Payloads : 9. Sub Atomic Reflection Analyser (SARA)

The SARA will image the surface using low energy neutral atoms (up to iron) in the energy range of 10 eV to 2 keV. It consists of a low energy neutral atom sensor and a solar wind monitor. The neutral atoms, after sweeping away the ambient charged particles by an electrostatic deflector, are converted to positive ions on an ionisation surface and then enter the sensor. The particle velocity
is measured by time of flight measurement and the energy and mass are deduced by electrostatic analyzer. The mass resolution is  such that H, O, Na–Mg, K–Ca and Fe group elements can be distinguished. Since the Moon does not have a magnetosphere or  atmosphere, neutral atom density in the Moon’s environment is extremely small, produced mainly by sputtering due to solar wind ions. The contribution due to micrometeorite vaporization and solar photon simulated desorption is estimated to be small in this low energy region of interest. LENA imaging of the neutral atoms will thus provide maps of the sputtered elements which can be converted into surface composition maps, making suitable corrections for the sputtering yield and the solar wind flux, which depends on the cosine of the solar zenith angle. The Moon does not have a magnetosphere but small magnetic anomalies, strong enough to shield them to solar wind have been observed. In these regions, the neutral atom density will be small as sputtering due to solar wind would be absent. Thus LENA can detect magnetic anolmalies. For these measurements it is desirable to monitor the solar
wind flux, which will be done by the SARA ion mass analyzer. The instrument and its operation is described in detail in the   paper by Bhardwaj (Bharadwaj A, Barabash S, Futaana Y, Kazama Y, Asamura K, Sridharan R, Holmstrom M, Wurtz P and Lundin R 2005 Low energy neutral atom imaging on the Moon with the SARA instrument aboard Chandrayaan-1 mission;)