O.G. de Lucio

A study of atmospheric aerosols from five sites in Mexico city using PIXE

Abstract Atmospheric aerosol samples collected in five sites in the Metropolitan Area of Mexico City were analyzed with proton induced X-ray emission (PIXE). Stacked Filter Units (SFU) of the Davis design were employed to obtain samples of fine aerosols (smaller than 2.5 μm Mean Aerodynamic Diameter) on polycarbonate filters. The study was conducted during the first half of 1996 (16 weeks, from 31st January to 6th June), exposing the filters once a week, from 8:00 to 14:00 h. PIXE analyses of the filters using a 2 MeV proton beam produced by the Instituto de Fisica, UNAM, 5.5 MV Van de Graaff accelerator were compared with those carried out using a 2.2 MeV proton beam from the 56 cm isochronous cyclotron at the Universidad de Chile, showing a good correlation between both laboratories. Analysis of variance of the measured elemental concentrations showed differences in the mean concentrations for several elements at the sampling sites. Comparisons with other studies are also given.

Ion beam analysis of ancient Mexican colored teeth from archaeological sites in Mexico City

Abstract Infant teeth with extremely rare colored enamel regions white, blue-gray and brown, have been analyzed by Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS). The teeth were part of human being sacrifices to deities of the Mexica culture, Mexico, corresponding to the Late Post-classic period (1325–1521 A.D. ). Comparisons to normal teeth from the same historical period indicate that the colored ones present larger mean amounts of Mn and Fe, while Zn and Sr do not differ too much. Possible inorganic compounds responsible for the different colorations are indicated.

Positron and electron impact double ionization of argon: How 1st- and 2nd-order mechanisms influence the differential electron emission

Double-to-single–ionization ratios for electron emission as a function of the angle are measured for 200, 500, and 1000 eV positron and electron impact on argon. Both the sign of the projectile charge and the impact energy are shown to influence the angular dependences. By combining these ratios for positron and electron impact, information about how first- and second-order double-ionization mechanisms interfere and contribute to the total differential electron emission at different collision velocities is obtained.

Total L-shell X-ray production cross sections by 400–700 keV proton impact for elements with 34 ≤ Z ≤ 53

Total L-shell X-ray production cross sections induced by protons with energies between 400 and 700 keV were measured for elements with atomic number Z between 34 and 53. The ECPSSR theory describes appropriately the results. This model modifies the plane wave born approximation by considering projectile energy loss (E), Coulomb deflection of the incoming ion (C), polarization and change in electron binding energies through a perturbed stationary states method (PSS) and relativistic values of target electron mass (R). A comparison is given with previously published data for proton energies below 1 MeV and 26 < or = Z < or = 53, based on a scaling obtained from a reduced velocity parameter zeta(L)R. The results show that the scaling for these atomic numbers and energy ranges is adequate and a semi-empirical expression to calculate those cross sections is proposed.

Differential ionization studies for positron impact

Recent studies of differential ionization by positron impact being performed at the University of Missouri-Rolla are described. Recoil ion-scattered positron and recoil ion-scattered positron-ejected electron coincidences are measured to provide doubly and triply differential ionization data (DDCS and TDCS) for e+ - Ar collisions. The DDCS and TDCS studies are for 750 and 200 eV impact respectively. For the TDCS studies the positron scattering angles are between approximately ± 5o and electrons ejected with energies less than 15 eV and angles between 90o ± 50o with respect to the beam direction are detected. Examples of TDCS data for the scattered positron and the ejected electron are presented.

Projectile Charge Effects in Differential Ionization by Positrons and Electrons

Differential data for single and double ionization of argon by positron and electron impact are presented and compared. For single ionization, coincidences between scattered projectiles, ejected electrons and recoil ions are measured as functions of the projectile scattering angle and energy loss. Differences associated with the sign of the projectile charge are indicated with regard to the scattering angle, the energy loss, and the relative intensities for binary and recoil events. Ejected electron‐recoil ion coincidences are also measured as a function of the observation angle along the beam direction. From these, double to single ionization ratios are determined and compared. Differences in the magnitudes and angular dependences for positron and electron impact are attributed to the interference between the TS‐1 and TS‐2 double ionization mechanisms.

Differential Measurements of Ionization and Excitation of H2 by 250 eV Positrons

Ejected electron-recoil ion and scattered projectile-recoil ion coincidences are measured for 250 eV positron impact on molecular hydrogen. The electron detector is also sensitive to photon emission. Various combinations of these coincidence spectra are used to extract differential information about ionization and excitation of H2.

Tagged fast neutron beams En > 6 MeV

Controlled flux of neutrons are produced through the 14N(d,n)15O nuclear reaction. Deuteron beams (2-4 MeV) are delivered by the CN-Van de Graaff accelerator and directed with full intensity to our Nitrogen target at SUGAR (SUpersonic GAs jet taRget). Each neutron is electronically tagged by the detection of the associated15O. Its energy and direction are known and “beams” of fast monochromatic tagged neutrons (En> 6 MeV) are available for basic research and applied work. MONDE is a large area (158 × 63 cm2) plastic scintillating slab (5 cm thick), viewed by 16 PMTs from the sides. Fast neutrons (MeV) entering the detector will produce a recoiling proton that induces a light spark at the spot. Signals from the 16 detectors are processed to deduce the position of the spark. Time logic signals from both the 15O detector and MONDE are combined to deduce a time of flight (TOF) signal. Finally, the position information together with the TOF yields the full momentum vector of each detected neutron.

Differential Cross Sections for Ionization of Argon by 1 keV Positron and Electron Impact

Differential information was generated by establishing coincidences and imposing conditions on data recorded for target ions, scattered projectiles, and ejected electrons, as a function of projectile energy loss and scattering angles; in order to describe the interaction between a positron (electron) 1 keV beam and a simple Ar jet. Single ionization triply differential cross section (TDCS) results exhibit two distinct regions (lobes) for which binary (events arising from 2-body interaction) and recoil (events which can only be produced by many-body interactions) interactions are associated. Results indicate that binary events are significantly larger for positron impact, in accordance with theoretical predictions. A similar feature is found for different energy losses and scattering angles. Intensity of the recoil lobe for both projectiles, positron and electron, is observed to depend on the energy loss and scattering angle. Also, it can be noticed that for positron impact the recoil interactions intensity is larger than that observed for electron impact.

Nuclear physics experiments with a windowless supersonic gas jet target

A new windowless gas target has been developed in Mexico. It is a supersonic gas jet flow produced inside a vacuum chamber which can be coupled to a regular beam line in an accelerator laboratory as a differential pumping system brings the pressure of the gas target system down to a microTorr, or better, at the connecting stage. In this work, we present the system as it was designed and constructed as well as the first results using air, Nitrogen and Argon.