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Piatti, Denise (2018) The Study of 22Ne(a,g)26Mg and 6Li(p,g)7Be Reactions at LUNA. [Ph.D. thesis]

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Abstract (italian or english)

In the current work the study of the 22Ne(a,g)26Mg and of the 6Li(p,g)7 Be reactions
are presented. Both reactions were investigated at LUNA, located under 1400 m of
Gran Sasso rock (Italy). The rock coverage guarantees an unprecedented reduction
of the cosmic rays background.
The 22Ne(a,g)26Mg reaction plays a fundamental role in the nucleosynthesis of s-
nuclei in low-mass AGB stars and in massive stars. Indeed, the 22Ne(a,g)26Mg reaction
is the competitor of the 22Ne(a,n)25 Mg reaction, an
effective neutron source for element synthesis through s-process. The ratio between
the rates of these two reactions is crucial to estimate the impact of
AGB stars and massive stars on the s-nuclei abundances.
Currently this ratio is affected by high uncertainty because of the wide
range of values proposed for the 22Ne(a,g)26Mg 395 keV resonance strength (10^(-15)
- 10^(-9) eV). All the results reported in literature are derived from indirect
measurements. The present study represents the first direct
measurement to estimate the strength of the 395 keV resonance and its impact on
the 22Ne(a,g)26Mg reaction rate.
Because of the low resonance strength values an high efficiency detector was
installed at the gas target beamline of LUNA400kV accelerator, where the Ne gas,
99% enriched in 22Ne, was irradiated with a 399.9 keV a-beam. No significant signal
was detected in the 22Ne(a,g)26Mg region of interest, thus an upper limit for the
395 keV resonance strength was estimated.
The 6Li is an important indicator of the stellar age and of mixing processes in stars.
Indeed it is progressively depleted by stars during the pre-main and main sequence
phase via both 6Li(p,g)7Be and 6Li(p,a)3He reaction. The 6Li(p,g)7Be reaction was
studied by many groups but a recent work renewed the interest on this reaction.
A resonance like structure was observed at Ecm ~ 195 keV (Ex ~ 5.8 MeV) which
is neither predicted by theoretical studies nor seen in previous experiments. The
new excited state could explain the angular distribution of the 6Li(p,a)3He reaction,
which requires both negative and positive parity levels contribution. In addition, the
new excited state may affect the Standard Big Bang Nucleosynthesis predictions and
the extrapolation to low energy of the 3He(a,g)7Be cross section. The present work
has the goal of investigating the existence of the 195 keV resonance and of measuring
the 6Li(p,g)7Be cross section down to low energies to better constrain the S-factor
extrapolation at energies of astrophysical interest. The measurement was performed
at the solid target beamline of LUNA400kV accelerator, where a devoted scattering
chamber was installed. Six targets of different composition and thickness were irradiated
at energies between 80 keV and 390 keV. In addition to the
HPGe detector, used to detect gamma-rays produced by the 6Li(p,g)7Be reaction, a
Si detector was put in place to detect the charged particles
produced by the 6Li(p,a)3He reaction. The
results of this thesis do not confirm the resonance observed at Ecm ~ 195 keV.

Abstract (a different language)

Nella presente tesi sono presentati gli studi sperimentali delle reazioni 22Ne(a,g)26Mg
e 6Li(p,g)7Be. Entrambe le reazioni sono state studiate a LUNA, che si trova sotto
1400 m di roccia del Gran Sasso, per aver un'efficiente schermatura dal fondo creato
dai raggi cosmici.
La reazione 22Ne(a,g)26Mg ha un ruolo fondamentale per la nucleosintesi dei nuclei
tramite il processo s in stelle AGB di piccola massa e nelle stelle massicce. Infatti
essa compete con la reazione 22Ne(a,n)25Mg, un’efficiente sorgente di neutroni per il
processo s. Il rapporto tra i rates di queste due reazioni è una quantità cruciale per
stimare l’impatto delle stelle AGB e delle stelle massicce sulle abbondanze dei nuclei
s. Attualmente questo rapporto non è noto con accuratezza poiché il contributo della
risonanza a 395 keV della reazione 22Ne(a,g)26Mg è ancora incerto. Tutti i risultati
di intensità riportati in letteratura per questa risonanza sono stati dedotti da misure
indirette. Il presente lavoro si propone di studiare il contributo della risonanza a 395
keV sul rate della 22Ne(a,g)26Mg con una misura diretta. Poichè i valori di inten-
sità per la risonanza stanno tra 10^(-15) eV e 10^(-9) eV un rivelatore ad alta efficienza
è stato installato sulla linea gassosa collegata all’acceleratore LUNA400kV. Il gas
Neon, arricchito al 99% in 22Ne, è sato irradiato con un fascio di particelle a con
un energia di 399.9 keV. Nessun segnale significativo è stato rilevato nella regione
energetica dello spettro di interesse per la reazione 22Ne(a,g)26Mg, perciò è stato
calcolato un limite superiore per l'intensità della risonanza a 395 keV.
Il 6Li è un importante indicatore dell'età stellare e dei processi di mescolamento nelle
stelle. Infatti esso viene progressivamente distrutto dalle stelle nella fase precedente e
durante la sequenza principale via le reazioni 6Li(p,a)3He e 6Li(p,g)7Be. La reazione
6Li(p,g)7Be è stata studiata da molti gruppi e un recente esperimento ha riacceso
l’interesse per questa reazione. Una risonanza è stata osservata a Ecm ~ 195 keV
(Ex ~ 5.8 MeV), né prevista da studi teorici né osservata in precedenti esperimenti.
Il nuovo stato eccitato potrebbe però spiegare la distribuzione angolare della reazione
6Li(p,a)3He, che richiede il contributo sia di livelli con parità positiva e negativa. Inoltre, il
nuovo stato eccitato potrebbe avere un effetto sulle predizioni della Standard Big Bang
Nucleosynthesis e sull’estrapolazione a bassa energia
della sezione d’urto
della reazione 3 He(a,g)7Be. Lo studio attuale ha lo scopo di investigare l’esistenza
della risonanza a 195 keV e di misurare la sezione d’urto della reazione 6Li(p,g)7Be
fino a basse energie così da meglio estrapolare il fattore astrofisico, S(E), a energie di
interesse astrofisico. La misura è stata svolta usando la linea solida dell’acceleratore
LUNA400kV dove una camera di scattering dedicata è stata montata. Sei bersagli
di diversa composizione e spessore sono stati irradiati a energie tra 80 keV e 390
keV. Assieme al rivelatore gamma HPGe, un silicio è stato usato per rivelare le
particelle cariche prodotte dalla reazione 6Li(p,a)3He. I risultati di questa tesi non
confermano la risonanza a Ecm ~ 195 keV.

Statistiche Download
EPrint type:Ph.D. thesis
Tutor:Broggini, Carlo
Supervisor:Broggini, Carlo
Ph.D. course:Ciclo 31 > Corsi 31 > FISICA
Data di deposito della tesi:16 November 2018
Anno di Pubblicazione:16 November 2018
Key Words:nuclear cross section, underground nuclear astrophysics, nucleosynthesis, gamma spctroscopy
Settori scientifico-disciplinari MIUR:Area 02 - Scienze fisiche > FIS/04 Fisica nucleare e subnucleare
Area 02 - Scienze fisiche > FIS/05 Astronomia e astrofisica
Struttura di riferimento:Dipartimenti > Dipartimento di Fisica e Astronomia "Galileo Galilei"
Codice ID:11333
Depositato il:07 Nov 2019 15:32
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