Spectroscopy of 7He in stopped pion absorption

Formation of the 7He heavy isotope was studied in the reactions of stopped pion absorption by light nuclei 9Be, 10,11B and 12,14C. Measurements were performed using the two-arm multilayer semiconductor spectrometer. Contrary to the results of other studies, we observed several narrow highly excited states (Ex > 16 MeV) of 7He. From the analysis of the continuous excitation spectrum in reactions 9Be(π−, d)X and 11B(π−, dd)X it was obtained first indication that the 4He+3n structure is not present in the ground state of 7He.


Introduction
Studies of the level structure of light neutron-rich isotopes near drip line are currently a field of great experimental activity. This information allows us to extend our understanding of the properties of nuclear matter under conditions of anomalously high N/Z ratios (N neutron number, Z proton number in nucleus).
An excited state E x = 2.9(3) MeV and = 2.2(3) MeV was observed for the first time in the p( 8 He,d) 7 He reaction at energy 50A MeV [2]. This level was also observed in other works (see table 1, references to earlier works are in compilation [1]). For other levels the experimental situation with the determination of their resonance parameters remains controversial (see table 1). Thus, indications of 7 He state with E x ~ 1 MeV were obtained in three works [3 5], while in other work this state was not observed. Note that no more than two low-lying excited states of 7 He were observed in any experiments. In the region of high excitations (E x 8 MeV) only wide levels ( ~ 10 MeV) were observed.
Theoretical predictions of level structure of 7 He are equally contradictory (see review [12]). Most of the models predict the same order of low lying levels: J = 3/2 -(ground state), 1/2 -, 5/2 -, 3/2 -. However, the values of resonance parameters in these models are quite different. Theoretical calculations of highly excited states are unknown to us.
Thus information on the level structure of 7 He is fragmented and contradictory. To improve this situation we conducted an experimental search for 7 He in the reactions of stopped pion absorption by light nuclei 9 Be, 10,11 B, 12,14 C. This paper presents an overview of these results. Note that some of these results were presented in paper [13].

Experiment
Measurements were carried out in the low energy pion channel of LAMPF with a two-arm multilayer semiconductor spectrometer [14]. The beam of negatively charged pions with an energy of 30 MeV was slowed down by the beryllium moderator and was then stopped in a thin target. The rate of pion stop in the target was about 6 10 4 s -1 . Measurements were made on targets Measurements were made on targets 9 Be, 10,11 B, 12,14 C. The 14 C enriched radioactive target consisted of 77% 14 C and 23% 14 C. Charged particles (p, d, t, 3,4 He) emitted after pion absorption in the targets were detected by two semiconductor telescopes oriented at an angle of 180 with respect to each other. The total sensitive thickness of each telescope was ~ 43 mm. This thickness permitted one to measure the total absorption of charged particles up to kinematical limits of the reaction.
The energy resolution (FWHM) was better than 0.5 MeV for single charged particles (p, d, t) and 2 MeV for double charged particles ( 3,4 He). The error of absolute energy calibration did not exceed 100 keV. In correlation measurements the missing mass (MM) resolution was 1 MeV for pairs of single charged pairs and 3 MeV for pairs hydrogen and helium isotopes.
The spectrometer and experimental techniques are described more detail in [14,15].

