Cesar Lattes...elementary particle man

Cesar Lattes
1924 to 2005

A little know South American physicist, Cesar Lattes, passed away in 2005 and was one the early experimenters involving sub-atomic particle physics and was the discoverer of the pi meson whose existence was firmly established in the 40's. This is a crude translation from Portuguese on some biographical material followed by, with generous permission, an article from Nature and Cesar Lattes's original paper, "Processes involving charged mesons", on the pi meson.


Cesare Mansueto Giulio Lattes was born in Curitiba the 11 of July of 1924, son of Giuseppe Lattes and D. Carolina Maria Rosa Lattes. Martha is married D. Siqueira Neto Lattes, has four children and nine grandsons. He made its studies, elementary schools in the American School of Curitiba between 1929 and 1933, and secondary one in the Average Institute Dante Alighieri, in São Paulo, of 1934 the 1938. He entered the Department of Physics of the College of Philosophy and Sciences and Letters of the USP, concluding the Bacharelado in 1943; he received from this University the Heading of Honoris Doctor Cause in 1948. Pensioner of the Federal University of Rio De Janeiro, the Brazilian Center of Physical Research and of the State University of Campinas is Titular Professor.

Its scientific career had beginning in middle of years 40, in then the Department of Physics of the College of Philosophy Sciences and Letters of the University of São Paulo, when it published scientific work on the abundance of nuclei in the universe, under the orientation of Gleb Wataghin.

Since then it had its on name the scientific results of the biggest repercussion and the initiatives of most fruitful for the progress of science in Brazil and the South America. The discovery of píon in 1947, in contribution with G.Occhialini and C.F.Powell, was the landmark in its career that if made to follow of the most significant consequences.

Of a side the discovery disclosed the particle, presumivelmente, responsible for the behavior of the nuclear forces. The reach of this fact exceeded the borders of basic science given the expectations that then coated any magnifying of knowledge in these domínios; the development of the nuclear energy, in the postwar period, demanded formularizations that alliviated it of onerous empirismo e, many times, risky with that it came if making. The artificial production of that particle, in 1948, still for Lattes but now in association with Eugene Gardner, in the just-constructed synchro-cyclotron of the University of California, in Berkeley, more marked the beginning of formidable race for the construction of accelerators and more powerful than the nuclear physics of the postwar period characterized.

Of another side, ample openings in the land of the institutionalization of science, in Brazil and the South America, had folloied this discovery directly, on to the return and definitive permanence of Lattes in the South American continent.

It leads a scientific group that in 1949 created the Brazilian Center of Physical Research, Institute that polarized and agasalhou initiatives as of the formation of the Institute of Pure and Applied Mathematics, of the Latin American School of Physics, the Latin American Center of Physics, while it was distinguished for the activity of research in international level, for the measures of modernization of the resumes of education of the physics and of formation of the staff that it today constitutes ponderável parcel of the operating scientific leadership in the physics Brazilian.

In the same year, together with bolivian colleagues, create in La Paz, the conditions to what she would come to be the Laboratory of Cosmic Physics, from an old station of meteorological comments, where gets the registers of the events that had led to the discovery of píon. Early this Laboratory if transformed into scientific center of the biggest international interest, sheltering in its dependences equipment and scientists of all the parts of the world that had written important chapters of the knowledge there on the cosmic radiation.

Both the institutions had resisted the hard tests of the time, having had the Brazilian Center of Physical Research been absorbed for the National Advice of Scientific and Technological Development of the Brazilian government, and the Laboratory of Chacaltaya, today Laboratory of Cosmic Physics, for the Universidad Mayor de San Andrés, constituting the main organism of its Institute of Physics.

Its performance in Brazil during the first years had, also, important paper in the catalização of the efforts that had led finally to the creation of the National Advice of Research - current National Advice of Scientific and Technological Development - in 1951. For the creation of an agency with its characteristics it fought of has the Brazilian scientific community very, constituted in its majority of researchers in biological sciences; they had come to enter into an alliance groups of interested in the development of the nuclear technology, but without being able of transaction with the bureaucracy, face to the scarce tradition and the lack of recognized scientific authority in those domínios. The National Advice of Research gave new impulse to the scientific and technological research in Brazil, having counted with Lattes in the composition of its first Managing Advice.

