PDS_VERSION_ID               = PDS3                                           
LABEL_REVISION_NOTE          = "                                              
         2007-02-11 MESS:mick V01 First Draft / Example;                      
         2007-08-13 MESS:mick V02 Keywords and Typos Fixed"                   
RECORD_TYPE                       = STREAM                                    
                                                                              
OBJECT                         = INSTRUMENT                                   
  INSTRUMENT_HOST_ID             = "MESS"                                     
  INSTRUMENT_ID                  = "XRS"                                      
                                                                              
  OBJECT                         = INSTRUMENT_INFORMATION                     
    INSTRUMENT_NAME                = "XRAY SPECTROMETER"                      
    INSTRUMENT_TYPE                = "XRAY SPECTROMETER"                      
    INSTRUMENT_DESC                = "                                        
                                                                              
    The Mercury Surface, Space Environment, Geochemistry and Ranging          
    (MESSENGER) mission is designed to orbit Mercury following one Earth      
    flyby, two flybys of Venus and three of Mercury.  It launched in          
    August 2004 and will use these flybys to achieve an orbit insertion       
    around Mercury in March 2011.  Initial data collection will begin         
    during the three flybys of Mercury, and will primarily consist of         
    global mapping and measurements of the surface, atmosphere and            
    magnetosphere composition.  MESSENGER will remain in orbit for the        
    rest of the nominal mission, which is scheduled to end in March           
    2012. Once in orbit around Mercury it will begin a series of              
    observations using multiple instruments. These observations will          
    provide data to answer questions about the nature and composition of      
    Mercury's crust, tectonic history, the structure of the atmosphere        
    and magnetosphere, and the nature of the polar caps.                      
                                                                              
    The X-Ray Spectrometer (XRS) experiment is comprised of three             
    identical gas proportional counters (GPC) that measure X-rays             
    emitted from the surface of Mercury in the energy range from about 1      
    to 10 keV.  X-rays in this energy range sample the planetary surface      
    to depths of a few tens of microns.  The GPCs each have a 10-cm2          
    active area and use both anti-coincidence wires and pulse shape           
    discrimination to minimize background.  Balanced filters are used to      
    resolve the lower energy X-ray lines from Mg, Al, and Si.  This           
    technique has been used previously on other orbital X-ray                 
    experiments flown on the Apollo 15, 16 and NEAR-Shoemaker missions.       
    One GPC has a thin aluminum filter, which filters out Si photons,         
    one has a thin Mg filter, which filters out Al and Si photons, and        
    the third GPC has no filter.  The energy resolution of the gas            
    counters is sufficient to resolve the higher energy lines from S,         
    Ca, Ti, and Fe.  A small Si- PIN detector is used as a solar              
    monitor, because the Sun is the source of the planetary X-ray             
    fluorescence.  The field-of-view (FOV) of the GPCs is 12 degrees.         
                                                                              
    Both the GPCs and the Si-PIN detector count individual photons,           
    producing an electronic pulse that is proportional to the energy of       
    the absorbed photon. Each valid count is stored in energy histograms      
    (spectra) that are 244 channels for the GPC and 231 channels for the      
    Si-PIN.  The accumulation time for these histograms will vary             
    depending upon proximity to the planet.  At periapsis the                 
    accumulation time will be 40 seconds and at apoapsis 450 seconds.         
                                                                              
    The XRS is comprised of three physical units.  The GPCs make up the       
    Mercury X- ray Unit (MXU).   The Solar Assembly for X-rays (SAX)          
    includes the Si-PIN diode, preamplifier, shaping electronics and          
    thermal electric cooler (TEC).  The analog and digital electronics        
    are contained in the Main Electronics for X-rays (MEX) unit.  The         
    SAX is located at the ''top center'' edge of the solar shield (see        
    Figure 1 in the XRS EDR SIS document).  The MXU and MEX are               
    co-located within the payload adapter ring.                               
                                                                              
    The XRS instrument is described in full detail in [SCHLEMMETAL2007].      
    "                                                                          
                                                                              
  END_OBJECT                     = INSTRUMENT_INFORMATION                     
                                                                              
  OBJECT                         = INSTRUMENT_REFERENCE_INFO                  
    REFERENCE_KEY_ID               =  "SCHLEMMETAL2007"                       
  END_OBJECT                     = INSTRUMENT_REFERENCE_INFO                  
                                                                              
END_OBJECT                     = INSTRUMENT                                   
END