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Investigation of neutron-induced reactions with the release of charged particles

Measuring the cross sections of neutron-induced reactions with the release of charged particles is of interest in several areas of science and technology. Thus, data is required for various structural materials, neutron absorbers and other nuclear technology applications. Accuracy and energy range requirements vary from 1 to 10 percent and from thermal to 8 and sometimes up to 20 MeV, respectively.

At FLNP JINR, in collaboration with Peking University (PRC), experimental and theoretical investigations of neutron-nuclear reactions, the products of which are charged particles, are carried out. The data obtained are important both for fundamental nuclear physics and nuclear astrophysics. Based on the results of measurements of cross sections and angular distributions of emitted charged particles, the parameters of the optical α-particle potential are refined and their dependences on the charge and mass numbers of the targets, as well as on the energy of the incident particles, are determined. The resulting sets of parameters are required for calculating cross sections and angular distributions of reaction products, including those on nuclei on which the experiment was not carried out.         In astrophysics, reaction cross sections (n, α) are very important for selecting scenarios for the nucleosynthesis of elements. It is believed that most elements heavier than iron are produced by neutron capture and beta decay (s- and r- processes), while rare proton-excess isotopes are produced mainly by photodissociation reactions (p-process). Measurements of reaction characteristics (n, α) are required for a better understanding of the s-process in the case of light nuclei and for heavy nuclei - for constructing the α-particle potential used to calculate reactions occurring in the p-process on unstable nuclei. Experiments can be carried out at IREN (En=th–100 keV); electrostatic accelerators EG-5 FLNP, EG-4.5 PKU, Beijing (En=3–6 MeV); tandem accelerator HI–13 CIAE, Beijing (En=8–11 MeV) and CSNS in China.

In experiments carried out at FLNP JINR, ionization chambers of various designs are used as a detector of protons and α-particles produced in the (n, p) and (n, α) reactions, since having a fairly good energy resolution, they allow to register charged particles in almost 4π geometry that is very important when working with small quantities of the substance under study (for instance, with radioactive targets separated by isotopes) and relatively small reaction cross sections. The experimental design is shown in Figure 1.

Figure 1. Diagram of the experimental facility

The currently used grid ionization chamber has a symmetrical common cathode structure and is also equipped with a special sample changer. A schematic view is shown in Figure 2.

Figure 2. Schematic view of the IC device and cathode 1 – cathode; 2 – Frisch grids; 3 – anode; 4 – fission chamber cathode; 5 – stainless steel body; 6 – rotating device for changing samples; 7 – valve; 8 – pressure gauge

At the EG-5 accelerator of the Frank Laboratory of Neutron Physics of JINR, measurements of the cross section of the 91Zr(n, α)88Sr reaction were carried out in the neutron energy range from 3.9 to 5.3 MeV. An ionization chamber with a double grid was used as a detector. The data obtained for the 91Zr(n, α)88Sr reaction can be used to verify the estimated data used in simulation of various processes, as well as in optimization of the parameters of theoretical models. A 2D spectrum of α-particles from the 91Zr(n, α)88Sr reaction at En=5.3 MeV, in the “forward” and “backward” directions without subtracting background events is shown in Figure 3. The measured cross sections for the 91Zr(n, α)88Sr reaction and the results of calculations in TALYS–1.9 are presented in Table 1. The energy dependence of the cross section for this reaction obtained by us is shown in Figure 4 in comparison with the estimated data and calculations in TALYS–1.9 with optimized parameters.  

Figure 3. 2D spectrum of α-particles from the 91Zr(n, α)88Sr reaction at En=5.3 MeV, direction “forward” (a), “backward” (b) without subtracting the contribution of background reactions.

Table 1. Measured (n, α) cross sections for the 91Zr(n, α)88Sr reaction and TALYS–1.9 results.



Cross section, mb



Amount (forward+backward)































Figure 4. Show the experimental cross sections for the 91Zr(n, α)88Sr  reaction in comparison with the results of calculations and using TALYS-1.9.

An important task is the investigation of resonance reactions with the emission of α-particles and in particular, the measurement and explanation of α-widths. The priority task is to clarify the nature of the anomaly of neutron resonances in the 147Sm(n, α)144Nd reaction. The investigation of this reaction started at FLNP JINR in the 1970s of the last century, when attention was drawn to the anomalously large value of the α-width of the resonance with E0 = 184 eV that significantly distorted the statistical distribution of the total α-widths. Further experiments showed that in this reaction, the α-widths averaged over the intervals of 200 – 500 eV are not constant, but indicate their noticeable increase with increasing neutron energy that clearly contradicts the statistical theory of compound states of the atomic nucleus.

Measurements at Oak Ridge confirmed the anomalous nature of the resonance with E0 = 184 eV in the 147Sm(n, α)144Nd reaction and also revealed several new anomalous resonances at higher energies. In addition, it was found that for resonances with spins 4 and En> 300 eV, the average values ​​of α-widths are three times larger than for resonances with spins 3, although according to theory, it should be the other way around, since for resonances with spins 4, the most intense α-decay into the ground state of the final 144Nd nucleus with spin 0+ is prohibited by the law of conservation of momentum and parity.

To clarify the nature of these anomalies, it is important to analyze the energy spectrum of α-particles in these resonances or to measure the averaged partial cross sections of this reaction in amplitude windows corresponding to α-transitions to the ground and excited states of the final nucleus. It may become one of the priority tasks for the high-aperture neutron source CSNS (China spallation neutron source).

Experimental study of angular and polarization correlations, as well as angular distributions of nuclear reaction products, obtaining more complete spectroscopic information about p-resonances, in particular, the values ​​and signs of the amplitudes of the widths of the input and output reaction channels, required both for interpreting the results of investigations of P-odd effects and checking the adequacy of the formalism used within the framework of the concept of a composite core seems to be a significant task. The P-even forward-backward correlations and anisotropy of angular distributions in the 14N(n, p)14C and 35Cl(n, p)35S  reactions will be measured in a wide range of neutron energies, including low-lying p-wave resonances and the data will be analyzed in conjunction with certain previously P-odd and P-even left-right correlations. Measurement of all of these correlations allows to determine the width amplitudes for various spins of the reaction channels and the matrix element of the weak interaction.