Nuclear reactions relevant to Big-Bang nucleosynthesis

Tišma, Isabela (author), Lipoglavšek, Matej (mentor), Mihovilovič, Miha (co-mentor)

The theoretical predictions of primordial abundances of the light stable isotopes ${}^{2}$H, ${}^{3}$He, ${}^{4}$He, and ${}^{7}$Li are largely constrained by the experimental uncertainties in the rates of nuclear reactions that occurred during BBN. The observed primordial deuterium abundance has reached a one percent precision, and the same level of precision should be achieved on the BBN-predicted result. The aim of this work was to improve our knowledge of the astrophysical S-factor for the ${}^{2}$H(p,$\gamma$)${}^{3}$He reaction, as it affects significantly the reaction rate and primordial deuterium abundance. The ${}^{2}$H(p,$\gamma$)${}^{3}$He reaction was studied in the energy range of interest for BBN. Its differential cross section at $135^{\circ}$, and $\gamma$-ray angular distributions at $90^{\circ}$ and $135^{\circ}$ were measured in a series of experiments using two deuterated titanium targets, and two HPGe detectors. The compositions of the targets were determined with the NRA and RBS techniques. The S-factor was obtained in the center-of-mass energy range $E$ = 97--210 keV. The achieved systematic uncertainty (10%) of the present measurements did not allow an evaluation of different theoretical S-factor calculations. Thus, more accurate data are needed at BBN energies. To check the lithium abundance problem, the effect of electron screening on the rates of nuclear reactions relevant for BBN was investigated. It was found that the electron screening effect is too small to have a notable impact on reaction rates in BBN temperature range, and hence also on the primordial abundances of light element isotopes. Additionally, the $E$ = 259 keV resonance in the ${}^{14}$N(p,$\gamma$)${}^{15}$O reaction was measured in normal and inverse kinematics, in order to confirm large electron screening in inverse kinematics reactions, as some previous studies suggest. The results obtained indicate smaller resonance strength values in inverse kinematics. The reason for this behaviour is currently unknown, and will be provided by the next generation of experiments.

• Language: English
• Keywords: low-mass nuclear reactions, electron screening, Big-Bang nucleosynthesis, elemental abundances, radiative capture
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2021
• PID: 20.500.12556/RUL-129887
• COBISS.SI-ID: 75686915
• Publication date in RUL: 09.09.2021

Secondary language

• Language: Slovenian
• Title: Jedrske reakcije pri nukleosintezi po velikem poku

Abstract

Teoretične napovedi primarnih vsebnosti izotopov lahkih elementov ${}^{2}$H, ${}^{3}$He, ${}^{4}$He in ${}^{7}$Li so v veliki meri omejene z eksperimentalnimi negotovostmi hitrosti jedrskih reakcij, ki so se dogajale med sintezo elementov po velikem poku. Primarna vsebnost devterija določena iz astronomskih opazovanj je dosegla natančnost enega procenta in podobno natančnost bi pričakovali tudi od teoretičnih napovedi nukleosinteze po velikem poku. Namen tega dela je bil izboljšati naše poznavanje astrofizikalnega S-faktorja za jedrsko reakcijo ${}^{2}$H(p,$\gamma$)${}^{3}$He, ki pomembno vpliva na hitrost reakcije in primarno vsebnost devterija. Dano reakcijo smo študirali v energijskem območju pomembnem za nukleosintezo po velikem poku. Diferencialni reakcijski presek in kotno odvisnost žarkov $\gamma$ pri kotih $90^{\circ}$ in $135^{\circ}$ smo izmerili v nizu eksperimentov z uporabo dveh devteriranih titanovih tarč in dveh germanijevih detektorjev. Sestavo tarč smo določili s tehnikama NRA in RBS. Astrofizikalni S-faktor smo določili za težiščne energije med $E$ = 97--210 keV in ga primerjali z dostopnimi teoretičnimi izračuni. Z dano sistematsko negotovostjo (10%) nismo mogli določiti, kateri teoretični model bolje opiše nove izmerjene podatke. Tako potrebujemo bolj natančne podatke v energijskem območju velikega poka. Za preveritev problema vsebnosti litija v vesolju, smo raziskali vpliv efekta elektronskega senčenja na hitrosti jedrskih reakcij pomembnih za nukleosintezo po velikem poku. Pokazali smo, da je efekt elektronskega senčenja premajhen, da bi imel zaznaven vpliv na hitrosti jedrskih reakcij v temperaturnem območju velikega poka in s tem na primarne vsebnosti izotopov lahkih elementov. Poleg tega smo izmerili jakost resonance pri $E$ = 259 keV v reakciji ${}^{14}$N(p,$\gamma$)${}^{15}$O tako v normalni kot tudi v inverzni kinematiki. S tem smo želeli potrditi velik efekt elektronskega senčenja v inverzni reakcijski kinematiki, kot so napovedale prejšnje študije. Naši rezultati kažejo nižje jakosti resonanc v inverzni kinematiki kot v normalni kinematiki. Takega obnašanja še ne razumemo in ga bomo poskusili razložiti z naslednjo generacijo eksperimentov.

• Keywords: reakcije med lahkimi jedri, elektronsko senčenje, nukleosinteza po velikem poku, vsebnosti elementov, sevalno zajetje