A photo-neutron source for time-of-flight measurements at the radiation source ELBE

Forward-looking insights in laser-generated ultra-intense γ-ray and neutron sources for nuclear application and science

Ultra-intense MeV photon and neutron beams are indispensable tools in many research fields such as nuclear, atomic and material science as well as in medical and biophysical applications. For applications in laboratory nuclear astrophysics, neutron fluxes in excess of 10²¹ n/(cm² s) are required. Such ultra-high fluxes are unattainable with existing conventional reactor- and accelerator-based facilities. Currently discussed concepts for generating high-flux neutron beams are based on ultra-high power multi-petawatt lasers operating around 10²³ W/cm² intensities. Here, we present an efficient concept for generating γ and neutron beams based on enhanced production of direct laser-accelerated electrons in relativistic laser interactions with a long-scale near critical density plasma at 10¹⁹ W/cm² intensity. Experimental insights in the laser-driven generation of ultra-intense, well-directed multi-MeV beams of photons more than 10¹² ph/sr and an ultra-high intense neutron source with greater than 6 × 10¹⁰ neutrons per shot are presented. More than 1.4% laser-to-gamma conversion efficiency above 10 MeV and 0.05% laser-to-neutron conversion efficiency were recorded, already at moderate relativistic laser intensities and ps pulse duration. This approach promises a strong boost of the diagnostic potential of existing kJ PW laser systems used for Inertial Confinement Fusion (ICF) research.

Laser-plasma interaction can provide alternative platform over conventional method for particle and photon beam generation. Here the authors demonstrate generation of gamma ray and neutron beams from intense laser interaction with near critical density plasma.

Fast neutron inelastic scattering from 7Li

The inelastic scattering of fast neutrons from 7Li nuclei was investigated at the nELBE neutron-time-of-flight facility. The photon production cross section of 478 keV γ-rays from the first excited state of 7Li was determined by irradiating a disc of LiF with neutrons of energies ranging from 100 keV to about 10 MeV. The target position was surounded by a setup of 7 LaBr3 scintillation detectors and 7 high-purity germanium detectors to detect the de-excitation γ-rays. A 235U fission chamber was used to determine the incoming neutron flux. The number of detected photons was corrected for the detection efficiency, multiple scattering and the time-of-flight dependent data acquisition dead time. The preliminary results show reasonable agreement with some previous measurments but are about 15 % below the recent data taken at the GELINA facility.

Measurement of the 16O(n, α)13C cross-section using a Double Frisch Grid Ionization Chamber

The 16O(n, α)13C reaction was proposed to be measured at the neutron time-of-flight (n_TOF) facility of CERN. To this purpose, a Double Frisch Grid Ionization Chamber (DFGIC) containing the oxygen atoms as a component in the counting gas coupled with a switch device in order to prevent the charge collection from the so-called γ-flash has been developed at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), in Germany. The first 16O(n, α)13C measurement without seeing the charge of the γ-flash at n_TOF has been performed in November 2018. After the electronics did not suffer from the y-flash any more, another huge charge collection was discovered. Due to the high instantaneous flux at the n_TOF facility [1] the amount of that induced charge from neutron induced background reactions was piling up so much that the recognition of 16O(n, α)13C reactions from that background was very difficult. For that reason another 16O(n, α)13C measurement at the time-of-flight facility nELBE at HZDR which has a low instantaneous flux [2], has been performed in April 2019. Both measurements from n_TOF and nELBE will be presented here.

New equipment for neutron scattering cross-section measurements at GELINA

Two new experimental setups are being developed at European Commission’s Joint Research Centre in Geel, Belgium. The scintillator array ELISA (ELastic and Inelastic Scattering Array) is for high-quality neutron scattering cross section and angular distribution measurements. It has the capability to separate neutron-and photon-induced events via pulse-shape analysis. Inelastic scattering can also be resolved from the elastic channel. The ELISA setup and data analysis procedure were validated by performing measurements using carbon and iron samples. The DELCO spectrometer (Detection of ELectrons from COnversion) is intended for inelastic neutron scattering cross-section measurements in cases where the detection of γ rays is not feasible. The current status of DELCO, results from the first tests, and future prospects will be discussed.

Determination of the fast-neutron-induced fission cross-section of 242Pu at nELBE

The fast-neutron-induced fission cross section of 242Pu was determined in the energy range of 0.5 MeV to 10MeV at the neutron time-of-flight facility nELBE. Using a parallel-plate fission ionization chamber this quantity was measured relative to 235U(n,f). The number of target nuclei was thereby calculated by means of measuring the spontaneous fission rate of 242Pu. An MCNP 6 neutron transport simulation was used to correct the relative cross section for neutron scattering. The determined results are in good agreement with current experimental and evaluated data sets.

Measurement of photo-neutron cross sections and isomeric yield ratios in the 89Y(γ,xn)89−x Y reactions at the bremsstrahlung end-point energies of 65, 70 and 75 MeV

The flux-weighted average cross sections of the 89Y(γ,xn; x=1–4)89−x Y reactions and the isomeric yield ratios of the 87m,gY, 86m,gY, and 85m,gY radionuclides produced in these reactions with the bremsstrahlung end-point energies of 65, 70 and 75 MeV have been determined by an activation and off-line γ-ray spectrometric technique using the 100 MeV electron linac in Pohang Accelerator Laboratory, Korea. The theoretical 89Y(γ,xn; x=1–4)89−x Y reactioncross sections for mono-energetic photonshave been calculated using the computer code TALYS 1.6. Then the flux-weighted theoretical values were obtaind to compare with the present data. The flux-weighted experimental and theoretical 89Y(γ,xn; x=1–4)89−x Y reaction cross sections increase very fast from the threshold values to a certain bremsstrahlung energy, wherethe other reaction channels open up. Thereafter it remains constant a while and then slowly decreases with the increase of cross sections for other reactions. Similarly, the isomeric yield ratios of 87m,gY,86m,gY and 85m,gY in the 89Y(γ,xn; x=2–4)89−x Y reactions from the present work and literature data show an increasing trend from their respective threshold values to a certain bremsstrahlung energy. After a certain point ofenergy, the isomeric yield ratios increase slowly with the bremsstrahlung energy. These observations indicate the role of excitation energy and its partitioning in different reaction channels.

Thermohydraulic safety issues for liquid metal cooled systems

In this paper recent developments of various techniques for single-phase and two-phase flow measurements with relevance to liquid metal cooled systems will be presented. Further, the status of the DRESDYN platform for large-scale experiments with liquid sodium is sketched.

Temporal Narrowing of Neutrons Produced by High-Intensity Short-Pulse Lasers

The production of neutron beams having short temporal duration is studied using ultraintense laser pulses. Laser-accelerated protons are spectrally filtered using a laser-triggered microlens to produce a short duration neutron pulse via nuclear reactions induced in a converter material (LiF). This produces a ∼3  ns duration neutron pulse with 10(4)  n/MeV/sr/shot at 0.56 m from the laser-irradiated proton source. The large spatial separation between the neutron production and the proton source allows for shielding from the copious and undesirable radiation resulting from the laser-plasma interaction. This neutron pulse compares favorably to the duration of conventional accelerator sources and should scale up with, present and future, higher energy laser facilities to produce brighter and shorter neutron beams for ultrafast probing of dense materials.