Results and discussion
A missing mass spectrum in the 9 Be( ,d)X reaction is shown in fig. 1. The sum of neutron and 6 He masses is taken as a reference point. To separate 7 He states and determine their parameters, we used the least square approximation in the fitting of the experimental spectrum by the sum of the Breit-Wigner resonances and N-particle phase-space distributions (N ≥ 3). A satisfactory description could be achieved by introducing three 7 He states: the ground state with the resonance parameters E r = 0.410(8) MeV and Г = 0.15(2) MeV [1], two excited states with the parameters listed in table 2, and four distributions over the phase volumes. Note that the absence of highly excited levels in these data may be due to the rapid growth of the continuous part of the inclusive spectrum. The obtained excitation energies coincide with the data of compilation [1] (see  table 1) within the errors of the measurements. At the same time, in our measurements, the width of the levels is noticeably narrower.
In the correlation measurements on the boron isotopes search for 7 He was carried out in the reactions 11  He masses is taken as a reference point. All spectra show structures associated with three-body channels involving 7 He in the ground and excited states. The spectra were described in the same way  Points with error bars denote the experimental data. Curve 1 summary spectrum; peaks are the Breit -Wigner distributions for the ground and excited states; distributions over phase volumes: 2 π + 9 Ве d + 6 Не + n, 3 π + 9 Ве d + 6 Не*(1.8 MeV) + n, 4 π + 9 Ве d + 5 Не +2n, 5 π + 9 Ве d + 4 Не +3n.
In our measurements the 11 B( ,dd)X and 11 B( ,pt)X reaction channels have the highest statistics. The spectra of levels observed in these reactions have almost the same structure. Three narrow states are observed in the range of low excitation. The existence of three states in the region 2.5 E x 7 MeV was predicted in several theoretical works [16 19]. Our data are close to the results of [19]: (E Be data. This probably is evidence for existence of more than three low lying excited states of 7 He. In the region of high excitation energies above the threshold of the decay 7 He t+t+n (E x = 11.9 MeV), three rather narrow states of 7 He are observed in both channels. In other experiments only wide states have been observed in this region (table 1). The nature of the observed us states is unclear.
The MM spectrum measured in the 10 B( ,pd)X reaction ( fig. 4) has the lower statistics in comparison with data on the 11 B. A satisfactory description is obtained by the inclusion of the three excited levels of 7 He (table 2). The parameters of these states are close to the corresponding values obtained on the 11 B. The absence of other levels in the description is due to insufficient statistics of the data.
Indications of the existence of a low-lying excited state of 7 He with E x 1 MeV were not obtained in any of the channels in the measurements on 9 Be, 10 B and 11 B. Nevertheless to test the hypothesis of existence this state, we carried out an alternative description of the experimental spectra (figs. 1 4) including the level with the parameters E x = 0.6 MeV and = 0.75 MeV, which was found in [3]. The upper bound for the contribution of this state is ~ 10%.
In addition to the discrete peaks corresponding to the formation of 7 He states one can see in figs. 1 4 the intensive continuous spectra. The common feature of these spectra is the absence of a significant contribution from the channels with three nucleons in the final state. This means that the probability to create a halo with three non-interacting neutrons ("true" halo) is very small.  Figure 2. MM spectrum for the reaction 11 B( ,dd)X. Points with error bars denote the experimental data. Curve 1 summary spectrum; peaks are the Breit -Wigner distributions for the ground and excited states; distributions over phase volumes: 2 π + 11 B d + d + 6 Не + n, 3 π + 11 B d + d + 5 Не + 2n, 4 π + 11 B d + d + 2d + 3n. The inset shows the spectrum obtained after the subtraction the sum of phase-space distributions. Figure 3. MM spectrum for the reaction 11 B( ,pt)X. Points with error bars denote the experimental data. Curve 1 summary spectrum; peaks are the Breit -Wigner distributions for the ground and excited states; distributions over phase volumes: 2 π + 11 B p + t + 6 Не + n, 3 π + 11 B p + t + 5 Не + 2n, 4 π + 11 B p + t + 4 Не + 3n, 5 π + 11 B p + t + 2d + 3n. The inset shows the spectrum obtained after the subtraction the sum of phase-space distributions. p + t + 6 Не + n, 3 π + 10 B p + t + 5 Не + 2n. The inset shows the spectrum obtained after the subtraction the sum of phase-space distributions.  Table 2. Parameters of excitation levels of 7 He obtained in the reactions of stopped pion absorption. 9 Be( ,d)X 11 B( ,dd)X 11 B( ,pt)X 10 B( ,pd)X The MM spectra obtained in the measurements on 12 C and 14 C are shown in figs. 5 7. Unfortunately due to insufficient energy resolution and low data statistics in these measurements only limited information on the level structure of 7 He could be obtained. In the reaction 12   peaks are the Breit -Wigner distributions for the ground and excited states; distributions over phase volumes: 2 π + 12 C p + 4 He + 6 Не + n, 3 π + 12 C p + 4 He + 5 Не + 2n, 4 π + 11 B p + 4 He + 4 Не + 3n. Figure 6. MM spectrum for the reaction 12 C( ,d 3 He)X. Points with error bars denote the experimental data. Curve 1 summary spectrum; peaks are the Breit -Wigner distributions for the ground and excited states; distributions over phase volumes: 2 π + 12 C d + 3 He + 6 Не + n, 3 π + 12 C d + 3 He + 5 Не + 2n.  Figure 7. a MM spectrum for the reaction ( ,p 4 He) on the radioactive target 14 С (points with error bars) and on 12 C target (shaded histogram); b MM spectrum for the reaction 14 C( ,t 4 He)X. Dots with error bars were obtained after the subtraction of 12 C background. Curve 1 summary spectrum; peaks are the Breit -Wigner distributions for the ground and excited states; distributions over phase volumes: 2 π + 14 C t + 4 He + 6 Не + n, 3 π + 14 C t + 4 He + 5 Не + 2n.
The MM spectrum obtained on the radioactive target 14 С is shown in fig. 7. The contribution of 12 С impurity is clear seen. The contribution of 12 C impurity was determined using the results from correlation measurements of p 4 He-pairs on the 12 C target. The MM spectrum for the reaction 12 C( ,p 4 He)X was normalized to the relative impurity contribution (23%) and was subtracted from the spectra in fig. 7a. The subtracted contribution is shown by the shaded histogram. The spectrum after impurity subtraction is shown in fig. 7b. In the obtained spectrum there are observed two excited states with parameters (E x , ) = (3.0(5) MeV, ~1 MeV) and ( 16 MeV,1.5 MeV). Note that the highly excited state with similar parameters was observed in the reaction 11 B( ,pt)X.

Conclusion
The overview of the data on the level structure of heavy helium isotope 7 He obtained in stopped pion absorption by light nuclei 9 Be, 10,11 B, 12,14 C is presented. In the region of low excitation energies, the existence of three States was observed simultaneously for the first time. The formation of narrow highly excited states was observed for the first time in four reaction channels.