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Description
In the Universe, the observed elements heavier than iron (Fe) are mostly produced by neutron-capture processes in the stars. These processes are defined as s (slow) and r (rapid) processes. The experimental measurement of the neutron capture cross sections is an actively working task, since it is an essential element in the stellar reaction rates calculations and thus, the possibility of reproducing the observed abundance of the elements in the universe. In stellar interiors, the plasma is in thermodynamic equilibrium and the particle velocities are described by a Maxwell–Boltzmann distribution. To calculate the reaction rate in the neutron capture processes it is common to work with the Maxwellian Average Cross Section (MACS). This parameter is defined as the reaction rate scaled by the most probable velocity of the Maxwell- Boltzmann distribution. For the s-process mainly, the MACS directly describes the reaction rate inside the stars, for a given temperature and neutron density. Hence, the importance of determining the MACS with the least possible uncertainty. A very accurate and direct MACS measurement, under certain conditions, can be performed by neutron activation analysis providing a neutron beam with the stellar spectrum i.e., a Maxwell-Boltzmann neutron spectrum (MBNS). Before any MACS measurement, a characterized neutron beam with a stellar spectrum is mandatory, and this is the main purpose of this work.
The $^7$Li(p,n)$^7$Be nuclear reaction was employed as neutron source. In the experimental measurement, neutron time of flight spectrometry (nTOF) was implemented to determine the neutron spectrum, using a 600 kHz proton pulsed beam at the Van de Graaff accelerator of the Legnaro National Laboratory of the National Institute of Physics Nuclear (LNL-INFN), in Padua, Italy.
A new approach to transform TOF spectra into energy spectra was implemented, using the detector response matrix. The proposed conversion method considers not only the mean moderation time of neutrons in the detector, but also its distribution in time. Simulations with MCNPX code to validate the conversion method were performed.
In order to measure directly the MACS with the activation method, a MBNS must be produced, as far as similar to the theoretical Maxwell-Boltzmann distribution. Mastinu et al. (2009) [1], proposed a method to produce a high-quality Maxwell-Boltzmann neutron spectrum at different thermal temperatures (kT). The method is based on the idea of "shaping the proton beam energy distribution to shape the neutron energy beam to a desired distribution". This method avoids the use of moderators.
To obtain a Maxwell-Boltzmann neutron spectrum with 28 keV of thermal temperature, an initial proton energy of 3170 keV and a 51 $\mu$m thickness aluminum (Al) foil, as proton energy shaper, were employed. Differential angular neutron energy distribution from 0 to 90 degrees in steps of 10$^{\circ}$ were measured to obtain the 0$^{\circ}$-90$^{\circ}$ integrated neutron spectra. Four Li-glass detectors were employed, placed at a 50 cm from the lithium target. The emitted neutron energy spectrum was obtained applying the proposed conversion method using the response matrix of the Li-glass detector. The expected MBNS has been measured and the obtained results will be presented.
[1] Mastinu, P. F., Martín Hernández, G., & Praena, J. (2009). A method to obtain a Maxwell-Boltzmann neutron spectrum at kT=30 keV for nuclear astrophysics studies. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 601(3), 333–338. https://doi.org/10.1016/j.nima.2009.01.005