Speaker
Description
The production of so-called ‘monoenergetic’ neutrons is standardized by ISO 8529 and can be achieved using ion accelerators such as the one at AMANDE facility from the Laboratory for micro-irradiation, neutron metrology and dosimetry (i.e. the LMDN from IRSN/Cadarache/France). The ions, typically proton or deuteron, are accelerated to speeds of up to a few MeV and are sent to a target containing abundantly a “well-chosen” nucleus of low atomic number (typically 2H, 3H or 7Li). The main energy at a given angle is then linked to the kinematics of the YZ(x,n)AB reaction allowing the neutron to be "ejected" from the compound nucleus (YZ+x). However, one or more ‘parasitic’ neutron production reactions are possible, particularly with deuteron beams. In this case, one or more additional groups of neutrons are present at different energies, for example from undesired presence of a different nucleus as the main one. These contributions can have several origins, sometime very surprising. For example, despite the care taken in target production, there have already been cases of nuclei migrating into the target support or the presence of unwanted nuclei such as 12C or 16O. Consequently, the complete absence of secondary production concerns only a small part of the available neutron fields. So, in order to establish the fluence energy distribution, it is necessary to be able to determine the fluence of each group and therefore to have the widest possible measurement range. One way of obtaining this quantity is to have a high-performance neutron spectrometer, which is however difficult to do in practice and required a long procedure. A more effective and easy way of identifying and evaluating neutron energy groups is to discriminate them by their time of flight. This technique, which can be done if the time of neutron production is known, is usually implemented using scintillators. Scintillators have three decisive advantages: high efficiency, very fast reaction time (~1ns) and the ability to detect and discriminate between photons and neutrons. Historical scintillator limitation is an energy threshold of around 1 MeV neutron. As a result, some of the contributions below this threshold may be poorly identified or not identified at all. Since 2018, the LMDN has been working on extending the lower time-of-flight limits down to 100 keV or better and within the same time, modernizing the measurement procedure. This work is being carried out under the behalf of the French National Metrological Institute (NMI, the "LNE"), which has designated the LMDN as a reference laboratory for the determination of neutron fluence and associated dosimetric quantities. Work carried out since 2018 has demonstrated that the new stilbene crystals have detection capabilities below 100 keV, enabling the detection threshold to be lowered by around a decade. Four scintillators (2 stilbenes and two EJ309) coupled to digital acquisition have been characterized by the LMDN in both neutrons and photons. The two aims of the project are: first to extend the time-of-flight references on AMANDE over the 100 keV - 1 MeV decade for wide and complex neutrons fields, and second to modernize the acquisition and analysis tools. These scintillators will determine the fluence energy distributions on AMANDE, using the time-of-flight technique if necessary. The energy range covered in a single measurement has been considerably increased and is now 100 keV to 22 MeV. The new functionalities provided by the developed system will give a much better assessment of these parasitic reactions, which are unfortunately still too often not considered or not well considered. The system is operational, as demonstrated by the time-of-flight measurement experiments carried out in September 2023 at Neutrons For Sciences Facility (NFS) and in November 2023 at the German NMI (PTB, Brunswick Germany). The entire system will be installed at the AMANDE facility early 2025. In this article, we will present the time-of-flight experiments carry out at NFS and PTB facility with a comparison between measure at some ISO 8529 reference energy. After this result, implementation on AMANDE will be presented with a particular attention to the 100 keV - 1 MeV energy range, since there are few, if any, equivalent measurements. After this, we will present the various secondary contributions observed and the process used to demonstrate their origin. Even the lack of contribution below 1 MeV could be an interesting result. A general assessment of the fluences of the secondary reactions observed and compared with the main reactions will be presented and we will finish by listing the potential reactions in relation to the beam configurations. The irradiation certificates delivered by the LMDN could provide new information for users. The improvement of these references will be an important contribution to scientific research and will allow better characterization of neutron detectors.
