Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

“Physics ofAuroral Phenomena", Proc. XXXIVAnnual Seminar, Apatity, pp. 103-1052011 £ J T \ Polar © Kola Science Centre, Russian Academy of Science. 2011 (П/M i Geophysical \JJ у Institute A NEW NEUTRON SP EC TROM ET ER W ITH NARROW D IAGRAM OF A C C E P TAN C E Yu.V. Balabin, E.A. Maurchev (Polar Geophysical Institute, Kola Science Center o f RAS, Apatity, Russia) Abstract. The instrument for spectrometry of neutrons has been designed. The original design scheme which is distinct from the widely spread Bonner type neutron spectrometer is used. Main difference is in angular selectivity that is absent in the Bonner spectrometer. The helium counters of thermal neutrons SNM-18 surrounded by a screen and moderator layers for various ranges of neutron energy are used as detectors. The protection from thermal neutrons ensures the complete shielding of the counter from selected directions that has allowed creating of a spectrometer accepting radiation only from the given direction. The energy diapason of a spectrometer makes from a thermal range up to hundreds MeV. The important feature of the instrument is its angular directivity: the angle of acceptation does not exceed 30-40 degrees. Construction of a spectrometer (with the detailed representation of all materials which are included in it) has been simulated on GEANT-4 package. This has allowed us to investigate response of the instrument to neutrons and other particles of various energies from 0.1 eV up to 300 MeV. The working experimental sample o f a spectrometer unit is manufactured and its calibration is carried out. Technical data of the instrument appeared in the consent with calculated. Measuring of the thermal neutron intensity at the ground has shown a significant anisotropy of neutron flux. Determination of a neutron spectrum over all the indicated energy diapason requires a solution of an inverse problem: deconvolution. 1. Construction of the spectrometer The principle of operation of the spectrometer sets up on a phenomenon of an effective moderation and reflection of neutrons by the materials containing of many hydrogen atoms (polyethylene, paraffin etc.). Only neutrons with energy more than some threshold depending on width of substance can overcome a stratum of such substance. On this principle widely spread Bonner type spectrometers are made [1-3]. However, they have an essential shortage - they are omni-directional. On the basis of the detector of thermal neutrons SNM-18 the spectrometer with a directional diagram about 30-40 degrees has been designed. A core of the spectrometer unit is SNM-18. The counter tube SNM-18 represents the cylinder in diameter of 32 mm and length 300 mm filled by 3He. The counter is protected from all directions, except for a rather narrow band along the cylinder (the reception window), a shield (the substance swallowing thermal neutrons with effectiveness more than 99 %). A thick stratum of polyethylene encloses the counter together with its shield. In such construction we have a narrow directed detector of thermal neutrons. As it will be said below the thick stratum of polyethylene is need to reflect neutrons totally. Both the stratum and protection from thermal neutron provide so good shield from backward neutrons. The operating unit of the spectrometer is shown on Fig.l. There is a petal directed detector of thermal neutrons in such construction. Direction sharpness is determined the reception window and equal ~ 40 degrees in petal cross-section; it is about 120-140 along the petal. For measuring neutrons of high energies the reception window of a unit is covered by a particular thickness polyethylene moderator. Only neutrons with energy higher than a particular threshold pass through. The package of such units is the neutron spectrometer of wide energy diapason. The package of a spectrometer may consist of 5-7 such detectors. The window of each of them is covered by a polyethylene slab of the given thickness. Thus the detector appears sensitive to neutrons above a particular threshold of energy. The complete spectrum of neutrons is obtained via a numerical solution of an inverse problem: deconvolution of the all detectors data. 2. Modeling of the spectrometer with GEANT-4 2.1. Geometry and the experiment concept The neutron spectrometer operation was modeled before manufacturing. To determine the detector response of the neutron spectrometer the geometry and material composition of it were implemented in GEANT-4 as real as possible. In the Fig. 1 visualization of a neutron spectrometer is presented. Fluxes of neutrons with energies over range 10‘‘-109 eV were used on each modeling pattern. The shield quality was checked (modeled) initially. It was found that the shield swallows all neutrons (effectiveness is more than 99 %). Than the counter tube was closed by polyethylene plates with various thickness (in figure these plates are not shown). In the first part of numerical experiments neutrons with a random initial position and angular distribution dropped on the detector surface bounded by sizes o f the counter (model approximation). In the second part of numerical experiments neutrons were started with the given incident direction (from 0 up to 90 degrees relative to vertical incident direction). 2.2. Methodology For the calculations presented in this work, the GEANT-4 simulation toolkit version 9.4 was used on openSuSe 11.2 operating system [4]. Additionally, the neutron data library G4NDL3.14 was installed, which includes cross sections 103