Lunar gamma-ray and neutron spectrometer (LGNS) for the Luna-26 Orbiter of the Luna-Resource-1 project
Instrument application 
The LGNS instrument is designed for remote study of the Moon's regolith using nuclear planetology methods with the aim to determine the regolith elemental composition on the basis of measurements of secondary neutron and gamma-ray radiation. The instrument will be mounted aboard the Luna-26 Orbiter of the Luna-Resource-1 project. The composition and designation of individual modules of the LGNS are presented in the table below. The LGNS prototypes are HEND operating on Martian orbit since 2001 as part of scientific payload of NASA’s Mars Odyssey mission, and MGNS designed for Mercury researches onboard the ESA BepiColombo mission.
| № |
Block or module name |
Designation |
| 1 |
Gamma-ray detector module LGNS-GD |
LGNS-GD is designed for recording and spectral analysis of γ-rays flux in the conditions of passive measurements of natural background of γ-radiation arising in the lunar soil under the influence of high-energy charged particles of cosmic rays. |
| 2 |
Neutron detectors module LGNS-ND |
LGNS-ND is designed for recording and spectral analysis of neutron flux in the conditions of passive measurements of the natural background of neutrons arising in the lunar soil under the influence of high-energy charged particles of cosmic rays. |
| 3 |
Electronics module LGNS-EM |
LGNS-EM is designed to generate secondary power for electronics and detector modules, to control their operation, to process and transmit the data of housekeeping and scientific telemetry to the Orbiter data storage subsystem, to control the instrument operation, to monitor and troubleshoot all systems and modules of the instrument. |
Object and research methods
The LGNS instrument is designed to measure secondary neutron and gamma-ray radiation arising in the upper subsurface layer of the lunar regolith under the influence of energetic particles of galactic cosmic rays (GCR). The main component of the GCR are protons with energies above 100 MeV. GCR particles, penetrating into the subsurface of the Moon, interact with the nuclei of rock-forming chemical elements and in the result of nuclear reactions form secondary charged particles, fast neutrons and excited nuclei of various isotopes. Excited nuclei of rock-forming elements emit gamma-rays with energies characteristic of the given nucleus.
As a result of chaotic wandering (diffusion), the formed fast neutrons collide with the nuclei of rock-forming elements, lose energy to some extent, can escape the surface and subsequently be registered by the instrument on the orbit. Since the neutron mass is very close to the mass of the hydrogen nucleus (proton), hydrogen is the best neutron moderator. Thanks to this, by conducting a spectral analysis of the flux of neutrons emerging from the regolith, it is possible to accurately determine the concentration of hydrogen nuclei in regolith (Figure 2). To solve this task, it is useful to know the mass fraction of the nuclei of other chemical elements included in its composition. Measuring the intensity of lines in the spectra of gamma-rays emitted from the celestial body surface (Figure 3), allows one to determine the abundance and prevalence of such chemical elements as H, O, Na, Mg, Al, Si, K, Ca, Fe, Th over the Moon’s surface.
Fig. 1. Dependence of the epithermal neutrons flux (counting rate in the neutron detector is indicated) from the lunar surface (different types of lunar soils are shown in different colors) as function of hydrogen concentration (H, in units of ppm = 10-6 by mass).
Fig. 2. Results of numerical modeling of the spectral density of gamma-ray flux from the surface of the Moon.
In addition to the cosmic rays, another cause of gamma-radiation from the surface of a celestial body is the presence of unstable isotopes of chemical elements (natural radioactivity). Some isotopes have a half-life long enough to remain since they were formed as a result of nuclear fusion billions of years ago. For example, the potassium isotope 40K has a half-life of 1.25 × 109 years. During the decay of the 40K nucleus, a 40Ar nucleus in the excited state is formed, which subsequently passes into the ground state with emission of a gamma-ray with an energy of 1.461 MeV. Intensity of the line with this energy in the measured spectrum reflects the concentration of the 40K isotopes in the regolith. Other chemical elements with natural radioactivity that can be detected by gamma spectroscopy of a celestial body surface include such elements as thorium and uranium. Some isotopes formed in the process of decay chains of these elements also emit gamma-lines. For example, thorium can be detected by a gamma line of 2.615 MeV emitted by one of the decay products - 208Tl. In addition to this gamma-line, several other strong gamma-lines appear in result of the 232Th decay process. In the 238U decay chain, the largest gamma-ray fluxes are recorded in the gamma lines 1.764, 1.120, and 0.609 MeV.
It is known that the relative content of K and Th (the so-called K/Th ratio) remains constant for all samples of lunar soil delivered to the Earth, and it also remains the same for the measurements made from orbit (Figure 3). However, one may expect that in the Moon polar regions this ratio may change because of the peculiarities of the surface formation processes in polar regions and a close proximity to the South pole of the giant ancient impact basin South Pole - Atkin. Measuring the K/Th ratio can provide important scientific information about the conditions of the Moon formation.
Figure 3. The difference in the K/Th ratios on the Martian and Lunar surfaces.
Expected results
Thanks to orbital measurements of neutron fluxes and gamma rays of different energies, LGNS will provide data on the substance composition (including hydrogen concentration) of the Moon at a depth of up to 1 meter. These data can be used to determine the origin of the lunar surface, to compare the composition of samples of lunar regolith returned to the Earth with the average composition at a depth of 1 meter and, also, for the exploration of minerals potentially suitable for the development of lunar resources. One of the most important minerals is water ice, probably present in the polar regions of the Moon.
