the first stage of the “btn-neutron” space experiment onboard the russian segment of the...

ISSN 00109525, Cosmic Research, 2010, Vol. 48, No. 4, pp. 285299. Pleiades Publishing, Ltd., 2010.Original Russian Text V.I. Tretyakov, I.G. Mitrofanov, Yu.I. Bobronitskii, A.V. Vostrukhin, N.A. Gunko, A.S. Kozyrev, A.V. Krylov, M.L. Litvak, M. LopezAlegria, V.I. Lyagushin,A.A. Konovalov, M.P. Korotkov, P.V. Mazurov, M.I. Mokrousov, A.V. Malakhov, I.O. Nuzhdin, S.N. Ponomareva, M.A. Pronin, A.B. Sanin, G.N. Timoshenko, T.M. Tomilina,M.V. Tyurin, A.I. Tsygan, V.N. Shvetsov, 2010, published in Kosmicheskie Issledovaniya, 2010, Vol. 48, No. 4, pp. 293307.

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1. INTRODUCTION

Investigation of neutron and gammaray background in the nearEarth space onboard manned andautomatic orbital spacecraft has started since the firstflights into space and is being continued at the presenttime. Such experiments were conducted and are carried out using both passive and active methods. In thefirst case measurements are made with passive detectors. Numerous experiments onboard the Mir orbitalstation [1, 2] and the SSNTD experiment onboard theInternational Space Station (ISS) [3] represent thistype of experiments. The data of detectors are processed onboard or on the ground to give integral information on particle spectra during the exposure, butthis method does not allow one to study time dynamicsof neutron and gammaray fluxes as a function of solaractivity level. The second, active method makes provision for carrying out onboard experiments using neutron and gammaray spectrometers with continuousdetection of radiation fluxes and transmission of measurement data through a radio channel to the Earth.The experiments DOSMIR onboard the Mir station[4] and DOSTEL onboard the ISS [5] were madebased on the second method. This method allows oneto get considerably larger amount of useful scientificinformation, though it requires more complex instru

mentation. Realtime measurements of the neutronand gammaray background allow one to solve successfully the problems of radiation safety of the crewand instrumentation during longterm space flights.The space experiment BTNNeutron whose firststage is in progress at the moment onboard the Russiansegment (RS) of the ISS belongs to the type of activeexperiments.

This space experiment BTNNeutron for measuring neutron fluxes at the nearEarth orbit in a widerange from thermal energies up to energies exceeding100 MeV was suggested as early as 1997 in the courseof discussion of the scientific program for the ISS.According to its original concept, proposed by theSpace Research Institute of Russian Academy of Sciences, the experiment was intended to detect neutronsin a wide energy range by several detectors of theinstrumental complex with a general title BTN (Russian abbreviation for the Onboard Neutron Telescope). These measurements should provide for acomplete set of data for physical interpretation andunambiguous identification of various components ofthe neutron radiation in the nearEarth space: neutronfluxes from the upper atmosphere, from the ISS constructions, and from solar proton events (SPE).

The First Stage of the BTNNeutron Space Experiment onboard the Russian Segment of the International Space Station

V. I. Tretyakov1, I. G. Mitrofanov1, Yu. I. Bobronitskii2, A. V. Vostrukhin1, N. A. Gunko3, A. S. Kozyrev1, A.V. Krylov4, M. L. Litvak1, M. LopezAlegria5, V. I. Lyagushin6, A. A. Konovalov1,

M. P. Korotkov2, P. V. Mazurov6, M. I. Mokrousov1, A. V. Malakhov1, I. O. Nuzhdin1, S. N. Ponomareva2, M. A. Pronin6, A. B. Sanin1, G. N. Timoshenko4, T. M. Tomilina2,

M. V. Tyurin7, A. I. Tsygan3, and V. N. Shvetsov41 Space Research Institute, Russian Academy of Sciences, Moscow, Russia

2 Blagonravov Institute of Machine Science, Russian Academy of Sciences, Moscow, Russia3 Ioffe Institute of Physics and Technology, Russian Academy of Sciences, St. Petersburg, Russia

4 Joint Institute of Nuclear Research, Dubna, Russia5 Johnson Space Flight Center, NASA, Houston, Texas, USA

6 Korolev Rocket and Space Corporation Energiya, Korolev, Moscow oblast, Russia7 Gagarin Center of Cosmonaut Training, Zvezdny, Russia

Received August 28, 2008

AbstractThe aims and tasks of the space experiment BTNNeutron onboard the Russian segment of theInternational Space Station are described. The experiment deals with detection of fast and epithermal neutrons, Xrays, and gamma rays. Characteristics and a short description of scientific instrumentation BTNM1for this experiment are presented, as well as the first results of operation of the apparatus onboard the stationduring the first two years of flight.

DOI: 10.1134/S0010952510040027

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In 2001 NASA launched to Mars the Mars Odysseymission for global exploration of the surface of the redplanet from a circular polar orbit. The payload of thisspacecraft included the complex of GRS gammarayspectrometer for studying the gammaray and neutronradiation of the Martian surface. The GRS complexconsisted of a gammaray detector, based on cooledhighpurity germanium, developed in Arizona StateUniversity, USA, of a neutron spectrometer for thermal and epithermal neutrons manufactured in theLosAlamos National Laboratory, USA and of theRussian detector HEND (High Energy NeutronDetector) of highenergy neutrons developed in theSpace Research Institute. Such measurements from anearMars orbit are possible due to the thin atmosphere of Mars, which freely lets cosmic rays passing tothe planet surface, and also almost without absorptionsecondary neutrons and gammarays produced in theMartian soil are able to move away into space. In thiscase the spectral density of the flux of secondarynuclear radiation is determined by the elemental composition of the soil substance.

At the present time the HEND instrument continues successfully operate onboard the Mars Odysseyspacecraft, and, according to NASA decision, themeasurements are planned to be prolonged at least upto October 2010. The data already obtained have demonstrated a high research potential of nuclear physicsmethods for studying the composition and structure ofthe upper layer substance of a celestial body bombarded by energetic particles of cosmic rays. Using thedata of neutron measurements made by the HENDinstrument and other instruments of the GRS complex it has been established that nearpolar regions ofMars located higher than 60 north and south latitudesrepresent permanent frosty soil with high content ofwater ice whose mass fraction can reach 5070% ofsoil [6]. The data accumulated for more than threeMartian years allowed one to investigate dynamics ofseasonal processes of condensation of atmosphericcarbonic acid snow on the surface of Mars [7]. Thedata of the HEND instrument also allowed one to estimate the contribution of the neutron component toradiation background at the Martian orbit under conditions of the quiet Sun and during SPEs. It was demonstrated that powerful SPEs could make the bulkcontribution to the total annual radiation dose on thesurface of Mars. Therefore, their forecasting and monitoring will be of primary importance for planningfuture interplanetary missions.

