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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Structure of the nucleus ¹¹⁴Sn using gamma-ray coincidence data

Oates, Sean Benjamin January 2015 (has links)
No description available.
2

Promene nuklearnih spektara pod dejstvom kosmičkog zračenja / Changes of nuclear spectra under the influence of cosmic radiation

Bikit Kristina 13 June 2015 (has links)
<p>U ovoj disertaciji su prikazani rezultati kompleksnih istraživanja uticaja<br />kosmičkog zračenja na nuklearne spektre na nivou mora.<br />Utvrđivanje specifičnih vremenskih intervala u vremenskom spektru,<br />kojima odgovaraju tačno određene grupe događaja indukovane<br />kosmičkim mionima, omogućava adekvatno odbacivanje ometajućih<br />događaja u željenim energetskim oblastima detektovanih spektara, pri<br />antikoincidentnom režimu rada ultraniskofonskih sistema. U prvom<br />eksperimentu prikazanom u ovoj disertaciji,&nbsp; istraživana je mogućnost<br />vremenskog razlaganja detektovanih događaja,&nbsp; pomoću koincidentnog<br />sistema u čijem sklopu se nalazi HPGe detektor i&nbsp; plastični scintilator, u<br />povr&scaron;inskoj laboratoriji. Ustanovljeno je da se promptni i zakasneli<br />koincidentni događaji između plastičnog &ldquo;veto&rdquo; detektora i<br />germanijumskog detektora mogu jasno razdvojiti u dve grupe, za oko<br />100 ns. Dodatno, zakočno zračenje i&nbsp; anihilacioni događaji mogu se<br />razdvojiti u vremenu od (n,n&rsquo;) događaja, iako svi ovi događaji pripadaju<br />grupi zakasnelih događaja. Takođe, registrovani su&nbsp; i&nbsp; značajno zakasneli<br />anihilacioni događaji, koji nastaju usled raspada zaustavljenih pozitivnih<br />miona.<br />Drugi eksperiment prikazan u ovoj disertaciji baziran je na ultra-niskofonskom HPGe spektrometru relativne efikasnosti 100%.<br />Dodatkom dva plastična scintilatora i brzo-sporog koincidentnog kola,<br />istraživani su koincidentni događaji između plastičnih scintilatora i<br />HPGe spektrometra. Ovaj spektrometarski sistem MIREDO <em>(Muon<br />Induced Rare Event Dynamic Observatory</em>) prvenstveno je namenjen<br />proučavanju procesa indukovanih kosmičkim mionima u različitim<br />materijalima. Analiza ovakvih interakcija može biti od značaja za ultra-niskofonske eksperimente. Rezultati dobijeni za tri ispitivana materijala,<br />pakovana u<em> Marinelli</em> sud, prezentovani su i diskutovani.<br />U trećem eksperimentu prikazanom u ovoj disertaciji ispitan je<br />potencijalni uticaj solarnih&nbsp; neutrina&nbsp; na izmerenu brzinu&nbsp; radioaktivnog<br />raspada,&nbsp; merenjem varijacija u brzini brojanja<br /><sup>3</sup>H&nbsp; metodom tečnog&nbsp;scintilacionog brojanja. Kori&scaron;ćenjem sofisticiranog tečnog scintilacionog&nbsp;spektrometra&nbsp;<em> Quantulus</em>&nbsp; ustanovljeno je da na merenje<br />visokoenergetskog dela&nbsp;<sup>3</sup>H spektra može značajno uticati nestabilnost<br />instrumenta. Oscilatorni karakter izmerenog visokoenergetskog dela<br /><sup>3</sup>H&nbsp;spektra&nbsp; je registrovan, ali sa veoma malom amplitudom (manjom od<br />0.5%), koja se ne može jednostavno objasniti samo nestabilno&scaron;ću<br />instrumenta.&nbsp; Kada je meren ukupan&nbsp;<sup>3</sup>H spektar, nisu nađene značajne<br />varijacije u brzini brojanja.<br />Već duže vreme je poznato da je niskoenergetsko gama zračenje<br />kontinualne distribucije prisutno na otvorenom prostoru, u vazduhu na<br />povr&scaron;ini Zemlje. U prethodnim istraživanjima pretpostavljano je da&nbsp; ovo<br />zračenje potiče skoro isključivo od gama fotona koji su emitovani usled<br />prirodne radioaktivnosti i potom rasejani u nazad od strane vazduha<br />iznad zemlje. U četvrtom eksperimentu prikazanom u ovoj disertaciji<br />pokazano je da&nbsp; je&nbsp; ovo zračenje (u energetskom regionu 30 keV-300<br />keV), sa maksimumom na oko&nbsp; 90 keV, u značajnoj meri&nbsp; proizvedeno<br />kosmičkim zračenjem, sa fluksom fotona od oko 3000 m<sup>-2</sup>s<sup>-1</sup>. Takođe,<br />ustanovljeno je da dozama op&scaron;te populacije doprinosi ovo sveprisutno<br />niskoenergetsko gama zračenja kosmičkog porekla, zajedno sa<br />odgovarajućim fluksom niskoenergetskih elektrona i da ove komponente<br />ukupnih doza indukovanih kosmičkim zračenjem na nivou mora nisu<br />zanemarljive.