Alpha Proton X-ray Spectrometer Instrument Description

Table of Contents

Instrument Overview

The Alpha/Proton/X-ray Spectrometer (APXS) is a foreign-provided copy of an instrument design flown on the Russian Vega, Phobos, and Mars '96 missions. Thus, the instrument has extensive, applicable flight heritage. The alpha and proton spectrometer portions were provided by the Max Planck Institute, Mainz Germany. The X-Ray spectrometer portion was provided by the University of Chicago. Integration of the instrument portions was the responsibility of the Principal Investigator.

This elemental analysis instrument consists of nine curium-244 alpha-particle sources and three detectors for each of three types of particles: backscattered alpha particles, protons, and X-rays. The APX spectrometer was placed against rocks and soils on Mars to determine the abundances of all elements except hydrogen, with a lower detection limit of about 0.1 weight percent. The analytical process is based on three interactions of alpha particles with matter: elastic scattering of alpha particles by nuclei, alpha-proton nuclear reactions with certain light elements, and excitation of the atomic structure of atoms by alpha particles leading to the emission of characteristic X-rays. The approach used is to expose material to a radioactive source that produces alpha particles with a known energy, and to acquire energy spectra of the alpha particles, protons and X-rays returned from the sample.

The basis of the alpha mode of the instrument is the dependence of the energy spectrum of alpha particles scattered from a surface on the composition of the surface material. The method has the best resolving power for the lighter elements (carbon and oxygen). A least-squares analysis of a complex spectrum from an unknown sample in terms of a library of known, pure element spectra determines the elements and their abundance in the sample. A characteristic of the alpha technique is that, due to variations in scattering intensity with atomic number, there is a minimum in the scattering probability for elements with atomic numbers between 9 and 14. This includes the important elements Na, Mg, and Al, so the alpha mode alone is insufficient for these elements. On the other hand, these elements produce protons when bombarded with alpha particles. The proton spectra for alpha particles interacting with elements with atomic numbers from 9 to 14 are very characteristic of the individual elements, reflecting the resonance nature of the nuclear interactions involved. The proton mode allows their detection and measurement. The alpha particles from the radiation sources are also an efficient source for production of characteristic X-rays. The addition of a third detector for X-rays therefore results in a significant extension of the accuracy and sensitivity of the instrument, particularly for the heavier, less abundant elements. In the X-ray technique, characteristic X-rays are emitted when the low electron orbit vacancies (in the K- and L-shells) produced by bombardment of atoms by alpha particles are filled by electrons from higher orbits.

The APXS electronics are mounted in the rover warm electronics box in a temperature-controlled environment. Cables leaving the electronics box connect the APXS electronics to the APXS sensor head, which contains the radioactive sources and particle detectors. The instrument sensor head is held by a robotic arm (deployment mechanism) attached to the back of the rover. The deployment mechanism, which places the APXS in contact with rock and soil surfaces, interfaces the APXS with the microrover. For the APXS to conduct a high quality measurement of a rock or soil sample, it must be placed with its front aperture ring in contact with the sample surface and the axis of the sensor head must be within 20 degrees of normal to the surface. The linkage is designed to allow the APXS to be placed at a variety of elevations above nominal ground level and at a variety of rotational orientations. The mounting of the APXS to the deployment mechanism permits about 20 degrees of compliance motion as the APXS is placed in contact with the sample. Three contact sensors on the deployment mechanism bumper ring indicate to the rover that the positioning is complete, thereby terminating the positioning motions. The sources and detectors are recessed in a cylinder 4 cm behind the bumper and the area of rock or soil that is analyzed with the instrument is a circle 5 cm across.

Instrument Id : APXS
Instrument Host Id : MPFR
PI PDS User Id : R. Rieder
Instrument Name : ALPHA PROTON X-RAY SPECTROMETER
Instrument Type : SPECTROMETER
Build Date : August 28, 1995
Instrument Mass : 570 grams
Instrument Length : 10.5 cm
Instrument Width : 8.0 cm
Instrument Height : 6.5 cm
Instrument Manufacturer Name : Max Planck and Univ. Chicago

Scientific Objectives

The Alpha-Proton X-Ray Spectrometer (APXS) will determine the dominant elements that make up the rocks and other surface materials at the landing site. A better understanding of these materials will address questions concerning the composition of the Martian crust and the processes that led to the formation and alteration of rocks and soils. This also provides some "ground truth" comparison for orbital remote sensing observations. Rover mobility allowed the APXS to characterize the composition of several rocks and soils in the vicinity of the lander.

Subsystems

The APXS has two subassemblies: (1) sensor head and (2) electronics box. The sensor head is mounted on the end of an arm on the rear of the rover called the APXS Deployment Mechanism. The electronics box is located inside the rover's Warm Electronics Box. The two subassemblies are connected by electronics cables.

