An investigation of a possible effect of reactor antineutrinos on the decay rate of 22Na

Van Rooy, Milton William (2015-04)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Reported claims of a solar neutrino influence on radioactive β - -decay rates prompted a series of three long term measurements that were carried out at the Koeberg nuclear power plant North of Cape Town to investigate the mirror reaction of reactor antineutrinos with a β + - source. The β + -emitter 22Na was exposed to a changing reactor antineutrino flux of the order of ~1012 cm-2 s -1 through the various phases of reactor operation, from off to on or vice versa, to investigate the antineutrino effect on its decay rate. 22Na sources were placed on the face of or inside a scintillation detector while γ-ray energy spectra were measured at source-toreactor-core distances of 17 m and 23 m for reactor unit #1 and unit #2 respectively. The analytical method employed uses ratios of the net-peak count rates of the photopeaks in the γ- ray energy spectra while the antineutrino flux changes during the various phases of reactor operation. These ratios reduce some potential systematic factors such as dead time, but should also remain constant during all phases of reactor operation and be independent of antineutrino flux if antineutrinos have no significant effect on the decay rate of 22Na. Measurement series 1 employed a LaBr3(Ce) detector. Results indicated an effect between reactor-off and –on corresponding to an antineutrino interaction cross section upper limit of the order of barns, assuming that antineutrinos are indeed the cause. Subsequently, two confirmation measurements were performed using a normal (cylindrical) NaI(Tl) detector as well as a NaI(Tl) well-counter. Results of these two measurements did not indicate any correlation with the reactor status. Measurements series 3 proved to be the most sensitive measurement for detecting changes in the decay rate of 22Na and an upper limit of ~10-25 cm 2 was estimated for the cross section of antineutrinos interacting with 22Na. This upper limit is one order of magnitude smaller compared to measurements series 1 and two orders of magnitude smaller compared to measurement series 2. It is also one of magnitude smaller than the effect reported by Jenkins et al. and two orders of magnitude smaller than observed by de Meijer et al. at the Delft reactor. Considering the systematic effects encountered during each measurement series we find no significant evidence for antineutrinos affecting β + -decay in 22Na.

AFRIKAANSE OPSOMMING: Berigte bewerings van son-afkomstige neutrinos se invloed op β - -verval het gelei tot drie lang termyn metings wat uitgevoer is by die Koeberg kernkrag stasie Noord van Kaapstad om die spieëldbeeld reaksie van antineutrinos met ‘n β + -bron te ondersoek. Die 22Na β + -bron is blootgestel aan ‘n veranderende reaktor antineutrino vloed van die order ~1012 cm -2 s -1 soos die reaktor deur verskeie fases van operasie gaan, vanaf afgeskakel na aangeskakel of omgekeerd, om die effek van antineutrinos op die bron se vervaltempo te ondersoek. 22Na bronne was teen die voorkant of binne ‘n sintillator detektor geplaas terwyl γ-straal energie spektra gemeet was by bron-tot-reaktor kern afstande van 17 m en 23 m vir reaktor eenheid #1 en #2 afsonderlik. Die analitiese metode gebruik verhoudings van die netto-piek teltempos van die fotopieke in die γ-straal energie spektra terwyl die antineutrino vloed verander tydens verskeie fases van reaktor operasie. Hierdie verhoudings verminder sekere potensiële systematiese effekte soos dooietyd en moet konstant bly tydens alle fases van reaktor operasie en moet ook onafhanklik wees van die antineutrino vloed indien antineutrinos geen effek het op die vervaltempo van 22Na nie. Die eerste reeks metings het gebruik gemaak van ‘n LaBr3(Ce) detektor. Resultate het ‘n effek getoon tussen reaktor-af en reaktor-aan wat ooreenstem met ‘n boonste limiet vir die interaksie kansvlak van antineutrinos met grootteorde van barns, indien antineutrinos wel die oorsaak is. Twee reekse van opvolgende metings is uitgevoer wat gebruik gemaak het van ‘n gewone (silindriese) NaI(Tl) detektor asook ‘n NaI(Tl) put detektor. Resultate van hierdie metings het geen korrelasie met die reaktor status gewys nie. Die derde reeks metings was die sensitiefste om veranderinge in die verval tempo van 22Na te wys en ‘n boonste limiet vir die interaksie kansvlak van antineutrinos op 22Na was beraam as ~10-25 cm 2 . Hierdie boonste limiet is een grootteorde kleiner in vergelyking met reeks 1 en twee grootteordes kleiner as reeks 2. Dit is ook een grootteorde kleiner as die waarde van Jenkins et al. en twee grootteordes kleiner as wat de Meijer et al. berig het by die Delft reaktor. In agneming van die systematise probleme wat ondervind was tydens die drie reeks metings vind ons geen beduidende effek van antineutrinos op die vervaltempo van 22Na nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97145
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