After launch of the Mars Odyssey mission, a spareflight unit of the HEND instrument has remained inSRI of RAS. It was suggested to use it as a detector unitof the BTNM1 instrumentation for carrying out afirst stage of the BTNNeutron space experiment.Such an approach also allows one to make synchro

nous measurements on nearEarth and Martian orbitsfor simultaneous stereoscopic observation of the solaractivity and for studying the neutron background variations.

Scientific instrumentation BTNM1 was designedand manufactured in SRI of RAS with participation ofSpecial Design Office of SRI, Blagonravov MachineBuilding Institute of RAS, Joint Institute of NuclearResearch, and RAS Institute of Medical and Biological Problems. Training of crew actions on installingthe detector unit on the outer surface of Zvezda module of Russian segment of the ISS during extravehicleactivity (extraVA) was performed in the GagarinCosmonaut Training Center. Apparatus control,acquisition and storing of primary telemetry andguidance data is made by the Flight Control Centerof TsNIIMaSh.

Assembling the research instrumentation onboardthe ISS by the crew members of the 14th basic expedition was the most important concluding stage of theexperiment preparation. First, this work includedinstalling the electronic block BTNME inside themodule Zvezda and laying internal cables for connection to service systems of the station. Second, duringtwo exits of the ISS crew into open space the detectionblock was installed on the outer surface of the Zvezdamodule and connected through hermetically sealedsockets to the electric circuits of the ISS Russian Segment. Because of the tight schedule of the extaVA during the first exit on November 23, 2006 astronauts succeeded only in fixing mechanically the detection blockand connecting the power supply cable. It should benoted that the crew proposed the solution of makingthis connection under critical conditions of substantialoverduration of scheduled staying in the open space.This proposal was accepted by the control group in theFCCM. Because of this solution the power supply forthe automatic system for thermal regime provision(ASTRP) was ensured, which allowed one to avoid itsfailure due to overcooling in the switchedoff state.During the second exit on February 22, 2007 the crewsuccessfully completed connection of the secondcommandtelemetry cable. After that, the instrumentation was switched on by a command from theground.

At present, the research instrumentation BTNM1is connected with systems of power supply and telemetry ensuring scientific measurements in the standardmode with everyday transmission of data and serviceinformation to the ground.

2. NEUTRONS IN THE NEARTERRESTRIAL SPACE

Neutrons at a height of the ISS are known to beproduced in three physical processes. First, energetic


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protons and ions of galactic cosmic rays and from theradiation belts of the Earths magnetosphere generateneutrons with energies of about 1020 MeV due tonuclear reactions in the upper atmosphere of theEarth. Thus produced secondary neutrons are slowedin the atmosphere matter, a part of them beingabsorbed in nuclear reactions of capture to form newnuclei. Another part is decayed, but a substantial fraction leaves the atmosphere [8], forming in the nearterrestrial space the socalled neutron albedo of theEarth. The flux and energy spectrum of albedo neutrons are variable in time and space, since they dependon the flux and energy of charged particles in the magnetosphere, as well as on density, temperature, andcomposition of the upper layers of the atmosphere.

Second, around the ISS the proper induced neutron emission is produced due to interactions of ofenergetic charged particles of the magnetosphere withthe matter of the station itself [9, 10]. These local neutrons fluxes are present around the station in theregion with a characteristic scale of the order of itsdimensions, and they are also variable along the station orbit due to irregularity of the flux of charged particles.

The third cause of the existence of neutrons in thenearterrestrial space is the solar activity. It is wellknown that during some powerful SPEs the fluxes ofhighenergy neutrons are generated in active regionson the Sun [11]. The neutron lifetime is about 15 min,which is comparable to the time of flight of relativisticparticles from the Sun to the Earths orbit. Therefore,a substantial fraction of highenergy solar neutron canreach the vicinity of the Earth.

Solar neutrons were detected for the first time during the solar flare on June 21, 1980 using the GRSinstrument installed on the NASA research satelliteSolar Maximum Mission (SMM) [12]. Later on, solarneutrons were recorded by the same instrument on theSMM [13], by the SONG instrument on the CoronasFspacecraft [14], and in joint measurements of groundbased neutron monitors and instruments of the Integral space observatory [15]. The energy spectra andfluxes of neutrons near the Earth were measured, andthe estimates of the total neutron flux generated in theactive region on the Sun were made. These estimatesare of great importance especially for the reason thatfluxes of the neutral particles (neutrons and gammarays) are not distorted by the interplanetary magneticfield, thus reflecting the processes of acceleration ofcharged particles up to relativistic energies in theactive regions of solar flares.

One should take into account that during EPS theproton component of cosmic rays increases substantially (by orders of magnitude). Accordingly, the fluxesof secondary neutrons from the Earths atmosphereand from the station itself also increase. Therefore, in

order to estimate the fraction of solar neutrons in thetotal number of recorded particles, it is necessary toanalyze in detail the contributions of all three sourcesof neutron radiation in the nearterrestrial space: theEarths atmosphere, the stations matter, and activeregions of the Sun.

3. GOALS OF THE EXPERIMENT AND EXPECTED RESULTS

3.1. Basic Goals of the Experiment

The BTNNeutron experiment has the followingbasic goals.

1) Construction of the map of secondary neutronradiation of the upper atmosphere of the Earth takinginto account the effects of longitude and latitude of themeasurement point, time of the day, and the state ofthe magnetosphere and upper atmosphere.

2) Measurement of the neutron component of solarradiation during SPEs.

3) Construction of a physical model of the localneutron background from the ISS under different conditions of its orbital flight.

There are some additional goals of the experimenton its first stage. They are as follows.

4) Synchronous measurements by the HENDinstrument in the Martian orbit and by BTNM1 inthe nearterrestrial orbit of the effects of increasedneutron component of the space radiation backgroundin the vicinity of Mars and the Earth during SPE.

