The inelastic scattering of fast neutrons on 56 Fe was investigated in different manners at the neutron time-of-flight facility n ELBE. The scattering cross section was determined via the measurement of the γ-ray production and by means of a kinematically complete double time-of-flight method. In a further measurement the γ-ray angular distribution was determined to correct the measured cross sections for anisotropy. The resulting inelastic scattering cross section determined from the photo production cross sections is in very good agreement with evaluations and previous measurements. In contrast, the result of the double time-of-flight measurement is about 10% lower than these data, giving a hint to neutron-γ-ray angular correlations in the process of inelastic neutron scattering.

The photodissociation of the deuteron is a key reaction in Big Bang nucleosynthesis, but is only sparsely measured in the relevant energy range. To determine the cross section of the d(gamma,n)p reaction we used pulsed bremsstrahlung and measured the time-of-flight of the neutrons. In this article, we describe how the efficiency of the neutron detectors was experimentally determined and how the modification of the neutron spectrum by parts of the experimental setup was simulated and corrected.

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A = 124, 128, 132, and 134 in nuclear resonance fluorescence experiments using the ELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIgS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides.

Neutron total cross sections of 197 Au and nat Ta have been measured at the nELBE photoneutron source in the energy range 0.1–10MeV with a statistical uncertainty of up to 2% and a total systematic uncertainty of 1%. This facility is optimized for the fast neutron energy range and combines an excellent time structure of the neutron pulses (electron bunch width 5ps) with a short flight path of 7m. Because of the low instantaneous neutron flux transmission measurements of neutron total cross sections are possible, that exhibit very different beam and background conditions than found at other neutron sources.

The dipole strength in the nucleus Pt-196 was investigated using two different experimental methods. The photon spectrum from the deexcitation of a state after cold neutron capture in Pt-195 is influenced by the dipole strength and nuclear level density in Pt-196 as is the gamma-ray spectrum from photon scattering on Pt-196. In a combined analysis of data from the research reactor in Budapest and the bremsstrahlung facility at the ELBE accelerator in Dresden, the GEANT4 code was used to calculate detector response and efficiency. Also the influence of non-nuclear scattered photons was determined and allows us to take into account the continuum of unresolved states. The statistical code gamma DEX was used to estimate branching ratios and compare simulated and experimental spectra. Using information from both experiments it was possible to obtain a temperature parameter of 600 keV for the constant temperature level density model. For the dipole strength a small extra strength over the tail of the giant dipole resonance in the region below the neutron separation energy was found. DOI: 10.1103/PhysRevC.87.044306

We describe the photo-neutron source at the superconducting electron accelerator ELBE of the Helmholtz-Zentrum Dresden-Rossendorf and present first experiments to determine cross sections of inelastic scattering of neutrons in the MeV range. We discuss analysis and results of photon-scattering experiments using the bremsstrahlung facility at ELBE. A consistent determination of the dipole strength function from the combination of photon scattering and radiative neutron capture is presented.

The neutron time-of-flight setup nELBE at the Helmholtz-Zentrum Dresden-Rossendorf uses an intense electron beam impinging on a liquid-lead target to produce neutrons in the energy range from about 10keV to 10MeV. This neutron source will be used to measure fast-neutron induced reactions with relevance for future nuclear transmutation facilities and nuclear waste management. The spatial profile, the intensity, the energy distribution and the time structure of the nELBE neutron beam have been investigated and the techniques how they were measured are explained in this work.

