L. F. S. Coelho

Electron-loss cross sections for heavy-ion beam probe using gold beams: experiment and theory

We present cross section experimental data required for modeling beam production, attenuation and energy loss in the heavy-ion beam probe technique, namely, electron-loss cross sections for anionic gold projectiles and Ar and N2 targets in the energy range from 30 keV to 1 MeV. The results agree well with low-energy measured values from the literature for the Ar target. The present results are also compared with literature results for the H− projectile and the Ar target, displaying similar velocity dependences. This similarity led to the use of a simple semi-classical model to calculate the projectile-electron-loss cross sections in a wide energy range. This model employs the total scattering cross sections for free electrons and the momentum distribution of the Au− electrons. The present experimental results are in good agreement with this model, allowing a reliable estimate of the cross section values up to 100 MeV. Our results indicate that previously calculated cross sections are overestimated by at least a factor of two. This previous overestimation of electron-loss cross sections leads to significant errors in the modeling of charge-state fractions of gold beams, with consequences in the optimization of beam energy resolution and intensity for HIBP with megaelectronvolts gold beams.

Electron-detachment cross sections of halogen negative-ion projectiles for inertial confinement fusion

Negative-ion beams have recently been suggested as sources of high-energy heavy atoms to be used as drivers for inertial confinement fusion (ICF). Owing to their electron affinities limited to a few eV, anions can be efficiently photo-detached in the vicinity of the fusion chamber, with the resulting high-velocity neutral projectiles following ballistic trajectories towards the hydrogen pellet target. Electron-detachment cross sections are needed as parameters to estimate the beam attenuation in the path from the ion source to the hydrogen pellet. Halogen anions are possible projectile choices. In this paper we present experimental data for total electron-detachment cross sections for F-, Cl-, Br - and I- ions incident on N 2 , in the 0.94-74 ke V u -1 energy range. Our measurements can benchmark theory on anion electron detachment at intermediate to high velocities. Comparison between different projectiles shows very similar collision velocity dependencies. A simple geometrical scaling is presented, providing an estimate for electron-detachment cross sections at the Me V u -1 energy range. The presented scaling indicates that the vacuum requirements due to the use of halogen anions for ICF are less critical than previously suggested.

A simple method for fast negative atomic and molecular ion studies in tandem accelerators

Abstract A simple method is proposed for measuring cross-sections of fast negative atomic and molecular ions colliding with atoms and molecules. The method, applicable to tandem accelerators, uses the stripping process of the negative ions, occurring at the gas target placed in the accelerator high-voltage terminal. The target (stripper) gas pressure, not directly measurable, is measured with an improved version of a method that we have recently proposed. The total destruction cross-sections of negative atomic or molecular ion projectiles may be obtained, as well as those for each specific destruction channel. As an example of the validity of the method, we measured the destruction cross-sections of Si2− in Ar, from 30 keV to 1.3 MeV laboratory energies.

A method to measure the pressure profile inside evacuated tubes with high voltage electrodes

Abstract We developed a method to determine the pressure distribution in the electrode region of electrostatic accelerators. Direct measurement of this distribution is not possible: the electrodes are maintained inside a high pressure vessel and the presence of vacuum gauges would disturb the beam optics. The method is based on measuring the energy distribution of the atomic ion beams and identifying features associated to collisions with residual gas molecules. The procedure is also applicable to any other evacuated tube in the presence of a static longitudinal electric field. The feasibility of the method was verified in a 1.7 MV commercial particle accelerator using its facilities (accelerator tubes, particle beam, stripper, etc.). The measurements also revealed characteristics of the field structure inside the accelerator tube and the occurrence of leaks.

Can Accelerators Accelerate Learning

The ‘Young Talented’ education program developed by the Brazilian State Funding Agency (FAPERJ) [1] makes it possible for high‐schools students from public high schools to perform activities in scientific laboratories. In the Atomic and Molecular Physics Laboratory at Federal University of Rio de Janeiro (UFRJ), the students are confronted with modern research tools like the 1.7 MV ion accelerator. Being a user‐friendly machine, the accelerator is easily manageable by the students, who can perform simple hands‐on activities, stimulating interest in physics, and getting the students close to modern laboratory techniques.

collisional destruction by He, Ne and Ar

We measured the total destruction cross sections of molecules in He and Ar , and in Ne targets (3.0 and 4.0 au). The present data agree well with extrapolated values extracted from previous results at lower projectile velocities for He and Ne targets. The target dependence of these cross sections, irrespective of the projectile velocity, was found to be essentially given by constant multiplicative factors, roughly associated to the target atomic size. A simple projectile scaling rule, known to work for light atomic and molecular projectiles, was also found to describe the cross sections well.