ISSN 0020-4412, Instruments and Experimental Techniques, 2015, Vol. 58, No. 3, pp. 337–344. © Pleiades Publishing, Ltd., 2015. Original Russian Text © G.A. Kononenko, A.G. Artukh, A.N. Vorontsov, D.A. Kyslukha, S.A. Klygin, V.E. Kovtun, V.V. Ostashko, Yu.N. Pavlenko, Yu.M. Sereda, B. Erdemchimeg, 2015, published in Pribory i Tekhnika Eksperimenta, 2015, No. 3, pp. 35–42.
NUCLEAR EXPERIMENTAL TECHNIQUE
Detection System of the COMBAS Fragment Separator G. A. Kononenkoa*, A. G. Artukha, A. N. Vorontsova, b, D. A. Kyslukhaa, c, S. A. Klygina, V. E. Kovtuna, V. V. Ostashkob, Yu. N. Pavlenkob, Yu. M. Seredaa, b, and B. Erdemchimega, d a
b
Joint Institute for Nuclear Research, ul. Joliot-Curie 6, Dubna, Moscow oblast, 141980 Russia Kiev Institute for Nuclear Research, National Academy of Sciences of Ukraine, pr. Nauky 47, Kyiv, 03680 Ukraine c Karazin Kharkiv National University, pl. Svobody 4, Kharkiv, 61022 Ukraine d National University of Mongolia, Nuclear Research Center, UlaanBaatar, Mongolia * e-mail:
[email protected] Received May 21, 2014
Abstract—The (∆E1, ∆E2, E) multidetector telescope has been developed and tested. The telescope is a compact combination of two 32-strip silicon ∆E1-, ∆E2-detectors and a scintillation E-detector, which is a 3 × 3 array of CsI(Tl) crystals. A timing signal can be obtained from any of the Si detectors for time-of-flight (TOF) measurements. Single-channel and 32-channel charge-sensitive preamplifiers have been developed and manufactured for the multidetector module to serve the ∆E- and Е-detectors with high efficiency and sensitivity. Reaction products obtained in the 40Ar (35 MeV/nucleon) + 9Ве reaction have been unambiguously identified in the experiment by mass number A and atomic number Z. DOI: 10.1134/S0020441215020207
1. INTRODUCTION Extraordinary variety of nuclear reactions with heavy ions and numerous possible combinations “ion−target nucleus” open up favorable prospects both for obtaining and forming secondary beams of accelerated radioactive nuclei with anomalous neutrons-to-protons ratio and for studying the structure of exotic nuclei. Reactions with heavy ions are very efficient in synthesis with unknown isotopes at the nuclear dripline or even beyond it. Mainly in-flight separators [1, 2] have been used today to obtain and separate secondary beams of radioactive nuclei with predetermined values of mass number A and atomic number Z. Nevertheless, using the in-flight separation techniques, it is impossible to obtain intense monoisotopic beams of secondary radioactive nuclei without significant losses of their intensity. It is known that ions—nuclear reaction products—obtained in exchange and fragmentation reactions have wide momentum distributions and different charge states Q. As a result, monoisotopic separation of nuclear reaction products cannot be ensured solely by magnetic rigidity Bρ, since magnetic rigidity Bρ of the separator is a function of a few parameters of analyzed particles, namely: Вρ ~ P/Q =Av/(Z – n), where B [T m] is the magnetic field of the separator; ρ [m] is the radius of the central track of the separated particle; P is the particle momentum; Q = (Z – n) is the charge state of an ion, which is equal to the differ-
ence of nuclear charge Z and number n of captured electrons; and v [m/s] is the velocity of transported particles. Therefore, a multiparameter detection system must be used for unambiguous identification by А and Z of nuclear reaction products, including their charge states Q. For this reason, the detection system of the COMBAS fragment separator is formed from a cascade of drift X- and Y-coordinate detectors with different thicknesses, which measure the ionization losses of particles ΔE with simultaneous signal generation for the TOF system and detector of residual energy absorption Еr,
Δ E ~ (Z / v ) 2, Δ E + Е r ~ ( A / v 2 )/2
and
TOF ~ (1/ v ) .
