Results 2015


TASK 1: PERSPECTIVE DETECTORS.
According to the plan of TASK 1 investigations for the 2nd year, studies were accomplished of innovated scintillation detector with the crystal of cerium bromide. It was experimentally shown that the usage of cerium bromide for gamma-ray spectroscopy allows to increase significantly the signal-to-noise ratio for the nuclear lines at the broad energy range up to 3 MeV in comparison with the lanthanum bromide crystal. Comparison between spectra of gamma-rays emission of analog samples of planetary substance under neutron irradiation, which are measured by the detector with cerium bromide and by the industrial detector with high purity germanium, has shown that the first one allows to identify and to evaluate practically all major nuclear lines, as were detectable by the second one. Using the results of the RSF project, the recommendations are developed for creation of the detectors of gamma-rays for space projects for studies the Moon, Mars, Venus and other celestial bodies.

In 2015 the laboratory researches for TASK 1 was also performed to study the possibility to use the silicon semiconducting PMT (SS-PMT) for scintillation light collection of gamma-ray and neutron detectors. The main design advantages of SS-PMT for space scientific instruments in comparison with the traditional electronic vacuum PMT (EV-PMT) are smaller mass and the absence of HV provision requirement for counts recording. It was shown that tested unit of SS-PMT does not have the necessary equality of amplitudes of photo-diodes, which would correspond to such resulted spectral resolution of the detector, which one should obtain provided the EV-PMT would use. Studying of opportunity to use the unit of SS-PMT for neutron detectors with the stylbene scintillator pointed out that much shorter processing time is necessary for SS-PMT count measurement to ensure the possibility of effective separation between counts of photons and counts of neutrons based on their pulse profiles. Therefore, the recommendations were suggested, as based on accomplished studies, for further development of SS-PMT for making them appropriated for spectrometers of gamma-rays and for detectors of high energy neutrons.

As part of the TASK 1 research, conditions were studied in the 2nd year of the project of so-call microphone effect in scintillation detectors of neutrons and gamma-rays with EV-PMT under the vibromechanical loading onboard the spacecraft. For a particular EV-PMT the level of vibration amplitude was found about 0.05 g for appearance of the effect. With increasing amplitude the effect is observed in broadening spectral range, and for the amplitude of 0.3 g the effect is observed practically at all frequencies of vibration from 300 Hz up to 1 kHz. Vibration tests will go on in the third year of the project for selection of the best PMT type for the perspective scintillation detector for space applications.


TASK 2: LABORATORY EXPERIMENTS FOR TESTING AND MODELING.
Studies for TASK 2 were performed at two specially created testing facilities in the Joint Institute for Nuclear Research (JINR). The first one TF-NP-1/2 was used to continue in the second your measurements, which were started in the 1st year of the project. Neutron albedo emission was measured from the analog sample of the planetary substance under irradiation with pulses of 14 MeV neutrons of pulsing neutron generator with the vacuum tube. The sampling target was a massive glass prism, which contains a layer of polyethylene at depth from 0 down to 60 cm, which simulate the layer of water ice in case of planetary condition. Obtained results prove the theoretical estimations of the sensitivity of the method of active neutron sensing for the presence of water bearing material down to the depth of 60 cm. These results were used for data analysis and interpretation of DAN experiment onboard the NASA rover Curiosity on the crater Gale on Mars.

