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Spectroscopic Study of 39Ca for Endpoint Nucleosynthesis in Classical
Novae
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Nuclear Physics in Astrophysics IX (NPA-IX)
Journal of Physics: Conference Series 1668 (2020) 012025
IOP Publishing
doi:10.1088/1742-6596/1668/1/012025
1
Spectroscopic Study of 39Ca for Endpoint
Nucleosynthesis in Classical Novae
JLiang
1,AAChen
1, M Anger3, S Bishop3, T Faestermann3,C
Fry2, R Hertenberger4, A Psaltis1, D Seiler3,PTiwari
2,H-F
Wirth4,CWrede
2
1Department of Physics and Astronomy, McMaster University, Hamilton, Ontario,
L8S 4M1, Canada
2Department of Physics and Astronomy, National Superconducting Cyclotron
Laboratory, Michigan State University, 640 S. Shaw Ln., East Lansing, MI, 48864,
USA
3Department of Physics, Technical University Munich, James-Franck-Str. 1, 685748,
Garching, Germany
4Faculty of Physics, Ludwig-Maximilians-Universit¨at M¨unchen, Munich,
Geschwister-Scholl-Platz 1, 80539 Munich, Germany
E-mail: LiangJ27@mcmaster.ca
Abstract. Classical novae are environments which can produce heavier elements up
to mass A ∼40. These nuclides at the endpoint of nova nucleosynthesis consist of
elements such as Ar, K, and Ca. There is an order of magnitude discrepancy with
the predicted and theoretical abundances of these endpoint nuclides produced in a
classical nova. The uncertainty in the theoretical 38K(p,γ)39Ca reaction rate has
been shown to affect the abundances by an order of magnitude or more. The only
direct measurement of this reaction rate was performed with the DRAGON facility at
TRIUMF; however additional spectroscopic data could aid the interpretation of this
data as well as motivate further study of this reaction rate. In this study, we present the
preliminary results of a spectroscopic study of 39 Ca using the 40Ca(d,t)39 Ca reaction
carried out at the Maier-Leibnitz Laboratory in Garching, Germany.
1. Introduction
Classical novae occur in interacting binaries consisting of a white dwarf (WD) and
a red dwarf/main sequence star that fills its Roche lobe. This mass transfer from
the companion star leads to an accretion disk on the white dwarf, and the material
is compressed and the bottom layer will become degenerate. Hydrogen burning via
the CNO cycle on this degenerate layer will result in thermonuclear runaway, which
ultimately leads to an explosive outburst. Temperatures can reach between 0.1 - 0.4
GK (depending on the size and rate of accretion), and the outburst can eject up to 10−4
-10
−5M.[1]
Nuclear Physics in Astrophysics IX (NPA-IX)
Journal of Physics: Conference Series 1668 (2020) 012025
IOP Publishing
doi:10.1088/1742-6596/1668/1/012025
2
Spectroscopic Study of 39Ca for Endpoint Nucleosynthesis in Classical Novae
In this process, heavier nuclides are produced through (p,γ), (p,α), and β−decays,
with the endpoint of this nucleosynthesis occurring near A ∼40.
Currently, observations of classical novae predict an order of magnitude
enhancement of endpoint elements, such as Ar, K, and Ca, relative to solar abundances;
however, simulations predict abundances closer to that of solar abundances [1, 2].
Sensitivity studies examining the effect of reaction rates on the abundances of
these elements have shown that the reaction 38K(p,γ)39Ca can change the abundances of
endpoint nuclides by an order of magnitude or more [3]. In temperatures characteristic of
this environment, this reaction rate is dominated by =0 resonances within the Gamow
window. These were previously identified as excited states in 39Ca at 6157(10), 6286(10),
and 6460(10) keV. This reaction rate was directly measured with the DRAGON facility
in TRIUMF, where the highest resonance was instead observed at a lower energy of
6450±+2
−1(stat.) ±1(sys.), keV and the other 2 resonances were unobserved, setting
upper limits for their respective resonance strengths [4].
High resolution spectroscopic studies of 39Ca could provide more information
on the various resonances that lie in the Gamow window, and illuminate additional
undiscovered states corresponding to low-capture resonances. To that end, we have
conducted a spectroscopic study of 39 Ca using the 40Ca(d,t)39 Ca reaction to populate
excited states in 39Ca relevant to astrophysics.
2. Experiment
The experiment was carried out at the Maier-Leibnitz Laboratory (MLL). The 2H
beam was accelerated to an energy of 22 MeV using the 14-MV MP-Tandem, which
impinged the production target: a 40 μg/cm2CaF2on a isotopically pure 12C target.
The tritons from the reaction were momentum analyzed using the Quadrupole-Dipole-
Dipole-Dipole (Q3D) spectrograph. The position data for the tritons was taken using
a position sensitive focal plane detector consisting of 255 electrically isolated cathode
strips arranged opposite to an anode wire in an isobutane gas filled chamber. When a
triton passed through the gas, it deposited energy via ionization of the surrounding gas,
inducing a charge on the cathode strips closest to the triton event. By observing the
amount of charge on specific cathode strips, the position of the triton could be deduced.
