Determination of the Multipolarity of Nuclear Electromagnetic Transitions Using a Magnetic Pair Spectrometer

An intermediate-image magnetic pair spectrometer has been modified so as to respond to positron-electron internal pairs emitted at large relative angles ($50°\lesssim \theta \lesssim 90°$) thereby making the pair-line transmission depend sensitively on the multipolarity of electromagnetic transitions above 2 MeV. The modification consists of a specially designed spiral baffle system which selects pairs emitted within 105° azimuthal sectors on opposite sides of the axis. Measurements are made of the net yield of an internal-pair-conversion coincidence line, both in the normal spectrometer operation (pairs with relative angles $0°\le \theta \lesssim 90°$) and with the spiral baffle installed, giving a reduction ratio $R_{\omega }=\frac{Y_{\mathrm{with}\mathrm{baffle}}}{Y_{\mathrm{without}\mathrm{baffle}}}$. Experimental ratios were determined for 14 known transitions including $E0\mathrm{}$, $E1\mathrm{}$, $M1\mathrm{}$, $E2\mathrm{}$, $M2\mathrm{}$, and $E3\mathrm{}$ multipoles between 3 and 7 MeV. Theoretical calculations were carried out on the spectrometer transmission, when using the baffle, for $E0\mathrm{}$, $E1\mathrm{}$ through $E4\mathrm{}$, and $M1\mathrm{}$ through $M4\mathrm{}$ transitions from nonaligned nuclei over a wide energy range. These transmissions were combined with previous calculations of the transmission without the baffle in order to derive curves of $R_{\omega }\mathrm{}\left(l\right)\mathrm{}$ versus transition energy for the various multipoles. A best fit to the experimental ratios for the known multipoles was made in the calculations by adjusting slightly the values of the mean spectrometer-entrace angle and the sector angle $\omega$ of the baffle. The various ratio curves thus obtained are spaced widely enough apart to allow clear multipole assignments to be made in most cases. For mixed transitions from aligned nuclei, calculations were made of correction factors to be applied to the experimentally determined ratios. It is shown how the correction factors can be derived from separate measurements of the angular distributions of the corresponding gamma rays. The method has been applied to a number of previously unassigned transitions in ${\mathrm{Be}}^{10}$, ${\mathrm{B}}^{10}$, ${\mathrm{C}}^{14}$, and ${\mathrm{N}}^{14}$ leading to new spin and parity information on certain levels in these nuclei. In particular, it is found that the ${\mathrm{Be}}^{10}$ 6.18-MeV level and the ${\mathrm{C}}^{14}$ 6.58-MeV level are both $0^{+}$ and the ${\mathrm{N}}^{14}$ 5.10-MeV level has odd parity.