Some modelisation aspects of the instrumental transformation of neutron kinetic energy into electrical impulse amplitude

The differential energy characterisation of neutron emission or fluxes may be called difficult especially when an approach to metrological standards is required. The use of somewhat complex computing programmes is unavoidable here since the process of selective detection and interpretation of measurement results involves: - Firstly of need to simulate by so-called MONTE CARLO techniques the responses of the various kinds of detector used (the detector responses are so dependent on changes in the environment that experimental calibration -too expensive to be easily repeated from one situation to the next- cannot be recommended); - secondly the need to answer pratically (in a discretized situation by matrix manipulations) to the problem, theoretically without a unique analytical solution, of resolving a Fredholm equation of the first kind. For the understanding, use and as necessary modification of the appropriate codes it is most important to have a very precise conceptual view of the mechanisms involved (in the sequence of direct instrumental transformations). For didactic purposes the present article attempts in a very simple case to give an idea, at increasing levels of complexity: stylisation, schematisation, modelisation, simulation, of the successive stage (neutron-proton energy transfer, analog conversion of ionising particle energy into electric impulse signal size) involved in the formation and understanding of an energy spectrum of recoil nuclei. All these instrumental distortions are made explicit in a kind of underlying two-dimensional ''density''; in its discretized form this is called the instrumental transfer matrix and its understanding and treatment are the ''keys'' to the deconvolution of the recorded spectrum, giving access to the differental distribution of the emitted neutron energies