Active detector gamma-ray and neutron (ADRON) for Luna-Resurs and Luna-Glob missions

 

Project Objectives Вверх

The task of the instrument is to measure local (near the landing site) elemental composition of the lunar surface using active neutron and gamma-ray spectrometry.

The experiment is currently under development within the Russian projects Luna-Glob and Luna Resurs with the Agreements between Federal Space Agency of the Russian Federation and Lavochkin Association. Launches of the spacecrafts are scheduled in 2017 and 2019.

ADRON is specifically aimed at measuring neutron and gamma-ray fluxes during interplanetary cruise to the Moon and at lunar surface.

ADRON is developed to study composition and content of major elements in the lunar soil in the immediate vicinity of the landing site, as well as to study stratified structure of the lunar soil with spatial resolution ~1 m.

Operation Principle Вверх

To achieve the stated goals we use neutron probing (bombarding of the soil with high-energy neutrons) together with neutron and gamma-ray spectroscopy (registering neutron and gamma-ray response in various energy ranges with high time resolution).

ADRON will be used in active experiment for pulse neutron probing of lunar matter onboard Landers. instrument will allow to recieve data of lunar surface composition up to 1 m deep immediately near stationary or moving automated lunar craft. This data will be used to study origin of lunar surface, to compare lunar samples having been brought back to the Earth with the average surface composition up to 1 m deep, as well as to search for minerals and resources, presumably available for mining.

ADRON instruments can be used in both active and passive modes, with active mode being the main one. In this mode neutron generator module emits powerful neutron pulses with energy of 14 MeV, every pulse up to 2 microseconds long consists of approximately 107 neutrons. These neutrons are emitted isotropically, and their significant part (around 30–50%) penetrates lunar soil immediately under the Lander to the depth of around 1 m. These neutrons interact with soil nuclei, and excite them via inelastic scattering reactions (being hit by a neutron a nucleus gets excited and then falls back to the ground state after emitting gamma-ray quantum with specific energy), and then are absorbed by these nuclei via capture reactions (being hit by a neutron a nucleus captures it, gets into excited state of another isotope, which then falls back to the base state after emitting gamma-ray quantum) (Fig. 1).

Fig.1.Physical scheme of neutron probing in order to define composition of the sample

Thus, emitted gamma-rays travel from the surface, and their spectrum represents nuclear composition of the lunar soil (Fig. 2). Remaining neutrons are moderated in the lunar soil and get back from the surface with energies close to the thermal energy of the soil, and their flux strongly depends on hydrogen content in the regolith. Hydrogen nuclei are protons, whose masses are about to that of neutrons, so that even small share of hydrogen in the regolith increases neutron moderation efficiency substantially.

Fig. 2.Epithermal neutron flux from the lunar surface (indicated is count rate registered by a neutron detector) dependence on hydrogen content in the latter (H in ppm)

The more hydrogen there is in the soil, the scarcer are epithermal neutrons and the more abundant are thermal neutrons in the outcoming flux. Epithermal neutrons flux falls rapidly with increase in hydrogen concentration, so that measuring epithermal neutron flux from the surface generated by high-energy neutron pulses shall be considered an effective way to estimate hydrogen (i.e. water) content in the lunar soil.

Pulsing mode of irradiating soil with neutrons allows to study stratified structures in the nuclei distribution down to the depth of around 1 m. Outcoming moderated neutrons reach the surface within considerable time (around 1 millisecond) due to diffusion, and stratified deposition of hydrogen influences the profile of outcoming neutron flux significantly.

Neutron generator does not work in passive mode. In this mode neutron and gamma-ray detectors measure natural flux of lunar neutron and gamma-ray emission, which emerges due to interactions with galactic cosmic rays or solar energetic particles emitted during solar proton events. Data obtained in passive mode will allow to assess natural neutron and gamma-ray background in the polar region where the spacecraft is expected to land and compare it to to the measurements made in other lunar regions as well as to the data obtained in orbit around the Moon. Gamma-ray measurements in the passive mode could be also used to estimate the abundances of primordial radioactive isotopes of K, Th, and U. It is well known that K and Th relative content remains constant for all lunar samples, which have been returned to the Earth earlier, and it also remains the same when measured from the orbit (Fig. 3), although one might expect that near the poles this ration may change due to peculiarities of polar regions' formation and their location close to the gigantic Aitken impact crater.

Fig. 3.Difference between distribution ratio of natural radioactive isotopes on surfaces of the Moon and Mars

Design and Mount Вверх

ADRON instrument is a unique development of IKI RAS and does not have counterparts abroad. Its prototype is DAN instrument, developed by IKI RAS for mount aboard Mars Scientific Laboratory automated rover (NASA). DAN shall run pulse neutron probing of Martian surface along the MSL path to look for regions with enhanced water content in the surface shallow layer. Prior to DAN acceptance as a part of MSL's scientific payload by NASA, it was compared to other variants of similar instruments developed by Western scientific centers. As a result, DAN was acknowledged to surpass similar foreign instruments in its lifetime and neutron pulse intensity. These advantages are applicable to the neutron generator ADRON-PNG in the same extent.

