Positron and Electron Impact Ionization of Atoms

Background
This work, funded by the National Science Foundation, studies and compares inelastic antiparticle-particle and particle-particle interactions.  Highly differential data, measured for positron and electron impact ionization of atoms and simple molecules, provide information about the interaction channels involved.  By performing measurements under identical conditions, differences associated with simply changing the sign of the projectile charge, i.e., the direction of the Coulomb field between the projectile and the target electrons and nucleus, can be identified.  These differences are absent in first-order theories such as the first Born approximation; therefore these data provide sensitive tests of more sophisticated theories.

Positrons (anti-electrons) offer a unique opportunity to study atomic collisions.  Using positrons one can study target ionization with the benefit that now the scattered projectile and the ionized electron are distinguishable.  This work concentrates on measuring fully differential ionization yields for both positron and electron impact in order to study charge 'and mass effects on the interaction kinematics.  In atomic collisions it is the strength of the interaction force (the Coulomb force) as well as the time of the interaction that determines what happens.  Therefore, all singly charged projectiles such as positrons, electrons, protons, or antiprotons traveling at the same speed generate identical probabilities that target electrons will be ionized or excited.  However, because of the different signs of their charges and their vastly different masses, the dynamics are quite different.  Changing the sign of the charge means that positive and negative projectiles will scatter "toward" or "away" from the atom.  Light masses, e.g., leptons, will scatter through large angles (many tens of degrees) whereas heavy masses such as protons have scattering angles of milliradians.  Another major difference is that atomic interactions always involve collisions with bound target electrons.  Therefore, equal mass collisions, i.e., lepton impact, can lead to the projectile transfering all of its kinetic energy to the target whereas impacting ions lose very little of their total energy.  By systematically changing the collision parameters such as charge and mass, these effects can be studied.  Another feature of this work is the investigation of multiple ionization by positron and electron impact.  Total cross sections have shown that positive and negatively charged projectiles have essentially the same probabilities for removing a single electron but negatively charged particles are twice as efficient at removing two electrons in a single collision.  Our double ionization studies have provided the first, and only, differential information about multiple ionization by antiparticles.  Also, by combining positron and electron impact data, we have been able to obtain differential information about how first and second order processes contribute and interfere. 

The experimental method used is to ionize atoms or molecules within a gas jet by a beam of positrons or electrons and then measure a) the energy loss as a function of scattering angle, b) the degree of ionization via the recoil target ion charge state, and c) the differential electron emission from the target.  One important experimental feature that is essential for the low-intensity positron beams is the use of two-dimensional, position-sensitive detectors for the scattered projectiles and ejected electrons.  These detectors allow data to be accumulated for a range of scattered projectile energies and angles and and for a wide rangle of ejection angles of the target electrons.  Coincidences between scattered projectiles and ejected electrons, combined with coincidences with singly charged target ions, yield fully differential information for single ionization as a function of momentum transfer.  For double ionization, by combining the positron and electron impact data information about the contributions and interference of first- and second-order double ionization mechanisms is extracted.

Technologically the positron studies are very challenging and on the edge of what can be achieved.  The signal rates are extremely weak; roughly six to ten orders of magnitude smaller than traditional studies for electron impact.   By using position sensitive particle detectors and a specially designed electrostatic energy analyzer, we have overcome these limitations and have obtained highly differential data.  Data are stored event-by-event and then sorted at a later time using different criteria.  Undergraduates have routinely been involved in this project. For example, the energy analyzer which made our first measurements possible was designed by an undergraduate, another student performed extensive trajectory analysis which helped improve the positron beam transmission, and one was responsible for much of the early data analysis.
 


 
Schematic of positon apparatus and examples of data that are being obtained.
Top left: Schematic of TDCS apparatus.  The 2D projectile detector shows scattered projectile energy losses along the horizontal direction and scattering angles in the vertical direction.  The small red dot shows the unscattered beam size and position.  Because the ejected electrons are detected above the interaction region, coincidences between ejected electrons and downward scattered projectiles indicate binary interactions while coincidences with upward scattered projectiles indicate recoil interactions.   By selecting a specific energy loss and scattering angle and sorting the electron data, triply differential yields for electron emission are obtained.  (See upper right figure.)

Top right: Doubly (left portion) and triply (right portion) yields for single ionization of molecular nitrogen by positrons and electrons.

Middle: By dividing the triply differential yields by the doubly differential yields, differential information about the relative importance of binary and recoil interactions can be obtained. 

Top right and bottom:  2D and 1D comparisons for positron and electron impact ionization of molecular nitrogen.
TDCS apparatus eplus eminus comparison
angular distributions
TDCS for N2