arXiv subjects

A. C. Larsen

Publications and source records attributed to A. C. Larsen.

At least 55 records · Page 3Linked to original sources

arXiv2018

Uncertainties in radiative neutron-capture rates relevant to the $A\sim 80$ $r$-process peak

The rapid neutron-capture process ($r$-process) has for the first time been confirmed to take place in a neutron-star merger event. A detailed understanding of the rapid neutron-capture process is one of the holy grails in nuclear astrophysics. In this work we investigate one aspect of the $r$-process modelling: uncertainties in radiative neutron-capture cross sections and astrophysical reaction rates for isotopes of the elements Fe, Co, Ni, Cu, Zn, Ga, Ge, As, and Se. In particular, we study deviations from standard libraries used for astrophysics, and the influence of a very-low $γ$-energy enhancement in the average, reduced $γ$-decay probability on the ($n,γ$) rates. We find that the intrinsic uncertainties are in some cases extremely large, and that the low-energy enhancement, if present in neutron-rich nuclei, may increase the neutron-capture reaction rate significantly.

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arXiv2017

Energy dependence of the prompt $γ$-ray emission from the $(d,p)$-induced fission of $^{234}\mathrm{U}^{}$ and $^{240}\mathrm{Pu}^{}$

Prompt fission $γ$-rays are responsible for approximately 5\% of the total energy released in fission, and therefore important to understand when modelling nuclear reactors. In this work we present prompt $γ$-ray emission characteristics in fission, for the first time as a function of the nuclear excitation energy of the fissioning system. Emitted $γ$-ray spectra were measured, and $γ$-ray multiplicities and average and total $γ$ energies per fission were determined for the $^{233}$U(d,pf) reaction for excitation energies between 4.8 and 10 MeV, and for the $^{239}$Pu(d,pf) reaction between 4.5 and 9 MeV. The spectral characteristics show no significant change as a function of excitation energy above the fission barrier, despite the fact that an extra $\sim$5 MeV of energy is potentially available in the excited fragments for $γ$-decay. The measured results are compared to model calculations made for prompt $γ$-ray emission with the fission model code GEF. Further comparison with previously obtained results from thermal neutron induced fission is made to characterize possible differences arising from using the surrogate (d,p) reaction.

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arXiv2017

Quasicontinuum $γ$-decay of $^{91,92}$Zr: benchmarking indirect ($n,γ$) cross section measurements for the $s$-process

Nuclear level densities (NLDs) and $γ$-ray strength functions ($γ$SFs) have been extracted from particle-$γ$ coincidences of the $^{92}$Zr($p,p' γ$)$^{92}$Zr and $^{92}$Zr($p,d γ$)$^{91}$Zr reactions using the Oslo method. The new $^{91,92}$Zr $γ$SF data, combined with photonuclear cross sections, cover the whole energy range from $E_γ \approx 1.5$~MeV up to the giant dipole resonance at $E_γ \approx 17$~MeV. The wide-range $γ$SF data display structures at $E_γ \approx 9.5$~MeV, compatible with a superposition of the spin-flip $M1$ resonance and a pygmy $E1$ resonance. Furthermore, the $γ$SF shows a minimum at $E_γ \approx 2-3$~MeV and an increase at lower $γ$-ray energies. The experimentally constrained NLDs and $γ$SFs are shown to reproduce known ($n, γ$) and Maxwellian-averaged cross sections for $^{91,92}$Zr using the {\sf TALYS} reaction code, thus serving as a benchmark for this indirect method of estimating ($n, γ$) cross sections for Zr isotopes.