Scientific director of the Brazilian Center of Physical Research since the foundation, and main scientific consultant in the first years of the Laboratory of Chacaltaya, leaves these incubencies in 1955 for one short season in the United States. Refusing the invitations honrosos, as to substitute the deceased Enrico Fermi in he commands of its Institute in the University of Chicago, returns to Brazil two years later creating, in the USP, a laboratory for studies of interactions the high energies in the cosmic radiation. He participates, in 1962, of the pioneering group that organized the State University of Campinas, moving to this city in the following year and giving beginning to the formation of its Institute of Physics. In short period this university conquered high concept in half Brazilian colleges student e, particular, its institute of physics is credited as of best in Brazil, surrounded of the great prestige and international projection.

Not obstante the singular repercussion of the discovery of píon, the contributions do not deplete, absolutely, in this memorable one made. Owner of rare versatility its works include contributions of the biggest merit in varied fields of the modern physics, since theoretical research on the origins and abundance of nuclear species in the universe and classic electrodynamics, until instrumental developments, in the area of nuclear emulsions, these last ones surrounded of favorables openings; as member of the group of Bristol, in the second half of years 40, it is participant of the shining sequence of developments that had culminated in the rise of nuclear emulsions, before precarious ionográficos log devices, to the category of measurement instruments. These works had not only made possible the discovery of píon, as physical properties. From 1962 it leads the meeting of Brazilian and Japanese groups in a long-range project on interactions the high energies in the cosmic radiation: the Brazil-Japan Contribution. Since then the pioneering results of this group, in domínios then are of the reach of the most powerful accelerators in operation or in project, they had gained high prestige in international the scientific ways, considered as promising openings for expansion them borders of the modern physics.

Member of the Brazilian Academy of Sciences, of the International Union of Pure and Applied Physics, of the Latin American Advice of Cosmic Rays, of Brazilian, American, German, Italian and Japanese the Societies of Physics, among others associations, occupied numerous times council member position, when it contributed with its experience and pioneering vision for the formularization of politics and lines of direction of action. He has been white of repeated homages on the part of official organizations and private in Brazil and the exterior and innumerable times paranymph or protector of contingents of new students was chosen, formandos in accurate and applied sciences. Between prizes, medals and comendas, received, in Brazil, the Prize Einstein of 1950, the Prize Fonseca Costa, of the CNPq, in 1958, the Medal Santos Dumont in 1989, the commemorative Medal of the 25 years of the SBPC and commemorative plate of the 40 years of this society, the symbol of the City of Campinas, in 1992, and many others. One is proud, particularly, of the initiative of sets of ten of Brazilian cities that had given to the name the municipal schools to it, libraries, squares, streets.

Its performance in the South American continent was recognized for the bolivian government, that granted the heading to it of honorary citizen of that country, in 1972, for the government of the Venezuela, that conferred it comenda Andrés Bello in 1977, and for the Organization of the American States, that the prize granted it Bernardo Houssay, in 1978; in 1987 it received the Prize from Physics of the Academy of the Third World.

Simple person, offers the heat of its privacy indistinctly how many they look it; she sees with accented concern the destorcidos uses of the scientific knowledge in the modern and manifest world its opinions without reverences, to the default of preconceptions and lesser interests. She observes with píons that she discovered. This will be, perhaps, the biggest gratuity that waits to receive from its life devoted to the progress of science and combat to the subdesenvolvimento.

Nature:

"Fifty years of the pi meson"

It was in 1947 that the existence of the pi meson was established. One of the authors of the research that led to its discovery was the Brazilian physicist César Lattes. Everybody knows that this was an important finding. However, who can tell what exactly are those pi mesons? What changes their discovery brought to physics?

