High-energy particle research with the synchrocyclotron of the laboratory of nuclear problems at the joint institute of nuclear studies

J. Nuclear Energy II, 1958. Vol. 7, pp. 129 to 168. Per&mm Press Ltd., London

HIGH-ENERGY PARTICLE RESEARCH WITH THE SYNCHROCYCLOTRON OF THE LABORATORY OF NUCLEAR PROBLEMS AT THE JOINT INSTITUTE OF NUCLEAR STUDIES* V. P. DZHELEPOVand B. M. PONTECORVO INTRODUCTION DURINGthe forty years existence of the Soviet State great progress has been made in the various fields of Natural Science and Technology. Particularly is this true of nuclear science which has developed rapidly during these years. In the Soviet Union, and in the most economically advanced foreign countries, the last 15 years of this development have witnessed the successful solution of a series of important problems concerning the application of atomic energy. Powerful atomic reactors and power stations have been built and a new field of technology, nuclear power, has appeared. This same period also saw the birth of a new field of physics, that of high and ultra-high energy particles, and the development of powerful accelerating machines. The history of this new field has been almost entirely confined to the last decade and, so far, it has contributed little to the solution of any practical problems. The main object of study in this new field of physics is the investigation of the nature and properties of elementary particles-nucleons, mesons, hyperons, and anti-particles-which, at present, are the simplest known structural components of matter. Various researches, using both cosmic rays and powerful accelerators, have shown that elementary particles can be created as a result of collisions involving particles with energies of hundreds and thousands of MeV. The principle aims of the physics of high and ultra-high energies are to provide a more complete description of elementary particles, to establish the laws of interaction between them, and to study the processes of mutual transformation of one kind of particle into another. At present as many as 26 elementary particles are known. However, in spite of the very considerable efforts of scientists throughout the world, the fundamental law of interaction between these particles has not yet been discovered. It will be readily appreciated, therefore, that in this new field of physics there exist wide and unexplored horizons. Moreover, past experience of the development of science has shown that, as a rule, on entering a new field of knowledge, scientists unexpectedly derive much that is of importance and which is usually of benefit to mankind. These are the great prospects of research with high-energy particles. In our article we will try to give a description of the fundamental scientific work accomplished by Soviet physicists using high-energy particles from the world’s * Translated

by D. L. ALLAN from Atomnaya Energiya 3, No. 11, 413 (1957). [Reprint No. AE 140.1 129

9

130

V. P.

DZHELEPOV and B. M. PONTECORVO

largest synchrocyclotron, which is in the Laboratory Joint Institute of Nuclear Studies.* THE STARTING THE

of Nuclear Problems at the

UP OF THE SYNCHROCYCLOTRON BEAM CHARACTERISTICS

AND

In this country, the systematic and detailed study of this field of physics began on 14 December 1949 in the Institute of Nuclear Problems of the Soviet Academy of Sciences, now the Laboratory of Nuclear Problems of the Joint Institute of Nuclear Studies. On that date, the 5 m synchrocyclotron came into operation, accelerating deuterons and alpha particles to energies of 280 and 560 MeV, respectively. Almost at the same time, a 250 MeV electron synchrotron came into operation at the Physical Institute of the Soviet Academy of Sciences.? The creation of powerful accelerators, with their large and complicated engineering structures, proved within the capabilities of only the most highly developed of the capitalist countries (the U.S.A. and Britain). The synchrocyclotron at the Joint Institute of Nuclear Studies therefore clearly shows the high level of industrial development that has been achieved by the U.S.S.R. The heavy expenditure in this country on the construction of powerful accelerators demonstrates the exceptional concern and attention which is given to the development of advanced science by the Communist Party, the Soviet Government, and the people. The basic operating principle of high-energy machines lies in the phase stability of the particles moving in a cyclic resonance accelerator discovered by V. I. VEKSLER (1944) and E. MACMILLAN(1945). The construction of a powerful synchrocyclotron was the result of almost three years intensive work by a large group of scientists and engineers from a number of research institutes. In the course of a few years this field attracted the attention of KURCHATOVand it ‘is to him that much of the credit for its ultimate success is due. The main equipment for the accelerator was made at the factories of the Ministry of Electrical Industries. Towards the end of 1950, the 5 m synchrocyclotron was modified for the acceleration of protons up to energies of about 500 MeV. (l) An important part of the first work done with this machine was that devoted to the study of the parameters of the high energy particle beams (intensity, energy, angular spread etc.). The main results of these experiments are given in Table 1. The nuclear research carried out on the 5 m synchrocyclotron will be described later. THE

RECONSTRUCTION

OF THE

ACCELERATOR

The reconstruction of the accelerator was begun in 1953 after an important programme of nuclear research with deuterons, c+particles, and 500 MeV protons had been completed. The pole pieces of the electromagnet were enlarged to 6 m and, as a result, the proton energy was increased to 680 MeV.(l) The building which * In this article it is our intention to give an exhaustive account of work with the 6 m synchrocyclotron. At the same time, the article is intended for a wider circle of readers than is usual. We have therefore aimed at a concisely written text well furnished with figures taken from the original publications. We do not include the large amount of work in which radiochemical methods were used. Much of this has been described in the sessions of the U.S.S.R. Academy of Sciences. See Report of the Conference of the U.S.S.R. Academy of Sciences on the Peaceful Uses of Atomic Energy, Moscow 1955, (Chemical Sciences Division), English language edition, Consultants Bureau, New York, 1955. Work using accelerators abroad is Moreover, it is our aim to present the work of Soviet physicists. not described. ,t In this article only work with the synchrocyclotron will be described.

131

High-energy particle research with the synchrocyclotron

TABLE1.-FUNDAMENTALBEAMPARAMETERS OFTHE5 m SYNCHROCYCLOTRON Type and energy of particles accelerated

Deuterons 280 MeV

cc-particles 560 MeV

1

0.025

Current at the Internal target &A) Beam density at a distance of 10 m from the magnet channel (cm-” see-I)

-

-

Protons 480 MeV

0.2-0.3 1.106 (& = 460 MeV)

Density of the neutron beam at the maximum of the angular distribution and at a distance of 2 m from the internal target (cm-8 see-I)

8.10'

2.106

5.106

Energy of the neutrons at the maximum of the energy distribution (MeV)

120

120

380

0.17

0.35

0.55

Stripping

Disintegration of cc-particles

Exchange interaction

Half-width of the neutron tribution (radians)

angular dis-

Type of process involved in the production of neutrons

housed the accelerator and the general appearance of the 6 m synchrocyclotron are illustrated in Figs. 1 and 2.” While the work of reconstruction proceeded in the synchrocyclotron hall an experimental area, which was well protected against radiation by massive concrete In addition, a meson laboratory and an enclosure for experiwalls, was constructed. ments with polarized beams of protons were built (Fig. 3). Ports were provided in the protective wall of the experimental area for the beam exits and for the installation of collimators. In the same year the accelerating conditions of the particles in the central field This almost doubled the external proton of the accelerator tank were improved. beam current, increasing it from 0.3 ,uA~) to O-5 ,uA.(*) From the accelerator tank, 14 different beams of particles, comprising protons, deuterons, y-rays, ,& and p* mesons(5s6) were led out into the atmosphere (Fig. 3). The extraction of these beams was achieved by using the regenerative deflection method which has been studied in detail by DMITRIEVSKY.‘~) The density of the proton beam which passes into the experimental area is increased as a result of focusing At a distance of 15 m from the exit port of the accelerator by quadrupole lenses .(*) tank, the beam density is 1.5 x log protons/cm2/sec. The high density of the proton beam made it possible to produce several beams of high energy particles by bombarding an external target: three beams of r-mesons * A detailed description of the machine has been given previously.‘1-3) [Figs. 1 and 2 are colour photographs which are not reproduced in this issue.]

132

V. P. DZHELEWV and B. M. PONTECORVO

t--l High -frequency

generator

High - vacuum

Vacuum chamber \

I

\

pump

Electromagnet \

Meson laboratory

\\

All\

/

I ’

0

III

II

__l_---,_____.

rf,&trot

Recording

FIG. 3.-Plan

of the 6 m synchrocyclotron

building,

___.,t__--'-

Zanefs

for

II

-ml

i,

-

II -

equipment

showing

the layout of the particle

beams.

II

133

High-energy particle research with the synchrocyclotron

(collimators 8, 9, and 10) and one polarized proton beam (collimator 6). (See Fig. 3). In order to increase the beam density of the charged particles, special focusing devices located in the gaps, of the deflecting magnets are being used in addition to the quadrupole lenses. These devices were designed in this laboratory.(g) Table 2 gives details of the energies and intensities of the various beams which have been available up to July 1957.