</p> / <p>In this dissertation results of complex research on cosmic-ray impact on&nbsp;nuclear spectra at sea level are shown.&nbsp; The appropriate selection of&nbsp;coincidence time interval &nbsp;in low-background experiments that are based&nbsp;on the rejection of anticoincidence background events is very important&nbsp;for reducing the influence of cosmic-ray muons on acquired spectral&nbsp;data. In&nbsp; the first&nbsp; experiment&nbsp; presented in this dissertation,&nbsp; performed by&nbsp;the coincidence system of an HPGe detector and a plastic detect or&nbsp; in a&nbsp;surface laboratory, the time resolution of the detected events is explored.&nbsp;It is&nbsp; found that the prompt and delayed coincidence events between a&nbsp;plastic veto detector and a &nbsp;Ge detector can be sharply divided for&nbsp;approximately 100 ns in two groups. In addition, the bremsstrahlung&nbsp;and annihilation events can&nbsp; be time-resolved from the (n,n&rsquo;) events,&nbsp;although all of these events belong to the group of delayed events. Also,&nbsp;substantially delayed annihilation events, which are caused by the&nbsp;<br />decays of stopped positive muons, were detected.<br />The second experiment shown in this dissertation is based on the 100%&nbsp;relative efficiency ultra-low-background HPGe spectrometer. With the&nbsp;addition of two plastic scintillators and a fast-slow coincidence circuit,&nbsp;the coincidence events between the plastic detectors and the HPGe&nbsp;spectrometer have been investigated. This&nbsp; MIREDO (Muon Induced&nbsp;Rare Event Dynamic Observatory) spectrometer system is primarily&nbsp;<br />developed for the study of cosmic muon induced processes in different&nbsp;materials. Exploration of such interactions can be important for ultra-low&nbsp;background experiments. Results derived for three samples, placed in a&nbsp;Marinelli beaker, are presented and discussed.<br />In third experiment shown in this dissertation,&nbsp; the potential influence of&nbsp;solar neutrinos on measured decay rate is investigated by&nbsp; the liquid&nbsp;scintillation measurement of the count rate variations of&nbsp;<sup>3</sup>H. Making use of the sophisticated Quantulus liquid scintillation spectrometer, it is&nbsp;found that the&nbsp; measurement of the high-energy tail of&nbsp;<sup>3</sup>H spectrum may be significantly influenced by instrumental&nbsp; instability.&nbsp; The oscillatory behavior of measured high-energy tail of <sup>3</sup>H spectrum&nbsp; is registered, but&nbsp;with very small amplitude (less than 0.5%), which cannot&nbsp; be easily&nbsp;<br />explained only by instrumental instability. When the total&nbsp;<sup>3</sup>H spectrum was measured, no significant variations in the count rate were found.&nbsp;<br />For a long time, it has been known that low-energy continuous gamma&nbsp;radiation is present in open air at the Earth&rsquo;s surface. In previous&nbsp;investigations it was assumed that this radiation is produced almost&nbsp;exclusively by gamma photons emitted due to the natural radioactivity,&nbsp;which are backscattered by air above ground.&nbsp; In the fourth experiment&nbsp;presented in this dissertation, it is&nbsp; shown&nbsp; that significant amount of this&nbsp;<br />radiation (related to energy region 30&nbsp; keV-300&nbsp; keV) that peaks at about&nbsp;90 keV, is produced by cosmic-rays, with the photon flux of about 3000&nbsp;m<sup>-2</sup>s<sup>-1</sup>.&nbsp; Also, it is found&nbsp; that the contribution of this omnipresent low&nbsp;energy gamma radiation of cosmic-ray origin, including the&nbsp;corresponding low-energy electron flux, to the doses of general&nbsp;<br />population are non-negligible components of overall doses induced by&nbsp;cosmic rays near sea level.</p>

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