π+-Meson Production Cross Section as a Function of Atomic Number

Abstract

TABLE I. Results for (y, n) reactions in O 18 and N 14 . QU N" Peak (Mev) 24.2 24.2 <rmax(millibarns) 11.4 2.84 Integrated cross section to peak position (Mev-barns) 0.0312 0.015a sharp increase at about 20 Mev, which may be the result of this effect. In Fig. 1 is shown the saturated specific activity curve for N 14 . The neutron yield at 22 Mev as determined from this curve is 2.38X10 4 neutrons/mole/r, which is lower than the value 6.5X10 4 obtained by Price and Kerst. 3 A curve drawn through the experimental points shows a small hump in the initial portion of this activity curve. Because of this unexpected behavior, dicyandiamide was irradiated. The nitrogen activity points obtained using it are shown in Fig. 1 as solid squares; they are in agreement with the previous results.The resulting cross section curve for N 14 (Fig. 2) shows a small peak at about 13 Mev as well as the larger one at 24.2 Mev. The falling off of the cross section curve immediately above 13 Mev, forming an apparent peak, may be caused by a (7, np) reaction in N 14 . The calculated threshold for this reaction, 12.5 Mev, is indicated in Fig. 2. In this process the neutron, presumed to be emitted first, leaves the N 13 nucleus in a highly excited state from which it decays by proton emission rather than by gamma-emission. The product nucleus is O 12 which is stable and therefore not detected in our experiment. While this process is not a true competition it has the effect of reducing the apparent (y, n) yield when this is measured by detecting the radioactivity of the product nucleus (in this case N 13 ). Such processes have been called "cascade processes" by Katz and Cameron. 6 Another mechanism is required to explain why the (7, n) cross section increases again above 17 Mev. Presumably this could be the result of the strong electric dipole absorption discussed earlier. Even for this portion of the curve it is probable that the cascade (7, np) competition strongly reduces the measured (7, n) yield leading to a smaller peak cross section in N 14 (2.8 mb) than, for instance, in O 16 (11.4 mb). This explanation requires a much larger (7, np) cross section in N 14 than in O 16 . Evidence for this is amply provided by Gaerttner and Yeater 7 who observed that the* (7, np) reaction in N 14 is almost 5 times that in O 16 for 100 Mev-bremsstrahlung. Further, the threshold for the (7, np) activity in O 16 occurs much higher (22.9 Mev) and thus a cascade process in oxygen would have a much smaller effect on the (y, n) cross section.The information from the oxygen and nitrogen cross section curves is shown in Table I. No attempt has been made to estimate the width at half-maximum. The ratio of the oxygen and nitrogen integrated (7, n) cross sections up to their peak positions is 2.08. In comparison the ratio of the total integrated cross sections obtained by Perlman and Friedlander 8 is 2.4 for 50-Mev and 2.2 for 100-Mev bremsstrahlung. The value obtained by Gaerttner and Yeater 7 is 1.9 for x-rays of maximum energy 100 Mev. It is seen that our results...

EXCITATION FUNCTION FOR POSITIVE PIONS PRODUCED AT 90 IN PROTON-CARBON COLLISIONS (thesis)

“…the absolute detection efficiency of the A -P delayed-coincidence equipment was 10 to 15 percent for all the positive pions entering the telescope and stopping in the third crystal.…”