5) Detection of cosmic gammaray bursts near theEarth in order to participate in the program of determining the direction to a flashed source of gammarayemission by the method of interplanetary triangulation.

6) Investigation of radiation resistance of the samples of new scintillation materials in order to estimatepossibilities of using them for detection of gamma raysand neutrons in future space experiments.

3.2. Expected Results of the First Stage of the Experiment

It is suggested in the course of performing the firststage of the BTNNeutron experiment to carry out thefollowing research studies and to obtain their practicalresults.

1) Investigation of the secondary neutron emissiongenerated in the upper atmosphere of the Earth underthe action of GCR as a function of longitude and latitude of the place of observation, hour of the day, stateof the atmosphere, and characteristics of charged particle fluxes in the radiation belts. Study of the effect ofincreased neutron background in the region of theSouth Atlantic Magnetic Anomaly (SAMA) and athigh geomagnetic latitudes for different phases of the

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solar cycle from the minimum in 20072009 years tothe maximum in 20122013.

2) Construction of a model of neutron generationin active regions of the Sun based on comparison of theresults of measuring the fluxes of highenergy neutronswith the data of measurements of xray and gammaray emissions, and detection of energetic protons byother spacecraft.

3) Creation of a prompt model of neutron environment onboard the ISS in order to control radiationconditions and to elaborate recommendations forminimization of the radiation dose rate for the crew.

4) Based on the BTNNeutron and HEND data,comparison, of the neutron component of the radiation background in the nearterrestrial orbit and atorbit around Mars in order to make a forecast of neutron contribution to the radiation environmentonboard an interplanetary vehicle in different periodsof solar activity.

5) Provision of the data about detection of cosmicgammaray bursts for the international program ofinterplanetary triangulation and determination ofcoordinates of burst sources on the celestial sphere.

6) Investigation of the effects of radiation damageof new scintillation crystals LaBr3 : Ce

3+, LaC13:Ce3+,

(Lu0.5Y0.5)AlO3 : Ce3+, (Lu0.7Y0.3)AlO3 : Ce

3+, (Lu2SiO5) :Ce3+, (YAlO3) : Ce

3+, (LuAl)O3 : Ce3+, and (LuY)SiO5 :

Ce3+ under space flight conditions in order to estimatetheir applicability in advanced space detectors ofgamma rays and neutrons, including those for subsequent stages of the space experiment BTNNeutron.

4. DESCRIPTION OF INSTRUMENTATION BTNM1

4.1. Composition of the BTNM1 Instrumentation

The BTNM1 scientific equipment includes twounits: detector assembly BTNMDMF with overalldimensions 594 462 220 mm, a mass of 11.5 kg,and maximal power consumption 14.7 W placed onthe outer surface of the Zvezda module and the internal electronic block BTNME with dimensions 255 265 115 mm, 3.4 kg mass, and maximal power consumption 3.4 W (Fig. 1).

The instrumentation also includes a set of cablesfor connecting equipment inside and outside the station with a mass of about 14.4 kg; two cassettes withdimensions 110 140 195 mm and masses 1.9 kg and0.8 kg, respectively, for placing spare electronic cardsof the electronic unit and for packing boards withdosimeters and samples of scintillators to be returnedto the ground after their exposure in orbit. The totalmass of BTNM1 equipment is more than 30 kg. Thetotal power consumption in the measurement mode isabout 18 W.

4.2. Construction of the Outer DetectorAssembly BTNMDMF

The detector unit BTNMD is installed inside aboxshaped frame BTNMF which serves severalfunctions.

First, the frame BTNMF ensures the optimalthermal regime for the BTNMD unit at different illumination conditions onboard the ISS. In order totransfer a released heat from the BTNMD unit to theBTNMF frame, copper bars with a crosssection of30 mm2 are used. The frame itself has for heat releasetwo oppositely located emitting radiators.

The ASTRP of the BTNMD unit has individualonboard power supply and operates independent ofother electronic circuits of the instrument. TheASTRP is tuned to switch internal heater on and offwhen temperature of the BTNMD unit body is equalto 23 and 18, respectively, which allows oneto keep the temperature of electronic circuits in therange higher than 23C. An additional heater with apower of 6 W is set in the BTNMD unit. It is activatedby a discrete command from the ground and usedunder conditions of strong overcooling of the unit,when heat released by the ASTRP cannot provide forthe admissible thermal regime of the instrument operation.

In order to reduce heat exchange with the externalhandrails of the Zvezda module on which the detectorassembly BTNMDMF is mounted, special heatinsulated bridge sleeves are installed on matching sitesof the BTNMF frame. The assembly of BTNMDunit and BTNMF frame is covered by multilayershield vacuum heat insulation (SVHI), excluding radiators, matching sites for docking with the adapter platform, and electric sockets. By this means, the externaldetector assembly is completely heatinsulated fromthe ISS and has an autonomous system of thermal regulation. In order to avoid overcooling of the BTNMDunit at the stage of installing it during extraVA, bothradiators of the frame are temporarily protected byheatinsulating jackets from SVHI. The jackets areremoved by the crew, when the detector unit has beeninstalled to its place, and power supply has been connected. In order to control the temperature regime ofthe BTNMF frame, two thermal sensors TP1805are arranged on one of its radiators and on the matching site. Their data are transmitted to the Earth as apart of service information practically at every orbit.

Numerical simulation of the operation of the external detector thermal conditions was performed at thestage of designing the equipment for various conditions of illumination, and the obtained results werechecked at the stage of testing the instrument in a thermal vacuum chamber of the checkout stand of SpaceResearch Institute of RAS. The data of measurements



after the beginning of flight tests of the BTNM1instrument confirmed the validity of technical solutions chosen to provide for an optimal thermal regimeof the unit at all flight modes of the ISS.

Second, the BTNMF frame also ensures protection of the detector and electronic units against various mechanical impacts. For example, in connectionwith necessity of damping the vibrations on the ISSboard that could result in electric noise due to microphone effect in photomultipliers, the BTNMD unit ismounted inside the BTNMF frame using four specialmechanical shock absorbers developed in collaboration with the Blagonravov Institute of Machine Science of Russian Academy of Sciences (Moscow).

