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BTN-Neutron-2 space experiment onboard Russian Orbital Segment of International Space Station

Project Objectives

The main task of the space experiment is to study neutron energy spectrum and their temporal and spatial distribution both inside and outside International Space Station in order to discriminate between solar neutron fluxes, albedo neutrons coming from the Earth's atmosphere, and neutrons born within the ISS elements. Another task is to study shielding properties of different materials, which can be used in the future for radiation shelters during interplanetary flights and manned expeditions to the Moon and Mars.

Thus, the objectives of BTN-Neutron-2 space experiment are:

• to study physical and technical parameters of different radiation protective shields of the inner detector module, which are basically special removable plates of various types as follows:
  • using hydrogen-bearing materials;
  • using boron-bearing materials with boron enriched with ¹⁰B isotope;
  • foils containing rare-earth elements and other materials, which effectively slow down and absorb neutrons;
• to advise on prospective design of radiation shelters in spacecrafts for interplanetary flights and manned expeditions to the Moon and Mars;
• to register neutrons and gamma-rays in the near-Earth space, thus contributing to the physical model describing the generation of albedo neutrons born in the Earth's atmosphere and local neutrons;
• to develop essential elements of protective collimator for next-generation neutron spectrometers.

Additionally, real-time monitoring of hard ionizing radiation and neutron fluxes in wide energy range, which can be performed during BTN-Neutron-2 experiment, will be used to warn the crew on the increased radiation background during powerful solar flares.

Space Neutron Sources

It is known that in the ISS' s orbit altitude three physical processes are responsible for generation of neutrons.

Firstly, solar and galactic cosmic rays generate primary neutrons with energies of about 10–20 MeV as a result of nuclear reactions with the nuclei of the Earth's upper atmosphere. These newborn neutrons are slowed down, part of them gets absorbed in nuclear reactions resulting in the emergence of new nuclei, another part decays, but the main part leaves the atmosphere, as if reflected, and enters the near-Earth space. For this reason, they were called 'neutrons albedo of the Earth'. This shell of neutrons surrounds the Earth permanently. Neutron flux and energy spectrum depend on the local flux density and the energy of primary cosmic rays' particles as well as on density, temperature, and composition of the upper atmosphere. These neutrons have energies between thermal up to several MeV.

Secondly, in the vicinity of and inside the ISS, as any other spacecraft, neutron radiation is generated due to interactions between energetic charged particles and materials the station is made of. This radiation has characteristic dimensions comparable with that of the station, and is also variable due to the variations of cosmic ray fluxes along the path of the station. The energy of these (local) neutrons may reach 100 MeV and even more, and the flux density exceeds that of albedo neutrons tenfolds. It is these neutrons that pose the highest danger for the crews in near-Earth orbits as well as in future interplanetary expeditions.

The third source of neutrons in the near-Earth space is solar activity. High-energy neutron fluxes are generated during some energetic solar proton events (or proton storms). Since a neutron's lifetime is about 15 minutes, which is long enough for a relativistic particle to travel the distance between the Sun and the Earth, a significant part of high-energy solar neutrons can reach Earth's immediate vicinity.

Solar neutrons were first detected during a solar flare on June 21, 1980, by GRS instrument onboard the research spacecraft SMM. Then, neutron fluxes from solar flares were detected by the said GRS experiment (SMM spacecraft), SONG experiment (Coronas-F solar observatory), INTEGRAL space observatory, and ground-based neutron monitors. Thanks to them, both energy spectra of neutrons in the Earth's vicinity and a total neutron flux born during the flare were estimated. Fluxes of neutral particles (neutrons and gamma rays), which are not distorted by the interplanetary magnetic field, represent acceleration of charged particles up to relativistic velocities in active regions of solar flares. Using observational estimates of solar neutron and gamma-ray fluxes, one can hope to assess the parameters of the active Sun region during the flare.

It is also worth to note that proton share of cosmic rays increases also during proton storms, therefore secondary neutron flux born in the terrestrial atmosphere or within the station grows up as well. Then, to single out solar neutrons' share in the total number of particles detected, one have to measure accurately all the three components of neutron environment in the near-Earth space: the Earth’s atmosphere, design of the SC and active regions of the Sun.

Fast neutron spectra, measured near the Earth, allow to assess the spectrum of accelerated protons in the solar flare region, which is an essential element for a self-consistent model of a solar flare. Detailed explanation of the powerful solar flares is as yet far from finalization, therefore BTN-Neutron measurements would contribute significantly to the solution of this task.

Instruments in Use

We propose to use BTN-M1 instrument, which has been successfully operating onboard the ISS from February 2007 (in the first stage on the BTN-Neutron experiment, see https://np.cosmos.ru/BTNm1-en.html) and a newly developed BTN-M2 equipment.

BTN-M2 tasks are as follows:

• to measure thermal, epithermal, and resonance neutron fluxes;
• to measure fast neutron fluxes and energy spectra;
• to measure X-ray and gamma-ray energy spectra;
• to study radiation hardness of removable protective shields of different compositions during a space flight.

Operation Principle

BTN-M2 exploits principle of registering neutrons and gamma-ray quants born or scattered in the Earth's atmosphere and in the materials and construction elements of the ISS, as well as those coming from the Sun or other space sources.

The ISS orbit allows to register all three components of high-energy radiation born in solar flares, which are gamma-rays, protons, and neutrons. Moreover, during powerful solar flares one can register thermal, epithermal, and fast neutrons, born in the Earth's atmosphere and in the station's elements.

The ISS is an optimal space platform to continue the first stage of the BTN-Neutron experiment (2007–2012) and to perform the second in 2016–2020 at the beginning of the solar activity cycle 24.

