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Experimental Physics: Methods and Apparatus PDF

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EXPERIMENT AL PHYSICS: METHODS AND APP ARA TUS METODIKA FIZICHESKOGO EKSPERIMENTA METOAHKA WH3HQECKOrO 8KcnEPHMEHTA The Lebedev· Physics Institute Series Editor: Academician D. V. Skobel'tsyn Director, P. N. Lebedev Physics Institute, Academy of Sciences of the USSR Volume 25 Optical Methods of Investigating Solid Bodies Volume 26 Cosmic Rays Volume 27 Research in Molecular Spectroscopy Volume 28 Radio Telescopes Volume 29 Quantum Field Theory and Hydrodynamics Volume 30 Physical Optics Volume 31 Quantum Electronics in Lasers and Masers Volume 32 Plasma Physics Volume 33 Studies of Nuclear Reactions Volume 34 Photomesonic and Photonuclear Processes Volume 35 Electronic and Vibrational Spectra of Molecules Volume 36 Photodisintegration of Nuclei in the Giant Resonance Region Volume 37 Electrical and Optical Properties of Semiconductors Volume 38 Wideband Cruciform Radio Telescope Research Volume 39 Optical Studies in Liquids and Solids Volume 40 Experimental Physics: Methods and Apparatus Volume 41 The Nucleon Compton Effect at Low and Medium Energies In preparation Volume 42 Electronics in Experimental Physics Volume 43 Nonlinear Optics Volume 44 Nuclear Physics and Interaction of Particles with Matter Volume 45 Experimental Physics: Programming and Computer Techniques Volume 46 Cosmic Rays and Interaction of High-Energy Particles Volume 47 Radio Astronomy Instruments and Observations Volume 48 Surface Properties of Semiconductors and DynamiCS of Ionic Crystals Volume 49 Quantum Electronics and ParamagnetiC Resonance Volume 50 E lectroluminescence Proceedings (Trudy) of the P. N. Lebedev Physics Institute Volume 40 EXPERIMENT AL PHYSICS ~ Methods and Apparatus Edited by Academician D. V. Skobel'tsyn Director, P. N. Lebedev Physics Institute A cademy of Sciences of the USSR, Moscow Translated from Russian CONSULTANTS BUREAU NEW YORK 1969 The Russian text was published by Nauka Press in Moscow in 1968 for the Academy of Sciences of the USSR as Volume 40 of the Proceedings (Trudy) of the P. N. Lebedev Physics Institute MeTO~llKa «{lu3H'IeCKOrO 8KCDepnMeHTa TPYALI op~eHa JIeHHHa CDH3nQeCKOrO HHcTHTYTa HMeHH II. H. JIe6eAeBa TOM 40 ISBN 978-1-4684-0675-7 ISBN 978-1-4684-0673-3 (eBook) DOI 10.1007/978-1-4684-0673-3 Library of Congress Catalog Card Number 69-12522 © 1969 Consultants Bureau, New York A Division of Plenum Publishing Corporation 227 West 17th Street, New York, N. Y. 10011 United Kingdom edition published by Consultants Bureau, London A Division of Plenum Publishing Company, Ltd. Donington House, 30 Norfolk Street, London W.e. 2, England All rights resen'ed No part of this publication may be reproduced in any form without written permission from the publisher CONTENTS Apparatus for Recording Neutral Particles by Reference to Decay Gamma Quanta . 1 Yu. A. Aleksandrov, A. V. Kutsenko, V. N. Maikov, and V. V. Pavlovskaya Magnetic Spectrometer for Charged Particles ...................... . 32 V. N. Maikov, V. A. Murashova, T. 1. Syreishchikova, Yu. Ya. Tel'nov, and M. N. Yakimenko Experimental Method of Determining the Efficiency Function of an Apparatus Containing a Magnetic Spectrometer ........................ . 57 V. F. Grushin and E. M. Leikin Positive Pion Stopping Detector .................... . 70 Yu. M. Aleksandrov, V. F. Grushin, and E. M. Leikin Absolute Sensitivity of a Thick-Walled Graphite Ionization Chamber for I-GeV Photons ............................................ . 75 1. N. Usova Statistics of Time Measurements Made by the Scintillation Method. 84 V. V. Yakushin A Cathode Stage with Amplified Feedback and Its Applications 137 V. V. Yakushin Wilson Chamber for Studying Photomeson Processes .......... . 164 V. P. Andreev, Yu. S. Ivanov, R. N. Makarov, V. T. Zhukov, V. E. Okhotin, and I. N. Usova Relative Monitor for a Wilson Chamber. 