The discovery of the pi meson was a fundamental step in the understanding of the sub-atomic world. Throughout the 20th century, there was a gradual change and increase in complexity of our ideas on the constitution of matter. Atoms are built of electrons and nuclei. The nucleus contains positive charge particles (protons) and other chargeless particles (neutrons). What binds protons and neutrons together to build the nucleus? They cannot be kept together by electric attraction – on the contrary, protons repeal each other. Gravitational forces, on the other side, are much smaller than the repulsive electrical forces. It was necessary to suppose the existence of a new kind of nuclear forces, stronger than the electric repulsion, to keep the nucleus particles together. In 1935, the Japanese theoretical physicist Hideki Yukawa proposed an explanation of nuclear forces. He suggested the existence of a new particle, with a mass about 200 times larger than that of the electron. The unknown particle was supposed to be emitted and absorbed by protons and neutrons. The exchange of those particles between the constituents of the atomic nucleus would produce a short-range attraction between them that would explain the stability of the nucleus. This particle received the name "meson" (from the Greek "mesos" = intermediate) because its mass was intermediate between those of the electron and of the proton. According to Yukawa's theory, mesons can only exist for a very short time. Outside the atomic nucleus, they were supposed to disintegrate in just one thousandth of a millionth of one second.

In 1937-38, Carl D. Anderson and Seth H. Neddermeyer found in the cosmic radiation, that continually reaches the ground, signs of something that looked like Yukawa's meson: it has the expected mass and disintegrated as Yukawa's particle was expected to disintegrate. For ten years, it seemed that everything fit in the scheme, and that there was a nice theory on the constitution of matter. In 1947, however, this peace was shaken. It became clear that the meson of Anderson and Neddermeyer did not behave as predicted by Yukawa's theory.

Mesons should be strongly absorbed by protons and neutrons, if they are to explain nuclear forces. It was predicted, therefore, that they should be easily captured by matter. However, a group of Italian physicists (Marcello Conversi, Ettore Pancini and Oreste Piccioni) observed that those mesons that had been found in cosmic radiation could pass through several hundred atomic nuclei without suffering any interaction. They had a very weak interaction with protons and neutrons – the opposite of what was expected.

Something was going wrong. That is where Lattes' group comes in. In 1946, a research group in Bristol, England, under the leadership of Cecil F. Powell, was studying the tracks produced by nuclear reactions in some special (thick and sensitive) photographic plates called "nuclear emulsions". Studying the tracks produced by protons and other charged particles in such emulsions, it was possible to find their mass and energy. Beppo Occhialini and César Lattes analyzed some emulsions of a new kind, that had been placed at the top of a mountain (the Pic du Midi). When the plates were developed and analyzed, they observed a large number of tracks produced by particles that at first they interpreted as being the known mesons. However, after a few days of detailed study, they found two special tracks of mesons that gradually reduced their speed in the emulsion, and finally stopped. At the end of those tracks, they observed that a new meson appeared. What could that be? There were several possible interpretations. Perhaps the meson had reacted with an atomic nucleus inside the emulsion, and after the interaction it could have been expelled with a larger speed. Perhaps the meson could have suffered some sort of transformation into another kind of meson. The two initial cases were insufficient evidence to reach any safe conclusion. In order to obtain a larger number of tracks, Lattes traveled to Bolivia, and put several nuclear plates at the top of Mount Chacaltaya, 5,500 m above the sea level. When the plates were developed, it was possible to find about 30 tracks of double mesons. A detailed study led to the determination of the meson masses, and it was possible to establish that there were indeed two types of mesons, with different masses.

One of the mesons was about 30% or 40% heavier than the other one. The heavier meson was able to disintegrate and to produce the lighter meson. The second particle was the one that was already known from the studies of Anderson and Neddermeyer. To distinguish it from the other one, it was called "mu meson" (nowadays, it is called "muon"). The primary meson, on the other side, was something new, unknown. It was called "pi meson", and its identification was announced in October 1947. Later tests showed that it strongly interacted with nuclei and that its characteristic properties were those required by Yukawa's theory. The particles that hold the nucleus together had been found.

The discovery of the pi meson was not merely a confirmation of a theory. It opened a whole new world to investigation. First, because it became clear that there were particles (muons) that had not been predicted; their role in nature was unknown. Secondly, because the study of cosmic rays soon led to the unexpected discovery of several other particles. In that same year, several tracks that did not correspond to anything known were detected.

