arXiv subjects

A. Bürger

Publications and source records attributed to A. Bürger.

14 recordsLinked to original sources

arXiv2013

Transitional $γ$ strength in Cd isotopes

The level densities and $γ$-ray strength functions of $^{105,106,111,112}$Cd have been extracted from particle-$γ$ coincidence data using the Oslo method. The level densities are in very good agreement with known levels at low excitation energy. The $γ$-ray strength functions display no strong enhancement for low $γ$ energies. However, more low-energy strength is apparent for $^{105,106}$Cd than for $^{111,112}$Cd. For $γ$ energies above $\approx$ 4 MeV, there is evidence for some extra strength, similar to what has been previously observed for the Sn isotopes. The origin of this extra strength is unclear; it might be due to $E1$ and $M1$ transitions originating from neutron skin oscillations or the spin-flip resonance, respectively.

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arXiv2012

Analysis of possible systematic errors in the Oslo method

In this work, we have reviewed the Oslo method, which enables the simultaneous extraction of level density and gamma-ray transmission coefficient from a set of particle-gamma coincidence data. Possible errors and uncertainties have been investigated. Typical data sets from various mass regions as well as simulated data have been tested against the assumptions behind the data analysis.

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arXiv2012

Primary gamma-ray spectra in 44Ti of astrophysical interest

Primary gamma-ray spectra for a wide excitation-energy range have been extracted for 44Ti from particle-gamma coincidence data of the 46Ti(p,t gamma)44Ti reaction. These spectra reveal information on the gamma-decay pattern of the nucleus, and may be used to extract the level density and radiative strength function applying the Oslo method. Models of the level density and radiative strength function are used as input for cross-section calculations of the 40Ca(alpha,gamma)44Ti reaction. Acceptable models should reproduce data on the 40Ca(alpha,gamma)44Ti reaction cross section as well as the measured primary gamma-ray spectra. This is only achieved when a coherent normalization of the slope of the level density and radiative strength function is performed. Thus, the overall shape of the experimental primary gamma-ray spectra puts a constraint on the input models for the rate calculations.

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arXiv2012

Nuclear Level Density and Gamma-Ray Strength Function of 43Sc

The nuclear level density and the gamma-ray strength function have been determined for 43Sc in the energy range up to 2 MeV below the neutron separation energy using the Oslo method with the 46Ti(p,alpha)43Sc reaction. A comparison to 45Sc shows that the level density of 43Sc is smaller by an approximately constant factor of two. This behaviour is well reproduced in a microscopical/combinatorial model calculation. The gamma-ray strength function is showing an increase at low gamma-ray energies, a feature which has been observed in several nuclei but which still awaits theoretical explanation.

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arXiv2012

AGATA - Advanced Gamma Tracking Array

The Advanced GAmma Tracking Array (AGATA) is a European project to develop and operate the next generation gamma-ray spectrometer. AGATA is based on the technique of gamma-ray energy tracking in electrically segmented high-purity germanium crystals. This technique requires the accurate determination of the energy, time and position of every interaction as a gamma ray deposits its energy within the detector volume. Reconstruction of the full interaction path results in a detector with very high efficiency and excellent spectral response. The realization of gamma-ray tracking and AGATA is a result of many technical advances. These include the development of encapsulated highly-segmented germanium detectors assembled in a triple cluster detector cryostat, an electronics system with fast digital sampling and a data acquisition system to process the data at a high rate. The full characterization of the crystals was measured and compared with detector-response simulations. This enabled pulse-shape analysis algorithms, to extract energy, time and position, to be employed. In addition, tracking algorithms for event reconstruction were developed. The first phase of AGATA is now complete and operational in its first physics campaign. In the future AGATA will be moved between laboratories in Europe and operated in a series of campaigns to take advantage of the different beams and facilities available to maximize its science output. The paper reviews all the achievements made in the AGATA project including all the necessary infrastructure to operate and support the spectrometer.

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arXiv2012

Observation of large scissors resonance strength in actinides

The orbital M1-scissors resonance (SR) has been measured for the first time in the quasi-continuum of actinides. Particle-gamma coincidences are recorded with deuteron and 3He induced reactions on 232Th. The residual nuclei 231,232,233Th and 232,233Pa show an unexpectedly strong integrated strength of $B_{M1} = 11-15 μ_{n}^{2}$ in the Egamma=1.0 - 3.5 MeV region. The increased gamma-decay probability in actinides due to the SR is important for cross-section calculations for future fuel cycles of fast nuclear reactors and may also have impact on stellar nucleosynthesis.

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arXiv2012

In-beam spectroscopic studies of $^{44}$S nucleus

The structure of the $^{44}$S nucleus has been studied at GANIL through the one proton knock-out reaction from a $^{45}$Cl secondary beam at 42 A$\cdot$MeV. The $γ$ rays following the de-excitation of $^{44}$S were detected in flight using the 70 BaF${_2}$ detectors of the Château de Cristal array. An exhaustive $γγ$-coincidence analysis allowed an unambiguous construction of the level scheme up to an excitation energy of 3301 keV. The existence of the spherical 2$^+_2$ state is confirmed and three new $γ$-ray transitions connecting the prolate deformed 2$^+_1$ level were observed. Comparison of the experimental results to shell model calculations further supports a prolate and spherical shape coexistence with a large mixing of states built on the ground state band in $^{44}$S.