TABLE

Z.-BEAM

INTENSITIES OF THE HIGH-ENERGY

PARTICLE BEAMS

FROM

THE 6

m

SYNCHROCYCLOTRONINTHEEXPERIMENTALAREA,ANDINTHEMESONANDPOLARIZATION LABORATORIES

Particles

Energy (MeV)

Collimator number

1Beam density (cm-” set-‘)

_ Protons Polarized protons Neutrons

Polarized neutrons

615 + 6

7 also 6 or 8

1.5 x 109

640 * 10 I 610

4 6

4 x 106 6 x lo5

Spread over spectrum 500 Q E, Q 650

11, 12, 13

Spread over spectrum 450 Q E, < 600

16

(3 t 4) x 10”

104

150 300 310 360

8 8 9 8

450 1000 1600 150

300 330 370

1 1 3

500 200 70

p+-mesons

90

8

20 f 30

p--mesons

25

17

60

10 < Ey < 600

12

3 x 103

rr+-mesons

y-rays from &meson

At the beginning of 1957 the running time of the accelerator was increased from 100 to 140 hr per week. (I) The increase in the intensity of the beams and the increase in the running time very considerably broadened the experimental possibilities of the synchrocyclotron. Work on such low probability effects as the creation of mesons by mesons close to the threshold energy became possible. Before describing the results of the work which was carried out with the synchrocyclotron at about the time of its coming into operation, we shall discuss that part of the work reported at the conference of the U.S.S.R. Academy of Sciences on the peaceful uses of atomic energy in 1955.(l”)

V. P. DZHELEPOVand B. M. PONTECORVO

134 SOME

EXPERIMENTS WITH 280 MeV DEUTERONS AND 560 MeV a-PARTICLES

In the 5 m synchrocyclotron, deuterons and a-particles were accelerated up to energies half as high again as the energies available in the accelerators of the United States. In this country it was therefore possible to study nuclear processes induced by these particles up to higher energies and to embark upon a series of exploratory experiments of a new type. In this section we will give a very brief account of the more important experiments carried out while the accelerator was operating with deuterons and a-particles (in the first half of 1950). The determination of the masses of mesons produced by particles of 500 MeV

During 1949 and 1950 there was considerable discussion about the existence in the cosmic radiation of mesons having different masses. A number of experiments on the 5 m synchrocyclotron were therefore planned which were designed to analyse the masses of mesons created by particles having energies of the order of 500 MeV. There were two groups of experiments using different methods. It was found that only 7r-mesons were produced by 560 MeV a-particles(11~12)and by 490 MeV protons.u3) It was estimated that if any mesons having different masses were created, the upper limit of their yield was 200 times less than the yield of negative n-mesons. Thefission of nuclei by r-mesons

In another experiment, an attempt was made to observe the fission of heavy elements by negative n-mesons which stopped in photographic emulsions loaded with uranium and tungsten. For the first time, this phenomenon was observed(14115) and in later work it was investigated in more detail.o@ The dissociation of 560 MeV u-particles into separate nucleons

In the experiments of DZHELEPOV et aZ.,(l’) internal targets of various materials were bombarded with 560 MeV u-particles. It was found that intense fluxes of neutrons and protons, having intermediate energies of about 120 MeV, were produced. These particles were emitted within a narrow cone about the direction of motion of the u-particles. Table 1 (column 2) gives some details of the flux, angular distribution and energy of the neutrons emitted in these experiments. At the same time, nuclear emulsions were exposed to the u-particle beam. It was found that a relatively high proportion of the nuclear disintegrations induced in the emulsion were accompanied by the emission of pairs of protons in a direction close to that of the incident uparticles and with an energy which was, on an average, equal to one quarter of the a-particle energy.(ls) From all these facts it may be concluded that, in a collision between an u-particle and a nucleus, the complete dissociation of the u-particles is quite probable-its component nucleons being projected in the direction of motion of the a-particle to form a comparatively narrow beam. From the yield of neutrons it was established that the disintegration of a-particles at 560 MeV is not very much less probable than the stripping of deuterons at 280 MeV (see Table 1). Deuteron stripping

The phenomerron of deuteron stripping was discovered at Berkeley in experiments with 190 MeV deuterons. This phenomenon was also studied by GAVRILOVSKY(~~)

High-energy particle research with the synchrocyclotron

135

at an energy of 280 MeV using a number of different targets. Some of the results he obtained are shown in Table 1 (column 1). The angular distribution of the neutrons produced in this process, and also the total cross section, were found to be in general agreement with calculations based upon the Serber model. The detection of hard y-rays from the synchrocyclotron target In the work of KOZODAEV and MARKOV, in which interior targets were bombarded with 560 MeV u-particles, hard y-rays were detected. These were apparently caused by the decay of TO-mesons formed in the bombardment.(20) The$ssion of nuclei by neutrons A new phenomenon, the fission by 120 MeV neutrons of nuclei in the middle region of the periodic system (dysprosium, erbium, and rhodium) was observed by DZHELEPOV et aLt21) At the time of this work the fission of silver by 560 MeV CCparticles was also established by radiochemical methods.(22) Preferential fission of nuclei in this region of the periodic table under these experimental conditions is in agreement with the theoretical model of GEILIKMAN~~~)and these experiments provide the first confirmation that this mechanism of fission takes place. The fission of heavier nuclei (2 > 73) by high-energy neutrons was investigated by DZHELEPOV(~]) and also by REUT et al.(24) Induced u-radioactivity The production of x-active nuclei by the bombardment of various target materials with 560 MeV a-particles was studied by BARANOV.(25) This work led to the discovery of new activities in the nuclei of gadolinium, terbium, dysprosium, produced in the bombardment of holmium, ytterbium, terbium, and erbium. A number of experiments were devoted to the study of nuclear disintegrations induced by n-mesons(26) and high-energy deuterons. t2’) In addition, the inelastic scattering of high-energy neutrons was studied by GOL’DANSKY et aZ.t2*)The emission of secondary neutrons (2g)from beryllium and lead in reactions induced by neutrons having energies between 120 and 380 MeV was also studied. ELASTIC

NUCLEON-NUCLEON

SCATTERING

One of the chief problems being studied in contemporary nuclear physics is that of nuclear forces-the forces of interaction between nucleons. The results of low-energy nucleon scattering experiments have shown that nuclear forces are not of an electromagnetic nature. They differ from electromagnetic forces by reason of their large magnitude, their very short range, and their complicated dependence on the spins of the particles. A number of other experimental facts have indicated yet another important property of these forces. This is their symmetry; the fact that the interactions between two neutrons and two protons are approximately equal. These facts lead to the far reaching hypothesis of the charge independence of the nuclear interaction between nucleons. At the basis of this hypothesis lies the supposition that the nuclear interactions between any pair of nucleons are identical for nucleons in states of the same total spin and orbital angular momentum. New information about the nuclear forces can be obtained from experiments on

136

V. P. DZHELEPOVand B. M. PONTECORVO

the elastic scattering of nucleons by nucleons at energies of hundreds of MeV, where the wavelength of the incident particles is less than the range of the nuclear forces (R < h/,uc = 1.4 x 1W13cm). Such high-energy experiments enable a much deeper study of the nuclear field of force to be made. However, in order to lind the extent to which the forces depend on spin, it is necessary to do nucleon-nucleon scattering experiments with polarized beams of particles as well as with unpolarized beams. It should be emphasized, that the methods used in the theoretical analysis of low-energy experimental data are quite invalid in the new energy region. Consequently in view of the absence of a valid theory of nuclear forces, methods of analysis based upon the hypothesis of charge independence have proved very useful. It is possible to progress a little further in the analysis of the results of high-energy experiments, and some new laws of interaction between nucleons have been revealed, as a result of introducing the concept of isotopic spin. t30) The effect of introducing isotopic spin is to split the number of possible states available to the system representing the collision of two high-energy nucleons into two groups having values of isotopic spin: T = 1 and T = 0. This suggests experiments designed to study interactions involving these two groups of ‘states separately. Information about interactions between two nucleons in states with T = 1 can be obtained from the study of the scattering of any of the pairs of nucleons: p-p, n-n, n-p. In order to obtain similar information about the interaction of nucleons in the states with T = 0, it is necessary to do experiments on the scattering of dissimilar nucleons (i.e. n-p scattering experiments). Because of the great importance of nucleon-nucleon scattering experiments, a broad programme of experimental work, centred upon the synchrocyclotron at the Laboratory of Nuclear Problems, was started in the autumn of 1950. In these experiments, nucleons having energies between 380 and 660 MeV were used-an energy region which, at that time, had not previously been studied. Elastic proton-proton scattering and the polarization of protons

In the work of E. SEGRB and 0. CHAMBERLAIN et al. (U.S.A.) in 1949, it was found that elastic proton-proton scattering was independent of both angle and energy over the energy range 150-340 MeV. This was evidence for the existence of a very strong interaction between two protons having energies of the order of hundreds of MeV. Beginning in 1952, this problem was studied in the synchrocyclotron laboratory at proton energies from 460 to 660 MeV. (~3~) For the first time, the pronounced anisotropy in high energy proton-proton scattering and the increase of the differential cross-section with decreasing scattering angle was observed. Similar results were obtained in a second group of experiments,(34p35)using another method. The experimental data from all these experiments,(31-35) are given for’ three incident proton energies, in Fig. 4. One should note not only the sharp anisotropy but also the fact that the (p-p) scattering cross-section remains practically constant over the energy region studied. (31~33)In order to derive a unique phase shift analysis of these experimental results, additional information from experiments on the scattering of polarized protons is required. Fig. 5 gives some results on the angular dependence of the polarization of the protons elastically scattered by protons at an energy of 635 MeV.‘36)

