Spin, Quadrupole Moment, and Mass of Selenium-75

Abstract

The microwave spectrum of OC${\mathrm{Se}}^{75}$ has been observed and from it the spin, quadrupole moment, and mass of ${\mathrm{Se}}^{75}$ were measured. Frequencies of the six observed lines of the $J=2\mathrm{}\to 3$ rotational transition give a nuclear spin of $\frac{5}{2}$, a quadrupole coupling constant $eqQ=946.0\mathrm{}$ Mc/sec, and a rotational constant $B_0=4081.926\mathrm{}$ Mc/sec. From these and other known properties of the OCSe molecule, one obtains the quadrupole moment of ${\mathrm{Se}}^{75}$ as $Q=1.1\mathrm{}\times {10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$±20 percent, and the mass ratio $\frac{(M_{{\mathrm{Se}}^{75}}-M_{{\mathrm{Se}}^{76}})}{(M_{{\mathrm{Se}}^{75}}-M_{{\mathrm{Se}}^{80}})}=0.199566\mathrm{}\pm 0.000030$. The odd-even mass difference for ${\mathrm{Se}}^{75}$ is 1.5±0.2 mMU. The observed spin disagrees with that expected from the shell model which predicts a spin of ½ or $\frac{9}{2}$. Three possible nuclear configurations which agree with the observed spin and also indicate a positive quadrupole moment are ${\left[p_{\frac{1}{2}}{\left(g_{\frac{9}{2}}\right)}^2\right]}_{\frac{5}{2}}$, ${{\left(g_{\frac{9}{2}}\right)}^3}_{\frac{5}{2}}$, and ${{\left(f_{\frac{5}{2}}\right)}^5}_{\frac{5}{2}}$. Magnetic moment measurements are needed to clearly distinguish between configurations. The sample material was obtained by bombarding arsenic with deuterons. Approximately 1½ micrograms of sample material were produced but only 0.07 microgram could be used in each spectroscopic run because of difficulties in synthesizing OCSe.