Typically, fission is a binary process in which a fissile nucleus splits into two fragments. In general, it is true for both spontaneous and forced nuclear fission. However, sometimes, instead of the standard “double fission,” a process of higher multiplicity is observed with the emission of three or more charged particles in outgoing circuits. The accompanying particles are lighter than the main fission fragments. An even rarer fission mode, accompanied by the emission of light fragments with probabilities ranging from 10–7 to 10–6, is quaternary fission (QF), in which a pair of light charged particles (LCPs) are simultaneously emitted in a single fission event. The QF mode can occur either from the decay of unstable particles among LCPs, such as 7Li*, 8Be, 9Be* (“pseudo-quaternary” fission) or from the independent emission of two LCPs (“true” quaternary fission. A schematic of these types of fission processes is presented in Fig. 1.
Figure 1. Schematic of different types of fission processes: binary (a), ternary (b) “pseudo” quaternary (c) and “true” quaternary (d) 
Data on ternary and quaternary fission are of interest to nuclear physics, for LCP particles are emitted in space and time very close to the fission point. Therefore, they are expected to provide information about the configuration of the breaking point. A good knowledge of their characteristics will also contribute to a better understanding of the emission mechanism of these particles. The second reason is that the nuclear industry requires accurate data on the yields of ternary fission, more precisely, the 3H and 4He particles, since the latter underlie the production of helium gas and especially radioactive tritium gas in reactors.
The main goal of this experiment is to study the emission probabilities and features of the energy distributions of ternary and quaternary particles, to implement experiments and to process experimental data on mass-energy and angular correlations in fission, accompanied, in addition to the production of two fragments, using the emission of a light fragment (ternary fission) or two such fragments (quaternary fission), search for hypothetical fission modes (quintuple fission, cluster collinear ternary decay).
Together with the Prague Technical University, an experimental facility was developed, consisting of Timepix-type pixel detectors with modernized electronic boards. Its diagram and operating principle are shown in Fig. 2. Timepix pixel detector is a promising development that has found a wide range of applications in various fields. Detectors of this type can provide multi-parameter event-by-event spectroscopic information (position, energy and time, type) for almost any charged particle. Additionally, when combined with event tracking analysis, it provides improved signal-to-noise ratio with high suppression of background and unwanted events. Detectors of this type allow to construct particle telescopes that are especially interesting for studying rare fission modes and direct observation of the decay of 8Be and 7Li, emitted as triple particles in the ground and excited states.
Figure 2. Diagram of the experimental facility and the principle of the experiment 
In 2021, rare fission mode measurements were implemented with a highly active (400 kBq) 252Cf spontaneous fission source using Timepix detectors with upgraded electronics boards. The experiment was carried out over 7 months; a large number of events of registration of triple particles with different cutoff energies were accumulated. Ternary species from 1H to C were registered and separated successfully. The ΔE-E 2D spectrum of light charged particles for one of the telescopes is shown in Fig. 3.
Figure 3. ΔE-E 2D spectrum of light charged particles for one of the telescopes 
Energy spectra were constructed for each type of particle. The results obtained for 1H, 2H and 3H are of particular interest since they were obtained for the first time. The energy spectrum for 1H is shown in Fig. 4. This distribution peak extends to 15.8 MeV that corresponds to an energy of 9.21±0.15 MeV with σ≈3.3 MeV and is very similar to the energy spectra of deuterons and tritons. Protons produced from background (n,p) and (α,p) reactions driven by fission neutrons can make a significant contribution to the proton yield of ternary fission. To analyze the contributions from various possible sources, the energy spectrum of protons was calculated using the Talys-1.9 programme. It was found that protons produced in the result of the (α,p) reaction can contribute to the energy spectrum of protons produced by ternary fission. A comparison of the calculated results for (α,p) with the experimental results is shown in Fig. 4. The expected energy spectrum of protons for reactions (α,p) is shown by a straight line. The results obtained in the experiment are shown in Table 1.
Figure 4. Energy spectrum of protons produced as a result of the reactions Al(α,p) (solid line) and ternary fission (dashed line) 
Table 1. Results obtained for all ternary fission particles Triple particles Number of events Cut-off energy (MeV) Energy (MeV) Sigma (MeV) Yield (104 α) H1 33434 5.5 9.21(0.15) 3.23(0.01) 0.0181 H2 12320 6.0 9.13(0.01) 3.59(0.01) 0.0067 H3 85498 6.5 8.96(0.01) 2.97(0.01 0.0463 4He 4.5206·106 9.5 15.99(0.01) 4.33(0.01) 1 6He 123186 10.5 12.44(0.01) 3.42(0.01) 0.02629 8He 7147 11.0 11.23(0.04) 3.21(0.04) 0.00152 Li 21844 20 15.66(0.04) 5.38(0.02) 0.00466 Be 47503 28 21.92(0.12) 5.95(0.04) 0.01013 B 4360 39 26.67(3.14) 7.50(0.79) 0.00093 C 12887 49 31.74(4.60) 8.09(0.97) 0.00275