Changes of nuclear spectra under the influence of cosmic radiation

Abstract

<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 />³H&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;³H spektra može značajno uticati nestabilnost<br />instrumenta. Oscilatorni karakter izmerenog visokoenergetskog dela<br />³H&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;³H 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^-2s^-1. 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;³H. Making use of the sophisticated Quantulus liquid scintillation spectrometer, it is&nbsp;found that the&nbsp; measurement of the high-energy tail of&nbsp;³H spectrum may be significantly influenced by instrumental&nbsp; instability.&nbsp; The oscillatory behavior of measured high-energy tail of ³H 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;³H 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^-2s^-1.&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>