The main measurements that are planned to be performed with the LGNS instruments and the corresponding results that will allow to study the properties of the moon substance are listed here below.
| Measurements |
Moon researches based on LGNS measurements |
| Spectrum of the secondary gamma-ray flux due to the irradiation of the Lunar surface by Galactic and Solar cosmic rays |
Map of the content of main rock-forming elements |
| Gamma-ray flux from natural radioactive isotopes K, Th, and U |
Map of the contents of natural radioactive isotopes K, Th, and U |
| Ratio of thermal neutron flux with medium and high energy neutron flux. |
Map of average hydrogen content in the regolith to a depth of ~ 1 m. |
| Measurement of natural radiation gamma-ray background of the Moon in conditions of active and quiet Sun. |
Measurement of radiation background at the orbit around Moon and comparison with measured data for landing sites in the polar regions and at middle latitudes |
The application area of the LGNS instrument is to conduct a space experiment on passive sensing of the lunar regolith from the Luna-26 spacecraft of the Luna-Resurs-1 project. The results obtained in the LGNS experiment will be used to construct a complete physical model of the processes of formation and evolution of the Moon's surface in polar regions. These results can also be of great practical importance: on the basis of LGNS measurements and other scientific instruments of the Luna-26 orbiter (Luna-Resurs-1), areas for the deployment of lunar space infrastructure will be selected. Chemical composition and water content in the polar lunar regolith will be assessed based on LGNS measurements. In future, on the basis of this assessment, space technologies will be developed to supply lunar manned and robotic complexes with water, oxygen and hydrogen directly on the spot.
Specifications
The composition of the instrument
The scientific instrument LGNS consists of the following modules:
- gamma-ray detector module (LGNS-GD);
- neutron detector module (LGNS-ND);
- Electronics module (LGNS-EM).
All modules are combined into one monoblock (see figure below).
LGNS-GD gamma-ray detector module
The main element of the detector is CeBr3 scintillation crystal, which has good sensitivity in the energy range of 300 keV - 6 MeV and spectral resolution of ≤ 5% (at 661 keV energy) at normal temperatures and low natural intrinsic background. To register scintillations generated in the crystal by gamma-rays, a photomultiplier, electronics of the divider and of the signal amplitude digitizer are used.
LGNS-ND neutron detector module
This module consists of three proportional detectors filled with 3He gas, two of which are shielded with cadmium shells with 0.5 to 1 mm thickness. One detector is placed in a polyethylene shell, which makes it possible to detect epithermal neutrons with energies in the range of 0.4 eV - 500 keV. Each detector has an aluminum case, which protects the detector from mechanical stress.
The high-energy neutron detector (> 400 keV) is based on an organic stilbene scintillator and is surrounded by a plastic scintillator with anti-coincidence logic to prevent charged particles from being registered by an internal detector. The module is placed in an aluminum casing to prevent from mechanical stress.
LGNS-EM electronics module
The electronics module consists of the following components:
- Analog electronics unit (charge sensitive amplifier, low threshold discriminator, input signal-shaping amplifier, peak detector, input signal converter to a digital code);
- High-voltage power supply unit contains six low to high voltage converters for power supply of detectors included in the modules of neutron and gamma-radiation detectors;
- Low-voltage power supply unit forms the necessary supply voltage - +5, +12, -12 V;
- Main electronics unit includes high-voltage power source control system, receivers and transmitters of external signals, the instrument control circuit, RAM for storing scientific and housekeeping data;
- Temperature Measurement System - temperature sensors are installed in the instrument case and at critical points of printed circuit boards. Temperature sensors are designed to measure temperature and record telemetry data.
Main parameters
Main parameters of LGNS instrument are given in the table below. The instrument is presented in Figure 4.
Main measuring functions of the LGNS detector modules are:
- -measurement of gamma-radiation spectra (4096 channels) in the energy range of 300 keV - 6 MeV with an accumulation time of 1 s and a time reference to universal time (UTC) not worse than 10 ms;
- -measurement of 16-channel spectra of thermal, epithermal and fast neutrons.
LGNS will be positioned at an outboard design element of the spacecraft (boom) to reduce the contribution to the recorded signals of secondary radiation arising in the SC structure elements under the influence of the GCR.
| Instrument mass : |
≤ 7 кг |
| Operational power consumption : |
< 6.5 W |
| Power supply : |
27 V |
| Dimensions : |
342.6 x 140.5 x 258 mm |
| Data transfer rate from the instrument : |
6 Mbps |
| Information capacity : |
< 770 MB / day |
| Storage of internal memory : |
2*512 KB |
| Instrument polling rate : |
1 time per 1 second |
| Time synchronization : |
Not worse than 10 ms. |
Figure 4. The appearance of LGNS.
Developers and subcontractors
Primary contractor ― Department 63, Space Research Institute of the Russian Academy of Sciences (IKI RAS).
LGNS Principle investigator – Dr. Igor Mitrofanov.
A.A. Blagonravov Institute for Engineering Science of the Russian Academy of Sciences
(IMASH, Moscow) |
Development of mathematical model of the instrument mechanical design; participation in the creation of a testing base for the LGNS instrument, in the preparation of methods for mechanical tests of models and support for testing models of the instrument. |
Joint Institute for Nuclear Research
(JINR, Dubna, Moscow region) |
Numerical modeling of MGNS counting parameters; participation in the development of MGNS physical scheme; preparation for and calibrations of the instrument with neutron and gamma-ray sources testbed. |
All-Russian scientific research Institute of mineral resources
(VIMS, Moscow) |
Development of scintillation blocks for registration of fast neutrons and gamma quanta. |
