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This volume contains the written proceedings of PPNS-2018, the international workshop on “Particle Physics at Neutron Sources” hosted by the LPSC in Grenoble, France from May 24th–26th, 2018.

Lifetimes of excited states of the 98;100;102Zr nuclei were measured by using the Generalized Centroid Difference Method. The nuclei of interest were populated via neutron-induced fission of 241Pu and 235U during the EXILL-FATIMA campaign. The obtained lifetimes were used to calculate the B(E2) transition strengths and β deformation parameters which were then compared with the recent theoretical predictions obtained with Monte Carlo Shell Model.

At the Institut Laue-Langevin in Grenoble, germanium-gated γ-γ fast-timing lifetime measurements of nuclear excited states in neutron-rich nuclei have been performed within a prompt γ-ray spectroscopy experimental campaign. We report on results obtained from the cold-neutron induced fission of 235U. The excited secondary fission products were stopped almost instantaneously within the thick target and the γ rays emitted were collected triggerlessly using the EXILL&FATIMA mixed array of HPGe and LaBr3(Ce) detectors. Precise lifetimes could be determined by analysing the γ-γ time difference spectra using the generalized centroid difference method. This picosecondsensitive method provides many advantages and is briefly explained. Still, the major source of systematic errors is related to the contribution of time-correlated Compton background. The EXILL&FATIMA results are discussed with respect to the typical energy-dependent timing behaviour of the background. According to the time response of the background, appropriate methods and a time correction for the sub-nanosecond regime are proposed.

The γ decays of excited states in the very neutron-rich nucleus 88Se have been observed following the cold neutron-induced fission of 235U at the PF1B facility of the Institut Laue-Langevin (ILL), Grenoble. The EXILL array was used to measure γ - γ - γ coincidences, which were then analysed to build a level scheme of 88Se. A low (2+1) energy hints at the onset of quadrupole deformation and the identification of possible members of a (2+2) band provide evidence for γ vibrations. Shell-model calculations using a 78Ni core reproduce the experimental decay scheme well, implying that interactions between particles in the π f5/2 p and vsd orbits are responsible for much of the collectivity present. The algebraic collective model has also been used to interpret the experimental data, showing that that the experimental spectrum is consistent with 88Se being a transitional nucleus, possessing weak static β deformation and γ instability.

Recent results on the structure of 47Ca will be presented. The nucleus of interest was populated via the cold-neutron capture 46Ca(n,γ) reaction, on a rare 46Ca target, during the EXILL experimental campaign at the nuclear reactor of Institut Laue- Langevin in Grenoble. High-resolution γ-ray spectroscopy, performed with a composite array of HPGe detectors, enabled the identification of new transitions deexciting states between the neutron-capture level and the ground state. Experimental data will be compared with a novel microscopic theoretical model, currently under development, specifically designed to describe the low-lying structure of odd-mass nuclei with one valence particle/hole outside a spherical doubly-magic core, using the Skyrme effective interaction self-consistently.

The first successful operation of a neutron interferometer with a separate beamrecombining crystal is reported. This result was achieved at the neutron interferometry setup S18 at the ILL in Grenoble by a collaboration between TU Wien, ILL, Grenoble, and INRIM, Torino. While previous interferometers have been machined out of a single-crystal block, in this work two crystals were successfully aligned on nanoradian and picometre scales, as required to obtain neutron interference. As a decisive proof-of-principle demonstration, this opens the door to a new generation of neutron interferometers and exciting applications. [ABSTRACT FROM AUTHOR]

At the PF1B cold neutron beam line at the Institut Laue Langevin, the EXILL array consisting of EXOGAM, GASP and ILL-Clover detectors was used to perform (n,γ) measurements at very high coincidence rates. About ten different reactions were measured in autumn 2012 using a highly collimated cold neutron beam. In spring 2013, the EXOGAM array was combined with 16 LaBr3(Ce) scintillators in the EXILL&FATIMA campaign for the measurement of lifetimes using the generalised centroid difference method. We report on the properties of the set-ups and present first results from both campaigns.

The neutron capture reaction at 77Se has been studied with cold neutrons in the course of the EXILL campaign at the high-flux reactor of the Institut Laue-Langevin Grenoble. A simulation of the detector array with Geant4 has been accomplished and evaluated. The detector response has been deduced and measured spectra were unfolded, which have been compared with simulations using γDex to determine strength functions.

The feasibility of retrieving accurate fission observables with a Ge-detector array around a fissile target placed in a cold neutron beam was tested. In three measurement campaigns performed at ILL with the EXILL setup, 235U and 241Pu targets were placed in the high flux cold neutron beam available at the PF1B neutron guide. Gamma-rays following fission were detected by an array of 16 Ge detectors. In the following study, part of data was analyzed as a proof of principle. A set of yields belonging to the Kr-Ba pair were extracted using a gamma-gamma coincidence technique. Preliminary results were compared to the predictions of two phenomenological models: GEF and FIFRELIN.

