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Alpha Proton X-ray Spectrometer (APXS)

NSSDCA ID: 2004-006C-03

Mission Name: Philae
Principal Investigator:Dr. Goestar Klingelhoefer


The Alpha-X-ray-Spectrometer (APXS) is designed to analyze the chemical composition of the surface of the comet nucleus for all major elements except hydrogen and helium by irradiating the surface with an alpha particle source and measuring the spectra of backscattered particles and alpha-induced X-rays. The scientific objective of the experiment is to determine the chemical composition of the Philae landing site and its potential alteration as the comet approaches the Sun. The data returned will be used to characterize surface and near-subsurface of the comet, the ratio of ice to dust, and the chemical compostion of the dust for comparison to known meteorites.

The APXS consists of a sensor head, deployment device, and electronics. The sensor head is in the shape of a cylindrical cup, 8.4 cm tall and 5.2 cm in diameter, and contains a high energy radioactive alpha source, 30 mCi (1.1 GBq) of Curium 244. It also holds collimators, six alpha particle detectors, one x-ray detector, and preamplifiers. The x-ray detector is in the center of the inside of the cup. Six alpha source portals are arranged concentrically around the x-ray detector, and the six alpha particle detectors are mounted concentrically around the sources at the periphery of the inside of the cup. The detectors are recessed into the cup slightly compared to the sources. The x-ray sensor is a high-resolution silicon drift detector with a resolution of about 160 eV at 6.4 keV. The end of the cup is surrounded by a ring that moves inward when it contacts the surface and opens two protective doors inside the opening of the cup. The backs of the doors act as calibration targets when the doors are closed. The distance between the detectors and the bottom of the cup is 4.0 cm, the nominal distance between the sample and detectors. The diameter of the hole in the bottom of the cup, and hence the sample viewed, is 3.8 cm. The preamplifiers are located in the sensor head. The electronics are mounted inside the lander body. The total instrument mass is 0.64 kg and it uses 1.5 W during operation.

The sensor head and deployment device are mounted in an opening in the floor of the balcony on the Philae lander. The deployment device can lower and raise the sensor head, placing it over the area to be sampled. The Curium source bombards the area with alpha particles and x-rays causing alpha and x-ray radiation to be emitted from the sample, allowing alpha backscatter spectroscopy (Rutherford scattering) and x-ray spectroscopy. The detectors measure this radiation to obtain energy dispersive spectra. The composition of only the uppermost atomic layers is measured. The measurement can last up to 10 hours. The composition of all major and many minor elements from carbon to nickel can be measured. The x-ray spectroscopy allows detection of all elements from sodium to nickel with concentrations above about 0.1 to 1 weight percent. The alpha spectroscopy is sensitive to low atomic number elements, such as carbon and oxygen, with concentrations above 0.1 weight percent. Hydrogen and helium cannot be measured. For most cases the docking position and surface morphology is not important to these measurements. Measurements can be made during the day when the non-volatile component should dominate, and at night when condensation on the surface may allow measurement of a volatile component. Measurements made at drilling sites will enable determination of the composition below the surface. Repetitive measurements at the same site over time will allow a determination of the effect of heliocentric distance and time evolution of the composition of the surface.

Alternate Names

  • APXS
  • Philae/APXS

Facts in Brief

Mass: 0.64 kg
Power (avg): 1.5 W

Funding Agency

  • European Space Agency (International)


  • Planetary Science: Small Bodies

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. Johannes BruecknerGeneral ContactMax-Planck-Institut fur
Dr. Goestar KlingelhoeferPrincipal InvestigatorJohannes Gutenberg

Selected References

  • Klingelhoefer, G., et al., The Rosetta Alpha Particle X-Ray Spectrometer (APXS), Space Sci. Rev., 128, 383-396, doi:10.1007/s11214-006-9137-3, 2007.
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