FORMATION OF C$^{11}$ IN NUCLEAR REACTIONS OF $\pi^-$ MESONS WITH OXYGEN AND NITROGEN*)

The formation of radioactive isotopes as a result of nuclear reactions is widely used for the registration of all kinds of radiation. In particular, for the registration of neutrons, protons, and other high-energy particles, the process of knocking neutrons out of C$^{12}$ nuclei with the formation of the $\beta^+$-active isotope C$^{11}$ is used (half-life 20.5 min, maximum energy of the $\beta^+$ spectrum about 1 MeV). It has recently been shown that this isotope is also formed in nuclear reactions of $\pi^-$ mesons with nuclei of oxygen and nitrogen. Thus, for the first time, the interaction of $\pi^-$ mesons with nuclei was registered by the formation of radioactive products as a result of such an interaction (i.e., the method of radiochemical indication of $\pi$ mesons was applied).

The equations of the reactions leading to the formation of C$^{11}$ have the form:

O$^{16}$ $(\pi^-;\ p,\ 4n)$ C$^{11}$ and (or) O$^{16}$ $(\pi^-;\ 5n)$ N$^{11}$ $\xrightarrow{\beta^+}$ C$^{11}$, N$^{14}$ $(\pi^-;\ 3n)$ C$^{11}$.

Negative $\pi$ mesons were obtained in a synchrocyclotron as a result of bombardment of the inner target with protons of energy 450 MeV. The mesons directed forward left the synchrocyclotron chamber through—

*) A. Turkevich and J. B. Niday, Phys. Rev. 84, 1253 (1951).

through a thin lucite window. A special channel in an iron shielding wall (180 cm thick) admitted only mesons with energies of 140–150 MeV. After passing through the channel, the mesons were deflected by a magnetic field through an angle of 45°. At the end of their path the mesons passed through two liquid scintillation counters connected in a coincidence circuit. When the magnitude of the magnetic field was changed from the value corresponding to a 45° deflection of mesons with an energy of 145 MeV, the coincidence counting rate decreased by about 50 times, confirming the monoenergetic character of the meson beam. Special measurements of the velocities of particles in the beam, using a counter based on the Cherenkov effect, confirmed that \(94 \pm 4\%\) of the particles in the beam were \(\pi\)-mesons \(\bigl((6 \pm 4\%)\ \mu^{-}\)-mesons and less than 1% electrons\bigr)).

The samples to be irradiated were placed in special iron shielding 30 cm thick between the magnet that deflected the \(\pi\)-mesons and the scintillation counter. The samples consisted of glass vessels 10 cm in diameter and about 4 liters in volume, filled with acidified water or a saturated solution of \(\mathrm{NH_4NO_3}\) (at \(\mathrm{pH} \simeq 3.5\)), with small additions of \(\mathrm{NaHCO_3}\). In all cases the range of the nuclear interaction of the \(\pi\)-mesons greatly exceeded their ionization ranges. Only about \(1/3\) of the mesons entered into nuclear interaction before slowing down; the remaining mesons reacted only after stopping.

After irradiation, the radiocarbon in the form of CO and \(\mathrm{CO_2}\) was displaced from the solutions by inert gases, passed over heated copper oxide for complete oxidation to \(\mathrm{CO_2}\), and absorbed in a dilute solution of caustic soda. The carbon was then precipitated as \(\mathrm{CaCO_3}\), and the activity of the precipitate was determined with Geiger counters.

To determine the background of \(\mathrm{C^{11}}\) formation due to the action of high-energy neutrons, experiments were carried out with the magnetic field deflecting the mesons switched off, i.e., under conditions in which the \(\pi\)-mesons did not strike the samples. The ratio of the background effect to the total effect did not exceed 10%.

Typical experiments lasted about 40 minutes, with the number of \(\pi^{-}\)-mesons passing through the sample not exceeding 2000 per second. The \(\mathrm{C^{11}}\) activity, reduced to the end of irradiation and to infinite irradiation, was about 100–150 counts/min when water was irradiated and about 200–300 counts/min when \(\mathrm{NH_4NO_3}\) was irradiated, i.e., it was quite sufficient for measurements.

Taking into account corrections for the decay of \(\mathrm{C^{11}}\) in the \(\mathrm{CaCO_3}\) samples themselves and the geometrical efficiency of the counting system, the authors concluded that the percentage of \(\mathrm{C^{11}}\) formation in the interaction of \(\pi^{-}\)-mesons with water is \(1.9 \pm 0.4\), and with \(\mathrm{NH_4NO_3}\), \(3.6 \pm 0.6\). Thus, the probability of formation of \(\mathrm{C^{11}}\) in the interaction \(\pi^{-} + \mathrm{O^{16}}\) is close to 2%, and in the interaction \(\pi^{-} + \mathrm{N^{14}}\), if one takes into account the relatively small atomic concentration of nitrogen in a saturated solution of \(\mathrm{NH_4NO_3}\), is approximately 4 times higher.

G. I.