Search for excited states in ${}^7\mathrm{He}$ with the ($d,p$) reaction

We have studied the properties of low-lying levels in ${}^7\mathrm{He}$ using the ${}^2\mathrm{H}$$($${}^6\mathrm{He}$$,p)$${}^7\mathrm{He}$ reaction at 11.5 MeV/u. This reaction probes the ${}^6\mathrm{He}{}_{g.s.}+n$ character of states in ${}^7\mathrm{He}$. The ground state was populated with a spectroscopic factor comparable to that obtained from ab initio calculations, supporting the tentative spin-parity assignment of ${3/2}^{-}$ in the literature. In addition to the ground state, a broad structure is observed between $E_X=2–3$ MeV, the excitation-energy range expected for the ${1/2}^{-}$ state in ${}^7\mathrm{He}$. No evidence was found for a lower-lying, first-excited state reported recently.

Figure 1

(a) Q-value spectrum for the ${}^2\mathrm{H}$$($${}^7\mathrm{Li}$,$p)$${}^8\mathrm{Li}$ reaction from $p\text{−}$${}^{7,8}\mathrm{Li}$ coincidence events. (b) Q-value spectrum for the ${}^2\mathrm{H}$${}^6\mathrm{He}$,$p)$${}^7\mathrm{He}$ reaction from $p\text{−}$${}^6\mathrm{He}$ coincidence events. In both cases the data are taken over the entire angular range covered by the proton detectors (${\theta }_{\mathrm{lab}}={109}^{°}$ to ${159}^{°}$). The histograms represent the Q-value dependence of the total detection efficiency.

Figure 2

(Color online) (a) Q-value spectrum for the ${}^2\mathrm{H}$$($${}^7\mathrm{Li}$,$p)$${}^8\mathrm{Li}$ reaction from $p\text{−}$${}^{7,8}\mathrm{Li}$ coincidence events corrected for detection efficiency. The dashed line represents the empirical continuum described in the text. (b) Q-value spectrum for the ${}^2\mathrm{H}$${}^6\mathrm{He}$,$p)$${}^7\mathrm{He}$ reaction from $p\text{−}$${}^6\mathrm{He}$ coincidence events corrected for detection efficiency. The solid line corresponds to the total fit to the spectrum described in the text. The thin, dot-dashed, and dashed lines represent the ground-state, broad resonance, and continuum contributions to the total fit as described in the text. (c) ${}^2\mathrm{H}$${}^6\mathrm{He}$,$p)$${}^7\mathrm{He}$ Q-value spectrum with fit (thick histogram) that includes the contributions of a low-lying first-excited state. The thin histogram illustrates the the expected contribution from the 600-keV state alone.

Figure 3

(a) Line shapes of states included in the analysis of the ${}^2\mathrm{H}$(${}^6\mathrm{He}$,$p)$${}^7\mathrm{He}$ data. The ground state, 600-keV state, and 2.6-MeV state are given by the thin solid, dot-dashed, and dashed lines, respectively. The profile of the broad state before including the effects of DWBA folding is given by the dotted line. (b) Plot of ${\chi }^2$ versus yield for the 2.6 MeV resonance.

Figure 4

Angular distributions for ${}^2\mathrm{H}$${}^6\mathrm{He}$,$p)$${}^7\mathrm{He}$. The circles correspond to the ground state, and the diamonds and squares to the possible broad excited state, with and without background subtraction, respectively. The curves represent DWBA calculations described in the text.

Figure 5

(a) Q-value spectrum for ${}^2\mathrm{H}$${}^6\mathrm{He}$,$p)$${}^7\mathrm{He}{}^{*}$ for ${}^7\mathrm{He}{}^{*}$$\to$${}^4\mathrm{He}$$+3n$ decays from ${}^4\mathrm{He}$ + p coincidences, not corrected for efficiency. The histogram represents the distribution of counts expected for the state at $E_X=2.6$ MeV shown in Fig. 3a, and is presented for purposes of illustration only. (b) ${}^4\mathrm{He}$ + p detection efficiency.