The dipole strength function and the nuclear level density of the compound nucleus Se-78 were studied in a combined analysis of a cold neutron capture experiment on Se-77 performed at the research reactor in Budapest and a photon-scattering experiment on Se-78 performed at the electron linear accelerator ELBE with bremsstrahlung produced at a kinetic electron energy of 11.5 MeV. In the combined analysis we developed the extreme statistical code gamma DEX for the simulation of radiative cascade deexcitations occurring in neutron capture and photon scattering. Comparisons of experimental and simulated neutron capture spectra allow us to estimate a temperature of T = 900 keV for the level density according to the constant-temperature model for Se-78. Using gamma DEX, we were also able to estimate ground-state branching ratios and intensities of inelastic transitions for states in Se-78 excited via photon scattering. In this way, we derived the photoabsorption cross section from 4 MeV up to the neutron separation energy from the measured photon-scattering data. The results obtained match the photoabsorption cross section derived from (gamma, n) measurements and show an enhancement of dipole strength around 9 MeV.

First results on the neutron-induced fission cross-section of Pa-231 for incident neutron energies E-n > 17 MeV are presented. The experiments were carried out with quasi mono-energetic neutrons produced in the reaction T(d, n)He-4. Corrections for low-energy neutron background produced in this reaction at incident deuteron energies E-d > 2 MeV are taken into account and based on experimental data obtained by two different techniques. Despite the relatively large error bars at the higher neutron energies, the new cross-section values meet the accuracy requirements set by the IAEA and will allow to remove the hitherto existing large spread between different previously published data. Recent cross-section calculations describe well the new experimental results, which are in consistency with cross-section values obtained in a particle-transfer reaction at excitation energies corresponding to neutron energies E-n < 10 MeV. (C) 2011 Elsevier Ltd. All rights reserved.

Using the methods of cold neutron capture and photon scattering the electric dipole strength function and the level density of the nuclei 78 Se and 196 Pt are investigated. Considering that the deexcitation process could be described by the same strength func- tions one could describe both experiments in a statistical model code. The report shows the data analysis as well as a new very fast statistical code, which was used to get the complete strength information up to the neutron separation energy.

A global parameterization is presented for the electromagnetic strength in heavy nuclei which gives a rather good fit to respective data in nuclei with mass numbers A between 50 and 240. It relies on a Lorentzian description of the isovector giant dipole resonance and it needs only a very small number of parameters to describe the electric dipole strength down to low excitation energy of importance for radiative capture processes. The resonance energies are chosen to be in accordance to liquid drop model parameters adjusted to ground state masses and to rotation invariant determinations of ground state deformation and triaxiality. By a straightforward use of this information a surprisingly smooth variation of. the GDR width. with A and Z is found and a full agreement to the predictions of the electromagnetic sum rule is assured. Predictions for radiative neutron capture cross sections compare well to respective data, when the proposed photon strength function is combined with standard prescriptions for the level density in the product nuclei.

The nELBE neutron time-of-flight facility has become operational at the ELBE superconducting linear accelerator at Forschungszentrum Dresden-Rossendorf. Fast neutrons in the energy range from ca. 0.1 to 10 MeV are produced by the pulsed electron beam from ELBE impinging on a liquid lead circuit as a radiator. The short beam pulses of ∼10 ps provide the basis for an excellent time resolution for neutron time-of-flight experiments, giving an energy resolution of about ≪ 1 % at 1 MeV with a short flight path of ∼ 5 m. The neutron intensity on target is ca. 2*104 n/s using an electron bunch charge of 77 pC and 100 kHz pulse repetition rate. A new superconducting RF injector that has been built at ELBE will increase the bunch charge to 2 nC.

The dipole strength function of 78 Se and 196 Pt are investigated by two different experimental methods, capture of cold neutrons in 77 Se and 195 Pt and photon scattering experiments on 78 Se and 196 Pt. Considering the different ways of excitation, the strength function deduced from the results are expected to agree.The report shows the status of the data analysis and presents first preliminary results.

