Numerical modeling of mapping of Martian epithermal neutron emission: Applications to FREND investigation onboard ESA’s Trace Gas Orbiter

Neutron telescope FREND is operating onboard ESA TGO mission from May 2018 till present. The instrument measures epithermal neutron flux from the Martian surface. Measured variations of the neutron flux may be used to determine hydrogen abundance in the soil usually interpreted as a water/water ice. The unique design of the instrument provides measurements of the neutron flux with a spatial resolution that is much higher than the resolution of previous neutron spectrometers operating on orbit around Mars on board the NASA Mars Odyssey spacecraft. It is known that the spatial resolution of a neutron spectrometer in an orbit around Mars is determined not only by the parameters of the detectors, presence or absence of additional modules to improve the resolution, the orbital altitude of the spacecraft, but also by the fact that Mars has an atmosphere that scatters some of the neutrons that emerge from the Martian surface. One of the ways to increase the spatial resolution of orbital neutron detectors is the passive collimation method using neutron-absorbing substances. In this paper we consider the results of numerical modeling of parameters of the collimated instrument in order to determine its spatial resolution, taking into account the presence of the Martian atmosphere. The contribution of neutrons passing through the collimator walls to the total counting rate and how they affect the estimation of the hydrogen/water concentration in the soil are also considered. Therefore, a method was developed to take into account the imperfect absorbing properties of the collimator and to clear the measured signal from the contribution of neutrons that passed through the collimator wall. Comparison of the spatial resolution of non-collimated and collimated instruments is discussed. Moreover, the results obtained with the conventional method of data processing are compared with the results of developed method, which takes into account the contribution of non-collimated neutrons. The advantage of the results obtained by the developed method is shown.