Suppression of the vacuum space-charge effect in fs-photoemission by a retarding electrostatic front lens

The performance of time-resolved photoemission experiments at fs-pulsed photon sources is ultimately limited by the e–e Coulomb interaction, downgrading energy and momentum resolution. Here, we present an approach to effectively suppress space-charge artifacts in momentum microscopes and photoemission microscopes. A retarding electrostatic field generated by a special objective lens repels slow electrons, retaining the k-image of the fast photoelectrons. The suppression of space-charge effects scales with the ratio of the photoelectron velocities of fast and slow electrons. Fields in the range from −20 to −1100 V/mm for Ekin = 100 eV to 4 keV direct secondaries and pump-induced slow electrons back to the sample surface. Ray tracing simulations reveal that this happens within the first 40 to 3 μm above the sample surface for Ekin = 100 eV to 4 keV. An optimized front-lens design allows switching between the conventional accelerating and the new retarding mode. Time-resolved experiments at Ekin = 107 eV using fs extreme ultraviolet probe pulses from the free-electron laser FLASH reveal that the width of the Fermi edge increases by just 30 meV at an incident pump fluence of 22 mJ/cm2 (retarding field −21 V/mm). For an accelerating field of +2 kV/mm and a pump fluence of only 5 mJ/cm2, it increases by 0.5 eV (pump wavelength 1030 nm). At the given conditions, the suppression mode permits increasing the slow-electron yield by three to four orders of magnitude. The feasibility of the method at high energies is demonstrated without a pump beam at Ekin = 3830 eV using hard x rays from the storage ring PETRA III. The approach opens up a previously inaccessible regime of pump fluences for photoemission experiments

РАДОН В ВОЗДУХЕ ЗДАНИЙ НАСЕЛЕННЫХ ПУНКТОВ НА ТЕРРИТОРИИ БРЕСТСКОЙ ОБЛАСТИ И ЭФФЕКТИВНЫЕ ДОЗЫ ОБЛУЧЕНИЯ НАСЕЛЕНИЯ, ОБУСЛОВЛЕННЫЕ РАДОНОМ-222 И ДОЧЕРНИМИ ПРОДУКТАМИ ЕГО РАСПАДА

The analysis of specific features of the geological structure within the Brest region territory has been performed, on the basis of which settlements for radon monitoring have been selected. The technique has been described and results of the volume activity of radon in air of living, public and industrial buildings, equivalent to equilibrium volume activity of radon (EROARn) and annual effective doses (ERn) population’s irradiation caused by radon and its decay products, have been shown. Average values in various administrative districts of Brest region are within the range from 15 to 60 Bq/m3 . The value of 200 Bq/m3 has been determined in one building of Kamenets district of Brest region. The maximum average of annual effective doses of population’s irradiation is from 6,8 to 13,9 m3v/year.Проведен анализ особенностей геологического строения территории Брестской области, на основе которого вы­браны населенные пункты для мониторинга радона. Описана методика и представлены результаты исследований объемной активности радона в воздухе жилых, общественных и производственных зданий, эквивалентной равновес­ной объемной активности радона (ЭРОАRn) и годовых эффективных доз (ERn) облучения населения, обусловленных радоном и его дочерними продуктами распада. Средние значения ЭРОАRn в различных административных районах Брестской области варьируются от 15 до 60 Бк/м3 . Значение ЭРОАRn более 200 Бк/м3 определено в одном помещении Каменецкого района Брестской области. Максимальные значения среднегодовых эффективных доз облучения насе­ления составляют от 6,8 до 13,9 мЗв/год.