Rutherford in Manchester .sx by J. E. GEAKE .sx Manchester College of Science and Technology .sx It is now 50 years since Rutherford , working in Manchester , conceived the idea that the atom had a small concentrated nucleus , and from this idea sprang the whole of our present-day knowledge of atomic structure and our exploitation of its consequences .sx This great landmark in physics was celebrated by holding the Rutherford International Jubilee Conference early in September .sx It was appropriate that the Conference should be held at Manchester University because , although Rutherford did valuable work at Cambridge and at McGill , it was his Manchester period which produced the most important results , and the discoveries with which his name is mainly associated .sx It was also appropriate that there were two parts to the Conference- a commemorative session in which some of the surviving members of Rutherford's Manchester team took us back by their reminiscences to those great days of the past , and also a full-scale conference setting out the present state of our knowledge of the nucleus .sx To keep up with a rapidly-changing subject such as this , one must not spend too long looking backwards .sx Of those closely associated with Rutherford in Manchester , Marsden , Darwin , Chadwick , Andrade and Niels Bohr were all present , and it was greatly regretted that William Kay , Rutherford's laboratory steward and personal assistant , to whom he acknowledged a great debt , did not live to be present at these celebrations ; he died in Manchester only a few months ago .sx The main commemorative session of the conference consisted of the reminiscences of Sir E. Marsden , Sir Charles Darwin and Professor Andrade , and this was followed by a ceremony at which honorary degrees were bestowed .sx During the week , delegates saw something of the local Derbyshire scenery , visited Jodrell Bank and A.E.I. at Trafford Park , were received by the Lord Mayor at a lavish reception in Manchester's impressive Victorian Gothic Town Hall , and rounded off the week at a special concert given by Sir John Barbirolli and the Halle@2 Orchestra- the source of another of Manchester's claims to renown .sx Concurrently with the Conference an exhibition of things associated with Rutherford was held- photographs , letters , models and , most interesting of all , some of his actual apparatus , including the piece said to have been his 'pet'- the superb piece of glass-blowing by Baumbach which made possible the spectral identification of 15a-particles as helium .sx The letters on view gave some interesting glimpses into the organization and economics behind the scene .sx There was Schuster's letter offering to hand his chair over to Rutherford ( then at McGill ) , Rutherford's answer making careful enquiries about the financial arrangements for research , and Schuster's detailed reply saying how he spent his annual grant for teaching and research ( all +450 of it !sx ) and by how much it was safe to overspend without getting into trouble .sx Rutherford was satisfied , and came in 1907 , and thus began the work in 'Tom Tiddler's field' , which was how Rutherford referred to one of the most celebrated research groups in the history of physics .sx Rutherford owed a considerable debt to Schuster for handing over to him a well organized and relatively well equipped laboratory and teaching department .sx While the glory of discovering the nucleus falls to Rutherford , it was entirely owing to Schuster that the work was done in Manchester .sx As early as 1906 Rutherford , then at McGill , had realized , from the observation that an 15a-particle beam was spread out slightly by passing through a mica sheet , that there must be surprisingly large electric fields within atoms , but it was not until 1911 that the idea of the nucleus was finally conceived .sx A trivial defect in an 15a-beam tube , which was cured empirically by inserting brass washers to confine the beam , suggested that 15a-particles were reflected by metals .sx Rutherford suggested to Marsden , a second-year student ( in those days undergraduates were given small research projects as part of their training ) , that he should follow this up .sx After some initial difficulties , because the available 15a-particle sources were too weak , Marsden eventually obtained a stronger source and did the experiment which is seen in retrospect to be one of the most profitable ever carried out .sx He directed a beam of 15a-particles at metal foils , and observed the range of angles at which they came off .sx The result was staggering ; although most of the particles were only deflected slightly , a few were turned through large angles , and a very few came almost back along their tracks .sx As Rutherford said later , it was as if one fired 15 in .sx shells at tissue paper , and found that occasionally they bounced back !sx Marsden told Rutherford what he had observed , Rutherford questioned him about the experiment to convince himself that it was all right , and that was all for several weeks , until a Sunday evening in the Autumn of 1911 .sx Rutherford had invited several of his research workers to supper in his house at Withington , as he often did , and while they were chatting after supper Rutherford suddenly came out with his first ideas about the atomic nucleus ; before they went home he asked one of them , Darwin , to check his hasty derivation of the scattering law to be expected when 15a-particles were deflected by point nuclei .sx They even discussed , on that first evening , the idea that , if the nucleus were not quite a point , departures from the law at close approach could yield information about nuclear structure .sx Although Rutherford did not live to see powerful enough scattering experiments performed , this is now the basis of modern methods of investigating the structure of nuclei and nucleons .sx In the months that followed Geiger and Marsden carried out more sophisticated scattering experiments than the one which had revealed the effect , and actually measured the angular distribution of the scattered 15a-particles .sx The results confirmed Rutherford's scattering law and therefore the validity of the assumptions he had made in deriving it , and led in 1913 to a group of three papers which laid the foundations of nuclear physics .sx The commemorative session of the conference produced reminiscences about several of Rutherford's group in Manchester ; of Moseley