4p detector
Del Mar Ventures offer new type of 4p detector, which can be used to detect charged particles in wide variety of applications. It's a high-resolution "microscope" for the investigation of the dynamics of atomic, molecular and nuclear reactions, particle-surface, laser-atomic-molecular interactions etc. The ultimate goal of such studies would be the realization of kinematically complete measurements where the momenta of all emerging particles are determined simultaneously with sufficient resolution.
The detector is a combined electrostatic and magnetic spectrometer with time and position sensitive particle detection capable to record more than one particle hitting detector at the same time or with a very short time gap (typically 10 ns) between subsequent hits. Object to be studied is placed inside detector. It can be a surface, or a region where interaction of different particles occur. We shall call it object in the following description.
At least one electrostatic acceleration field and an additional preferably homogeneous magnetic field cover the space between the object in question and the detector. In most common configuration both fields are homogeneous fields with field lines along the shortest line between object and detector. With such an arrangement of electrostatic and magnetic fields it is not necessary that the detector covers the complete half sphere above the object in order to reach a complete detection of particles independent of their energy and direction.
Emitted particles are additionally accelerated by the electric field towards the detector and any transversal momentum results in a rotational motion of the particles around the magnetic fields line. This way even particles with a considerably large transversal momentum (i.e. the momentum component perpendicular to the shortest line between object and detector) are focused by the magnetic field and transported to the detector because of the particles initial longitudinal momentum and the applied electric field.
From the distance and from the angle of the position of detection, measured with respect to the shortest line between object and detector, the magnitude of the transversal momentum can be deduced. From this distance and from the measured time of flight the momentum and the direction of the emitted particle can be extracted unambiguously.
One of the examples of application of 4p detector is described below.
The integrated recoil-ion electron spectrometer.
The spectrometer is based on a well-localized inherently cold atomic jet target ensuring the "perfect" preparation of the initial target momentum and the energetic state if molecules are used. Any projectile beam in the presence of a well-defined electric and magnetic field configuration crosses this atomic jet. This way, ions and electrons created in any collision are guided onto position sensitive microchannel plate detectors (MCP). From the final positions and their time of flight (TOF) in the apparatus the trajectories of ions and electrons can be calculated and their initial momenta are obtained.
Figure above shows a schematic drawing of the present spectrometer. A uniform electric field along the ion beam is generated in the space between two parallel ceramic plates covered with burnt-in resistive layers. Recoil-ions and with energies below 50 eV are accelerated into the forward and backward directions, respectively. An additional solenoidal magnetic fields forces electrons on spiral trajectories, thus ensuring a solid angel DW e of 4p for electrons with transverse energies E e^ below 30 eV and a lager solid angel of about 20% for E e^ £ 60 eV. The flight time of electrons and recoil-ions, measured by a triple coincidence with the scattered projectiles, give information on their longitudinal momenta and on the recoil-ion charge state. The transverse momenta and azimuthal emission angels are calculated from the positions on the corresponding detectors.
 |
|
 |