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Volume 77, 1948-49
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Some Aspects of Experimental Nuclear Physics.

The subject, for our purpose, can be divided into sections:—




—The development of equipment


—The study of particles


—The study of the nucleus

These divisions, of course, overlap, but they will serve to direct our thought.


In natural radioactivity we have transmutations of unstable nuclei into others that may or may not be stable—thus the uranium series, starting with uranium, changes through a series of elements, one of which is radium, and ends with a particular isotope of lead. Thorium gives a similar series; actinium (a branch product from the uranium series) gives a third. The only other unstable nuclei known on earth are certain isotopes of potassium, rubidium, and lutecium, which give off electrons, and samarium, an α-emitter. Their activity is very weak, i.e., they have long lives.

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In 1919, however, arising out of observations by Dr. Marsden, Rutherford demonstrated what was termed the disintegration of nitrogen, in which collisions of α-particles (which are the nuclei of He atoms) with N nuclei resulted in the emission of protons (which are nuclei of hydrogen). Later, as Blackett's cloud-chamber photographs show, this was recognised as a true transmutation, i.e., the α-particle is first captured by the N nucleus, a proton is then ejected and an oxygen isotope forms the remnant: N14+He4→O17+p1 (N14αpO17)

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In 1932—the bonanza year of modern experimental physics—Cockroft and Walton, in Rutherford's laboratory at Cambridge, showed the transmutation of Li, using accelerated protons:— Li7+p1→2He4 (Li7p, 2α)

—the first artificial transmutation of matter.

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In that same year, 1932, the positron (the positive counterpart of the electron) and the neutron (a neutral particle of mass just greater than the H atom) were discovered and, further, Curie and Joliot found artificial radioactivity—e.g. that aluminium, bombarded by α-rays, became radioactive. From aluminium an unstable nucleus of phosphorus was formed which decayed with the emission of the newly-discovered positron:— 13Al27+2He415P30 +0n1 (period=half life—195 sec.)

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(The neutron at first eluded detect on.) The P30 is a new isotope of phosphorus too unstable to exist in nature. This activity is of a new type—the natural radioactive nuclei emit α's or β's, none give off positrons.

A tremendous field was thus opened up. The neutron was shown by Fermi to be particularly prolific in such actions (owing to its ease of penetration into the highly charged nucleus) and it was possible to bombard materials with α-rays or with accelerated particles such as p (or its ally, the deuteron d, i.e. the nucleus of deuterium or heavy hydrogen, the isotope of ordinary hydrogen, mass 2) or with neutrons, say, from the convenient (Rn+Be) source (Beryllium filings in a Rn tube, using the α-rays):—

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Be9+He4→C12+n1 or finally with photons from γ-rays: Be9+γ→Be8+n1.

The discharge-tube method, using accelerated nuclei as ions, was of prime importance because it was obviously possible to control both the particle used and the energy applied. Cockroft and Walton obtained their high tension by a process of voltage multiplication. Van de Graaff in 1931 had developed a belt type of electrostatic generator and this was further improved for use in accelerating tubes of the Cockroft-Walton type where the ions received a step-wise increase of energy down the tube. These generators now can produce some millions of volts with an output of some milliamps.—a useful power of ∼ 10 kw. Such generators have also been made in pressure tanks (to prevent sparkover) and form a medium-size apparatus (∼20 ft.). In 1934, Lawrence, who had been experimenting with phased acceleration, conceived the idea of bending the particles by a magnetic field into a circular or spiral path and imparting then the phased acceleration across the gaps of D's in high vacuum. The particles starting from the centre would be speeded up; their bending would then become less, so that a spiral path resulted; calculation showed that the increase of speed corresponded to the increase of path so that each semi-turn of the spiral took the same time. This time is very short ∼ 10-7sec.; it is therefore a radio-frequency voltage of some megacycles that must be applied to the two D's to accelerate bursts of ions in phase with it. The first model built by Lawrence had pole pieces ∼ 1 foot across. A small “laboratory” size has been described by Kruger et al. (Phys. Rev. 51). The largest yet made is the one at Berkeley, California, with pole pieces 15 ft. across. Liverpool and Cambridge possess cyclotrons (of average size) and the one at Birmingham (where two of my former students are working) is just nearing completion. There is, as yet, none in the Southern Hemisphere, though probably one is planned for Australia. The cyclotron has the advantage that the energy given to the various particles can be much greater than in the linear accelerator (up to 100 Mev., cf. with, say, 10 Mev.) and the useful yields (in current) are in general higher, e.g., ∼ 3 m.amp. giving ∼ 30 kw. in the beam pulses. Further, the cyclotron can be extended and its limitations partly removed. It is limited by reason of the increase in the mass of the particles as they are speeded up. This upsets the phasing relative to the accelerating field and results in a

