd$(α,pγ)^{109}$In reaction using a combination of the Oslo and the shape methods. Both quantities are consistent with those of neighboring Cd and Sn nuclei, but show substantial discrepancies with currently available model predictions. In contrast to earlier observations in the neighboring isotopic chains, $^{109}$In does not exhibit any significant enhancement of the dipole strength near the neutron separation energy. To interpret this feature, random-phase time-blocking approximation calculations have been performed for $^{109}$In and the neighboring $^{110,112}$Sn nuclei. The experimental data were also employed to estimate cross sections and rates of the radiative neutron- and proton-capture reactions, $^{108}$In($n,γ)$$^{109}$In and $^{108}$Cd($p,γ)$$^{109}$In, respectively, with the reaction code TALYS. Our ($p,γ)$ cross section is in excellent agreement with direct measurements over a wide range of proton energies, while the ($n,γ)$ cross section demonstrates notable deviations from predictions in the JINA REACLIB library. The new results on the statistical properties of $^{109}$In provide valuable constraints that may help address the problem of large model uncertainties compromising the accuracy of astrophysical $p$-process simulations.

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arXiv2025

A saga on the $γ$-decay branching ratio of the Hoyle state

The radiative branching ratio of the Hoyle state is crucial to estimate the triple-$α$ reaction rate in stellar environments at medium temperatures of $T=0.1$ to 2 GK. Knowledge of the $γ$-decay channel is critical as this is the dominant radiative decay channel for the Hoyle state. A recent study by Kibédi et al. [Phys. Rev. Lett. 125, 182701 (2020)] has challenged our understanding of this astrophysically significant branching ratio and its constraints. The main purpose was to perform a new measurement of the $γ$-decay branching ratio of the Hoyle state to deduce the radiative branching ratio of the Hoyle state, an additional objective was to independently verify aspects of the measurement conducted by Kibédi et al. For the primary experiment of this work the Hoyle state was populated by the $^{12}\textrm{C}(p,p')$ reaction at 10.8 MeV at the Oslo Cyclotron Laboratory. The $γ$-decay branching ratio was deduced through triple-coincidence events between a proton populating the Hoyle state and the subsequent $γ$-ray cascade. An independent analysis of the 2014 data published by Kibédi et al. has been carried out. From the main experiment of this work, a $γ$-decay branching ratio of the Hoyle state was determined as $Γ_γ^{7.65}/Γ^{7.65}=4.0(3)\times 10^{-4}$, yielding a radiative branching ratio of $Γ_{\textrm{rad}}/Γ=4.1(4) \times 10^{-4}$. The reanalysis of the 2014 experiment in this work yielded $Γ_γ^{7.65}/Γ^{7.65}=4.5(6)\times 10^{-4}$, with a radiative branching ratio of $Γ_{\textrm{rad}}/Γ=4.6(6) \times 10^{-4}$. The measurements of the radiative branching ratio of the Hoyle state in this work is in excellent agreement with several recent studies, as well as the previously adopted ENSDF average of $Γ_{\textrm{rad}}/Γ=4.16(11)\times 10^{-4}$.

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arXiv2023

Nuclear level densities and $γ-$ray strength functions of $^{111,112,113}$Sn isotopes studied with the Oslo method

The $^{111,112,113}$Sn isotopes have been studied with ($p,d γ$), ($p,p^{\prime} γ$), and ($d,p γ$) reactions to extract the nuclear level densities (NLDs) and $γ$-ray strength functions (GSFs) of these nuclei below the neutron separation energy by means of the Oslo method. The experimental NLDs for all three nuclei demonstrate a trend compatible with the constant-temperature model below the neutron separation energy while also being in good agreement with the NLDs of neighboring Sn isotopes, obtained previously with the Oslo-type and neutron evaporation experiments. The extracted microcanonical entropies yield $\approx 1.5$ $k_B$ entropy of a valence neutron in both $^{111}$Sn and $^{113}$Sn. Moreover, the deduced microcanonical temperatures indeed suggest a clear constant-temperature behavior above $\approx$ 3 MeV in $^{111,113}$Sn and above $\approx$ 4.5 MeV in $^{112}$Sn. We observe signatures for the first broken neutron pairs between 2 and 4 MeV in all three nuclei. The GSFs obtained with the Oslo method are found to be in good agreement below the neutron threshold with the strengths of $^{112,114}$Sn extracted in the ($p,p^{\prime}$) Coulomb excitation experiments.