The focal plane was then calibrated using the position spectrum of a 32 S target, since
the excitation energies of 31S populated by the 32S(d,t)31S are known, therefore allowing
us to convert the positions of the tritons to the triton energies (and by conservation of
energy the residual nucleus energies).
3. Analysis and Results
Peaks corresponding to energy levels in 39Ca were fitted using a Gaussian function
modified with an exponential. This combined function produces an asymmetric
Gaussian that accounts for the asymmetry from the low energy tail of tritons. The triton
Nuclear Physics in Astrophysics IX (NPA-IX)
Journal of Physics: Conference Series 1668 (2020) 012025
IOP Publishing
doi:10.1088/1742-6596/1668/1/012025
3
Spectroscopic Study of 39Ca for Endpoint Nucleosynthesis in Classical Novae
Table 1. Preliminary important astrophysical resonance energies and new state of
39Ca determined in the current work compared to previously evaluated values. New
state at 6446 keV in bold.
NDS evaluated energy (keV) This work (keV) (±)stat
6451 (2) 6474(1)
6446(1)
6286 (10) 6301(1)
6157 (10) 6160.4(6)
spectrum (Figure 1) contains a significant background from the fluorine content in the
production target, which produced tritons through the 19 F(d,t)18F reaction. However
the distribution of tritons was continuous and slowly varying over the focal plane, and
could be approximated linearly on local scales underneath the peaks. Thus, a linear
function was also added into the fitting function.
The centroids of these asymmetric Gaussian peaks gives information regarding the
positions of the tritons on the focal plane, however they must be calibrated in order to
find the corresponding energies of the tritons. This is done by using the 32S calibration
target to populate known states in 31S. These known states - and therefore the energies
of the tritons from those states - are assigned to the peaks on the position spectrum,
and a polynomial function is calculated with a Markov chain Monte Carlo [5] fitting to
convert the position data to the energies of the triton. From there, using conservation
of energy and momentum, the energy of the residual nucleus in the 39Ca spectrum can
be found for each respective peak in the triton position spectrum. Preliminary energies
and a new state discovered at 6446(1) keV in this work are tabulated in Table 1.
There is a systematic difference of ∼+10 keV when comparing the excitation
energies from this work to that of the previous literature values. This difference could
be explained by the usage of more current mass measurements: Since this analysis
depends heavily on the Q-value of the reaction, the energies determined are extremely
sensitive to the masses of each nuclide in the reaction. Between the 2003 and 2012
Atomic Mass Evaluations (2003AME, and 2012AME respectively) [6], the mass excess
of 39Ca changed by -8 keV, increasing the Q-value by 8 keV. This effect systematically
shifts the resonance energies. When the 2003AME mass table is used in the calculation
of the residual energy in the 39Ca nucleus, the weighted averages are within error of
tabulated values prior to the DRAGON measurement, as seen in Table 2. No difference
was observed when using the 2016AME [7] compared to the 2012AME.
One potential explanation to the DRAGON measurement’s anomalous results for
the state found at 6450±+2
−1(stat.) ±1(sys.) keV is that the new state found in this
work at 6446(1) keV is being observed instead. Additional measurements on 39 Ca have
been performed with the 39 K(3He,t)39Ca reaction at the Triangle Universities Nuclear
Laboratories (TUNL) and will verify the existence of this new resonance and determine
Nuclear Physics in Astrophysics IX (NPA-IX)
Journal of Physics: Conference Series 1668 (2020) 012025
IOP Publishing
doi:10.1088/1742-6596/1668/1/012025
4
Spectroscopic Study of 39Ca for Endpoint Nucleosynthesis in Classical Novae
Table 2. Preliminary resonance energies of 39Ca determined in the current work using
2003AME compared to evaluated values prior to the DRAGON measurement. New
state recalculated with 2003AME in bold.
NDS evaluated energy (keV) This work (keV) (±)stat
6460 (10) 6459.2(6)
6431(2)
6286 (10) 6289(1)
6157 (10) 6150.7(1)
the spin and parity of this resonance. It will also serve to increase precision on known
states.
4. Conclusions
In this work, a spectroscopic study of 39Ca using the 40 Ca(d,t)39Ca reaction has shown a
new state in 39Ca in the Gamow window. In addition, with more current mass measure-
ments, the resonance energies calculated have changed relative to previous literature
values tabulated in ENSDF. Future work at TUNL will be carried out to verify the
excited states of 39Ca in the Gamow window. Once verified, a remeasurement of the
astrophysically important resonance strengths of the 39Ca(p,γ)39Ca reaction (i.e. reac-
tions populating 5/2+and 7/2+states in 39 Ca) is recommended.
Nuclear Physics in Astrophysics IX (NPA-IX)
Journal of Physics: Conference Series 1668 (2020) 012025
IOP Publishing
doi:10.1088/1742-6596/1668/1/012025
5
Spectroscopic Study of 39Ca for Endpoint Nucleosynthesis in Classical Novae
Figure 1. Position spectrum of tritons, spectrometer angle of 20 degrees. Significant
number of tritons from 19F(d,t)18F causes locally linear background, however peaks
from 39Ca can be easily seen atop the background.
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