Experimental Physics: Methods and Apparatus

194 Pages·1969·7.78 MB·English

by  Yu. A. Aleksandrov

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