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In recent years JRC-IRMM has been investigating fission cross-sections of 240,242Pu in the fast-neutron energy range relevant for innovative reactor systems and requested in the High Priority Request List (HPRL) of the OECD/Nuclear Energy Agency (NEA). In addition to that, prompt neutron multiplicities are being investigated for the major isotopes 235U, 239Pu in the neutron-resonance region using a newly developed scintillation detector array (SCINTIA) and an innovative modification of the Frisch-grid ionisation chamber for fission-fragment detection. These data are highly relevant for improved neutron data evaluation and requested by the OECD/Working Party on Evaluation Cooperation (WPEC). Thirdly, also prompt fission γ-ray emission is investigated using highly efficient lanthanide-halide detectors with superior timing resolution. Again, those data are requested in the HPRL for major actinides to solve open questions on an under-prediction of decay heat in nuclear reactors. The information on prompt fission neutron and γ-ray emission is crucial for benchmarking nuclear models to study the de-excitation process of neutron-rich fission fragments. Information on γ-ray emission probabilities is also useful in decommissioning exercises on damaged nuclear power plants like Fukushima Daiichi to which JRC-IRMM is contributing. The results on the 240,242Pu fission cross section, 235U prompt neutron multiplicity in the resonance region and correlations with fission fragments and prompt γ-ray emission for several isotopes will be presented and put into perspective.

LICORNE is a new neutron source recently installed at the tandem accelerator of the Institut de Physique Nucléaire d'Orsay, where a Li7-beam is used to bombard a hydrogen-containing target to produce an intense forward-directed neutron beam. The directionality of the beam, which is the unique characteristic of LICORNE, will permit the installation of γ-ray detectors dedicated to the investigation of fission fragment de-excitation which are unimpeded by neutrons from the source. A first experimental program will focus on the measurement of prompt γ-ray emission in the neutron-induced fission of fertile and fissile isotopes at incident neutron energies relevant for the core design of Generation-IV nuclear reactors. Other potential uses of the LICORNE facility for both fundamental and applied physics research are also presented.

We present new spectral data of prompt γ-ray emission from the spontaneous fission of 252Cf. This work was performed in direct response to an OECD/NEA high priority data request. We discuss the impact of our new data on evaluated nuclear data tables not only for this nuclide, but also for 238U and 241Pu, which are always produced in a reactor. Furthermore, we will show results from our investigation of prompt γ-ray emission from the reaction 235 U(nth, f), measured in at the Centre for Energy Research of the Hungarian Academy of Sciences in Budapest, Hungary. Spectral data obtained with three different detectors are consistent and led to an uncertainty on total energy and multiplicity considerably smaller than requested by the OECD/NEA.

For the development of future nuclear fission applications and for a responsible handling of nuclear waste the a-priori assessment of the fission-fragments’ heat production and toxicity is a fundamental necessity. The success of an indispensable modelling of the fission process strongly depends on a good understanding of the particular mechanism of scission, the mass fragmentation and partition of excitation energy. Experimental observables are fission-fragment properties like mass- and energy-distributions, and the prompt neutron as well as γ-ray multiplicities and emission spectra. The latter quantities should preferably be known as a function of fragment mass and excitation energy. Those data are highly demanded as published by the OECD-NEA in its high priority data request list. With the construction of the double (v, E) spectrometer VERDI we aim at measuring pre- and post-neutron masses directly and simultaneously to avoid prompt neutron corrections. From the simultaneous measurement of pre- and post-neutron fission-fragment data the prompt neutron multiplicity may then be inferred fully correlated with fragment mass yield and total kinetic energy. Using an ultra-fast fission event trigger spectral prompt fission γ-ray measurements may be performed. For that purpose recently developed lanthanum-halide detectors, with excellent timing characteristics, were coupled to the VERDI spectrometer allowing for a very good discrimination of fission γ-rays and prompt neutrons due to their different time-of-flight.

In this paper we report on a new study of prompt y-rays from the spontaneous fission of 252Cf. Photons were measured in coincidence with fission fragments by employing four different Ianthanide halide scintillation detectors. Together with results from a previous work of ours, we determined characteristic parameters with high precision, such as the average y-ray multiplicity vγ = (8.29 ± 0.13), the average energy per photon ey = (0.80 ± 0.02) MeV, and the total γ-ray energy release per fission Eγ.tot = (6.65 ± 0.10) MeV. The excellent agreement between the individual results obtained in all six measurements proves the good repeatability of the applied experimental technique. The impact of low-energy photons, i.e., below 500 keV, on prompt fission y-ray spectra characteristics has been investigated as well by comparing our results with those taken with the DANCE detector system, which appears to suffer from absorption effects in the low-energy region. Correction factors for this effect were estimated, giving results comparable to ours as well as to historical ones. From this we demonstrate that the different techniques of determining the average γ-ray multiplicity, either from a properly measured and normalized spectrum or a measured multiplicity distribution, give equivalent and consistent results. [ABSTRACT FROM AUTHOR]

In this paper we present results from the first high-precision prompt-γ-ray spectral measurements from the reaction 24lPu(nth, f). Apart from one recent experiment, no data are reported in the literature for this fissioning system, which motivated a new dedicated experiment. We have measured prompt-fission y rays with three cerium-doped LaBr3 (two 5.08 cm x 5.08 cm and one 7.62 cm x 7.62 cm) and one CeBr3 (5.08 cm x 5.08 cm) scintillation detectors, which all exhibit excellent timing and good energy resolution. The average γ-ray multiplicity was determined to be vγ = (8.21 ± 0.09) per fission, the average energy to be ∊γ = (0.78 ± 0.01) MeV, and the total energy to be Eγ, tot = (6.41 ± 0.06) MeV as the weighted average from all detectors. Since the results from all detectors are in excellent agreement, and the total released γ energy is modestly higher than the one in the present evaluated nuclear data files, we suspect that the underestimation of the prompt-γ heating in nuclear reactors is due to fast-neutron-induced fission on 238U or rather from fission induced by γ rays from neutron capture in the construction material. [ABSTRACT FROM AUTHOR]

