| Abstract | Direktno opažanje interakcije čestica tamne tvari sa zemaljskim detektorom još uvijek je neriješen eksperimentalni problem. Iz teorijskih razmatranja očekuje se godišnja, sinusoidalna modulacija signala čestica tamne tvari jer se sustav Sunce - Zemlja giba prema hipotetiziranom halou tamne tvari naše galaksije. Zemlja ima dodatno, relativno gibanje u tom sustavu, pri kojem ljeti ide prema halou, a zimi od haloa. Modulacija signala moguća je i na dnevnoj razini, s obzirom da je planet Zemlja u jednom dijelu dana između detektora i izvora, a to pak ovisi o tom koliki je udarni presjek čestica tamne tvari i prolaze li kroz Zemlju neometano ili se ipak dio toka deponira tj. smanji. Na Odjelu za fiziku Sveučilišta u Rijeci, razvijen je optomehanički senzor koji je modificirani Michelsonov interferometar, u kojem je jedno od zrcala zamijenjeno silicij nitridnom membranom (Norcada, Canada), presvučenom tankim slojem platine (13nm), osjetljivom na vrlo sitne podražaje (jako malu silu ili tok ˇcestica). Promjene u ponašanju membrane se očituju u promjenama pruga interferencije, koje se mogu detektirati tehnikom homodinom.
U ovom radu kalibriran je detektor i karakteriziran njegov odziv na promjene temperature, tlaka i nezavisnog izvora zračenja. Nadalje, prikupljani su podaci u razdoblju od deset mjeseci kako bi se ispitalo postoje li dnevne i/ili godišnje modulacije signala te postoji li mogućnost uporabe senzora za pojedinačnu detekciju čestica. Podaci su analizirani u posebno razvijenom programu razvijenom u jeziku LabVIEW. Naši rezultati upućuju na veliku osjetljivost silicij nitridne membrane koja pokazuje
odziv na temperaturu (povećanje od 1 ◦C smanjuje rezonantnu frekvenciju za oko 10 Hz) i tlak. Ukupni, desetomjesečni set podataka pokazuje povećanje rezonantne frekvencije od oko 150 Hz koje se ne dovodi u vezu s utjecajem temperature i tlaka. Iako je ovaj rezultat daleko od konačne potvrde početne hipoteze i neosporne detekcije čestica tamne tvari, činjenica je da se membranom detektira sila na membranu, odnosno zračenje nepoznatog porijekla koje ima periodičan karakter. Ta periodičnost blago korelira s hipotezom o godišnjoj i dnevnoj modulaciji signala toka ˇcestica tamne tvari iz galaktičkog haloa.
Potrebno je izvršiti dodatni set mjerenja za ispitivanje sinusoidalnosti perioda frekvencije. Detektiran je i vrlo intrigantan događaj, sa drastičnim povećanjem amplitude membrane, a za koji su odbačene mogućnosti da je posljedica atmosferskog, seizmološkog, ljudskog i drugih faktora. Pretpostavka je da se radi o pojedinačnoj detekciji čestice, koja bi mogla
biti i čestica tamne tvari. Za daljnje utvrđivanje valjanosti rezultata potrebna su dodatna i duža mjerenja, modifikacije eksperimentalnog postava i automatiziranje analize korištenjem programa Python i superračunala BURA. |
| Abstract (english) | Direct observation of the interaction of dark matter particles with a terrestrial detector is still an ongoing experimental problem. From theoretical considerations the signal of dark matter particles is expected to have an annual, sinusoidal modulation because the Sun - Earth system moves towards the hypothetical halo of the dark matter of our galaxy. Planet
Earth has an additional, relative movement in that system, in which it goes to the halo in the summer and from the halo in the winter. Signal modulation is also possible on a daily basis, given that, in one part of the day, planet Earth is positioned between the detector and source, and this in turn depends on the impact cross section of the dark matter particles
and whether they pass through the Earth undisturbed or a part of the flow is deposited, i.e., reduced. At the Department of Physics of the University of Rijeka, an optomechanical sensor was developed which is a modified Michelson interferometer, in which one of the mirrors is replaced by silicon nitride membrane (Norcada, Canada), coated with a thin layer of platinum (13 nm), sensitive to very small stimuli (very little force or particle flow). Changes in membrane behavior are manifested as changes in interference fringes, which can be detected by the homodyne technique. In this thesis, the optomechanical sensor is calibrated and its response to temperature and pressure change and an independent radiation source is characterized. Furthermore, data were collected in a period of ten months to examine whether daily and/or annual signal modulations exist and if it is possible to use this sensor for individual particle detection. Data were analyzed in a
specially developed program developed in the LabVIEW language. Our results indicate the high sensitivity of the silicon nitride membrane showing temperature (an increase of 1 ◦C reduces the resonant frequency by about 10 Hz) and pressure response. The total, tenmonth data, set shows an increase in the resonant frequency of about 150 Hz which is not
related to the influence of temperature and pressure. Although this result is far from the final confirmation of the initial hypothesis and the indisputable detection of dark particles, the fact is that the membrane detects the force on the membrane, i.e., the radiation of the unknown source which has a periodic character. This periodicity correlates slightly
with the hypothesis of the annual and daily modulation of the dark matter particle flow signal from the galactic halo. An additional set of measurements must be performed to examine the sinusoidal characteristics of the frequency period. Furthermore, a drastic increase in membrane amplitude was detected and the possibility that it is a consequence
of atmospheric, seismological, human and other factors was eliminated. The assumption is that it is perhaps an individual particle detection, which could be and a particle of dark matter. Additional measurements are required to further validate the results, as well as modifications of the experimental setup and automation of the data analysis using Python ii
and the BURA supercomputer. |
