| Abstract | Dozimetrija primjenom spektroskopije elektronske spinske rezonancije (ESR) utemeljena je na detekciji radikala nastalih pri izlaganju ionizirajućem zračenju. Metoda ima široku primjenu u kemiji, fizici, biologiji i mnogim interdisciplinarnim područjima, a važnost joj raste i u području retrospektivne dozimetrije. U retrospektivnoj dozimetriji važna je brza rekonstrukcija nepoznate apsorbirane doze kod žrtve slučajne radiološke ili nuklearne nesreće. Pomoću spektroskopije ESR apsorbirana doza se određuje direktnim mjerenjem radikala nastalih u materijalu pronađenom u blizini žrtve ili na samoj žrtvi zračenja. Lijevana kalcijeva natrijeva silikatna stakla masovno se primjenjuju u građevinskoj i automobilskoj industriji, te su stoga potencijalno dobar retrospektivni dozimetar. U ovom istraživanju lijevana kalcijeva natrijeva silikatna stakla su upotrijebljena kao detektori malih doza zračenja metodom spektroskopije ESR. Prema dosadašnjim istraživanjima lijevano natrijevo kalcijevo silikatno staklo ima dobar odziv u širokom rasponu doza, s donjom granicom detekcije 2 Gy. U ovom radu, ispitan je ESR odziv uzoraka lijevanoga natrijeva kalcijeva silikatnog stakla ozračenih dozama u rasponu od 0,2–20,0 Gy. Mjerenja su pokazala da se ESR spektar ozračenih uzoraka sastoji od dvije vrste signala i to signala nastalih kao posljedica zračenja (engl. radiation-induced signal, RIS) i signala koji ne nastaju zračenjem, odnosno pozadinskih signala (engl. background signal, BKS). U rasponu niskih doza zračenja do 2 Gy, udio RIS ESR signala manji je od udjela BKS ESR signala, te se stoga RIS ESR signal preklapa s BKS ESR signalom. Ovo otežava određivanje apsorbirane doze standardnim metodama rekonstrukcije temeljenih na doznoj ovisnosti vrh-vrh amplitude IPP (engl. peak-to-peak amplitude) RIS signala. U ovom radu ispitana je promjena položaja g-vrijednosti ESR spektralne linije ozračenih uzoraka u odnosu na g-vrijednost BKS spektralne linije koja je mjerljiva i u rasponu doza nižim od 2 Gy. Nadalje, pokazano je da IPP amplitude RIS i BKS signala imaju različitu temperaturnu ovisnost te da je rast signala obrnuto proporcionalan s temperaturom. Razlika u temperaturnoj ovisnosti ova dva signala omogućuje preciznije očitanje g-vrijednosti pri nižim temperaturama. Rezultati ovih istraživanja pokazali su se upotrebljivim za odvajanje BKS od RIS signala u rasponu malih doza čime doprinose poboljšanju u detekciji malih doza zračenja nakon neplaniranog izlaganja ionizirajućem zračenju te u slučajevima nepoznatog broja izloženih osoba. U tu svrhu metoda je testirana na uzorku ozračenom nepoznatom dozom te je pokazana mogućnost očitanja vrijednosti doze zračenja koja je znatno niža od do sada mogućeg očitanja u uzorcima lijevanoga natrijeva kalcijeva silikatnog stakla. |
| Abstract (english) | The electron spin resonance (ESR) method is widely used in the field of chemistry, physics, biology and many interdisciplinary fields, and the application is growing also in the field of retrospective dosimetry. ESR detection of dosimetric signals originating from radicals induced under the influence of ionizing radiation allows to determine the dose absorbed by the casualties following catastrophic, large-scale radiological events. The ESR technique have been applied to a wide range of radiological studies, including nuclear bomb detonation (e.g., Hiroshima and Nagasaki), nuclear power plant accidents (e.g., Chernobyl), radioactive pollution (e.g., Mayak plutonium facility), and in the future could include terrorist events involving the dispersal of radioactive materials. For retrospective dosimetry, immediate action to evaluate dose to individuals following radiation exposure is required to assess deterministic biological effects and to enable rapid medical triage. The triage dose levels are differently defined in the literature and for purpose of this study triage levels defined in MULTIBIODOSE project have been used. Low triage dose level is less than 1 Gy when it is unlikely to develop symptoms of acute radiation syndrome (ARS) and no immediate care is required. The medium level implies doses 1-2 Gy when victim may experience mild or delayed ARS symptoms and follow-up care may be necessary and high for the doses higher than 2 Gy when moderate to urgent care may be required. The retrospective dosimetry measurement techniques aim to perform direct dose evaluations for individuals who, as members of the public, are most unlikely to be carrying a dosimeter for radiation monitoring purposes in the event of a radiation incident. Hence attention has focused on biological or physical materials they may have in their possession that could be used as surrogate dosimeters. For ESR measurements this includes material within the body (such as bone or tooth enamel) requiring invasive procedures, a materials collected