$\beta$-delayed $\gamma$ decay of ${}^{26}\text{P}$: Possible evidence of a proton halo

Background: Measurements of $\beta$ decay provide important nuclear structure information that can be used to probe isospin asymmetries and inform nuclear astrophysics studies.

Purpose: To measure the $\beta$-delayed $\gamma$ decay of ${}^{26}\text{P}$ and compare the results with previous experimental results and shell-model calculations.

Method: A ${}^{26}\text{P}$ fast beam produced using nuclear fragmentation was implanted into a planar germanium detector. Its $\beta$-delayed $\gamma$-ray emission was measured with an array of 16 high-purity germanium detectors. Positrons emitted in the decay were detected in coincidence to reduce the background.

Results: The absolute intensities of ${}^{26}\text{P}\beta$-delayed $\gamma$ rays were determined. A total of six new $\beta$-decay branches and 15 new $\gamma$-ray lines have been observed for the first time in ${}^{26}\text{P}\beta$ decay. A complete $\beta$-decay scheme was built for the allowed transitions to bound excited states of ${}^{26}\text{Si}$. $ft$ values and Gamow-Teller strengths were also determined for these transitions and compared with shell-model calculations and the mirror $\beta$ decay of ${}^{26}\text{Na}$, revealing significant mirror asymmetries.

Conclusions: A very good agreement with theoretical predictions based on the USDB shell model is observed. The significant mirror asymmetry observed for the transition to the first excited state $\left(\delta =51\left(10\right)\%\right)$ may be evidence for a proton halo in ${}^{26}\text{P}$.

Figure 1

Schematic view of the experimental setup. The thick arrow indicates the beam direction. One of the 16 SeGA detectors was removed to show the placement of the GeDSSD.

Figure 2

Particle identification plot obtained for a selection of runs during the early portion of the experiment, before the beam tune was fully optimized. The energy loss was obtained from one of the PIN detectors and the time of flight between the same detector and the scintillator placed at the focal plane of the A1900 separator. A low-gain energy signal in the GeDSSD condition was used. The color scale corresponds to the number of ions.

Figure 3

$\gamma$-ray spectrum observed by the SeGA array in coincidence with a $\beta$ particle in the GeDSSD. Photopeaks have been labeled by the emitting nucleus and its energy rounded to the closest keV integer. Peaks labeled with one (two) asterisks correspond to single (double) escape peaks.

Figure 4

(Upper panel) Energy calibration of SeGA $\gamma$-ray spectra using the $\beta$-delayed $\gamma$ rays emitted by ${}^{24}\text{Al}$. The solid line is the result of a second degree polynomial fit. Energies and uncertainties are taken from [46]. (Lower panel) Residuals of the calibration points with respect to the calibration line.

Figure 5

SeGA photopeak efficiency. (Top panel) Results of a geant4 simulation [solid line (red)] compared to the efficiency measured with absolutely calibrated sources (black circles) and the known ${}^{24}\text{Mg}$ lines (empty squares). The simulation and the ${}^{24}\text{Mg}$ data have been scaled to match the source measurements. (Bottom panel) Ratio between the simulation and the experimental data. The shaded area (yellow) shows the adopted uncertainties.

Figure 6

(Top panel) Example of a typical fit to the 1960-keV peak, using the function of Eq. (2). The dashed line corresponds to the background component of the fit. (Bottom panel) Residuals of the fit in terms of the standard deviation $\sigma$.

Figure 7

$\beta -\gamma -\gamma$ coincidence spectrum gating on the 1797-keV $\gamma$-rays (blue). The hatched histogram (green) shows coincidences with continuum background in a relatively broad region above the peak gate. The background bins are 16-keV wide and are normalized to the expected background per 2 keV from random coincidences. The strongest peaks corresponding to $\gamma$ rays emitted in coincidence are indicated.

Figure 8

Selected sample of $\beta -\gamma -\gamma$ coincidence peaks gating on different $\gamma$ rays: (a) 989 keV, (b) 1401 keV, (c) 2024 keV, and (d) 2341 keV. The hatched histogram shows normalized coincidences with continuum background in a relatively broad region above the peak gates. The background bins are 8-keV wide and are normalized to the expected background per keV.

Figure 9

(Left) ${}^{26}\text{P}$ decay scheme as deduced from the experimental data acquired in the present work. $\gamma$-ray transition labels correspond to the absolute intensities. $\beta$-decay branches corresponding to each populated level are also given (red). The branches to the unbound $3^{+}$ state and the particle unbound states (asterisks) were taken from literature [8, 33]. (Right) ${}^{26}\text{Si}$ levels populated in ${}^{26}\text{P}\beta$ decay obtained from a USDB shell-model calculation. Level energies are given in keV.

Figure 10

Summed Gamow-Teller strength distribution of the $\beta$ decay of ${}^{26}\text{P}$ up to 5.9 MeV excitation energy. The results of the present experiment are compared to previous results [8] and Shell-Model calculations. A quenching factor $q^2=0.6$ was used in the theoretical calculation.