Abstract
The cyclotron-based fast neutron source at NPI produces mono-energetic neutron fields up to 35 MeV neutron energy using the p + 7Li(carbon backing) reactions. To be applied for activation cross-section measurements, not only the intensity of neutron peak, but also the contribution of low-energy continuum in the spectra must be well determined. Simulations of the spectral flux from present source at a position of irradiated samples were performed using CYRIC TOF-data validated in the present work against LA150h by calculations with the transport Monte Carlo code MCNPX.Simulated spectra were tested by absolute measurements using a proton-recoil telescope technique. The recoil-proton spectrometer consisted of a shielded scattering chamber with polyethylene and carbon radiators and the ΔE1-ΔE2-E telescope of silicon-surface detectors located to the neutron beam axis at 45° in the laboratory system. Si-detectors were handled by usual data acquisition system. Dead-time – and pulse-overlap losses of events were determined from the count rate of pulse generator registered during duty cycle of accelerator operation. The proton beam charge and data were taken in the list mode for later replay and analysis.The calculations for 7 Li(p,n) and 12 C(p,n) reactions reasonably reproduce CYRIC TOF neutron source spectra. The influence of neutron source set-up (proton beam dimensions, 7 Li-foil, carbon stopper, cooling medium, target support/chamber and the geometry-arrangement of irradiated sample) on the spectral flux is discussed in details.
Highlights
The quasi-monoenergetic p-7Li source with neutron energies between 18 to 36 MeV has been build up at NPI [1]
The proton beam from the NPI isochronous cyclotron U120M strikes the self-supporting 7Li foil backed by carbon disk which serves as a beam stopper
The neutron flux of 6.108 n/cm2/s is calculated for 38 MeV /10 μA proton beam and for the target-to-sample distance of 50 mm
Summary
The quasi-monoenergetic p-7Li source with neutron energies between 18 to 36 MeV has been build up at NPI [1]. The 7Li(p,n) reaction on thin 7Li foil target is commonly utilized for activation cross sections measurement at higher (20 MeV) neutron energies. To carry out the integral validation- and/or the cross-sections measurements, the neutron spectral flux at sample positions needs to be known. The spectral neutron yield from the source reaction p +7 Li(C backing) was measured by TOF technique at proton energies of 20, 25, 30, 35 and 45 MeV (spectral data measured by the CYRIC group are available from Fig. 2 of the paper [2]). Present work is devoted to the experimental test the MCNPX simulation of flux-density gradient and other geometry aspects of spectral flux at sample positions (in the vicinity of neutron source), using the proton-recoil-telescope method
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