## Abstract

The cross-section for the photodisintegration $^{12}$C($\gamma $, 3$\alpha $) has been determined for $\gamma $-ray energies up to about 60 MeV from a study of 2500 stars in nuclear emulsions. The methods used in selecting and identifying the stars are described, and full details are given of the corrections (for escape, observer efficiency, etc.) required for converting the experimental data into cross-section values. The cross-section exhibits at least five resonances, situated at $\gamma $-ray energies (E$_{\gamma}$) of 17$\cdot $3, 18$\cdot $3, 21$\cdot $9, 24$\cdot $3 and 29$\cdot $4 MeV, and a strong minimum at E$\gamma \ \sim $ 20$\cdot $5 MeV. This behaviour suggests that a well-defined compound nucleus is formed, the minimum near 20$\cdot $5 MeV resulting from ($\gamma $, n) and ($\gamma $, p) competition. furthermore, the finer details of the cross-section data are consistent with current knowledge of the $^{12}$C level structure. The integrated cross-section is 1$\cdot $21 $\pm $ 0$\cdot $16 MeV mb for E$_{\gamma}$ < 20$\cdot $5 MeV, a further 2$\cdot $8 $\pm $ 0$\cdot $4 MeV mb for 20$\cdot $5$\leq $E$_{\gamma}$ < 42 MeV, and < 0$\cdot $2 MeV mb for 42$\leq $E$_{\gamma}$ < 60 MeV (where the symbol mb denotes 10$^{-27}$ cm$^{2}$). As a subsidiary result of the main work, the existence of the reaction $^{13}$C($\gamma $, n) 3$\alpha $ has been established.