CLOUD CHAMBER PHOTOGRAPHS OF THE COSMIC RADIATION CD.ROCHESTER «nd J.G.WILSON THE PHYSICAL LABORATORIES, UNIVERSITY OF MANCHESTER FOREWORD by Professor P. M. S. BLAC KETT, F. R. S. PERGAMON PRESS LTD . LONDON1952 Published in Great Britain by Pergamon Press Ltd, 2, 3 & 5 Studio Place, London S. W. 1 Printed by Universitätsdruckerei H. Stürtz AG., Würzburg VI Contents—Acknowledgments Section V: V-particIes The first photographs of V-particle decay 102 Fast charged V-particle 120 Fast neutral V-particles 104 Slow charged V-particle 121 Slow neutral V-particles 106 Nuclear encounters in which particles are identi- _ , . . , , fied which appear to have masses of the order XT Ί Neutral V-decays in which one secondary can be , f ηηπ 199 identified 108 0t 1W °W· lZl The origin of neutral V-particles 116 Section VI A heavy cosmic ray primary at 95,000 altitude 124 ACKNOWLEDGMENTS A truly representative collection of photographs would not have been possible without the active cooperation of workers in all parts of the world, and we are very greatly indebted to all those from whom we requested photographs, and who without exception gave us every assistance. Our particular thanks are due to Dr. G. R. EVANS, of Aberystwyth, through whom we were able to obtain photographs taken by the late Professor E. J. WIL­ LIAMS, and to the Director of the Science Museum, South Kensington, who supplied prints of early photographs by Professor D. SKOBELTZYN, of which the original negatives no longer exist. We are most grateful to Professors ANDERSON, BLACKETT, BRODE, COCCONI, FRETTER, GREGORY, HAZEN, JOHNSON, KUNZE, LEPRINCE-RINGUET, NEY, NISHINA, W. M. POWELL, ROSSI, SALVINI, STREET and THOMPSON, and Drs. BUTLER, COHEN, DAUDIN, EVANS, HAYWARD, HODSON, JOPSON and ROSSER, all of whom supplied photographs. We are also indebted to the editors and publishers of the following publications, for permission to reproduce photographs first published in their pages : the American Journal of Physics, Annales de Physique, Comptes Rendus de Γ Académie des Sciences, the Journal of the Franklin Institute, Nature, the North-Holland Publishing Co., il Nuovo Cimento, the Philosophical Magazine, the Physical Review, the Princeton University Press, the Pro ceedings of the Physical Society of London, the Proceedings of the Royal Society, Reviews of Modern Physics, Sigma Books Ltd., and the Zeitschrift für Physik. Finally we wish to record our thanks to our colleagues at Manchester for their interest and advice throughout the preparation of the book, and in particular to Dr. C. C. BUTLER and Mr. K. H. BARKER, with whom we have had invaluable discussions on the inter pretation and presentation of the photographs. Many of the photographs used were prepared by Mr. HUGH MARTIN, and we are most grateful to him for his careful and patient work. G.D.R. J.G.W. vu FOREWORD by Professor P. M. S. BLACKETT, F.R.S. Nobel Laureate THE LAST TWO decades have seen an increasing use of two experimental methods, the cloud chamber and the photographic emulsion, by which the tracks of individual sub­ atomic particles of high energy can be studied. Progress in the study of these particles would have been extremely slow were it not for these two methods, so contrasted in their technique but so similar in their results. All but one of the now known unstable elementary particles have been discovered by one or other of these techniques ; the positive and nega­ tive μ-mesons and the various types of charged and uncharged V-particles by the cloud chamber ; the positive and negative π-mesons and the κ- and τ-mesons by the photographic emulsion. Only the jr°-meson was first discovered by other means, although a few months later it was also found in photographic emulsions. Since both methods give us pictures of what single particles do—and what any particle does is much dependent on chance—the pictures obtained are all different in detail and often very complicated. An important step in any investigation using these methods is the interpretation of a photograph, often of a complex photograph, and this involves the ability to recognise quickly many different types of sub-atomic events. To acquire skill in interpretation, a preliminary study must be made of many examples of photo­ graphs of the different kinds of known events. Only when all known types of event can be recognised will the hitherto unknown be detected. Such new events may be extremely rare, and it is important that, when found, the chamber record should prove to be technic­ ally suitable for accurate measurement. Thus an essential, and by no means always an easy, task which faces the user of the cloud