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The low-lying dipole strength of the deformed nucleus 156Gd was investigated in the energy region from 3.1 MeV to 6.2 MeV using the method of nuclear resonance fluorescence (NRF). The NRF experiments were performed at the Darmstadt High Intensity Photon Setup (DHIPS) at Technische Universitat Darmstadt using unpolarized continuous-energy bremsstrahlung and at the High-Intensity γ-ray Source (HIγS) at Duke University using quasi-monoenergetic and linearly-polarized photon beams. The combination of both experiments allows to separate electric and magnetic contributions and to determine absolute transition strengths for individual excited states as well as averaged quantities over narrow excitation energy regions. The investigated energy regions cover the region of the scissors mode as well as the low-energy part of the Pygmy Dipole Resonance. This is the first experiment where both of these excitation modes as well as the region in between has been successfully studied in a deformed heavy nucleus using the NRF method.

Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink–Axel hypothesis and the related concept of so-called photon strength functions to describe γ-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasi-monochromatic photon beams and a newly implemented γ–γ coincidence setup. This technique does not assume a priori the validity of the Brink–Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus $^{128}$Te were obtained for γ-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink–Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink–Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold (Sn=8.78MeV) for the studied case of $^{128}$Te.

International audience; The nature of the high-lying final levels of the 96Ygsβ decay, one of the three most important contributors to the high-energy reactor antineutrino spectrum, has been investigated in high-resolution γ-ray spectroscopy following the β decay as well as in a campaign of inelastic photon scattering experiments. The comprehensive approach establishes 1− levels associated with the Pygmy Dipole Resonance as high-lying final levels in the β decay. Branching ratios extracted from β decay complement photon scattering and allow the absolute E1 excitation strength to be determined for levels populated in both reactions. The combined data represents a comprehensive approach to the wavefunction of the 1− levels below the Qβ value, which are investigated in the Quasiparticle Phonon Model. The calculations reveal that the components populated in β decay contribute only with small amplitudes to the complex wavefunction of these 1− levels. A comparison of the β decay results to data from total absorption γ-ray spectroscopy demonstrates a good agreement between both measurements.

In this paper we propose a pulse shape based method for monitoring the interior temperature of a CsI(Tl) crystal in order to correct the temperature dependence in the energy calibration of the corresponding detector system. The gain dependence on temperature of the CsI(Tl) detector was measured using both, a photomultiplier tube (PMT) and an avalanche photodiode (APD) readout photosensor. The analysis shows that the gain shifts due to temperature variations can be corrected to a precision of better than 1% with both the PMT and the APD, well below the CsI(Tl) intrinsic energy resolution for ~1 MeV γ-rays.

A new setup for the characterization of γ - ray detectors has been installed at the NEPTUN photon tagger facility of TU Darmstadt. The tagging technique used at NEPTUN provides a quasi monoenergetic photon source up to about 20 MeV by selecting single γ - ray energies within a bremsstrahlung spectrum. The energy is freely selectable by changing the tagging condition. The detector response function (DRF) of γ - ray detectors for quasi monoenergetic incident photons can be measured. This allows to investigate DRFs of various photon detectors as a function of the incident γ - ray energy. Simulations of DRFs that are intensively used in the analysis of nuclear physics experiments can be tested and compared to experimental data. The experimental setup is presented and the measurement of the DRF of a large volume high-purity Germanium detector is described as an example.

A new approach for constraining the low-energy part of the electric dipole Photon Strength Function (E1-PSF) is presented. Experiments at the Darmstadt High-Intensity Photon Setup and the High Intensity γ → -Ray Source have been performed to investigate the decay properties of 130Te between 5.50 and 8.15 MeV excitation energy. In particular, the average γ-ray branching ratio to the ground state and the population intensity of low-lying excited states have been studied. A comparison to the statistical model shows that the latter is sensitive to the low-energy behavior of the E1-PSF, while the average ground state branching ratio cannot be described by the statistical model in the energy range between 5.5 and 6.5 MeV.

The applicability of LaBr3:Ce detectors for high count rate γ - ray spectroscopy is investigated. A 3 in.×3 in. LaBr3:Ce detector is used in a test setup with radioactive sources to study the dependence of energy resolution and photo peak efficiency on the overall count rate in the detector. Digitized traces were recorded using a 500 MHz FADC and analysed with digital signal processing methods. Good performance is obtained using standard techniques up to about 500 kHz counting rate. A pile-up correction method is applied to the data in order to further improve the capabilities at even higher rates with a focus on recovering the losses in efficiency due to signal pile-up. It is shown that γ - ray spectroscopy can be performed with only moderate lossen in efficiency and high resolution at count rates even above 1 MHz and that the performance can be enhanced in the region between 500 kHz and 10 MHz by using the applied pile-up correction techniques.

The Darmstadt High-Intensity Photon Setup (DHIPS) for photon-scattering and photodissociation experiments at the superconducting electron linear accelerator S–DALINAC has been improved. We report on its present performance. The developments include the installation of a segmented radiator for an online evaluation of the beam energy and the construction of a second photon-scattering setup with a Clover detector to deduce parity assignments. The enhancement of the existing photon-scattering setup with an additional 100% HPGe detector and some minor changes are described as well as the installation of two setups for photon-induced activation. In addition, we present a comparison of experimental data to Monte Carlo simulations using the codes Geant 3.21 and GEANT4.

A new approach to pulse shape classification for neutron detectors of type BC501A has been investigated. The method is based on the fuzzy c-means (FCM) algorithm, which allows finding clusters of similar shapes in a set of digitized detector pulses. The aim of the method is to provide principal pulse shapes, which further can be used to apply a pulse shape based particle identification. Since the method is not adapted to the case of liquid scintillator signals it is of general use and can also be applied to signals of other detector types. A detailed study of the quality of the FCM method for the search of principal pulse shapes in BC501A liquid scintillators is presented using a 500 Msample/s 12 bit digitizer and a 252Cf neutron source. A comparison to principal pulse shapes extracted using time-of-flight information proves the applicability of the method. Finally, an example of a pulse shape discrimination method based on the extracted principal pulse shapes is presented and compared to the well-known integration method.

A new photon tagging spectrometer was built at the superconducting Darmstadt electron linear accelerator (S-DALINAC). The system is designed for tagging photons in an energy range from 6 to 20 MeV with the emphasis on best possible energy resolution and intensity. The absolute energy resolution of photons at 10 MeV is expected to be about 20 keV. With scintillating fibres as focal-plane detectors a maximum rate of tagged photons of 10 4 keV-1s-1 will be achieved. Detailed design studies including Monte Carlo simulations are presented, as well as results for the measured tagged photon energy profile of the system realized so far. This photon-tagging facility will allow to determine the photon absorption cross-sections as a function of excitation energy and to study the decay patterns of nuclear photo-excitations in great detail.