As it is known, there is still no generally accepted theory of the strong interaction. Important results in this area can be obtained with the investigations of the interaction of neutrons with the lightest nuclei - protons, deuterons, ^{3}He, others. One of the most important results obtained by F.L. Shapiro was the investigation of the energy dependence of the cross section of the reaction and the prediction of the occurrence of the excited level of ^{4}He with 0^{+}spin. In the 1970-1980s of the last century, these investigations were still carried out under the supervision of E.I. Sharapova.

To describe the properties of the lightest nuclei, it is necessary to study the ** NN** interaction. At low energies, the interaction of nucleons occurs with an orbital momentum l = 0, that is, in the

**-state. Since the spins of the neutron and proton are equal to 1/2,**

*S***-interaction is possible in the singlet (**

*np*^{1}S

_{0}) and triplet (

^{3}S

_{1}) states. The triplet spin state corresponds to the isotopic spin T = 0. In this state, there is a bound state of the neutron and proton - the deuteron with a bond energy of 2224 keV. Pairs of identical particles can only interact in the singlet

^{1}S

_{0}state, since the triplet state is prohibited by the Pauli principle. The interaction in the

^{1}S

_{0}state for the three possible pairs of nucleons (

**and**

*nn, np***) corresponds to the interaction with isotopic spin T = 1.**

*pp*For ** NN** interactions at low energies, the theory of effective radius (TER) was developed. According to TER, the interaction amplitude

**for a certain spin state can be expressed in terms of two parameters: the scattering length**

*F***and the effective radius**

*a***.**

*r*_{0}(1)

Scattering lengths and effective radii for ** np** and

**interactions are measured with good accuracy. Determining the parameters of the**

*pp***interaction is a much more difficult task, since there is no neutron target.**

*nn*NN interaction in the singlet state at very low energies is described using the so-called virtual level, the physical meaning of which is unclear. Based on ideas about the virtual level, it was concluded that the singlet deuteron does not occur. However, a strict scattering theory does not prohibit the occurrence of a bound state of two nucleons in a singlet state. Currently, the theory of strong interactions is based on quantum chromodynamics. Recently, a number of research papers have been presented that essentially predict the occurrence of a bound state of two nucleons in the ^{1}S_{0 }-state [1-4].

The interaction of neutrons with protons in the ^{1}S_{0 }-state can be described using resonance with negative energy. The resonance energy is determined by the scattering length ** a **and the effective radius

**:**

*r*_{0}(2)

This formula was first obtained by S.T. Ma in 1953 [5]. Since the singlet scattering length is negative, the resonance energy is also negative, that is, the resonance is below the deuteron photodisintegration threshold. The interaction of nucleons in the singlet state can be considered as a manifestation of dibaryon resonance [6-8]. The following estimate of the MeV resonance energy was obtained in [9] that is consistent with formula (2).

If this is a subthreshold resonance (that is, a short-lived quasi-stationary state), then the following processes are possible: a) emission of a cascade of gamma quanta after the capture of neutrons by protons; b) resonant scattering of gamma quanta using deuterons. The idea of describing the interaction of neutrons with nuclei using resonances with negative energy is widely used in neutron physics. F. L. Shapiro and his co-authors applied this idea to describe the interaction of neutrons with ^{3}He that subsequently results in the discovery of the excited level of ^{4}He.

Thus, at present, the task of searching for a dineutron and a singlet deuteron, that is, bound states of two neutrons and a neutron and a proton with a common spin of zero, becomes increasingly urgent. Due to the principle of isotopic invariance, their energies should be close (see Fig. 1). There is apparently no bound state of the two protons, since there is Coulomb repulsion between them. A singlet deuteron can manifest itself in the radiative capture of neutrons by protons with the release of a cascade of two gamma rays. The cross section of such a process is about 10^{4} times smaller than the cross section with the emission of one gamma quantum corresponding to the main transition. We have made attempts to search for such a process. The measurements were carried out in Budapest [10]. In the spectrum of gamma rays there are lines that can be interpreted as a manifestation of the cascade [11]. Their further research is required.

Fig. 1. Sum of masses of different pairs of nucleons and the expected position of the singlet state

The idea of the occurrence of a dineutron was presented back in 1948 by N. Fizer [12]. He estimated the dineutron lifetime (1-5 sec) and the maximum bond energy (3 MeV). An estimate of the bond energy was obtained from the condition of ** β**-decay of a dineutron into a deuteron. To date, a range of experimental indications have been obtained in favor of the occurrence of the dineutron [13-20].

A dineutron can occur in the reaction. FLNP staff members have proposed a way to search for this reaction and to determine the bond energy of a dineutron by registering a proton in a counter filled with deuterium [21].

The problem of the occurrence of a dineutron is related to the problem of the occurrence of neutral nuclei. A number of interesting results have also been obtained in this area recently [22–26]. It should be noted that according to a number of theoretical papers, the occurrence of neutral nuclei is impossible without the occurrence of a dineutron.

Thus, at present, there are a number of papers dedicated to this topic and new experiments will be carried out.

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