Powell's group found evidence for a kind of meson twice as heavy as pions. They were called "tau mesons" at first, and nowadays they are called kappa mesons. In the same year (1947), Clifford Butler and George Rochester observed V shaped tracks. They could be explained assuming the existence of new neutral particles (without electric charge) that produced no track and that disintegrated into one positive and one negative particle. In the following years, physics was flooded by new, unexpected particles. It was difficult to understand their properties, at that time. Robert Oppenheimer introduced the phrase "sub nuclear zoo" to describe this new world of particles. Among the exotic animals of this zoo, there were particles heavier than protons (the so-called "hyperons"), of several different types. The new "animals" were first studied in cosmic rays, but powerful particle accelerators were soon built – each one more powerful than the former – and they allowed the creation and investigation of those particles in the laboratory.

The discovery of the pi meson was something more than finding one special particle. It heralded the beginning of a deep revision of the physical concepts on the structure of matter. The large variety of particles that were discovered in the following years challenged the very concept of "elementary particle" as something indivisible, simple. It led to the search for a substructure for protons, mesons and the other particles. The theory of quarks would never arise without the stimulus of those empirical discoveries, that began 50 years ago.

From Nature, here is Cesar Lattes's original paper: Nature 159: 694-7, May 24, 1947

"PROCESSES INVOLVING CHARGED MESONS"

By

DR. C. M. G. LATTES,

H. MUIRHEAD,

DR. G. P. S. OCCHIALINI

and DR. C. F. POWELL

H. H. Wills Physical Laboratory,

University of Bristol

IN recent investigations with the photographic method1,2, it has been shown that slow charged particles of small mass, present as a component of the cosmic radiation at high altitudes, can enter nuclei and produce disintegrations with the emission of heavy particles. It is convenient to apply the term ‘meson’ to any particle with a mass intermediate between that of a proton and an electron. In continuing our experiments we have found evidence of mesons which, at the end of their range, produce secondary mesons. We have also observed transmutations in which slow mesons are ejected from disintegrating nuclei. Several features of these processes remain to be elucidated, but we present the following account of the experiments because the results appear to bear closely on the important problem of developing a satisfactory meson theory of nuclear forces.

In identifying the tracks of mesons we employ the method of grain-counting. The method allows us, in principle3, to determine the mass of a particle which comes to the end of its range in the emulsion, provided that we are correct in assuming that its charge is of magnitude |e|. We define the ‘grain-density’ in a track as the number of grains per unit length of the trajectory. Knowing the range-energy curve for the emulsion4, we can make observations on the tracks of fast protons to determine a calibration curve showing the relation between the grain-density in a track and the rate of loss of energy of the particle producing it. With this curve, the observed distribution of grains along the track of a meson allows us to deduce the total loss of energy of the particle in the emulsion. The energy taken in conjunction with the observed range of the particle then gives a measure of its mass.

We have found that the above method gives satisfactory results when, in test experiments, it is applied to the determination of the mass of protons by observations on plates developed immediately after exposure. The errors in the observed values, based on grain-counts along individual tracks, are only a little greater than those corresponding to the statistical fluctuations associated with the finite number of grains in a track. As we have previously emphasized, however, serious errors arise when the method is applied to the plates exposed for several weeks to the cosmic rays2. These errors are due mainly to the fading of the latent image in the time elapsing between the passage of the particle and the development of the plate.

We have attempted to allow for fading by determining a calibration curve for each individual plate by grain-counts on the tracks of a number of protons, chosen at random from those originating in 'stars'. Such a calibration curve corresponds to an average value of the fading of the tracks in the plate. While we thus obtain improved mean values for the mass of particles of the same type, as shown by test measurements on the tracks of protons other than those used in making the calibration, the individual values are subject to wide variations. In no case, however, have mass determinations by grain-counts of particles, judged to be protons from the frequency of the small-angle scattering, given values exceeding 2,400 me or less than 1,300 me.

