New Target Cryostat for Experiments with Negative Muons

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D13-98-146

V.F.Boreiko, V.M.Bystritsky, V.I.Datskov, A.N.Fedorov, V.N.Pavlov, V.A.Stolupin, A.Del Rosso1, R.Jacot-Guillarmod1, F.Mulhauser1, L.A.Rivkis2

NEW TARGET CRYOSTAT FOR EXPERIMENTS WITH NEGATIVE MUONS

Submitted to «Nuclear Instruments and Methods*

'institute of Physics, University of Fribourg, Switzerland 2A.A.Bochvar All-Russia Research Institute of Inorganic Materials, Moscow, Russia 30-07 1998 Introduction

The study of processes yielding charge nonsymmetric muonic molecules like pxZ (z = p,d,t and Z = He, Li, Be) and nuclear fusion started about fifteen years ago. Theoretical and experimental interest have increased these past five years [1-13]. Due to some experimental difficulties, the muon catalyzed fusion (pCF ) is essentially measured in deuterium-helium mixtures. The nuclear fusion from the charge nonsymmetric molecule pd 3He may occur via two different channels,

pd3He -4 a + p(14.7 MeV) + p (1) -4 psLi +7(16.4 MeV), (2) whereas the pd4He molecule fuses via a single channel

pd*He -4 p6U + 7(1.48 MeV). (3) The proposed apparatus should be able to easily detect those fusion products. In addition, the 6.85 keV soft X rays resulting form the decay of the pd4He and pd3He molecules should be measured, as well as the muon decay electrons. The detection of so many different types of particles, with totally different paths in matter, is a non-trivial constraint in the design of the apparatus. The different processes which occur when a muon enters a deuterium-helium mix ture are strongly dependent on the mixture density. Hence, the gas mixture pressure and temperature are essential parameters. A special temperature control system is required to keep them as stable as possible. In addition, the level of impurities (N2, 0a, H20 and C02) in the deuterium-helium mixture has to be very low, to avoid competing processes. The total amount of impurities should not exceed 500 ppb. A gas filling system has been constructed to allow such a purity. The study of pCF in a deuterium-helium mixture requires an integrated set of devices (target, gas supply, detectors and temperature control system) that provide the optimal conditions [12]: a pressure range of 0-10 atm and a temperature range of 21-300 K. The set-up presented in this paper was tested at the Paul Scherrer Institute (PSI), Switzerland, during a two weeks measurement. Pure deuterium and a mixture of deuterium-helium at pressures of 5.6-6.4 atm were used at a temperature of about 32 K.

The cryostat

Experimental studies of muon catalyzed fusion in deuterium-helium mixtures re quire some special cryogenics and vacuum design for the development of the target. Figure 1 shows a general view of the cryostat. The cryogenic gas target (1) is enclosed inside a vacuum chamber (2) made of 2 mm thick stainless steel. The target body is made of the aluminium alloy ADI (Al> 99.3%, Fe< 0.3%, Si< 0.3%,

1 Cu< 0.05%, Mn< 0.025%, Zn< 0.1%, Ti< 0.05%, Mg< 0.05%). It is a sphere of internal diameter of 66 mm, which gives a volume of % 250 cm3. The use of this aluminium alloy is important to suppress almost completely background from characteristic X rays. Aluminium X-ray energies substantially differ from the energy of the 6.85 keV line, whereas stainless steel X rays have an energy very close to this line. There are 5 Kapton windows around the target (1). The choice of window diam eters is due to the size of the detectors located behind them. The window thickness was chosen after several tests with varying gas pressure and temperature. A high pressure in the target allows a good stopping rate of the incident muons and high experimental rates. Construction constraints have lead to a maximum pressure of the deuterium-helium mixture below 10 atm at a temperature of about 32 K. The muon beam entrance window has a 40 mm diameter and is 135 pm thick. The four re maining windows are in the planes perpendicular to the muon beam. Three windows. 45 mm diameter and 135 pm thick, have been designed to install the Si detectors (4) in order to measure the 14.7 MeV protons. The last window, 15 mm diameter and 55 pm thick, was set-up to install a Ge detector (Ge) to measure the 6.85 keV X rays. With the above mentioned dimensions a gas pressure of 13 atm was reachable at a temperature of 77 K. The diffusion of deuterium or helium through the windows is negligible. So, no corrections for the target density variations are necessary during the measurements. The Kapton windows are sealed with indium gaskets and stainless Steel flanges (3). The silicon detectors (4) (diameter 42 mm, thickness 4 mm), which record the. fusion protons, are attached to the target body immediately behind the Kapton win dows. Air-tight connectors are provided in the vacuum chamber wall for high voltage power supply of the Si detectors and read-out of their signals. The temperature of Si detectors will never be lower than 40 K [14]. Below this temperature the output signal for a given energy decreases by about a factor five compared to the same energy for temperatures above 40 K. To decrease the heat flow to the target due to the thermal radiation of the vacuum chamber walls, a thermal shield (5) has been installed inside the chamber. The shield is made of copper with a thickness of 1 mm. Behind the Si detectors the shield (6) is made of 0.3 mm aluminium. The shield on the muon beam path and in front of the Ge detector, is made of 12 pm thick Mylar with 1 pm A1 coating. The vacuum chamber (2) has two Kapton windows: one along the muon beam axis (40 mm diameter, 55 pm thick) and the second in the direction of the Ge detector (20 mm diameter, 25 pm thick). These windows are glued with epoxy and scaled with stainless steel flanges. In addition, 0.5 mm thick aluminium windows (6) are placed in the vacuum chamber walls in front of the outer detectors. These windows are sealed with flanges and viton gaskets. The external detectors are plastic scintillators for the detection of the muon decay electrons as well as neutron detectors, BGO, and Ge detectors.

