MESONS FROM 335-MeV $\gamma$ RAYS

Mesons with kinetic energies up to 150 MeV have been obtained at the large synchrotron in Berkeley[^1][^2]. This accelerator gives a beam of quanta with a maximum energy of 335 MeV. The half-width of the beam is 0.0135 radians, and its intensity at a distance of 1 meter from the target reaches 3500 roentgens per hour (the working intensity is one half of this value).

Fig. 1. Arrangement of the apparatus for recording mesons.
1—lead collimator; 2—brass collimator; 3—photographic plates; 4—lead absorber; 5—carbon target.

In the first experiments on the detection of mesons, the plates were placed directly in the beam 180 cm from the accelerator target. Mesons were born chiefly in the glass. In this case, the total blackening

Fig. 2. Angular distribution of meson tracks in the energy interval 43–59 MeV. The distribution of ρ-mesons is the result of superposing the distribution of π-mesons on the uniform background of μ-mesons.

of the plates severely limited the time of their exposure. Subsequently the experimental conditions were changed: meson production already took place in a carbon target of cylindrical shape (see Fig. 1), around which stacks of photographic plates were arranged radially. The latter were separated from the cylinder by lead absorbers of various thicknesses, making it possible to determine the energy of the mesons. A beam of γ-rays

...was collimated by a lead block with a 2.5 cm aperture and a brass collimator, which protected the plates from scattered electrons.

The mesons found in the plates were classified, depending on the character of the end of the meson track, as:

1) σ-mesons, producing stars of one, two, or more tracks,
2) π-mesons, forming μ-mesons, and
3) ρ-mesons, whose tracks either end with a recoil nucleus track, or have nothing at the end.

In constructing the mesons, the authors relied on the results of a magnetic analysis of mesons carried out at the phasotron3, according to which the number of negative π-mesons in the plates is equal to \(1.37\sigma\), and 95% of the positive π-mesons undergo \(\pi \to \mu\) decay. The ratio

\[ \alpha = \frac{\pi^-}{\pi^+} \]

for the number of counted mesons of all groups \(N = 1053\) \((\sigma = 403;\ \pi\mu = 327;\ \rho = 323)\) was found to be \(1.7^{+8}_{-3}\%\). (In the first experiments, values \(\alpha = 10\) and 7.5 were erroneously obtained.)

Within considerable statistical errors (\(\pm 25\%\) up to 100 Mev and \(\pm 50\%\) in the interval \(E_{\text{mes}}\) from 100 to 150 Mev), \(\alpha\) remains constant for mesons of all energies.

Fig. 3. Distribution of mesons by energy at the maximum γ-ray energy \(E_m = 335\) Mev. Data for 803 σ- and π+-mesons. The area under the curve is \(1.8 \cdot 10^3\) tracks/cm² target. The lower energy limit is due to the fact that, when calculating the energy of mesons, the center of the carbon target is taken as the place of their production.

The angular distribution of the σ-mesons formed (Fig. 2) is close to the law \(\sin^2 \theta\), where \(\theta\) is the angle between the direction of the γ-ray and the σ-meson track, which, when the geometry of the apparatus is taken into account, corresponds to an isotropic distribution. The same distribution is obtained for π-mesons associated with \(\pi\mu\)-decay. The angular distribution of μ-mesons with respect to the direction of the γ-beam is, practically, isotropic.

The distribution of mesons with respect to energy is calculated by the formula:

\[ \frac{dN}{dE}=4r\frac{dR}{dE}\frac{1.37\sigma+\tau\mu}{A\cdot t}P, \]

where \(N\) is the number of mesons formed per \(1\ \mathrm{cm}\) of target length, \(E\) is the meson energy, \(r\) is the radius of the center of the emulsion stack, \(R\) is the meson range in glass or emulsion, \(\sigma\) and \(\mu\) are the numbers of tracks on the area \(A\) of the emulsion, and \(t\) is the effective thickness of the emulsion (coefficient 2.4).

The form factor \(P\) varies from 1.8 to 4.3 depending on the meson energy. The absolute value of the cross section for meson formation of all energies (\(\sigma_m\)) was found after determining the number of quanta corresponding to 1 roentgen. The number of \(\gamma\)-quanta was defined as the ratio of the total energy of the \(\gamma\)-rays to the maximum energy of a \(\gamma\)-quantum. \(\sigma_m\) for a carbon nucleus was found to be \(5\cdot 10^{-28}\ \mathrm{cm}^2\).

The isotropic angular distribution of mesons obtained by the authors agrees with the theoretical one\(^4\) when the pseudoscalar theory is used for the calculations; the scalar theory of mesons gives a distribution according to the law \(\sin^2\theta\). The experimentally obtained value of \(\sigma_m\) is close to the calculated value of the cross section for a scalar meson, which, however, is not sufficiently conclusive because of the large errors in its determination.

According to the calculations of Brueckner and Goldberger\(^5\), the theoretical value of \(\pi^-/\pi^+\) depends only weakly on the type of meson and is close to the observed value.

B. R.

References

1. McMillan F. L. and J. M. Peterson, Science 109, 438 (1949).
2. E. M. McMillan, J. M. Peterson and S. White, Science 110, 579 (1949).
3. F. L. Adelman and S. B. Sones, Phys. Rev. 75, 1468 (1949).
4. L. Foldy, Phys. Rev. 76, 372 (1949).
5. K. A. Brueckner and M. L. Goldberger, Phys. Rev. 76, 1725 (1949).