Detectors used in the experiment onboard the ISS make a wide field-of-view survey of the sky. Moreover, BTN-M1 measures the fluxes of thermal, epithermal, and fast neutrons from the Earth's atmosphere and the ISS elements outside the pressurized habitant module, while BTN-M2 does the same inside the module.

The ISS advantage is that a detectors module and protective shields modules with various shielding materials can be developed and delivered to the station during the experiment.

Design and Mount

Technical, technological, and scientific groundwork in the registration of neutron and gamma-rays in the near-Earth orbit as well as in the studies of radiation hardness of prospective scintillation crystals, that were carried out at the first stage of the experiment, has allowed to move further to the second stage, that will employ BTN-M2 scientific equipment in 2016–2020.

Detectors of two types will be used in BTN-M2 instrument to register neutrons. First, proportional counters on the basis of helium isotope (3He) with moderators of different thickness would catch thermal, epithermal, and resonance neutrons. Currently developed detectors of this type have high efficiency (approximately 10%). Proportional counters will be used for: thermal neutron detector (CTN), epithermal neutron detector (CETN), and resonance neutron detector (CRN). Scintillation detector using stilbene will register fast neutrons (SCN) with protection from charged particles on the bases of organic scintillator.

To register gamma-rays in the energy range of 30 keV – 10 MeV a LaBr₃ crystal scintillation detector (SCG) will be used.

BTN-M2 will ensure the following physical measurements:

• high-energy neutron energy spectrum in 16 channels with energies from 1 to 10 MeV with sensitivity not less than 20 counts*s^-1/(neutron•s^-1•sm^-2);
• thermal neutron flux in the energy range of 0.001–0.1 eV with sensitivity not less than 30 counts *s^-1/(neutron•s^-1•sm^-2);
• epithermal neutrons flux in the energy range of 0.1–1.0 eV with sensitivity not less than 10 counts *s^-1/(neutron•s^-1•sm^-2);
• resonance neutrons flux in the energy range of1.0 eV – 1 MeV with sensitivity not less than 10 counts *s^-1/(neutron•s^-1•sm^-2);
• gamma quanta energy spectrum in 4096 channels in the energy range of 30 keV – 10 MeV with effective area not less than 30 cm² and energy resolution not less than 3% (at 663 keV).

BTN-M2 equipment will be mounted inside the ISS's pressurized habitant module.

Expected Results and Their Application

Scientific information obtained during the experiment will be used to build a model describing the formation of the Earth albedo neutrons, as well as a model of the local neutron background distribution inside ISS at various points.

Experimental data gathered throughout the experiment will help to reveal the patterns which rule solar neutron emission formation at all stages of the solar activity.

Measurements made during the 2nd stage of the experiment involving different variants of the radiation protective shields will be used to define requirements to the materials for radiation shelters lowering radiation hazards for the crew during interplanetary flights, and to test elements of the shielding collimator for the next-generation neutron spectrometer.

We expect the following scientific and technological results:

• experimental data on the Earth albedo neutron fluxes; we measured fluxes' dependence on intensity of the primary emission, observational point, zenith angle, Sun-ward direction, and state and composition of the upper atmosphere; a physical model describing scattering, absorption, and thermalization of neutrons in the Earth's atmosphere will be built on the basis of these observations;
• energy spectra and high-energy neutron fluxes coming from solar flares will be measured, thus allowing to build a model of solar flare evolution and charged and neutral particles acceleration;
• experimental data on neutron fluxes inside ISS with regard to different shields of the BTN-M2 equipment will be used to test methods of protecting the crew and equipment from radiation. This will help to make further recommendations for radiation protection when planning interplanetary manned space stations;
• experimental results will be also used to define the requirements for standard radiation control equipment (instruments for neutron measurements) for interplanetary expeditions.

Then, during the second stage of the BTN - Neutron-2 space experiment a decision on the commencement of the third stage thereof is expected to be made, which would involve large neutron telescope. This instrument, the development of which will make full use of the previous results and experience, would open a new branch of space studies — solar neutron astronomy.

Novelty, Quality Assessment in Comparison to Similar Russian and Foreign Studies

Suggested experiment within the 2nd stage of the BTN-Neutron-2 space experiment has the following novel elements in comparison to similar foreign and Russian studies:

• for the first time neutron time profiles and energy spectra in the near-Earth space will be measured by detectors with identical parameters, mounted both inside and outside a spacecraft, which will allow a highly reliable discrimination between and studies of each of the three components of the neutron field in the near-Earth orbit: solar neutrons, Earth's atmospheric albedo neutrons, and local neutrons born within the ISS integral parts;
• this method will allow to build a model of neutron and gamma-ray background in the ISS's vicinity, which would include energy spectra and neutron and gamma-rays fluxes;
• studying the efficiency of various radiation shielding methods against neutrons will allow to elaborate and test methods to build future radiation shelters for interplanetary spacecrafts as well as for habitats on lunar and martian surface.

As to the quality level of the results of the suggested experiment, it does not have equal counterparts either in Russian or in foreign studies in the field of neutron physics in the near-Earth's space.

Main Parameters

Developers and co-executors

Funding Organization ― Open Joint-Stock Company S.P. Korolev Rocket and Space Corporation Energia.

Primary contractor, space experiment organization, and instrument designer ― Space Research Institute of the Russian Academy of Sciences (IKI RAS).

Principal Investigator ― Dr. Igor Mitrofanov.

Project Timeline

1. 2011– 1st quarter of 2012 — preliminary design review.

2. 2012 – 2016 - development of the instrument.

3. end of 2016 - beginning of flight tests.