chamber method, is to maintain a high standard of technique over long periods of time. If once the level drops too far, reliable inter­ pretation and accurate measurement become impossible. I think the guiding object in this compilation by Drs. WILSON and ROCHESTER of a volume of carefully selected cloud chamber photographs, is to facilitate the acquiring of these two essential skills, that of interpretation and that of recognition and attainment of high technical quality. I believe that the two authors have succeeded brilliantly in their task, and that the book will prove a quite indispensable aid to all those research workers who are investigating sub-atomic events by the cloud chamber method. More­ over to all students of the physics of elementary particles, even if they are not themselves engaged in original investigation, the book should prove an invaluable means of acquiring a clear picture of the various sub-atomic processes, and so should prove a great aid to physical understanding. To many non-physicists too I think the book should make a definite appeal, if only by the visual beauty of the patterns of the tracks woven by these energetic elementary particles. It should do more than this, however, for it must surely help us to make clear that this world of sub-atomic events is one which can be easily visualised and understood without the aid of complicated mathematics or the mastery of deep theories. If one asks why some of these complicated events happen, one may be led into the subtle intricacies and uncertainties of modern fundamental theoretical physics, but if the experimenter contents himself with asking how they happen, then these pictures, and the attached commentaries, are an ideal guide to the world of energetic elementary particles. Manchester, Dec. 1951. VITI PREFACE IT IS NOW rather more than ten years since the "Atlas typischer Nebelkammerbilder" of GENTNER, MAIER-LEIBNITZ and BOTHE was published. This volume covered the whole field of application of the cloud chamber, and illustrated in a striking way the varied contributions made by cloud chamber investigations in many branches of the physics of the atomic nucleus and of cosmic rays. When, some two years ago, the possibility of a revised edition of the " Atlas " was under consideration, it became clear that the advances of the last ten years were so extensive that representative photographs covering all applications of cloud chambers could no longer be appropriately brought into a single volume. It was decided to publish two volumes, and we were invited to prepare one of these, which would cover the application of the cloud chamber method to problems of cosmic ray physics. In this volume it has been our aim to include both particular photographs of historic importance and typical examples of the principle phenomena of the subject, weighting our selection towards topics in which the cloud chamber method is at present making, and may be expected to continue to make, important contributions. We accordingly give relatively few examples of pure electron-photon cascade phenomena, and of the characteristic behaviour of the abundant /j-meton, and by contrast, many of high energy nuclear interactions, and in particular of the recently discovered V-particles. The cloud chamber is an instrument of considerable versatility, and in the legends attached to the photographs it has seemed useful to add brief descriptions of construction, control, operating conditions and performance, which will give some indication of the wide varia tions of design which have proved of value under different conditions. In addition, although technical features of operation do not in general lend themselves to illustration in single photographs, we include a section on problems of selection and of technical quality as they are related to interpretation. Any record of cloud chamber application would be misleading which did not adequately stress the element of critical interpretation which is necessary if photographs are to yield the full information contained in them. With this in view, we have tried to make the legends to photographs illustrate the process of interpretation as well as the accepted description of the central phenomenon. The legends are therefore long, and in many cases discursive, and occasionally offer alternative interpretations which are not resolved in the photographs or in the additional data available. We hope, however, that they will, for readers who are interested, provide a useful series of exercises in problems of inter pretation which are fairly typical of those encountered in cosmic ray applications. We offer this volume as a