The Atmospheric X-ray Imaging Spectrometer

Concept and Capabilities

The Atmospheric X-ray Imaging Spectrometer (AXIS) measures the intensities and energy spectra of 3 keV to 100 keV bremsstrahlung x-rays that are generated as energetic electrons penetrate the atmosphere. By continuously remote-sensing these x-rays over a wide area from limb to limb beneath UARS, AXIS provides a "global" monitor of atmospheric energy input. The AXIS data set is thus complementary to the more direct, but localized energy input monitor provided by the PEM particle spectrometers. Compared to x-ray instruments flown aboard satellites on previous missions (Imhof et al., 1980; Mizera, et al., 1984; Calvert et al., 1985), AXIS measures atmospheric x-rays over a broader energy range with finer energy resolution. These attributes are vital for accomplishing the UARS mission objectives.

The AXIS instrument is shown in Figure 2. In orbit, the earth would be off the top of the photograph. The x-ray entrance apertures are behind the rectangular horns at the top of the instrument. The large conical structure is a reflector and shield protecting a large radiator plate from sun and earth-shine. This radiator provides passive cooling for the x-ray detectors.

AXIS consists of an array of 16 cooled silicon detectors, 8 in each sub-unit (AXIS 1 and AXIS 2). Each 0.5 cm x 5 cmsup2 detector is collimated by a series of tantalum and tungsten shields to have a roughly conical field-of-view with a response width of 8 degrees full-width at half maximum (FWHM) (Figure 3). The eight detectors in each camera are oriented to view the top of the atmosphere from nadir to horizon perpendicular to the UARS ground track (Figure 4). AXIS 1 views to the -Y (sunlit) side of UARS, AXIS 2 to the +Y (dark) side. Additional characteristics of the instrument are provided in Table 1. A more detailed description of AXIS design and performance features may be found in Chenette et al., (1992).

Each detector in AXIS is connected to an individual analog charge amplifier and pulse-shaping amplifier module. The energy of each detected x-ray is measured by pulse height analysis of the signals from each of these modules. A 128-channel pulse-height distribution is accumulated continuously for each detector over the x-ray energy range from 3 to 100 keV. To optimize both time and energy resolution this 128-channel pulse-height distribution is summed and put into the telemetry in three ways. For the central 10 of the 16 detectors, AXIS provides an integral flux every 1/2 second. For all detectors it provides a 4-channel spectrum every second, and a 32-channel spectrum spaced quasi-logarithmically in energy every 8-seconds. The range of each of these sums over the 128 channels can be modified by commands from the ground.

Images of the atmospheric x-ray emission over the AXIS energy range are built up by integration as the orbital motion of UARS sweeps the AXIS field of view along the ground track. These images are 16 pixels across and a number of pixels along the satellite track which is limited only by the total image accumulation time. At the UARS orbital speed of ~7 kilometers per second the 8-second data can yield spatial resolution better than 60 km along the direction of satellite motion. Across the ground track the 8 degrees field of view yields a spatial resolution of better than 100 km near the nadir.


	  TABLE 1.  AXIS Operating Parameters

X-ray Response

Energy Range                              3 to 100 keV
Energy Resolution                         ~1.5 keV FWHM
Number of Energy Channels                 128 linear compressed to:
			                 	  32 per pixel every 8 s
			                 	  4 per pixel every 1 s
			                	  1 per pixel every 1/2 s
				                  (for central 10 pixels)

Geometrical Factor                       0.07 cmsup2 sr for each of 16 pixels
Field of View                            Full earth coverage from limb to limb
Angular Resolution                       8 degrees FWHM per pixel
Spatial Resolution                       <100 km 
Accumulation Interval                    0.5, 1, and 8 seconds
Temporal Coverage                        Continuous day and night



