LICORNE was comissioned in June 2013 and a first program of experiments was started in July 2013 on the measurement of the prompt gamma-ray emission in the process of nuclear fission. Since then we have identified at least four areas of physics where LICORNE can perform unique types of experiments addressing both fundamental and applied nuclear physics problems.
PROMPT GAMMA AND NEUTRON EMISSION IN FISSION
The study of prompt gamma emission in fission is important for both a better understanding of the fission process and the provision of data for the gamma-heating phenomena in fast reactor cores.
About 10% of the energy released in the core is in the form of gamma rays and can travel larger distances (many centimeters) from the initial reaction site. During reactor operation gamma heating processes dominate for all non-fissile materials in the reactor core (e.g. structural materials, core shielding, reactor instrumentation, etc.). When the reactor is shut down, gamma heating is the dominant energy deposition process for all core materials and thus the problem of gamma heating is directly related to reactor safety.
Currently, benchmark models of gamma heating in reactor cores underestimate heating effects by up to 30% when compared to experimental results. This is mainly due to insufficiently accurate nuclear data and possibly also deficiencies in the modeling. The gamma rays originating from neutron capture, inelastic scattering and beta decay of the fission fragments are fairly well understood. However, about 40% of this energy is prompt gamma emission (< 1 ns) and here, the available data in evaluated data-bases have the potential for improvement and measurements are at the top of the high priority nuclear data list of the NEA/OECD. Since future generation IV reactor concepts will mostly use fast neutron spectra, prompt gamma emission data in fast neutron induced fission are very important. However, currently very little data of this kind exists.
STUDIES OF EXOTIC NEUTRON RICH NUCLEI
LICORNE coupled to the MINORCA array can be used to study exotic neutron rich nuclei produced in fast neutron induced fission.
The use of the fission process to produce nuclei far from stability and study their excited states via gamma ray spectroscopy has become a topic of great interest in recent years. This is the main principle behind the ALTO radioactive beam facility at the IPN Orsay which has been developed specifically for these purposes. The isotope separation on line (ISOL) technique uses photofission of 238U to produce exotic nuclei and selectivity is achieved by transport of the fragments, laser ionisation and A/Q separation to perform unique identification. However, this selectivity comes at a price, which is the loss of prompt decay information and a beta decay occurring during transport leaving the nucleus several nucleons closer to stability when it is studied.
The study of prompt decay from fast neutron-induced fission of 238U therefore represents a complimentary technique to the photofission and ISOL already carried out at the ALTO facility. Nuclei farther from stability can be reached ; however, the selection of a particular nucleus is more difficult and must be performed via detecting triple gamma coincidences and/or using isomer tagging. It thus requires a very high efficiency gamma ray spectrometer such as the MINORCA array. The possibility of using 238U(n,f) to perform detailed spectroscopy has only recently become possible thanks to LICORNE which has a natural directionality lacking in conventional neutron sources. The cones of neutrons produced in LICORNE exit the spectrometer without directly hitting the Germanium detectors.
This offers some considerable advantages over other experimental setups. Firstly, the compound nucleus is the most neutron rich that can be realistically produced and is 3 neutrons further than stability that thermal induced fission of 235U. Secondly, the neutron beam can be pulsed so time correlations can be exploited to tag isomeric states. Thirdly, nuclei in the refractor region around A118 where masses are symmetric are populated. Fourthly, the very high efficiency of the MINORCA array is ideal for detecting high-fold (> 2) coincidences for the gamma rays emitted in fission which have an average multiplicity of around 8.
FAST NEUTRON CAPTURE AND STUDIES OF THE STATISTICAL PROPERTIES OF HOT NUCLEI
The LICORNE directional fast neutron source coupled to the MINORCA array can be used to study fast neutron capture reactions. The emitted gamma ray cascades, particularly those involving two gamma rays, can then be detected to study the fundamental statistical properties of excited nuclei – nuclear level densities and gamma ray strength functions.