After the program of measurements with TF-NP-01/02 was accomplished, the thick target was re-built to the new one of TF-NP-04 with additional layers of alloys of aluminum and iron. Also, a layers of polymer were added to simulate the presence of chlorine in the target. The set of layers was selected to simulate in the first approximation the content of the main elements in the martian soil ((~27% of silicon, ~38% of oxygen, ~13% of iron, ~8% of sodium, ~5% of aluminium, ~5% potassium and ~2% of magnesium). The comprehensive testing program was started at TF-NP-04 at the 2nd year of the project with detection of secondary neutron and gamma-rays from the target under irradiation from the pulsing neutron generator with vacuum tube. Time profiles of gamma-ray lines emission are measured in respect to the moment of neutron pulse generation. It is shown that analysis of this emission allows to determine the elementary composition of irradiated substance. This testing program is planned to continue at the third year 2016 of the project implementation, and in addition to the currently available pulsing neutron generator with the vacuum tube the new purchased pulsing neutron generator with gas-filled tube will be used, which will allow to measure gamma-ray lines of is expected to be used together with the currently available generator with the vacuum tube. The generator with vacuum tube allow to measure the gamma-ray lines from the capture reactions only, while the generator with gas-filled tube allows to measure both the set of lines from capture reactions as well as the set of lines from in-elastic scattering. The comprehensive reference library of nuclear lines of soil constituting elements and their isotopes is necessary for confident evaluation of elementary composition of the planetary substance under the bombardment by galactic cosmic rays or by irradiation by neutrons from pulsing generator. In the second year of the Project the experimental program of measurements in the testing facility TF-NP-03 has been initiated, as part of TASK 2 investigations, for characterization of gamma-ray emission from analog samples of planetary substance in the flux of epithermal and thermal neutrons. Energy and intensity of lines of sodium chloride were measured. In the 3rd year of the Project this program will be continued for oxides of the main soil constituting elements, such as SiO2, Al2O3, TiO2, MgO, CaO2, Cr2O3, etc.


TASK 3: ANALYSIS OF PLANETARY SOIL COMPOSITION.
Numerical simulation of nuclear processes (TASK 3 of the Project) is the necessary part of experimental studies of the content of planetary substance by the methods of nuclear science. For getting the conclusive results of space studies by nuclear methods, the package of codes for numerical calculations should be validated using the comparison with laboratory data on the ground. As part of TASK 3 researches, calculations were performed by such codes for neutron albedo of glass think target of TFNP- 01/02 with a layer of polyethylene under irradiation by pulsing neutron generator with the vacuum tube. Good agreement was achieved between calculated values and the experimental data for secondary neutron emission from TF-NP-01/2. After that more complex simulation was performed for the secondary neutron and gamma-ray emission for the glass target with layers of aluminum and iron allows and chlorine polymer on the facility TF-NP-04. Good agreement between measurements and calculations for neutrons and gamma-rays of the TF-NP-04 facility proved the high confidence of numerical models of nuclear processes in the planetary substance. As part of TASK 3 researches, data of LEND experiment onboard the NASA LRO mission were studied using the numerical calculations for neutron emission from the surface of the Moon based on the twolayer model of water ice bearing regolith in the subsurface. If was found from this analysis that the substance of the polar crater Cabeus contains at least 0.54% mass fraction of water ice, provided the upper most dry layer is absent, and this fraction goes up to as much as 11%, provided the uppermost dry layer is as much thick as 100 cm.

Numerical calculation for TASK 3 investigation were performed on high-speed personal computers mainly, which, however, did not allow to get the numerical result for data interpretation of particular single measurement faster than after several days. This situation was particularly unfavorable for conduction of DAN experiment on Mars – after data receiving at the radio session with the rover, one needed about three days for their scientific interpretation. Rover might go about several hundreds of meters from the spot of measurements during this time. This situation gas changed drastically at the end of the 2nd year of the project, when the cluster calculation system CCS-NP was purchased by RSF funds of this project. Already performed testing calculations for numerical modeling of neutron an gamma-ray emission of celestial bodies shown that CCS-NP decreases the time of calculation by a factor of 60, which allows to perform practically synchronous the data analysis along the process of measurements. Comprehensive study of DAN data was performed for active neutron sensing of Mars soil from the NASA rover Curiosity using the cluster CCS-NP. It was found for the sample of 412 individual active measurements along the 11 km traverse that 78% of measured spots have homogeneous distribution of water in the subsurface, and its average content within the uppermost layer of 60 cm is 2.1 ± 0.5%. For 22% other measured spots the data confidently point out on the two-layer distribution of water in the subsurface, and the upper most layer for these spots contains 2 – 3% of water similarly to spots with the homogeneous water distribution. One fraction of spots with two layers, which constitute 8% of the total sample, has the increased content of water 5.6 ± 2.7% at the bottom layer at the depth of 27 ± 18 cm. Another fraction, which constitute 14% of the total sample, has the decreased content of water 1.2 ± 0.5% in the bottom layer at the depth of 14 ± 7 cm. It is proposed for explanation of these findings that crater Gale has distinct regions with high and low content of ground water, which correspond to different sedimentary layers produced at dry and wet past epochs of Martian history at deposition processes at wet and dry environment, respectively.