High-energy particle research with the synchrocyclotron

137

Elastic neutron-neutron scattering By using original methods, DZHELEPOVet a1.,t3’)and GOLOVINand DZHELEPOV,(~*) working at the Laboratory of Nuclear Problems in the years 1952 and 1952-56, were able to obtain the first data on the scattering of neutrons by neutrons. They determined the differential cross-sections of elastic neutron-neutron scattering at energies of 300 and 590 MeV. It was found that the differential cross-sections for

6

0

fOO” 20’

30”

CO”

50’

88’

70’

80’

8,cenfre FIG. 4.-El&tic

proton-proton

scattering

90” of moss system

at 460, 560 and 660 MeV.

elastic (n-n) and (p-p) scattering were equal (within the limits of experimental error) for the same nucleon energy over the whole range of angles studied (Fig. 6). This indicates that the total cross-sections for (n-n) and (p-p) interactions are equa1,(3gr40) immediately confirming the charge symmetry hypothesis in the high-energy region. It follows, therefore, that all the important deductions pertaining to the interaction between two protons are true also of the interaction between two neutrons.

138

V. P. DZHELEPOVand B. M. PONTECORVO

10 f

10'20" 30' 40' 50" 60' 70' 80' 90' 0, cent& of mass system FIG. 5.-Polarization P(0) as a function of angle in elastic proton-proto! scattering at 635 MeV. The analysis of the data indicates a noticeable contribution from trlplet F-state scattering.

o 4” (e),E,,=300 MeV[37] -6&e), ,!,,I 300 MeV @n-Soviet

data]

e, Cm tre FIG. L-Elastic

neutron-neutron

of

mass system

scattering at 300 and 590 MeV.

High-energy

particle

research

with the synchrocyclotron

139

Elastic neutron-proton scattering

The main point of experiments on the scattering of neutrons by protons is that they give information about the nuclear interaction between non-identical particles. The first experiments on the scattering of high-energy neutrons by protons were those

9

6

0

20”

40”

60”

80°

loo0

120”

140”

160” 160”

9,centI-e of ma.53System FIG. 7.-Elastic

neutron-proton

scattering

at 380 and 580 MeV.

The 580 MeV data provide evidence that states with orbital angular momentum, I = 5, contribute to the scattering. The sharp rise in the low-angle scattering at 580 MeV is apparently connected with the increased probability for inelastic scattering processes (meson production) in the energy interval 380-590 MeV. These give the scattering a diffraction character.

of E. SEGR&et al. at Berkeley in 1948, with neutrons having energies of 40 and 90 MeV. Strong scattering in the backward direction (in the centre of mass system) was observed, giving direct proof that the nuclear forces acting between a neutron and a proton are of an exchange character. Using the synchrocyclotron at the Laboratory of Nuclear Problems, the scattering of neutrons by protons was systematically studied in the neutron energy region 380 to 580 MeV.(41-44) The results of these experiments are shown in Fig. 7. The experiments with 380 MeV neutrons (1950-51) showed that the differential cross-sections for (n-p) scattering were practically constant over a wide range of angles on either side of 90” and were of relatively large magnitude. From this, it

140

V. P. DZHELEPOVand B. M. PONTECORVO

follows that the interaction in the .neutron-proton system, as in the proton-proton system, is very large. In fact, these fundamental nucleon scattering experiments have established that, at high energies, the interaction between any pair of nucleons is characterized by a very large intensity.

10° 0, laboratory system FIG.

%-Exchange

scattering of neutrons by protons and deuterons at 380 MeV.

Both at 380 and at 580 MeV,( 43y44)the experimental results clearly show the exchange character of the interaction between neutrons and protons, and they demonstrate that exchange forces are important up to an energy of 600 MeV. It should be noted that none of the experimental results on high-energy nucleonnucleon scattering contradict the hypothesis of charge independence of the nuclear forces. As a result of an analysis of the nuclear scattering data in terms of this hypothesis,‘30,42) evidence was found of the difference in the behaviour of the interaction cross-sections as a function of energy and angle for nucleons in states of different isotopic spin (2’= 0 and T = 1). This reveals the difference in the character of the interaction between nucleons in the two states. Exchange

scattering

of neutrons by deuterons at 380 MeV

Although the scattering of unpolarized neutrons by protons has revealed the important role played by exchange forces in nuclear interactions, no conclusions can be drawn from this type of experiment regarding the spin dependence of these forces. An original method of studying this problem was proposed by POMERANCHUK.(~~)

High-energy particle research with the sync!lrocyclotron

141

He suggested comparative observation of fast protons produced in exchange collisions of high-energy neutrons with deuterons and with free protons. In experiments of this kind,(46) it was found that at low scattering angles the cross-sections for the emission of high-energy protons as a result of the exchange scattering of neutrons by deuterons were significantly less than the corresponding cross-sections when neutrons were scattered by free protons (Fig. 8). It was concluded from this fact that, in (n-p) collisions of an exchange type, there is a considerable probability for the simultaneous exchange of spins. In other words, the contributions of spin exchange and spin non-exchange forces to the exchange interaction between a neutron and a proton are of the same order.

0

40”

60”

80”

140” 100" 120" 0, centre of mass system

FIG.9.-Elastic proton-deuteron scattering at 660 MeV. The total cross-section of elastic (p-d) scattering is about 2-3 per cent of the total cross-section for (p-d) interaction at this energy. Elastic proton-deuteron from light nuclei

scattering and the direct ejection of deuterons by protons

It had already been established in experiments performed in 1952 with 460 MeV protons(47) that, in large angle elastic (p-d) scattering, energies of up to 300 MeV are given to the deuterons. Very complete data on the differential cross-section of (p-d) scattering as a function of angle were obtained by LEKSIN(@) at a proton energy of 660 MeV (Fig. 9). The proton scattering in the angular range from 130” to 150” (in the centre of mass system) deserves particular attention. In this angular range, the deuterons are projected forwards with energies of from 500 to 600 MeV; energies which exceed the deuteron binding energy by a factor of several hundred. These facts suggest a mechanism of collective three body forces between the nucleons. Special experiments were set up on the 6 m synchrocyclotron in order to study the collisions of high-energy protons with quasi-deuteron groups within nuclei. By bombarding light nuclei with 675 MeV protons, AZHGIREI et aZ.(4g)observed deuterons having energies of about 600 MeV which were emitted in the forward direction at small angles to the direction of the incident beam (Fig. 10). This result corresponded exactly to the emission of deuterons in elastic (p-d) collisions at the same energy and angle of observation. A qualitative explanation of this phenomenon can be given if it be assumed that strongly interacting short-lived groups of nucleons

142

V. P. DZHELEPOVand B. M. PONTECORVO

c= velocity

of

light

Deuterons ,

5000

FIG. IO.-Momentum

6000

Hp

spectrum of protons kind deuterons emitted from lithium at an angle of 7.6” to the incident 675 MeV proton beam. The deuterons are emitted as a result of collisions between the protons and quasideuteron groups within the nucleus. At the top right is shown the momentum spectrum of deuterons from elastic @d) collisions for the same angle and proton bombarding energy.

V. P. DZHELEPOV and B. M. PONTECORVO

144

the results found at lower energies: for T = 1 states, the elastic scattering crosssection is constant; for T = 0 states, the cross-section decreases with increasing energy.(42) This indicates that the magnitude of the interaction between nucleons in states with T = 0 decreases with energy. Summarizing the results of the experiments on the elastic scattering of nucleons by nucleons and deuterons, the following statements concerning the main features of nucleon-nucleon interactions can be made : (1) Strong forces of a number of different types (central, tensor, exchange forces) are involved in the interaction between nucleons at high energies. They depend in a complex way upon the spins of the nucleons. (2) The contributions to the interaction of the different types of force are about equal. (3) The interaction between nucleons is charge independent. Clearly, any future theory of nuclear forces must account for these experimental facts. THE

INTERACTION

OF

MESONS

AND

NUCLEONS

The problem of nuclear forces cannot be completely separated from the study of the properties of all the elementary particles. It is well known that there is a connexion between elementary particles and the field of force. For example, the coulomb interaction between charged particles is effected through the intermediary of photons, which are the quanta of the electromagnetic field. That is why the properties of photons are closely connected with the character of the electromagnetic force which acts between charged particles. Similarly, it was immediately established that the properties of r-mesons were closely connected with nuclear forces between nucleons. Clearly, therefore, the study of the properties of mesons is very important. Meson theory, that is, the formal theory of the meson field, has its origin in the supposition first proposed by H. YUKAWA that nuclear forces are dependent upon mesons. This idea is correct but, unfortunately, meson theory is still in a relatively early stage of development. For the present, when any specific form of the meson theory, at best, gives only a qualitative agreement with experiment, the fundamental problem of high-energy physics is the gathering of data for a phenomenological description of the interaction between the different particles. A consistent meson theory cannot be constructed without some general picture which is able to unify the various phenomena of elementary particle physics. Therefore, the first step in the construction of a meson theory of nuclear forces is to obtain a phenomenological description of the interactions between these particles, and particularly those involving r-mesons and nucleons. In both the U.S.S.R. and in other countries, work proceeded along many different lines in order to obtain this information. Besides studying nucleon-nucleon scattering at different energies and angles of observation, the scattering of mesons by nucleon& and the creation of mesons in nucleon-nucleon collisions and by mesons and y-rays were also studied. The scattering of r-mesons by nucleons and complex nuclei A great step forward in the study of the properties of r-mesons was made in the period 1950-1954 when W. PANOFSKY and J. STEINBERGER determined their spin and parity, and E. FERMI et al. determined the differential cross-sections of rr-meson-

145

High-energy particle research with the synchrocyclotron

nucleon scattering for energies up to 200 MeV. The study of the scattering of mesons by nucleons began at the Laboratory of Nuclear Problems in 1954 after well collimated at beams of n-mesons became available. t6) The aim was to obtain information rr-meson energies above 200 MeV where there was practically no data.

