Measurements of prompt gamma-rays from fast-neutron induced fission with the LICORNE directional neutron source

At the IPN Orsay we have developed a unique, directional, fast neutron source called LICORNE, intended initially to facilitate prompt fission gamma measurements. The ability of the IPN Orsay tandem accelerator to produce intense beams of $^7$Li is exploited to produce quasi-monoenergetic neutrons between 0.5 - 4 MeV using the p($^7$Li,$^7$Be)n inverse reaction. The available fluxes of up to 7 × 10$^7$ neutrons/second/steradian for the thickest hydrogen-rich targets are comparable to similar installations, but with two added advantages: (i) The kinematic focusing produces a natural neutron beam collimation which allows placement of gamma detectors adjacent to the irradiated sample unimpeded by source neutrons. (ii) The background of scattered neutrons in the experimental hall is drastically reduced. The dedicated neutron converter was commissioned in June 2013. Some preliminary results from the first experiment using the LICORNE neutron source at the IPN Orsay are presented. Prompt fission gamma rays from fast-neutron induced fission of $^{238}$U, $^{232}$Th and $^{235}$U were measured by two different techniques. An ionisation chamber containing 10 mg samples of $^{238}$U and $^{235}$U to provide a fission trigger was used in conjunction with BaF$_2$ and LaBr$_3$ scintillator gamma detectors to detect fission fragment-gamma-ray coincidences. In the second part of the experiment thick targets of around 50g of $^{238}$U and $^{232}$Th were used, with a high efficiency calorimeter to tag on fission events by requiring both high sum energy and multiplicity

The LICORNE Neutron Source and Measurements of Prompt γ-rays Emitted in Fission

International audienceThe emission of prompt gamma rays is one of the least measured and least well-understood parts of the fission process. Knowledge of prompt fission gamma spectra, mean energies and multiplicities are important for reactor gamma heating and hence linked to reactor safety. At the IPN Orsay we have developed a unique, directional, fast neutron source called LICORNE, intended initially to facilitate prompt fission gamma measurements. The ability of the IPN Orsay tandem accelerator to produce intense beams of 7Li is exploited to produce quasi mono-energetic neutrons between 0.5 - 4 MeV using the p(7Li, 7Be)n inverse reaction. The available fluxes of up to 7×107 neutrons/second/steradian are comparable to existing installations, but with two added advantages: (i) The kinematic focusing produces a natural neutron beam collimation which allows placement of gamma detectors adjacent to the irradiated sample unimpeded by source neutrons. (ii) The background of scattered neutrons in the experimental hall is drastically reduced. The dedicated neutron converter was commissioned in June 201

Development of a kinematically focused neutron source with the p(7Li,n)7Be inverse reaction

Directional beams of neutrons can be produced if a nuclear reaction which emits neutrons is initiated in inverse kinematics with a heavy ion projectile bombarding a light target. In this paper we investigate the use of the p(7Li,n)7Be inverse reaction to produce kinematically focused, quasi-mono energetic neutron beams with a view to development of such an unusual neutron source for fundamental and applied nuclear physics studies. An experiment was carried out to validate the concept and test the viability of two types of hydrogen-rich solid targets: polypropylene and TiH2. Neutron time-of-flight/energy spectra at 3 m distance from the source have been measured at 7Li bombarding energies of 13.5, 15, 15.5, 16, and 17 MeV, and neutron backgrounds from parasitic reactions have been characterised. The neutron angular distribution in the laboratory has been measured at 15 MeV. A Monte-Carlo code based on two-body relativistic kinematics has been developed and validated by comparisons with the experimental data. Code-based extrapolations have then been used to deduce neutron energy spectra and maximum neutron fluxes available for future irradiation of samples placed in the neutron beam at small distances. For neutrons produced with thin (4 micron) and thick (28 micron) polypropylene targets the maximum available fluxes are calculated to be 10^7 n/s/sterad and 7 x 10^7 n/s/sterad respectively. The development of a dedicated facility to produce kinematically focused neutrons is discussed.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Comparative measurement of prompt fission gamma-ray emission from fast-neutron-induced fission of 235U and 238U

Prompt fission γ -ray (PFG) spectra have been measured in a recent experiment with the novel directional fast-neutron source LICORNE at the ALTO facility of the IPN Orsay. These first results from the facility involve the comparative measurement of prompt γ emission in fast-neutron-induced fission of 235U and 238U. Characteristics such as γ multiplicity and total and average radiation energy are determined in terms of ratios between the two systems. Additionally, the average photon energies were determined and compared with recent data on thermal-neutron-induced fission of 235U. PFG spectra are shown to be similar within the precision of the present measurement, suggesting that the extra incident energy does not significantly impact the energy released by prompt γ rays. The origins of some small differences, depending on either the incident energy or the target mass, are discussed. This study demonstrates the potential of the present approach, combining an innovative neutron source and new-generation detectors, for fundamental and applied research on fission in the near future.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Prompt fission γ-ray spectra characteristics - a first summary

In this work we give an overview of our investigations of prompt γ-ray emission in nuclear fission. This work was conducted during the last five years in response to a high priority nuclear data request formulated by the OECD/NEA. The aim was to reveal data deficiencies responsible for a severe under-prediction of the prompt γ heating in nuclear reactor cores. We obtained new prompt fission γ-ray spectral (PFGS) data for 252Cf(SF) as well as for thermal-neutron induced fission on 235U(nth,f) and 241Pu(nth,f). In addition, first PFGS measurements with a fast-neutron beam were accomplished, too. The impact of the new data and future data needs are discussed.JRC.G.2-Standards for Nuclear Safety, Security and Safeguar

LICORNE: A new and unique facility for producing intense, kinematically focused neutron beams at the IPN Orsay

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