Title Measurement of the 58Ni neutron capture cross section Title (croatian) Mjerenje udarnoga presjeka za uhvat neutrona na izotopu 58Ni Author Petar Žugec Mentor Damir Bosnar (mentor)Committee member Tamara Nikšić (predsjednik povjerenstva)Committee member Damir Bosnar (član povjerenstva)Committee member Neven Soić (član povjerenstva)Committee member Nicola Colonna (član povjerenstva) Committee member Matko Milin (član povjerenstva)Granter University of Zagreb Defense date and country 2014-12-19, Croatia Scientific / art field, NATURAL SCIENCES Universal decimal classificationUDC ) 53 - Physics Abstract The (n, γ) cross section of 58Ni – of importance for the nuclear astrophysics, as well as for nuclear technologies – was measured at the neutron time-of-flight facility n_TOF at CERN. For the measurement, two C6D6 liquid scintillation detectors were used, specifically optimized to provide as low neutron sensitivity as possible. At n_TOF, the highly luminous white neutron beam covers 12 orders of magnitude in energy – from 10 meV to 10 GeV. It is produced by a pulsed beam of 20 GeV protons impinging on a massive Pb spallation target, yielding a total of 2 x 10^15 neutrons per pulse. Through an evacuated beamline the neutrons are transported towards the experimental area at a distance of approximately 185 m from a spallation target. In the process the beam is collimated by the set of two collimators, while charged particles are removed by a sweeping magnet. The neutron flux is actively monitored during the measurement by means of a silicon based neutron beam monitor SiMon, relying on 6Li(n, t)α reaction. The measurement is performed by detecting the prompt capture g-rays from 58Ni(n, γ) reaction. The kinetic energy of captured neutron is calculated from its time-of-flight, which is determined from a time-stamp of a detected γ-ray, relative to the start-signal caused by an intense γ-flash following every proton pulse hitting the spallation target. In order to calculate a capture yield from a measured data, the well-established Pulse HeightWeighting Technique was applied. A capture yield was analyzed in the energy range between 27 meV and 400 keV. The resolved resonance region was analyzed up to 122 keV by means of a multilevel R-matrix code SAMMY. Within the resolved resonance region 51 capture resonances were identified and their parameters reported. By complementing these results with the data from the unresolved resonance region – analyzed by the specialized code SESH – the Maxwellian averaged cross sections (MACS) were calculated for the stellar temperatures of kT = 5–100 keV. The new results call for the revaluation of the cross section data presently available throughout the various evaluation libraries, while revealing a significant impact on the final 58Ni abundance in the massive stars. In calculating the capture yield a significant effort has been paid to the clear identification of the separate background components. While the environmental and the so called empty-frame background are easily and regularly measured, the neutron background – caused by the neutrons elastically scattered off the sample itself – has been identified for the first time at n_TOF by means of the high-precision GEANT4 simulations. The simulated results were extensively compared against the available experimental data, confirming that they may be used with high degree of confidence for reaching the new precision standards in analyzing the capture data from n_TOF.
Abstract (croatian) Udarni presjek za uhvat neutrona na izotopu 58Ni – od važnosti za nuklearnu astrofiziku i tehnologije vezane uz nuklearnu fiziku – mjeren je na n_TOF postrojenju neutronskog vremena proleta u CERN-u. Kao mjerni instrumenti korištena su dva C6D6 tekuća scintilacijska detektora, posebno optimizirana kako bi osigurali najnižu izvedivu neutronsku osjetljivost. Intenzivan snop neutrona s n_TOF postrojenja proteže se 12 redova veličine u energiji – od 10 meV do 10 GeV. Snop se proizvodi izlaganjem masivne olovne spalacijske mete pulsnom protonskom snopu energije 20 GeV, pri čemu se oslobađa približno 2 x 10^15 neutrona po pulsu. Evakuirani cjevovod vodi snop prema eksperimentalnoj prostoriji udaljenoj približno 185 m od spalacijske mete. U međuvremenu snop se kolimira dvama kolimatorima i oslobađa nabijenih čestica snažnim magnetom. Tok neutrona aktivno se mjeri tijekom eksperimenta tzv. neutronskim monitorom snopa baziranim na siliciju, koji se oslanja na 6Li(n, t)α reakciju. Mjerenja neutronskih uhvata provode se detekcijom trenutno emitiranih g-zraka iz 58Ni(n, γ) reakcije. Kinetička energija uhvaćenog neutrona računa se iz njegova vremena proleta, koje se određuje iz vremena detekcije γ-zrake s obzirom na početni signal izazvan snažnim γ-bljeskom uzrokovanim udarom protonskoga snopa o spalacijsku metu. U svrhu izračuna vjerojatnosti uhvata iz eksperimentalnih podataka, pojedinim događajima pridruženi su odgovarajući težinski faktori na temelju visine mjerenih signala. Vjerojatnost uhvata analizirana je u energijskom rasponu od 27 meV do 400 keV. Područje raspoznatljivih rezonanci analizirano je do 122 keV primjenom programskoga koda SAMMY koji koristi formalizam R-matrice za obradu podataka. Unutar ovog područja izdvojena je i parametrizirana 51 rezonanca u udarnome presjeku za uhvat neutrona. Nadopunivši ove rezultate podacima iz područja neraspoznatljivih rezonanci – analiziranog posebnim programskim kodom SESH – izračunati su maxwellovski usrednjeni udarni presjeci za zvjezdane temperature od kT = 5–100 keV. Najnoviji rezultati ukazuju na potrebu za ponovljenim vrednovanjem udarnih presjeka dostupnih u različitim bazama podataka. Rezultati također pokazuju značajan utjecaj na zastupljenost konačnog sadržaja 58Ni u masivnim zvijezdama. Pri izračunu vjerojatnosti uhvata značajna pažnja posvećena je jasnoj identifikaciji izdvojenih doprinosa pozadinskim događajima. Pozadina uzrokovana prirodnim i induciranim zračenjem iz eksperimentalnog okruženja te pozadina uzrokovana prolaskom neutronskog snopa kroz eksperimentalnu prostoriju lako se i redovito mjere. No tzv. neutronska pozadina uzrokovana raspršenjem neutrona na samome mjerenom uzorku po prvi put u povijesti n_TOF postrojenja jasno je identificirana GEANT4 simulacijama visoke preciznosti. Simulirani rezultati uspoređeni su s dostupnim eksperimentalnim podacima, čime je potvrđeno da se s pouzdanošću mogu koristiti za postizanje nove razine preciznosti u analizi mjerenja neutronskih uhvata s n_TOF postrojenja.
Keywords
n_TOF
neutron time of flight
58Ni neutron capture cross section
GEANT4 simulations
neutron background
Keywords (croatian)
n_TOF
vrijeme proleta neutrona
udarni presjek za uhvat neutrona na 58Ni
GEANT4 simulacije
neutronska pozadina
Language english URN:NBN urn:nbn:hr:217:359507 Study programme Title: Physics Type of resource Text Extent 147 str. File origin Born digital Access conditions Open access Terms of use Created on 2017-08-22 09:53:53