I

150

I

200

I

25-o

I

I

300

350

COO

Er,MeV

FIG. 12.-Total

cross-sections for the interaction of nf and v--mesons with hydrogen and deuterium. The “resonance” behaviour of the cross-section near 190 MeV is characteristic of the meson-nucleon interaction in states of isotopic spin and total angular momentum 3/2. At energies Elab > 300 MeV the contribution to the scattering of states of isotopic spin 3 is noticeable.

A number of experiments were devoted to studying the energy dependence of the total cross-section of & and 7~~ meson interactions in the energy range between 140 and 400 MeV(53-56) (Fig. 12) and to studying the angular distribution of mesons scattered by hydrogen in the reactions & + p-+ T+ + p; CT-+ p--+ T- + p; X- + p + no + n at different meson energies: 176, 200, 240 and 270 MeV;(56’57) 307 MeV;(58-61) 330 MeV;(62) and 360 MeV. (63) Some of the results obtained are shown in Figs. 13 and 14. In experiments on the angular distribution of n-mesons scattered by hydrogen, scintillation counters and photographic plates were used, 10

v. ~~‘f)zmt~~ov

0

30’

SO”

90”

and B. M. ~NTECORVO

120° 0, centre

FIG. 13.-Angular

distributions

of moss system

of +-mesons elastically at different energies.

scattered

by hydrogen

The figure shows that near the “resonance” energy (190 MeV) the angular symmetrical about 90”. At energies above “resonance” forward scattering

distribution is predominates.

147

High-energy particle research with the synchrocyclotron

The results of these experiments, particularly the observed fact that the crosssections for the interaction of 7r+and W- mesons with deuterons are equal, confirm for the meson-nucleon system the principle of charge symmetry and also the more rigid principle of charge independence. Although the principle of charge independence has a natural place in the meson theory of nuclear forces, it should be stressed that the deductions about the validity of this principle were made without appealing to meson theory. They were made on the strength of the phenomenological mesonnucleon scattering data and data obtained on meson production by nucleons (discussed below).

I

0

30”

60”

FIG. 14.-The scattering ofn-mesons (a)p++p~TT++P,(b)~-++P’+n From

JO”

r20”

150*

e

MO0

by hydrogen at 307 MeV for the processes: +Y+Y+n,(C)~-+p--+n-+P.

the data shown in this and the preceding figures, a value for the coupling constant of the meson-nucleon interactionf2 N 0.1 was obtained.

It has been experimentally confirmed that for energies up to 300 MeV the mesonnucleon interaction is particularly strong for states of isotopic spin and total angular momentum equal to 3/2. At the same time, the scattering cross-section in this state reaches the maximum possible value at a n-meson energy of about 190 MeV. The meson-nucleon interaction is therefore said to be of a resonance character. The resonance may be connected with the structure of nucleon@*) but, at present, there is no way of confirming this. It can be concluded from the work on the high-energy meson-nucleon interaction’53T55)that the contribution to the scattering of states with isotopic spin 4 becomes noticeable for E, > 300 MeV. Since one can now measure with high accuracy the angular distribution for the

V. P. DZHELEPOV and B. M. PONTECORVO

t

I

50

FIG. 15.-The

100

energy

150 200 250 dependence of the total cross-sections rr-mesons and light nuclei.tBS)

300 350 for the interaction

400 I& ,Mel Y between

The curves recall the energy dependence of the total, cross-sections for the interaction between ST+-and n--mesons with nucleons. Analysis shows that the interaction of n-mesons with nuclei is basically an interaction with the individual nucleons in the nucleus.

8, centre of mass system FIG. 16.-Inelastic

scattering

of n--mesons

by carbon

and lead.

An analysis of the results showed that there is a correlation between the energy defect and the scattering angle of the meson. This is evidence that inelastic-scattering ofn-mesons in the angle interval 60”-180’ proceeds preferentially by the single scattering of the rr-mesons the differential cross-section for by the individual nucleons in the nucleus. For comparison, elastic scattering of 240 MeV &-mesons in hydrogen is given (broken line).

High-energy particle research with the synchrocyclotron

149

scattering by hydrogen of n+-mesons, it is possible, for the first time, to account, not only for s- and p-states in the phase shift analysis, but also for d-states. From such an analysis, the radius of the meson-nucleon interaction was found to be about 7 x lo-i4 cm. All the data obtained with the synchrocyclotron at the Laboratory of Nuclear Problems(56~57~5g~60~62~63) are consistent with a single value of the coupling constant f2, roughly equal to 0.1.

IT=mesons E =330 MeV

0, centre of muss system FIG. 17.-Elastic At low angles the behaviour completely different

scattering

of TT+-and n--mesons

by helium.

of the two differential elastic scattering cross-sections due to the effect of the coulomb interaction.

is

A number of experiments were devoted to the study of the interaction of mesons with complex nuclei. In these experiments many different detection techniques were used: scintillation counters,(65-67) photographic plates,(68~6g)Wilson Cloud Chambers,(70) and diffusion chambers. (‘it’s) Some of the results of these experiments are shown in Figs. 15-17. The data obtained from work on the inelastic scattering of r-mesons by the nuclei of photographic emulsion (6g) show that attractive forces operate between nuclei and r-mesons at 160 MeV, corresponding to a potential well depth of approximately 25 MeV.

150

V. P. DZHELEP~Vand B. M. PONTECORVO

The production of mesons by m&eons

(a) Early experiments. Many experiments on the synchrocyclotron were devoted to the problem of single production of charged and neutral n-mesons in collisions between nucleons. Clearly, the creation of charged mesons in nucleon-nucleon collisions is a more complex process, from a theoretical point of view, than that of the scattering of mesons by nucleons. In order to obtain a reasonably complete picture of meson production in nucleon collisions, both qualitative and quantitative details of the process were studied by a number of different methods. In collisions involving two nucleons, mesons can be produced in the following ways : n+n+TO+n+n

P+P+~“+P+P p +p+n+

fp

n+n-+n-+p+n

+ n

n+n+n-fd

p+p-tr++d

n+p-+55-++n+n n +p-+r-

+p

n +p-tnO

+ n +p

+p

n+p-tnO+p

In 1951 and 1952, the 5 m version of the synchrocyclotron was used to obtain information about the creation of no-mesons in (n-p) and (n-n) collisions(73) at a neutron energy of 400 MeV. In addition, the reactions p + p -+ 7~~+ p + P(‘~-‘~) andp+p-tn++d(77) were studied at a proton enegy of 460 MeV. Processes leading to the formation of no mesons by the interaction of both protons(7817g)and of neutrons(80) with complex nuclei were also investigated. From all this work the following information was obtained. (1) From the results of some very difficult but accurate measurements of the y-ray spectrum arising from the decay of no-mesons produced by bombarding complex nuclei ‘with 460 MeV protons, it was concluded that the mesons are created mainly in a p-state.(78p7g) (2) Measurements of the probability of TO-meson production by nuclei of different atomic weight enabled an estimate to be made of the mean free path of the mesons in nuclear matter.(73y74) (3) The production of no-mesons by the bombardment of nucleons and complex nuclei by neutrons was studied for the first time.(81) (4) It was shown that, near threshold, the creation of no-mesons in neutronneutron collisions was forbidden,(73) in agreement with the similar result obtained at Berkeley for the collision of two protons. (5) The experiments on TO-meson production in (n-p) and (n-n) collision leads to the following general scheme for the processes of n-meson formation near threshold.(73) Probable Processes .

pi-p-n+ n+n+nn+p+?rO

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Improbable p+p+ n+n-+

151

with the synchrocyclotron

Processes 7T” 770

It was pointed out for the first time that the low probability for the production of strong mesoncharged mesons in (n-p) collisions was a result of the particularly nuclear interaction in the state with isotopic spin and angular momentum equal to

t\ \ \ \ \

\ \ \ \ \ \

0 -Non-Soviet

data

i

/

L

100

150

200

FIG. lg.-Energy dependence of the total cross-section for the reaction p + p---f v++ d. E, is the energy of the meson in the centre of mass system. The maximum is due to the interaction between the meson and the nucleon in a state of isotopic strong “resonance” spin and total angular momentum 3/2.