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static equilibrium orbit being reached beyond which further acceleration cannot be achieved. If, however, either the magnetic field or the accelerating potential be varied to suit, i.e. to fit in with the spiralling burst of particles, then further acceleration is possible until the particles, owing to their high speed, finally radiate energy as fast as they acquire it. The limit then is the input of energy. This phasing development (suggested by Veksler in Russia and by McMillan at Berkeley) is called the synchroton principle. The effect has been demonstrated but not yet applied to the cyclotron.

The cyclotron can be used for massive particles (i.e. of atomic mass). It cannot be used for electrons because of their rapid acceleration and the consequent increase of their mass with the speed. For these, Kerst has developed the betatron, in which the electrons are accelerated by the electric field associated with a changing magnetic field. An A.C. magnet is used with a frequency of 600; the growing electric field in a cycle accelerates electrons from the gun till they reach an equilibrium orbit; by using a central core of iron dust which saturates before the iron outside, the flux inside this orbit is diminished towards the end of the cycle and the electrons spiral in on to a target. If a thin target is used the X-rays generated are in a very narrow, forward beam, e.g., in the 100 Mev., 60 cycle, betatron built at the American General Electric laboratory, the beam has a breadth of only 4 to 6 in. at 11 ft. It thus produces a concentrated beam of X-rays of energy much greater than any so far known except in cosmic rays. Later modifications, to allow of more economic use of the flux variation, indicate the possibility of a 250 Mev. machine.

Another modification of this idea is the race-track synchrotron proposed by Crane in which ½ Mev. electrons will be accelerated as in a betatron, but instead then of circling in an equilibrium orbit they will be accelerated further by an r.f. field on the straight legs of the track. This field will be automatically frequency-modulated by the electron beam itself and the whole acceleration related to the more slowly varying magnetic field so as to maintain the equilibrium path in the active part of the cycle.

Looking at these machines from the point of view of New Zealand developments, we realise first that physics on these lines has inevitably taken an engineering turn of a very specialised type. For high-energy ions the cyclotron principle is supreme; for high-energy X-rays the betatron. For medium energy, a Herb pressure E.S. generator should give good service and be within reach of our resources. The second point that emerges is that such machines, to justify the investment, need not only a competent design staff of physicists and engineers, but also a permanent running staff of physicists and technicians. University resources in Physics will have to be on a different scale altogether from the existing miserable provision.

The machines so far described produce, in general, a special type of result, viz., accelerated particles in a more or less convenient or concentrated form so that unwanted results such as × or γ rays may be largely screened out. There are, however, two other sources of high-energy particles, of different type. The first is a natural one—cosmic

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rays—where, by and large, we must take what nature provides and plan to catch interesting events. The second is an artificial one, but still more or less uncontrolled as regards its radiations—I refer to the fission pile. Here from the complex of actions going on there results a veritable bath of high-intensity and penetrating radiations— neutrons, α-rays, β-rays, positrons—of varying energies. While not theoretically impossible, it is at present impracticable to isolate any of these, say, n's of a desired energy. The pile may still be used, however, for suitable reactions; it produces a very high intensity of neutrons, for example, and forms a major instrument for transmutations. The size of the pile depends mainly upon the material used for slowing down the neutrons—the graphite piles used during the war were of the small-house size, but much smaller ones can be used with, say, heavy water, the manufacture of which requires mainly electrical power. This seems a feasible project for New Zealand in either form, and many of the new radioactive isotopes will be required here for plant, animal, and human physiology—branches of knowledge basic to our agriculture and to our medicine. Thus the radioactive form of phosphorus, mentioned earlier, behaves chemically like ordinary phosphorus, but its distribution in a plant or an animal can be traced by reason of its radioactivity. Minute ray “counters” have been designed to do this work or, of course, photographic plates can be used where suitable. Similarly radioactive carbon, iron, cobalt, nitrogen, potassium, manganese, sodium, calcium, copper, etc., may be employed—the only requirements are (1) that the life be long enough for the particular process being studied, and (2) that the radiation be energetic and intense enough to be detectible.