A novel method for direct electronic “fast-timing” lifetime measurements of nuclear excited states via γ-γ coincidences using an array equipped with N very fast high-resolution LaBr3(Ce) scintillator detectors is presented. The generalized centroid difference method provides two independent “start” and “stop” time spectra obtained without any correction by a superposition of the N(N – 1)/2 calibrated γ-γ time difference spectra of the N detector fast-timing system. The two fast-timing array time spectra correspond to a forward and reverse gating of a specific γ-γ cascade and the centroid difference as the time shift between the centroids of the two time spectra provides a picosecond-sensitive mirror-symmetric observable of the set-up. The energydependent mean prompt response difference between the start and stop events is calibrated and used as a single correction for lifetime determination. These combined fast-timing array mean γ-γ zero-time responses can be determined for 40 keV < Eγ < 1.4 MeV with a precision better than 10 ps using a 152Eu γ-ray source. The new method is described with examples of (n,γ) and (n,f,γ) experiments performed at the intense cold-neutron beam facility PF1B of the Institut Laue-Langevin in Grenoble, France, using 16 LaBr3(Ce) detectors within the EXILL&FATIMA campaign in 2013. The results are discussed with respect to possible systematic errors induced by background contributions.

Levels of 186Re have been studied in the thermal neutron capture reaction with an enriched 185Re target. Evaluation of spectrum measured with GAMS5 allowed to obtain energies and intensities of more than 500 γ-lines assigned to 186Re. Most of the obtained transitions have been placed in the model-independent level scheme of the doubly odd 186Re nucleus, taking into account the available data of earlier experiments as well as the results of recent 187Re(p, d)186Re reaction measurements. Structure of the 186Re low-lying levels has been analysed in terms of the particle-plus-rotor coupling model.

Measurement of γ radiation following slow neutron capture on 68Zn and 70Zn using very efficient germanium array EXILL was done at Institute Laue-Langevin in Grenoble. New spectroscopic information for 69Zn (including doorway state) and first results for 71Zn (accurate energy of β decaying isomer) were obtained. Binding energies for 68Zn, 69Zn and 71Zn were determined with 5-to-70 times higher accuracy than presented in the literature.

Within the EXILL campaign a large and efficient cluster of Ge-detectors was installed around a very well collimated neutron beam. This has allowed to carry out rather complete spectroscopic studies close to the line of stability using the (n,γ) reaction. Neutron rich isotopes were produced by neutron induced fission and prompt spectroscopy was carried out. The isotope selection in this setup was based on a partially known level scheme and the use of triple coincidences. The latter is limiting the statistical sensitivity in the case of weak production yields. Based on the experiences of these campaigns we are currently developing a new setup: FIPPS (FIssion Product Prompt Spectroscopy). This setup combines a collimated neutron beam, a highly efficient cluster of Ge detectors, a gas filled magnet and auxiliary detectors. The presence of the gas filled magnet will allow us to identify fission products directly and should give access to a new quality of studies if compared to the EXILL campaign. The EXILL campaign and the FIPPS project are presented.

A combination of germanium detectors has been installed at the PF1B neutron guide of the ILL to perform the prompt spectroscopy of neutron-rich nuclei produced in the neutron-capture induced-fission of 235U and 241Pu. In addition LaBr3 detectors from the FATIMA collaboration have been installed in complement with the EXOGAM clovers to measure lifetimes of low-lying excited states. The measured characteristics and online spectra indicate very good performances of the overall setup.

One way to explore exotic nuclei is to study their structure by performing γ-ray spectroscopy. At the ILL, we exploit a high neutron flux reactor to induce the cold fission of actinide targets. In this process, fission products that cannot be accessed using standard spontaneous fission sources are produced with a yield allowing their detailed study using high resolution γ-ray spectroscopy. This is what was pursued at the ILL with the EXILL (for EXOGAM at the ILL) campaign. In the present work, the EXILL setup and performance will be presented.

A cold neutron induced fission experiment recently took place at the Institute Laue-Langevin (ILL) in Grenoble. The neutron beam was provided by the nuclear reactor facility at ILL and the detector setup that was used for the γ-spectroscopy of the fission products consisted mainly of the detectors of the EXOGAM array [1], thereby the name of the campaign is EXILL. The main purpose of our measurement was to investigate the nuclei in the region with N = 50 close to 78Ni as well as the nuclei close to the N = 82 shell closure. In this paper, the motivation of the experiment is described as well as the experimental setup and the status of the ongoing data analysis.

New research facilities like MEGa–Ray (Livermore) or ELI–NP (Bucharest) will provide within the next years (2013–2016) photon beams of unprecedented quality with respect to both photon intensity (total flux ~ 1013 γ/s) and spectral intensity (~ (104–106)/eVs), thus exceeding the performance of existing facilities by severalorders of magnitude. This tremendous progress will be enabled by Compton-backscattering of an intense laser off a high-quality electron beam, in conjunction with novel refractive bremsstrahlung beams focusing γ optics and efficient monochromatization techniques. We envisage to employ these γ beams for photofission studies on extremely deformed nuclear states of actinides, investigating their multiple–humped potential energy landscape in a highly selective way. Transmission resonances in the prompt fission cross section from the (superdeformed) second and (hyperdeformed) third potential minimum will be studied, where the fission decay channel can be expressed as a tunnelling process of these gateway states through the multiple–humped fission barrier.

