Update of the radiation environment measurement results aboard ExoMars TGO in May 2018-June 2020

Abstract

The dosimetric telescope Liulin-MO [1] for measuring the radiation environment onboard the ExoMars TGO is a module of the Fine Resolution Epithermal Neutron Detector (FREND) [2].

Here we present recent results from measurements of the charged particle fluxes, dose rates and estimation of radiation quality factors and dose equivalent rates at ExoMars TGO science orbit (circular orbit with 400 km altitude, 74⁰ inclination, 2 hours orbit period), provided by Liulin-MO dosimeter from May 01, 2018 to June 10, 2020.

Since now the dosimeter has measured the dosimetric parameters of the galactic cosmic rays (GCR). Solar particle events were not registered. The measurements were taken during the declining and minimum of the Solar activity in 24^th Solar cycle.

Compared are the time profiles of particles and neutron detections from Liulin-MO, FREND/TGO and HEND/Odyssey [3] measured for the period May 2018 –December 2019.

Liulin-MO contains two dosimetric telescopes arranged at two perpendicular directions [1]. The parameters, provided by Liulin-MO simultaneously for two perpendicular directions have the following ranges: absorbed dose rate from 10^-7 Gy h^-1 to 0.1 Gy h^-1; particle flux in the range 0 - 10⁴ cm^-2 s^-1; energy deposition spectrum and coincidence energy deposition spectrum in the range 0.08 - 190 MeV.

Similar to FREND, HEND/Odyssey is a neutron spectrometer based on ³He proportional counters but with smaller sensitivity and without collimation as in FREND [3], [4].

The fluxes and dose rates recorded in the perpendicular detectors B(A) and D(C) of Liulin-MO and count rates of Oulu neutron monitor (http://cosmicrays.oulu.fi/) for the period from 1 May 2018 to 10 June 2020 are shown in Figure 1. An increase of all quantities – flux, dose rate and Oulu count rate due to solar cycle modulation is observed. In this period in two perpendicular directions B(A) and D(C) the average values are: dose rate 14.8±1.5/15.4±1.5 microGy h^-1, planar flux 3.11/3.21 cm^-2 s^-1, quality factor Q 3.5±0.26, dose equivalent rate 1.61±0.33/1.66±0.34 mSv d^-1.

In Figure 2 is shown the comparison between the normalised (relatively to the mean daily value for the full period) daily mean Liulin-MO data and HEND data accumulated in energy channels 1 – 8, most sensitive to the charge particles of GCR. These profiles demonstrate good correlation both on a short time scale (days) and long term scale (months). The total amplitude of variations observed during 20 months of TGO mapping is about 8%. Figure 3 presents the comparison between the normalised FREND total counts rate and HEND data accumulated in the energy channels 9 – 16, mostly populated with counts from Martian neutron albedo (which is produced by GCR by interaction with matter in the shallow subsurface). These curves also show high correlation with each other both on short and long term scales.

The obtained data from May 2018 to April 2019 show that 1) An increase of the dose rates and fluxes is observed from May 2018 to March 2020 which corresponds to the increase of GCR intensity during the declining of the solar activity. From March to June 2020 the measured values are practically equal, corresponding to the minimum of the solar activity; 2) The time profiles of GCR daily variations measured by Liulin-MO and HEND demonstrate good correlation. The comparison between the normalised FREND total counts rate and HEND data accumulated in the energy channels mostly populated with counts from Martian neutron albedo also show high correlation with each other. The difference in the long term variations between the profiles of GCR and neutron albedo might be addressed to how the solar modulation changes directly the GCR flux and how it changes emission of neutron albedo by interaction with a matter in the Martian shallow subsurface.

Acknowledgements

The work in Bulgaria is supported by the Contract No. 4000117692/16/NL/NDe funded by the Government of Bulgaria through an ESA Contract under the PECS (Plan for European Cooperating States) and by Project No. 129 for bilateral projects of the National Science Fund of Bulgaria and Russian Foundation for Basic Research. The work in Russia is supported by Grant 19-52-18009 for bilateral projects of the National Science Fund of Bulgaria and Russian Foundation for Basic Research in part of HEND data analysis, and Russian Science Foundation Grant 19-72-10144 in part of FREND data analysis.

References

[1] Semkova, J., et al: Charged particles radiation measurements with Liulin-MO dosimeter of FREND instrument aboard ExoMars Trace Gas Orbiter during the transit and in high elliptic Mars orbit, Icarus 303, 53–66, 2018, https://doi.org/10.1016/j.icarus.2017.12.034

[2] Mitrofanov, I., et al: Fine Resolution Epithermal Neutron Detector (FREND) onboard the Trace Gas Orbiter, Space Sci Rev, August 2018, 214:86, https://doi.org/10.1007/s11214-018-0522-5

[3] Mitrofanov, I., et al: Maps of Subsurface Hydrogen from the High Energy Neutron Detector, Mars Odyssey, Science, 2002, Volume 297, Issue 5578, 78-81

[4] Mitrofanov I.G. et al: Search for water in Martian soil using global neutron mapping by the Russian HEND instrument onboard the US 2001 Mars Odyssey spacecraft, Solar System Research, 2003, Volume 37, Issue 5, 366-377