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arXiv2023

Direct measurement of hexacontatetrapole, $\textbf{E6}$ γ decay from $^{\textbf{53m}}$Fe

The only proposed observation of a discrete, hexacontatetrapole ($E6$) transition in nature occurs from the T$_{1/2}$ = 2.54(2)-minute decay of $^{53m}$Fe. However, there are conflicting claims concerning its $γ$-decay branching ratio, and a rigorous interrogation of $γ$-ray sum contributions is lacking. Experiments performed at the Australian Heavy Ion Accelerator Facility were used to study the decay of $^{53m}$Fe. For the first time, sum-coincidence contributions to the weak $E6$ and $M5$ decay branches have been firmly quantified using complementary experimental and computational methods. Agreement across the different approaches confirms the existence of the real $E6$ transition; the $M5$ branching ratio and transition rate have also been revised. Shell model calculations performed in the full $pf$ model space suggest that the effective proton charge for high-multipole, $E4$ and $E6$, transitions is quenched to approximately two-thirds of the collective $E2$ value. Correlations between nucleons may offer an explanation of this unexpected phenomenon, which is in stark contrast to the collective nature of lower-multipole, electric transitions observed in atomic nuclei.

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arXiv2023

Nuclear level densities and $γ$-ray strength functions in $^{120,124}$Sn isotopes: impact of Porter-Thomas fluctuations

Nuclear level densities (NLDs) and $γ$-ray strength functions (GSFs) of $^{120,124}$Sn have been extracted with the Oslo method from proton-$γ$ coincidences in the ($p,p^{\prime}γ)$ reaction. The functional forms of the GSFs and NLDs have been further constrained with the Shape method by studying primary $γ$-transitions to the ground and first excited states.The NLDs demonstrate good agreement with the NLDs of $^{116,118,122}$Sn isotopes measured previously. Moreover, the extracted partial NLD of 1$^{-}$ levels in $^{124}$Sn is shown to be in fair agreement with those deduced from spectra of relativistic Coulomb excitation in forward-angle inelastic proton scattering. The experimental NLDs have been applied to estimate the magnitude of the Porter-Thomas (PT) fluctuations. Within the PT fluctuations, we conclude that the GSFs for both isotopes can be considered to be independent of initial and final excitation energies, in accordance with the generalized Brink-Axel hypothesis. Particularly large fluctuations observed in the Shape-method GSFs present a considerable contribution to the uncertainty of the method, and may be one of the reasons for deviations from the Oslo-method strength at low $γ$-ray energies and low values of the NLD (below $\approx1\cdot10^{3}-2\cdot10^{3}$ MeV$^{-1}$).

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arXiv2022

High precision proton angular distribution measurements of $^{12}$C(p,p') for determination of the $E0$ decay branching ratio of the Hoyle state

Background: In stars, carbon is produced exclusively via the $3α$ process, where three $α$ particles fuse to form $^{12}$C in the excited Hoyle state, which can then decay to the ground state. The rate of carbon production in stars depends on the radiative width of the Hoyle state. The radiative width can be deduced by combining three separately measured quantities, one of which is the $E0$ decay branching ratio. The $E0$ branching ratio can be measured by exciting the Hoyle state in the $^{12}$C$(p,p')$ reaction and measuring the pair decay of its Hoyle state and first $2^+$ state. Purpose: To reduce the uncertainties in the carbon production rate in the universe by measuring a set of proton angular distributions for the population of the Hoyle state ($0^+_2$) and $2^+_1$ state in $^{12}$C in $^{12}$C$(p,p')$ reactions between 10.20 and 10.70 MeV, used in the determination of the $E0$ branching ratio of the Hoyle state. Method: Proton angular distributions populating the ground, first $2^+$, and the Hoyle states in $^{12}$C were measured in $^{12}$C(p,p') reactions with a silicon detector array covering $22^\circ<θ<158^\circ$ in 14 energy steps between 10.20 and 10.70 MeV with a thin ($60\ μ$g/cm$^2$) $^{nat}$C target. Results: Total cross-sections for each state were extracted and the population ratio between the $2^+_1$ and Hoyle state determined at each energy step. By appropriately averaging these cross-sections and taking their ratio, the equivalent population ratio can be extracted applicable for any thick $^{12}$C target used in pair-conversion measurements. Conclusions: We present a general data set of high-precision $^{12}$C$(p,p')$ cross-sections that make uncertainties resulting from the population of the $2^+_1$ and $0^+_2$ states by proton inelastic scattering negligible for any future measurements of the $E0$ branching ratio in $^{12}$C.