Many of the reactions of transmutations leave the nucleus excited, i.e., with extra energy which it may emit as γ-ray and some of the new isotopes will be of value as γ-ray sources which may either be applied or inserted or, in favourable cases, differentially secreted in a particular organ requiring this form of treatment.

The tracer method also introduces a new technique in all wear problems and in chemistry for quick analysis of gases, liquids or solids—such problems as transport numbers, diffusion in solids. adsorption, gaseous diffusion or absorption, etc., can be elegantly followed using the counter technique provided that the different mass of the isotope plays no predominant part.

II.—Development of Equipment.

I have already indicated the high importance of design in physics. This, of course, is no new thing—it characterises all good experimental work—and a high place must be given to apparatus design in assessing the honours for advances in physics. Wilson's cloud-chamber has given an intimate insight into atomic processes; Lawrence's cyclotron has played and will play an important part in progress. Kapitza is another design genius whose interest, like that of Lawrence, seems to be in design itself. The development of apparatus must go hand in hand with actual research and the competent designer is worthy of his hire at a goodly wage. This is even more the case to-day, since many of the modern machines are large and failures in design more

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expensive. A complete mass spectograph will probably cost about £2,000, a Cockroft-Walton accelerator for 500 kv. about £3,000, a cyclotron of small size about £30,000. Much smaller items of apparatus are equally important—the simple counter tube plays a highly important part in modern physics and photographic plate technique using particle tracks in the emulsion is a still simpler method of increasing importance, because, like the cloud-chamber, it gives a picture in time and space. An improved ion source may increase the efficiency of a large machine 100 per cent, and raise it to a highly productive level.

In addition to design of apparatus, there is also the testing of it, e.g., it is necessary to know its efficiency. Ion yields are simply measured with a Faraday cylinder—in the ion accelerators this is usually included round the target. Neutron yields are more difficult to measure. An elegant solution (due to Amaldi and Fermi) is to slow all the neutrons down by collisions with protons in hydrogen-bearing molecules such as water, hydrocarbon oils, etc. Such light particles, by the laws of collision, absorb most of the neutron energy, reducing them ultimately to thermal velocity, i.e., in temperature equilibrium with the medium. A detector which reacts with thermal neutrons, such as Rh foil, is activated at varying distances from the source and gives thus a measure of the slow neutron density. Integration through a sphere yields the answer.

As an example of modern design, I should like to mention the powerful new apparatus for neutrons called the velocity spectrometer. This selects neutrons of a certain velocity range from a composite beam. Neutrons emitted from sources are nearly always fast; their energy is of the order of Mev's., and a 25 Mev. neutron has a velocity ⅓ that of light. Many of the interesting reactions of n's with matter occur with slower neutrons—from thermal velocities (105 cms./sec. or ·03 ev. energy) upward to 1,000 ev. No primary sources of these exist—they must be produced by slowing down—usually in paraffin wax. If, now, a neutron source (e.g., a deuteron beam on a Li target) has the ion beam pulsed, say, at 50 cycles, then we get bursts of fast neutrons from the target and of the slowed neutrons from the paraffin wax around the target—these bursts occurring with the 50-cycle periodicity. If the detector (which consists of an ionisation chamber plus amplifier) be now similarly pulsed, the two can be phased so that the detector lags behind the source by the time taken for neutrons of a particular slow velocity to pass from source to detector. Thus velocities can be selected by varying the distance, the frequency, and the phasing. So far, energies up to 1,000 ev. have been dealt with (v ∼ 5X107 cms./sec.) and the apparatus developed has 16 ranges, with this upper limit. The idea originated both in England and in the United States of America, and while in both, countries the first development was unsatisfactory, it has now been successfully worked out in the States so that experiments, normally taking months, can be done in days and accurate data on absorption of slow n's by different elements is now pouring out in a quick succession of papers.

This development for n's brings us naturally to my third division of the subject:—

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III.—Study of Particles.