In these circumstances it is not possible to place serious reliance on the masses of individual mesons determined by grain-counts; and we employ the method, in the present experiments, only to distinguish the track of a meson from that of a proton. In searching a plate, an experienced observer quickly learns to recognize the track of a meson by inspection, provided that its range in the emulsion exceeds 100m. Nevertheless, we regard it as established that a particular track was produced by a meson only if both the grain-density and the frequency of the Coulomb scattering correspond to the values characteristic of a particle of small mass. We have con-sidered the possibility' that as a result of a rare combination of circumstances we might, in spite of the above precautions, wrongly attribute the track of a proton to a meson of mass less than 400 me. It is difficult to give a numerical estimate of the probability of making such an error, but we believe it to be very small.

Secondary Mesons

We have now made an analysis of the tracks of sixty-five mesons which come to the end of their range in the emulsion. Of these, forty show no evidence for the production of a secondary particle. The remaining twenty-five lead to the production of secondary particles. Fifteen of them produce disintegrations with the emission of two or more heavy particles, and from each of the remaining ten we // observe a single secondary particle. Of these latter events, the secondary particle is in four cases a hydrogen or heavier nucleus ; in four other cases the identification is uncertain, and in the last two cases it is a second meson.

Fig. 1 is a reproduction of a mosaic of photomicrographs which shows that a particle, m1, has come to the end of its range in the emulsion. The frequent points of scattering and the rapid change of grain-density towards the end of the range show that the track was produced by a meson. It will be seen from the figure that the track of a second particle, m2, starts from the point where the first one ends, and that the second track also has all the characteristics of that of a particle of small mass. A similar event is shown in Fig. 2. In each case the chance that the observation corresponds to a chance juxtaposition of two tracks from unrelated events is less than 1 in 109.

Grain-counts indicate that the masses of the primary particles in Figs. 1 and 2 are 350 ± 80 and 330 ± 50 me, respectively ; and of the secondary particle in Fig. 1, 330 ± 50 me, the limits of error corresponding only to the standard deviations associated with the finite numbers of grains in the different tracks. All these values are deduced from calibration curves corresponding to an average value of the fading in the plate, and they will be too high if the track was produced late in the exposure, and too low if early. We may assume, however, that the two-component tracks in each event were produced in quick succession and were therefore subject to the same degree of fading. In these circumstances the -measurements indicate that if there is a difference in mass between a primary and a secondary meson, it is unlikely that it is of magnitude greater than 100 me. The evidence provided by Fig. 2 is not so complete because the secondary particle passes out of the emulsion, but the variation in the grain density in the track indicates that it was then near the end of its range. We conclude that the secondary mesons were ejected with nearly equal energy.

We have attempted to interpret these two events ,in terms of an interaction of the primary meson with a nucleus in the emulsion which leads to the ejection of a second meson of the same mass as the first. Any reaction of the type represented by the equations

ANZ + m0-1 ® BNZ-2 + m0+1 or ANZ + m0+1 ® CNZ+2 + m0-1 (1)

in which A represents any stable nucleus known to be present in the emulsion, involves an absorption// of energy, in contradiction with the fact that the secondary meson is observed to have an energy of about 2 MeV.

A second process, represented by the equation:

Ag47 + m0-1 ® XZ + Y45-Z + m0+1 , (2)

in which X and Y represent two nuclei of approximately equal charge number, may be energetically possible, but' the chance of it occurring in conditions where the total energy of the two recoiling nuclei id of the order of only a few million electron-volts is remote. It is therefore possible that our photographs. indicate the existence of mesons of different mass5,6,7. The evidence provided by grain counts is not inconsistent with such an assumption. We have no direct evidence of the signs of the charges carried by the two mesons, except that the one secondary meson which comes to the end of its range in the emulsion does not lead to a disintegration with the emission of heavy particles. If, however, we assume that the transmutation corresponds to the interaction of the primary meson with a light nucleus, of a type represented by the equation

Cl126 + m0-1 ® Be124 + m0+1 , (3)

the difference in mass of the two mesons must be of the order of 60 me, according to estimates of the mass of the beryllium nucleus.