Engineering Parameters
Sensors:                      High resolution, cooled, lithium-drifted silicon
Electron Rejection Methods:   Sweeping magnets, collimators, and passive tungsten and       
                              tantalum shielding
Cooling:                      Passive, two-stage thermal radiator
                              Detector Operating Temperature:<200 degrees K
Central View Direction:       22.5 degrees from nadir towards UARS +X
Operating Time Restrictions:  none
Weight:                       44.3 kg (95.7 lbs)
Power:                        12.2 W at 28 VDC (exclusive of heaters)

The major physical components of AXIS are indicated in Figure 5. The instrument is constructed on a machined base plate which is oriented in the horizontal (X,Y) plane. The spacecraft mounting interface and the other components of AXIS are attached to this plate. The detectors of AXIS 1 and AXIS 2 are contained within a pair of housings which are bolted together and held by brackets over the +Z (nadir) side of the base plate. The collimators, with special magnets at their throats, help to shield the detectors from electrons. Analog electronics are mounted in boxes near the detectors on the outside of each housing. The large truncated cone on the +Y side provides a secondary thermal radiator and is a sun-and-earth shield for the primary thermal radiator which lies at the small end of the cone. The primary radiator cools the detectors to below -80 degrees C by passive radiation to space. Cooling improves the energy resolution by reducing the thermal leakage current in the detectors.

Operating Modes

AXIS is designed to provide continuous measurements of the atmospheric bremsstrahlung x- rays. Normally these measurements are interrupted only by daily operation of the on-board electronic calibration system built into the instrument, which takes slightly more than two minutes. Other modes are built into the instrument to permit testing of the microprocessor computer system and memory, the spacecraft command and telemetry interfaces, and other systems. These modes were designed to support instrument testing and initial integration with the spacecraft and are not used on-orbit except for diagnostic purposes.

Instrument Characterization and Calibration

The accurate determination of x-ray source spectra depends on the calibration of both the angular response function and the energy response function of each detector. The angular response function is the result of the mechanical design and construction of the collimators and fixtures that hold the detectors. The energy response function results from a combination of the detector characteristics and its supporting analog and digital electronics.

The field of view of each detector is determined by the entrance "pinhole" in a tantalum plate, a series of tantalum plate baffles, a thick tungsten cup which surrounds each detector, and the detector itself (Figure 3). The position and shape of the angular response function for each pixel were measured directly in an extensive instrument calibration program using laboratory x-ray sources. An example is shown in Figure 6. A model response function was developed from the instrument design specifications. This model was parameterized and used to fit the measured response function using a least-squares technique. The look direction unit vectors for each pixel in UARS coordinates and the measured geometrical factors were obtained using this calibration and fit procedure.

The efficiency for detecting x-rays in the field of view of any pixel also depends on the x-ray energy. At low energies the efficiency is limited by absorption in the thin foils that are installed in AXIS to shield the detectors from light and to provide thermal control. At high energies the efficiency is limited by the finite detector thickness coupled with the decline of the photoelectric absorption coefficient with increasing x-ray energy. The calculated efficiency as a function of x- ray energy is shown in Figure 6 for a typical pixel. At low energies the responses differ slightly for different pixels because of changes in the effective thickness of the beryllium foil at the aperture. Validation checks of this calculation were performed during the AXIS calibrations.

The energy calibration was measured over the expected operating temperature range and over a range of instrument operating conditions to verify the stability of the energy conversion electronics. These calibrations were based on x-ray line spectra from radioactive elements like Americium-241 and were supplemented at low energies by Kx-line measurements taken during the angular response function calibrations. Final electronic gain coefficients and noise resolution figures were measured for each pixel during the final instrument-level thermal vacuum tests.

In flight, the gain and noise of the AXIS analog and PHA electronics system is monitored daily with a precision electronic pulse calibrator activated by ground command. The results of these calibrations are incorporated in the analysis of the flight data via calibration data files. The electronic calibration has shown the dominant sources of uncertainty in the measurement of x-ray energy to be the intrinsic detector energy resolution (~1.5 keV) at low energies and the energy channel quantization (<7 keV) at high energies.

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