Understanding the cosmic abundance of22Na lifetime measurements in23Mg C.Fougères12F .de Oliveira Santos1N. A. Smirnova3C.Michelagnoli15 and GANIL-

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Understanding the cosmic abundance of 22Na: lifetime

measurements in 23Mg

C. Fougères1,2,∗,F. de Oliveira Santos1,∗∗,N. A. Smirnova3,C. Michelagnoli1,5, and GANIL-

E710 /AGATA collaborations

1Grand Accélérateur National d’Ions Lourds (GANIL), CEA/DRF-CNRS/IN2P3, Caen, France

2Physics Division, Argonne National Laboratory, Lemont, USA

3LP2IB, Université de Bordeaux, CNRS/IN2P3, Gradignan, France

4Institut Laue-Langevin, Grenoble, France

Abstract. Simulations of explosive nucleosynthesis in novae predict the pro-

duction of 22Na, a key astronomical observable to constrain nova models. Its

gamma-ray line at 1.275 MeV has not yet been observed by the gamma-ray

space telescopes. The 20Ne/22Ne ratio in presolar grains, a possible tool to iden-

tify nova grains, also depends on 22 Na produced. Uncertainties on its yield in

classical novae currently originate from the rate of the 22Na(p, γ)23Mg reac-

tion. At peak novae temperatures, this reaction is dominated by a resonance

at ER=0.204 MeV, corresponding to the Ex=7.785 MeV excited state in 23Mg.

The resonance strengths measured so far disagree by one order of magnitude.

An experiment has been performed at GANIL to measure the lifetime and the

proton branching ratio of this key state, with a femtosecond resolution for the

former. The reactions populating states in 23Mg have been studied with a high

resolution detection set-up, i.e. the particle VAMOS, SPIDER and gamma

tracking AGATA spectrometers, allowing the measurements of lifetimes and

proton branchings. We present here a comparison between experimental results

and shell-model calculations, that allowed us to assign the spin and parity of

the key state. Rather small values obtained for reduced M1 matrix elements,

|M(M1)|.0.5µN, and proton spectroscopic factors, C2Sp<10−2, seem to be

beyond the accuracy of the shell model. With the reevaluated 22Na(p, γ)23Mg

rate, the 22Na detectability limit and its observation frequency from novae are

found promising for the future space telescopes.

Many nuclei are synthesized in explosive stellar environments, like classical novae, su-

pernovae and neutron star mergers. Novae are transient astronomical events and the most

frequent explosive events in our galaxy, after X-ray bursts. These explosions happen in a

close binary stellar system consisting of a white dwarf accreting hydrogen-rich matter from

its companion. This matter is progressively compressed at the white dwarf surface until the

conditions required for hydrogen combustion are reached. Nucleosynthesis takes place dur-

ing this explosive stage while a part of the new material is being ejected in the interstellar

medium. Beyond this well understood picture, large uncertainties remain in our knowledge

of novae, for example the white dwarf initial conditions (mass, luminosity), the accretion

rate and its composition, the amount of admixed white dwarf material with the accreta, the

∗e-mail: cfougeres@anl.gov

∗∗e-mail: oliveira@ganil.fr

arXiv:2210.14336v2 [nucl-ex] 7 Dec 2022

ejected mass, to list a few [1]. After the explosion, gas is crystallized, imprinting the ejecta

composition into grains. The isotopic composition of presolar grains of a putative nova origin

shed some lights into these stellar explosions [1–3]. Furthermore, novae are thought to end

as supernovae of type Ia [4] which are used as standard candles to determine distances and,

so, to estimate the cosmic expansion. Therefore, astronomical observables are required to

improve our understanding of novae: 22Na has revealed itself as a key candidate.

