Bernard T. Feld

Mesons and the structure of nucleons: Part II. The nucleon isobar and pion dynamics

Abstract An “atomic” model of the physical nucleons, previously applied to the nucleon ground states, is extended to describe the excited isobar nucleon state, of angular momentum and isotopic spin 3 2 . The model is used to compute the cross sections for resonant photoproduction of pions and Compton scattering of photons on protons. Other, nonresonant processes are taken into account in a phenomenological fashion, and the computed cross sections for the aforementioned processes are compared with available experimental data. The model is also used as a guide for the phenomenological interpretation of other high-energy processes—in particular, photodisintegration of the deuteron in the region of the photomeson threshold, and the resonant π− − P interaction at ≈1 Bev.

Mesons and the structure of nucleons

Abstract An atomic model of the physical nucleons is considered in which the structure is represented as a nucleon core surrounded by a pion cloud in a p 1 2 state, the system being in a state of isotopic spin 1 2 . From the pion wave function, constructed on this basis, the static properties of the physical nucleons—magnetic moments, charge distributions, mass difference—are derived and compared with observation. Modifications are considered which could bring the predictions of this model into better accord with the experiments. It is shown that the core must be given properties, both of size and of structure, which deviate considerably from those of the Dirac nucleons. Some discussion is given of the relationships of the atomic model to meson field theories.

The consequences of nuclear war

At best, civilization would be crippled; at worst, it would be annihilated—and yet current strategic doctrines consider such a risk to be acceptable

Nucleon polarization resulting from π-meson production

SummaryIn this paper we derive the polarization of the nucleons resulting from the three «fundamental,» meson-nucleon reactions π+N→N′+π′+,γ+N→N′+π′,π+D→N+N′. It is shown that the observed angular distributions of these reactions permit of two types of solutions—a «Fermi» solution, in which the important interaction is in the meson-nucleon intermediate state ofJ=3/2+,T=3/2; and a «Yang» solution, which requires a strong interaction in the stateJ=1/2+,T=3/2. These solutions can be distinguished on the basis of the polarization of the recoil nucleons. Tables are given for the predicted polarizations in the second and third of the above reactions (the first has previously been discussed byFermi). The greatest difference between the two solutions is predicted in the photoproduction of neutral mesons. However, the kinematics of the reactions are such that the most likely case for detection of the polarized nucleons is the third of the above reactions; in this case, however, the difference between the predicted polarizations for the two solutions is only in the sign.RiassuntoNel presente lavoro deriviamo la polarizzazione dei nucleoni risultanti da tre «fondamentali» reazioni mesone-nucleone: π+N→N′+π′, γ+N→N′+π′, π+D→N+N′. Si dimostra che le distribuzioni angolari di queste reazioni osservate consentono due tipi di soluzioni—una soluzione di «Fermi» in cui l’interazione importante avviene nello stato mesone-nucleone intermedio conJ=3/2+,T=3/2; e una soluzione di «Yang» che richiede una forte interazione nello statoJ=1/2+,T=3/2. Queste soluzioni possono distinguersi in base alla polarizzazione dei nucleoni di rinculo. Si danno tabelle delle polarizzazioni previste nella seconda e la terza delle reazioni sopra indicate (la prima è stata già precedentemente discussa daFermi). La massima differenza fra le due soluzioni si prevede per la fotoproduzione di mesoni neutri. Le cinematiche delle reazioni sono, tuttavia, tali che la maggior facilità di rivelare i nucleoni polarizzati è offerta dalla terza delle reazioni di cui sopra; in questo caso, però, la differenza tra le polarizzazioni previste è solo nel segno.

Isotopic spin seleci n rules and pari y nor-conservation in the decay if the s range particles

SummaryThe experimental situation is reviewed on the decay ratios of the θ-meson and hyperons from the point of view of testing the validity of the suggested δT = ± 1/2 selection rule. Assuming charge-independence, it is shown that the observed decay ratios of the Σ-hyperons require a very large admixture of δT = ± 3/2 decay, provided parity is conserved in the decay. However, if the requirement of parity conservation is discarded, the hyperon decay ratios can be accounted for, in a most natural fashion, while still preserving the δT =± 1/2 selection rule. This, explanation leads to the prediction of large asymmetries in the angular distributions from the decay of polarized hyperons. Furthermore, these asymmetries are very sensitive to the degree of time-reversal in variance of the decay interaction. A table is given for the expected asymmetry parameters in parity non-conserving hyperon decays with and without time-reversal invariance.RiassuntoSi passano in rassegna i risultati sperimentali sui rapporti di decadimento dei mesoni θ e degli iperoni allo scopo di saggiare la validità della proposta regola di selezione δT= ± 1/2. Assumendo l’indipendenza dalla carica si dimostra che i rapporti di decadimento degli iperoni Σ osservati richiedono una assai abbondante presenza di decadimenti δT = 3/2, purchè nel decadimento sia conservata la parità. Tuttavia, se si rinuncia alla conservazione della parità, si puÒ render conto in modo del tutto naturale dei rapporti di decadimento degli iperoni, pur conservando la regola di selezione δT= 1/2. Questa spiegazione porta alla predizione di grandi asimmetrie nella distribuzione angolare dei decadimenti degli iperoni polarizzati. Inoltre, tali asimmetrie sono molto sensibili al grado di invarianza dell’interazione di decadimento, rispetto all’inversione del tempo. Si dà una tabella dei parametri di asimmetria previsti nei decadimenti di iperoni senza conservazione della parità con o senza invarianza rispetto all’inversione del tempo.

