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X-Ray Fluorescence

NSSDCA ID: 1971-063A-09

Mission Name: Apollo 15 Command and Service Module (CSM)
Principal Investigator:Prof. Isidore Adler


This experiment, carried in the scientific instrument module (SIM) of the Command and Service Module (CSM) on the Apollo 15 mission, was used for orbital mapping of the lunar surface composition and X-ray galactic observations during the transearth coast. The instrument was designed to observe the intensity and characteristic energy distribution of the secondary, or flourescent, x-rays produced by the interaction of solar x-rays with the lunar surface. A typical quiet Sun X-ray spectrum produces measurable x-ray responses from elements up to about atomic number 14 (silicon), higher solar activity can result in flourescence of higher atomic number elements. The instrument consisted of two separate assemblies, the spectrometer processor, which also held the alpha spectrometer experiment and had a total mass of 59 kg, and the solar X-ray monitor. The instrument consumed 26 W during operation and 21 W during standby (including the heaters).

The spectrometer processor assembly comprised three large-area proportional counters with state-of-the-art energy resolution, a set of large-scale filters for energy discrimination among the characteristic X rays of aluminum, silicon, and magnesium, and a data handling system for count accumulation, for eight-channel pulse-height analysis, and for relaying the data to the spacecraft (SC) telemetry system. Temperature of the apparatus was maintained by heaters. The large-area proportional counters were collimated to fields of view of about 60 deg full width half maximum (although the actual field of view shows a complicated response function) and yielded a resolution on the lunar surface of 111 by 148 km. Timing was done using the spacecraft 100 pps clock. The three counters each had a 0.0254 mm thick beryllium window with an area of 25 square centimeters, detector 1 was unfiltered, detector 2 had a magnesium filter, and detector 3 had an aluminum filter. Both filters were foils in the range of 0.005 - 0.0075 mm thick. The counters were filled to a pressure of 1 atmosphere with 90% argon, 9.5 % carbon dioxide, and 0.5% helium. A stable power supply provided a bias voltage of 2250 V for the proportional counters. The three counters pointed towards the lunar surface during lunar orbital operations and were pointed towards X-ray sources in space during cruise operations.

The spectrometer assembly was equipped with two calibration sources emitting characteristic K radiation, a magnesium K-alpha source and iron 55 (which decays to manganese 55). The calibration sources are mounted on a rod which can be rotated into the field of view of the proportional counters. This was scheduled to be done every 16 minutes for a period of 64 seconds.

The solar monitor assembly was designed to provide a simultaneous measure of the incident solar x-ray flux in order to account for variations in solar activity and provide an estimate of the flux at the lunar surface and was mounted on the opposite side (sector IV) of the CSM facing the Sun during operation. The assembly consisted of a proportional counter with a 3.0 mm wide window cut into a 4.19 mm thick aluminum block overlain by a 2.36 mm thick tungsten block, cut to give a 106 degree field of view. The window is covered with a 0.025 mm beryllium foil shield. This turned out to be insufficient shielding and the monitor would be saturated during operation. On Apollo 16 a second beryllium filter was added. Behind the window was a proportional counter filled to one atmosphere with 90% argon, 9.5% carbon dioxide, and 0.5% helium. The solar monitor had its own high voltage power supply and analog electronics.

The instrument had four modes of operation. In normal mode, the three counters measured in seven equal-range channels over an energy range of 0.75 - 2.75 keV and the solar monitor measured in seven equal-range channels from approximately 1.0 to 3.0 keV. In the calibration mode, the calibration sources were measured by detectors 1, 2, and 3. The third mode was the same as the normal mode except the energy range boundaries of detector 1 were doubled to 1.5 to 5.5 keV, and the boundaries of each channel were also doubled. The final mode used the same spectral range as the third mode but detectors 1, 2, and 3 measured the calibration sources.

The Apollo 15 X-ray spectrometer was turned on some 84 hours into the flight on the third orbit and returned the first data at roughly 1:00 UT on July 31, 85 hours 10 minutes GET (ground elapsed time, or time since launch) on the fourth orbit. From 84 to 102 hours the orbit was about 15 x 110 km. After 102 hrs GET the orbit was circularized to roughly 110 x 110 km. Over 100 hours of observations of the lunar surface were made. The coverage between the Apollo 15 and Apollo 16 missions overlapped between about 50 and 100 degrees and the results showed that the measurements were consistent for the two experiments. Galactic X-ray observations were made during trans-Earth coast. The solar x-ray monitor became overloaded near the sub-solar points so the data are not useful, modifications were made for the Apollo 16 flight. Otherwise the experiment performed nominally and allowed production of compositional maps of 20% of the lunar surface.

Alternate Names

  • Apollo15CSM/X-RayFluorescence
  • S161
  • X-Ray Flourescence
  • X-Ray Fluorescence Spectrometer

Facts in Brief

Power (avg): 26 W


  • Planetary Science: Geology and Geophysics

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Mr. L. E. JamesGeneral ContactNASA Johnson Space Center
Dr. Paul GorensteinOther InvestigatorHarvard College
Dr. Jacob I. TrombkaOther InvestigatorNASA Goddard Space Flight Center
Dr. Herbert GurskyOther InvestigatorHarvard College
Dr. Albert E. MetzgerOther InvestigatorNASA Jet Propulsion
Prof. Isidore AdlerPrincipal InvestigatorNASA Goddard Space Flight Center

Selected References

  • Adler, I., et al., Apollo 15 X-ray Fluorescence Experiment Preliminary Report, in -- Proc. Third Lunar Sci. Conf., 3, 2157-2178, MIT Press, Cambridge, MA, 1972.
  • Adler, I., et al., Apollo 15 and 16 X-ray fluorescence experiment, Space Sci. Instrum., 1, No. 3, 305-316, Aug. 1975.
  • Adler, I., et al., X-ray fluorescence experiment, in Apollo 15 Prelim. Sci. Rept., NASA SP-289, 17-1 to 17-17, Wash., D.C., 1972.

Related Information at NSSDCA

Apollo 16 X-Ray Flourescence Experiment

X-Ray Data Tables Online (Lunar Data Project)

Lunar Data Project

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