2 The target is filled with deuterium, helium, or their mixture through a small stainless steel tube (7). The target is cooled by liquid helium (for a temperature range between 21-70 K) or liquid nitrogen (70-300 K), flowing through the target heat exchanger (8). A second heat exchanger (9) is placed on the mounting flange (10), to cool the shield and thus, the different parts of the cryostat are cooled by the same coolant flow. Coolant is evacuated by a vacuum pump of throughput 20 1/s.

The temperature control system

A temperature monitoring and control system was implemented to regulate the target, the thermal shield and the Si detectors. The regulation is based on the com pensation method. The target temperature is periodically measured and compared with the preset value. If any noticeable difference is detected, the target heating power increases or decreases depending on the variation of the temperature. The temperature is measured by calibrated carbon TVO thermometers [15], mounted di rectly on the different devices. To increase the measurement accuracy, the current and voltage leads of all thermometers have thermal anchors, which are glued1 to the appropriate points. The thermometers were calibrated with a special tester in the temperature range 4-300 K by the method described in Ref. [15]. The calibration accuracy is ±0.02 K in the temperature range 4-70 K and ±0.1 K in the range 70-300 K. At our coolant evacuation rate, the target temperature is regulated by an electric heater made of 0.15 mm diameter constantan. The contact between the heater and the target is ensured by the use of the thermally conducting glued1. The heater resistance is 92 ft at a temperature of 32 K. Figure 2 shows the diagram of the thermal stabilization of the system. The tem perature read-out information, i.e., the voltage at the sensor (TS), is fed into the PC through a CAMAC crate. The temperature corresponding to a given sensor voltage results from the following series expansion:

T = fC ± 7G x (1000/A) ± ... ± #7 % (1000/R)* ± ..., (4) where K, are the polynomial coefficients found from the calibration of the thermome ter and R is the thermometer resistance at that temperature. When the target tem perature is above or below the preset level, the target heating power changes by increasing or decreasing the voltage at the heater (H), using a digital-to-analog con verter (DAC). The measuring system works via a four cycles process. The first cycle activates the shield sensor (SS) and measures the voltage. The second and third cycles operate the same way on the target (TS) and detector (DS) thermometers. The fourth cycle measures the target pressure gauge (PG) voltage. Therefore the target pressure and temperature are obtained and determine correctly the experimental conditions during

1 Sty cast 1266

3 the measurement. The multiplexer (M) has several other input possibilities which are used to measure the pressure at different locations in the gas supply system, e.g., in the deuterium-helium mixing volume (MV). The multiplexer (M), the stabilized power supply (not shown in Fig. 2), and the amplifier (A) are located directly on the cryostat external wall, in one single unit. They are connected to the CAMAC module by a 15 m cable. It should be mentioned that the algorithm of the control program can stabilize the thermodynamic parameters of the target by using either the thermal sensor (TS) or the target pressure gauge (PG). The results of a test experiment with pure deuterium and a deuterium-helium mixture indicate that this target temperature control system ensures a stable oper ation at 32.00 ±0.05 K. Temperature stability of 0.1 K is sufficient for our physics goals.