185 V. p. Andreev, T. 1. Kovaleva, and I. N. Usova APPARATUS FOR RECORDING NEUTRAL PARTICLES BY REFERENCE TO DECAY GAMMA QUANTA Yu. A. Aleksandrov, A. V. Kutsenko, V. N. Maikov, and V. V. Pavlovskaya An apparatus for the recording of correlated 'YY coincidences, designed in the high-energy electron laboratory of the Lebedev Physics Institute of the Academy of Sciences (FIAN), re cords and measures the energy of two 100 to 650 MeV 'Y quanta simultaneously incident on its detectors; this apparatus may thus be employed in order to study any processes in which such 'Y quanta are formed. The detectors are formed by Cerenkov total-absorption spectrometers with an energy resolution of between ±19 and ±9.59'c over the range indicated. The energy of each of the cor related'Y quanta is measured in the presence of coincidences with a resolving time of ""5'10-9 sec. The resultant energy values are recorded in the form of a numerical code in an inter mediate memory based on ferrite cores. The form of the recording is such that the appearance of a correlated event is strictly related to the energy of each of the 'Y quanta. The resultant in formation is then printed and converted to perforated cards suitable for subsequent analysis of the results on an M-20 computer. CHAPTER I RANGE OF NUCLEAR REACTIONS STUDIED AND PRINCIPAL REQUIREMENTS OF THE APPARATUS When studying the interaction of elementary particles it is frequently essential to record short-lived neutral mesons decaying into two 'Y quanta (1To meson and 1'/ meson). These par ticles can only be recorded by reference to their decay products, i.e., by detecting one or two 'Y quanta. It may be easily shown that the simultaneous detection of two decay 'Y quanta has a num ber of advantages over other methods, as it offers the possibility of studying a large number of processes under conditions in which other methods are practically useless. By way of example, let us consider the advantages of the method in question for studying the photogeneration of 1To mesons in hydrogen. 1 2 Yu. A. ALEKSANDROV ET AL. For recording the process r + p -'>-1tO + P (1) it is sufficient to determine any two parameters, for example, the escape angle and the energy of the recoil proton. It was in this way that the first precision information regarding the angu lar and energy distributions of 'lr0 mesons in this reaction was obtained. However, the regions e:r e;. of very small and very large meson escape angles s 30° and 2: 150° in the system of the center of inertia (CIS) have hardly been studied at all. This is because the corresponding re coil protons in the laboratory system of coordinates (LS) escape at angles close to 0 and 180°. e; In addition to this, the case s 30° corresponds to very low-energy protons (Ep ::s 10 MeV for Ey i:::i 500 MeV), and it is almost impossible to record these. Yet it is the photogeneration of 'lr0 mesons at large and small angles which is of the greatest interest, since the more com plex intermediate states in the meson-proton system should make their greatest contribution in this case. The recording of a 'lr0 meson by reference to its decay I' quanta offers the possibility of carrying out investigations at any 'lr°-meson escape angles. The kinematics of the decay of a 'lr0 meson are such that the most likely event is that two I' quanta flying off symmetrically with respect to the 'lr0 meson escape direction will be recorded. The angle between the quanta in the LS will be close to the minimum (critical) angle O!cr at which I' quanta may be emitted: • C1 cr =-m- ec-2 . sm2 ~ (2) ~ where Err is the total energy of the 'lr0 meson and mrr its mass. The energy range Err = 300-650 MeV corresponds to critical angles O!cr = 53.5-24°, i.e., I' quanta arising from the decay of 'lr0 mesons generated at 0° in the LS (which corresponds to err ° = 0°) will make fairly large angles with the axis of the beam of primary I' quanta, and will thus be easy to record. Thus, the simultaneous detection of two I' quanta arising from the decay of 'lr0 mesons enables us to study the photogeneration of 'Ir ° mesons in an angular range very close to 0 and 180°, for which other methods prove extremely difficult. A second very important advantage of the method under consideration is the possibility, in principle, of distinguishing the desired process from a background of other competing pro cesses ultimately leading to the appearance of a particle