The neutron time of flight facility nELBE, produces fast neutrons in the energy range from 0.1 MeV to 10 MeV by impinging a pulsed relativistic electron beam on a liquid lead circuit [1]. The short beam pulses (∼10 ps) and a small radiator volume give an energy resolution better than 1% at 1 MeV using a short flight path of about 6 m, for neutron TOF measurements. The present neutron source provides 2 ⋅ 104 n/cm2s at the target position using an electron charge of 77 pC and 100 kHz pulse repetition rate. This neutron intensity enables to measure neutron total cross section with a 2%–5% statistical uncertainty within a few days. In February 2008, neutron radiator, plastic detector [2] and data acquisition system were tested by measurements of the neutron total cross section for 181Ta and 27Al. Measurement of 181Ta was chosen because lack of high quality data in an anergy region below 700 keV. The total neutron cross – section for 27Al was measured as a control target, since there exists data for 27Al with high resolution and low statistical error [3].

At the Forschungszentrum Dresden-Rossendorf a new neutron time-of-flight [Klug07] facility has been set up. Fast neutrons in the energy range from 0.1 MeV to 10 MeV are produced using pulsed electron beams from a superconducting electron linear accelerator[Gabriel00]. Short beam pulses of less than 10 ps allow high-resolution time-of-flight experiments with the aim to determine interaction cross sections of neutrons with reactor structural materials and actinides at energies matching the neutron energies in fast reactors [Salvatores08]. Following experiments using a thermionic electron injector a new superconducting radiofrequency injector [Arnold07] has been built which will allow average beam currents of 0.5 mA at a repetition rate of 500 kHz. Meanwhile, first experiments on inelastic neutron scattering cross sections on Fe and total neutron cross sections on aluminium and tantalum have been performed as benchmark experiments. While photons from the de-exciting transitions are being detected using a 16element BaF2 scintillator array, neutrons are measured with five low-threshold plastic scintillation detectors [Beyer07]. Beam normalisation is done using a calibrated U fission chamber. The preparation of actinide targets for neutron induced fission cross section measurements is under way. Experimental Setup The radiation source ELBE (Electron Linear Accelerator with High Brilliance and Low Emittance) at Forschungszentrum Dresden-Rossendorf (FZD) makes use of a superconducting continuous-wave electron linear accelerator delivering beams with energies up to 50 MeV. The high-intensity beam with average beam currents of up to 1 mA at pulse repetition rates up to 26 MHz serves as a driver for the production of various secondary beams. Figure1 shows the layout of the accelerator facility and the beam lines for secondary radiations. Table 1 lists the available secondary beams and their primary applications. As one of the secondary beams, an intense neutron beam is being produced by bremsstrahlung photons via the (γ,n)-process inside a liquid-lead circuit. The lead acts threefold as the electron-to-bremsstrahlung convertor, as the source of evaporation neutrons, as well as the heat removal medium in order to cope with the designed average electron beam power of * Corresponding author: a.wagner@fzd.de Fig. 1: Layout of the accelerator facility ELBE. Two superconducting accelerator structures are located inside the accelerator hall and the liquid-lead loop is situated on the upper right side inside the neutron hall. A new superconducting radio-frequency injector is already installed (SRF gun) which will allow for increase in electron bunch charge by a factor of 15 as compared to the standard thermoionic injector. Recently, a 100 TW laser systems has been set up which serves for laser-Thomson electron-scattering experiments and laser-plasma particle acceleration experiments. about 50 kW. Monte Carlo simulations using MCNP and GEANT4 were performed to characterise the neutron and photon intensities as well as time and energy distributions, and to optimise the neutron transport and shielding of the experimental setup. The design of the liquid-lead loop is shown in Fig. 2. The design focuses on combining a small active volume of neutron production with a correspondingly high local heat load of about 5 kW / g. As one of the possible solutions, a liquid-lead loop operated at a temperature of about 630 K has been selected which is also favourable by means of induced radioactivity caused by various (,xn)and (,p)-reactions as compared to e.g. liquid mercury. Nevertheless, the liquid-lead loop has to be stored during shutdown phases inside a lead shielding of 200 mm thickness in order to grant access to the vault for maintenance and other experiments [Seidel07]. While using lead as material for efficient neutron production (neutron separation energies Sn from 6.7 to 8.1 MeV) the beam dump is made from pure aluminium (>99.9%) resulting in low induced activities and low neutron generation (neutron separation energy of Al: Sn=13.1 MeV). The short beam pulses (about 5 ps FWHM) delivered by the superconducting electron accelerator provide the basis for an excellent time resolution for neutron time-of-flight experiments while the pulse repetition rates can be varied between 100 kHz and 26 MHz according to the demands of the experiments. With the existing electronically pulsed thermionic electron source electron