The gas supply system

The gas supply system of our cryogenic target has to be adapted to the projected experiments [10]. Figure 3 shows the schematic diagram of the gas supply system. Two molecular sieve traps (MSI) and (MS2), connected in series, are used to purify the hydrogen isotopes. The traps are made of stainless steel vessels of 34 mm diameter, 230 mm long, with 2 mm thick walls. They are filled with 2 mm CaA-type molecular sieve granules, with 0.5 nm pore size. To prevent penetration of this material in the gas supply system, compressed 10 fim metal-fiber filters are placed at both ends of the traps. The traps are prepared for use by heating to 350°C for about 12 hours. The traps are ready to be used when the pressure of the residual gases does not exceed 2 x 10-5 torr, when the pumping system is running. Then, deuterium can be send through the traps at a flow rate of about 30 cm3/s. Early test measurements [16,17] indicate that the system of two traps in series allows an impurity concentration below 200 ppb. Impurities like N2, 02, H20, C02 or others can be accepted in hydrogen or helium to a level of 500 ppb. The accuracy should be better than 100 ppb. Helium isotopes are purified by the trap (IM1) filled with the powdered intermetallic compound (IMG) ZrCrFe [18]. IMG ingots were produced by arc smelting of pure metals (99.9% Zr, 99.8% Cr, 99.8% Fe) in an inert atmosphere of pure argon. Homogeneity was ensured by remelting the ingots four times. The ingots were hy drogenated by 99.999% hydrogen and dehydrogenated by heating several times until they were transformed into powder. Pieces of 0.2 mm nominal diameter were selected for the IMG traps. The trap (IM1) is a stainless steel vessel of 25 mm diameter, 180 mm long, and walls of 2 mm thick. Both ends of the trap are covered by 8 mm filters, in a similar way as for the other traps. The trap is prepared by heating to 750°G and pumping. When the pressure of the residual gases is about 10~5 torr, the trap is cooled l.o

4 room temperature. Then the IM(' is saturated with deuterium and the trap is again heated to 750°G and evacuated. The activation cycle is considered completed if the pressure of the residual gases in the trap does not exceed 10_;> torr at 750°C. Helium purification takes place at a temperature of 350 400°C. The (IM2) trap is installed to recover *3 IIe from the deuterium-helium mixture at 1 lie end of the measurement period. It is filled with the IMG ZrCoo.sXio.5 [10]. Its design and activation procedures are similar to the one of (IM1). The ((’T) trap is used to analyse the total impurities concentration in the gas mixture. Its 10 cm3 volume is filled with activated charcoal of AG-1 size. As well as for the others traps, filters are placed on both ends. The conditioning of the trap is performed at 200°C. It is considered ready to use when the residual pressure is below !0-> torr. The target cryostat, the chamber, and gas supply system are evacuated by a turbo molecular pump (TP)2 and a roughing pump (FP)3 installed in series. A vacuum in the target and all the pipes of the gas supply system of about 10-S torr is achieved. To avoid penel ration of oil vapour into the system, a liquid nitrogen cold trap (LX f) is placed in the inlet of the roughing pump. A safety valve (VSV) is installed to prevent, the break of the Kapton windows when the gas mixture pressure exceeds a preset level. In case of over-pressure, the gas is evacuated into a 2 l vessel (SV) via the (VGS) valve. The gas supply system includes some metallic flexible hoses and stainless steel tubes. Only metallic bellows valves are used, t.o let the system operate up to 200 atm and to 200°C. Before using the gas supply system for preparing the mixtures, it is heated and pumped until the remaining outgasing rate is below 10-' I torr/h. Such a rate guarantees the requested mixture purity. To mix deuterium and helium gases in a chosen concentration ratio, with an accuracy better than 0.5%, we use a vessel (MV) of a well known volume. V=1043.3 ± 0.3 cm3. The gas pressure in the (MV) is measured bv a pressure gauge (PG2). To measure the partial pressures of the different, gases and the total pressure of the mixture, several strain gauges with an accuracy of about 0.2% are installed in the system. Mixing is done by filling volume (MV) with a requested fraction (pressure) of helium. The helium flows from the bottle (HHl or IIB2) through a certain number of valves and the (IM l) trap into the volume (MV). The helium pressure in the (IMl) trap is measured with the gauge (PG3). With a helium flow rate of about 30 cm3(STP)/s [18], the IMG trap purifies the helium below I pph. The chosen amount of deuterium is then send to the volume (MV). The deuterium flows from the bottle (l)B) through different valves and the (MSI) and (MS2) traps. The gas pressure in t he traps is measured by a. manometer (M) and a pressure gauge (PG5). Once the volume (MV) is filled with the two gases, the contents of the vessel is allowed to mix for several hours to provide homogenous mixture. Before filling the