analogous or similar in properties to the one being detected. For example, on studying reaction (1) at energies of E y > 322 MeV, the single generation of ~ mesons is accompanied by the paired generation of charged and neu tral mesons, which also leads to the appearance of recoil protons. The separation of the recoil protons in reaction (1) from this background is quite a difficult problem, especially if one con siders that reaction (1) is studied in bremsstrahlung beams having a continuous 'Y-quantum en ergy spectrum. The corrections introduced into a number of experiments of this type are very indefinite, and this reduces the accuracy of the results obtained. When using the method based on recording two correlated I' quanta for studying process (1), a background process leading to the appearance of 'lr0 mesons in the final state is, for ex ample, the paired photoexcitation of 'lr0 mesons (for E y > 322 MeV): (3) However, the maximum energies of the 'Ir ° mesons so formed will be smaller than the energies of the 'lr0 mesons generated in reaction (1) at the same angle, and hence the critical angles be tween the decay I' quanta will also differ. For example, for Ey = 500 MeV, the critical angles APPARATUS FOR RECORDING NEUTRAL PARTICLES 3 for 'ITo mesons generated in reactions (1) and (3), respectively, equal By using this fact it is in practice quite easy to set up a geometry such that the recording of any of the 'ITo mesons generated in reaction (3) is in principle impossible (the angular aperture between the decay 'Y-quantum detectors should not exceed 45°). Nevertheless, even after ensuring the fulfillment of this condition, reaction (3) may still be recorded on account of the incidence of'Y quantum from the decay of different 'ITo mesons in the two detectors. Cases (1) and (3) are then distinguished by means of 'Y-quantum spectrom etry. The hardest to distinguish are the improbable cases in which photogeneration of the two 'ITo mesons occurs in the direction of the 'Y detectors, and this is followed by asymmetric decay. Quantitative consideration of the kinematics of these processes shows that they may be clearly distinguished by using 'Y spectrometers with good energy resolution as detectors and by vary ing the upper limit of the bremsstrahlung spectrum. In describing our experimental results in the following sections, we shall show that the recording of TJ mesons by this method has advan ° tages analogous to those considered in the case of 7r mesons. CHAPTER II A TOTAL-ABSORPTION CERENKOV GAMMA SPECTROMETER § 1. Characteristics of the Existing Form of the Spectrometer The total-absorption Cerenkov spectrometer is intended for recording and measuring the energy of 80-650 Me V 'Y-quanta. The use of a spectrometer in experiments involving the re cording of 'Y quanta arising from the decay of neutral particles ('IT 0, TJ) also presupposes its in clusion in fast-acting coincidence circuits (with a resolving time of the order of a few nano seconds) with other analogous spectrometers or other particle counters. All this necessitates ensuring a high resolving time of the system as a whole, as well as good spectrometric charac teristics. In the present form of the spectrometer [I, 21, good spectrometric and time character istics were achieved by simultaneously employing two types of photomultiplier ("spectrometer" and "time" types) and thus forming two independent channels for the electrical pulses arising from the same source, namely, the Cerenkov radiation in the radiator. In addition to this, the separation of the "spectrometric" and "time" functions of the apparatus is very convenient under the conditions of a physical experiment involving the recording of "fast" coincidences and, a "slow" amplitude analysis of the events. The "spectrometric" channel used one Soviet-made spectrometric photomultiplier (FEU- 49). The use of one multiplier instead of the usual several eliminated the necessity of having special electronic circuits for summing the pulses and obviated problems associated with choos ing photomultipliers of the same sensitivity, as well as simplifying the construction and adjust ment of the apparatus and increasing the reliability