Thesis (PhD)--Stellenbosch University, 2015. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/97145 |
| Date | 04 1900 |
| Creators | Van Rooy, Milton William |
| Contributors | Papka, Paul, Smit, Frederick David, de Meijer, Robert Johan, Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 152 pages : colour illustrations |
| Rights | Stellenbosch University |
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