312, i.e., in the same state that was in evidence in the experiments on the interaction between r-mesons and nucleons.(s2) (6) In experiments on the bombardment of hydrogen by 460 MeV protons,, it was shown that the inhibition of the reaction p +p-+ no + p +p, observed at low energies, disappeared with the increase in energy.(74-76) This observation is in agreement with the work of J. MARSHALL et al. The analogous observation that at high energies the inhibition of no-meson production in (n-n) collisions also disappears was made (83) after the reconstruction of the synchrocyclotron had increased the energy of the neutron beam to 580 MeV.

152

V. P. DZHELEPOV and B. M. PONTECORVO

(7) The reaction p + p + 7~++ d was studied in detail at a proton energy of 460 MeV. The angular distribution indicated that mesons from this reaction are created preferentially in the p-state. In comparison, the emission of mesons in the d-state was negligible. (b) The energy dependence of the meson production cross-sections and the angular distribution of the mesons. After a beam of 680 MeV protons had been obtained from the synchrocyclotron, all the processes of meson creation which have been mentioned above were studied by direct or indirect means. As a result of experiments

0

15’

30’

45”

60”

75’

90’

I m

e

I

a

135

1500

I

I

165’

HO’

9, ten tre of mass sys tern FIG. lg.-The n + p + ?r” + The ordinate the differential

angular distribution of n-mesons emitted in the reactions p + p + r+ + d and d at incident proton and neutron energies of 650 and 600 MeV, respectively. for the second reaction is in relative units. The identical behaviour of cross-sections confirms the principle of charge independence for the interaction of elementary particles.

on the creation of 7~*-mesons in proton-proton collisions at energies of 460-660 MeV, it was established for the first time cE4)that the energy dependence of the total crosssection of the reaction p + p 4 z-f + d was of a resonance character (Fig. 18). An analysis of the angular distribution of the mesons emitted in this reaction (Fig. 19) showed that, in the energy range studied, the mesons are created mainly in the pstate. The same deduction was made from the results (85y86)ofthe angular distribution of To-mesons produced in the reaction n + p -+ rr”+ d at a neutron energy of 600 MeV (Fig. 19). From the point of view of charge independence this reaction is identical with the reaction p + p--t YT++ d. In addition to these reactions, the reaction p + p -+ 7~++ n + p was also studied (*‘) at a proton energy of 657 MeV (Fig. 20).

High-energy particle research with the synchrocyclotron

FIG. 20.-Angular

distribution

350 400 FIG. 21.-The

of &-mesons from the reaction proton energy of 657 MeV.

p + p --f n+ + p + n at a

420 440 460 480 500

620Ep,MeV

540

580

153

energy

dependence of the total cross-section for &meson production in (p-p) and (p-n) collisions. momentum of aO-mesons in centre of mass system &-proton energy; pmar maximum (units mc). $4 increases with proton energy more rapidly than opn because meson production in (p-p) collisions near threshold is inhibited.

The production of no-mesons in reactions induced by the bombardment of hydrogen and deuterium with both protons(88-g1) and neutrons(83~g2) was studied for incident energies up to 680 MeV. It was found (88ygo)that at 660 MeV the cross-section ratio e$/o;i has a value approximately equal to two. This value of the ratio is predicted by the charge independence hypothesis when the final state of the meson-nucleon system has isotopic spin equal to 3/2.tg3)

154

V. P. DZHELEPOV and B. M. PONTECORVO

The energy dependence of the z-O-meson production cross-sections for (p-p) and (p-d) collisions was studied using both interna1(sg3g4)and external(gO~gl)proton beams. In Fig. 21 the results are given of some very accurate measurements of the total cross-section for To-meson production in (p-p) collisions.(8g) The behaviour of the total cross-section as a function of energy was completely unexpected. The angular distribution of no-mesons formed in nucleon-nucleon collisions was studied at energies of 445, 500 and 555 MeV@s)and 660 MeV.(*s~89~93) It was found that the angular distribution, which is anisotropic at low energies, is practically isotropic at 660 MeV. This is apparently connected with the appearance, at energies near 600 MeV, of a strong interaction between the &meson and one of the nucleons in the final state. Experiments on the angular distribution of the y-rays from the decay of nomesons produced in proton collisions with nuclei, gave the interesting result that the distribution is asymmetric@sP90~94) (Fig. 22). From this work it was found that 57 1 46 2 izi 4 5 5 B x 4

0

30"

60"

JO" 120" 15-O" HO0 8,centre of mcLs.ssJ5tem

distribution of y-rays from the decay of +mesons produced in the scattering of 660 MeV protons by carbon. &effective angle in the centre of mass system of the colliding nucleons. The decrease in y-ray intensity at low angles is due to the fact that the nucleus is a thick absorber for both the #-mesons and the incident nucleons. FIG. 22.-Angular

mean free path of the mesons and protons in nuclear matter was small compared with nuclear dimensions. Information about the absorption of mesons in nuclear matter was also obtained from measurements of the yield of no-mesons from different nuclei bombarded by protons@) and neutrons.(80s3) (c) The energy spectra of particles emitted in inelastic collisions of two m&eons. The spectra of particles emitted in inelastic collisions of nucleons with nucleons and nuclei was studied by a number of different experimental’ methods. A magnetic pair-spectrometer was used to study the spectra of y-rays from the decay of TOmesons produced in (p-p) collisions tg6) (Fig. 23) and in collisions of protons with nucleiW9’) (Fig. 24). A magnetic analyser was used to study the secondary protons the

High-energy

particle

research

I

with the synchrocyclotron

!OG FIG. 23.-Energy

spectrum at O0 of y-rays from the decay of G-mesons collisions at a proton energy of 660 MeV.

produced

in (p-p)

The spectrum gives the angular and energy distribution of &mesons which cannot observed directly because of the very short lifetime of the #-meson (7 < IO-r5 set).

be

?? ? ? I

Frc. 24.-Energy

spectrum

of the y-rays from the decay of &-mesons produced by 660 MeV protons on carbon: (a) at 0”; (b) at 180”. The maximum energy of the spectrum at about 600 MeV corresponds to head on collisions in which one of the two decay y-rays receives practically all of the energy.

156

V. P. DZHELEPOV and B. M. PONTECORVO

td

II COUU

FIG. 25.-Momentum

spectrum, in the laboratory system of co-ordinates, particles from (p-p) collisions.

sum Hp, kgauss,cm of secondary

The spectrum implies that the following reactions occur: (1) elastic p-p scattering (proton peak at 4260 kG cm); (2) the reactionp + p ---tv+ + d, giving two deuteron groups at 2880 and4520kGcm;(3)thereactionsp+p~?r++p+nandp+pp-to+p+pwhich gives a continuous spectrum of protons.

High-energy particle research with the synchrocyclotron

0 Y

0

157

V. P. DZHELEPOV and B. M. PONTECORVO

L

tnn

I””

FIG. 27.-Energy

‘V”

.wn ““1

distribution of rr+- and rr--mesons, produced by 660 MeV protons carbon at an angle 24’ to the direction of the incident beanxoOr~

on

The small yield of n--mesons is due to the fact that whereas n+-mesons are created in both (p-p) and in (p-n) collisions, r-- mesons can only be created in (p-n) collisions. The production of charged mesons in (JHZ) collisions is a relatively ineffective process.

High-energy

particle

research

with the synchrocyclotron

159

and deuterons emitted from (p-p) collisions cg8)(Fig. 25). A magnetic analyser was also used in experiments on charged mesons from (p-p) collisions(gg~lOO) (Fig. 26) and from collisions of protons with nuclei (101J02)(Fig. 27). Finally, for the analysis of the spectra of charged mesons produced in (n-p) collisions a photographic plate method was used(lo3) (Fig. 28). The analysis made of the spectra of the decay y-rays from no-mesons, produced in the bombardment of light nuclei by 47&660 MeV protons, showed that as the proton energy increased, a softening of the spectrum occurred and the angular

I

I

1 I I j 80 fO0 120 Er, MeV FIG. 28.-Spectrum of &- and n--mesons produced by 600 MeV neutrons falling on hydrogen. The angle of observation is 90”. The equal yields of n+- and F- mesons (within the experimental errors) confirms the principle of charge synime?ry.

0

10

I 60

distribution of the mesons produced in the (n-p) collisions became less anisotropic.(g6) Apparently, the reason for this is the appearance, as the proton energy is increased, of a strong interaction between the z-O-meson and one of the nucleons. The sanie methods were used to obtain the first information on the angular and energy distributions of $-mesons produced in collisions between 660 MeV protons and hydrogen.@j) Analysis of the spectra of rr+-mesons from the reaction p + p-+ rr+ + p + n, at proton energies of 556 and 657 MeV, showed that the matrix element of the reaction was proportional to the meson momentum in this energy region.(ss) An interesting result was obtained from measurements of the fluxes of charged(lo2) and neutraPo4) mesons emitted from different nuclei when bombarded by protons. It was found that the ratio of the number of charged mesons to the number of nomesons was equal to two for nuclei with zero isotopic spin. This confirms once again the charge independence of the nuclear forces. This principle applies to mesons having both low and high energies. In conclusion, it should be remarked that such a systematic study of the processes of neutral and charged meson creation is at the present time quite unique.

