Measurement of Spontaneous 14C Emission from 223Ra with an Enge Split-Pole Magnetic Spectrograph *

The first experimental evidence for the spontaneous emission of particles much heavier than 4He emerged in 1984 when Rose and Jones' discovered in an ingeniously simple experiment that ²²³Ra, an α-emitter with a half-life of 11.4 days, occasionally emits ¹⁴C nuclei of 29.8 MeV energy. The branching ratio relative to α-decay was measured to be (8.5 ± 2.5) X 10^-l0. ²²³Ra occurs in the natural-decay chain of ²³⁵U, with ²²⁷Ac (T_1/2 = 21.7 years) being a convenient precursor to ²²³Ra. In their pioneering experiment Rose and Jones used a 3.3 γCi source of ²²⁷Ac with ²²³Ra in secular equilibrium as a member of the ²²⁷Ac main decay sequence: ²²⁷Ac(β^-) → ²²⁷Th(α) - ²²³Ra(α) — ^2l9Rn(α) → ²¹⁵Po(α) → ^21lPb(β^- ) → ^21lBi(α) → ²⁰⁷Tl(β^-) → ²⁰⁷Pb (stable). Energy loss and total energy of particles emitted from the source were measured with a △E-E silicon surface-barrier detector telescope. Although no mass determination was possible, certain events could be uniquely identified as carbon nuclei. The measured kinetic energy was consistent with the Q-value for ^l4C emission from ²²³Ra. In addition, a comparison of Gamow penetrability factors favored ^l4C emission over any other carbon isotope.