Detekcija neutrona u atmosferi

• Main Author: Poje Sovilj, Marina
• Other Authors: Planinić, Josip, Vuković, Branko
• Format: Doctoral or Postdoctoral Thesis
• Language: Croatian
• Published: Sveučilište u Zagrebu. Prirodoslovno-matematički fakultet. Fizički odsjek. 2012
• Subjects: PRIRODNE ZNANOSTI. Fizika; NATURAL SCIENCES. Physics; Fizika; Physics; info:eu-repo/classification/udc/53(043.3); kozmičko zračenje; neutroni; brzina doze; detektori nuklearnih tragova; cosmic radiation; neutrons; dose rate; nuclear track etched detectors; North Pole; Dane
• Online Access: https://dr.nsk.hr/islandora/object/pmf:1762; https://urn.nsk.hr/urn:nbn:hr:217:283309; https://repozitorij.unizg.hr/islandora/object/pmf:1762; https://repozitorij.unizg.hr/islandora/object/pmf:1762/datastream/PDF

Nakon prvih mjerenja fotonske doze kozmičkog zračenja s poluvodičkim detektorima u zrakoplovima na visinama leta od 7 km i 10 km pristupa se pripremi pasivnih detektora nuklearnih tragova za detekciju neutrona u atmosferi. Korišteni su detektori nuklearnih tragova LR 115 i CR – 39 u kombinaciji s boronskom folijom kao neutronskim konverterom; u nuklearnoj reakciji, 10B (n, α) 7Li, nastaju alfa čestice koje proizvode trag na površini plastičnog materijala detektora. Posebno se istražuje osjetljivost neutronskih detektora te se izvodi njihovo baždarenje za različita područja energije neutrona. Baždarenje detektora na termalne neutrone izvedeno je u nuklearnom reaktoru Instituta Jozef Štefan, te posebno u CERF – u (CERN), gdje se koristi referentno neutronsko polje za baždarenje aktivnih i pasivnih dozimetara; neutronsko polje se napaja na sekundarnoj zraci (H6) sa Super Proton Sinhrotrona (SPS). Mjerenja neutronske doze u zrakoplovima izvode se već nekoliko godina na letovima koji povezuju Zagreb s najvećim gradovima Europi, kao i na interkontinentalnim letovima. Najudaljeniji letovi su bili, primjerice, Zagreb – Paris – Buenos Aires (s prosječnom brzinom neutronske doze od 2,46 μSv/h), Zagreb – Frankfurt – Tokyo (2,58 μSv/h s prosječnom brzinom neutronske doze od 2,46 μSv/h) i Zagreb – Paris – Mexico City (2,35 μSv/h s prosječnom brzinom neutronske doze od 2,46 μSv/h). Podatci o brzini doze pokazuju da su letovi udaljeniji od ekvatora imali veću dozu (Tokyo, Buenos Aires), dok je manja doza neutronskog zračenja dobivena za let bliže ekvatoru (Mexico City). Na letovima izmežu Europe i Dalekog Istoka, zrakoplovi koriste rute koje su blizu sjevernom polu, i gdje su izmjerene brzine neutronske doze bile oko 5 μSv/h. Dakle, zapažen je utjecaj geomagnetskog efekta na brzine neutronske doze u zrakoplovima; na ekvatoru je najveći otklon nabijenih čestica kozmičkog zračenja i najveće smanjenje doze, dok je taj otklon zanemariv na većim geografskim širinama. Naša mjerenja su pokazala takožer utjecaj solarne aktivnosti na zrakoplovne letove i značajno povećanje doze u dane jakih sunčevih bljeskova odnosno protonskih erupcija; koristeći eksperimentalne podatke o povećanju doze u zrakoplovu od 80 % odrežen je udio solarnog zračenja od 8,3 % u ukupnom kozmičkom zračenju za razdoblje normalne aktivnosti Sunca. Udio neutronske komponente u ukupnom kozmičkom zračenju na zrakoplovnim visinama iznosi oko 50%. Razvijena metoda mjerenja neutronskog zračenja s detektorom nuklearnih tragova primijenjena je na površini Zemlje u različitim dijelovima Hrvatske na 36 lokacija. Jednogodišnja mjerenja neutronske doze pokazala su ovisnost doze o nadmorskoj visini mjernog mjesta te je dobiven signifikantan pozitivni koeficijent korelacije (većoj nadmorskoj visini pripada veća brzina neutronske doze); na nadmorskoj visini od 89 m (mjerna lokacija Osijek) neutronska brzina doze iznosila je 109,7 nSv/h, dok je prosječna neutronska brzina doze na visinama do 200 m iznosila 100 nSv/h. Na istim lokacijama mjerena je i brzina doze fotonskog zračenja. Za obje izmjerene komponente zračenja, neutronskog i fotonskog, izražena je mapa zračenja kao prostorna distribucija kozmičkog zračenja na tlu RH. Detektori nuklearnih tragova posebno su baždareni za dozimetriju neželjenih neutrona u radioterapiji s linearnim akceleratorima (LINAC). Mjerenja neutronske doze izvedena su na linearnim akceleratorima od 15 MeV i 18 MeV u Kliničkom bolničkom centru u Osijeku. Usporedo s detektorima nuklearnih tragova izlagan je i aktivni detektor Thermo BIOREM FHT 752, u svrhu pouzdane procjene ambijentalne neutronske doze za pacijente tijekom radioterapije, kao i u svrhu radiološke zaštite medicinskog osoblja. LINAC od 18 MeV dao je veću neutronsku dozu: u radnoj prostoriji sa zaštitnom oplatom od betona i čelika, s pasivnim detektorom izmjerena je neutronska doza od 12 μSv/h, dok je aktivni detektor BIOREM pokazao dozu od 15,5 μSv/h. Nakon navedenih mjerenja doze neutronsko zračenje je atenuirano dodatnom zaštitom od hidrogeniziranog materijala (baritni beton). After the first