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The present paper presents works carried out about designing of multiwire proportional chambers with a 3He converter and a multistep avalanche chamber with a solid-state gadolinium converter. Both detectors are intended for a position-sensitive registration of thermal neutrons at neutron diffraction investigation.
MULTIWIRE PROPORTIONAL CHAMBER WITH A 3HE-BASED GAS
CONVERTER
The detector is designed for modernization of DN-2 diffractometer, arranged on the beams of the pulse reactor IBR-2 Laboratory of Neutron physics of the Joint Institute of Nuclear Research, Dubna, Moscow region. This diffractometer is intended for structural investigations of complex crystals. A two-coordinate multiwire proportional chamber (MWPC) consisting of three electrodes arranged one over another within a single gas volume is selected as the detector The sensitive volume of MWPS is 260x130x12 mm3. The MWPS is placed within a seal-tight box made from aluminum with a 6 mm-thick entrance window. The box's dimensions are 580 x 440 x 130 mm 3. The net gas volume of tile box is 4,5 liter.
The mixture of 3He and (10-20%) of propane is used as a gaseous mixture of the detector. The operation pressure of the detector is 4 atm. Apart from the gaseous additive of MWPS the propane is used to reduce the path of protons forming as a result of the reaction n + 3He - p + 3He + 764 KEV.
For long-term operation of the detector the Ni/SiO2 getter-based gas recovery system is developed. To purify the gas from electronegative impurities the former is pumped through the getter at room temperature by force. The duration of purification process doesn't exceed 20 min. The operation time of the detector between purification processes is 2-3 months.
The basic parameters of detectors are: Sensitive volume - 260x130x12 mm, Gaseous mixture - 3He + (10-20%) of propane, Entrance window- 6 mm Al, Operation pressure - 4 atm, Detection efficiency (at λ=1.8A)-70%, Spatial resolution-<3mm (FWHM), Time resolution (by anode signal)- 20ns, Counting rate- 105 1/s.
MULTISTEP AVALANCHE CHAMBER WITH a SOLID-STATE
GADOLINUM CONVERTER.
A multistep avalanche chambers (MSAC) consists of a conventional MWPC placed within a single gaseous volume with additional grid electrodes forming preamplification and drift spacing. Gadolinium converter settles down directly ahead of a preamplification grid. MSAC registers electrons escaping off in a back hemisphere. In the preamplification gap, there forms an electron-photon avalanche, which is transferred to MWPC for consequent amplification and registration. Due to the exponential amplification in the preamplification gap there is implemented a tie of coordinates to the entrance point of particles to the detector volume. The spatial resolution of MSAC can be improved by the amplitude analysis and the consequent mathematical processing of events up to 0,2-0,3 µm. The gas amplification coefficient of MSAC is of the order 106-7 which provides registration of single electrons with practically zero energy. Thus, the registration efficiency of the MSAC-based detector is mainly defined by converter’s characteristics.
We have lead works on modeling calculations of efficiency of registration of thermal neutrons by foil converters from natural gadolinium and its 157 isotopes.
The basic parameters of detectors are: Sensitive volume - 200x200 mm, Gaseous mixture - Ar + (2%) of acetone, Operation pressure - 1 atm, Detection efficiency (at λ=1.8A)-35%, Spatial resolution-<1mm (FWHM), Time resolution (by anode signal)- 20ns, Counting rate-1061/s.
