The light neutron-rich nuclei lying in the vicinity of the drip line are interesting for the study of structure and properties of exotic nuclear states. The properties of these states are important for the development of nuclear models and refinement of the nucleon-nucleon potentials.
Among nucleon-stable isotopes the He has a record ratio of the neutron number to proton number: N/Z = 3. Valence nucleons in He are bound stronger than in He (S ( He) = 2.14 MeV, S ( He) = 0.973 MeV ), therefore cluster structure of He is a He+n+n+n+n system and He rms radius is smaller than He one. The ground state of He was previously considered as a system consisting of an inert -particle core, surrounded by four valence neutrons, occupying the p shell . However, it was shown experimentally [3,4] and theoretically [5,6], that along with the (p ) component the wave function of the ground state, can contain a noticeable admixture of other components (p ) (s ) , (p ) (d ) and (p ) (p ) .
Excitation levels He were observed in several experiments (see compilation of world data  and review ), however, the statistical reliability of the data is low. Probably this is the main reason of considerable uncertainty in the energy of the first excited state: E = 2.7 3.6 MeV. Another reason may be due to the fact that the peak observed in the experiments is a superposition of two states 2 resonance and a soft dipole resonance with J = 1 . Therefore, the difference in the results may be due to the different population of these channels. However, evidence of this assumption requires better statistics of measurements. In several works [1,4,8,9] higher excitations were observed. The observed spectrum is limited to the E = 7.5 MeV . But these results can be considered only as indications on the existence of He states due to low statistics.
Use of the reaction of stopped pion absorption by light nuclei was successful in the study of the level structures of heavy helium isotopes He [10 12]. In present work this method has been used for He in the measurement of the reaction Bе( , p)X.
The experiment was performed in the low energy pion beam of meson factory LANL using the two arm semiconductor spectrometer .
A beam of 30 MeV negatively charged pions traversed a beryllium moderator and stopped in the thin target ( 24 mg cm ). The rate of pion stopping in targets was about 6 10 s . The measurements were carried out on the isotope-pure target Be. The contribution of uncontrolled impurities was 1%. Charged particles, emitted after pion absorption in the target were detected by two semiconductor telescopes arranged at an angle of 180 with respect to each other. The energy resolution (FWHM) for single-charged particles (p, d, t) was better than 0.5 MeV in the whole energy range up to the kinematic limits of the reaction . The error of absolute energy calibration did not exceed 100 keV .
3. Results and Discussion
A search for the He excited states was performed on the peaks in the missing mass spectrum (MM) obtained in inclusive measurements of the reaction Be( ,p)X. The measured spectrum is shown in fig. The mass of He is taken as a reference point.
Peaks formed due to the appearance of the ground state and the excited states of He in the two-body reaction channels are well seen. To determined these states we used the method of least squares in describing the experimental spectra by the sum of n–particle distributions over phase space (n 3) and Breit–Wigner distributions for excited states. A statistically satisfactory description of the experimental spectrum can be obtained by introducing the ground state and two excited states of He with parameters (E , ) presented in the Table. Also table includes data of the compilation  and results of the later works.
It is seen that parameters of the first excited state obtained in the present work coincide with the last data in the limits of errors of the measurements. The small width level is hardly compatible with the assumption that the observed peak is a superposition of two states. At the same time, the position of the second excited level coincides with the results of earlier works . However, the level observed us is significantly narrow in comparison with data from .
|E , MeV||, MeV||E , MeV||,MeV|
|1, compilation||2.7 3.6||0.6 0.2||4.36 0.20||1.3 0.5|
|15 He(p, p')X||3.62 0.14||0.3 0.2||5.4 0.5||0.3 0.5|
|16, H( He, p) He||3.6 3.9||0.5||5.3 5.5|
|this work, ( Ве, p) X||3.8 0.2||0.3 0.1||4.6 0.3||0.3 0.1|
The position of the first excited level is consistent with the predictions of theoretical works [17 19]. At the same time, we found no theoretical predictions the second level, which coincides with our result.
The excited states of heavy helium isotopes He were studied in reaction Be( ,p)X. The state of He with E = 4.6(3) MeV and Г = 0.3(1) MeV has been observed in the first time.
This work was supported by the Russian Ministry of Education and Science grant N3.4911.2017/VU and program of increasing the competitive ability of National Research Nuclear University MEPhI (agreement with the Russian Ministry of Education and Science of August 27, 2013, project no. 02.a03.21.0005).