In combination with magnetic analysis, this variant of the detection system allows one to simultaneously measure the whole spectrum of transported particles both for light long-range products (within the limits of the E-detector thickness) and for short-range high-Z heavily ionizing particles (only a few forward ∆E-detectors are used). Analysis shows that a set of parameters Вρ, ∆E, Еr, and TOF is sufficient for unambiguous identification by А and Z of reaction products, including charged states of their ions Q. The aim of this work is to develop a set of detecting modules (∆E1, ∆E2, Er), which could provide a sufficient spatial angle of detection, a high angular resolution, and unambiguous identification of the whole
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3
2
4 5
1
(a)
(b)
Fig. 1. External appearance of the detector telescope with the preamplifiers, which was mounted on the 180-mm-diameter vacuum-tight plug, (a) after its housing was removed: (1) Х-coordinate strip detector, (2) Y-coordinate strip detector, (3) squeezing strap with vacuum rubber seals, (4) frame–holder with the 3 × 3 array of CsI(Tl) detectors, and (5) preamplifiers; (b) the telescope as a complete assembly. The diameter of the housing is 135 mm, and its height is 170 mm.
spectrum of reaction products transported toward the output focal plane of the COMBAS separator. Each module is a compact telescopic combination of two 32-strip coordinate (X, Y) silicon ΔE-detectors and a scintillation CsI(Tl) E-detector composed of a 3 × 3 array of CsI(Tl) crystals, which provides a means for obtaining the timing signals from the strip detectors for TOF measurements. Special charge-sensitive preamplifiers were developed for servicing the hybrid Si + CsI(Tl) telescope. 2. TELESCOPE DESIGN The telescopic module is mounted in the vacuum inside the inlet chamber of the COMBAS fragment separator [2]. The detector system of the telescope consists of two silicon drift X- and Y-coordinate strip ∆Е-detectors with a large area (66 × 66 mm each) and the 3 × 3 array of total-absorption scintillation Е-detectors, which is located behind the strip detectors. The strip detectors have been designed both for spectrometric and angular measurements of particles and for determining the profile of their distribution in the position of the achromatic output focus of the fragment separator. The total-absorption scintillation detectors are required for energy measurements of long-range reaction products passing through both strip detectors. The external appearance of the telescope is shown in Fig. 1. Thin (380 μm) strip Si detector 1 (Fig. 1а) is located the first downstream of the beam. It is oriented
so that its strips lie in the vertical plane (the Х-coordinate detector). At a distance of 10 mm past detector 1, there is 1-mm-thick strip Si detector 2. It is installed so that its strips are orthogonal to detector 1 (the Y-coordinate detector). The dimensions of the strip detectors in the plane are 66 × 66 mm. Each of them contains 32 strips with a length of 60 mm and a width of 1.9 mm. The strip pitch is 2 mm. In the telescope, the strip detectors are inserted in the connectors on fiberglass boards, which are fixed in place on a rectangular frame with a window. The frame is fastened to two racks, which are fixed in position on the vacuum-tight plug disk via an insulating gasket. The detectors were produced by the Ioffe Physical Technical Institute, Russian Academy of Sciences (St. Petersburg) and the Research Institute of Materials Science and Technology (Zelenograd, Moscow, Russia). The array of nine total-absorption scintillation detectors is placed past the Y-coordinate strip detector at a distance of 10 mm. Each of them is composed of a CsI(Tl) scintillator crystal viewed by a silicon p–i–n photodiode (PD). We used unpackaged p–i–n diodes with surface area S = 10.6 × 11.6 mm2 and sensitive layer thickness d0 = 380 mm. PD capacitance value is СD ≈ (1 pF/cm × S/d0) ≈ 41 pF. A ceramic package with contacts mounted inside it was manufactured separately. A PD was fixed in place and soldered in it and, thereafter, poured with epoxy resin. For the exit and entrance windows of the scintillator and the PD to be matched, a CsI(Tl) crystal was
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Telescope
U-400M cyclotron
Target
eam mary b
Pri
Bρ3
Bρ2 Bρ1
Bρ1 < Bρ2 < Bρ3
Fa
Fig. 2. COMBAS fragment separator disposed in the experimental hall of the U-400M cyclotron at the Joint Institute for Nuclear Research.
