Activities in Light Nuclei from Nitrogen Ion Bombardment

Abstract

LETTERS TO THE EDITORcomes nearer to c\ as the temperature is decreased (i.e., as the mean free path becomes longer) and as the sensitivity of the receiver is increased. In this picture, the sharp rise in velocity at intermediate temperatures is caused by the disappearance of the high temperature excitations (bosons or rotons). As soon as these have disappeared, the velocity of heat pulses should become ci/y/3, and remain there until the mean free path for phonon-phonon collision becomes long, at which temperature it should rise to near c\. The pressure dependence should be in the direction found experimentally, since (as mentioned by Maurer and Herlin) the number of phonons at a given temperature decreases with pressure, and hence any long mean-free-path effect should increase with pressure.The shape of the received pulse at low temperatures and at all pressures observed shows the behavior reported by Pellam and Scott 9 and by Atkins and Osborne. At the lowest temperatures, where the velocity is high and does not vary rapidly with temperature, the pulse shape is essentially that reproduced in the letter of Atkins and Osborne. The rise is fairly sharp, with a well-defined foot, but the tail is extremely long. At the intermediate temperatures, coincident with the rapid drop in velocity with temperature, the tail becomes shorter, the rise longer, and the foot less well defined, so that the pulse becomes symmetrical and assumes a roughly Gaussian shape. At the higher temperatures the entire pulse becomes well defined. The extremely large dispersion in the intermediate-temperature region may possibly be due to the interaction of the phonons and the higher temperature excitations, since in this region they are competing for dominance.

Nuclear astrophysicists at war

“…This required a sufficiently high neutron flux; having worked on the poloniumberyllium neutron sources for the ignition of a bomb explosion, Fowler was familiar with the issue. Since beryllium-9 is a rare element in the stellar environment, he proposed the 13 C(α,n) reaction as a possible neutron source in helium burning, with 13 C being a product of the CNO cycles in the preceding hydrogen-burning phase of stars. Since the neutron flux was low, Fowler envisioned it as the neutron source for the slow or s-process occurring in stellar helium burning.…”

“…This was especially the case when considering underwater tests, 9 which involved higher densities and hydrogen contents, as well as in the first tests of the hydrogen bomb, 10,11 where the possibility of the fusion of oxygen isotopes 16 O+ 16 O was also considered. To experimentally clarify the troubling situation, a dedicated accelerator was built at Oak Ridge National Laboratory (ORNL) in the early 1950s 12 that made it possible to measure fusion cross‐sections for 14 N+ 14 N, 16 O+ 16 O, and other reactions of medium‐heavy nuclei 13,14 . Alexander Zucker (1924−2017), one of the young scientists who was to measure the effective cross‐sections and who would later be director of ORNL, noted that for security reasons, they were not directly told why there was interest in these data.…”

“…To experimentally clarify the troubling situation, a dedicated accelerator was built at Oak Ridge National Laboratory (ORNL) in the early 1950s 12 that made it possible to measure fusion crosssections for 14 N+ 14 N, 16 O+ 16 O, and other reactions of medium-heavy nuclei. 13,14 Alexander Zucker (1924−2017), one of the young scientists who was to measure the effective cross-sections and who would later be director of ORNL, noted that for security reasons, they were not directly told why there was interest in these data. They were, therefore, told indirectly, which certainly increased the pressure on these young scientists.…”

Cooling the atmosphere

“…It is impossible to reach such temperatures unless fission bombs or thermonuclear bombs are used which greatly exceed the bombs under consideration. 5 A cyclotron at Oak Ridge 6 and a high current injector with 350 mA beam current at Livermore 7 were used in the postwar years to confirm experimentally the assumptions in the Teller report on the 14 N + 14 N fusion cross section 8 and the radiative proton capture on 14 N, 9 respectively. The critical point, however, was not the probability for fusion reactions occurring at the high temperature conditions reached during the first millisecond of the ignitions, but the efficiency of radiative cooling processes through electron induced bremsstrahlung and Compton effect.…”