tribute to Professor P. M. S. BLACKETT, F.R.S., Director of the Physical Laboratories in the University of Manchester, an outstanding pioneer in the application of the cloud chamber as a tool of precision to cosmic ray investigation and to whom we owe, as pupils of many years standing, an exhaustive training in the art of cloud chamber operation and in the discipline of interpretation of cloud chamber photographs. Dec. 1st. 1951. G.D.R, J.G.W. SECTION I TECHNICAL FEATURES OF OPERATION Diffused cloud-chamber tracks 3 Diffused cloud-chamber tracks Plates 1, 2, 3 and 4. Photographs of tracks of cosmic-ray particles in which the ions have been allowed to diffuse before condensation has taken place, illustrating the formation of drops, the distribution of ions in the track, and the efficiency of conden sation. Plate 1. R, B. BRODE, Berkeley, Rev. Mod. Phys. 11, 222 (1939). [See also D. R. CORSON and R, B. BRODE, Phys. Rev. 53, 773 (1938).] The chamber was 30 cm. in diameter and 6 cm. in It will be noted that the main delay which allowed dif depth, illuminated from the side, the illuminated region fusion of the ions was 0-2sec. The drops were allowed being 2 cm. deep. The gas filling was mainly nitrogen, to grow for 0045sec. before being photographed. The at a pressure of approximately one atmosphere, with duration of the light flash was kept small, i.e. 0-005 sec, ethyl alcohol-water mixture as condensant. The cham in order that the free fall of the drops, in this time ber was counter-controlled and was operated in a interval, should be only a small fraction of the appar- magnetic field of 2300 gauss. The tracks were photo ant diameter of the drops. The delay of 0-2 sec. was graphed with a single Elmar lens of focal length 50 cm., sufficient to allow clusters up to about 250 drops in with an image reduced in the ratio 1:7-5, the apparent size to be counted. diameter of the drops in the chamber being 0-2-0-4 mm. The general features of the distribution of ions in The calculated diameter of the drops from observations a track are well shown in these photographs. The of the rate of fall in the chamber was 004 mm. wide fluctuation in the density of ions along the track The tracks were about 5 mm. broad in the chamber, may be noted. Clusters such as the large one on the and the time intervals used in photographing them track at the extreme right-hand side are caused by were as follows: secondary electrons with energies of several kilovolts. Arrival of particle t = 000 sec. A non-contemporary track can be seen crossing one Clearing field removed t = 0 010 sec. of the diffused tracks. Expansion completed t = 0-200 sec. Lights on t = 0-245 sec. These tracks are reproduced 1-2 times their size Lights off t = 0-250 sec. in the chamber. l* 4 Diffused cloud-chamber tracks Preferential sign condensation Plate 2. W. E. HAZEN, Berkeley. Photograph published in" Cosmic Ray Physics" by D. J. X. MONT GOMERY. Princeton University Press (1949) Plate VI. The photograph shows a small electron shower of diffused tracks in which the positive ion columns have been separated from the negative ion columns by leaving the electrostatic clearing field on during the period between the passage of the shower and expan sion. The positive ion columns are the left-hand members of the pairs of tracks. The chamber was coun ter-controlled and contained air at a pressure of 90 cm. Hg. with an alcohol-water mixture as con densant. A vertical temperature gradient of about 0-5° C. per 30 cm. in the chamber resulted in an increase of supersaturation from top to bottom, and the effect was a gra dual increase in the number of negative ions upon which conden sation took place with the change in the super saturât ion. Practically complete condensation has occur red on the positive ions at even the lowest value of super saturât ion. (Note that if the condensant had been water alone, complete con densation would have occurred first on the negative ions.) The gross distortion of the tracks is a consequence of their age. Preferential sign condensation is an important feature of cloud chamber work, particularly when the ionization produced by a par ticle is determined from drop counts. It is necessary that com plete condensation shall have taken place on the ions to be counted. When ions of either sign are un- separated (as in Plate 1) there is no direct method of establishing that complete condensation has indeed taken place. If, however, the ions are separated, as in the present example, into columns of positive and negative ions, it is possible to establish criteria by which conditions in which com plete condensation has taken place in the more