whom Sir Charles Darwin ( who worked with him ) described as the hardest-working person he had ever known , and who was an expert in finding a meal in Manchester at 3 a.m. ; of Niels Bohr who was a very comforting theoretician with great skill in bridging the gap between startlingly new theoretical concepts and classical ideas ; of Robinson , a keen music-hall addict- and indeed of the music-hall origin of the correct intonation to Rutherford's nickname of 'Papa' .sx While these reminiscences of the physics of 50 years ago were appropriate and entertaining , it was right that most of the time at the conference should be concerned with the physics of the present .sx There were nearly 200 contributed papers , and for those who want a detailed picture of the present state of nuclear physics these papers will shortly be published as a 750-page volume .sx The conference sessions , however , consisted of the presentation of invited papers , each intended to summarize a different aspect of the subject .sx Thirty years ago Rutherford said , " It is my personal conviction that if we knew more about the nucleus , we should find it much simpler than we suppose .sx I am always a believer in simplicity being a simple fellow myself .sx " The subject at present seems a long way from this simplicity ; parts of the conference seemed to be in a foreign language , and at one point there were so many rival theories that they were referred to by reference numbers .sx Perhaps we need another Rutherford .sx The main topics reviewed included nuclear forces , nuclear structure , and the interactions with outside particles from which most of the evidence for nuclear properties is obtained .sx There was also a paper on the limitations and possibilities of the instruments for nuclear investigation , and another , rather off the main line , on cosmological dating by nuclear methods .sx It has long been understood that the attractive forces between nucleons ( the neutrons and protons which comprise nuclei ) were somehow concerned with the interchange of a particle ( the 15p-meson or pion ) between them .sx There has also been evidence that sometimes two pions are in transit between the interacting nucleons at the same time , and the possibility of this occurrence modifies the force to be expected ; although the theory of this process is still an unsolved problem , models describing the resulting behaviour have been proposed .sx What has only recently been confirmed- in fact it was announced at this conference- is that occasionally three pions at a time are involved .sx These three pions may actually be joined together transiently as a compound particle during the interchange process ; indeed , theoreticians have been invoking a compound particle of this type for some time .sx There now seems to be evidence for its existence .sx A nuclear model which has been surprisingly long-lived and successful is the shell model , which was first proposed 25 years ago .sx This assumes nucleons to occupy energy levels , obey quantum-number selection rules , and group themselves into closed shells in a manner analogous to the electrons outside the nucleus .sx This theory was given a new lease of life by adding the concept of nucleon spin , which undergoes coupling with the nucleon 'orbital' motion .sx The presence of any nucleons in addition to the numbers which comprise closed shells will tend to distort the otherwise spherical shape , but these distortions were ignored in the approximate treatment of the problem .sx If there are only a few nucleons more ( or less ) than complete shells the mean distortion is indeed small , but the theory has been extended to include vibrations about this mean shape .sx With larger numbers of extra nucleons , mid-way between the numbers comprising complete shells , the nucleus is much more distorted , and rotational modes become important .sx With these larger numbers of extra nucleons it is no longer practicable to treat them singly and only their collective behaviour is considered .sx The way nucleons are arranged in a nucleus , and especially in the surface regions of heavy nuclei , is another topic of current interest .sx Some workers consider that nucleons tend to be found singly or in pairs in the nuclear surface , while others believe that there is more than a random chance of their being found in groups of four , although the grouping may be of a very transitory nature , the particles perhaps remaining associated for 10 :sx -22: of a second or so .sx Indeed , it is known that if a single particle , say a neutron , hits a nucleus it may result in the ejection of an 15a-particle ( an assembly of 2 protons and 2 neutrons) .sx However there was a vigorous argument at one session of the conference as to whether this 15a-particle existed in the nuclear surface and was knocked out by the neutron , or whether the incident neutron simply collected three more particles and itself became part of the resulting 15a-particle .sx The evidence seems to be in favour of the former idea- that the four particles were already associated before ejection .sx Soon after Rutherford came to Manchester he and Geiger , using Geiger's new 15a-particle counting techniques , were able to make the first measurements of the half-lives of radioactive elements .sx Nearly 20 years later , when Aston measured the relative abundances of the isotopes in lead ( the end-points of radioactive decay series ) from a lead-uranium ore , Rutherford realized that this , combined with his half-life measurements , could yield estimates both of the age of the earth ( i.e. the time since solidification ) and of the time since the actual formation of the heavy elements .sx Rutherford's results increased the estimated time-scale for the Earth's development by a factor of more than 10 over the currently accepted estimates due to Kelvin , and this advance produced the newspaper headline 'Doomsday Postponed' .sx Apart from Rutherford's assumption that the amount of :sx 235:U initially formed was at the most equal to that of :sx 238:U , modern cosmochronologists would agree with him .sx It is now believed that :sx 235:U was produced initially in greater abundance than :sx 238:U , and this , plus minor changes in the accepted values of other constants , pushes the estimated time since the formation of the heavy elements ( loosely called the age of the galaxy ) up from Rutherford's estimate of 3.4 x 10 :sx 9: years to about 20 x 10 :sx 9: years .sx