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Description
The unbound heavy helium isotope $^{9}$He was discovered in pion double charge exchange reaction $^{9}$Be($\pi^{-}$,$\pi^{+}$)$^{9}$He at E$_{\pi}$ = 194 MeV [1]. Despite a significant number of experiments performed to date, the problem of the level structure of $^{9}$He remains open [2, 3]. In particular, the energy and quantum numbers of the ground state are undefined. The studied range of excitation energies is limited to 5 MeV, while the parameters of states lying above 3 MeV do not agree in different works. In this situation, new experimental information is needed to resolve existing contradictions and discover new levels.
In this work a search for $^{9}$He was performed in the reaction of stopped pion absorption $^{11}$B($\pi^{-}$,$pp$)X. The experiment was taken at low energy pion channel of LANL with two-arm multilayer semiconductor spectrometer. In these measurements missing mass resolution was 1 MeV and error of absolute energy calibration did not exceed 0.1 MeV. These values were determined from measurements of the reaction $^{10}$B($\pi^{-}$,$pp$)$^{8}$He carried out in the same experimental run. The studied excitation energy ranges up to about 50 MeV.
In these measurements we do not observed $s$-wave resonance just above threshold in $^{9}$He. The lowest lying state in our measurements has the following resonant parameters: E$_{r}$ =1.3(3) MeV and $\Gamma \leq$ 0.5 MeV, which are in agreement with a number of experiments [2, 3]. Highly excited state with E$_{r}$ =10.5(2) MeV and $\Gamma$ = 1.5(5) MeV has been observed for the first time.
- K.K.Seth et al. // Phys. Rev. Lett. 1987. V.58. P. 1931.
- I.Tanihata, H.Savajols, and R. Kanungo // Progr. Part. Nucl. Phys. 2013. V. 68. P. 215.
- H.T.Fortune // EPJA 2018, V. 54. P. 51.