The first excited level of 96Mo was investigated in a high-statistics experiment using the 95Mo(n, γγ) cold neutron capture reaction. The measurements used the high cold neutron flux from the research reactor at Institut Laue-Langevin and employed the highly-efficient EXILL array to detect γ-ray coincidences. The recorded statistics allow identification of decay branches with only a small relative intensity including the E3 decay. With the knowledge of the newly measured branching ratio and the known transition probability, the lifetime of the level was determined and, subsequently, the strength of the other decay branches of the octupole phonon were calculated. The extracted electromagnetic decay strengths are compared to the systematics of the stable even–even molybdenum isotopes and values calculated in a Skyrme-force based quasiparticle random phase approximation and in a cluster approach. Additionally, the decay branch to the low-lying quadrupole isovector level was observed. [ABSTRACT FROM AUTHOR]

The structure of the 131Sn nucleus was studied at the Institut Laue-Langevin by prompt-delayed -ray spectroscopy across the 0.3 μs, 23/2-isomeric state at 4670 keV, following cold-neutron-induced fission of 235U and 241Pu targets. New -ray transitions are observed and the results are compared with theoretical calculations performed with the Hybrid Configuration Mixing Model. The latter suggests that the majority of the states can be well-described in terms of couplings between single-hole degrees of freedom and 132Sn core excitations, pointing to a robust neutron shell closure at N = 82. [ABSTRACT FROM AUTHOR]

The low-spin structure of the 41Ca isotope, populated by neutron capture, has been investigated by high resolution -spectroscopy techniques. The experiment was performed at the PF1B cold neutron facility at the Institut Laue-Langevin (Grenoble, France) employing the EXILL array, consisting of 46 HPGe crystals. Several new γ rays have been observed and two excited states were newly established. [ABSTRACT FROM AUTHOR]

The γ decay of the one-proton valence nucleus 133Sb is studied using the cold-neutron induced fission of 235U and 241Pu targets. The experiment is performed at the ILL reactor in Grenoble, using a highly efficient HPGe array, also coupled to fast LaBr3(Ce) scintillators. High-spin excited states above the 16.6 μs isomer are observed, and the lifetime of the 13/2+ and 15/2+ states is measured by fast-timing techniques, revealing a complex nature of the wave functions. The experimental results are well-interpreted by a newly developed microscopic model which takes into account the coupling between the valence proton and excitations (both collective and noncollective) of the doubly magic 132Sn core. [ABSTRACT FROM AUTHOR]

High- and medium-spin structures have been investigated in neutronrich 96Y isotope produced in the cold-neutron-induced fission of 235U and 241Pu targets. The study significantly extended the existing level scheme of this isotope. In the current work, particular attention was devoted to the structure which has been identified above the long 9.6 s, (8+) isomer, up to the excitation energy of 4813 keV. The angular correlation analysis, together with obtained decay pattern, allowed to propose spin values for some of the newly observed states. [ABSTRACT FROM AUTHOR]

Medium-spin, yrast excitations in nucleus 83As have been studied in neutron-induced fission of 235U. The experiment took place at the Institut Laue-Langevin in Grenoble where gamma rays were registered using the EXILL detector array. Recently, we proposed new tentative (9/2+) spin assignment for 2777 keV level in 83As, which is now discussed in the context of deformation showing up in 78Ni region. [ABSTRACT FROM AUTHOR]

Preliminary results are presented from a γ-spectroscopy study of low-spin states of 41,49Ca isotopes, produced by neutron capture on a Ca target, at very high coincidence rates. The experiment was performed at the PF1B cold-neutron facility at ILL (Grenoble, France), using the EXILL array, consisting of EXOGAM, GASP and ILL-Clover detectors. [ABSTRACT FROM AUTHOR]

Preliminary results are presented from a recent campaign of experiments, performed at the PF1B cold-neutron facility at ILL (Grenoble, France), in which γ-ray spectroscopy of neutron-rich nuclei produced in neutron induced fission on 235U and 241Pu targets was performed. By using the EXILL array, consisting of EXOGAM, GASP and ILL-Clover detectors, and a digital triggerless data acquisition system, γ coincidences over several microseconds time windows were studied. This allowed to obtain new information on nuclei around 132Sn, such as 132Te, 133Sb and 130Sn, in which microsecond isomeric states occur. Preliminary results on 210Bi, populated by cold-neutron capture, are also shown. The data offer the possibility to investigate the couplings between core excitations and valence particles, around the doubly magic nuclei 132Sn and 208Pb. [ABSTRACT FROM AUTHOR]