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arXiv2017

Low-energy enhancement and fluctuations of $γ$-ray strength functions in $^{56,57}$Fe: test of the Brink-Axel hypothesis

Nuclear level densities and $γ$-ray strength functions of $^{56,57}$Fe have been extracted from proton-$γ$ coincidences. A low-energy enhancement in the $γ$-ray strength functions up to a factor of 30 over common theoretical E1 models is confirmed. Angular distributions of the low-energy enhancement in $^{57}$Fe indicate its dipole nature, in agreement with findings for $^{56}$Fe. The high statistics and the excellent energy resolution of the large-volume LaBr$_{3}$(Ce) detectors allowed for a thorough analysis of $γ$ strength as function of excitation energy. Taking into account the presence of strong Porter-Thomas fluctuations, there is no indication of any significant excitation-energy dependence in the $γ$-ray strength function, in support of the generalized Brink-Axel hypothesis.

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arXiv2017

Is the generalized Brink-Axel hypothesis valid?

Experimental results of the $^{237}$Np($d, p γ)^{238}$Np reaction are presented, which verifies the generalized Brink-Axel (gBA) hypothesis for $γ$ transitions between states in the quasi-continuum. The gBA hypothesis holds not only for specific collective resonances, but for the full dipole strength below the neutron separation energy. We discuss the validity of the gBA hypothesis also for lighter systems like $^{92}$Zr where the concept of a unique $γ$-ray strength function ($γ$SF) is problematic due to large Porter-Thomas fluctuations. Methods for studying the $γ$SF and the fluctuations as function of excitation energy are presented.

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arXiv2016

Nature of low-lying electric dipole resonance excitations in 74Ge

Isospin properties of dipole excitations in 74 Ge are investigated using the (α,α'γ) reaction and compared to (γ,γ) data. The results indicate that the dipole excitations in the energy region of 6 to 9 MeV adhere to the scenario of the recently found splitting of the region of dipole excitations into two separated parts: one at low energy, being populated by both isoscalar and isovector probes, and the other at high energy, excited only by the electromagnetic probe. Relativistic quasiparticle time blocking approximation (RQTBA) calculations show a reduction in the isoscalar E1 strength with an increase in excitation energy, which is consistent with the measurement.

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arXiv2016

Study of the 238U(d,p) surrogate reaction via the simultaneous measurement of gamma-decay and fission probabilities

We investigated the 238U(d,p) reaction as a surrogate for the n + 238U reaction. For this purpose we measured for the first time the gamma-decay and fission probabilities of 239U* simultaneously and compared them to the corresponding neutron-induced data. We present the details of the procedure to infer the decay probabilities, as well as a thorough uncertainty analysis, including parameter correlations. Calculations based on the continuum-discretized coupled-channels and distorted-wave Born approximations were used to correct our data from detected protons originating from elastic and inelastic deuteron breakup. In the region where the fission and gamma-decay probabilities compete, the corrected fission probability is in agreement with neutron-induced data, whereas the gamma-decay probability is much higher than the neutron-induced data. The performed statistical-model calculations are not able to explain these results.

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arXiv2016

Completing the nuclear reaction puzzle of the nucleosynthesis of 92Mo

One of the greatest questions for modern physics to address is how elements heavier than iron are created in extreme, astrophysical environments. A particularly challenging part of that question is the creation of the so-called p-nuclei, which are believed to be mainly produced in some types of supernovae. The lack of needed nuclear data presents an obstacle in nailing down the precise site and astrophysical conditions. In this work, we present for the first time measurements on the nuclear level density and average strength function of $^{92}$Mo. State-of-the-art p-process calculations systematically underestimate the observed solar abundance of this isotope. Our data provide stringent constraints on the $^{91}$Nb$(p,γ)^{92}$Mo reaction rate, which is the last unmeasured reaction in the nucleosynthesis puzzle of $^{92}$Mo. Based on our results, we conclude that the $^{92}$Mo abundance anomaly is not due to the nuclear physics input to astrophysical model calculations.

