Experimental Procedure

Advances in particle coating processes have enabled the coating of low-density polystyrene latex spheres with a number of different conducting polymers suitable for acceleration [6]. The projectiles are synthesised at the University of Sussex [7,8]. The conducting polymer is deposited as an ultrathin overlayer (less than 10% by mass) onto the surface of a polystyrene latex sphere, producing a well-defined core-shell morphology. Each sample has a fairly monodisperse size distribution clearly apparent in Figure 1, an electron micrograph of a PEDOT coated polystyrene latex of size 1.8 |im diameter. The non-conducting core latex is readily doped with different elements and the versatility of core size and coating thickness enables a range of projectile diameters and compositions to be synthesised.

The particles' ability to retain surface charge makes them suitable for use in an electrostatic accelerator, and they represent better substitutes for carbonaceous cosmic particles than traditional projectiles. They mimic the carbon-rich composition of cosmic dust grains and their densities are comparable, although cosmic grains are believed to contain more dense elements they are also believed to be micro-porous. The controllable size enables a range of velocities to be investigated and a mean projectile mass to be accurately obtained for each sample. The method of acceleration with the 2MV electrostatic accelerator is discussed in [5]. The laboratory CDA model was dimensionally identical to the flight instrument and had a representative rhodium target; its operation is given in [9], The accelerated particles were within the same velocity and mass ranges as those expected from interplanetary space (e.g. up to -37 km s"1 and 10"19 - 10"15 kg.).

In the Van de Graaff the velocity (v) and particle charge (q) is measured for each event through detection along the beam line. The mass (m) of the impacts can be determined from the accelerating voltage (V):

For flight events the velocity is obtained from empirically calibrated rise times of the charge signals on the instrument.

The mass spectra are obtained in terms of time-of-flight and must have an atomic mass scale applied. For this work we assumed that only single ionization has occurred. The method adopted has been to identify two known features within each spectrum to "anchor" the mass scale. Sodium and potassium, as contaminants within the system were always present and clearly identifiable within the complex spectra of these low-density projectiles. The resulting calibration was verified for accuracy through comparison with the impact time as recorded by the target signal.

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