High‐Energy Neutron Detectors Employing Liquid Scintillators

Two liquid scintillation type neutron detectors with efficiencies of the order of 5% for counting 130‐Mev neutrons have been studied. A fairly monoenergetic source of neutrons was obtained from the photodisintegration of deuterons. Estimates were made of the efficiencies of the counters as a function of bias. Experiments indicate: (1) an appreciable effect of the products of neutron reactions with carbon; (2) the efficiency can be more accurately predicted for a small counter although the behavior of large counters can be qualitatively predicted; (3) the attenuation due to a lead shield in the beam depends upon the neutron energy, the bias, and the geometry; (4) lead shielding on the sides is relatively unimportant if kept thin.

Mesons and the struture of nucleons: Part III. Pion-nucleon scattering

Abstract The “atomic” model of the physical nucleons has been used in a direct computation of the scattering of free pions by nucleons. The computation is analogous to the computation of the scattering of electrons by hydrogen atoms. Neglecting any pion-pion interaction, p-wave scattering phase shifts are computed, in the Born approximation and also by an improved technique, for various assumptions concerning the interaction between a pion and the nucleon “core”. The model is shown to be capable of reproducing the resonant scattering in the (3,3) isobar state, while predicting much smaller phase shifts for the other states. Our results differ from the Chew-Low theory, in the Born approximation, only in that we predict a small, positive phase-shift for scattering in the (1,1) state; the experiments appear to favor our prediction. We conclude that the same pion-nucleon interaction, which accounts for the properties of the ground (physical) nucleon state, leads to the observed low-energy pion-nucleon scattering.

The Menace of a Fission Power Economy

The worlds mounting reliance on nuclear fission power as a source of energy presents several problems to society. Some of them are discussed in the following three articles. Bernard T. Feld, a physicist at the Massachusetts Institute of Technology, examines the general danger arising out of industrial production and dispersion of plutonium, which is also the raw material for atomic weapons. Lawrence Scheinman, professor of government and associate director of the Peace Studies Program at the Center for International Studies, Cornell University, looks specifically at the problem of safeguarding nuclear materials from diversion or theft. And Philip P. Micklin, assistant professor of geography at Western Michigan University, considers the still unsolved problem of how to store for centuries or longer the extremely hazardous high-level radioactive wastes from nuclear power plants. Drs. Feld and Scheinman presented their reports at the 1973 Pugwash Conference on Science and World Affairs at Aulanko, Finland. ...

The Preparation of Pressed Ra+Be Neutron Sources

A method for preparing pressed Ra+Be fast neutron sources is described in detail. Such sources can be made to contain as much as 5 g of Ra. They have the advantage of small volume and constancy of neutron output. The yield of fast neutrons per sec. per g of Ra can be obtained from the relationship yield=1.7×107MBeMBe+MRaBr2.

On the « Quasi-Elastic Diffraction » Scattering of High-Energy Protons.

SummaryThe observations of Cocconiet al., on the « quasi-elastic diffraction » scattering of (10 ÷ 25) GeV/c protons by nucleons, are explained as resulting from « isobar » excitation of the target nucleons. The experiments are shown to provide evidence on the excitation of the first three levels. Some physical arguments are presented concerning the nature of the excitation process and its energy and angular dependence.RiassuntoLe osservazioni di Cocconi et al., sullo scattering della « diffrazione quasi-elastica » dei protoni di (10 ÷ 25) GeV/c su nucleoni, vengono spiegate come il risultato di una eccitazione « isobara » dei nucleoni del bersaglio. Mostriamo che gli esperimenti forniscono prove della eccitazione dei primi. Presentiamo alcuni argomenti fisici concernenti la natura del processo di eccitazione e la sua dipendenza dall’energia e dall’angolo.