Showing papers by "Ralf Srama published in 2008"

Low-charge detector for the monitoring of hyper-velocity micron-sized dust particles

Abstract: A new low-noise beam detector was developed, assembled and tested for the Heidelberg dust accelerator facility. The detector was used to determine in situ the charge, speed and mass of individual dust grains flying through a highly shielded metal cylinder with a length of 200 mm and integrated with a charge-sensitive amplifier Amptek model A250F/NF. Micron-sized latex and iron particles were fired at speeds between 5 and 50 km s−1. The detector characterizes dust particles with a primary charge of 1 fC, a speed of 20 km s−1 and a size of 0.1 µm with a signal-to-noise ratio of 6. The noise of the integrated detector system is typically 0.15 fC (950 electrons) in a bandwidth from 2 kHz to 10 MHz. The new detector allows the control and selection of particles either with a lower surface potential (low-conductive surfaces of polyaniline-coated polystyrene particles), or smaller grains with very small primary charges (sub-micron-sized grains with speeds above 10 km s−1).

The Dust Halo of Saturn's Largest Icy Moon, Rhea

Abstract: Saturn's moon Rhea had been considered massive enough to retain a thin, externally generated atmosphere capable of locally affecting Saturn's magnetosphere. The Cassini spacecraft's in situ observations reveal that energetic electrons are depleted in the moon's vicinity. The absence of a substantial exosphere implies that Rhea's magnetospheric interaction region, rather than being exclusively induced by sputtered gas and its products, likely contains solid material that can absorb magnetospheric particles. Combined observations from several instruments suggest that this material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.

Characteristics of a dust trajectory sensor

Abstract: Trajectories of cosmic dust particles are determined by the measurement of the electrical signals that are induced when a charged grain flies through a position-sensitive electrode system. A typical dust trajectory sensor has four sensor planes consisting of about 16 wire electrodes each. Two adjacent planes have orthogonal wire directions. The sensor is highly transparent and mechanically robust, provides a large sensitive area, large field of view, and can, at least in principle, achieve unlimited precision. While a sensor model had already undergone limited testing in the dust laboratory, its response as a function of position and angle of incidence of the trajectory and as a function of sensor dimensions was generally unknown. To better understand its characteristics, the operation of a sensor model consisting of three planes and seven wires per plane was simulated using the COULOMB computer program. We show that the response of the reduced model can be applied to a model with more planes and more wires per plane. The effect of a trajectory's position and angle on the signal strength is discussed as well as the influence of geometrical parameters such as wire diameter, distance between wire planes, and wire length. We found a greater effect of the wire diameter on the signal strength, and a lesser effect of the plane distance, than expected. A set of similarity rules is provided for the design of a larger sensor. Finally, we discuss the optimization of the sensor for different applications.

Measurement of temperature after hypervelocity collision of microparticles in the range from 10 to 40 km/s

Abstract: The temperature recorded immediately after hypervelocity collision of microparticles comprising iron and nickel with a silver-coated piezoelectric plate was analyzed using photomultipliers of different spectral response characteristics. The conversion rate between the velocity and temperature is estimated to be ∼900 K/km∕s in the velocity range of 10–40 km/s. This rate is greater than that reported earlier.

Position Sensitive Element for Hypervelocity Microparticles Using a Piezoelectric Plate

Abstract: The propagation of transverse waves generated acoustically in a piezoelectric element by colliding it with hypervelocity microparticles was studied. The propagation times were measured by a set of multiple electrodes on the surface of the element. The coordinates that the particles struck were determined by combining the propagation times and the velocity of the waves. By using the position-sensitive element, significant deviations between the prior indicated and actually measured orbits were observed. The potential of the present element as part of a steering system is discussed.

German Lunar explaration orbiter (LEO): providing a globally covered, highly resolved, integrated, geological, geochemical, and geophysical data base of the moon

Abstract: The German initiative for the Lunar Exploration Orbiter (LEO) originated from the national conference “Exploration of our Solar System”, held in Dresden in November 2006 Major result of this conference was that the Moon is of high interest for the scientific community for various reasons, it is affordable to perform an orbiting mission to Moon and it insures technological and scientific progress necessary to assist further exploration activities of our Solar System Based on scientific proposals elaborated by 50 German scientists in January 2007, a preliminary payload of 12 instruments was defined Further analysis were initated by DLR in the frame of two industry contracts, to perform a phase-zero mission definition The Moon, our next neighbour in the Solar System is the first choice to learn, how to work and live without the chance of immediate support from earth and to get prepared for further and farther exploration missions We have to improve our scientific knowledge base with respect to the Moon applying modern and state of the art research tools and methods LEO is planed to be launched in 2012 and shall orbit the Moon for about four years in a low altitude orbit

Response of piezoelectric lead zirconate titanate detector to oblique impact with hypervelocity iron particles

Abstract: A cosmic dust detector using piezoelectric lead zirconate titanate (PZT) is currently being developed for use onboard a spacecraft for the BepiColombo mission. The characteristics of the PZT detector were studied by carrying out hypervelocity impact measurements with iron particles supplied by a Van de Graaff accelerator. The measurements with particle velocities of less than 5 km/s showed a linear relationship between the output voltages obtained from the detector and the particle momenta. This linear relationship obtained was almost independent of the impact angle between the particle and the PZT surface.

Measurement of incident position of hypervelocity particles on piezoelectric lead zirconate titanate detector

Abstract: A cosmic dust detector for use onboard a satellite is currently being developed by using piezoelectric lead zirconate titanate (PZT). The characteristics of the PZT detector have been studied by bombarding it with hypervelocity iron (Fe) particles supplied by a Van de Graaff accelerator. One central electrode and four peripheral electrodes were placed on the front surface of the PZT detector to measure the impact positions of the incident Fe particles. It was demonstrated that the point of impact on the PZT detector could be identified by using information on the time at which the first peak of the output signal obtained from each electrode appeared.