Data Set Overview : This dataset contains two tables of time-ordered, raw event count rates acquired by the Apollo 16 X-Ray Fluorescence Spectrometer (XRFS) from 20 April to 24 April 1972 during lunar orbit. The tables include counts for all channels of the three proportional counters (unfiltered, magnesium filter, aluminum filter) and the solar monitor. The XRFS was carried aboard the Apollo 16 Command and Service Module and took measurements from the third to the sixty-first lunar orbit. XRFS measured the abundances of the elements magnesium, aluminum, and silicon. It also also had magnesium K alpha and Fe55 calibration sources. Features as small as 30 kilometers across can sometimes be detected in these data. In all, about 20% of the Moon's surface was studied by this experiment on Apollo 15 and 16. Data : Both data tables are in fixed-width, ASCII format. The first table A16_XRFS_72_NORMAL_MODE.TAB contains data from the XRFS when it was set to normal modes with all three lunar-surface counters measuring the same energy ranges. The second table, A16_XRFS_72_EXTENDED_MODE.TAB, contains data from the XRFS when it was set to extended mode with detector 1 (unfiltered) measuring with the energy boundaries for each channel doubled. The tables provide the measurement mode of the instrument, the time, orbit, spacecraft position and orientation, and the event counts, normally for 8-second intervals, for the three proportional counters with beryllium windows and the solar monitor. The normal energy range for the first 7 channels of the first three counters (detector 1 unfiltered, detector 2 magnesium filter, and detector 3 aluminum filter) was between 0.75 and 2.75 keV, broken into seven equal energy ranges: 0.75-1.04, 1.04-1.32, 1.32-1.61, 1.61-1.89, 1.89-2.18, 2.18-2.46, and 2.46-2.75 keV. These seven channels are numbered from 1 to 7, respectively, in the data tables. The solar monitor, numbered detector 4 in the data tables, measured the range from approximately 1 to 3 keV, divided into seven equal intervals. An eighth channel recorded all x-ray events with energies higher than the upper level. The XRFS instrument was switched between four measuring modes: normal (16), calibration (96), normal with energy boundaries for detector 1 doubled (144), and calibration with energy range for detector 1 doubled (224). Therefore normal scientific data are interleaved with calibration data in each table. The tables include subspacecraft selenographic latitudes and longitudes. The selenographic coordinate system is used to refer to locations on the surface of the Moon, where reference satellite crater 'Moesting A' is located at latitude 3.2 South and longitude 5.2 West. The prime meridian, equator, and poles for this lunar coordinate system are analogous to the planetographic coordinate system for Earth. For descriptions of the pre- and in-flight calibration tests and the data reduction process for the XRFS instrument, refer to the 'Apollo 15 and 16 X-Ray Fluorescence Spectrometer - Experiment Calibration' report in the DOCUMENT directory of this dataset. This dataset description was provided by the NASA Space Science Data Coordinated Archive (NSSDCA).

Confidence Level Overview : This dataset was peer-reviewed in December 2007 and was accepted for the PDS archive pending resolution of liens which were completed in June 2016. Data Coverage and Quality : Restoration from Magnetic Tape ------------------------------ These data were restored and made available as part of the Lunar Data Project at the NSSDCA with assistance from Dr. Michael Bielefeld of the Space Telescope Science Institute. The original source for this dataset is NSSDCA Collection PSPG-00086, Apollo 16 Lunar Orbit X-Ray Data on Magnetic Tape. A set of Fortran programs, developed by Dr. Bielefeld to extract and process data from the magnetic tapes, are included in the DOCUMENT directory of this dataset to help the user with the preliminary analysis of the XRFS data. Data Gaps and Possible Contamination ------------------------------------ There are gaps in the data tables. Regular gaps occurred in each orbit as the spacecraft crossed the unilluminated hemisphere (about half an orbit). Also the instrument alternated between the normal and extended modes which caused data gaps when it was in the other mode. Finally several large data gaps occured when the instrument was turned off or pointed away from the target for various reasons: Orbit# Ground Elapsed Time 6-8 323849-336095 6-7 309065-317491 10-17 331487-382673 20-21 404247-413336 24-25 433024-440226 28-29 461801-466571 32-33 490579-497781 36-37 519356-526558 39-41 541570-555335 44-45 576910-584112 48-49 604528-611509 50-53 615083-640578 56-57 662169-669355 The scientific-mode data point immediately after a calibration mode may have some contamination because the shutter lid was not completely in place. The effect increased the flux by about 40% relative to comparable spectra, but it was considered negligible given that the data were ratioed and the number of these elevated data points were few. This effect was observed late in analysis and thus was not considered in published results. When analyzing the XFRS data, the recommendation is to filter out any scientific data point (mode 16 or 144) whose time stamp is within a window of 9 seconds of the previous calibration data point (mode 96 or 224). Energy Ranges, Field of View and Surface Resolution --------------------------------------------------- There is some discrepancy in the overall energy range of the detector and the individual channels and in the field of view and surface resolution in the published literature. For a discussion of the discrepancies, see the report 'Apollo 15 and 16 X-Ray Fluorescence Spectrometer - Energy Channels and Field of View/Surface Resolution Discrepancies' in the DOCUMENT directory.