Dynamic Albedo of Neutrons (DAN)

PI: Igor Mitrofanov, Space Research Institute (IKI), Russia

The Dynamic Albedo of Neutrons (DAN) instrument is an active/passive neutron spectrometer that measures the abundance and depth distribution of H- and OH-bearing materials (e.g., adsorbed water, hydrated minerals) in a shallow layer (0.5-1 m) of Mars' subsurface along the path of the MSL rover. The experiment is contributed by the Federal Space Agency of Russia.

Scientific Investigation

Hydrogen is easily detected by nuclear methods. Even a small amount of hydrogen in the soil significantly increases the efficiency of moderation of secondary high energy neutrons, because in each collision, an H nucleus removes about half the neutron's energy. The High Energy Neutron Detector (HEND), part of the Gamma-Ray Spectrometer suite on the Mars Odyssey orbiter, has detected a very large deficit of epithermal and high-energy neutron emission over extensive northern and southern provinces on Mars, which has been interpreted to indicate the presence of dirty water ice. Data from orbit have a spatial resolution of about 600 km.

The next step in nuclear studies related to the search for water on Mars will be carried out at the planet's surface by DAN. While the orbital measurements rely on natural neutron emission induced by galactic cosmic rays, the DAN instrument will carry a source of neutrons. Neutron activation analysis is a far more powerful technique for detecting hydrogen and studying its vertical distribution in the soil than the passive method. The DAN instrument will be the first to use this technique for determining the soil composition of another planet. In its active mode, the DAN instrument will determine the hydrogen content of martian soil along the path of the rover by measuring the time-decay curve (the dynamic albedo) of the neutron flux from the subsurface induced by a pulsing source of neutrons. Following the pulse, less-moderated neutrons with epithermal energies escape during the first tens of microseconds, while thermalized neutrons leave the subsurface after hundreds of microseconds. The amplitude and shape of the die-away time profile strongly depends on the content and depth distribution of hydrogen-bearing materials.

The science objectives of the DAN investigation are as follows:

• Detect and estimate the abundance of hydrogen in the subsurface throughout the surface mission

• Investigate the upper 0.5 m of the subsurface and determine the possible layering structure of hydrogen-bearing materials

• Track the variability of hydrogen abundance in the upper soil layer (~1 m) during the mission by periodic analysis

• Track the variability of neutron radiation background (neutrons with energy < 100 keV) during the mission by periodic analysis

Instrument Description

The DAN instrument consists of two separate units: the detectors and electronics (DAN-DE), and the pulse neutron generator (DAN-PNG). Both are mounted on the rear panel of the rover chassis about 1 m apart and 0.8 m above the ground. The DAN-PNG produces 1-microsecond pulses (FWHM) of emission of about ten million 14.1-Mev neutrons each, at frequencies up to 10 Hz. The DAN detectors are He-3 proportional counters. One of the detectors is surrounded by a cadmium enclosure to prevent registration of thermal neutrons and is able to detect neutrons with energies above thermal, up to and including epithermal neutrons. The second detector registers both thermal and epithermal neutrons (<0.1 MeV). The detectors measure the die-away curve with resolution in both time and energy (16-channel energy spectrum × 64 lognormal time bins for each detector). The instrument can also be used in a passive mode in which the DAN-PNG is inactive and the detectors integrate measurements into energy bins.

Operations Strategies

In active mode, DAN measures the time decay curve (the "dynamic albedo") of the neutron flux from the subsurface induced by its pulsing 14 MeV neutron source (see figure below). The DAN instrument is expected to be used during rover traverses (e.g., during short stops at ~1 m intervals) and while the rover is parked. Short-duration (< 2 min) active measurements will provide a rough estimate of the water-equivalent hydrogen distribution with an accuracy of ~1% by weight. Longer-duration (~30 min) active measurements are necessary to derive the vertical distribution of water-equivalent hydrogen with an accuracy of 0.1-0.3% by weight.

Passive measurements may be scheduled whenever the rover is awake for the purpose monitoring the average soil water content, the natural neutron background, and the temporal evolution of solar particle events.

Numerical simulations of the neutron count rate versus time (i.e.,die-away curve) for the unshielded (left) and shielded (middle) detectors as a function of water abundance. The right panel shows the difference between the two count rates.

Additional Information

Litvak, M. L., et al. (2008), The Dynamic Albedo of Neutrons (DAN) experiment for NASA's 2009 Mars Science Laboratory, Astrobiology, 8, 605-612.