Lunar Dust Detector

Description

The purpose of the Lunar Dust Detector Experiment (also referred to as the Dust, Thermal, and Radiation Engineering Measurements package, or DTREM) on the Apollo Lunar Surface Experiments Package (ALSEP) was to assess the long term effects of the lunar environment on silicon solar cells by measuring power output reduction caused by high-energy cosmic particle and ultraviolet radiation damage and dust accumulation, and to measure temperatures, including reflected infrared brightness temperatures in computing lunar surface temperatures. Two other objectives were to determine if pre-irradiation of the cells before deployment and irradiation on the lunar surface followed a simple superposition and if protective cover glass was necessary on the cells.

The Dust Detector had two components - a sensor package mounted on a housing on top of the central station sun shield and a printed circuit board located within the central station that interfaced with a preamplifier and the power distribution unit of the ALSEP data subsystem. The sensor package had three 10-ohm-cm n-on-p silicon solar cells, each 1 x 2 cm. The power output of each cell varied from 0 to 150 mV. They were mounted on the top horizontal surface of the dust detector housing. One cell had a 0.15 mm thick blue filtered glass cover for protection against particles and ultraviolet radiation. A second cell had the same protective glass but was pre-irradiated with 10E15 electrons of 1 MeV energy. And one cell had no cover glass and was not pre-irradiated. The energy threshold for the covered cells was 175 keV electrons and 4.25 MeV protons and for the uncovered cell 173 keV electrons and 60 keV protons. Three temperature sensors were also mounted on the housing, one beneath the solar cells and two, one internally and one externally, on a vertical side of the housing. The sensors are high-precision nickle resistance thermometers with a range of 84 to 408 K.

Data from this experiment consisted of the photovoltaic output of the solar cells and temperatures measured by the three thermistors (internal temerature, cell temperature, and external infrared temperature) as a function of time during daytime conditions. The voltage drop across a precision 1.00 ohm resistor was measured to determine the photovoltaic output. Measurements were made over the course of each lunar day, returned to Earth, and stored on magnetic tape.

The Dust Detector was originally designed to measure only dust accumulation but was expanded to include the particle, radiation, and temperature studies when it was determined from Surveyor engine firings on the surface that the dust accumulation would not be as heavy as anticipated. (The original dust accumulation design was flown on the Apollo 12 mission.) The expanded experiments were flown on the Apollo 11, 14, and 15 missions and were identical to each other except that the Apollo 14 and 15 experiments had a pre-irradiated cell covered by a 0.15 mm glass shield in place of the normal cell covered by 0.51 mm glass on Apollo 11.

The Dust Detector began operation with deployment of the ALSEP on July 31, 1971. The instrument was turned off when ALSEP operations were terminated on 30 September 1977. Performance of the equipment was nominal.

In general the irradiated cells showed a power drop of 1 percent per year and the nonirradiated cell 3.5 percent per year. The uncovered cell had a total power drop of about 16 percent in the first year before leveling off to a decline of roughly 5 percent per year. A major solar particle event in August 1972 caused a drop of approximately 7 percent in the uncovered cell.