ADRON exploits new type of scintillation detector on the basis of LaBr3 to register gamma-ray emission. This detector possesses extremely high spectral resolution around 3% at 662 keV in comparison to other scintillation detectors, thus gamma detector unit will comply with the most advanced world scientific achievements in the field.

ADRON-LR includes detector module (Fig. 4), pulsed neutron generator (Fig. 5).

Fig. 4.DE module of the ADRON instrument

Fig. 5.Pulsed generator of the ADRON instrument

ADRON makeup is shown in the Table 1.

Table 1

Unit or module name Task
1. ADRON-PNG pulsed neutron generator ADRON-PNG is a separate module, aimed at generating neutron pulses with energies around 14 MeV to irradiate lunar surface in the landing site
2. ADRON-DN Neutron detector unit ADRON-DN is an integral part of ADRON-DE module, and it is aimed at registration of epithermal and thermal neutron fluxes in active working mode together with the generator, as well as in passive working mode while measuring natural neutron background at the landing site.
3. ADRON-GD Gamma detector unit ADRON-GD is an integral part of the ADRON-DE module, and its task is to register and carry on spectral analysis of the gamma-ray flux in active working mode together with the generator as well as in passive working mode while measuring natural neutron background at the landing site.
4. ADRON-ME electronics unit ADRON-ME is an integral part of the ADRON-DE module. Its task is to generate secondary power supply for instrument's units, to control its performance, to process and transmit scientific and service telemetry data to the spacecraft, to monitor and testing of the subsystems and nodes of the instrument.

The instrument will measure the following parameters (see Table 2) during lunar cruise phase and at its surface.

Table 2

Physical quantity Measured parameters Measuring unit
Thermal and epithermal neutron flux Neutron count rate in the energy range 0.025 eV – 1.0 keV STN detector
Epithermal neutron flux Neutron count rate in the energy range 0.4 eV – 1.0 keV SETN detector
Gamma-ray spectrum Gamma-ray spectrum in the energy range 100 keV – 8 MeV with energy resolution not less than 3% at 662 keV line of 137Cs with temporal resolution up to 1 microsecond GRS detector

For ADRON-LR-DE module serially produced proportional counters on the basis of 3He are used (to register neutrons). Similar devices were also used in Russian HEND, NS HEND, DAN, MGNS scientific instruments (developed by IKI RAS). ADRON-LR-DE electronics is based on design developed for the instruments mentioned above and successfully tested for long-term work in space and/or during ground tests. Main differences of the ADRON-LR-DE module from DAN-DE module concern scintillator on the basis of LaBr3 crystal.

DE unit includes following nodes:

  • two detectors STN and SETN on the basis of proportional neutron counters. Detectors differ from each other by their external shields made either of cadmium (for SETN) or lead (STN).
  • scintillation unit, comprising cylinder-shaped LaBr3 detector and photomultiplier.
  • electronics nodes.

Main parameters Вверх

Mass : 3.8 kg — ADRON-LR-DE
2.6 kg — ADRON-LR-PNG
Power consumption : 4.5 W — DE
14 W — PNG
Dimensions : 270 x 215 x 110 mm — DE
340 x 125 x 45 mm — PNG
Energy rangec : 0.025 eV – 1.0 keV (neutrons)
100 keV – 6 MeV (gamma rays)
Energy resolution (gamma rays) : 3% at 662 keV
Time resolution : from 1 mks
Surface resolution : ~3 m
Depth resolution : ~1 m
Telemetry : 20 Mb per day

Developers and Co-Executors Вверх

Funding Agency — Federal Space Agency of Russian Federation.

Primary contractor — Space Research Institute of the Russian Academy of Sciences (IKI RAS).

Principal Investigator — Dr. Igor Mitrofanov.

Works on ADRON-LR project are led on the basis of the State contract №361-9870/09 from March 16, 2009 (R&D theme MSP «Luna-Resurs»).

Works on ADRON-LR project are planned for 2009–2015 (development, tests, assembling, and instrument delivery) and 2015–2018 (operating and data processing).

 
Federal State Unitary Enterprise 'N.L. Dukhov All-Russia Research Institute of Automatics'
(VNIIA, Moscow)
Neutron generator module development for ADRON instrument.
N.M. Fedorovsky All-Russian Scientific Research Institute for Mineral Raw Materials
(Moscow)
Development of scintillation units for neutron and gamma registration.
Joint Institute for Nuclear Research
(Dubna, Moscow region)
Numerical modeling of ADRON counting parameters; participation in the development of ADRON physical scheme; preparation for and calibrations of the instrument.
A.A. Blagonravov Institute for Engineering Science of the Russian Academy of Sciences
(IMASH, Moscow)
Development of mathematical model of instrument mechanical structure; participation in the development of testing facilities for ADRON instrument in accordance with project requirements; development of mechanical test program; support of the instrument testing.
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