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The beta decay of 31Ar, produced by fragmentation of a 36Ar beam at 880 MeV/nucleon, was investigated. Identified ions of 31Ar were stopped in a gaseous time projection chamber with optical readout allowing to record decay events with emission of protons. In addition to b{eta}-delayed emission of one and two protons we have clearly observed the beta-delayed three-proton branch. The branching ratio for this channel in 31Ar is found to be 0.07(2)%.
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Background: The Doppler broadening of $gamma$-ray peaks due to nuclear recoil from $beta$-delayed nucleon emission can be used to measure the energies of the nucleons. This method has never been tested using $beta$-delayed proton emission or applied to a recoil heavier than $A=10$. Purpose: To test and apply this Doppler broadening method using $gamma$-ray peaks from the $^{26}$P($beta pgamma$)$^{25}$Al decay sequence. Methods: A fast beam of $^{26}$P was implanted into a planar Ge detector, which was used as a $^{26}$P $beta$-decay trigger. The SeGA array of high-purity Ge detectors was used to detect $gamma$ rays from the $^{26}$P($beta pgamma$)$^{25}$Al decay sequence. Results: Radiative Doppler broadening in $beta$-delayed proton-$gamma$ decay was observed for the first time. The Doppler broadening analysis method was verified using the 1613 keV $gamma$-ray line for which the proton energies were previously known. The 1776 keV $gamma$ ray de-exciting the 2720 keV $^{25}$Al level was observed in $^{26}$P($beta pgamma$)$^{25}$Al decay for the first time and used to determine that the center-of-mass energy of the proton emission feeding the 2720-keV level is 5.1 $pm$ 1.0 (stat.) $pm$ 0.6 (syst.) MeV, corresponding to a $^{26}$Si excitation energy of 13.3 $pm$ 1.0 (stat.) $pm$ 0.6 (syst.) MeV for the proton-emitting level. Conclusions: The Doppler broadening method has been demonstrated to provide practical measurements of the energies for $beta$-delayed nucleon emissions populating excited states of nuclear recoils at least as heavy as $A=25$.
A multi-hit capacity setup was used to study the decay of the dripline nucleus 31Ar, produced at the ISOLDE facility at CERN. A spectroscopic analysis of the beta-delayed three-proton decay of 31Ar is presented for the first time together with a quantitative analysis of the beta-delayed two-proton-gamma-decay. A new method for determination of the spin of low-lying levels in the beta-proton-daughter 30S using proton-proton angular correlations is presented and used for the level at 5.2 MeV, which is found to be either a 3+ or 4+ level, with the data pointing towards the 3+. The half-life of 31Ar is found to be 15.1(3) ms. An improved analysis of the Fermi beta-strength gives a total measured branching for the beta-3p-decay of 3.60(44) %, which is lower than the theoretical value found to be 4.24(43) %. Finally the strongest gamma-transitions in the decay of 33Ar are shown including a line at 4734(3) keV associated to the decay of the IAS, which has not previously been identified.
We present for the first time precise spectroscopic information on the recently discovered decay mode beta-delayed 3p-emission. The detection of the 3p events gives an increased sensitivity to the high energy part of the Gamow-Teller strength distribution from the decay of 31Ar revealing that as much as 30% of the strength resides in the beta-3p decay mode. A simplified description of how the main decay modes evolve as the excitation energy increases in 31Cl is provided.
Resonances just above the proton threshold in 30S affect the 29P(p,gamma)30S reaction under astrophysical conditions. The (p,gamma)-reaction rate is currently determined indirectly and depends on the properties of the relevant resonances. We present here a method for finding the ratio between the proton and gamma partial widths of resonances in 30S. The widths are determined from the beta-2p and beta-p-gamma decay of 31Ar, which is produced at the ISOLDE facility at the European research organization CERN. Experimental limits on the ratio between the proton and gamma partial widths for astrophysical relevant levels in 30S have been found for the first time. A level at 4688(5) keV is identified in the gamma spectrum, and an upper limit on the proton to gamma width of 0.26 (95 % C.L.) is found. In the two-proton spectrum two levels at 5227(3) keV and 5847(4) keV are identified. These levels are previously seen to gamma decay and upper limits on the gamma to proton width of 0.5 and 9, respectively, (95 % C.L.) are found, where the latter differs from previous calculations.
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 $^{26}$P and compare the results with previous experimental results and shell-model calculations. Method: A $^{26}$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 $^{26}$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 $^{26}$P $beta$-decay. A complete $beta$-decay scheme was built for the allowed transitions to bound excited states of $^{26}$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 $^{26}$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 ($delta=51(10)%$) may be evidence for a proton halo in $^{26}$P.
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