Venturing into the vast expanse of cislunar space, early space missions aimed to unravel the mysteries of cosmic dust. Among these pioneering experiments was the Cosmic Dust Detector, designed to quantify the flux of dust particles and analyze their energy and momentum. A key technical detail of this instrument lies in the ratio 1000/31.6, a figure crucial to understanding its energy detection capabilities. This article delves into the intricacies of the Cosmic Dust Detector, shedding light on the significance of this ratio and its role in space exploration.
The Cosmic Dust Detector, a compact package measuring 8.6 x 15.2 x 15.2 cm, was engineered to provide valuable data on the density, mass, and velocity distributions of cosmic dust particles. Mounted on the spacecraft with detectors oriented perpendicularly to the probe-Sun line, this 1.60 kg experiment was equipped with two distinct types of sensors: one for energy and another for momentum. This dual-sensor approach allowed for a comprehensive analysis of the impacting particles.
At the heart of the energy measurement system was a light flash detector. This ingenious sensor operated on the principle that the intensity of light emitted during micro-crater formation is directly related to the kinetic energy of the impacting particle. The detector utilized a photomultiplier, a highly sensitive light detector, with a diameter of 3.49 cm. To shield the photocathode from unwanted light, a thin 800 angstrom aluminum layer was deposited on its surface. The photomultiplier’s output was meticulously configured with outputs taken from the anode, the tenth dynode, and the ninth dynode. Crucially, these outputs were scaled in a precise ratio: 1000:31.6:1.
This 1000/31.6 ratio is not arbitrary; it represents the relative sensitivity levels of the different dynode outputs. The anode circuit, with the highest sensitivity (represented by 1000 in the ratio), fed into a 5-bit counter designed to capture even the faintest light flashes, indicative of low-energy impacts. The tenth and ninth dynode outputs, with progressively lower sensitivities (31.6 and 1 respectively), were connected to 3-bit and 2-bit counters, respectively. This tiered sensitivity allowed the detector to measure a wide range of energy levels, from a minimum detectable energy of approximately 0.005 erg up to higher energy impacts without saturating the detection system. The 1000/31.6 ratio, therefore, was instrumental in enabling the instrument to accurately characterize the energy distribution of cosmic dust particles encountered in space.
Complementing the energy detector, the momentum impact detector employed a piezo-electric crystal microphone. Mechanically coupled to the photomultiplier tube’s sensitive surface, this microphone responded to the momentum imparted by impacting particles. Its output drove a parallel resonant amplifier tuned to 100 kHz, achieving a substantial voltage gain of 80 dB. This amplified signal then fed into both a 2-bit counter and another amplifier stage, further boosting the signal by 20 dB before feeding into another 2-bit counter. With a trigger circuit designed to capture only the initial pulse, this system was sensitive enough to detect momentum thresholds as low as approximately 0.0001 dyne sec.
While the experiment faced initial challenges, such as anomalously high counting rates in the light flash detector possibly due to sunlight leakage, and erratic microphone outputs likely from outgassing, the fundamental design and the strategic 1000/31.6 ratio in the energy detector remained crucial to its scientific objectives. The Cosmic Dust Detector, with its carefully engineered sensors and the insightful application of sensitivity ratios like 1000/31.6, provided valuable early data on the nature of cosmic dust in the space environment, paving the way for more advanced studies in planetary science and space exploration.