The only meson theory, to our knowledge, which assumes the existence of mesons of different mass is that of Schwinger8. It is visualized9 that a negative vector meson should have a very short life. and should lead to the production of a pseudo-scalar meson of the same charge but lower mass, together with a quantum of radiation. It will therefore be of great interest to determine whether the secondary meson, in trans. mutations of the type we have observed, is always emitted with the same energy. If this is so, we must assume that we are dealing with a more fundamental type of process than one involving particular nuclei such as is represented in equation (3). If, as an example of such a process, we assume that the momentum of the secondary meson appearing in our experiments is equal and opposite to that of an emitted photon, the total release of energy in the transmutation is of the order of 25 MeV.

In recent communications10,11 very radical conclusions have been drawn from the results of observations on the delayed coincidences produced by positive and negative mesons in interactions with light and heavy nuclei12,13. It is assumed that a negative meson, at the end of its range, falls into a K orbit around a nucleus. In the. case of a heavy nucleus, it is then captured, giving rise to a disintegration with the emission of heavy particles. With a light nucleus, on the other hand, it is regarded as suffering b-decay before being captured, so that, like a positive meson, it can produce a delayed coincidence. The conclusion is drawn that the nuclear forces are smaller by several orders of magnitude than has been assumed hitherto. Since our observations indicate a new mode of decay of mesons, it is possible that they may contribute to the solution of these difficulties.

Emission of Mesons from Nuclei

Fig. 3 shows a mosaic of photomicrographs of a disintegration in which six tracks can be distinguished radiating from a common centre. The letters at the edge of the mosaic indicate whether a particular track passes out of the surface of the emulsion, s, into the glass, g, or ends in the emulsion, e. The grain-density in tracks a and c indicate that the time between the occurrence of the disintegration and the development of the plate was sufficiently short to avoid serious fading of the latent image.

The track marked f suffers frequent changes in direction due to scattering, and there is a very rapid change in the grain-density in moving along the trajectory. These two features, taken together, make it certain that the track was produced by a light particle, and grain counts give an estimate for the mass of 375 ± 70 me 14.

We have now observed a total of 1,600 disintegration 'stars', in each of which three or more charged particles are ejected from a nucleus. Of these, 170 correspond to the liberation of an amount of energy equal to, or greater than, that in the 'star' represented in Fig. 4; but only in two cases can we identify an// emitted particle as a meson. We cannot conclude, however, that the emission of mesons in such disintegrations is so rare as these figures suggest. If a meson is emitted with an energy greater than 5 MeV., it is likely to escape detection in the conditions of d our experiments. Mr. D. H. Perkins, of the Imperial College of Science and Technology, has shown that, in the B1 emulsion, the grain-density in the track of a meson becomes very small at energies greater than 2 MeV., and we must anticipate a similar result in the C2 emulsion at higher energies. Our observations are therefore not inconsistent with the view that the ejection of mesons is a common feature of the disintegration of nuclei by primary particles of great energy, and that the present instance, in which the velocity of ejection has been exceptionally low so that an identification of the particle has been possible, is a rare example. It is possible that the example of meson production recently described15 is due to a similar process, produced by a primary particle of higher energy, in which some of the heavier fragments emitted on the disintegration have escaped detection because of the depth inside the lead plate at which the event occurred.

The disintegration shown in Fig. 3 may be the representative of a type, common in the high atmosphere with particles of great energy. In the present instance the energy of the primary particle must have been of at least 200 MeV., and, if its mass was equal to or less than that of a proton, it would not have been recorded by the emulsion.

Disintegrations Produced by Mesons

The observation of the transmutations of nuclei by charged mesons has led to the suggestion of a method for determining the mass of these particles based on observations of the total energy released in the disintegration1,2. In attempting to apply the method, we meet the difficulty of identifying the particular type of nucleus undergoing disintegration and of taking account of any ejected neutrons which will not be recorded by the emulsion. A photograph of such a disintegration which, at first sight, appears to allow us to draw' definite conclusions, is shown in Fig. 4. In the photograph, the tracks of four heavy particles can be distinguished, of which the short tracks a1, a2, and a3 end in the emulsion ; a1 and a2 were certainly produced by a-particles, and grain-counts show that a3 is due to a proton. The observations are therefore consistent with the equation

N147 + m0-1 ® 2He42 + H11 + H11 + 4n10 ; (4)

or, less probably, to a similar equation involving the emission of a deuteron or a triton in addition to the particles of short range.

