Angular Distribution ofn−pScattering with 90-Mev Neutrons

Abstract

O NE of the important experiments that became feasible as soon as the 184-inch cyclotron started to operate was the measurement of the angular dependence of n-p scattering. As is well known, experiments of this type acquire particular significance when the de Broglie wavelength of the neutron is comparable with the range of nuclear forces. FIG. l.The neutron beam 1 of the cyclotron has an angular distribution well described by Serber's 2 stripping theory. We assume that the energy distribution of the neutrons is also that predicted by this theory, i.e., has a maximum at 90 Mev; the corresponding wave-length in the center of mass system is X = 0.95 X 10~1 3 cm.We have used these neutrons to measure the n-p cross section as a function of the angle of scattering 0, in the CM system, between 70° and 170°. Figure 1 shows a schematic layout of the apparatus. The neutron beam produced by stripping 180-Mev deuterons on a Be target was coilimated to about 2.5-cm diameter and scattered by paraffin or polyethylene targets which were larger in diameter than the neutron beam. The recoil protons used for the measurement were detected by a telescope of three proportional counters in coincidence pointing towards the scatterer. The primary neutron beam was monitored either by the protons scattered by an auxiliary hydrogenous target or by a bismuth fission chamber.The telescope was made insensitive to protons below a certain energy by the Al absorber A. The thickness of the FIG. 2. Plan view of experimental arrangement. THE EDITORabsorber was adjusted so that protons of energy less than 66COS 2 [(TT-0)/2] Mev could not enter the third counter. Thus we made sure that if the recoil protons detected originated from elastic collisions of neutrons on hydrogen, the impinging neutron had an energy greater than 66 Mev. (We have neglected some small relativity correction.) By taking background measurements with a carbon scatterer and with no scatterer we distinguished the protons due to n-p scattering and those due to {n-p) reactions on C and air. Table I gives the cross section as a function of 0. Smaller and larger angles are being investigated but the results will not be available for some time.From what we have said above it is apparent that our measurements give only a number proportional to (da/ do))Aa) where « is the solid angle; however, we have normalized our figures by the following method: we have hypothetically assumed that da/dw is the same for 0 and ir-0 and we have made the total scattering cross section 0.083 X10" 24 cm 2 as reported by Cook, McMillan, Peterson, and Sewell. 8 Our assumption on the behavior of the cross section for 0<9O° is not in disagreement with some preliminary observations at 0 <90° by us and with some cloudchamber data communicated to us by Dr. W. Powell, but is by no means established. The data are also plotted in Fig. 2 with the relativistic corrections.In order to avoid some sources of systematic error in these experiments, we have checked the following:1. Plateau of the coincidence counting rate ...

Diffusion nucléon-nucléon en présence de forces non centrales - I. Diffusion neutron-proton.

“…-Les expériences de collisions neutron-proton, effectuées aux faibles énergies, montrent que la section efficace différentielle possède pratiquement la symétrie sphérique. Mais lorsque l'on a bombardé des protons et des deutérons avec des neutrons de 90 MeV [1], [2], on s'est aperçu de la nécessité d'introduire des forces non centrales pour rendre compte des résultats expérimentaux.…”

“…Nous pourrions décomposer ce vecteur -¡(J, m;;) suivant les vecteurs harmoniques sphériques YmJL1 [2], [3], [4] ; mais la force tenseur S12 nẽ conserve pas le moment orbital L : elle conserve la parité (-1 )L. Donc les vecteurs propres de l'hamiltonien seront construits à partir de vecteurs harmoniques sphériques de même parité.…”

“…On effectue un développement en polynômes de Legendre de l'onde plane puis, à l'aide des coefficients de Clebsch-Gordan [9], il est aisé de développer l'onde incidente en vecteurs harmoniques sphériques. La forme asymptotique des fonctions sphériques de Bessel permet d'écrire cette dernière onde sous une forme analogue à celle de l'onde, totale : , ' Nous choisissons maintenant les coeffioients A,, BJ Cj de telle manière que les coefficients des termes en e-fb dans les expressions (1) et (2) soient identiques. Ceci conduità un système linéaire immédiatement résolu.…”