pulse charges of up to 80 pC are realized allowing for a moderate average electron beam current of 16 μA at a repetition rate of 200 kHz. Fig. 2: Three-dimensional rendering of the liquid-lead loop installed at the ELBE facility. For reasons of radiation protection the platform containing the lead loop can be lowered into a lead housing by means of a remotely controlled spindle lifter. The intense electron beam impinges from the front left while the beam transport system is not shown here. The lead flow is driven by means of a magneto-hydrodynamic pump thus avoiding mechanical contact with the fluid and avoiding mechanical feedthroughs. Thermal insulation, heating units, and support systems are not shown. The beam dump is made from aluminium surrounded by lead in backward and radial directions. A new superconducting radio-frequency injector is already installed which in the near future will allow for an increase in electron bunch charge by a factor of 15 and repetition rates of 500 kHz. The short beam pulses together with the small neutron-production volume in the lead circuit allow for an energy resolution of about 1% with a flight path of 6 m when using a fast detector stop signal (e.g., 1 ns for 1.5 MeV neutrons). Neutrons emerging from the liquid-lead radiator are shaped into a beam using a 240 cm long cylindrically symmetric collimator made from borated polyethylene and lead, greatly reducing scattered-neutron background and photons at the target position. Around the target, an array of 16 BaF2 scintillation detectors for the detection of secondary photons has been installed. Each detector consists of two 190 mm long prisms with hexagonal bases of 53 mm inner radius glued together and read out on both ends using fast photomultiplier tubes (PMT). The double-sided readout helps to reduce background signals stemming from anode dark-currents, it improves the energy resolution, and it permits the determination of the longitudinal hit position. The signals from the PMTs are processed in † Hamamatsu Photonics K.K., http://www.hamamatsu.com, PMT: R2059-01 selected with quartz windows. dedicated VME-based readout units allowing for pulse-shape discrimination of charged particles and photons. Fig. 3: Sketch of the setup used in neutron inelastic scattering experiments. Part a) shows the top view, and part b) shows the side view of the detector and target arrangements. Solid (dashed) lines refer to neutron (photon) tracks. Electron-optical elements are not shown. Drawing is not to scale. Scattered neutrons are detected using proton-recoil plastic scintillation detectors at a distance of about 1 m from the target. The plastic scintillation detectors are strips of 1000 mm length and 42 mm x 11 mm cross section read-out double-sided using the same PMTs as described above. The trigger threshold is selected to detect single photo-electrons permitting detection limits for neutrons as low as 30 keV for detection efficiencies above 10%. The design, operation, and calibration of these detectors are described in a previous publication [Beyer07]. The system is optimized for high time resolution of the time-of-flight detectors reaching about 600 ps FWHM for the BaF2-detectors and about 860 ps FWHM for the plastic scintillation detectors in order to allow a compact setup ensuring high beam repetition rates with low pulse-to-pulse overlap. Scattered neutrons and photons originating from the BaF2-array are being suppressed by borated polyethylene absorbers in the direction of the neutron detectors. The distribution of random background has been determined in measurements without target. Neutron flux determination is done using a calibrated U-235 fission chamber [Gayther90]. The time resolution obtained using photo-induced fission amounts to about 4 ns. The data-acquisition system is controlled by a VME-based computer running the real-time operating system LynxOS and the versatile data acquisition system MBS. The readout of the double-sided BaF2-detectors is done using CAEN V874b calorimeter units providing t energy information, while the plastic-scintillation neutron-detector information is processed by in-house made constant-fraction discriminators. CAEN V1190a time-to-digital converters provide the timing information for both detectors. he ‡ Eljen Technology, http://www.eljentechnology.com, EJ-200 scintillator. § Creative Electronics Systems, http://www.ces.ch, RIO3 8064 single board computer. ** GSI Multi-Branch System, http://daq.gsi.de. †† CAEN s.p.A., http://www.caen.it Experiments Several experiments on interactions of fast neutrons with materials of interest for fast reactors have been performed. As a preliminary example, the inelastic scattering on iron with natural isotopic composition has been studied. Although iron is not a material of interest for transmutation studies, its importance as structural material demands high-precision data for the interaction of fast neutrons occurring in fast reactors, transmutation facilities, or accelerator-driven systems [Salvatores08]. The detector setup is shown in Figure 3. List-mode data are taken for coincident signals in the BaF2-array and the neutron-detector array selecting primarily reactions with at least one photon and one neutron in the exit channel. Time-of-flight calibration is done using the photon flash stemming from scattered bremsstrahlung inside the radiator. The target consisted of a cylindrical slab of natural iron (isotopic composition: 5.85% Fe, 91.75% Fe, 2.12% Fe, 0.28% Fe) w

