$\beta$-delayed neutron and $\gamma$-ray spectroscopy of ${}^{17}$C utilizing spin-polarized ${}^{17}$B

Excited states in ${}^{17}$C were investigated through the measurement of $\beta$-delayed neutrons and $\gamma$ rays emitted in the $\beta$ decay of ${}^{17}$B. In the measurement, three negative-parity states and two inconclusive states were identified in ${}^{17}$C above the neutron threshold energy, and seven $\gamma$ lines were identified in a $\beta$-delayed multiple neutron emission of the ${}^{17}$B $\beta$ decay. From these transitions, the $\beta$-decay scheme of ${}^{17}$B was determined. In particular, a de-excitation 1766-keV $\gamma$ line from the first excited state of ${}^{16}$C was observed in coincidence with the emitted $\beta$-delayed neutrons, and this changes the previously reported $\beta$-decay scheme of ${}^{17}$B and level structure of ${}^{17}$C. In the present work, the $\beta$-NMR technique is combined with the $\beta$-delayed particle measurements using a fragmentation-induced spin-polarized ${}^{17}$B beam. This new scheme allows us to determine the spin parity of $\beta$-decay feeding excited states based on the difference in the discrete $\beta$-decay asymmetry parameters, provided the states are connected through the Gamow-Teller transition. In this work, $I^{\pi }=1/2^{-}$, $3/2^{-}$, and ($5/2^{-}$) are assigned to the observed states at $E_{\mathrm{x}}=$ 2.71(2), 3.93(2), and 4.05(2) MeV in ${}^{17}$C, respectively.

Figure 1

Arrangement of the high-energy and low-energy neutron detector arrays.

Figure 2

Schematic layout of the setup around a Pt stopper, showing NaI(Tl) and Ge $\gamma$-ray detectors and a $\beta$-NMR system.

Figure 3

Block diagram for the $\beta$-delayed neutron and/or $\gamma$-ray measurement combined with the AFP-NMR technique. $T_{\mathrm{B}}$, $T_{\mathrm{R}}$, and $T_{\mathrm{C}}$ are the duration of the ${}^{17}$B beam bombardment; the application of the $B_1$ field; and the $\beta$, neutron, and $\gamma$ particle detection. $T_{\mathrm{R}}$ and $T_{\mathrm{C}}$ are fixed, while $T_{\mathrm{B}}$ is not fixed and remains open until a ${}^{17}$B particle is implanted. The $\beta$-decay rate $Y_{\beta }$, as the function of the ${}^{17}$B implantation rate, of the new beam-waiting mode was compared with that of the fixed beam-on and -off cycle mode. For details, see the text.

Figure 4

$\gamma$-Ray energy spectra obtained with the Ge detector in coincidence with the $\beta$ ray. The spectra were obtained with the counting periods $T_{\mathrm{C}}$ of (a) and (b) 18 ms and (c) 250 ms.

Figure 5

Obtained $\gamma$-ray energy spectra measured with the Ge detector by gating with (a) $\beta$-ray energies $E_{\beta }>10$ MeV, (b) neutron multiplicity $M_{\mathrm{n}}\ge$ 1, and (c) $\gamma$-ray energy $E_{\gamma }<$ 400 keV measured with NaI(Tl) detectors.

Figure 6

Time evolution of the photopeak counts of the observed $\beta$-$\gamma$ lines measured with the Ge detector in the ${}^{17}$B $\beta$ decay. The vertical bar shows the statistical error, and the horizontal bar represents the time-slice window. The dotted line curve shows the result of the ${\chi }^2$-fitting analysis using an exponential function with the known half-life of ${}^{17}$B, $t_{1/2}=$ 5.08(5) ms; plus a constant is used. Values $R$ indicated in each panel show the ratio of the exponential component to the total counts. The spectrum of (a) 217(2) keV is obtained for an additional coincidence with a $\beta$ ray having $E_{\beta }>10$ MeV measured with the NaI(Tl) detector. Furthermore, the spectra (q) and (r) are obtained with a measurement conducted separately by expanding $T_{\mathrm{C}}$ to 250 ms for investigating components with a long lifetime.

Figure 7

Observed $\gamma$ lines in the decay chain initiated by the ${}^{17}$B $\beta$ decay. The observed and identified $\gamma$ lines are shown by open arrows.

Figure 8

TOF spectra obtained for the $\beta$-delayed neutrons emitted in the ${}^{17}$B $\beta$ decay with (a) high-energy and (b) low-energy neutron detector arrays. The solid curve shown in panel (a) is the result of a least ${\chi }^2$-fitting analysis. The decomposed components are represented by dashed curves.

Figure 9

Obtained $\gamma$-ray energy spectra with (a) NaI(Tl) detectors and (b) the Ge detector in coincidence with $\beta$-delayed neutrons with multiplicity $M_{\mathrm{n}}=$ 1.

Figure 10

TOF spectrum obtained with the neutron detector array in coincidence with a 1767(6)-keV $\gamma$ peak observed with NaI(Tl) detectors.

Figure 11

$\beta$-Ray energy spectra (i.e., Kurie plot) measured with NaI(Tl) detectors in coincidence with (a) the 1.86(1)-MeV $\beta$-delayed neutrons in the ${}^{17}$B $\beta$ decay and (b) 1.76-MeV $\beta$-delayed neutrons in the ${}^{15}$B $\beta$ decay. Solid curves show the result of geant simulations, in which the end-point energies $E_{\beta }^{\max }=$ 19.7(1) MeV and 17.9(2) MeV were deduced, respectively. For comparison with the simulation results, $\beta$-ray yields are normalized to unity at $E_{\beta }=$ 2 MeV.

Figure 12

Values of ${\chi }_{\nu }^2$ plotted as a function of the mean spin polarization calculated for all possible sets of $A_{\beta }$ values in the ${}^{17}$B $\beta$-decay transition to the observed levels in ${}^{17}$C at $E_{\mathrm{x}}=$ 2.71(2), 3.93(2), and 4.05(2) MeV. Classification according to the $I^{\pi }$ for $E_{\mathrm{n}}=$ 2.71(2) MeV is indicated by dotted ellipses. The dashed ellipse shows the most probable sets of the $A_{\beta }$ allocation.

Figure 13

Decay scheme of ${}^{17}$B constructed in the present work. Unassigned levels $E_{\mathrm{x}}=$ 4.78(2) and 6.08(3) MeV are also shown.

Figure 14

Comparison of the obtained $\beta$-decay feeding excited states in ${}^{17}$C with shell-model calculations, with positive and negative parity states indicated by dashed and solid lines, respectively. The negative-parity states in ${}^{13}$C and ${}^{15}$C are also shown.