Third, electric plugs X1 and X2 of the RS50 typeare arranged on the BTNMF frame. Cables fromplugs of the BTNMD unit and from thermal sensorsare connected to them from inside. The X1 plug servesfor transmitting scientific and telemetry data to theBTNME unit and for receiving the commands ofcontrol from it. The X2 plug is designed to providepower supply to electronic circuits and ASTRP (Automatic system of Thermal Regulation Providing) of theBTNMD unit. The BTNMF frame has also a metalized bridge that is fixed to a support arm, thus providing for ground connection of the external detectorunit to the stations mainframe through locks of theadapter platform.

In order to accomplish scientific tasks it wasrequired that the BTNMD unit at standard installation would be oriented in such a way that the axis of

unit scintillator should be directed to zenith. Oneshould add to this the requirements of storing theinstrument temporarily inside the ISS, possibility ofmounting and dismantling the equipment while working in a space suite, and securing access to the handrails after installation of the unit. Taking these requirements into account, designers of the RocketSpaceCorporation Energiya have developed a specialassemblageandinstallation device on which theBTNMDMF assembly was installed. This deviceincluded the support arm, adapter platform, and threequickdetachable locks.

4.3. The Unit BTNMD for Detection of Neutrons and Gamma Rays

The detector unit BTNMD is a spare flight specimen of the HEND instrument (Fig. 2). This instrument is a spectrometer with four independent neutrons detectors. Three detectors of epithermal neutrons (in Fig. 2 they are designated as SD, MD, andLD) represent proportional gas counters filled with3He, while the fourth detector of neutrons (marked inFig. 2 as SC) is manufactured from stilbene C14H12 surrounded by active anticoincidence shield of CsI : Tl3+

crystal.

Neutrons in the counters based on 3He are detectedthrough the reaction of capture with production of aproton and a tritium nucleus (triton):

n + 3H p + 3H + 764 keV, (1)

External cables

Shock absorbers

Thermal bars

BTNME

BTNMD

BTNMF

Internalcables

Internalcables

Pressure connectors ISS wall

Mounting frame

Thermal sensors

Thermal insulatorsConversion board

X1X2X3X4

X1

X2

X3

X4

X1X2

Plugs of the internal electronic block:

Plugs of the external detector unit:

X1X2

power supply scientific data and telemetry electric power supply and commands

command interface RS422 interface BITS212 scientific data and telemetry

Fig. 1. Block diagram of scientific equipment.

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the products of this reaction being detected due to ionization caused by them. The energy of 764 keVreleased in this reaction is divided between the tritonand proton in the proportion 1 : 3, i.e., the triton carries away 191 keV (25%), and the proton carries awaythe energy of 573 keV (75%). If the reaction occursnear the counter wall, the particles can leave thedetecting volume, only a part of the total energy beingreleased in the counter. Therefore, when neutrons aredetected by a 3He counter, the spectrum of energyrelease has a characteristic shape with two peaksbeginning from 191 keV (the reaction threshold) andat energy of 764 keV (the complete absorption peak).The crosssection of capture reaction (1) increases ininverse proportion to the relative velocity of a neutronand 3He nucleus. Therefore, the efficiency of neutrondetection increases with decreasing energy of neutrons, and it becomes maximal for thermal neutrons.

Signals from the counter come to the input of achargesensitive preamplifier and then to an analogdigital converter. Measurement of the energy spectrum of reaction (1) allows one to separate reliably theevents associated with neutron detection from thecontribution of charged particles and/or electricnoise.

Three detectors have polyethylene shells of different thickness, which provide different degree of moderation for incident neutrons down to thermal energiesat which their detection is most efficient. Eachcounter is also surrounded by a shield from cadmiumfoil that does not virtually let neutrons with energies



signals about detection of epithermal neutrons in proportional counters SD, MD, and LD; signalSC/IN/N about detection of highenergy neutrons inthe inner scintillator SC/IN; signal SC/IN/G aboutdetection of gamma rays in the same scintillator; andsignal SC/OUT about detection of photons and/orcharged particles in the outer scintillator. Completeprocessing of all above signals is performed in theBTNMD unit: from primary amplification and analogdigital conversion in 16 spectral channels up toformation of the frames with scientific and telemetrydata to be transmitted to the BTNME unit. For atime of accumulation of an individual frame sixsummed 16channel spectra of counts are formed inthe unit for every signal. Duration of the accumulationtime of the spectra is determined by a command fromthe ground and can vary from 1 to 256 seconds.

The logic of a flash cell is also realized in the BTNMDdetection unit, which allows one, during accumulation of a current frame, to record two integral profilesof signals SC/IN/G (with a time resolution of 1.0 s)and SC/OUT (with a resolution of 0.25 s). In the case,when the current counting rate of these signalsexceeded the background counting rate by a factorpreset by command, these profiles are added to aframe of scientific information. The instrument canalso be used in a regime when the profiles are recordedat formation of every frame, independent of thecounting rate.

The BTNMD unit has only internal time. Thedata of measurements are referenced to the ISSonboard time in the BTNME unit which every fiveminutes synchronizes the instruments internal timeusing the time marker received from the onboard unitof the payload server. Three modes of operation arespecified by logic of the unit operation: onduty, basicresearch, and shortcut research with a veto put upondetection of gamma ray bursts. In the onduty modeno scientific measurements are performed. The unitselectronic circuits execute the received commands,make measurements of temperature at six points of theinstrumentation, and send frames with service information to the telemetry system. In the basic researchmode, which is also the mode of operation by default,the BTNMD unit forms the frames with scientificdata, including all six physical signals and gammarayprofiles with time resolutions of 1.0 s and 0.25 s. Theduration of accumulation of aggregate spectra ofcounts for each signal (duration of forming a separateframe) is determined by periodicity of injection of synchronization signal from BTNME to the BTNMDunit. Upon reception of this signal, the logic block ofthe BTNMD unit terminates accumulating the spectra and recording the profiles of counts, transmits theformed frame to the BTNME unit, and beginsrecording a new frame.

In the third shortcut mode, all six physical signalsare also measured, but no profiles of gamma ray signalsare recorded, which ensures some economy in the useof telemetry resources on the station. This mode isused when there is a limitation on the volume of dailytransmitted data. The complete technical descriptionof the instruments electronics and software is presented in [18].