put into an intricate shape consisting of a rectangular parallelepiped with dimensions of 18.0 × 18.0 × 3.5 mm and a pyramidal part. The pyramidal part is 3.6 mm in height, its upper base is 18 × 18 mm in size, and the dimensions of its lower base, coinciding with the dimensions of the PD window, are 11 × 11 mm. The total scintillator volume is 5.1 cm3. The side walls of the crystal are treated with a sand cloth (no. 400) until the surface becomes matte and, thereafter, are coated with a diffuse light-proof material, by way of which we use 2 or 3 layers of 0.1-mm-thick Teflon band. The front surface of the crystal facing toward the beam is treated with a finer (no. 1500) sand cloth with lacquer petroleum and is coated with a Mylar film 2– 5 μm thick. The solvent is used, because it helps avoid coarse damages to the crystal surface and makes its relief more uniform. The crystal surface adjacent to the PD is polished until it shines. Thereafter, the scintillator is coupled to a PD by means of an optical adhesive. Nine crystals with PDs coupled to them are combined into a 3 × 3 array and fixed in place in frame−holder 4 (Fig. 1а), which is located near the drift strip Si detectors. The detectors are placed in cells with connectors on a common printed circuit board. Charge-sensitive preamplifiers 5 are also attached to this board by means of connectors. The printed circuit INSTRUMENTS AND EXPERIMENTAL TECHNIQUES
board with the detectors and the preamplifiers is attached to the carrying frame−holder, which has a window for crystals. Four squeezing straps with vacuum rubber seals 3 are inserted in the grooves on the face part of the carrying frame. They can move over the grooves of the frame and be rigidly fixed in position on it. They are used for mechanical fixation of the detectors. The carrying frame with the scintillation detectors is attached with screws to the back side of the rectangular frame with the strip detectors. The signals from each strip of the Х and Y detectors, as well as from their common electrodes, are fed over short cables to the corresponding (Х or Y) vacuum 50-pin OSRS50TV connectors. The RSGSP 50-V counterparts (the feedthrough connectors) are attached to the disk of the vacuum-tight plug. The bias voltages for each detector are supplied through two sealed SRG-50-82fv connectors, which are also fixed in place on the disk. The output signals of the scintillator detector preamplifiers are read out over a separate OSRS50TV connector. The output signals from the common electrodes of the strip detectors, the bias voltage (+60 V) of the CsI(Tl) detector PDs, the test signal, and the supply voltage for the scintillation detector preamplifiers also arrive at this connector. The telescope is fixed in place in vacuum on the flange of the inlet chamber, which is located in the Vol. 58
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(a)
(b)
Fig. 3. Output noise signals from the spectrometric amplifier (a) with the preamplifier being connected to it and (b) without the preamplifier.
position of output achromatic focus Fa of the COMBAS fragment separator (Fig. 2). 3. CHARGE-SENSITIVE PREAMPLIFIERS The detector module has 75 spectrometric channels each of which is equipped with its own chargesensitive preamplifier and allows individual readout. Since the capacitances of the scintillation detectors (≈41 pF) and isolated strips (≈30 pF for a 380-μm detector and ≈11 pf for a 1-mm detector) are small, their preamplifiers were designed according to the simplest classical scheme [3, 4]. It consists of the input stage with cascode connection of JFET-pnp transistors [5] and a composite emitter follower at the output. The preamplifiers are produced on elements of surface mounting in the form of small modules with pin connectors and dimensions of 25 × 11 mm. From the side of the common electrodes, the capacitance of the strip detectors is much greater: ≈1000 pF for 380-μm detectors and ≈360 pF for 1-mm ones. Therefore, several charge-sensitive preamplifiers having a higher gain at open negative feedback were produced for detecting the signals from the common electrodes. Their input stage is similar to the stage described above; however, the load resistor in their cascode collector is replaced with an active load based on a single transistor (a current generator) [6], and a buffer element is set at the output. BF861A field-effect transistors with a low working drain current (5 mA) are used in the input stages of the preamplifiers for the scintillation detectors operating in vacuum. This has allowed the power consumption from the power supplies for one preamplifier to be only 100 mW. BF862 transistors are used in the preamplifiers for the strip detectors. The detector module contains preamplifiers for only nine CsI(Tl) + PD detectors. The preamplifiers
for the strip detectors are inserted in individual sockets with pin connectors inside cassettes with 32 detectors in each cassette. The cassettes are located beyond the vacuum chamber of the reception facility of the COMBAS fragment separator and are connected to the respective connectors of the detectors by means of short (20 cm) cables. Adjacent to them, there are two preamplifiers connected to the common electrodes of the strip detectors. The preamplifiers and the detectors were tested on a setup consisting of standard spectrometric facilities and, in addition, of a vacuum chamber containing the telescope and a shielded electronic module in which a necessary preamplifier was set via the connector. Channels 2700 2600 2500 2400 2300 2200 30
40
50
60
70
80 UB, V
Fig. 4. Position of the 241Am α-particle peak with Eα = 5.485 MeV vs. the bias voltage applied to the p–i–n diode. The diode was irradiated from the back side.
