Direct observation of three-neutron emission from $^7$He$^*$ and the search for the trineutron

Abstract

Three-neutron emission from $^7$He has been directly measured for the first time, following neutron knockout from a $^8$He beam at 156 MeV/nucleon. A resonance-like structure at $2.08(4)$ MeV above the $^4$He+$3n$ threshold [$E_x=2.68(4)$ MeV] with a width of $3.9(2)$ MeV was observed and deduced to arise predominately from the predicted $J^π=3/2^{-}_2$ level. The three-neutron invariant-mass spectrum was reconstructed and found to peak at around 1 MeV and could, through complete simulations incorporating neutron-neutron correlations, be very well described by the sequential decay of $^7$He$^*$ via the $2_1^+$ excited state of $^6$He. No evidence was found for any significant three-neutron correlations beyond those expected from well-established two-body interactions, including a trineutron resonance.

Figures (4)

• Figure 1: Schematic view of the experimental setup. The incident $^8\mathrm{He}$ beam was tracked by two drift chambers (BDC1, BDC2) onto the liquid hydrogen target system (MINOS). Recoil protons from the $(p,pn)$ reaction were tracked using a cylindrical time projection chamber surrounding the target, and their energies were measured by an array of 36 NaI(Tl) crystals. The charged fragments $^4\mathrm{He}$ ($^6\mathrm{He}$) were momentum analyzed using the SAMURAI spectrometer and two drift chambers (FDC0, FDC2), and detected in a plastic scintillator array (HODO). The forward-going beam-velocity neutrons were detected by the multi-element NeuLAND (single-wall) and NEBULA (two separated walls) plastic scintillator arrays, located approximately 11 m and 14 m downstream of the target, respectively.
• Figure 2: Relative-energy spectra for (a) $^6$He+$n$ and (b) $^4$He+$2n$ events. The red line shows the best fit (see text). The vertical error bars are statistical only.
• Figure 3: Relative-energy ($E_{\alpha3n}$) spectrum for $^4$He+$3n$ events. The red solid line shows the best fit up to 5 MeV using a BW lineshape (see text). The inset shows the transverse momentum distribution ($P_X$) of the knocked-out neutron for $^7$He g.s. ($E_{^6\rm {He}}$$_{-n}<1$ MeV in Fig. \ref{['fig:1n2n']}a) and for the present $^7$He$^*$ structure ($1<E_{\alpha3n}<5$ MeV) normalized to the total number of events, with Gaussian fits to the central region to guide the eye. The resolution in $P_X$ (FWHM = 38 MeV/$c$) is also shown for comparison (dashed line). The vertical error bars are statistical only.
• Figure 4: Relative-energy spectra for $^{4}$He+3$n$ events with $E_{\alpha3n}<5$ MeV: (a) $3n$ energy $E_{3n}$; (b) energy, $E_{\alpha2n}$, of the $^{4}$He+2$n$ subsystem for the $^{4}$He+3$n$ events; (c) normalized $3n$ energy $\varepsilon_{3n}=E_{3n}/E_{\alpha3n}$. The lines correspond to simulations of different decay scenarios (see text). The inset of panel (a) shows the $E_{3n}$ spectrum for events with $5<E_{\alpha3n}<8$ MeV. The vertical error bars are statistical only.