4.4. Electronic Unit BTNME

The BTNME electronic unit includes electronicboards designed for matching electric interfaces of theBTNMD unit that obey the requirements of the MarsOdyssey spacecraft and the interfaces of electric systems of the ISS Russian segment. The external interfaces of the BTNME block provide for power supply,transmission of control commands and telemetry, andtranslation of time synchronization signals from theISS systems. The internal interfaces provide for powersupply, transmission of control commands, receptionof the frames of scientific data, and synchronization offrame formation for the BTNMD unit.

The following functional tasks are run by the BTNMEunit.

1. Generation and transmission of a logical signalSYNC, according to which the recording of the preceding frame of measurements in the detector unitBTNMD is terminated, this frame is read out, andrecording of the next frame begins. The time intervalbetween successive signals SYNC can be set by a digital command within the limits from 1 to 256 s. Thechoice of this interval determines the instrumentstime resolution (accumulation time) and the amountof scientific data.

2. Readout of scientific data from the external unitBTNMD, storage and transmission of telemetry datato BITS212 in the mode of direct transmission (DT)to the ground. The internal memory of the BTNMEunit is 1 MB. When the time of one measurement is setto be 1 s (this corresponds to 1s periodicity of sendingSYNC), the total daily amount of data (including thescientific and service data) is about 2.7 MB. In order toensure uninterrupted measurements, one needs in thiscase to have at least 3 DT sessions per day. The modeof measurements, at which the accumulation time isequal to 60 s, is optimal for operation with the ISS RSsystems. In this case the total time of discharging thedaily data of about 1 MB through the BITS212 systems of transmission is of order of 3 min.

3. Reception and execution of discrete commandscoming from the onboard BITS212 system andreception of digital commands through RS422 interface coming from the payload server controlling scientific instrumentation of the ISS RS. A part of thesecommands is transmitted for execution to the external

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unit, the other commands are executed by the BTNME unit itself.

4. Arrangement of special boards with passivedetector assemblies (PDAs) including crystals foradvanced scintillation detectors and passive dosimeters. These PDAs are used at the first stage of the spaceexperiment for studying radiation resistance of thescintillators and estimating their applicability todesign instrumentation for other space experiments,including that for subsequent stages of the BTNNeutron space experiment.

Electric plugs X1X4 of the RS50 type are placedon the frame of BTNME, and cables from the ISS RSonboard systems are connected to them from inside.The X1 plug serves for power supply, through X2 thedigital commands and time synchronization signalsare received from the payload server, the X3 plug servesfor transmission of discrete commands and telemetrydata to BITS212. The plug X4 realizes communication with the external detector unit, including reception of scientific and telemetry data and transmissionof control commands.

5. INSTRUMENTATION CONTROLAND DATA TRANSMISSION

Commands of two types control the BTNM1instrumentation: discrete (relay) and digital commands. The BTNMD detector unit is controlled byinternal digital commands through the RS422 interface. These commands are generated and recievedfrom the electronic block BTNME come from there.This block is controlled by external digital commands(also through RS422 interface) from the payloadserver and by external discrete commands from theSystem of Onboard Automation Control (SOAC) ofthe ISS RS. The external digital commands change themode of BTNMD operation, set measurementparameters (thresholds of the discriminator, levels ofhigh voltage on the detectors, accumulation time,logic of anticoincidence, etc.), determine the operation conditions for ASTRP, and so on. The externaldiscrete commands such as Switch on BTNME,Switch off BTNME, The 1st halfset, The 2ndhalfset, Data array discharge, End of data arraydischarge serve for controlling the BTNME unit andfor putting the BTNMD unit into a research modeunder conditions, when it is impossible to send digitalcommands from the payload server to it, or when it isnecessary to make diagnostics of operation of theunits logical digital interface in emergency situations.When it is impossible to use digital commands for conversion of the BTNMD unit into the research modeand its adjustment, provision is made for a possibilityof switching it on in the shortcut research mode with

default settings using a cyclogram of discrete commands.

The BTNM1 instrumentation uses the standardtelemetry system BITS212 of the ISS for transmission to the ground of data measurements. The frameswith scientific and service data are accumulated in the1MB native storage of the BTNME and are transmitted to BITS212 and next to the ground in radiocommunication sessions. Depending on the mode ofoperation of instrumentation the structure and size ofa single telemetric frame with scientific and servicedata are variable. In the onduty mode the size of aframe of BTNMD is 56 bites; the research mode withpermanent detection of bursts corresponds has theframe size 512 bits; and in the shortcut research modethis size id equal to 212 bites. When one sets the accumulation time equal to 60 s, the total data stream isequal to 160 kB per day and 850 kB per day for the onduty mode and basic research mode, respectively. Theelectronic block BTNME adds to every frame formedby BTNMD its own heading with a telemetry size of64 bites. These additional data include temperatures ofthe BTNMF frame and inside the BTNME unit, aswell as service data about the state of the latter.

6. MATHEMATICAL MODELING OF DETECTORS AND PHYSICAL

CALIBRATIONS

When developing the BTNMD unit, numericalcalculations based on the Monte Carlo method weremade in order to evaluate the sensitivity of neutrondetectors. In this mathematical simulation, carriedout with participation of the Ioffe PhysicalTechnicalInstitute and Joint Institute of Nuclear Research(JINR), both selfmade programs and commonlyused MCNPX code distributed by the Oak RidgeNational Laboratory, USA (http://rsicc.ornl.gov)were used. The aim of simulation was to constructresponse functions of the neutron detectors in theentire energy range from the cadmium threshold0.4 eV up to 10 MeV and to compare the results ofnumerical calculations with experimental values thathad been obtained for separate values of neutronenergy. Figure 3 presents calculated curves of theresponse functions for proportional counters and stilbene scintillator. One can see that the instrumentdetects neutrons in a wide energy range from the cadmium threshold (about 0.4 eV) to energies higher than10 MeV with sufficiently high efficiency.

In order to verify the calculations, physical calibrations of the instruments detectors were carried out inthe Laboratory of Neutron Physics of JINR (Dubna).These calibrations included both measurements withthe use of isotope neutron sources based on Cf252andPuBe and measurements with the neutron beam



from a lithium target on the linear accelerator of protons. In the last case the neutron energy was determined by geometry of scattering of protons by lithium.For different angles it was equal to 100, 500, and900 keV. The detector was installed on the neutronbean axis at distances 50 cm and 1.5 m from the lithium target. During all measurements the incident fluxof neutrons was measured by a neutron monitor. Figure 4 presents a comparison of calculated curves withphysical measurements for proportional counters andstilbene scintillator.