The yields, energy ranges, spatial distributions, masses, and the moments of the various particles—α, e_, e+, p, d, n, u, γ—from the various reactions makes a huge branch of nuclear physics. I can only mention it here. In addition, there is the particularly important aspect of the reaction of these particles with other similar ones, usually bound in matter, e.g., with protons in hydrogen atoms, deuterons in deuterium, α's in helium, etc. The classic case of this, of course, was the study of α particles by Rutherford and his co-workers, which both established the nuclear nature of the atom and also discovered transmutation, thus pioneering the whole development of nuclear physics. No less important are the scattering of protons, neutrons and deuterons, especially with the simpler nuclei, because these give vital information on the forces acting between the particles that constitute nuclei.

Such experiments need again either natural sources (where available) or sources from the machines mentioned previously. They employ also the detecting devices of counters, cloud-chambers, photographic plates, along with amplifier technique.

The scattering of neutrons has been of special interest partly because of the newness of the neutron and of its intriguing nature as a massive neutral particle, mainly, of course, because of its fundamental character as a nucleon or nuclear particle. Although neutral, it possesses a magnetic moment and is thus subject to magnetic fields, e.g., the field in saturated iron causes a better transmission of those neutrons with moments parallel to it and thus results in a beam which is partially polarised.

Another aspect of scattering is of interest. In X-rays we know the action of a crystal in scattering a beam of X-rays in preferred directions determined by the crystal lattice and by the wave-length or energy of the X-rays. On wave ideas we think of this in terms of wave-length, in particle terms (photons) we relate it to the energy. Electron diffraction showed that electrons also partook of the nature of waves and could also (apart from their charge) be regarded in either way. Any particles are similar; λ=h/p=h/mv, so that given m and v, λ can be calculated. Thus protons, neutrons, etc., show also the properties which we have hitherto associated with waves and will thus be specially diffracted by crystalline materials. This feature has recently been studied with neutron scattering; it can affect the distribution of n's appreciably, but since the λ is approximately the same as for X-rays and n's are much more difficult to control, it has not opened any new path of investigation comparable with the electron microscope.

IV.—Study of the Nucleus Itself.

This final division of the subject is concerned with the internal economy of the nucleus. Its charge was determined by Rutherford and by Chadwick: its approximate size (∼ 10−13cm.) resulted from the same experiments. Of course, “size” in relation to a particle in

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Physics must be specially defined because one cannot find any sharp edge on which to put measuring tools or their equivalent! Thus in the α-scattering experiments of Rutherford and of Chadwick, the distance to which the ordinary inverse-square law held gave a maximum measure for the radius, viz.~ 10−13cm. The elastic scattering of fast neutrons (which have no special re-actions with nuclei) gives a closer measurement because the neutrons get further in. Thus Sherr, using Li bombarded with 10 Mev. d's obtained 25 Mev. n's and isolated the action of the fastest ones by using as a detector the reaction C12 (n 2n)C11 which has a threshold of 20.4 Mev. From his results he could calculate the nuclear radius.

The magnetic moments of nuclei have been determined by Rabi using beams of atoms and of molecules in magnetic fields. The nuclei are (ideally) oriented by a divergent field (the polariser) and then will be transmitted by a similar one (the analyser). By applying, in between, a uniform field to hold them and at right angles to the orientation an A.C. field, the nuclei are made to precess so that they will not pass the analyser. The effect is actually statistical; the moment can thus be evaluated. For nuclei in ordinary materials a new elegant method based on the above has been devised by Bloch and by Purcell and is called nuclear induction. In this the nuclei in ordinary matter, e.g., protons in water, are made to precess by applying a resonance field at right angles to an aligning one. The induction produced by the precessing nuclei is then picked up in a receiving coil perpendicular to the precession coil. In this way the magnetic moment may be studied, its relaxation time, etc. In Purcell's method the resonance absorption in a cavity resonator is measured as the aligning field is altered in value.

The angular momentum of the nucleus can be derived from refined developments of spectroscopy.

Masses are measured by the mass spectrograph—separation of isotopes is also possible by this means.