160

V. P. DZHELEPOVand B. M. PONTECORVO

Creation of rr-mesons by mesons It has been known for a long time that r-mesons can be created in collisions between r-mesons and nucleons (multiple production). However, the processes of creation of mesons by mesons have, up to now, only been studied at energies well above the threshold for meson production. The study of these processes close to threshold was recently undertaken at the Laboratory of Nuclear Problems. The multiple production of mesons was investigated with a liquid hydrogen targetuo5)

ET, MeV

FIG. 29.-The cross-section for the creation of charged mesons by r-mesons falling on and r-+p+ hydrogen near the threshold of the reactions: n-+p+rr”+w-+p TT++ r- + n.‘105) The calculated threshold for r-meson production in these reactions is 170 MeV. The curve gives the results of calculations by Franklin based on the theory of Chew and Low.

using scintillation counters and with complex nuclei(ro6) using photographic plates. In Fig. 29 are given the first data on the energy dependence of the combined probabilities for the occurrence of the reactions: n-T + p -+ no + nT + p and .rrT + p + 3-f++ 3~~ + n. Attempts to calculate these probabilities using meson theory were unsuccessful. The experimental data are in significantly better agreement with the curve calculated by D. FRANKLIN* on the basis of the theory of G. Chew and F. Low, than with other calculations. Conclusion From the results obtained by studying all the different processes discussed above, it is possible to form a reasonable phenomenological picture of the interaction between mesons and nucleons for energies up to 700 MeV. The main characteristics of this interaction which must be taken into account in future theories are as follows: (a) The interactions between mesons and nucleons are charge independent. (b) The meson-nucleon interaction is especially strong in the state with isotopic spin and angular momentum equal to 3/2. * Phys. Reu. 105,1101 (1957).

FIG. 32-Apparatus neutrons.

used for studying the scattering of high energy protons by protons Scintillation counter telescopes record both secondary particles.

and

p. 160

FIG. 33.-Hodoscope counter system used in experiments on the scattering of high energy n-mesons by hydrogen. The array of control counters subtends a solid angle of 2 steradians. The resolving time of the system is 2. 1Om8sec. By using a pulsed voltage supply, ordinary gas-filled counters can be used to record the high energy particles in the presence of the intense background radiation from the accelerator.

FIG. 34.-High pressure diffusion chamber used to study the interaction of high energy photons with protons and helium nuclei. The diameter of the chamber is 300 mm and it is filled with hydrogen or helium at a pressure of 20 atmospheres. A pulsed magnetic field of 1600 oersteds is applied over the sensitive volume of the chamber.

FIG. 35.4

litre propane

bubble chamber designed for recording processes of elementary particles.

the interaction

and decay

FIG. 36.-Wilson cloud chamber and magnet used to study the interaction of high energy of sensitive volume: diameter 400 mm, height mesons with atomic nuclei. Dimensions 100 mm. Pulsed magnetic field strength over sensitive volume: 13,500 oersteds. A deflecting electromagnet can be seen in the background.

FIG. 37.-General view of apparatus for studying inelastic collisions between polarized protons and protons. The secondary particles are recorded by large scintillation counters. The liquid hydrogen in the hydrogen-deuterium target (surrounded by a casing in the photograph) can be retained in the 3.9 cm diameter cylinder for 50 hours.

FIG. %-Magnetic spectrometer used in the experiments on the reaction n + p + 4 + d. The apparatus records both reaction products simultaneously. The deuteron is detected by proportional counters and a simultaneous determination of its momentum is made in the magnetic field. The ~9 mesons are detected by recording the decay y-quanta using a scintillation counter telescope.

FIG. 39.-Liquid hydrogen bubble chamber (capacity: 1 litre) and associated equipment used to study the interaction of high energy particles with hydrogen.

FIG. 40.-Triple magnetic lens for focusing high energy charged particle beams. The focal length of the lens for protons having a momentum of 1200 MeV/c is 2.5 m, the aperture is 80 mm, and the magnetic field gradient is 2000 oersteds/cm.

FIG. 41 .-Twelve

channel 140 MC/$ pulse amplifier for amplifying the scintillation outputs. The gain of each channel is 40 dB.

counter

FIG. 42.-Reprojector used for the determination of the spatial distribution of the charged particle tracks in stereophotographs of the events recorded by the Wilson chambers, and the diffusion and bubble chambers. The screen diameter is 400 mm.

High-energyparticle research with the synchrocyclotron

161

P-MESONS It is known that p-mesons are not nuclear-interacting particles: their interaction with nucleons is significantly weaker than the n-meson-nucleon interaction. However, it would be interesting to discover if there exist relatively weak nuclear forces, dependent upon the virtual emission of a pair of mesons (p*, PO). An attempt was made to detect the hypothetical PO-mesons emitted during the bombardment of nuclei by 670 MeV protons (lo’). An upper limit for the probability of producing such particles equal to 1O-4 times the emission probability of n-mesons was deduced from these experiments. It follows, therefore, that the contribution to the nuclear forces of a paired interaction (p*, 1~~)is negligible. Great interest in the decay of p-mesons was aroused after it had been discovered in the U.S.A. that parity was not conserved in weak interactions. This point was investigated, using photographic plates, by studying the asymmetry of electrons emitted in the decay process p+-f e+ + v + Y’relative to the direction of emission of the ,u-mesons in the process ni- --t ,& + v. The angular distribution of the electrons was determined with high accuracy and the results ‘confirm the non-conservation of parity.(lOs) According to the two component neutrino theory of LANDAU(~O~) the neutrino has zero mass and is polarized along the direction of motion. There is considerable interest in testing this theory experimentally. In some difficult photographic plate experiments, the asymmetry of the electrons emitted in p+-meson decay was determined as a function of the electron energy. (no) It was found that the asymmetry is greater for the more energetic part of the spectrum, in agreement with the predictions of the two component neutrino theory. Experiments on this fundamental problem are still proceeding. STRANGE PARTICLES In the Laboratory of Nuclear Problems the processes leading to the creation of R”-particles were also studied. Theoretical calculations both in the U.S.S.R. and in other countries led to the prediction of the simultaneous creation of a heavy meson and a hyperon. (111) This was eventually confirmed by experiments with the Cosmotron in the U.S.A. The hypothesis of simultaneous creation of ho-particles and K-mesons is contradicted by the reaction: nucleon + nucleon--t nucleon + R”, but not by the reaction: nucleon + nucleon -+ A0 + ho. An attempt was made (112)to create ho-particles by bombarding the internal target of the synchrocyclotron with 680 MeV protons, an energy quite sufficient for the formation of two ho-particles. The experiment showed that A”-particles were not created under these conditions. This result gives considerable support to the supposition that A”-particles and heavy mesons are created simultaneously. It also allows a deduction to be made about the magnitude of the isotopic spin of the KOmeson, which takes part in the interaction between a A”-particle and a nucleon, It follows that the K”-meson and its anti-particle k” are not identical. THE

INTERACTION WITH

OF HIGH-ENERGY COMPLEX NUCLEI

PARTICLES

A priori, one cannot answer the question: is the interaction of x-mesons and nucleons with nuclei the sum of the paired interactions with the individual nucleons in the nuclei? The main reason for the interest in interactions of n-mesons and 11