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5 target, both it and its associated service lines are evacuated and flushed with a small amount of the gas mixture from the volume (MV). Then the target, is filled with the gas mixture to the required pressure (between 0 and 10 atm), through the valve (VTI). with the deuterium-helium mixture. It will be evacuated via the (V'l'O) valve. At the end of the measurement period, the Tie is recovered from the deuterium- helium mixture. The mixture comes from the target through the valve (V’l'O) up to the trap (IM2). The IMG trap absorbs the deuterium and thus practically pure Tie leaves the trap and reaches the 2 I vessel (HR). To be able to re-use the trap (I.M2), the absorbed deuterium is removed by heating the trap to 750° G and pumping the released gas until a pressure below 10" ’ ton is reached. The amount of impurities is determined by the (GT) trap. A sample of the target mixture is pumped through the trap (GT) submerged in liquid nitrogen. The trap is then evacuated to a residual gas pressure below 10-:* torr. When this pressure is reached, the trap is heated to room temperature to release the impurities absorbed by the activated charcoal. The measurement of the impurity composition of the deuterium-helium mixture in the target is performed with a quadrupole mass spec trometer (RGA)V The amount of impurities is determined by measuring the pressure with the gauge (PG6). The accuracy of this procedure is of the order of 10 ppb [20].

Operation

Figure 4 shows a time history of a target cryostat operation. The time interval is divided into five parts, each one clearly showing the characteristics of the respective target operation stage for each of the three cryostat components, namely the target, the shield, and the detectors. The pre cooling (a) of the cryostat with liquid nitrogen lasts less than 2 hours. At the end the target temperature is about 70 K. During the change to liquid helium (b), the cooling is interrupted, which results in a warming of all the parts inside the vacuum chamber. As can be seen in (c), switching from one cooling system to the other decreases slightly the cooling rate. The minimum temperature of each part of the cryostat, can be reached in about 6 hours. The thermal equilibrium established in the cryostat is characterized by a fixed temperature difference between the target, the Si detectors, and the thermal shield. The next step (d) is the filling of the target, with the deuterium helium mixture. The equilibrium also results from the choice of the target heating power. It is im portant that at this stage the temperature difference between the Si detector and the target remains constant, and the Si detector temperature is kept above 40 K. It takes a short time (e) to shift the thermal control system to a mode corresponding to a higher target temperature. The natural warming time of the cryostat is over 2 days. A faster warming of the cryostat (f) is possible by stopping the circulation of the 1

1 Vacscan by Spectra

6 coolant and by filling up the vacuum chamber with helium to a pressure of about 200 torr. In this case the target warming time is about one day. The results of the two weeks test run indicate that the average consumption of liquid helium is 0.6 1/h to keep the target at a temperature of 32 K, with a cooling power of 0.45 W in the target volume.

Conclusion

The experimental set-up presented here corresponds to our needs, showing that /rCF experiments with deuterium-helium mixtures can be performed with such an apparatus. The target system has shown high reliability during its use. The possibility of using liquid nitrogen for temperatures between 70 and 300 K and liquid helium for lower temperatures allows a reduced operating cost. The same statement applies for the small consumption (0.6 1/h) of liquid helium. With such consumption, one 450 1 dewar is sufficient for one month of operation. Long running time reduce the number of target changes and the loss of 3He. All the different operating parameters, such as temperature, pressure, and concentration, are measurable with good accu racy. Furthermore, they can be changed fairly easily. The flexibility in pressure and temperature, in choice of target gas, and in available detectors makes this apparatus useful for many experiments in muon and pion physics. The authors are indebted to S. Mango and B. van den Brandt for the fruitful discussions in the development of the cryostat and for their help during the tests, and to P. Knowles for proofreading the manuscript. The authors are grateful to the deputy director of the Laboratory of Nuclear Problems of the Joint Institute for Nuclear Research (Russia) V.B. Brudanin and to the direction of the Institute of Physics of the Fribourg University (Switzerland) for their constant interest and support to the construction of the apparatus. This work was supported by the Swiss National Science Foundation.

7 10 9 7

He Dz+He

LHe

Figure 1. Schematic diagram of the target cryostat. (1) Target Body, (2) Vacuum Chamber, (3) Flange, (4) Silicon Detector for the 14.7 MeV fusion protons, (5) Thermal Shield, (6) Aluminium Window, (7) Gas In let, (8 ) Heat Exchanger, (9) Tubing for cooling liquid, (10) Shield Mounting Flange, (E) Plastic scintillators for the muon decay electrons, (7 ) the 7 ray detector for the. 16.4 MeV fusion 7 , (Ge) Germanium detector for the 6.85 keV X rays.