We present results from new spectral prompt γ-ray measurements from the spontaneous fission of 252Cf. Apart from one recent experiment, about four decades have passed since the last dedicated experiments were reported in the literature. Hence, there was a need for a revision. We have measured prompt fission γ rays with both cerium-doped LaBr3 and CeBr3 scintillation detectors, both of which exhibit excellent timing and good energy resolution. The emission yield was determined to be vγ = (8.30 ± 0.08)/fission and vγ = (8.31 ± 0.10)/fission, with the average energy of εγ = (0.80 ± 0.01) MeV and εγ = (0.80 ± 0.01) MeV and total energy of Eγ,tot = (6.64 ± 0.08) MeV and Eγ,tot = (6.65 ± 0.12) MeV, with the LaBr3 and CeBr3 detectors, respectively. Since the results from the two detectors are in excellent agreement and confirm the historical data, but not those in the present evaluated nuclear data files, we strongly recommend an update. [ABSTRACT FROM AUTHOR]

Abstract: Recently developed cerium-doped lanthanum bromide (LaBr3: Ce) scintillation detectors have shown to possess promising properties with respect to the detection of γ-rays compared to previously known materials. In this work however, we demonstrate how these detectors may be used to obtain information not only about γ-rays, but also about neutrons, i.e. thinking”inside” and”outside” the box, respectively. For this purpose γ-rays were detected in coincidence with fission fragments and both their energy and their time-of-flight relative to the instant of a fission event is recorded. By evaluating the time-of-flight distributions of γ-rays, identified as decays of excited states after population by inelastically scattered neutrons inside the scintillation crystal as well as other surrounding materials, we show that it is possible to acquire knowledge from and about the spectrum of incident neutrons. We give three examples for conceivable applications, used to determine geometrical profiles, cross sections and neutron spectra, respectively. [Copyright &y& Elsevier]

The knowledge of prompt fission γ-ray emission has been of major interest in reactor physics for a few years. Only few experimental spectra were published until now for fast-neutron-induced fission, and measurements would be also valuable in order to improve our understanding of the fission process. A simple experimental method was used to measure the first prompt fission γ-ray spectra up to 10 MeV. In this approach, the γ rays are measured with a bismuth germanate (BGO) detector which offers two significant advantages with respect to other γ-ray detectors: a high peak-to-total ratio and a high efficiency. The prompt fission neutrons are rejected by the time-of-flight technique between the BGO detector and a fission trigger given by a fission chamber. Prompt fission γ-ray spectra were measured for 1.6 ± 0.1, 5.1 ± 0.2 and 15.0 ± 0.6 MeV neutron-induced fission on 238U at the CEA, DAM, DIF Van de Graaff accelerator; average multiplicity and mean photon energy per fission were deduced from the spectra. [ABSTRACT FROM AUTHOR]

Systematics from 2001, describing prompt-fission γ-ray spectra (PFGS) characteristics as a function of mass and atomic number of the fissioning system, was revisited and parameters were revised, based on recent experimental results. Although originally expressed for spontaneous and thermal-neutron induced fission, validity for fast neutrons was assumed and applied to predict PFGS characteristics for the reaction n+238U up to incident neutron energies of En=20MeV. The results from this work are in good agreement with corresponding results from both model calculations and experiments. [ABSTRACT FROM AUTHOR]

In this paper we present results from measurements of prompt γ rays from the thermal neutron induced fission of 235U. Photons were measured in coincidence with fission fragments with cerium-doped LaCl3 and LaBr3 as well as CeBr3 scintillation detectors, which offer an intriguing combination of excellent timing resolution and good resolving power. The spectra measured with all employed detectors are in excellent agreement with respect to their shapes. Characteristic parameters were extracted for a γ-energy range from 0.1 to 6.0 MeV and the results obtained with several detectors were averaged. From that, the average emission yield of prompt-fission γ rays was determined to be vy = (8.19 ± 0.11) per fission, the average energy per photon to be Ey = (0.85 ± 0.02) MeV, and the total energy to be Ey.tot = (6.92 ± 0.09) MeV. The uncertainties are much lower than the 7.5% requested for the modeling of advanced nuclear reactor cores. Estimating the influence of γ rays with energies between 6 and 10 MeV on the values determined in this work revealed a negligible deviation of the order of the found uncertainties. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we demonstrate the employment of scintillation detectors for the identification of prompt fission from in contrast to other reactions. We present the properties of cylindrical 1.5in.×1.5in. detectors in terms of intrinsic activity, energy resolution, intrinsic full peak efficiency and intrinsic timing resolution. In addition, we show results from the measurement of emitted in coincidence with the spontaneous fission of 252Cf, which lead us to the conclusion that the properties of the detectors used in this work, in particular the good timing resolution and a reasonably good energy resolution, are more than just sufficient for the assessment of prompt in future nuclear reactors as requested by the OECD. Hence, in our opinion, detectors, compared to those made out of crystals, represent an appropriate and quasi low-cost choice for the above mentioned applications. [Copyright &y& Elsevier]