experiences with measurements of photon dose radiation at the aviation altitudes of 7 km and 10 km respectively, we started to prepaire solid state nuclear track etched detectors for detecting neutrons in the atmosphere. We used the LR 115 and CR – 39 detectors in combination with boron foil as a neutron converter in the nuclear reaction 10B (n, α) 7Li. Sensitivity enhancing of the detectors is of special interest, as well as their calibration to neutrons of different energies. The calibration of the neutron detector to thermal neutrons was performed at the nuclear reactor facility at the Josef Stefan Institute. The detectors were especially calibrated in the CERF high energy reference field facility (CERN). This field is the neutron reference field for calibration of both passive and active detectors. The field is installed on one of the secondary beam lines (H6) from the Super Proton Synchrotron (SPS). Neutron dose measurements were performed on long commercial flights connecting the Zagreb airport with the capital cities in Europe or in the World for several years. For example the longest flights were: Zagreb – Paris – Buenos Aires (with the average measured neutron dose rate of 2,46 μSv/h), Zagreb – Frankfurt – Tokyo (with the average measured neutron dose rate of 2,58 μSv/h) and Zagreb – Paris – Mexico City (with the average measured neutron dose rate of 2,35 μSv/h). The measured neutron dose rate clearly showed that flights with routes further away from the equator (for example Tokyo and Buenos Aires) are exposed to higher dose rates, while flights closer to the equator are protected due to geomagnetic field wich results in smaller dose rates (Mexico City). On the flights between Europe and Far East, airplanes use the so – called polar routes which are very close to the North Pole, where the measured neutron dose rates are higher – around 5 μSv/h. Therefore, the geomagnetic effect on the measured dose rate is clearly shown: at the equator and lower latitudes, due to the nature of the Earth’s magnetic field, charged particles of cosmic radiation are deflected more than in the polar region and higher latitudes. Some of the measurements showed influence of solar activity on the dose rates. Due to high solar activity (coronal mass ejections, solar proton flares) the measured doses were higher. In one of such experiments, solar activity enhanced the dose rates in the aircraft by 80 %, and we succeeded to determine that the solar component accounts for 8,3% of the total cosmic radiation during quiet period of solar activity. At the aircraft altitudes the neutron component accounts for almost 50% of total cosmic radiation. Methods developed for the neutron dose measurements are applied for determinig the neutron component of cosmic radiation on the ground oft he Republic of Croatia in dependance on the altitude of the measuring station. In one year of measuring period, the detectors were set on 36 locations throughout Croatia. The measured average dose rates on the locations up to 200 m height above sea level, were 100 nSv/h; at the measuring station in Osijek, which is located at 89 m above sea level, the measured neutron dose rate is 109,7 nSv/h. We could derive a linear regression equation for the relationship between the dose rate and the height above sea level. Significant and positive correlation coefficient was obtained. On the same measuring locations photon dose rates were measured, as well. For both measured components of natural radiation, radiation maps were build. Neutron detectors are especially calibrated and used for unwanted neutron radiation during radiotherapy at the linear accelerator (LINAC). Measurements were performed on two linear accelerators in the Clinical hospital center in Osijek. Two linear accelerators have different beam energies: one has the energy of 15 MeV and the other of 18 MeV. Simultaneously with our passive detectors, we have used the active detector Thermo BIOREM FHT 752 with the goal to estimate the neutron dose as better as possible. A neutron dose assessment during radiotherapy on a high energy linear accelerator is important for patients but also for medical personnel in order to plan and perform all the actions for the radiation protection during their working career in a correct way. Of course, measurements on the 18 MeV accelerator gave higher values than the ones performed on the 15 MeV accelerator. In the working area around the accelerator, that is shielded with steel and concrete panels, the measured dose rates with passive detectors were 12 μSv/h, and 15,5 μSv/h with the BIOREM active detector. Based upon the results of our first measurements, a request for additional construction interventions (with the barite concrete) was approved in order to improve the protection of patients and medical personnel from the unwanted and unplanned neutron dose.