<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, istraživana je mogućnost<br />vremenskog razlaganja detektovanih događaja, pomoću koincidentnog<br />sistema u čijem sklopu se nalazi HPGe detektor i plastični scintilator, u<br />površinskoj laboratoriji. Ustanovljeno je da se promptni i zakasneli<br />koincidentni događaji između plastičnog “veto” detektora i<br />germanijumskog detektora mogu jasno razdvojiti u dve grupe, za oko<br />100 ns. Dodatno, zakočno zračenje i anihilacioni događaji mogu se<br />razdvojiti u vremenu od (n,n’) događaja, iako svi ovi događaji pripadaju<br />grupi zakasnelih događaja. Takođe, registrovani su i 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 neutrina na izmerenu brzinu radioaktivnog<br />raspada, merenjem varijacija u brzini brojanja<br /><sup>3</sup>H metodom tečnog scintilacionog brojanja. Korišćenjem sofisticiranog tečnog scintilacionog spektrometra <em> Quantulus</em> ustanovljeno je da na merenje<br />visokoenergetskog dela <sup>3</sup>H spektra može značajno uticati nestabilnost<br />instrumenta. Oscilatorni karakter izmerenog visokoenergetskog dela<br /><sup>3</sup>H spektra je registrovan, ali sa veoma malom amplitudom (manjom od<br />0.5%), koja se ne može jednostavno objasniti samo nestabilnošću<br />instrumenta. Kada je meren ukupan <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 />površini Zemlje. U prethodnim istraživanjima pretpostavljano je da 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 je ovo zračenje (u energetskom regionu 30 keV-300<br />keV), sa maksimumom na oko 90 keV, u značajnoj meri proizvedeno<br />kosmičkim zračenjem, sa fluksom fotona od oko 3000 m<sup>-2</sup>s<sup>-1</sup>. Takođe,<br />ustanovljeno je da dozama opšte 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 nuclear spectra at sea level are shown. The appropriate selection of coincidence time interval in low-background experiments that are based on the rejection of anticoincidence background events is very important for reducing the influence of cosmic-ray muons on acquired spectral data. In the first experiment presented in this dissertation, performed by the coincidence system of an HPGe detector and a plastic detect or in a surface laboratory, the time resolution of the detected events is explored. It is found that the prompt and delayed coincidence events between a plastic veto detector and a Ge detector can be sharply divided for approximately 100 ns in two groups. In addition, the bremsstrahlung and annihilation events can be time-resolved from the (n,n’) events, although all of these events belong to the group of delayed events. Also, substantially delayed annihilation events, which are caused by the <br />decays of stopped positive muons, were detected.<br />The second experiment shown in this dissertation is based on the 100% relative efficiency ultra-low-background HPGe spectrometer. With the addition of two plastic scintillators and a fast-slow coincidence circuit, the coincidence events between the plastic detectors and the HPGe spectrometer have been investigated. This MIREDO (Muon Induced Rare Event Dynamic Observatory) spectrometer system is primarily <br />developed for the study of cosmic muon induced processes in different materials. Exploration of such interactions can be important for ultra-low background experiments. Results derived for three samples, placed in a Marinelli beaker, are presented and discussed.<br />In third experiment shown in this dissertation, the potential influence of solar neutrinos on measured decay rate is investigated by the liquid scintillation measurement of the count rate variations of <sup>3</sup>H. Making use of the sophisticated Quantulus liquid scintillation spectrometer, it is found that the measurement of the high-energy tail of <sup>3</sup>H spectrum may be significantly influenced by instrumental instability. The oscillatory behavior of measured high-energy tail of <sup>3</sup>H spectrum is registered, but with very small amplitude (less than 0.5%), which cannot be easily <br />explained only by instrumental instability. When the total <sup>3</sup>H spectrum was measured, no significant variations in the count rate were found. <br />For a long time, it has been known that low-energy continuous gamma radiation is present in open air at the Earth’s surface. In previous investigations it was assumed that this radiation is produced almost exclusively by gamma photons emitted due to the natural radioactivity, which are backscattered by air above ground. In the fourth experiment presented in this dissertation, it is shown that significant amount of this <br />radiation (related to energy region 30 keV-300 keV) that peaks at about 90 keV, is produced by cosmic-rays, with the photon flux of about 3000 m<sup>-2</sup>s<sup>-1</sup>. Also, it is found that the contribution of this omnipresent low energy gamma radiation of cosmic-ray origin, including the corresponding low-energy electron flux, to the doses of general <br />population are non-negligible components of overall doses induced by cosmic rays near sea level.</p>
| Identifer | oai:union.ndltd.org:uns.ac.rs/oai:CRISUNS:(BISIS)94514 |
| Date | 13 June 2015 |
| Creators | Bikit Kristina |
| Contributors | Mrđa Dušan, Vesković Miroslav, Todorović Nataša, Puzović Jovan |
| Publisher | Univerzitet u Novom Sadu, Prirodno-matematički fakultet u Novom Sadu, University of Novi Sad, Faculty of Sciences at Novi Sad |
| Source Sets | University of Novi Sad |
| Language | Serbian |
| Detected Language | Unknown |
| Type | PhD thesis |
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