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Counts 800 600
59.54 keV
26.34 keV
400 FWHM = 2.8 keV 200 0 200
100
300 Channels
Fig. 5. 241Am X-ray spectrum measured by the Si p–i–n PD with capacitance CD = 41 pF, area S = 10.6 × 11.6 mm, and thickness 300 μm.
When the noise characteristics of all preamplifiers were measured during the tests, they were disconnected from the detector (CD = 0) and tuned to gain Gp ≈ 1 V/pC, which corresponded to the sensitivity in terms of energy units GE ≈ 44 mV/MeV for the Si detector. The mean inherent noise of the charge-sensitive preamplifiers for the strip and scintillation detectors was 2 keV. The preamplifier noise reduced to the input in the form of the equivalent noise charge (ENC) in terms of rms electrons was
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sured parameter UN (rms). We estimated value UN (rms) from oscillograms of the noise, by measuring the maximum deviations in the time interval of observations of peak-to-peak noise voltage amplitude U(peak-to-peak) and assuming the peak factor to be U(peak-to-peak)/UN(rms) = 6. Peak-to-peak amplitude U(peak-to-peak) of the noise voltage was measured at the output of the BUI-3K spectrometric amplifier having a time constant of 1 μs and gain Gamp = 100 with the preamplifier being connected and, thereafter, disconnected. For illustration, Fig. 3 presents two oscillograms of the noise signals, obtained while testing one of the preamplifiers for strips: the output noise of the BUI3K spectrometric amplifier with the preamplifier being connected to it (Fig. 3а) and without the preamplifier (Fig. 3b). The peak-to-peak amplitudes of these signals were UN1(peak-to-peak) = 23 mV and UN2(peak-to-peak) = 8 mV, respectively. According to these data, the rms value of the noise voltage is UN(rms) ≈ 3.6 mV, and ENC ≈ 225 rms electrons. After conversion for the silicon detector, the full width at half-maximum (FWHM) of the noise amplitude distribution in terms of the energy units is FWHM = 2.355 × ε × ENC = 2.355 × 3.61 × 225 ≈ 1.9 keV,
ENC = U N ( rms ) / G pG ampq e ,
where ε = 3.61 eV is the energy spent on the production of a pair of charged particles in silicon. The preamplifier noises for the common electrodes were 7.5 keV at an input capacitance of 360 pF and 22 keV at 1000 pF.
where UN(rms) is the rms noise voltage at the output of the spectroscopy amplifier, Gamp is the gain of the spectrometric amplifier, and qe is the electron charge. Since the measured noise signal is white noise with the Gaussian distribution and a zero mean value, its standard deviation σ is equal to the rms value of mea-
4. DETECTOR CHARACTERISTICS 4.1. CsI(Tl) Scintillator Detectors p–i–n PDs with the lowest possible dark current were selected for the scintillation detectors. Before a
Counts 3000 60
Co
1.17 MeV
2250
1.33 MeV 1500 FWHM = 4.8%
750 0 100
200
300
400
500
600
700
800 Channels
Fig. 6. 60Co γ-ray spectrum measured by the CsI(Tl) + PD detector with a crystal volume of 5.1 cm3. INSTRUMENTS AND EXPERIMENTAL TECHNIQUES
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FWHM, % 4.0
FWHM, % 2.0
3.5
1.5
3.0
1.0
2.5
0.5
2.0
0
0
4
8
12
16
20
24
1.5 1.0
0
2
4
6
28 32 Strip no.
