Alpha Proton X-ray Spectrometer (APXS)
The Mars Pathfinder
Rover Alpha Proton X-ray Spectrometer (APXS) is designed to measure the elemental
chemistry of martian surface material to help gain a more detailed understanding of the
geologic and geochemical processes taking place within Mars and on the surface and to
allow inferences as to the role played by reactions with the atmosphere and water. The
instrument consists of a sensor head which is placed against a sample by a deployment
mechanism mounted on the rover chassis. The sensor head is placed on a sample (rock or
soil) for a period of 10 hours. It can then be moved to a different sample for another
measurement. A color imager mounted on the back of the rover takes pictures of all the
samples studied.
APXS General Description
The APXS consists of three parts, the sensor head, the sensor deployment mechanism, and the electronics box. The sensor head is cup-shaped and has dimensions of 52 x 71 x 35 mm, taking up a total volume of ~85 cubic cm. It is mounted on the deployment mechanism on the rear of the rover outside the warm electronics box. The sensor head contains a radioactive alpha particle source (curium) and three detectors: a telescope containing two silicon detectors, for alpha particles and protons, and a silicon-PIN X-ray detector and pre-amplifier. Collimators are situated in front of the detectors, and have been designed for a nominal distance between sample surface and collimator front face of 4 cm. This distance is, however, not very critical and may in a real situation vary by as much as 0.5 cm.
The principle of the APXS technique is based on measurements of three interactions of alpha particles with the sample. Rutherford backscattering of the alpha particles gives information on composition, particularly on the abundance of C and O. Production of protons by alpha particles impinging on the sample is sensitive to F, Na, Mg, Al, Si, and S. X-ray emission from recombination of atomic shell vacancies due to alpha particle bombardment yields information on the abundance of heavier elements, Na and above. A combination of all three measurements allows determination of the abundance all elements except hydrogen at concentration levels above typically a fraction of one percent.
Alpha Particle Source
Nine source holders in a ring-type geometry contain Cm-244 (curium 244, half-life of 18.1 years) with a total intensity of 50 millicuries. The curium is protected by a motor-driven shutter of 0.2 mm thick stainless steel blades and very thin (~200 nm thick) foils of alumina and VYNS. The alpha particles (of known energy) from the curium bombard the sample material. The particles interact with the sample in three ways. Elastically scattered alpha particles are picked up by the alpha particle detector. Protons from alpha-proton nuclear reactions are measured by the proton detector. The X-ray detector samples X-rays produced by excitation of the atomic structure by the alpha particles.
Details of the Alpha-Proton Detector
The silicon detectors for the alpha particles and protons are in a telescope arrangement. The front silicon detector has a thickness of 35 mm, which will completely stop alpha particles of energy 6.5 MeV or less. The maximum energy in the backscatter spectrum of Cm-244 is 5.80 MeV, so none of these backscattered alpha particles can reach the second detector. However, the first particle detector is transparent to protons with energies greater than 1.6 MeV. The second silicon detector is over 320 mm thick and will stop protons with energies up to 6 MeV. Both detectors register these protons and the sum of the signals will give the total proton energy. With the help of threshold discriminators and coincidence logic, events caused by alpha particles can be distinguished from events caused by protons and thus alpha spectra and proton spectra can be recorded separately. Low energy (less than 1.8 MeV) protons can register as alpha events, but the significant portion of the proton range is above 2 MeV and alpha events occur at a much higher rate than proton events so the alpha spectra is not noticeably altered. The thick detector also acts as an active anticoincidence shield against cosmic ray protons.
Details of the X-Ray Detector
The X-Ray instrument consists of a silicon PIN photodiode X-ray detector mounted on a beryllia substrate, the front end of a charge sensitive preamplifier, and a temperature sensor, enclosed in a hermetically sealed metal container filled with inert gas. A thin (8 micron) beryllium window in the front enables entry even for very low energy X-rays. A tungsten collimator assures the X-ray detector analyzes the same sample area as the alpha and proton detectors and shields the detector from the X-rays and other gamma-rays coming directly from the Cm-244 sources. The output from the first stage of the preamplifier is fed into the charge sensitive preamplifier sitting on top of the sensor head.
The total measurement time required for a single sample is about 600 minutes. For all three instruments the energy spectrum of the detections is recorded allowing a determination of the elemental chemistry of the sample. The instrument is a copy of the instrument flown on the Russian Vega and Phobos missions, with the alpha and proton spectrometers being supplied by the Max Planck Institute in Germany and the x-ray spectrometer by the University of Chicago.