non-invasively (such as clippings taken from finger- or toenails) and artefacts within their personal belongings (such as clothing, fabrics, plastics, glass, sugar, touch screens of mobile phones). The use of electron spin resonance (ESR) on detection of ionizing radiation-induced radicals in different types of glasses is a well-established technique in radiation dosimetry. Nowadays, the float glass is the most widely used form of glass and makes more up to other types 96% of output. Float soda lime silicate glass is used principally in building and automotive industries. It is used in various application such as in car glass, windows, doors, sun roofs, copy v machines, computers, etc. This is very useful for retrospective dosimetry since it is easily found at places of the nuclear and radiological mass-casualty incidents and possible terrorist events involving the dispersal of radioactive materials. As a retrospective dosemeter, the float soda-lime glass has proven to possess many required qualities due to the specific radiation-induced ESR signal with a linear dose response (generally ascribed to an oxygen hole center) and the detection limit (DL) of 2 Gy. Additionally, ESR signal decay is about 10 up to 35% at room temperature during the first 24–48 h after irradiation, after which the signal remains stable. A major effort in recent years has been devoted to improving the lower detection limit for retrospective dosimetry purposes because the useful low dose radiation induced signal (RIS) is superimposed over the background signal, BKS. Regarding triage dose levels several methods for correct determination of dosimetric signal magnitude in retrospective ESR dosimetry have been proposed in the literature. Deconvolution method is based on mathematical simulation of BKS and RIS that approximates measured spectra. The approach based on the best fit of the experimental spectrum with a set of Gaussian lines, spectrum-simulated lines, and experimental reference spectra is used in intercomparison organized among participants of European Radiation Dosimetry Group (EURADOS). For the retrospective dosimetry purposes, the widely used method is ESR spectrum subtraction approach based on background ESR signal subtraction from ESR signal of irradiated samples. Also, there are other techniques like selective saturation method and second derivative method. The subject of investigation presented in this disertation is ESR dosimetry of transparent float soda-lime glass in the low dose range, 0.2-20.0 Gy, for retrospective dosimetry purposes. Investigation of the ESR spectrum of the float soda-lime glass in the low dose range shown that it consists of two main components, background signal, BKS, and ionising radiation induced signal, RIS. In the low dose range ESR spectrum of the RIS in float soda-lime glass overlaps with the ESR spectrum of the BKS. Therefore, in the low dose range, the dose reconstruction by the established methods based on the dose dependence of peak to peak amplitudes, IPP, of RIS and BKS is very difficult and ambiguous. For the purpose of improvement of dose reconstruction in the range 0-10 Gy, we have investigated the ESR parameters of two components (RIS and BKS) of soda-lime glass ESR spectrum irradiated in the Co-60 beam. After establishing the exact g-values of the BKS and ESR signal of irradiated samples at the position of maximum amplitude, we have investigated the change of g-effective, geff, combination of the two reference signals g-values, value of ESR signal of irradiated samples and observed regular change of geff value of spectral lines after vi increasing the gamma irradiation dose. This change is measurable even in the dose range below 2 Gy, which is published detection limit of the float soda-lime samples. The experiment shown that the geff value does not depends on temperature. Furthermore, it is shown that the IPP amplitude of RIS signal exhibits different temperature dependence than IPP amplitude of BKS signal and that both IPP amplitudes increase linearly with the temperature decrease. At the low dose regime combination of the observed dose dependence of the geff value and temperature dependence of the IPP amplitudes of irradiated and non-irradiated float soda-lime samples, indicate that performing ESR measurements at the lower temperature can increase sensitivity of dose reading. From obtained results according to the proposed model, it can be concluded that the geff value dose dependence is useful tool in dose reconstruction and decomposing of RIS and BKS signal. This enables contribution to the improvement of the detection limit in the low dose range, 0-10 Gy. The method is tested on a sample irradiated with unknown, blind, dose and the application of the geff value analysis proved the improvement of the detection limit of the float soda-lime samples. |