Fig. 8. Strip-by-strip resolution FWHM of the 1-mmthick detector (UB = 150 V, I0 = 950 nA). The 226Ra source was in vacuum.
8 10 Detector no.
Fig. 7. Resolution FWHM of the array of nine CsI(Tl) + PD detectors. The 226Ra source was in vacuum.
spectra is shown in Fig. 5. This spectrum was recorded for a PD with a dark current of 3 nA and bias voltage UB = 60 V. The spectrometric section consisted of the above-described strip preamplifier—a BUI-3K spectrometric amplifier with gain K = 100 and time constant τ = 1 μs. Energy resolution Δ E (FWHM)/ E for the 59.54 keV peak was ≈5%.
diode was placed in the ceramic package, bias voltage UB providing full depletion of the p–n junction was measured for each diode. To do this, the diodes were irradiated from the back side with α particles from a 241Am source, and the position of the α particle peak in the amplitude spectrum was measured at different bias voltages. The results of these measurements are presented in Fig. 4 from which it is apparent that the full depletion regime begins at a bias exceeding 50 V.
The quality of the manufactured scintillators was estimated by their energy resolution R = Δ E (FWHM)/ E for the full-energy γ-ray peak at 1332 keV (60Co). One more reference PD was used for this purpose: a crystal under investigation was coupled to it. The PD for crystal testing was selected so that its reverse current was minimal (3 nA). When fixed in place in the ceramic package, it was not coated with epoxy resin. A small invariable air gap was maintained between the tested crystal and the sensitive surface of the diode.
After each PD was enclosed in a ceramic package, soldered, poured with epoxy resin, and, thereafter, tested for the noise level and the dark current value. The noise properties of a PD were estimated by the FWHM of the 59.54 keV X-ray peak in the 241Am spectrum from a tested PD. An example of such X-ray Counts 500 226 88
400
Ra
4.79 MeV
222 86
Rn
5.5 MeV
300 210 84
200 100
226 88
Ra
218 84
214 84
Po
Po
7.7 MeV
6.0 MeV
Po FWHM = 0.9%
5.3 MeV
4.6 MeV
0 1000
1500
2000 Channels
Fig. 9. 226Ra α-particle spectrum obtained from a single strip of the 1-mm-thick detector. INSTRUMENTS AND EXPERIMENTAL TECHNIQUES
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dE, channel 2500 40
Q=Z−2 Ar+16
Q=Z−1
39
2000
38
K+18
Cl+16 36
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1500
Ar+17
Cl+16
32 +15
S
30 28
1000
Al+12 28
22
500
20
P+14
Si+13
24 Mg+11 Na+10
Ne+9
2000
2500
3000 Er, channel
Fig. 10. Identification matrix of nuclear reaction products with atomic numbers 4 < Z < 20, obtained in reaction 40Ar (E = 35 MeV/nucleon) with a 9Be target (15 mg/cm2). The reaction products were extracted by the COMBAS fragment separator and transported toward the output focus at the position of the ΔE–E-detector telescope.