The lifetime of the 22Na radioisotope (τ=2.6 yr) makes it a good radioactive tracer of no-

vae [7] since: (i) the lifetime is longer than the duration of the opaque phase following the ex-

plosion (on timescales of hours), (ii) the decay time is short enough to ensure space-time cor-

relation. Hence, 22Na is a promising candidate for observing γ-ray emissions from novae [8].

The emitted γrays at Eγ=1.275 MeV (and 0.511 MeV) are expected from hours to months

after the explosion [9]. They were searched for by the missions CGRO/COMPTEL [10] and

INTEGRAL/SPI [11], leading only to upper limits for the ejected amount of 22Na (left Fig. 1).

New space telescopes are in preparation, namely COSI [12] and e-ASTROGAM [13], with

3−

10 2−

10 1−

10 1 10

(meV)

0.204MeV

γω

2−

1−

) M

-9

(10

INDIRECT

Stegmuller (1996)

Sallaska (2010)

Integral

Comptel

Ex (MeV)

23Mg

22Na+p

3/2+

7.586

7.581

7.770

7.782

7.785 5/2+7/2+

11/2+

7.803 5/2+

7.855 7/2+

8.016 5/2+7/2+

0.204

0.274

Jπ

0.1 < T9< 0.4

ER(MeV)

Novae

5/2+

9/2+

0.222 IAS

0.189

0.201

0.435

Figure 1. Left: The expected 22Na mass ejected from a nova is given as a function of the strength of the

ER=0.204 MeV resonance, from simulations with the MESA code [5]. Colored lines mark the Integral

and Comptel space telescope lower sensitivity limits. Right: Levels scheme of 23Mg and resonance

energies ERof the proton capture reaction. The Gamow window of novae is shown with the red zone.

sensitivities 20 times higher than INTEGRAL. The radioisotope 22Na can be also measured,

indirectly, by the over-abundance of the daughter nucleus 22Ne in presolar grains. Neon noble

gas is not expected to easily condense into dust grains and, so, the presence of 22Ne implies

in-situ β+decays of 22Na. A 22Ne excess was observed [14]. Therefore, it is critical to well

quantify 22Na nucleosynthesis in novae.

ONe novae are predicted to be the main site of 22Na production [6]. In these sites,

the matter is composed of a mixture between the transferred proton rich gas and the white

dwarf envelope with mainly oxygen and neon. The nucleosynthesis involves H to Ca iso-

topes, proton capture reactions and β+decays [1]. The production pathway of 22Na is

20Ne(p,γ)21Na(p,γ)22Mg(β+)22Na at high temperatures and 20Ne(p,γ)21Na(β+)21Ne(p,γ)22Na

otherwise. The thermal window of novae is [0.05, 0.5] GK. The destruction of 22Na proceeds

via β+decays and 22Na(p,γ)23Mg. The latter reaction is the least known, resulting in a factor

of 10 uncertainty in the predicted amount of 22Na ejected (left Fig. 1). Since the direct capture

contribution was found to be negligible [15, 16], the total reaction rate is determined by sev-

eral narrow resonances in the Gamow window < σv >tot=Pi(2π

µ(22Na,p)kBT )3

2~2(ωγ)iexp(−ERi

kBT )

(right Fig. 1). A direct measurement at astrophysical energies showed that a resonance at

ER=0.204 MeV dominates the 22Na(p,γ)23Mg rate [17]. However, its contribution disagrees

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Understandingthecosmicabundanceof22Na:lifetimemeasurementsin23MgC.Fougères1;2;,F.deOliveiraSantos1;,N.A.Smirnova3,C.Michelagnoli1;5,andGANIL-E710/AGATAcollaborations1GrandAccélérateurNationald'IonsLourds(GANIL),CEA/DRF-CNRS/IN2P3,Caen,France2PhysicsDivision,ArgonneNationalLaboratory,Lemont,USA3LP...

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Understanding the cosmic abundance of22Na lifetime measurements in23Mg C.Fougères12F .de Oliveira Santos1N. A. Smirnova3C.Michelagnoli15 and GANIL-

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