7. START OF THE SPACE EXPERIMENT AND FIRST RESULTS

7.1. Primary Processing of Measurement Data

The instrumentation was first activated on February 26, 2007. The mode of shortcut research measurements with default settings was switched on by discretecommands. The time reference was made according tothe time of switching on the detector unit. At present,according to the telemetry data, electronic circuits ofthe detector and electronic units operate in the normalmode, and temperatures of both the units are withinthe admissible limits.

The scientific data of the space experiment BTNNeutron transmitted from the ISS board are stored inthe database of the Information Retrieval System anddatabases of space experiments of the ISS RS in theFlight Control Center FCCM. Associated data ofnavigation and ballistic support are also stored there,as well as the data of service telemetry coming to theFCC via independent communication channels.

At the moment, taking into account losses of information during transmission of the data to the ground,

the total information content (scientific data andtelemetry) without failures exceeds 90%. From theFCCM the entire information of the BTNM1instrumentation is copied to the database of the BTNNeutron experiment in the Space Research Institute.The mean monthly amount of the telemetry and scientific data varies from 40 to 60 MB.

The coordinates are referred based on the calculations of orbits parameters of the ISS in the geodeticcoordinate system according to twoline elements offormat TLE NORAD presented by NASA for publicaccess through the Internet [http://www.spacetrack.org/tle_format.html] which are updated onceper day using the data of radar measurements. Sincethe fluxes of neutron and gamma ray emission areanisotropic in the vicinity of the ISS, the data on ISScurrent orientation are included in accompanyinginformation. These data are generated by the RS navigation system. Orbital and orientation parameters forshort segments of the orbit above groundbased information points are recorded by the tracking service ofFCCM several times per day in the form of asset ofISS coordinates, components of the velocity vector,

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Fig. 4. Comparison of calculated response function curveswith results of experimental calibrations for proportionalcounters (upper plot) and stilbene scintillator (lower plot).Squares and triangles show the data of measurements andcalculated curves, respectively.

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orientation quaternion, and onboard calendar time inseconds from January 6, 1980. They are copied intothe experiments databases in FCCM and SRI,extrapolated to the starting moment of a given frame ofmeasurements, and ascribed to it in the SRI database.

Primary processing of the data includes their representation in the form of profiles of the counting ratesof neutrons and gamma rays, and instrumental spectraof counts. Coordinate maps of the background of neutron and gamma ray emission in the vicinity of the ISSare constructed by way of referring the profiles to theISS coordinates.

7.2. First Scientific Results

Three basic scientific products of the space experiment were created at its first stage:

uninterrupted sequence of instrumental spectraof counts in all detectors of the instrument with anexposure time of 60 s, which is synchronized with theuniversal time;

uninterrupted profiles of integral counting ratesin all detectors of the instrument with a resolution of60 s, with reference to the universal time; and

maps of counting rates in the neutron andgamma ray detectors in the geographic coordinateswith spatial resolutions of 5 5 degrees in longitudeand latitude.

No manifestations of solar activity were observed inthe entire period 20072008 of performing the experiment, therefore, all the profiles of counting rate variability represented only variations of the backgroundenvironment along the ISS orbit. Figure 5 presents anexample of the time profiles of counts for detectors

SD, MD, and LD for oneday time interval on January 17, 2008.

The variable character of the profile with a periodclose to the ISS orbital period is clearly seen. The profiles of signals from fast neutrons and gamma rays inthe scintillations detector the same periodic character.The periodicity of these signals is explained by the ISSpassage through the northern and southern circumpolar regions of the Earths magnetosphere, where theflux of charged particles is increased. This is due to thefact that when one approaches the poles the fluxes ofsecondary neutrons and gamma rays both from theupper layers of the Earths atmosphere and from thestation itself increase. In addition to these periodicpeaks, the enhanced counting rates are observed on allprofiles when the station flies over the SAMA. As iswell known, a substantial depression of the magneticfield is observed in the region of this anomaly. Therefore, in its vicinity the fluxes of charged particles fromthe radiation belts can reach the upper boundary of theatmosphere.

Figure 6 presents maps of averaged counting ratesof neutrons in different detectors of BTNM1 constructed based on data of the instrument operationduring one year. The maps include the data for epithermal neutrons 0.4 eV1 MeV (MD counter), fastneutrons with energies 300 keV10 MeV (signal fromstilbene SC/IN/N), and gamma rays in the energyrange 300 keV5 MeV (scintillator CsI : Tl3+ of detector SC/OUT).

Due to the ISS orbit inclination such maps can beconstructed only for the latitude belt from 51 south to51 north. The substantial enhancement of countingrates for neutrons and gamma rays in the SAMAregion is very bright in the maps. Also remarkable are

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Fig. 5. Time profiles of neutron counting rate for proportional counters.



enhanced counting rates in nearpolar regions to thesouth of Australia and to the north from Canada,where charged particles from the radiation belts penetrate into the upper atmosphere in the vicinity of themagnetic poles. The asymmetric picture at high latitudes is explained by inclination of the geomagnetic

dipole axis to the Earths rotation axis. A large numberof passages of the station through the SAMA regionallow one to make a detailed scanning of the neutronfield in the region. The penetration of charged particles from the radiation belts into the atmosphere in theanomaly region is known to be intensified in the solarminimum periods, when the intensity of galactic cosmic rays in especially high in the inner region of theSolar System [19]. In 20072009 (at the minimum ofsolar activity) the contribution of SAMA into formation of the Earths neutron albedo is quite significant.Therefore, the data of measuring the neutron radiation in the SAMA region seem to be of extreme importance: the next opportunity of such measurements willoccur no earlier than in 2018.

Based on the data of neutron measurements in theenergy range from the cadmium threshold of 0.4 eV toenergies of about 1 MeV, averaged estimations of theneutron component of the radiation dose rate wereobtained at the place of BTNM1 instrumentationarrangement onboard the ISS for regions with different geographical coordinates along the station orbit(Fig. 7).