Figure 6 presents the typical 60Co γ-ray spectrum obtained from one of the crystals of the CsI(Tl) + PD detectors comprising the telescope array. The distribution of the resolutions of the CsI(Tl) + PD telescope detectors is shown in Fig. 7. The resolution was determined as the FWHM of the peak of 7687 keV α particles from a 226Ra source. The detectors were exposed to the α-particle source in vacuum. 4.2. Strip Detectors The full depletion voltage was measured for both detectors according to the procedure used for the PDs. It was 120 V for the 380-μm strip detector and 150 V for the 1-mm detector. For these voltages, the dark current (I0) was 540 nA for the thin detector and 0.95 μA for the 1-mm detector. The strip-by-strip resolution of the detectors was measured using a collimated 226Ra source at the facility in the vacuum box. The mean resolution was 1% for both detectors. The strip-by-strip resolution of the 1-mm detector is shown in Fig. 8. As was the case with the CsI(Tl) + PD scintillation detectors, the resolution was determined as the FWHM of the 7687 keV αparticle peak from the 226Ra source. Figure 9 presents the α-particle spectrum of the 226Ra source, obtained from one of the strips in the 1-mm detector irradiated in vacuum. INSTRUMENTS AND EXPERIMENTAL TECHNIQUES
5. CONCLUSIONS The (∆E1, ∆E2, E) multidetector measuring telescope capable of providing a significant spatial angle of detection and unambiguous identification by А and Z of reaction products transported toward the output focal plane of the COMBAS fragment separator has been developed and tested. The module is a compact telescopic combination of two 32-strip silicon ∆E-detectors and the 3 × 3 array of CsI(Tl) + PD scintillation E-detectors, which provides a means for obtaining a timing signal from any of the strip detectors for TOF measurements. Charge-sensitive preamplifiers have been developed and manufactured for the multidetector module to serve the silicon ∆E-detectors and the CsI(Tl) + PD scintillation Е-detectors with high efficiency and sensitivity. The quality of the isotope identification of nuclear reaction products obtained in reaction 40Ar (35 MeV/nucleon) + 9Ве using the COMBAS fragment separator and the described detection system is illustrated by Fig. 10. From the array presented in Fig. 10, it is apparent that a combination of the COMBAS fragment separator with the designed telescopic measuring module ensures a high resolution and makes it possible to unambiguously determine mass numbers А and atomic numbers Z of nuclides and charge state Q of Vol. 58
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their ions for a wide spectrum of nuclear reaction products. The detection system of the COMBAS fragment separator, produced from a complex of such telescopic measuring modules, seems to be very promising for measuring rare multiparticle decays of exotic nuclei. Investigations of these processes may be efficient if the detection system is capable of detecting particles in a cone of the maximum spatial angle and the high angular resolution of correlated clusters in multiparticle decay of an unstable nucleus. By its identification ability and data quality, this complex is expected to possess unique possibilities in studying the structure of exotic nuclei obtained in reactions with ultralow yields near the nuclear dripline. A high granularity of each detector module can be used to good effect to study the angular, energy, and time correlations in multiparticle decays of exotic nuclei.
Sereda, Yu.M., Shchepunov, V.A., Szmider, J., Teterev, Yu.G., Bondarenko, P.G., Rubinskaya, L.A., Severgin, Yu.P., Myasnikov, Yu.A., Rozhdestvenski, B.V., Konstantinov, A.Yu., Koreniuk, V.V., Sandrev, I., Genchev, S., and Vishnevsky, I.N., Nucl. Instrum. Methods Phys. Res., A: Accel., Spectr., Detect., Assoc. Equip., 1999, vol. 426, p. 605, DOI:. doi 10.1016/ S0168-9002(98)01383-7. 2. Artyukh, A.G., Sereda, Yu.M., Klygin, S.A., Kononenko, G.A., Teterev, Yu.G., Vorontsov, A.N., Kaminski, G., Erdemchimeg, B., Ostashko, V.V., Pavlenko, Yu.N., Litovchenko, P.G., Kovtun, V.E., Koshchii, E.I., Foshchan, A.G., and Kislukha, D.A., Instrum. Exper. Tech., 2011, vol. 54, no. 5, p. 668. 3. Fabris, L., Madden, N.W., and Yaver, H., Nucl. Instrum. Methods Phys. Res., A: Accel., Spectr., Detect., Assoc. Equip., 1999, vol. 424, p. 545. 4. Bertuccio, G., Rehak, P., and Xi, D., Nucl. Instrum. Methods Phys. Res., A: Accel., Spectr., Detect., Assoc. Equip., 1993, vol. 326, p. 71.
ACKNOWLEDGMENTS We thank V.V. Avdeichikov and E.A. Shevchik for their fruitful help in manufacturing the scintillation detectors and testing them on heavy ion beams.
5. Nemchinov, V.M., Nikitaev, V.G., Ozhogin, M.A., and Lyakhovich, V.V., Usiliteli s polevymi tranzistorami (Amplifiers with Field-Effect Transistors), Moscow: Sovetskoe Radio, 1980.
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6. Gusev, V.G. and Gusev, Yu.M., Elektronika (Electronics), Moscow: Vyshaya Sshkola, 1991.
1. Artukh, A.G., Gridnev, G.F., Gruszecki, M., Koscielniak, F., Semchenkov, A.G., Semchenkova, O.V.,
Translated by N. Goryacheva
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