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arXiv2022

Pulse Shape Discrimination of low-energy nuclear and electron recoils for improved particle identification in NaI:Tl

The scintillation mechanism in NaI:Tl crystals produces different pulse shapes that are dependent on the incoming particle type. The time distribution of scintillation light from nuclear recoil events decays faster than for electron recoil events and this difference can be categorised using various Pulse Shape Discrimination (PSD) techniques. In this study, we measured nuclear and electron recoils in a NaI:Tl crystal, with electron equivalent energies between 2 and 40 keV. We report on a new PSD approach, based on an event-type likelihood; this outperforms the charge-weighted mean-time, which is the conventional metric for PSD in NaI:Tl. Furthermore, we show that a linear combination of the two methods improves the discrimination power at these energies.

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arXiv2022

Electric monopole transition from the superdeformed band in $^{40}$Ca

The electric monopole ($E0$) transition strength $ρ^2$ for the transition connecting the third 0$^+$ level, a "superdeformed" band head, to the "spherical" 0$^+$ ground state in doubly magic $^{40}$Ca has been determined via $e^+e^-$ pair-conversion spectroscopy. The measured value, $ρ^2(E0; 0^+_3 \to 0^+_1)~=~2.3(5)\times10^{-3}$, is the smallest $ρ^2(E0; 0^+ \to 0^+)$ found in $A<50$ nuclei. In contrast, the $E0$ transition strength to the ground state observed from the second 0$^+$ state, a band head of "normal" deformation, is an order of magnitude larger, $ρ^2(E0; 0^+_2 \to 0^+_1)~=~25.9(16)\times~10^{-3}$, which shows significant mixing between these two states. Large-Scale Shell Model (LSSM) calculations were performed to understand the microscopic structure of the excited states, and the configuration mixing between them; experimental $ρ^2$ values in $^{40}$Ca and neighboring isotopes were well reproduced by the LSSM calculations. The unusually small $ρ^2(E0; 0^+_3 \to 0^+_1)$ value is due to destructive interference in the mixing of shape-coexisting structures, which are based on several different multiparticle-multihole excitations. This observation goes beyond the usual treatment of $E0$ strengths, where two-state shape mixing cannot result in destructive interference.

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arXiv2021

Comprehensive test of the Brink-Axel hypothesis in the energy region of the pygmy dipole resonance

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The $γ$-ray strength functions (GSFs) extracted from primary $γ$-decay spectra following charged-particle reactions with the Oslo method and with the Shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance.

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arXiv2020

The radiative width of the Hoyle state from $γ$-ray spectroscopy

The cascading 3.21 MeV and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in $^{12}$C, excited by the $^{12}$C(p,p$^{\prime}$) reaction at 10.7 MeV proton energy. From the proton-$γ$-$γ$ triple coincidence data, a value of ${Γ_{\rm rad}}/Γ=6.2(6) \times 10^{-4}$ was obtained for the radiative branching ratio. Using our results, together with ${Γ_π^{E0}}/Γ$ from Eriksen et al., Phys. Rev. C 102, 024320 and the currently adopted $Γ_π(E0)$ values, the radiative width of the Hoyle state is determined as $Γ_{\rm rad}=5.1(6) \times 10^{-3}$ eV. This value is about 34% higher than the currently adopted value and will impact on models of stellar evolution and nucleosynthesis.