Information on the photon strength in heavy nuclei with mass A > 150 will be given and compared to respective data. The photon strength function is a very important ingredient for statistical model calculations – especially when these are used to describe neutron capture. Several schemes for a transmutation of radioactive waste favor nuclear reactions with fast neutrons and these also influence the performance of various reactor types proposed to deliver nuclear energy together with only small quantities of such waste. Reactions with fast neutrons are less well studied as compared to those induced by thermal neutrons. As they are not easily accessible experimentally, reference is often made to calculations using the statistical model. Photon emission probabilities are needed as input to such calculations aiming for predictions on fission to capture ratios. Results of approximate calculations for radiative neutron capture will be presented.

At the superconducting electron linear accelerator ELBE at Forschungszentrum Dresden-Rossendorf the neutron time-of-flight facility nELBE has become operational. Fast neutrons in the energy range from 200 keV to 10 MeV are produced by the pulsed electron beam from ELBE impinging on a liquid lead circuit as a radiator. The short beam pulses of 10 ps provide the basis for an excellent time resolution for neutron time-of-flight experiments, giving an energy resolution of about <1% at 1 MeV with a short flight path of 5 m. By means of a “double-time-of-flight” setup the (n,nâγ) cross section to the first excited state of 56Fe has been measured over the whole energy range without knowledge about cross sections of higher-lying levels. Plastic scintillators were used to detect the inelastically scattered neutron and BaF2 detectors to detect the correlated γ-ray.

The nELBE beamline at Forschungszentrum Dresden-Rossendorf (FZD) provides intense neutron beams by stopping primary electrons in a liquid lead target, where neutrons are produced by bremsstrahlung photons via (γ,n) reactions. With the aim to increase the neutron yield through the enhancement of the electron beam energy (from the current 40 MeV limit up to 50 MeV), as well as to minimize several sources of background that are presently affecting the measurements, a new neutron beam-line and a new, larger neutron experimental room have been designed. The optimization of the neutron/photon ratio, the minimization of the backscattered radiation from the walls and the possibility to have better experimental conditions are the main advantages of the new design. To optimize the beamline, extensive simulations with the particle interaction and transport code FLUKA have been performed. Starting from the primary electron beam, both the photon and neutron radiation fields have been fully characterized. To have a cross-check of the results, the calculated values of the neutron yields at different energies of the primary beam have been compared both with an independent simulation with the MCNP code and with analytical calculations, obtaining a very satisfactory agreement at the level of few percent. The evaluated radiation fields have been used to optimize the direction of the new neutron beamline, in order to minimize the photon flash contribution. A general overview of the new photo-neutron source, together with all the steps of the optimization study, is here presented and discussed.