heavily condensed column may be recognised. This photograph is reproduced at its correct size in the chamber. Plate 2 Diffused cloud-chamber tracks 5 Ionization in hydrogen Plates 3 and 4. W. E. HAZEN, Berkeley. (Unpublished.) The photographs show the diffused and sign-separated tracks of an electron and of a slow, singly-charged, heavy particle in hydrogen at a pressure of 90 cm. Hg. Each photograph is approximately natural size. The excellence of the chamber conditions is shown by the fact that the columns of negative ions (the left-hand members of the pairs) are almost as dense as the positive columns, while the density of background drops is small. The quality of the drop images, which is very high at the centre of each picture, falls off somewhat at the edges. The lines across the photographs are clearing-field wires. Plate 3 Plate 4 Plate 3. The track of an electron of energy lOMeV. The of the drop densities on the two photographs is very electrostatic field was perpendicular to the magnetic roughly of the expected order of magnitude. field, and the axis of the camera made an angle of about 40° with the direction of this field. Plate 4. This photograph shows the track left by a slow, The ionization produced by this electron was near massive, cosmic-ray particle of charge e. The ioni the minimum ionization for a fast charged particle zation is about 2-5 times minimum, indicating a in hydrogen. The electrons shown in Plate 2 also velocity of about 0-6 e. A non-contemporary track ionize near their minimum value, but it will be seen crosses the diffused track: it is near minimum that the density of ionization along comparable tracks ionization and shows no separation in the electro is much greater in nitrogen than in hydrogen. The static clearing field, and has been produced by a theoretical ratio of the primary ionizations in nitrogen particle traversing the chamber after supersaturation and hydrogen at N.T.P. is about 5:1, and the ratio has been established. 6 Relationship in time and space of counter-controlled tracks Relationship in time and space of counter-controlled tracks Plates 5, 6, 7 and 8. G. D. ROCHESTER and C. C. BUTLER, Manchester. (Unpublished.) These photographs were taken in a chamber 30 cm. in diameter, 9 cm. deep, and filled to a pressure of 1-5 atmospheres with 80% argon and 20% oxygen. The condensant was a 3:1 alcohol-water mixture. Across the chamber was a lead plate 3 cm. in thick ness, faced above and below with 1-8 mm. chromium-plated brass sheets. Photographs 5, 6 and 7 were taken without magnetic field; photograph 8 was taken in a field of 7100gauss. The electrostatic clearing field was removed before expansion. Plate 5 Plate 6 Plates 5 and 6. These photographs, taken under good appearance typical of their energy ; the electron in the conditions of condensation and illumination, show upper part of the chamber is of a few kilovolts and tracks of different age. In each case the particle which the one in the lower part, a few MeV. The increase actuated the expansion is probably a meson. The par in ionization and scattering along the track of the ticle which triggers the chamber produces a track slower electron as it comes to rest in the gas of the which, for a particular chamber, operated under chamber is well shown. definite expansion conditions, has a well-defined In Plate 6 the control track is to the right-hand width. In the present chamber the width of such tracks side of the chamber. The other particle which has was about 0-7 mm., corresponding to a delay between formed a more diffuse track is also a meson. The width the arrival of the particle and the onset of conden of this track is about twice the width of the control sation of about 0-01 sec. If this type of track is called track and hence it must be pre-control of about four the controlled track, tracks of different age may be times the age. Both tracks show appreciable distortion termed pre- or post-control, according to whether they within 1 or 2 cm. of the lead plate. The distortion have arrived before or after the controlled track. probably arises from gas motion present in the cham Plate 5 shows four tracks. The meson which triggered ber before expansion, and is, as would be expected, the chamber is the vertical track: the other three more pronounced on the older track. Old tracks from tracks are all post-control since they are narrower than slow electrons may also be seen to the right-hand side the controlled track. Of these, one is a fast cosmic-ray of the chamber; these have clearly been formed long particle and two are electrons. The electrons have an before the passage of the control track.