The constitution of the nucleus is adequately explained in term of n's and p's, these particles in some form being held together by very strong, short-range forces between n-n, n-p and p-p against the electrostatic repulsion of the protons. From the emission of α's by the heavy radioactive atoms and from transmutations like those of Li and B, this α-grouping, (2n+2p), would seem to be somewhat differentiated as a sort of closed group. Gamow conceived the potential diagram of a nucleus, and in this we can represent levels of energy for the particles, both occupied and excitation levels. The latter may be found from the so-called “resonance” energies of bombarding particles, i.e., energies at which the particle has a much larger chance of capture by the nucleus. Such level ideas lead to a sort of spectroscopy of the nucleus, in analogy with that of the atom; it governs energy of transitions (with γ-ray emission, i.e., photons), probability of transition, life in the excited state; and the reverse of these, such as probability of absorption of a photon, p, n, α, d, etc (large absorption being known as “resonance” Absorption).

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Of the actual forces between the nucleons very little can be definitely stated. The modern theory uses the meson or mesotron (a particle of mass ∼ 200 mass of the electron and ∼ 1/9 mass of the proton) as the energy vehicle between the nucleons, but mathematicians cannot yet formulate a satisfactory theory. This is the urge behind the extension of accelerating apparatus ever to higher and higher energies. The mesotron appears in cosmic rays; its intrinsic energy (mc2) is ∼ 100 Mev. and energy of at least this amount must be available in nuclear particles such as neutrons and protons before we can hope to create mesotrons and thus study the postulated method of interchange between nucleons. Nuclear forces are investigated mainly from scattering experiments, but also from the build-up of nuclei, and this brings us to the subject of stability of nuclei.

All the natural radioactive processes, together with the transmutations produced by capture and the evidence of stability shown by the isotopes existing in nature, enable a theory of stability of nuclei to be formed and predictions to be made (within limits) of the effects of possible changes, i.e., the course of nuclear reactions (a parallel to chemical equilibria). The new phenomenon of fission has added an interesting chapter to this—here the addition of one particle can cause a fundamental splitting of the new nucleus:—



92U238+γ→92U239*→fission or with α or p.

The fissionable atoms known are U, Th, Pa, Np, and Pu and the energy needed varies in the different cases; neutrons are the most efficient bombarding particle. Various modes of splitting exist, along with the simultaneous emission of n's. The new nuclei first formed are strongly β-active, emitting high-energy electrons. The fission particles are projected in opposite directions with very great energy ∼ 100 Mev. each. Such high energies (relative to the low chemical-action energies characteristic of combustion, ∼ 10 cv. compared with 108) combined with a relatively high proportion of atoms changed produce the tremendous energy release that characterises the so-called “atomic” bomb. In the pile process, the kinetic and radioactive energy of fission is degraded into low temperature heat, but if it could be used directly (e.g., in a dream turbine) and the high temperature of the primary particles thus made effective, it would give a highly efficient engine. Again, the order of energy in all the nuclear actions is very large—thus radon gives, per gram, energy at the rate of 25 h.p. for 5½ days, so that if, some day, a suitable α-product can be economically made, this might well form the energy source for small-scale engines. Such energy-development work will need a very general institute, embracing or associated with, most lines of nuclear work and using mathematicians, physicists, chemists, engineers and technicians—its outlook must be broad and general, though its lines of work may be restricted as a matter of financial limitation.

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I have, I hope, said sufficient to indicate the rapid advances made in this subject—advances that will help greatly to extend the control by man of the resources of nature, that have already widened the range of atomic material with which he can operate and that will put greater and more convenient power sources at his disposal. New Zealand has a special interest in the subject, in view of the outstanding work of Lord Rutherford in placing it on sure foundations (and some of his disciples are still among us!); New Zealand must also have an interest in it because of its very fundamental nature, such that it marks a real epoch in the history of physics and, indeed, of mankind. Some of the branches of the subject such as cosmic rays, mass spectra, nuclear moments can be attacked with relatively simple apparatus, but the greater part of the work lies with high-energy particles and demands both the use of expensive machines and, as I have mentioned, the employment of a fairly permanent staff to operate them. In the fundamental side of all this work the University must play its part, and for this it will need greatly increased resources; the applications to medicine, agriculture, and engineering must be the realm of Government planning. The two must go hand in hand, for the one thing certain is that this latest advance of Physics is fraught with such possibilities, for good and for evil, that New Zealand cannot stand aside and neglect it.