Neutron spectrum determination of accelerator-driven d(10)+Be neutron source using the multi-foil activation technique

Highlights

• Development of accelerator-driven neutron sources and neutron fields.

• Fast neutron spectrometry using the multi-foil activation technique.

• Nuclear data measurement and integral validation.

• Neutron fields for neutron activation analysis and interdisciplinary research.

Abstract

New neutron field of the compact accelerator-driven fast neutron source based on the cyclotron and beryllium target station was studied and developed at the NPI CAS. Neutrons were generated using the deuteron bombardment on 8 mm thick beryllium layer by the 10.3 MeV deuteron beam. Broad neutron energy spectra of the d(10)+Be source reaction at two source-to-sample distances (14 mm and 154 mm) were determined for the first time at the NPI. Sets of eight activation materials comprising Au, Co, Ti, In, Al, Fe, Ni, and Nb were irradiated in the d(10)+Be neutron field, and activated dosimetry foils were analysed using the nuclear gamma-spectroscopy technique (HPGe detector) and reaction rates were measured. Neutron energy spectra of the d + Be source reaction for 10.3 MeV deuterons were reconstructed employing the modified version of the SAND-II unfolding code. For 9.7 μA deuteron beam, the fast neutron flux of developed d(10)+Be neutron field reached the value of 1.4 × 10¹⁰ cm⁻²s⁻¹ at a distance of 14 mm from the source target and 4.1 × 10⁸ cm⁻²s⁻¹ at a distance of 154 mm. The novel d(10)+Be neutron field with energy range up to 15 MeV is convenient for neutron cross-section data validation, experimental simulation of the fast neutron spectrum of experimental nuclear reactors, applications of neutron activation analysis, and radiation hardness tests of materials important for nuclear energetics.

Introduction

Currently, more than 220 research reactors are in operation all over the world (RRDB IAEA, 2021). Small research reactors with thermal power up to 1–10 MW and corresponding neutron flux approx. of 10¹² cm⁻²s⁻¹ are used for isotope production, neutron radiography, material irradiation, neutron activation analysis, nuclear data provisioning, boron neutron capture therapy, transmutation silicon doping, and transmutation gemstone colouration (RRDB IAEA, 2021). However, some of the small research reactors built in the last quarter of the 20th century are gradually decommissioned because they have reached the end of their lifetime period. Moreover, the operation and construction of research reactors is quite expensive. Today, the cheaper and smaller alternative to nuclear reactors seems to be the compact accelerator-driven neutron sources. They can be used for the most of the above mentioned research activities realised at the research reactors. Just as the research nuclear reactors, the compact accelerator-based neutron sources are also capable to be involved in interdisciplinary studies. Depending on used neutron source reaction, the neutron fields provided by the accelerator-driven neutron sources represent the useful tools e.g. for application of neutron activation analysis. For such purposes, the neutron sources that utilize the deuteron or proton beams with energies up to 10–20 MeV and thick targets are more convenient. Many compact neutron sources in the world uses the beryllium targets.

Nuclear Physics Institute of the Czech Academy of Sciences (NPI CAS) in Rez operates the compact accelerator-driven fast neutron sources (CANAM, 2021). These sources are primarily focused on nuclear data measurement and validation, especially for the future fusion energetics. To extend their experimental utilizations mostly towards some more traditional research reactor applications (such as the neutron activation analysis), the neutron field generated by the d + Be source reaction for 10.3 MeV deuteron beam has been studied recently. The deuteron induced reaction on beryllium target was selected for these purposes due to intensity reasons and on account of fact that the many compact neutron sources of broad energy spectra operated in the world are built-on the d + Be source reaction and thick Be-targets (Allisy et al., 1989; Cierjacks, 1983).

The conventional compact accelerator-driven neutron sources utilize mostly the thick beryllium layers that benefit from high thermal stability (melting point of 1 287 °C (Winter, 2019)) under the load of the intensive charged particles beam. The deuteron beams are preferred over the proton beams due to higher value of the neutron yield from the deuteron bombardment of beryllium. The d + Be interaction is the best neutron source for neutron radiotherapy (Wyckoff et al., 1976); with suitable layers of moderators it is utilized for the boron neutron capture therapy. The d + Be neutron sources also provide the important neutron fields for material research, isotope production, and silicon transmutation doping. It seems to be the really important tool for the neutron activation analysis as well.

The main neutron producing reactions from the deuteron bombardment of beryllium (thick target), that form the neutron spectrum, are listed in Table 1. The table also includes the reaction energy (Q-value) and threshold energy. The (d,n) deuteron stripping process produces the high energy spectrum component at E_n > 0.8 × E_d (Allisy et al., 1989). The multi-body break-up interactions (d,αn), (d,np), and (d,n2p) form the broad maximum around 0.4 × E_d. The inelastic deuteron scattering and three-body interactions (d, 2n) and (d,np) create the low energy fraction of spectrum below 2 MeV and the peak at an energy of 0.8 MeV (Cierjacks, 1983; Allisy et al., 1989).

The most of research works related to the d + Be source reaction come from the 70s and 80s of the last century, and they mainly include the results published by Lone et al. (1977), Weaver et al. (1973), Saltmarsh et al. (1977), Brede et al. (1989), Graves et al. (1979), Meulders et al. (1975), Madey et al. (1977), Bonnett and Parnell (1982), Meadows (1993), and Waterman et al. (1979), who were concerned with the spectral flux distributions and neutron yields measurement.

Based on the d + Be neutron spectra measurements carried out by M.A. Lone and J.P. Meulders, the empirical equation for fluence-averaged energy of neutrons ${\bar{E}}_{\text{n}}$ above 2 MeV in the forward direction was derived (Lone et al., 1977; Meulders et al., 1975; Cierjacks, 1983):

$${\bar{E}}_{\text{n}}=0.4\times E_{\text{d}}-0.3\mathrm{f}\mathrm{o}\mathrm{r}{E}_{\text{d}}>10\mathrm{M}\mathrm{e}\mathrm{V},$$

and it is valid for deuteron beam energy E_d above 10 MeV.

On the other hand, C.J. Parnell found that fluence-averaged energy ${\bar{E}}_{\text{n}}$ of neutrons above 0.5 MeV for thick targets is about (Parnell, 1972; Allisy et al., 1989):

$${\bar{E}}_{\text{n}}=0.42\times E_{\text{d}}\mathrm{f}\mathrm{o}\mathrm{r}{E}_{\text{d}}>7.5\mathrm{M}\mathrm{e}\mathrm{V},$$

and this empirical equation can be used for deuteron beams with energies higher than about 7.5 MeV.

Similarly, J.W. Meadows estimated the averaged neutron energy ${\bar{E}}_{\text{n}}$ for his measurement of the d + Be neutron spectra for deuteron energies between 2.6 and 7.0 MeV by expression (Meadows, 1993):

$${\bar{E}}_{\text{n}}=1.210+0.277\times E_{\text{d}}.$$

The illustrations of experimentally found shapes of the d + Be neutron spectra measured by J.P. Meulders and M.A. Lone using the time-of-flight (TOF) method and scintillation probes are depicted in Fig. 1, Fig. 2 (Meulders et al., 1975; Lone et al., 1977). Beside the measurements of the thick targets yields and spectra, the neutron emission spectra from the thin targets were investigated as well, and double-differential neutron production cross-section was determined. Example of double-differential cross-section of the d + Be reaction measured by Y. Iwamoto is given in Fig. 3 (Iwamoto et al., 2009).