Athens Neutron Facility Characterized for Precision Nuclear Data

A review article published in EPJ Techniques and Instrumentation details the operational capabilities and recent research activities of the neutron facility located at the National Center for Scientific Research “Demokritos” in Athens, Greece. The study, authored by R. Vlastou, provides a comprehensive overview of how the facility produces quasi-monoenergetic neutron beams and how these beams are utilized for high-precision cross-section measurements by the Nuclear Physics group at the National Technical University of Athens. The facility operates at the 5.5 MV Tandem T11/25 Accelerator Laboratory, where specific nuclear reactions are employed to generate neutron beams across a wide energy spectrum, ranging from approximately 120 keV to 21 MeV. This capability is critical for fundamental research in nuclear physics and astrophysics, as well as for practical applications in medicine, nuclear technology, dosimetry, and industry, particularly in the design of Generation-IV reactors and Accelerator Driven Systems.

The production of quasi-monoenergetic neutron beams relies on four distinct nuclear reactions, each yielding neutrons within specific energy ranges. The 7Li(p,n)7Be reaction produces neutrons in the range of approximately 120 to 650 keV, while the 3H(p,n)3He reaction generates neutrons between 2.0 and 5.3 MeV. Higher energy beams are produced via the 2H(d,n)3He reaction, which yields neutrons in the 4.0 to 11.4 MeV range, and the 3H(d,n)4He reaction, which produces neutrons between 15 and 20 MeV. These beams are considered quasi-monoenergetic because their energy spectra consist of a single line with an energy width much less than the energy itself, although real sources also produce parasitic neutrons from thermal regions up to a few MeVs. The target assemblies for all these reactions are air-cooled during proton or deuteron beam irradiations, with beam currents maintained at approximately 700 nA to 1 μA.

Characterizing these neutron beams is essential for ensuring the reliability of cross-section measurements, especially in the absence of time-of-flight capabilities. The facility employs the multiple foil activation technique in combination with extensive Monte Carlo simulations to determine the energy dependence of the neutron beam flux. The maximum flux has been determined to be of the order of 10^5 to 10^6 n/cm^2/s, using reference reactions such as 27Al(n,α), 197Au(n,2n), and 93Nb(n,2n). The variation of the neutron beam flux is monitored in real-time using a BF3 detector placed at a distance of 2 to 3 meters from the neutron-producing target, while a BC501A scintillator detector is used to investigate the neutron beam energy distribution. Spectra from the BF3 monitor are stored at regular intervals of approximately 100 seconds during the irradiation process to ensure data integrity.

For the low-energy 7Li(p,n)7Be reaction, a 7LiF target on an aluminum backing is used with proton beams in the energy range of 1.9 to 2.4 MeV to produce purely monoenergetic neutrons at zero degrees. At proton energies above 2.4 MeV, neutron emission to the first excited state in 7Be becomes possible, producing a second group of monoenergetic neutrons. The BC501A scintillator detector was utilized to investigate these neutron energy spectra by rejecting gamma pulses and reconstructing the recoil energy spectra in the liquid scintillator. This method allowed for the energy calibration of the detector in the low-energy region, confirming the production of purely monoenergetic neutrons at specific proton beam energies.

The 3H(p,n)3He reaction utilizes a titanium-tritiated target consisting of a 2.1 mg/cm2 Ti-T layer on a 1 mm thick copper backing for heat conduction. Proton beams in the energy range of 3.4 to 6.5 MeV are used to produce neutrons between 2 and 5.3 MeV. While purely monoenergetic neutrons are generated in the range of approximately 2 to 3 MeV, higher energies introduce parasitic neutrons from (p,n) reactions on molybdenum, titanium, copper, carbon, and implanted deuterium. Extensive MCNP5 simulations coupled with NeuSDesc results were performed to investigate these parasitic neutrons, and preliminary results for a 3.8 MeV proton beam leading to 2.5 MeV neutrons were presented using the GEANT4 code to account for all interactions of the proton beam with the experimental setup.

For the 2H(d,n)3He reaction, a deuterium gas cell pressurized to approximately 1 atm is used, with neutrons lying in the energy range of 4 to 11.2 MeV corresponding to deuteron beam energies of 0.8 to 8.2 MeV. The energy spectrum and characteristics of the beam were studied using the multiple foil activation technique in combination with the SULSA code, as well as with the BC501A liquid scintillator and deconvolution of its recoil energy spectra using the DIFBAS code. Results from investigations at neutron beam energies of 5.5, 6.3, 7.0, 8.0, 8.5, 9.0, 9.5, and 10 MeV indicated that beams at 5.5 and 6.3 MeV are monoenergetic, while above 7 MeV, parasitic low-energy neutrons start to appear around 4 MeV and become more significant as the main neutron beam energy increases.