162

V. P. DZHELEPOVand

B. M. PONTECORVO

nucleons with nuclei is that one can obtain unexpected information about nuclear forces which cannot be obtained from nucleon-nucleon scattering experiments. We have already mentioned the large amount of work which has been done on the interaction of nucleons and r-mesons with complex nuclei. Due to shortage of space we cannot give even a brief account of the many different ways in which this reaction is being studied. References to some of this work are included in the references.013-121) METHOD AND APPARATUS The comprehensive programme of nuclear research using high-energy particles and y-rays, which was carried out on the synchrocyclotron, required the development of new methods for recording particles and also the construction of complicated experimental equipment. We cannot give here a detailed account of the large amount of experimental equipment which was used in the work with the synchrocyclotron over a period of 8 years by the various groups of physicists. However, it should be stressed that the difficulty, novelty, and wide scope of the work on high-energy particles called for the solution of many new problems both scientific and technological. In order to illustrate this side of the work we will describe some of the equipment used. In all, there were about fifty pieces of equipment. During experiments, these were usually situated in the paths of the particle beams either in the main experimental area (Fig. 30*), or in the meson and polarization laboratories. They were operated by remote control from whichever room accommodated the output recording instruments. Crystal and liquid scintillation counter telescopes were widely used in the work on the scattering of nucleons and n-mesons by nucleons and nuclei. They were used in conjunction with coincidence circuits having resolving times of down to 1O-8 sec. Fig. 31 shows a typical layout for experiments on the scattering of 7r-mesons by protons, and Fig. 32 is a photograph of a typical piece of equipment for the study of nucleon-nucleon scattering. In the same experiments highly efficient hodoscope systems of several hundred Geiger counters with a pulsed voltage supply were used (Fig. 33). Other equipment used included: diffusion chambers with diameters up to 400 mm filled with hydrogen and helium at pressures up to 20 atm (Fig. 34), propane bubble chambers, and a 400 mm Wilson Cloud Chamber provided with a magnetic field (Fig. 36). Special target assemblies filled with liquid hydrogen or deuterium were often used in experiments designed to study the scattering of particles and the creation of mesons. A general view of one of these installations is shown in Fig. 37. Most of the work on the production of charged and neutral n-mesons in nucleonnucleon and nucleon-nucleus collisions was performed with the aid of multi-channel magnetic spectrometers used in conjunction with gas or scintillation counters (Fig. 38). The linear dimensions of the pole pieces of the spectrometer electromagnets are ~1 m and the field in the gaps (~10-15 cm) is 15,000-17,000 oersteds. All of these magnets weighed 35-50 tons. Roughly the same parameters apply to the magnets used with Wilson Cloud Chamber and the diffusion chambers. Bubble chambers filled with liquid hydrogen (Fig. 39) were used in the experiments on the production of charged meson by neutron-proton collisions. * Pig. 30 is a colour photograph which is not reproduced in this issue.

164

V. P. DZHELEPOV and B. M. PONTECORVO

The elementary processes caused by the interaction of high-energy particles with nucleons and nuclei were observed in photographic emulsions and by the use of photographic plate cameras. Microscopes provided with goniometers and stages with very accurate magnifying scales were used for scanning the plates. A special light mask with an illuminated grid was designed and made in the laboratory for marking the emulsions in the camera. Before beginning a series of experiments it is necessary to focus the high-energy beam of particles. For this purpose electromagnetic quadrupole lenses were used and one of these is illustrated in Fig. 40. A considerable array of specialized electronic equipment was constructed to meet the experimental requirements (e.g. wide band amplifiers, multi-channel analysers etc). The need also arose for stereoscopic photographic apparatus, re-projecting equipment, and stereocomparitors, which were required for photographing and reproducing the tracks of particles in the Wilson Cloud Chamber and the diffusion and bubble chambers. Photographs of some typical instruments, designed and constructed in the laboratory, are shown in Figs. 41 and 42. Radiochemical methods were also used to a considerable extent in the work on the 6 m synchrocyclotron.* CONCLUSION

In summarizing the experimental work with the 6m synchrocyclotron of the Laboratory of Nuclear Problems, one can say that it has made a valuable contribution to high-energy physics. It has widened out knowledge of this field of contemporary physics and, at the same time, it has shown the way to new and important problems for study. It is largely due to this work that the young science of high-energy physics has taken its place alongside other sciences in this country. This work has shown that there has been collected together a fine school of trained Soviet scientists who have mastered contemporary physics, and of engineers and other specialists, who have familiarized themselves with a new technology. During 1956 and 1957, one of the problems which appeared in the organization of the Joint Institute of Nuclear Studies was the creation of a pool of physicists on behalf of the twelve countries which are associated with the work of this Institute. In the glorious fortieth year of the October Revolution, the inauguration, at the high-energy laboratory of the Joint Institute of Nuclear Studies, of the giant 10 BeV proton synchrotron opens up great prospects for the further development of contemporary nuclear physics. REFERENCES 1. EFREMOV D. V., MESHCHERIAKOV M. G., MINTSA. L., DZHELEPOV V. P., IVANOVP. P., KATYSHEV V. S., KOMARE. G., MALYSHEVI. F., MONOSZONN. A., NEV~AZHSKY I. KH., POLIAKOV B. I. and CHE~TNOI A. V. CERN Symposium on Hi;rh Energy Particle Accelerators and Pion Physics 1, 148, CERN, Geneva (1956). 2. ME~HCHERIAKOV M. G., C&~TNOI A. V., DZHELEPOV V. P., KATYSHEVV. S., KROPINA. A., ZAMALODCHIKOV B. I., DMITRIEVSICY V. P., GRIGOR’EVE. L., REUT A. A., SAVENKOVA. L., VAKHRAMEEV A. G., TOMILINAT. N., BATIUNIAV. V., IGNATENKO A. E. and IUROV S. N. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1955). 3. MINTS A. L., NEVIAZHSKYI. KH. and POLIAKOVB. I. CERN Symposium on High-Energy Particle Accelerators andPion Physics Vol. 1, p. 419. CERN, Geneva (1956). * See footnote on page 131.

High-energy particle research with the synchrocyclotron

165

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9 (1956). Peaceful, Edition, Sciences

Division; English Language Edition, p. 1. Consultants Bureau, New York (1955). 11. BOGACHEVN. P. Report of the Institute of Nuclear Problems U.S.S.R. Academy of Sciences

(1950). 12. GERASIMOVAR. I., GUREWCH I. I. and MUKHIN K. N. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950). 13. B~GACHEV N. P. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1951). 14. PERFILOV N. A. and IVANOVAN. S. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950); Zh. eksp. teor._fiz. 29, 551 (1955); PERFILOVN. A., LOZHKY 0. V. and SHAMOVV. P. Ibid. 28, 656 (1955). 15. BELOVITSKYG. E., SUKHOVL. V., ROMANOVAT. A. and FRANK I. M. Report of the Institute of Nuclear Problems, U.S.S.R. Academy ofsciences (1950); Zh. eksp. teor. jiz., 28, 729 (1955). 16. PERFILOVN. A. et al. Trud. radieo. inst. Akad Nauk. SSSR. 7, 3-98 (1956). 17. DZHELEPOV V, P., KAZARINOVIv. M., GAVRILOVSKYB. V. and GOLOVIN B. M. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950). 18. MESHCHERIAKOVM. G., GRIGOR’EV E. L., B~GACHEV N. P. and SOROKOL. M. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950). 19. GAVRILOVSKYB. V. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950). 20. KOZODAEVM. S. and MARKOV A. A. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950). 21. DZHELEPOVV. P., GOLOVINB. M. and KAZARINOVI. M. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950, 1951). Report 22. KURCHATOVB. V., MEKHEDOVV. M., KUZNETSOVAM. IA. and KUTCHATOVAL. N. of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950); KURCHATOVB. V., MEKHEDOV V. N., BORISOVA N. I., KUZNETSOVA M. Ia., KURCHATOVA L. N. and CHISTIAKOVL. V., Conference of the U.S.S.R. Academy of Sciences on the Peaceful Uses of Atomic Energy, Moscow (1955). Chemical Sciences Division; English Language Edition, p. 111. Consultants Bureau, New York (1955). 23. GEILIKMANB. T. Report of the U.S.S.R. Academy of Sciences (1950).

24. REUT A. A., SELIVANOVG. I. and IAR’EV V. V. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950); GOLDANSKYV. I., PEN’KINA V. S. and TARUMOVE. Z. Ibid. (1951); Dokl. Akad. Nauk SSSR 101, 1027 (1955); Zh. eksp. teor.fir. 29, 778 (1955). 25. BARANOVS. A. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1950). 26. MESHCHERIAKOV M. G., BOGACHEVN. P., NEGANOVB. S. and PISKAREVE. V. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1951); GRIGOR’EV E. L. and SOROKOL. M. Ibid. (1950); ZHDANOVA. P. and SOKOLOVAZ. S. Report of the Institute of Electronics, U.S.S.R. (1950); ZHDANOVA. P. and SOKOLOVAZ. S. Ibid. (1952).

27. SOLOV’EVAL. P. Zh. eksp. teor.jz. 31, 12 (1956); ZHDANOVA. P. and LEPEKHINA F. G. Report qfthe Institute of Electronics, U.S.S.R. (1951).

166

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28.GOL’DANSKY V. I., KOVAL’SKY A. N., PEN’KINA V. S. and TARUMOVE. Z.

29.

30. 31.

32. 33. 34.

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30, 7 (1956). 56. MUKHIN A. I., OZEROVE. B. and PONTECORVO B. M. Zh. eksp. teor. jTz. 31, 371 (1956) B. M. Zh. eksp. teor.,fiz. 31, 550 (1956). 57. MUKHINA. 1. and PONTECORVO 58. KOZODAEVM. S., SULIAEVR. M., FILIPPOVA. I. and SCHCHERBAKOV lu. A. Dokl. Akad. Nauk SSSR 107,236 (1956). 59. DUL’KOVA L. S., ROMANOVAT. A., SOKOLOVA1. B., SUKHOVL. V., TOLSTOVK. D. and SHAFRANOVA M. G. Dokl. Akad. Nauk SSSR 107, 43 (1956); DUL’KOVA L. S., SOKOLOVA 1. B., SHAFRANOVA M. G. Dokl. Akad. Nauk SSSR 111, 992 (1956). 60. ZINOV V. G. and KORENCHENKO S. M. Zh. eksp. teor.$z. In press. 61. GRIGOREVE. L. and MITIN N. A. Zh. eksp. teor.fiz. 31, 37 (1956). S. M. In course of preparation. 62. ZINOV V. G. and KORENCHENKO 63. MITIN N. A. and GRIGOR’EVE. L. Zh. eksp. teor.jiz. 32, 440 (1957). 64. TAMMI. E., GOL’FANDlu. A. and FAINBERG V. IA. Zh. eksp. teor.fiz. 26, 649 (1954). 65. IGNATENKO A. E., MUKHIN A. I., OZEROVE. B. and PONTECORVO B. M. Dokl. Akad. Nauk

SSSR 103, 395 (1955). 66. IGNATENKOA. E., MUKHIN A. I., OZEROVE. B. and PONTECORVO B. M.