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arXiv2020

Improved precision on the experimental E0 decay branching ratio of the Hoyle state

Stellar carbon synthesis occurs exclusively via the $3α$ process, in which three $α$ particles fuse to form $^{12}$C in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the $α$ threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the $E0$ decay branching ratio of the Hoyle state, and the current $10$\% uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The $E0$ branching ratio was deduced from a series of pair conversion measurements of the $E0$ and $E2$ transitions depopulating the $0^+_2$ Hoyle state and $2^+_1$ state in $^{12}$C, respectively. The excited states were populated by the $^{12}$C$(p,p^\prime)$ reaction at 10.5 MeV beam energy, and the pairs were detected with the electron-positron pair spectrometer, Super-e, at the Australian National University. The deduced branching ratio required knowledge of the proton population of the two states, as well as the alignment of the $2^+_1$ state in the reaction. For this purpose, proton scattering and $γ$-ray angular distribution experiments were also performed. An $E0$ branching ratio of $Γ^{E0}_π/Γ=8.2(5)\times10^{-6}$ was deduced in the current work, and an adopted value of $Γ^{E0}_π/Γ=7.6(4)\times10^{-6}$ is recommended based on a weighted average of previous literature values and the new result. The new recommended value for the $E0$ branching ratio is about 14% larger than the previous adopted value of $Γ^{E0}_π/Γ=6.7(6)\times10^{-6}$, while the uncertainty has been reduced from 9% to 5%.

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arXiv2019

$E0$ transition strength in stable Ni isotopes

Excited states in $^{58,60,62}$Ni were populated via inelastic proton scattering at the Australian National University as well as via inelastic neutron scattering at the University of Kentucky Accelerator Laboratory. The Super-e electron spectrometer and the CAESAR Compton-suppressed HPGe array were used in complementary experiments to measure conversion coefficients and $δ(E2/M1)$ mixing ratios, respectively, for a number of $2^+ \rightarrow 2^+$ transitions. The data obtained were combined with lifetimes and branching ratios to determine $E0$, $M1$, and $E2$ transition strengths between $2^+$ states. The $E0$ transition strengths between $0^+$ states were measured using internal conversion electron spectroscopy and compare well to previous results from internal pair formation spectroscopy. The $E0$ transition strengths between the lowest-lying $2^+$ states were found to be consistently large for the isotopes studied.

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arXiv2019

Table of electronic factors for E0 electron and electron-positron pair conversion transitions

A new tabulation of electronic factors is reported for electron conversion for elements of Z from 5 to 126 and electronic factors for electron-positron pair conversion for elements of even Z from 4 to 100. The electronic factors for electron conversion, $Ω_{CE}$(E0), were calculated using a modified version of the CATAR program developed by Pauli and Raff with a relativistic-Hartree-Fock-Slater approach (Pauli and Raff, 1975). The electronic factors for electron-positron pair conversion, $Ω_{IPF}$(E0), were calculated using the model developed by Wilkinson (1969). The data tables presented here cover all atomic shells up to R2 and transition energies from 1 keV to 6000 keV and from 1100 keV to 8000 keV for pair conversion. A comparison with previous electronic factor tabulations is presented. Ratios of experimental Ω(E0) values for 83 E0 transitions in 8$\leq$Z$\leq$98 are compared to this tabulation. Two examples of how to use the tabulation to extract E0 strengths are also included.

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arXiv2019

First-excited state $g$ factors in the stable, even Ge and Se isotopes

Transient-field $g$-factor measurements in inverse kinematics were performed for the first-excited states of the stable, even isotopes of Ge and Se. The $g$ factors of $^{74}$Ge and $^{74}$Se were measured simultaneously using a cocktail beam, which eliminates most possible sources of systematic error in a relative $g$-factor measurement. The results are $g(^{74}{\rm Se})/g(^{74}{\rm Ge})=1.34(7)$, $g(^{70}{\rm Ge})/g(^{74}{\rm Ge}) = 1.16(15)$, $g(^{72}{\rm Ge})/g(^{74}{\rm Ge})=0.92(13)$, $g(^{76}{\rm Ge})/g(^{74}{\rm Ge})=0.88(5)$, $g(^{76}{\rm Se})/g(^{74}{\rm Se})=0.96(7)$, $g(^{78}{\rm Se})/g(^{74}{\rm Se})=0.82(5)$, $g(^{80}{\rm Se})/g(^{74}{\rm Se})=0.99(7)$ and $g(^{82}{\rm Se})/g(^{74}{\rm Se})=1.19(6)$. The measured $g$-factor ratios are in agreement with ratios from previous measurements, despite considerable variation in previous reported absolute values. The absolute values of the $g$ factors remain uncertain, however the Rutgers parametrization was used to set the transient-field strength and then compare the experimental $g$ factors with shell-model calculations based on the JUN45 and jj44b interactions. Modest agreement was found between experiment and theory for both interactions. The shell model calculations indicate that the $g(2^+_1)$ values and trends are determined largely by the balance of the spin carried by orbital motion of the protons.