Calibration

The flight sensor head and flight alpha sources were cross calibrated against the laboratory sources before launch. A laboratory unit, identical to the flight instrument, is being used in ongoing laboratory calibration tests to produce the elemental libraries and to establish instrument performance.

Preliminary composition results were obtained [RIEDERETAL1997B] using just X-ray energy spectra and calibration curves of peak areas versus concentration for several standards measured for each element. Conversion of combined alpha, proton, and X-ray energy spectra to elemental abundances requires further instrument calibration, which is still underway. Alpha and proton modes will be recalibrated at Martian pressures.

Refinements of the calibration have been reported in meetings [BRUECKNERETAL1998], [BRUECKNERETAL1999], [DREIBUSETAL1999], and [WAENKE1999].

Operational Considerations

The X-ray detector, when running from batteries at night, provided excellent data, with resolution of the X-ray peaks even better than were obtained during the calibration in the laboratory. Most of the X-ray data on Mars were obtained during the night, when the ambient temperature was between -50 and -85 degrees C. Because of the low leakage current at these temperatures, and superb stability of the X-ray electronics over the entire temperature range, excellent data were obtained. The gain stability was better than 0.2 channel and constant resolution of about 260 eV FWHM at Fe-Kalpha line was observed over the entire period of the mission. After the rover battery died in the early morning of Sol 56, the noise levels increased, which increases the uncertainty of elemental abundances of the low atomic number elements like sodium and magnesium.

Alpha and proton modes provided excellent data on the Martian rocks and soils throughout the mission.

On the order of 10 hours integrated measurement time produces high-quality alpha and proton spectra, and about 3 hours are required for high-quality X-ray spectra. All modes (alpha, proton, X-ray) are measured simultaneously when the APXS is powered on and measuring.

Detectors

Detector geometry is shown in [RIEDERETAL1997A]. From a distance of 3.9 cm from the rock or soil sample (deployment mechanism in contact), the sample diameter viewed by the detectors is 5 cm.

  1. The X-ray detector is a Si-PIN photodiode in a hermetically sealed package with a berrylium window.

  2. The alpha detector made of silicon, 35 micrometers thick, which stops alpha particles of 6.5 MeV.

  3. Behind the alpha detector is a proton detector made of silicon, thick enough to stop protons of energies up to 6 MeV.

Electronics

The APXS electronics consists of three analog channels (one for each mode: X-ray, alpha, and proton). Each channel has its own preamplifier. The detectors and X-ray preamplifier are the only electronics in the sensor head; the rest are in the warm electronics box on the rover. Digital-to-analog converters control the gain of each of the analog amplifiers. Measurement of the pulse amplitudes is done using a 10-bit analog-to-digital converter. An 8-bit microcontroller board with battery backup responds to: commands from the rover, signals from the analog electronics to perform signal amplitude analysis and channel storage, and timer signals to increment a time counter and make temperature measurements. Details of the electronics are provided in [RIEDERETAL1997A].

Operational Modes

In APXS Night Mode, the rover kept the APXS powered on after a rover power shutdown occurred, by connecting the APXS directly to the battery bus. In normal operating mode, this latch between the APXS and the battery bus was not connected and the APXS could only be powered on while the rover was powered on.

Measured Parameters

During each measurement session four spectra are accumulated, each containing up to 256 channels. Each channel consists of two bytes, organized as an event counter (each channel can contain a maximum of 65535 counts; the channel number corresponds to the amplitude of the event, i.e. the energy of the registered particle/photon. The first spectrum, called the "alpha spectrum", contains events registered by the alpha detector only. The second spectrum, called the "proton spectrum", contains events registered simultaneously by the alpha and the proton detector (the amplitude being the sum of the amplitude of both signals). The third spectrum ("X-ray spectrum") contains events registered by the X-ray detector and the fourth spectrum ("background spectrum") contains events registered by the proton detector only. This fourth spectrum is essentially cosmic ray background events. Further details are provided in [RIEDERETAL1997A].

Other parameters measured are the temperature of the X-ray preamplifier in the sensor head, temperature of the alpha detector preamplifier in the rover's warm electronics box (°C), and the sampling duration of the alpha, proton, and X-ray measurements (hh:mm:ss).

Related Information

The following links provide some additional information about topics related to the Mars Pathfinder mission. You must be connected to the Internet for most of these links to work, since they are located at the Central Node of the Planetary Data System.

Mission

Mars Pathfinder

Targets

PDS Welcome to the Planets: Mars
PDS High Level Catalog: Mars

Instrument Host

Mars Pathfinder Rover

Instrument

Mars Pathfinder Rover Cameras

Data Set Descriptions

APXS Raw Data
APXS Oxide Abundances
Rover Camera Raw Data
Rover Camera Calibrated Images and Mosaics
Rover Engineering Data

References

BRUECKNERETAL1998
BRUECKNERETAL1999
DREIBUSETAL1999
RIEDERETAL1997A
RIEDERETAL1997B
WAENKE1999