Zahlavi

Neutrons, Charged particles

Fast neutron generators (FNG) based on the reactions of the accelerated protons (cyclotron U120-M) with the Li and Be targets present the experimental base for the research activities of our group. The irradiation of the Li target with protons of energies ranging from 20 MeV to 35 MeV produces quasi-monoenergetic neutron spectra (half of the produced neutrons in the peak – 18-33 MeV depending on the proton energy, typical peak neutron flux 109 n/cm2/s). The Be target irradiated with 35 MeV neutrons produces continuous neutron spectrum of high intensity (1011 n/cm2/s in the closest irradiation position).

We are mainly focused on the experimental validation of the neutron cross-section libraries for the materials used as neutron monitors (Au, Bi, Co, Nb, Tm, …) and construction material (Fe, Cr, W, Ta, …) in the future thermonuclear technologies (IFMIF-DONES, ITER). The validation is based on the comparison of the evaluated cross-sections (libraries) with the experimental results. These are obtained by the irradiation of the studied material with well-defined quasi-monoenergetic or continuous spectrum neutron beams and subsequent analysis of the induced activities using gamma-spectrometry. The pneumatic tube system is used for the transport of the studied samples from the irradiation position to the gamma-spectrometry laboratory. Few seconds transport time allows the study of short-lived isotopes (l1/2>10s).

The detailed knowledge of the source neutron spectra and intensity is crucial for our experiments and validations. The calibration measurements of the neutron spectra are performed by several methods (Proton Recoil Telescope, Time-Of-Flight, neutron activation analysis). Furthermore, for the p-Li source the total number of peak neutrons produced during the irradiation is determined by the measurement of the 7Be production in the Li foil using gamma spectrometry (ca. 2%). The measurements of the neutron yields are extrapolated to the positions of irradiated samples (few cm from the converter) using Monte Carlo simulations with the MCNPX code and LA150H or JENDL4.0/HE libraries.

The neutron Time-Of-Flight method is based on the pulsation of the cyclotron beam (bunches are extracted at each turn, with frequency ranging from 20 to 25 MHz). Due to the effect of the frame overlap the neutron spectrum at lower energies cannot be determined, but the measurements of its high energy part are accurate. High energy neutron transmission through various materials is performed using Time-Of-Flight – measurements of (n,tot) reaction cross-section (on eg. oxygen).

The p+Be generator produces intense fluxes of continuous-spectrum neutrons (with the maximum energy of 34 MeV) which are used for the neutron hardness tests, for the studies of the primary radiation damage by neutrons, multidisciplinary research, etc. Various electronic components supposed to operate in the neutron flux are tested here (eg. readout electronics for the ATLAS/CERN calorimeter in the cooperation with Max Planck Institute, Munchen). Longer irradiations with intense neutron fluxes produce enough radiation defects to be detected by the Positron Annihilation Lifetime Spectrometry (cumulative flux of 1016 n/cm2 is necessary). For the studied materials the primary radiation damage in the units of the displacement per atom (dpa) can be determined.

In 2018, the collimation system for the neutron beam was designed. The collimated neutron beams make possible the measurements in which the studied sample is exposed to the high neutron flux while the online detectors are placed in few orders of magnitude lower neutron fluxes. Two detection systems are being constructed on the newly acquired collimated neutron beam:

1) the evacuated chamber with the Si telescopes on the rotating table will be used for the studies of the (n,charged particle) reactions and

2) the array of four HPGe detectors placed around the studied sample will serve to study the (n,*g) reactions.

The modulation of the cyclotron beam by the microcontroller allows the measurements of the gamma radiation from isotopes with decay times down to ms as well as the studies of the delayed neutrons from the fission process. The neutrons from the fission products are moderated by the polyethylene setup and detected with the array of the BF3 detectors. The experimental results from the two types of experiments are used to validate the fission model calculations (eg. GEF) at higher neutron energies.

Our group is taking part in the development of the neutron target station and preparation of first day experiments at the NFS@SPIRAL2 (Ganil, France). In the frame of the research infrastructure SPIRAL2-CZ we prepared the specialized irradiation and measurement station for the studies of the material activated with proton and deuteron beams at the NFS. The irradiation, transport and the measurement of the irradiated samples with the HPGe detector is fully automatized.

The studies of the activation by the charged particles is important in various research fields (eg. international energetic projects). In cooperation with the theoreticians from IFIN-HH (Magurele, Romania), the department performs the activation analysis of the selected materials (Al, Cu, Nb, Ni, Fe, …) with the deuteron beams up to 20 MeV. The results bring important information for the reaction databases for future IFMIF-DONES and similar accelerator based facilities. The results are showing us that the whole complex of the processes has to be taken in account at the description of the experimental results – direct reactions, pre-equilibrium and compound nucleus processes. The results have initiated the intensive improvement of the nuclear models, which is important for the description of the reactions that cannot be measured experimentally at the moment. The improved theoretical models provide safer future energetics and better understanding of the processes of the nucleosynthesis, eg. the explosion of the supernovas.