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arXiv2019

Restricted spin-range correction in the Oslo Method: The example of nuclear level density and $γ$-ray strength function from $^{239}\mathrm{Pu}(\mathrm{d,p}γ)^{240}\mathrm{Pu}$

The Oslo Method has been applied to particle-$γ$ coincidences following the $^{239}\mathrm{Pu}$(d,p) reaction to obtain the nuclear level density (NLD) and $γ$-ray strength function ($γ$SF) of $^{240}\mathrm{Pu}$. The experiment was conducted with a 12 MeV deuteron beam at the Oslo Cyclotron Laboratory. The low spin transfer of this reaction leads to a spin-parity mismatch between populated and intrinsic levels. This is a challenge for the Oslo Method as it can have a significant impact on the extracted NLD and $γ$SF. We have developed an iterative approach to ensure consistent results even for cases with a large spin-parity mismatch, in which we couple Green's Function Transfer calculations of the spin-parity dependent population cross-section to the nuclear decay code RAINIER. The resulting $γ$SF shows a pronounced enhancement between 2-4 MeV that is consistent with the location of the low-energy orbital $M1$ scissors mode.

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arXiv2019

Impact of restricted spin-ranges in the Oslo Method: The example of (d,p)$^{240}\mathrm{Pu}$

In this paper we present the first systematic analysis of the impact of the populated vs. intrinsic spin distribution on the nuclear level density and $γ$-ray strength function retrieved through the Oslo Method. We illustrate the effect of the spin distribution on the recently performed $^{239}\mathrm{Pu}$(d,p$γ$)$^{240}\mathrm{Pu}$ experiment using a 12 MeV deuteron beam performed at the Oslo Cyclotron Lab. In the analysis we couple state-of-the-art calculations for the populated spin-distributions with the Monte-Carlo nuclear decay code RAINIER to compare Oslo Method results to the known input. We find that good knowledge of the populated spin distribution is crucial and show that the populated distribution has a significant impact on the extracted nuclear level density and $γ$-ray strength function for the $^{239}\mathrm{Pu}$(d,p$γ$)$^{240}\mathrm{Pu}$ case.

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arXiv2018

The gamma-ray strength function of $^{89}$Y and $^{90}$Y

In this work, we present new data on the $^{89}$Y($γ$,n) cross section studied with a quasi-monochromatic photon beam produced at the NewSUBARU synchrotron radiation facility in Japan contributing torwards resolving a long standing discrepancy between existing measurements of this cross section. Results for $γ$-ray strength function below threshold obtained by applying the Oslo method to $^{89}$Y($p,p'γ$)$^{89}$Y coincidences combined with the $^{89}$Y($γ$,n) data this providing experimental data for the $γ$-ray strength function of $^{89}$Y for $γ$ energies in the range of $\approx 1.6$ Mev to $\approx$ 20 MeV. A low-energy enhancement is seen for $γ$-rays below $\approx 2.5$ MeV. Shell-model calculations indicate that this feature is caused by strong, low-energy $M1$ transitions at high excitation energies. The nuclear level density and $γ$-ray strength function have been extracted from $^{89}$Y($d,p γ$)$^{90}$Y coincidences using the Oslo method. Using the ($γ,n$) and ($d,pγ$) data as experimental constraints, we have calculated the $^{89}$Y($n,γ$)$^{90}$Y cross section with the TALYS reaction code. Our results have been compared with directly measured (n,$γ$) cross sections and evaluations. The $N=50$ isotope $^{89}$Y is an important bottleneck in the s-process and the magnitude of the $^{89}$Y(n,$γ)$ cross section is key to understanding how s-process stars produce heavy isotopes.

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