Observation of an Excited State inH7ewith Unusual Structure

Abstract

The transfer reaction p͑ 8 He, d͒ 7 He has been studied by correlational measurements, and an excited state of 7 He was observed (E ‫ء‬ 2.9 6 0.3 MeV, G 2.2 6 0.3 MeV) which decays mainly into 3n 1 4 He. Most likely, it has a structure with a neutron in an excited state coupled to the 6 He core which itself is in the excited 2 1 state. [S0031-9007(99)09012-2] 23.20.En, 27.20. + n, 21.45. + v We report on such a novel step, in the experiments with beams of exotic nuclei, as the investigation of transfer reactions. Namely, we studied the p͑ 8 He, d͒ 7 He reaction for the spectroscopy of 7 He.It is well known that nuclei have excited states. There is a famous exception from this rule-the absence of excited states in 3 He and 3 H. Another kind of exception was 7 He. This nucleus was investigated for 30 years in many reactions with stable beams, and no excited states were found. As a result, 7 He began to be considered as a nucleus which may not have excited states. It can be explained by a large width for decay 7 He ‫ء‬ ! n 1 6 He. The ground state of 7 He is a well established resonance that decays into n 1 6 He.Radioactive nuclear beams are the most promising tool to study as neutron-rich systems as 7 He. Since the projectiles in reactions are already neutron rich, reaction mechanisms are simpler than those with stable beams, cross sections are higher, and physical backgrounds are lower. We used a beam of 8 He at 50A MeV, that was produced by the fragment separator RIPS at RIKEN, and studied the p͑ 8 He, d͒ 7 He reaction with the CH 2 and C targets. To study transfer reactions with beams of exotic nuclei, a special detection system, the RIKEN telescope, was designed (Fig. 1). It represents a stack of solidstate position-sensitive detectors (strip detectors) that have large area and annular hole. Using this telescope, we detected deuterons at small angles in the laboratory system (10 ± -25 ± ) corresponding to a high cross section.In addition to the deuterons, we detected other particles emitted from the decay of 7 He. Neutrons were measured by the neutron walls of plastic scintillators, while charged particles were bent in the dipole magnet and detected by the drift chamber and the plastic scintillators' hodoscope (Fig. 1). These parts of the detection system allowed us to study spectra of deuterons detected in coincidences with 6 He, 4 He, and neutrons. Other detectors in Fig. 1, the beam scintillators and multiwire proportional counters, were used for identification of each beam particle, determination of its energy, and for its tracking.The resulting deuteron spectra are presented in Figs. 2 and 3 as a function of energy in the center of mass of 7 He relative to the n 1 6 He threshold. In each graph, the upper histogram with pronounced peak corresponds to measurements with CH 2 target; the lower structureless histogram shows total background from materials other than protons in the target (it was obtained with the C and empty targets). The cutoff of spectra at energy of ϳ30 MeV reflects the energy range meas...

Spectroscopy ofHe7using theBe9(HeRenzi1, Raabe2, Randisi3 et al. 2016Phys. Rev. C5010View full textAdd to dashboardCite

“…Three other experiments showed a resonance with an excitation energy in good agreement with Ref. [13]: the 9 Be( 15 N , 17 F) 7 He transfer reaction [3], 8 He(p,d) 7 He at 15.6 MeV/nucleon [6] and (d, 3 He) reaction with 8 Li beam of 76 MeV [12]. In the latter two measurements the decay of the 7 He 5/2 − state into α + 3n was also confirmed.…”

“…However, in our work, a confirmation of spin of the resonance could not be achieved experimentally. Prior works [4,6,[11][12][13] inferred the spin of the observed 7 He excited state, 1/2 − or 5/2 − , through the determination of its decay mode, 6 He + n or 4 He + 3n. Here, due to the limited angular resolution of the Lamp detector, scarce information is available on the charged particles from the 7 He decay, precluding its isotopic identification.…”

“…The detection of the 6 He nucleus excludes this state as the 5/2 − state observed in Ref. [13]. In the work of Skaza et al [6], the excitation energy spectrum of 7 He was described by assuming a resonance at 0.9(5) MeV ( = 1.0(9) MeV).…”

“…In earlier experimental studies [3,6,13,45] phase-space contributions played an important role in interpreting the 7 He excitation energy spectrum. As for the 7 Li spectrum analysis, the few-body components considered here are those with 8 Be g.s.…”

“…[22] for a review), no excited state was found. Korsheninnikov et al [13] were the first to identified an excited state in 7 He. They investigated the p( 8 He ,d) 7 He reaction at 50 MeV/nucleon observing a resonance at 2.9(3) MeV, with a width = 2.2(3) MeV.…”