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Register a new userLow-energy $^{23}$Al $beta$-delayed proton decay and $^{22}$Na destruction in novae
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The radionuclide $^{22}$Na is a target of $gamma$-ray astronomy searches, predicted to be produced during thermonuclear runaways driving classical novae. The $^{22}$Na(p,$gamma$)$^{23}$Mg reaction is the main destruction channel of $^{22}$Na during a nova, hence, its rate is needed to accurately predict the $^{22}$Na yield. However, experimental determinations of the resonance strengths have led to inconsistent results. In this work, we report a measurement of the branching ratios of the $^{23}$Al $beta$-delayed protons, as a probe of the key 204--keV (center-of-mass) $^{22}$Na(p,$gamma$)$^{23}$Mg resonance strength. We report a factor of 5 lower branching ratio compared to the most recent literature value. The variation in $^{22}$Na yield due to nuclear data inconsistencies was assessed using a series of hydrodynamic nova outburst simulations and has increased to a factor of 3.8, corresponding to a factor of $sim$2 uncertainty in the maximum detectability distance. This is the first reported scientific measurement using the Gaseous Detector with Germanium Tagging (GADGET) system.
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The $^{22}$Ne(p,$gamma$)$^{23}$Na reaction is the most uncertain process in the neon-sodium cycle of hydrogen burning. At temperatures relevant for nucleosynthesis in asymptotic giant branch stars and classical novae, its uncertainty is mainly due to a large number of predicted but hitherto unobserved resonances at low energy. Purpose: A new direct study of low energy $^{22}$Ne(p,$gamma$)$^{23}$Na resonances has been performed at the Laboratory for Underground Nuclear Astrophysics (LUNA), in the Gran Sasso National Laboratory, Italy. Method: The proton capture on $^{22}$Ne was investigated in direct kinematics, delivering an intense proton beam to a $^{22}$Ne gas target. $gamma$ rays were detected with two high-purity germanium detectors enclosed in a copper and lead shielding suppressing environmental radioactivity. Results: Three resonances at 156.2 keV ($omegagamma$ = (1.48,$pm$,0.10),$cdot$,10$^{-7}$ eV), 189.5 keV ($omegagamma$ = (1.87,$pm$,0.06),$cdot$,10$^{-6}$ eV) and 259.7 keV ($omegagamma$ = (6.89,$pm$,0.16),$cdot$,10$^{-6}$ eV) proton beam energy, respectively, have been observed for the first time. For the levels at 8943.5, 8975.3, and 9042.4 keV excitation energy corresponding to the new resonances, the $gamma$-decay branching ratios have been precisely measured. Three additional, tentative resonances at 71, 105 and 215 keV proton beam energy, respectively, were not observed here. For the strengths of these resonances, experimental upper limits have been derived that are significantly more stringent than the upper limits reported in the literature. Conclusions: Based on the present experimental data and also previous literature data, an updated thermonuclear reaction rate is provided in tabular and parametric form. The new reaction rate is significantly higher than previous evaluations at temperatures of 0.08-0.3 GK.
We have developed a technique to measure beta-delayed proton decay of proton-rich nuclei produced and separated with the MARS recoil spectrometer of Texas A&M University. The short-lived radioactive species are produced in-flight, separated, then slowed down (from about 40 MeV/u) and implanted in the middle of very thin Si detectors. The beam is pulsed and beta-p decay of the pure sources collected in beam is measured between beam pulses. Implantation avoids the problems with detector windows and allows us to measure protons with energies as low as 200 keV from nuclei with lifetimes of 100 ms or less. Using this technique, we have studied the isotopes 23Al and 31Cl, both important for understanding explosive H-burning in novae. They were produced in the reactions 24Mg(p,2n)23Al and 32S(p,2n)31Cl, respectively, in inverse kinematics, from stable beams at 48 and 40 MeV/u, respectively. We give details about the technique, its performances and the results for 23Al and 31Cl beta-p decay. The technique has shown a remarkable selectivity to beta-delayed charged-particle emission and would work even at radioactive beam rates of a few pps. The states populated are resonances for the radiative proton capture reactions 22Na(p,g)23Mg and 30P(p,g)31S, respectively.
The proton-proton momentum correlation functions ($C_{pp}(q)$) for kinematically complete decay channels of $^{23}$Al $rightarrow$ p + p + $^{21}$Na and $^{22}$Mg $rightarrow$ p + p + $^{20}$Ne have been measured at the RIKEN RI Beam Factory. From the very different correlation strength of $C_{pp}(q)$ for $^{23}$Al and $^{22}$Mg, the source size and emission time information were extracted from the $C_{pp}(q)$ data by assuming a Gaussian source profile in the correlation function calculation code (CRAB). The results indicated that the mechanism of two-proton emission from $^{23}$Al was mainly sequential emission, while that of $^{22}$Mg was mainly three-body simultaneous emission. By combining our earlier results of the two-proton relative momentum and the opening angle, it is pointed out that the mechanism of two-proton emission could be distinguished clearly.
Two-proton relative momentum ($q_{pp}$) and opening angle ($theta_{pp}$) distributions from the three-body decay of two excited proton-rich nuclei, namely $^{23}$Al $rightarrow$ p + p + $^{21}$Na and $^{22}$Mg $rightarrow$ p + p + $^{20}$Ne, have been measured with the projectile fragment separator (RIPS) at the RIKEN RI Beam Factory. An evident peak at $q_{pp}sim20$ MeV/c as well as a peak in $theta_{pp}$ around 30$^circ$ are seen in the two-proton break-up channel from a highly-excited $^{22}$Mg. In contrast, such peaks are absent for the $^{23}$Al case. It is concluded that the two-proton emission mechanism of excited $^{22}$Mg is quite different from the $^{23}$Al case, with the former having a favorable diproton emission component at a highly excited state and the latter dominated by the sequential decay process.
Remarkable results have been published recently on the $beta$ decay of $^{56}$Zn. In particular, the rare and exotic $beta$-delayed $gamma$-proton emission has been detected for the first time in the $fp$ shell. Here we focus the discussion on this exotic decay mode and on the observed competition between $beta$-delayed protons and $beta$-delayed $gamma$ rays from the Isobaric Analogue State.
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