of its operation. The possibility of using a single photomultiplier arises from the good intrinsic resolution of "'6% and the comparatively large area of the photocathode (diameter 150 mm). The "time" channel uses FEU-36 photomultipliers with a maximum spread of "'2 nsec in the time of flight of the photoelectrons and a high amplification factor. The number of multip- 4 Yu. A. ALEKSANDROV ET AL. Hers was chosen in such a way as to give 100% radiation-recording efficiency in the working 'Y quantum energy range. Experiment has shown that the present version of the Cerenkov spectrometer (total-ab sorption type) has the best spectrometric characteristics of all those found in the literature (the energy resolution varies from ±19 to ±10% in the range 100-600 MeV) and fairly good time characteristics (resolving time 4-5 • 10-9 sec). An extremely important property of the appa ratus is, moreover, the separation of the recording function from the spectrometric analysis of the radiation. § 2. Spectrometric Characteristics R a d i at 0 r • The radiator employed was TF-1 lead glass containing 53 % of lead oxide PbO. The radiator was made in the form of a truncated cone 240 mm high (10.1 t units) and diameters 260 and 300 mm (10.9 and 12.6 t units, respectively). In order to improve the conditions for collecting the Cerenkov light, the side and ends of the radiator were carefully polished and surrounded by a reflector of polished aluminum. In contrast to a cylinder, a radiator in the form of a truncated cone should give total internal re flection of the greater part of the light emitted. For a cylindrical radiator, in fact, the condi tion of total internal reflection (sinO! = n-1, where O! is the angle made with the normal to the reflecting surface) can only be satisfied over the whole lateral surface for relativistic particles ({3 = 1) passing along the axis of the cylinder, since the angle of Cerenkov emission is deter mined by the relation cos e = (n{3)-1. In the case of a conical radiator, however, the dimensions quoted ensure the maintenance of this condition for particles passing at angles of ±5° to the axis of the radiator. Spectrometric Photomultiplier [3]. Tests were carried out on several samples of the FEU-49 with an NaI-TI crystal (diameter 30 mm, h = 20 mm) and a collimated Cs137 source. The pulses from the photomultiplier were amplified and analyzed in a 100-chan nel AI-100 amplitude analyzer. The Uniformity of the sensitivity of the photocathode was veri fied roughly by means of a crystal placed in turn at the center of the photocathode and on the periphery. The FEU-49 multipliers tested had a fairly uniform sensitivity of the photocathode, the amplitude difference between the two points never exceeding 10-15%. The time stability of the FEU-49 characteristics was specially checked. Over a period of 8-h operation the change in amplitude and resolution never exceeded 0.5% after a 20-min heating. The FEU-49 photomultiplier was very insensitive to weak magnetic fields. Thus, the earth's magnetic field, which usually affects spectrometer photomultipliers, had little effect on the resolution of the FEU-49 for different orientations of the multiplier to the field. The ampli tude of the pulses from the photomultiplier in the horizontal and vertical positions differed by 2.5%. Screening the multiplier with a Permalloy screen annealed at 8000 at atmospheric pres sure completely eliminated this difference. Optical Contact of the Photomultiplier with the Radiator. Inorder to transfer the light from the radiator to the photocathode of the photomultiplier without loss of intensity, a lubricant with a good transmission in the range of spectral sensitivity of the photo multiplier and a refractive index close to that of the radiator and the entrance window of the photocathode must be used" between them. These requirements are excellently satisfied by the usually employed liquid and viscous intermediate media, which have good optical properties, but require constant fairly reliable mechanical pressure on the multiplier. The use of insol uble adhesives eliminates the necessity of mechanical fixing, but makes the apparatus more dif ficult to