It is obvious that the maximum dose of neutronswas observed in the SAMA region: in 2007 the rate ofneutron dose accumulation in this region was 56 Svper hour, which exceeds the accumulation rate on theequator almost by a factor of 25. Our estimations ofradiation dose rate accumulation according to BTNM1 data are compared in Table with the results ofmeasurements in the Japan experiment BBND [20] ina hermetic compartment of the laboratory module ofthe ISS American segment, and with estimated neutron component of the radiation dose obtained fromthe results of measurements with the HEND instrument onboard Mars Odyssey orbiting around Mars.

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Fig. 6. Maps of averaged counting rates of epithermal neutrons in the MD detector (top map), fast neutrons in stilbene (middle map), and gamma rays in CsI:Tl3+ (bottommap). Color scales show counting rates in the detectors.Irregularities in the map for the MD detector are caused bystrong noise pollution of the signal and by cutting the noisein the process of map construction.

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Fig. 7. A map of estimated neutron component of the radiation dose rate onboard the ISS according to the BTNNeutron data for the period February 2007March 2008.

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As a part of additional task of the experiment ondetection of cosmic gamma ray bursts an analysis wasmade of gamma ray profiles obtained by the CsI:Tl3+

detector with time resolutions of 1 s and 0.25 s. Figure8 presents an example of detection of a gamma rayburst on December 15, 2008 by the BTNM1 instrument and by gamma ray detectors of other spacecraft.It is well seen that variability of the luminosity profileof the gamma ray burst detected in the BTNNeutronexperiment on board the ISS is in good correspondence with those on other spacecraft.

Since neutrons are mostly produced due to interaction of GCR with the Earths atmosphere (for BTNM1 on the ISS) and with the surface of Mars (forHEND onboard Mars Odyssey), one can estimate theGCR flux variation from variations of albedo neutrons. At the moment, the Sun is at a minimum of itsactivity, and, accordingly, the intensity of galactic cosmic rays is maximal [21]. Figure 9 presents the plots ofincreased neutron fluxes generated by GCR in orbitsaround Mars and around the Earth. The data of theHEND experiment were taken for the period 20022009 and for the nearequatorial region Solis Planum,where variability of neutron fluxes is caused only byvariation of the GCR flux, and not by local seasonal

variations of the soil composition. In the case of BTNM1the data are taken for the period 20082009, and forcircumpolar (4551 north and 4551 south) andnearequatorial (15 south north) regions.

The same figure presents the plot of measurementsof the neutrons produced by GCR in the Earthsatmosphere according to the data of a neutron monitor at the Antarctic station McMurdo for the period20022009 [22]. It is well seen that for BTNM1 thetrend is the brightest in circumpolar regions, whereGCR penetrate into the atmosphere most strongly.

CONCLUSIONS

The first stage of the space experiment BTNNeutron with instrumentation BTNM1 allowed us in20072009 to measure neutron background outsidethe ISS pressurized compartment and to investigate itsspatial and time variations. It is planned in 20102012to continue these studies when the next 24th solaractivity cycle will be on the rise. Of special interest willbe the data on neutron fluxes detected during solarflares, SPE, and in periods of disturbances of theEarths magnetosphere. At the same time, the work oncomparison of variations in the neutron fluxes andGCR intensity with analyzing simultaneously the data

Measurements of the neutron dose rate onboard the ISS and near Mars

Experiment, dates of measurements, solar activity leve

Measurement conditions, detectors, range of measurements

Estimation of dose rate accumulation at various segments of the orbit,

Sv per hour

BBND experiment onboard the ISS, Mar 23, 2001Nov 15, 2001, maximum of the 23rd cycle

Inside the ISS, 6 sensors 3He and one scintillation detector, 0.03 eV15.0 MeV

3.5 (equator) 4.6 (nearpolar segments) 12.0 (SAMA)

Experiment BTNNeutron on the ISS, Feb 26present time, A half of the solar activity cycle

Outside the ISS, 3 sensors 3He and one scintillation detector, 0.4 eV10.0 MeV

0.2 (equator) 0.8 (nearpolar segments) 5.0 (SAMA)

The HEND experiment onboard Mars Odissey, Oct 23, 2001present time, maximum of solar activity

Outside Mars Odyssey, 3 sensors 3He and one scintillation detector, 0.4 eV10.0 MeV

0.81.1



of other space experiments and groundbased neutronmonitors. After the beginning of a campaign on detection of gamma ray bursts, it is planned to include thespace experiment BTNNeutron into the international network for localization of gamma ray bursts(GCN Network, [http://gcn.gsfc.nasa.gov]) based onthe method of interplanetary triangulation for determination of directions to the sources of gamma raybursts by comparing the data from different spacevehicles.

At the moment some variants are elaborated tocontinue the experiment with reequipment of the ISS

by the set of instrumentation BTNM2. As a prototypeof the detector unit it is suggested to use the neutronand gammaray spectrometer NS HEND which isunder design at the moment in the Space ResearchInstitute for the Russian interplanetary automaticsample return mission PhobosGrunt. This instrumentis developed for detection of neutrons in a wide energyrange from thermal energies up to energies of about 15MeV and for measurement of the energy spectrum ofgamma rays in the energy range from 100 keV up to 10MeV. The new advanced crystal scintillator on thebasis of LaBr3 is used in this instrument for detection

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Fig. 8. An example of detection of a gamma ray burst on December 15, 2008 according to the data of BTNM1 onboard the ISS(at the top), the KONUS instrument onboard the Wind spacecraft (in the middle), and SPLACS instrument onboard the Integral spacecraft (at the bottom).

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of gamma rays. Radiation resistance of the samples ofthis scintillator was tested under space flight conditions at the first stage of the BTNNeutron experimentin accordance with the program of expeditions ISS14and ISS15 [23]. This scintillator has uniquely highspectral resolution (about 3% for energy of 662 keV).It will give a possibility of measuring basic lines of thegamma ray background both from the upper atmosphere of the Earth and from ISS construction elements. These data will allow one to separate the components of neutron radiation of the atmosphere andISS materials in the common neutron field around thestation.

