Excited states of $^{39}$Ca and their significance in nova nucleosynthesis

• Main Authors: Setoodehnia, Kiana, Marshall, Caleb, Longland, Richard, Kelley, John H, Liang, Johnson, Chaves, Federico Portillo
• Format: Journal Article
• Language: English
• Published: 01-11-2018
• Subjects: Physics - Nuclear Experiment
• DOI: 10.48550/arxiv.1811.00398

Background: Discrepancies exist between the observed abundances of argon and
calcium in oxygen-neon nova ejecta and those predicted by nova models. An
improved characterization of the $^{38}$K($p, \gamma$)$^{39}$Ca reaction rate
over the nova temperature regime ($\sim$ 0.1 -- 0.4 GK), and thus the nuclear
structure of $^{39}$Ca above the proton threshold (5770.92(63) keV), is
necessary to resolve these contradictions.
Purpose: The present study was performed to search for low-spin proton
resonances in the $^{38}$K $+$ $p$ system, and to improve the uncertainties in
energies of the known astrophysically significant proton resonances in
$^{39}$Ca.
Method: The level structure of $^{39}$Ca was investigated via high-resolution
charged-particle spectroscopy with an Enge split-pole spectrograph using the
$^{40}$Ca($^{3}$He, $\alpha$)$^{39}$Ca reaction. Differential cross sections
were measured over 6 laboratory angles at 21 MeV. Distorted-wave Born
approximation calculations were performed to constrain the spin-parity
assignments of observed levels with special attention to those significant in
determination of the $^{38}$K($p, \gamma$)$^{39}$Ca reaction rate over the nova
temperature regime.
Results: The resonance energies corresponding to two out of three
astrophysically important states at 6154(5) and 6472.2(24) keV are measured
with better precision than previous charged-particle spectroscopy measurements.
A tentatively new state is discovered at 5908(3) keV. The spin-parity
assignments of a few of the astrophysically important resonances are
determined.
Conclusions: The present $^{38}$K($p, \gamma$)$^{39}$Ca upper limit
thermonuclear reaction rate at 0.1 -- 0.4 GK is higher than that determined in
[Physical Review C 97 (2018) 025802] by at most a factor of 1.4 at 0.1 GK.