CALIBRATION STUDIES ON A PIEZOELECTRIC SENSING DIAPHRAGM FOR DETECTION OF MICROMETEORITES IN SPACE

Laboratory measurements conducted on 100 kHz piezoelectric transducers mounted on a diaphragm similar to the micrometeorite sensors flown on Vanguard III, Explorer VIII, Mariner II and IV are described, which show that the stresses induced at impact are coupled to the piezoelectric sensor predominantly by a bending mode in the diaphragm, rather than by a longitudinal compression radiating from the impact region. Particles impacting at velocities ranging from 10 cm s-1 to 6 km s-1 induce similar vibration modes. The momentum imparted to the diaphragm in this mode, resolved along a normal to the surface, will equal the sum of the momenta of the impacting particle and of the rebounded particle (or ejected matter at hypervelocities), and a piezoelectric crystal mounted with its sensitive axis normal to the surface will develop a signal proportional to the total momentum exchange at impact. Such a system will, therefore, bear resemblance to a ballistic device; the fraction of this momentum which falls within the frequency range of the sensing crystal may, however, be affected by the area of contact and by the contact time of the particle. It is shown that the contact time of small beads can lead to significant errors of calibration.