The second stage of the experiment is planned to bestarted in 20122013. The BTNM2 instrumentationwill be arranged in the pressurized compartments ofthe multipurpose laboratory module (MLM) of the

ISS RS. It will include also a set of shields against neutrons based on hydrogenous and boroncontainingcompounds. The efficiency of these shields will betested onboard the ISS. Comparison of counting ratesin the detectors for different variants and directions ofshielding will allow one to estimate the spatial andangular distribution of the neutron background radiation in the MLM hermetic compartment. Joint processing of measurements made by BTNM1 andBTNM2 instruments inside and outside the ISS pressurized compartments will allow one to construct athreedimensional model of the neutron and gammaray components of the radiation background. Fullscale tests of variants of the radiation protectionagainst neutrons onboard the spacecraft will be performed in order to elaborate source data for creation of

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Fig. 9. Trends of neutron fluxes produced by GCR according to the HEND instrument data taken onboard Mars Odyssey (curve 1 inthe upper plot) and as measured by BTNM1 onboard the ISS (curves 2, 3, and 4 are for the southern, northern, and equatorialregion, respectively). The lower plot presents the neutron flux variation according to the data of a neutron monitor of the American Antarctic station McMurdo.



onboard radiation shelters for the crew of longtermlunar and interplanetary missions.

REFERENCES

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2. Benton, E.R., Benton, E.V., and Frank, A.L., PassiveDosimetry aboard the MIR Orbital Station: ExternalMeasurements, Radiat. Meas., 2002, vol. 35, pp. 457471.

3. Palfalvi, J.K., Akatov, Yu., et al., Cosmic ParticleInduced Reaction Detection with SSNTD StacksExposed onboard of the International Space Station,10th Intern. Conf. on Nuclear Reaction Mechanisms,Varenna, Italy, June 913, 2003.

4. Vana, N., Schoner, W., Fugger, M., and Akatov, Yu.,DOSIMIRRadiation Measurements inside theSoviet Space Station MIRFirst Results, International Space Year Conference, Munich, 1992, p. 193.

5. Beaujean, R., Burmeister, S., Petersen, F., andReitz, G., DOSTEL Dosimetry Data as a Part of DOSMAP, 7th Workshop on Radiation Monitoring for theInternational Space Station, Paris, France, 24 September, 2002.

6. Mitrofanov, I.G., Anfimov, D.S., Kozyrev, A.S., et al.,Maps of Subsurface Hydrogen from High Energy Neutron Detector, Science, 2002, vol., 297, pp. 7881.

7. Litvak, M.L., Mitrofanov, I.G., Kozyrev, A.S., et al.,Seasonal Variations of Neutron Flux in the MartianCap Regions according to Data of the Russian Instrument HEND of the NASA 2001 Mars Odyssey Mission,, Astron. Vestn., 2003, vol. 37, no. 5, pp. 413422.

8. Nymmik, R.A., A Model of Particle Fluxes and EnergySpectra of Solar Cosmic Rays, Kosm. Issled., 1993,vol. 31, no. 6, pp. 5159.

9. Panasyuk, M.I., Bogomolov, A.V., Bogomolov, V.V.,et al., Background Fluxes of Neutrons in Near Space:Experimental Results of SINP, Preprint of SkobeltsynInst. of Nuclear Phys., Moscow State Univ., Moscow,2000, no. 9/613.

10. Sevastyanov, V.D., Tarnovskii, G.B., andLyagushin, V.I., Measurement of the Neutron EnergySpectrum on the Mir Orbital Station, Kosm. Issled.,1997, vol. 35, no. 2, pp. 216220. [Cosmic Research,pp. 201205].

11. Ramaty, R. and Mandzhavidze, N., Theoretical Models for HighEnergy Solar Flare Emission, in Proc. AIPConference High Energy Solar Phenomena. New Era of

Spacecraft Measurements., Ryan, J.M. andVestrand, W.T., Eds., New York: American Institute ofPhysics, 1994.

12. Chupp, E.L., Forrest, D.J., Ryan, J., et al., A DirectObservation of Solar Neutrons Following the 01:18 UTFlare on 1980 June 21, Astrophys. J., 1982, vol. 263,pp. 9599.

13. Chupp, E.L., Solar Neutron Observations and TheirRelation to Solar Flare Acceleration Problems, SolarPhysics, 1988, vol. 118, pp. 137154.

14. Kuznetzov, S.N., Kudela, K., Myagkova, I.N., et al.,First Experience with SONGM Measurements onBoard CORONASF Satellite, Indian J. Radio SpacePhysics, 2004, vol. 33, pp. 353357.

15. Watanabe, K., et al., Solar Neutron Events of 2003October November, The Astrophys. J., 2006, vol. 636,pp. 11351144.

16. Bogomolov, A.V., Britvich, G.I., Myagkova, I.N., andRyumin, S.P., Identification of Neutrons against theBackground of Gamma Rays when Recording Them byDetectors Based on CsI:Tl+3, Prib. Tekh. Eksp., 1996,vol. 39, no. 1, pp. 1319.

17. Bogomolov, A.V., Gudima, K.K., Myagkova, I.N.,et al., Reconstruction of Neutron Spectra from DataObtained with the Use of Scintillation Detectors Basedon CsI:Tl+3, Vestn. Mosk. Cos. Univ., Ser. 3: Fiz.,Astron., 1994, vol. 35, no. 3, p. 81.

18. Mokrousov, M.I. et al., Instrumentation BTNM1 forSpace Experiment BTNNeutron on the Russian Segment of the International Space Station. In press.

19. Krymskii, G.F., Cosmic Rays and nearEarth Space,SolnechnoZemnaya Fizika, 2002, no. 2, pp. 4245.

20. Chishiki, A., Matsumoto, H., Koshiishi, H., et al.,Analysis of the Neutron Radiation Environment insidethe International Space Station as Obtained by a Bonner Ball Neutron Detector, 2nd Intern. Workshop onSpace Radiation Research, Nara, Japan, 1115 March,2002.

21. Ishkov, V.N., The Current 23 Cycle of Solar Activity: ItsEvolution and Principal Features, Proceedings of ISCSSolar Variability as an Input to the Earths Environment, ESA SP535, Sept. 2003, p. 103104.

22. Bieber, J.W., Clem, J., et al., LongTerm Decline ofSouth Pole Neutron Rates, J. Geophys. Res., 2007,vol. 112.

23. Owens, A., The Effect of Long Term Radiation Exposure on Inorganic Scintillator Flown on the International Space Station, Nuclear Instr. and Methods inPhysics Research. In press.

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the first stage of the “btn-neutron” space experiment onboard the russian segment of the...

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