arXiv subjects

T. W. Hagen

Publications and source records attributed to T. W. Hagen.

At least 19 recordsLinked to original sources

arXiv2022

Evolution of the $γ$-ray strength function in neodymium isotopes

The experimental gamma-ray strength functions (gamma-SFs) of 142,144-151Nd have been studied for gamma-ray energies up to the neutron separation energy. The results represent a unique set of gamma-SFs for an isotopic chain with increasing nuclear deformation. The data reveal how the low-energy enhancement, the scissors mode and the pygmy dipole resonance evolve with nuclear deformation and mass number. The data indicate that the mechanisms behind the low-energy enhancement and the scissors mode are decoupled from each other.

nucl-ex↗

arXiv2020

Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes

Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes $^{142,144-151}$Nd which exhibit such a crossover. These results represent to date the most complete data set of nuclear level densities for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of $\approx 170$ at an excitation energy of 7.5 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement.

nucl-ex↗

arXiv2019

Nuclear level densities and gamma-ray strength functions of $^{180,181,182}$Ta

Particle-$γ$ coincidence experiments were performed at the Oslo Cyclotron Laboratory with the $^{181}$Ta(d,X) and $^{181}$Ta($^{3}$He,X) reactions, to measure the nuclear level densities (NLDs) and $γ$-ray strength functions ($γ$SFs) of $^{180, 181, 182}$Ta using the Oslo method. The Back-shifted Fermi-Gas, Constant Temperature plus Fermi Gas, and Hartree-Fock-Bogoliubov plus Combinatorial models where used for the absolute normalisations of the experimental NLDs at the neutron separation energies. The NLDs and $γ$SFs are used to calculate the corresponding $^{181}$Ta(n,$γ$) cross sections and these are compared to results from other techniques. The energy region of the scissors resonance strength is investigated and from the data and comparison to prior work it is concluded that the scissors strength splits into two distinct parts. This splitting may allow for the determination of triaxiality and a $γ$ deformation of $14.9^{\circ} \pm 1.8^{\circ}$ was determined for $^{181}$Ta.

nucl-ex↗

arXiv2019

Evidence for coexisting shapes in $^{98}$Zr through lifetime measurements

The lifetimes of first excited 2$^+$, 4$^+$ and 6$^+$ states in $^{98}$Zr were measured with the Recoil-Distance Doppler Shift method in an experiment performed at GANIL. Excited states in $^{98}$Zr were populated using the fission reaction between a 6.2 MeV/u $^{238}$U beam and a $^{9}$Be target. The $γ$ rays were detected with the EXOGAM array in correlation with the fission fragments identified in mass and atomic number in the VAMOS++ spectrometer. Our result shows very small B(E2;2$_1^+$ $\rightarrow$ 0$_1^+$) value in $^{98}$Zr thereby confirming the very sudden onset of collectivity at $N=60$. The experimental results are compared to large-scale Monte Carlo Shell model and beyond mean field calculations. The present results indicate coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.

nucl-ex↗

arXiv2018

Experimental $γ$-decay strength in $^{59, 60}$Ni compared with microscopic calculations

Nuclear level densities and $γ$-ray strength functions have been extracted for $^{59, 60}\rm{Ni}$, using the Oslo method on data sets from the $^{60}$Ni($^{3}$He,$^{3}$He$^{\prime}γ$)$^{60}$Ni and $^{60}$Ni($^{3}$He,$αγ$)$^{59}$Ni reactions. Above the neutron separation energy, S$_n$, we have measured the $γ$-ray strength functions for $^{61}$Ni and $^{60}$Ni in photoneutron experiments. The low-energy part of the $^{59,60}$Ni $γ$-ray strength functions show an increase for decreasing $γ$ energies. The experimental $γ$-ray strength functions are compared with $M1$ $γ$-ray strength functions calculated within the shell model. The $E1$ $γ$-ray strength function of $^{60}$Ni has been calculated using the QTBA framework. The QTBA calculations describe the data above $E_γ\approx$ 7 MeV, while the shell-model calculations agree qualitatively with the low energy part of the $γ$-ray strength function. Hence, we give a plausible explanation of the observed shape of the $γ$-decay strength.

nucl-ex↗

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.

nucl-ex↗

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.

nucl-ex↗

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.

nucl-ex↗

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.

nucl-ex↗

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.

nucl-ex↗

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.

nucl-ex↗

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.

nucl-ex↗

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.

nucl-ex↗

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.

nucl-ex↗

arXiv2014

Shell-gap reduced level densities in $^{89,90}$Y

Particle-$γ$ coincidences from the $^{89}$Y($p,p' γ$)$^{89}$Y and $^{89}$Y($d,p γ$)$^{90}$Y reactions were utilized to obtain $γ$-ray spectra as function of excitation energy. The Oslo method was used to extract the level density from the particle-$γ$ coincidence matrices. The impact of the $N=50$ shell closure on the level densities is discussed within the framework of a combinatorial quasi-particle model.

nucl-ex↗

arXiv2014

Evidence for dipole nature of the low-energy $γ$ enhancement in $^{56}$Fe

The $γ$-ray strength function of $^{56}$Fe has been measured from proton-$γ$ coincidences for excitation energies up to $\approx 11$ MeV. The low-energy enhancement in the $γ$-ray strength function, which was first discovered in the ($^3$He,$αγ$)$^{56}$Fe reaction, is confirmed with the ($p,p^\primeγ$)$^{56}$Fe experiment reported here. Angular distributions of the $γ$ rays give for the first time evidence that the enhancement is dominated by dipole transitions.

nucl-ex↗

arXiv2014

Observation of double pygmy resonances in $^{195,196}$Pt and enhanced astrophysical reaction rates

Our measurements of $^{195,196}$Pt $γ$-strength functions show a double-humped enhancement in the $E_γ= 4-8$ MeV region. For the first time, the detailed shape of these resonances is revealed for excitation energies in the quasicontinuum. We demonstrate that the corresponding neutron-capture cross sections and astrophysical reaction rates are increased by up to a factor of 2 when these newly observed pygmy resonances are included. These results lend credence to theoretical predictions of enhanced reaction rates due to such pygmy resonances and hence are important for a better understanding of r-process nucleosynthesis.

nucl-ex↗

arXiv2013

Neutron-skin thickness of $^{208}$Pb, and symmetry-energy constraints from the study of the anti-analog giant dipole resonance

The $^{208}$Pb($p$,$nγ\bar p$) $^{207}$Pb reaction at a beam energy of 30 MeV has been used to excite the anti-analog of the giant dipole resonance (AGDR) and to measure its $γ$-decay to the isobaric analog state in coincidence with proton decay of IAS. The energy of the transition has also been calculated with the self-consistent relativistic random-phase approximation (RRPA), and found to be linearly correlated to the predicted value of the neutron-skin thickness ($ΔR_{pn}$). By comparing the theoretical results with the measured transition energy, the value of 0.190 $\pm$ 0.028 fm has been determined for $ΔR_{pn}$ of $^{208}$Pb, in agreement with previous experimental results. The AGDR excitation energy has also been used to calculate the symmetry energy at saturation ($J=32.7 \pm 0.6$ MeV) and the slope of the symmetry energy ($L=49.7 \pm 4.4$ MeV), resulting in more stringent constraints than most of the previous studies.

nucl-ex↗