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Gamma Ray/Neutron Spectrometer (GR/NS)

NSSDCA ID: 2007-043A-03

Mission Name: Dawn
Principal Investigator:Dr. William C. Feldman


The gamma-ray/neutron spectrometer (GR/NS) is designed to map major element (O, Si, Fe, Ti, Mg, Al, Ca) and trace element (U, Th, K, H, Gd, Sm) composition of the surfaces of Vesta and Ceres in order to help determine their geological composition. Additionally, the GR/NS will be able to detect hydrogen, indicating the presence of water, to determine the level of hydration of the asteroidsí crusts. Abundances can be measured during the 130 days at Vesta and Ceres when Dawn is within 130 km of the surface. The GR/NS is an integrated system held in a 25.7 x 18.0 x 20.3 cm box mounted on the nadir (asteroid) facing deck of the Dawn spacecraft. At the top of thebox is the gamma-ray sensor, a squared-off version of the scintillator flown on Lunar Prospector. The gamma ray sensor consists of a 4 x 4 array of 1.0 x 1.0 x0.75 cm cadmium zinc telluride (CZT) sensors facing in the nadir direction. Below this is the neutron detector, consisting of a 7.6 x 7.6 x 5.08 cm slab of bismuth germinate (BGO) connected to a 7.6 cm diameter photomultiplier tube. Both sensors are surrounded on five sides by four segments of a 2.5 cm thick borated plastic scintillator (BC454) anticoincidence shield (ACS). The nadir and spacecraft facing segments of the ACS are laminated with 2 mm of lithium 6 glass scintillator (GS20) optically coupled to the BC454 and surrounded on their sides by 1 cm thick sheets of lithium 6 loaded polyethylene to provide a separation between incoming thermal and epithermal/fast neutrons. Each segment of the ACS is attached to a 2.5 cm diameter photomultiplier tube. The BGO also acts as an active shield for the CZT sensors to minimize spacecraft contribution. The front end electronics classifies each event detected into a category dependent on the combination of photomultiplier detections and readings from the CZT detector array. Event data is sent back in a 3 kb/s data string with an accumulation time of 60 s. Multiple signals from each event can allow determination of direction and enable distinction between asteroid generated and spacecraft generated radiation and between fast, epithermal, and thermal neutrons. Separation of the GS20 events from the BC454 events is done using time-domain filters. The pulse height of the first pulse of a pair of a fast neutron event can be used to estimate the energy. Gamma ray spectra are recorded separately.

Alternate Names

  • Dawn/GR/NS
  • GR/NS
  • GRaND
  • Gamma Ray and Neutron Detector (GRaND)
  • urn:nasa:pds:context:instrument:dawn.grand

Facts in Brief

Mass: 10.5 kg
Power (avg): 9 W


  • Planetary Science: Small Bodies

Additional Information

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



NameRoleOriginal AffiliationE-mail
Dr. William C. FeldmanPrincipal InvestigatorLos Alamos National

Selected References

  • Prettyman, T. H., et al., Gamma-ray and neutron spectrometer for the Dawn mission to 1 Ceres and 4 Vesta, in IEEE Trans. Nuclear Sci., 50, No. 4, 1190-1197, doi:10.1109/TNS.2003.815156, Aug. 2003.
  • Prettyman, T. H., et al., Gamma-ray and neutron spectrometer for the Dawn mission to 1 Ceres and 4 Vesta, Space Sci. Rev., 163, 371-459, doi:10.1007/s11214-011-9862-0, 2011.
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