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arXiv2016

Experimentally constrained ($p,γ$)$^{89}$Y and ($n,γ$)$^{89}$Y reaction rates relevant to the $p$-process nucleosynthesis

The nuclear level density and the $γ$-ray strength function have been extracted for $^{89}$Y, using the Oslo Method on $^{89}$Y($p,p' γ$)$^{89}$Y coincidence data. The $γ$-ray strength function displays a low-energy enhancement consistent with previous observations in this mass region ($^{93-98}$Mo). Shell-model calculations give support that the observed enhancement is due to strong, low-energy $M1$ transitions at high excitation energies. The data were further used as input for calculations of the $^{88}$Sr($p,γ$)$^{89}$Y and $^{88}$Y($n,γ$)$^{89}$Y cross sections with the TALYS reaction code. Comparison with cross-section data, where available, as well as with values from the BRUSLIB library, shows a satisfying agreement.

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arXiv2016

The structure of low-lying states in ${}^{140}$Sm studied by Coulomb excitation

The electromagnetic structure of $^{140}$Sm was studied in a low-energy Coulomb excitation experiment with a radioactive ion beam from the REX-ISOLDE facility at CERN. The $2^+$ and $4^+$ states of the ground-state band and a second $2^+$ state were populated by multi-step excitation. The analysis of the differential Coulomb excitation cross sections yielded reduced transition probabilities between all observed states and the spectroscopic quadrupole moment for the $2_1^+$ state. The experimental results are compared to large-scale shell model calculations and beyond-mean-field calculations based on the Gogny D1S interaction with a five-dimensional collective Hamiltonian formalism. Simpler geometric and algebraic models are also employed to interpret the experimental data. The results indicate that $^{140}$Sm shows considerable $γ$ softness, but in contrast to earlier speculation no signs of shape coexistence at low excitation energy. This work sheds more light on the onset of deformation and collectivity in this mass region.

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arXiv2016

First observation of low-energy γ-ray enhancement in the rare-earth region

The γ-ray strength function and level density in the quasi-continuum of 151,153Sm have been measured using BGO shielded Ge clover detectors of the STARLiTeR system. The Compton shields allow for an extraction of the γ strength down to unprecedentedly low γ energies of about 500 keV. For the first time an enhanced low- energy γ-ray strength has been observed in the rare-earth region. In addition, for the first time both the upbend and the well known scissors resonance have been observed simultaneously for the same nucleus. Hauser-Feshbach calculations show that this strength enhancement at low γ energies could have an impact of 2-3 orders of magnitude on the (n,γ) reaction rates for the r-process nucleosynthesis.

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arXiv2016

Statistical properties of $^{243}$Pu, and $^{242}$Pu(n,$γ$) cross section calculation

The level density and gamma-ray strength function (gammaSF) of 243Pu have been measured in the quasi-continuum using the Oslo method. Excited states in 243Pu were populated using the 242Pu(d,p) reaction. The level density closely follows the constant-temperature level density formula for excitation energies above the pairing gap. The gammaSF displays a double-humped resonance at low energy as also seen in previous investigations of actinide isotopes. The structure is interpreted as the scissors resonance and has a centroid of omega_{SR}=2.42(5)MeV and a total strength of B_{SR}=10.1(15)mu_N^2, which is in excellent agreement with sum-rule estimates. The measured level density and gammaSF were used to calculate the 242Pu(n,gamma) cross section in a neutron energy range for which there were previously no measured data.

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arXiv2015

Validity of the generalized Brink-Axel hypothesis in $^{238}$Np

We have analyzed primary $γ$-ray spectra of the odd-odd $^{238}$Np nucleus extracted from $^{237}$Np($d,pγ$)$^{238}$Np coincidence data measured at the Oslo Cyclotron Laboratory. The primary $γ$ spectra cover an excitation-energy region of $0 \leq E_i \leq 5.4$ MeV, and allowed us to perform a detailed study of the $γ$-ray strength as function of excitation energy. Hence, we could test the validity of the generalized Brink-Axel hypothesis, which, in its strictest form, claims no excitation-energy dependence on the $γ$ strength. In this work, using the available high-quality $^{238}$Np data, we show that the $γ$-ray strength function is to a very large extent independent on the initial and final states. Thus, for the first time, the generalized Brink-Axel hypothesis has been experimentally verified for $γ$ transitions between states in the quasi-continuum region, not only for specific collective resonances, but also for the full strength below the neutron separation energy. Based on our findings, the necessary criteria for the generalized Brink-Axel hypothesis to be fulfilled are outlined.