dismantle. APPARATUS FOR RECORDING NEUTRAL PARTICLES 5 We tested several types of optical contacts with corresponding fixing systems. Experiment showed the undesirability of working with liquids and viscous substances, particularly in the case of prolonged operation. Despite fairly elastic fixing of the multipliers, the optical contact deteriorated with time. In the final version of the Cerenkov spectrometer we used a grease best satisfying the conditions both of light collection and of long-term stability of the optical contact. This was a water-soluble adhesive [4] based on the epoxy resin glycerin diethyleneglycerol (DEG-1 type) and hardener ethylene diamine in a weight proportion of approximately 10: 1. In order to prepare the adhesive, the resin and hardener were mixed. The mixture re tained the consistency of a viscous liquid for 40-60 min. The joint between the parts to be ce mented together was effected in the same way as in the case of ordinary optical greases. The adhesive hardened fully at room temperature after 20 h (approximately). This kind of adhesive ensures good light conduction (the same as vaseline oil), a steady, reliable optical contact, and at the same time good mechanical fixing of the photomultiplier. If necessary, the cemented parts may be separated by dissolving the adhesive in water without harming their efficiency. The time required for this operation diminishes with increasing water temperature and washing intensity, and increases with increasing area of the cemented surfaces and diminishing thick ness of the cementing layer. Thus, an FEU-36, 40-mm in diameter, may be dismantled by im mersing the join in water at room temperature for 6-7 h, while for an FEU-49 with a diameter of 150 mm this requires several days. Calibration and Characteristics of the Apparatus. The spectrometric properties of the Cerenkov spectrometers were studied and calibration was carried out in a monochromatic electron beam obtained by the deflection and magnetic focusing of electrons formed in a thin target bombarded by synchrotron 'Y quanta [5]. Smooth variation of the mag netic field of the f3 spectrometer enabled electron beams of preassigned energy (80-650 MeV) to be obtained in this way. The dispersion of the beam in the collimated window of a spectrom eter 80 mm in diameter and of a given geometry was (according to a preliminary survey) ±1.5%, which was much less than the expected energy resolution of the spectrometer. The intensity of the electron beam was "'103 electrons per pulse in the spectrometer win dow. For calibration purposes the pulse of 'Y radiation, and hence the duration of the electron beam, were "drawn out" to 0.5 sec (working frequency of the accelerator one pulse in 6 sec). Immediately in front of the entrance window of the spectrometer, a telescope comprising two scintillation counters and serving as monitor was placed. The conditions of calibration are indicated in Fig. 20 of Chapter VI. The calibration consisted mainly of determining the resolv ing power and the amplitude of the output pulse as functions of the electron energy Ee. The spectrometer was calibrated for electron energies between 100 and 600 MeV. The results of a study of the spectrometric characteristics of the apparatus are presented in Figs. 1 and 2. We see from Fig. 1 that the amplitude of the output pulse is a linear function of the energy up to "'600 MeV. The linearity of the apparatus over a wide frequency range indicates that the radiator dimensions selected ensure the absorption of nearly all the energy of the electron photon shower. However, in view of the finite size of the radiator, it would be natural to expect a slight deviation from linearity on raising the energy, owing to the different conditions governing the absorption of the last generations of the cascade shower in the course of its development. The observed linearity of the apparatus is evidently due not so much to the strict constancy of the effects of total absorption and proportional light collection as to the fact that any deviations from these tend to compensate each other [6]. For slight deviations there is clearly a mutual

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