Zh. eksp. teor. j?z.

31, 545 (1956). 67. IVANOVV. G., OSIPENKOV V. T., PETROVN. 1. and RUSAKOVV. A. Zh. eksp. teor. jiz. 31, 1097 (1956). 68. MITIN N. A. and GRIGOR’EVE. L. Dokl. Akad. Nauk SSSR 103, 219 (1955). 69. KUDRINL. P. and NIKOL’SKYB. A. Dokl. Akad. Nauk SSSR 111, 795 (1956). 70. DZHELEPOV V. P., IVANOVV. G., KOZODAEVM. S., OSIPENKOV V. T., PETROV N. I. and RUSAKOV V. A. Zh. eksp. teor.jiz. 31,923 (1956). 71. KOZODAEVM. S., SULIAEVR. M., FILIPPOVA. 1. and SHCHERBAKOV lu. A. Zh. eksp. teor.fiz. 31, 701 (1956). 72. KOZODAEVM. S., SULIAEVR. M., FILIPPOVA. I. and SHCHERBAKOV lu. A. Zh. eksp. teor.

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89. PROKOSHKIN Iu. D. and TIAPKINA. A. Zh. eksp. teor.fiz. 32, 750 (1957). 90. BALASHOVB. D., ZHUKOV V. A., PONTECORVO B. M. and SELIVANOVG. I. Report of the Institute of Nuclear Problems, U.S.S.R. Academy qf Sciences (1955). 91. SOROKOL. M. Zh. eksp. teor.jiz. 30, 296 (1956). 92. DZHELEPOVV. P., OGANE~IANK. 0. and FLIAGINV. B. Zh. eksp. teor. fiz. 29, 886 (1955). 93. LAPIDUSL. I. Report of the Institute of Nuclear Problems, U.S.S.R. Academy qf Sciences (1955). 94. PROKOSHKIN Iu. D. and TIAPKINA. A. Report on the Institute of Nuclear Problems, U.S.S.R. Iv. D. CERN Symposium on High-Energy Particle Academy qf Sciences (1955); PROKOSHKIN Accelerators andPion Physics Vol. 2, p. 385 CERN, Geneva (1956). 95. BAIAKOVIu. D. and TIAPKINA. A. Zh. eksp. teor.fiz. 32, 953 (1957). 96. BAIUKOVIu. D., KOZODAEVM. S. and TIAPKINA. A. Zh. eksp. teor. jiz. 32, 667 (1957). 97. BAWKOV Iu. D., SINAEVA. N. and TIAPKINA. A. Zh. eksp. teor.,fiz. 32,385 (1957). 98. MESHCHERIAKOV M. G., NEGANOVB. S., VZOROVI. K., ZRELOVV. P. and SHABUDINA, F.

Dokl. Akad. Nauk SSSR 109,499 (1956). 99. MESHCHEIUAKOV M. G., ZRELOVV. P., NEGAOVB. S., VZOROV I. K. and SHABUDINA. F. Zh. eksp. teor._fiz. 31, 45 (1956). 100. MESHCHERIAKOV M. G., PLIGINIu. S., SHALAMOVIA. IA. and SHEBANOVV. A. Zh. eksp. teor.jz. 31, 560 (1956). 101. MESHCHERIAKOV M. G., VZOROVI. K., ZRELOVV. P., NEGANOVB. S. and SHABUDINA. F. Zh. eksp. teor.jiz. 31, 55 (1956). 102. MESHCHERIAKOV M. G., PLIGINIv. S., SHALAMOVIA. IA. and SHEBANOVV. A. Zh. eksp. teor.fiz. 31, 987 (1956); Ibid. In press. 103. SIDOROVV. I. Zh. eksp. teor.jiz. 28, 727 (1955). 104. PROKOSHKIN Iu. D. and TIAPKINA. A. Zh. eksp. teor.fiz. In press. 105. ZINOV V. G. and KORENCHENKO S. M. In course of preparation. 106. KFUWTSKY V. V. and REUT A. A. Dokl. Akad. Nauk SSSR 112,232 (1957). 107. NOVIKOVA. N., PONTECORVO B. M. and SELIVANOV G. I. Zh. eksp. teor.jiz. 29, 889 (1955). 108. GUREVICHI. I., MISHAKOVAA. P., NIKOL’SKYB. A. and SURKOVAL. V. Zh. eksp. teor. fiz. In press. 109. LANDAUL. D. Zh. eksp. teor.jiz. 26, 649 (1957). 110. VAISENBERG A. 0. and SMIRNITSKY V. A. Zh. eksp. teor._fiz. In press. 111. PONTECORVO B. M. Reports of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1951, 1953); Zh. eksp. teor.fiz29, 140 (1955). 112. BALANDINM. P., BALASHOVB. D., ZHUKOVV. A., PONTECORVO B. M., and SELIVANOV G. I. Zh. eksp. teor.fiz. 29, 265 (1955). 113. SHAMOVV. P. and L~ZHKIN 0. V. Zh. eksp. teor.fiz. 29, 286 (1955); PERFILOV N. A. and OSTROUMOV V. I. Dokl. Akad. Nauk SSSR 103, 227 (1955); OSTROUMOV V. I. Dokl. Akad. Nauk SSSR 103,409 (1955); IVANOVAN. S., PERFILOV N. A. and SHAMOVV. P. Dokl. Akad.

Nauk SSSR 103, 573 (1955). 114. GRIGOR’EVE. L. Zh. eksp. teor._fiz. 28,761 (1955). 115. MESHCHERIAKOV M. G., NU~USHEVS. B., and STOLETOV G. D. Zh. eksp. teor.$z. 31,361(1956). 116. ALPERSV. V., BARKOVL. M., GERASIMOVA R. I., GUREVICHI. I., MUKHIN K. N., NIKOL’SKY B. A. and TOPORKOVA E. Z. Zh. eksp. teor.jiz. 30,1025 (1956); ALPERSV. V., BARKOVL. M., GERASIMOVA R. I., GIJREV~CHR. I., MISHAKOVAA. P., MUKHIN K. N. and NIKOL’SKYB. A. Zh. eksp. teor.jz, 30, 1034 (1956). 117. RETJTA. A., KORENCHENKOS. M., IUR’EV V. V. and PONTECORVO B. M. Dokl. Akad. Nauk

SSSR 102, 723 (1955). 118. GRIGOR’EVE. L. and SOLOVEVAL. P.

Zh. eksp. teor.jiz. 31,932 (1956). 119. KISELEVV. S. and FLIAGINV. B. Zh. eksp. teor. fiz. 32, 957 (1957); DZHELEPOVV. P. KAZARINOVIu. M., GOL~~IN B. M. and FLIAGINV. B. Report of the Institute of Nuclear Problems, U.S.S.R. Academy of Sciences (1953). 120. KUMEKIN Iu. P. Zh. eksp. teor.$z. In press. 121. PONTECORVO B. M. and IGNATENKO A. E. Report of theznstituteof Nuclear Problems, U.S.S.R. Academy of Sciences (1951); MOSKALEVV. I. and GAVRILOVSKY B. V. Dokl. Akad. Nauk SSSR p. 972 (1956).

FIG. I.-Mechanical chopper to obtain short neutron pulses.@3’~‘*4’ The transverse rotor revolves at a speed of up to 25.000 revs/min through a frictional drive from a motor.

FIG. 2.-The rotor of the mechanical chopper shown in Fig. 1. The rotor has a double-slot system: straight to allow the passage of high energy neutrons; and curved for small energy neutrons.

p. 168

FIG. 3.-Mechanical chopper with a longitudinal rotor.cz5j The rotor by an air turbine at a speed of up to 25,000 revs/min.

is driven

0

c~

c~

FIG. 6.-Multi-electrode

fission chambers, Above-chamber Below-chamber

with a large amount

of fissionable

with 22 mg of 239Pu.i25) with 280 mg of z3sU.“05’

isotopes.

FIG. 7.-Lead The neutron

cube of a spectrometer for slowing down neutrons.C50) The cube is 2 x 2 x 2.3 m. source is a zirconium-tritium target, bombarded with deuterons from an accelerator tube. At the top-part of the target with an electrical shutter. On the right in the centre of the cube may be seen a neutron-detector, placed in the measuring channel.