The 3H(d,n)4He reaction employs the same titanium-tritiated target used in the 3H(p,n)3He reaction, producing neutrons in the region of 15.3 to 20.9 MeV at corresponding deuteron beam energies of 2.0 to 4.3 MeV. The study of neutron energy spectra generated by deuterons on this target was carried out using Monte Carlo simulations with the MCNP5 code, utilizing the NeuSDesc output file. Several foils were irradiated at different energies, most corresponding to reference reactions, to extract the experimental neutron beam fluence. The detailed geometry of the experimental setup for each irradiation was simulated, and the results were compared with experimental data. For example, at 17.9 MeV, the experimental and simulated neutron fluence in successive foils showed very good agreement, indicating that the simulations can be trusted for estimating neutron fluence.

The neutron facility has been extensively used over the past 15 years by the NTUA group for the measurement of (n,2n) and occasionally (n,3n), (n,p), and (n,α) reaction cross sections on several isotopes of americium, hafnium, germanium, iridium, and gold. These measurements were conducted using neutron beams in the energy region from approximately 7 to 11 MeV produced via the 2H(d,n)3He reaction and from 15 to 21 MeV derived via the 3H(d,n)4He reaction. High-purity foils were stacked between two aluminum reference foils for irradiation, and the induced gamma-ray activity of the samples and reference targets was measured offline by HPGe detectors. The activity measurements were carried out at a distance of approximately 10 cm from the detector window to minimize pile-up and true coincidence summing effects, with a 152Eu point source used to determine the absolute efficiency of each detector.

Theoretical calculations accompany the experimental measurements to investigate model parameters in statistical model calculations. The study of the measured reactions is usually accompanied by theoretical cross-section calculations in the framework of the Hauser–Feshbach theory, using the EMPIRE and TALYS codes. These calculations are carried out in a wide energy range for the measured data of each reaction as well as for data reported in the literature. The reactions on gold and iridium are of particular interest because the (n,2n) reactions on 191Ir and 197Au lead to the population of both ground and isomeric states of the residual nuclei. The theoretical study of these reactions, due to the existence of high-spin isomeric states, serves as a powerful tool for obtaining information on the structure of the involved nuclei.

In the case of 197Au(n,2n) reactions, cross sections were measured in the energy range of 9.0 to 10.5 MeV and 15.3 to 20.9 MeV. This data, along with data for competing reactions such as (n,3n), (n,elastic), (n,α), (n,p), and (n,total), were simultaneously reproduced in a satisfactory way by both EMPIRE and TALYS calculations. The high angular momenta treatment in the EGSM level density model in the EMPIRE code helped to reproduce the experimental values of the isomeric cross sections, while in TALYS, a small increase of the spin cut-off parameter was sufficient to successfully reproduce the cross section of the isomeric state along with all other competing channels.

Similarly, for 191Ir(n,2n) reactions, cross sections were measured in the energy range of 10 to 11.3 MeV and 15.3 to 20.9 MeV. These data, along with data for competing reactions and other isotopes like 193Ir, were reproduced satisfactorily by EMPIRE and TALYS calculations. It is noteworthy that these calculations were performed with a similar combination of model parameters that had also been successfully used for the neighboring 197Au nucleus. This similarity is not surprising since both gold and iridium nuclei belong to the transitional region from well-deformed to spherical nuclei near the shell closure Z = 82, where high-spin intruder configurations result in high-spin isomeric states unable to communicate with neighboring states.

The tandem accelerator at NCSR “Demokritos” is currently under a major upgrade through the CALIBRA/EYIE program, with the beam current expected to be considerably increased in the near future. The neutron beams have been characterized using the multiple foil activation technique and extensive simulations, and they have been used for cross-section measurements of neutron-induced reactions. The investigation of these reaction cross sections is followed by theoretical calculations implementing the EMPIRE and TALYS codes. In the near future, these techniques will be applied again for the investigation of the upgraded neutron beams, while neutron activation measurements will continue to be an important research project of the Nuclear Physics group at the National Technical University of Athens. The facility’s ability to produce quasi-monoenergetic neutrons and characterize them with high precision remains a vital resource for advancing nuclear data and supporting various technological applications.

Subject of Research: Characterization of quasi-monoenergetic neutron beams and activation cross-section measurements at NCSR Demokritos

Article Title: The neutron facility at NCSR ‘Demokritos’ and neutron activation research activities of NTUA

Article References: Vlastou, R. (2023). The neutron facility at NCSR ‘Demokritos’ and neutron activation research activities of NTUA. EPJ Techniques and Instrumentation, 10(1), Article 4. https://doi.org/10.1140/epjti/s40485-023-00091-8

Image Credits: AI Generated

DOI: 10.1140/epjti/s40485-023-00091-8

Keywords: neutron physics, nuclear data, cross-section measurements, quasi-monoenergetic neutrons, NCSR Demokritos, NTUA, Monte Carlo simulations, activation technique, tandem accelerator, nuclear reactions, neutron, facility