Absolute cross sections for electron scattering from organic molecules relevant to the structure of biological macromolecules

U ovoj disertaciji proučavana je interakcija elektrona srednjih energija, 40 eV – 300 eV, sa molekulima analognim delovima DNK (furan, 3-hidroksitetrahidrofuran, pirimidin) i proteina (formamid i N-metilformamid). Opisana je eksperimentalna procedura za merenje relativnih i apsolutnih efikasnih preseka (metod relativnih protoka) za elastično rasejanje elektrona na biomolekulima, kao i merenje raspodele kinetičke energije pozitivnih jona. Apsolutni diferencijalni preseci (differential cross sections – DCSs) za elastično rasejanje elektrona na molekulima su predstavljeni tabelarno i grafički, u funkciji ugla rasejanja i upadne energije elektrona. Takođe su opisani relevantni teorijski proračuni (IAM, SCAR, SCARN, SCARND) sa kojima su eksperimentalni rezultati bili upoređeni. Dobijeno je veoma dobro slaganje eksperimentalnih i teorijskih efikasnih preseka, a mala neslaganja su viđena samo na malim uglovima i nižim upadnim energijama elektrona. Takođe, u radu je korišćena prva Bornova aproksimacija za procenu doprinosa vibracionih neelastičnih preseka za rasejanje elektrona na molekulu furana, pošto eksperimentalna merenja (s obzirom na energijsku rezoluciju) ne mogu odrediti ovaj doprinos, niti SCARND teorija uračunava ove neelastične procese. Ova aproksimacija je potvrdila da je moguće odstupanje od apsolutnih eksperimentalnih elastičnih preseka u okviru apsolutne greške merenja. Generalno, apsolutni diferencijalni preseci za sve molekule su slični po obliku i na apsolutnoj skali. Uporedili smo rezultate za furan, 3HTHF i THF na 100 eV. Ovi rezultati su vrlo bliski na apsolutnoj skali, pokazujući sličnu redistribuciju rasejanih elektrona na sve 3 mete. Takođe su upoređeni i rezultati za formamid i NMF. Dodatno, ugaona zavisnost razlike apsolutnih preseka za ova dva molekula je upoređena sa objavljenim presecima za metan (CH4), pokazujući jako dobro slaganje i potvrđujući primenljivost principa gradivnih blokova za procenu preseka za elastično rasejanje elektrona u ovom domenu energija. Dobijanje diferencijalnih efikasnih preseka za rasejanje elektrona na biomolekulima je važno za testiranje teorijskih modela koji se primenjuju za proračun sudarnih procesa. Takođe, pouzdano izmereni diferencijalni preseci predstavljaju važne ulazne parametre za Monte Carlo simulacije depozicije energije u živoj materiji usled dejstva jonizujućeg zračenja. Treba istaći da je pouzdan proračun radijacionog oštećenja proizvedenog visokoenergijskim česticama bitan deo istraživanja povezanih sa kancer terapijom.The experimental investigation of electron interaction with biomolecules that are analogues to building blocks of DNA (furan, 3-hydroxytetrahydrofuran and pyrimidine) and proteins (formamide, N-methylformamide), at medium incident electron energies 40 eV – 300 eV, is presented in this thesis. An experimental procedure for the measurement of both relative and absolute cross sections (by using relative flow method) for elastic electron scattering from biomolecules, as well as the measurement of kinetic energy distribution of positive ions, has been described. Absolute differential cross sections (DCSs) for elastic electron scattering from all investigated molecular targets are tabulated and graphically presented as a function of both the scattering angle and the incident electron energy. Also, relevant theoretical models (IAM, SCAR, SCARN, SCARND) have been described and the obtained calculated results compared with our experimental results. The theoretical and the experimental cross sections are in a very good agreement, with some deviations at small scattering angles and low incident electron energies. Furthermore, the first Born approximation has been used in the present work in order to estimate a contribution of vibrationally inelastic DCSs for furan molecule, since neither experimental measurement (due to the energy resolution) or SCARND calculations can resolve this contribution. The present calculations confirm that a possible deviation of the experimental absolute elastic DCSs should be within reported absolute errors. Generaly, absolute differential cross sections for all investigated molecules are similar both in the shape and on the absolute scale. We have compared results for furan, 3HTHF, and THF at 100 eV, which are very close on the absolute scale, thus showing a similar redistribution of scattered electrons for all three targets. The results for formamide and NMF have been also compared. Additionally, the angular dependence of a distinction between the cross sections for these two molecules is compare with previously published DCSs for CH4, showing a very good agreement and thus confirming a valiablity of building blocks approach in determination of DCSs for elastic scattering in this incident energy range. The present results are of interest for investigation of radiation damage in living tissue upon exposure to high energy radiation and this experimental study is important for fundamental understanding of electron interaction with biomolecules in the medium incident electron energies. The obtained cross section data set is of interest for estimation and modeling of products formation induced by electron interaction within a biological sample. Electron scattering cross sections are input parameters for energy deposition modeling in Monte Carlo simulations. Insight and accurate estimation of radiation damage produced by high energy particles represents an important part of research connected with cancer therapy