Grain-counts on the track of the particle of long range, d, which passes out of the emulsion, indicate that if it was produced by a proton, the initial energy of the particle was about 15 MeV. Alternatively, if the particle was a deuteron, its energy was 30 MeV.; or, if a triton, 45 MeV. In any case, we can determine the minimum energy which must be attributed to the emitted neutrons if momentum is to be conserved in the disintegration. As a result, we find a minimum value for the mass of the primary meson of 240 me.

The value determined by grain-counts is also 240 ± 50 me. In view of the recent results of experiments on delayed coincidences, referred to previously12,13, such results must, for the present, be accepted with great reserve. We must expect the liberation of an amount of energy of magnitude 100 MeV. in any nucleus to lead to the ejection of several particles, some of which may be neutrons. There is therefore no firm basis for assuming, that the disintegration represented in Fig. 4 corresponds to the disintegration of a nucleus of nitrogen rather than one of silver or bromine. Indeed, the delayed coincidence experiments suggest that the second assumption is the more probable. When a sufficient number of observations with loaded plates has been accumulated, it may be possible to draw more definite conclusions from observed regularities in the modes of disintegration of particular types of nuclei.

A detailed account of the experiments will be published elsewhere.

1 Perkins, D. H., Nature, 159, 126 (1947).

2 Occhialini and Powell, Nature, 159, 186 (1947).

3 Bose and Choudhuri, Nature, 148, 259 (l941) ; 149, 302 (1942).

4 Lattes, Fowler and Cuer, Nature, 159, 301 (l947).

5 Hughes, Phys. Rev., 69, 371 (l946).

6 Leprince-Ringuet and L'Héritier, J. Phys. et Rad.,

7, 65 (1946). 7 Bethe, Phys. Rev., 70, 821 (1947).

8 Schwinger, Phys. Rev., 61, 387 (1942).

9 Wentzel, Rev. Mod. Phys., 19, 4 (l947).

10 Fermi, Teller and Welsskopf, Phys. Rev., 71, 314 (1947).

11 Wheeler, Phys. Rev., 71, 320 (l947).

12 Conversi, Pancini and Piccioni, Phys. Rev., 71, 200 (l947).

13 Sigurgeirsson and Yamakawa, Phys. Rev., 71, 310 (l947).

14 Zhdanov, Akad. Nauk. 0dtel. Bull. Ser. Phys., 3, 734 (1938) ; 4, 272 (l939). C.R. (U.S.S.R.), 28, 110 (1940).

15 Rochester, Butler and Runcorn, Nature,159, 227 (1947).

Fig. 1. Observation by Mrs. I. Roberts. Photomicrograph with Cooke X 45 'fluorite' objective. Ilford 'Nuclear Research', boron-loaded C2 emulsion. ml is the primary and m2 the secondary meson. The arrows, in this and the following photographs, indicate points where changes in direction greater than 2° occur, as observed under the microscope. All the photographs are completely unretouched.

Fig. 2. Observation by Miss M. Kurz. Cooke X 45 'fluorite' objective. Ilford 'Nuclear Research' emulsion, type C2, boron-loaded. The secondary meson, m2, leaves the emulsion.

Fig. 3. Observation by Mrs. I. Roberts. Photomicrograph with Cooke X 45 'fluorite' objective’. Ilford 'Nuclear Research', boron-loaded C2 emulsion. The track (b) dips steeply and its apparent grain density is greater than the true value through foreshortening. Both (b) and (c) were probably produced, by a-particles.

Fig. 4. Observation by Mrs. I. Roberts. Cooke X 95 achromatic objective. Ilford ‘Nuclear Research’ emulsion, type C2, lithium-loaded.