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arXiv2011

Evolution of the pygmy dipole resonance in Sn isotopes

Nuclear level density and $γ$-ray strength functions of $^{121,122}$Sn below the neutron separation energy are extracted with the Oslo method using the ($^3$He,$^3$He$^\primeγ$) and ($^3$He,$αγ$) reactions. The level densities of $^{121,122}$Sn display step-like structures, interpreted as signatures of neutron pair breaking. An enhancement in both strength functions, compared to standard models for radiative strength, is observed in our measurements for $E_γ\gtrsim 5.2 $ MeV. This enhancement is compatible with pygmy resonances centered at $\approx 8.4(1)$ and $\approx 8.6(2)$ MeV, respectively, and with integrated strengths corresponding to $\approx1.8^{+1}_{-5}%$ of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in $^{116-119}$Sn. Experimental neutron-capture cross reactions are well reproduced by our pygmy resonance predictions, while standard strength models are less successful. The evolution as a function of neutron number of the pygmy resonance in $^{116-122}$Sn is described as a clear increase of centroid energy from 8.0(1) to 8.6(2) MeV, but with no observable difference in integrated strengths.

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arXiv2011

The SiRi Particle-Telescope System

A silicon particle-telescope system for light-ion nuclear reactions is described. In particular, the system is designed to be optimized for level density and gamma-ray strength function measurements with the so-called Oslo method. Eight trapezoidal modules are mounted at 5 cm distance from the target, covering 8 forward angles between theta = 40 and 54 degrees. The thin front dE detectors (130 micrometer) are segmented into eight pads, determining the reaction angle for the outgoing charged ejectile. Guard rings on the thick back E detectors (1550 micrometer) guarantee low leakage current at high depletion voltage.

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arXiv2010

Radiative strength functions in 163,164Dy

The nuclei 163,164Dy have been investigated using the Oslo method on data from the pick-up reaction 164Dy(3He,alpha gamma)163Dy and the inelastic scattering 164Dy(3He,3He' gamma)164Dy, respectively. The radiative strength functions for both nuclei have been extracted, and a small resonance centered around Eg ?3 MeV is observed in both cases. The parameters of this so-called pygmy M1 resonance (the scissors mode) are compared to previous results on 160,161,162Dy using the Oslo method, and to data on 163Dy measured by the Prague group using the two-step cascade method. In particular, the integrated reduced transition probability B(M1) of the pygmy resonance is compared with neighboring dysprosium isotopes. We also observe an enhanced strength in the region above gamma energy around 5 MeV in 164Dy. Possible origins of this feature are discussed.

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arXiv2010

Level densities and $γ$-ray strength functions in Sn isotopes

The nuclear level densities of $^{118,119}$Sn and the $γ$-ray strength functions of $^{116,118,119}$Sn below the neutron separation energy are extracted with the Oslo method using the ($^3$He, \,$αγ$) and ($^3$He,$^3$He$^\primeγ$) reactions. The level density function of $^{119}$Sn displays step-like structures. The microcanonical entropies are deduced from the level densities, and the single neutron entropy of $^{119}$Sn is determined to be $(1.7 \pm 0.2)\,k_B$. Results from a combinatorial model support the interpretation that some of the low-energy steps in the level density function are caused by neutron pair-breaking. An enhancement in all the $γ$-ray strength functions of $^{116-119}$Sn, compared to standard models for radiative strength, is observed for the $γ$-ray energy region of $\simeq (4 -11)$ MeV. These small resonances all have a centroid energy of 8.0(1) MeV and an integrated strength corresponding to $1.7(9)\%$ of the classical Thomas-Reiche-Kuhn sum rule. The Sn resonances may be due to electric dipole neutron skin oscillations or to an enhancement of the giant magnetic dipole resonance.

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arXiv2009

Structure of 55Ti from relativistic one-neutron knockout

Results are presented from a one-neutron knockout reaction at relativistic energies on 56Ti using the GSI FRS as a two-stage magnetic spectrometer and the Miniball array for gamma-ray detection. Inclusive and exclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the orbital angular momentum of the populated states. First-time observation of the 955(6) keV nu p3/2-hole state in 55Ti is reported. The measured data for the first time proves that the ground state of 55Ti is a 1/2- state, in agreement with shell-model calculations using the GXPF1A interaction that predict a sizable N=34 gap in 54Ca.

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arXiv2007

Collapse of the N=28 shell closure in $^{42}$Si

The energies of the excited states in very neutron-rich $^{42}$Si and $^{41,43}$P have been measured using in-beam $γ$-ray spectroscopy from the fragmentation of secondary beams of $^{42,44}$S at 39 A.MeV. The low 2$^+$ energy of $^{42}$Si, 770(19) keV, together with the level schemes of $^{41,43}$P provide evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that $^{42}$Si is best described as a well deformed oblate rotor.

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arXiv2007

A Database for Fast Access to Particle-Gated Event Data

In nuclear physics experiments involving in-flight fragmentation of ions, usually a large number of different nuclei is produced and various detection systems are employed to identify the species event by event, e.g. by measuring their specific energy loss and time-of-flight. For such cases -- not necessarily limited to nuclear physics -- where subsets of a large dataset can be identified using a small number of measured signals a software for fast access to varying subsets of such a dataset has been developed. The software has been used successfully in the analysis of a one neutron knock-out experiment at GANIL.

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