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arXiv2015

Low-energy enhancement in the γ-ray strength functions of $^{73,74}$Ge

The $γ$-ray strength functions and level densities of $^{73,74}$Ge have been extracted up to the neutron separation energy S$_n$ from particle-$γ$ coincidence data using the Oslo method. Moreover, the $γ$-ray strength function of $^{74}$Ge above S$_n$ has been determined from photo-neutron measurements, hence these two experiments cover the range of E$_γ\approx$ 1-13 MeV for $^{74}$Ge. The obtained data show that both $^{73,74}$Ge display an increase in strength at low $γ$ energies. The experimental $γ$-ray strength functions are compared with $M1$ strength functions deduced from average $B(M1)$ values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in $^{73,74}$Ge are adopted in the calculations of the $^{72,73}$Ge(n,$γ$) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement.

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arXiv2015

Observation of a first-order pairing phase transition in atomic nuclei

Experimental nuclear level densities at excitation energies below the neutron threshold follow closely a constant-temperature shape. This dependence is unexpected and poorly understood. In this work, a fundamental explanation of the observed constant-temperature behavior in atomic nuclei is presented for the first time. It is shown that the experimental data portray a first-order phase transition from a superfluid to an ideal gas of non-interacting quasiparticles. Even-even, odd-$A$, and odd-odd level densities show in detail the behavior of gap- and gapless superconductors also observed in solid-state physics. These results and analysis should find a direct application to mesoscopic systems such as superconducting clusters.

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arXiv2015

Experimental level densities of atomic nuclei

It is almost 80 years since Hans Bethe described the level density as a non-interacting gas of protons and neutrons. In all these years, experimental data were interpreted within this picture of a fermionic gas. However, the renewed interest of measuring level density using various techniques calls for a revision of this description. In particular, the wealth of nuclear level densities measured with the Oslo method favors the constant-temperature level density over the Fermi-gas picture. From the basis of experimental data, we demonstrate that nuclei exhibit a constant-temperature level density behavior for all mass regions and at least up to the neutron threshold.

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arXiv2014

Level densities and thermodynamical properties of Pt and Au isotopes

The nuclear level densities of $^{194-196}$Pt and $^{197,198}$Au below the neutron separation energy have been measured using transfer and scattering reactions. All the level density distributions follow the constant-temperature description. Each group of isotopes is characterized by the same temperature above the energy threshold corresponding to the breaking of the first Cooper pair. A constant entropy excess $ΔS=1.9$ and $1.1$ $k_B$ is observed in $^{195}$Pt and $^{198}$Au with respect to $^{196}$Pt and $^{197}$Au, respectively, giving information on the available single-particle level space for the last unpaired valence neutron. The breaking of nucleon Cooper pairs is revealed by sequential peaks in the microcanonical caloric curve.

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arXiv2014

Large low-energy $M1$ strength for $^{56,57}$Fe within the nuclear shell model

A strong enhancement at low $γ$-ray energies has recently been discovered in the $γ$-ray strength function of $^{56,57}$Fe. In this work, we have for the first time obtained theoretical $γ$ decay spectra for states up to $\approx 8$ MeV in excitation for $^{56,57}$Fe. We find large $B(M1)$ values for low $γ$-ray energies that provide an explanation for the experimental observations. The role of mixed $E2$ transitions for the low-energy enhancement is addressed theoretically for the first time, and it is found that they contribute a rather small fraction. Our calculations clearly show that the high-$\ell$ ($=f$) diagonal terms are most important for the strong low-energy $M1$ transitions. As such types of $0\hbarω$ transitions are expected for all nuclei, our results indicate that a low-energy $M1$ enhancement should be present throughout the nuclear chart. This could have far-reaching consequences for our understanding of the $M1$ strength function at high excitation energies, with profound implications for astrophysical reaction rates.

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