iT mesons produced in an internal wolfram target bombarded by 330-Mev protons in the 184-inch cyclotron are absorbed in a high pressure hydrogen target. The resulting gamma-ray spectrum is analyzed outside the shielding of the cyclotron by means of a 30-channel electron-positron pair spectrometer. The principal results are as follows. (1) The gamma-rays result from two competing reactions: Tr~-\-p->n+y and TT-\~p->w+7r°; 7r°-->2y. (2) The ratio between the 7r° yield to the single gamma-ray yield is = 0.94±0.20. (3) The mass difference between the x~ meson and the TT° meson is given by 10.6rb2.0 electron mass,es. (4) The TT mass is 275.2±2.5 electron masses. The large mass difference between x~ and x° precludes the conclusion that the unexpectedly small 7r° to y ratio is due to the small amount of momentum space available for ir° emission. It rather indicates that 7r° emission is slowed down by the nature of the coupling of the 7r° field to thenucleons. The experiment has been repeated by substituting D2 for H2 in the vessel. The result is that the reaction ir~-\-D-+2n and w~-\~D^>2n-\-y compete in the ratio 2 : 1. The reaction TT~-\-D-»2w+7r° is absent.
The angular and energy distributions of the fast charged particles emerging from nuclei bombarded by 90-Mev neutrons have been investigated. The results indicate that the nature of the collision process is determined predominantly by interactions of the bombarding particle with single nucleons rather than with the struck nucleus as a whole: angular distributions are found to be peaked in the forward direction for all emergent particles, the degree of peaking increasing with energy. Deuterons are found to be more concentrated in the forward direction than are protons. In the case of carbon, particles traveling in the forward direction with energies greater than 20 Mev consist of 60 percent protons, 36 percent deuterons, and about 4 percent tritons.
750R. M. STERNHEIMER excitation of electron 1 into higher p states, while S\ and d\ are s and d functions, respectively, which describe the excitation of electron 2. The first-order overlap SE^i does not contribute to q' because so(l) is orthogonal to pi{\) and ^'i(l), and similarly po(2) is orthogonal to Si(2) and d\{2). However, \Fi 2 contributes three second-order terms to q f which arise from |j>i(l)] 2 , [j>'i(l)] 2 , and E^i(2)] 2 . Second-order terms of the type ^i 2 are also obtained from the simultaneous Pi m excitation of a core electron in a p state and the valence electron. A similar class of terms is obtained using the P 2 m part of e 2 /r 12 . These terms were not evaluated because of the difficulty of determining the functions of type pi, s h p f h and d\. Thus pi and Si satisfy a set of two simultaneous differential equations. The same applies for p\ and d\.. The numerical solution of these sets would be much more complicated than the solution of Eqs. (5A) and (24A) which involve a single unknown function. However, there seems to be no reason to believe that the two-electron terms would be appreciably larger than the one-electron excitation terms which were shown to be verv small for the case of CI.Measurements have been made of the (d,2n) cross sections of the nuclear species Ti 48 , Cr 52 , and Fe 56 . Results are given for incident deuterons in the energy region 1-20 Mev.
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 ...
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