The grand challenge of low energy nuclear physics is to fully understand the nature of nucleonic matter well enough to predict the properties of nuclei, especially those at the extremes of instability where experimentation is difficult to impossible. A key application of such knowledge is the origin of observed elements in our solar system and the universe, in which reaction networks involving tens of thousands of extremely short-lived nuclei must be modeled. Fundamental to the prediction of nuclear properties is a complete understanding of the strength function (SF) : the ability of a nucleus to absorb and emit photons of a given frequency, sometimes colloquially called the color of the nucleus. The generalized Brink hypothesis gives that any collective decay mode of the nucleus is independent of excitation energy (or temperature) and spin of the compound system (i.e., the SF is independent of the excitation energy and spin). Differences in SFs in various energy regions have been observed, however measurements are often generated by different reactions and methods, and therefore these discrepancies could be method-dependent, reaction-dependent, or an indication of a breakdown of the Brink-Axel hypothesis. Further, known discrepancies exist between SFs measured using reactions with significantly different angular momentum transfer, and with SFs extracted using the Principle of Detailed Balance. It is therefore imperative to test the validity of the Brink hypothesis in a reaction-independent and method-independent fashion. The goal is to determine the SF using multiplicity-two events populating low-lying levels and compare this to the existing measurements of the compound system populated via thermal neutron capture to explore the effects of the disparate angular momentum population distributions afforded by the different incident neutron energies.
Thermal neutrons overwhelmingly undergo s-wave capture, transferring at most 1 unit of angular momentum into the nucleus whereas higher energy neutrons undergo p-wave or higher capture, transferring several units of spin. Ordinarily, measurement of high-energy (n,gamma) reactions is difficult because the cross section is many orders of magnitude lower than at thermal energies. Thus, any background thermal flux produced by scattering of neutrons in the room environment will overwhelm the primary signal. The TSC method provides discrimination for high-energy events, by requiring that the two rays detected add up to the known excitation energy, in this case defined exactly by the neutron energy plus the neutron binding energy ; Thermal capture cannot produce two rays that sum to this higher energy.
MEASUREMENT OF THE NEUTRON RESPONSES OF ADVANCED DETECTORS
LICORNE will soon use the p(11B,11C)n reaction to extend the neutron energy range from 0.5 MeV up to around 12 MeV. This then opens up the possibility to use these neutrons for testing the neutron response of advanced detectors over the neutron energy range encountered in typical nuclear reactions.
Several instruments dedicated to the physics program of SPIRAL 2 facility are now at the end of their R&D program and prepare the construction of demonstrators or of their final setups. This is the case for the PARIS detector community (Photon Array for studies with Radioactive Ion and Stable beams) where the construction of the demonstrator (about 25% of the total array) has already begun and it should be available for use in physics experiments at the beginning of 2015.
The PARIS detector is intended to detect prompt γ-rays over a wide energy and multiplicity range as emitted in fusion-evaporation, fusion-fission, deep-inelastic and transfer reactions. In these reactions, the gamma rays are almost always accompanied by the emission of neutrons. The timing performance of the PARIS phoswich detectors (time resolution below 1 ns) should allow simultaneous detection of both gamma and neutrons with near-perfect identification via time-of-flight (TOF) discrimination, even at very small distances (10-30 cm). However, to undertake such discrimination a good knowledge of the detector response to both gammas and neutrons is obviously necessary.
To complete the characterization of the PARIS cluster we will measure the response to neutrons in the energy range 0.5 to 12 MeV. The ultimate goal will be to know the response of the PARIS cluster detectors sufficiently well so that neutron/gamma discrimination and neutron energy determination can be carried out by time-of-flight over short distances for a large range in neutron energies typically emitted in heavy ion reactions.
The LICORNE neutron source will operate in pulsed white-source/mono-energetic mode, producing fast neutrons between the energies of 0.5 – 12 MeV using the p(11B,11C)n reaction. The kinematic curves for the outgoing neutrons are given in Figure 1. This inverse binary reaction has a maximum angle of emission for the neutrons in the Laboratory Frame which makes that the neutron beam is forward focused. As an important consequence, the room background will be low and uncorrelated interactions from neutrons undergoing multiple scatterings in the experimental hall is expected to be minimal especially since there is no roof, which allows scattered neutrons to escape. The choice between white-source or mono-energetic modes is made by using either a thick PP target (100 µm) or a thin PP target (several µm). A scan in neutron energy (mono-energetic mode) can be performed by varying the incoming 11B energy.