8 Bus CAMAC

Figure 2. Funct ional diagram of t he temperature control system. (TS) Temperature Sensors of the target, (SS) Shield thermometer. (1)S) Si Detector temperatim? Sensor, (H) Heater, (PCI) Pressure Gauge, (M) Multiplexer. (A) Ampli fier, (ADC) Analog to Digital Converter. (DAC) Digital to Analog Converter. (PC) Personal Computer.

9 EXHAUST LNT

RGA

RG IG

Helium In/Out

Figure 3. Schematic diagram of the gas supply system. HB2 HB1 (T) Target, (VC) Vacuum Chamber, (VSV) Safety Valve, (SV) Safety Volume. (MV Mixing Volume, (HR) Helium Receiver, (DB) Deuterium Bottle. (HB) Helium Bot tle, (IM) Trap with IMC, (MS) Trap with Molecular Sieve, (CT) Trap with activated Charcoal, (LNT) Liquid Nitrogen Trap, (PC) Pressure Gauge, (M) Manometer. (RG) Roughing Gauge, (IG) Ionization Gauge, (RGA) Residual Gas Analyzer, (FP) Rough ing Pump, (TP) Molecular Turbo Pump, (NV) Needle Valve. a b c d e f 250 n

—Shield

150-

— Detector

-To rget 100-

t i i i r i i i i r hours

Figure A. Temperatures of various parts of the target system during operation.

11 References

1. V.B. Belyaev, et al., J1NR communication, 1)15-92-323, Dubna. 1992. 2. S.S. Gershtein and V.V. Gusev, Hyp. Inter., 82 (1993) 185. 3. Y. Kino and M. Kamimura, Hyp. Inter., 82 (1993) 195. 4. V.B. Belyaev, O.I. Kartavtsev, V.I. Kochkin and E.A. Kolganova, Phvs. Rev.. A52 (1995) 1765. 5. V.B. Belyaev, O.I. Kartavtsev, V.I. Kochkin and E.A. Kolganova, Z. Phys.. 1)41 (1997) 239. 6. V.I. Korobov, Hyp. Inter., 101/102 (1996) 333. 7. W. Czaplinski, A. Kravtsov, A. Mikhailiv and N. Popov, Phys. Lett., A219 (1996) 86 . 8 . V.B. Belyaev, et al., Nukleonika, 40 (1995) 85. 9. F.M. Penkov, Yad. Fiz., 60 (1997) 1003 [Phys. Atom. Nucl., 60 (1997) 897]. 10. R. .Jacot-Guillarmod, V.M. Bystritsky et al., PSI Proposal, R-96-01, PSI. 1996. 11. P. Ackerbauer, D.V. Balin et al., PSI Annual Report, Annex 1, PSI, 1996, p. 32. 12. C. Petitjean, D.V. Balin, V.A. Gartzha et al., PSI Proposal, R-94-05.1, PSI, 1997. 13. V.M. Bystritsky and F.M. Penkov, JINR preprint, E15-97-329, Dubna, 1997. 14. M. Martini and T.A. MacMath, Nucl. Inst. Meth., 79 (1970) 259. 15. V.I. Datskov, L.V. Petrova and G.P. Tsvineva, JINR communication, R8-87- 604, Dubna, 1987. 16. V.M. Bystritsky, et al., Pribory i Tekhnika Eksperimenta, No. 6 (1981) 27. 17. V.M. Bystritsky, et al., Pribory i Tekhnika Eksperimenta, No. 4 (1984) 46. 18. A N. Perevesenzev, B.M. Andreev and I.L. Selivanenko, Vysokochistye Veschestva, No. 1 (1990) 122. 19. B.A. Kolachev, R E. Shalin and A.A. Ilin. Hydrogen accumulated allois, Hand book, Moscow, Metallurgiya, 1994, p. 129 (in Russian). 20. K.N. Zinoveva, Zavodskaya Laboratoriya, 21 (1955) 30.

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Boreiko V.F. et al. D13-98-146 New Target Cryostat for Experiments with Negative Muons

An experimental set-up, including a cryostat with temperature control and gas supply systems has been developed to study muon catalyzed fusion in mixtures of hydrogen and helium isotopes. The target body is made of aluminium with five kapton windows 55-135 pm thick. The working temperature range of the target is 21-300 K, the pressure is 0-10 atm. The coolant is either liquid helium or liquid nitrogen, according to the experimental conditions. The average consumption of liq uid helium at a target temperature of 32 K is 0.6 1/h, providing 0.45 W of cooling power. The temperature control system ensures an accuracy of ±0.05 K in the temperature range 21-70 K. Hydrogen isotopes are purified by molecular sieves (CaA), helium isotopes by intermetallic compound ZrCrFe.

The investigation has been performed at the Laboratory of Nuclear Problems, JINR.

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