INTRODUCTIONI N the last few years study of the laws of (:J decay has been the object of many experimental and theoretical investigations. As a consequence, the form of the nuclear (:J-decay interaction is now well-established. We know that (:J decay violates parity conservation completely, and can be written V-A for electron (negaton) emission (TEN 58) . 1 The fact that parity is not conserved has enlarged the number of possible experiments on nuclear (3 decay: The measurement of the correlation between the direction of the (:J particle and of the circular polarization of photons emitted after the (:J decay, for example, has become a new source of information. This increase in number of experimental possibilities, together with our knowledge of the interaction law, has given nuclear (:J decay a new aspect: It can be applied to the study of nuclear structure. In the same way in which we can use knowledge of the electrodynamic interaction to measure E1, M1 matrix elements, etc., we can use knowledge of the (:J interaction in many cases to measure the nuclear matrix elements involved in the transition.For the case of allowed transitions, where there are only two nuclear matrix elements J1 and J CT, this scheme has been applied successfully in many cases. Recently, a combination of measurements of half-life, transition energy, spectrum shape factor, f:J-'Y angular correlation and (:J-circularly polarized ')'-angular correlation has given a unique determination of the nuclear parameters involved in the first-forbidden (:J decay of Sb 124 (HA 60, ST 60). Similarly, a study of the spectrum of the first-forbidden (:J decay of Pr 144 and its log-ft value has yielded values for the two nuclear matrix elements relevant for the transition (PO 59). For allowed transitions, Konopinski (KON 59) has recently given a summary of the present status of our knowledge; no such account exists for forbidden (:J decay. In many recent papers, various aspects of forbidden (:J decay have been discussed. Particularly, the papers of Kotani [e.g., (KOT 59b)] have dealt with the problems encountered in the investigation of nuclei by studying first-forbidden (:J decay. However, access to these papers can often be obtained only by studying the classical papers on (:J decay, as, for example, Konopinski (KON 41).In the light of these developments and in view of the fact that in various laboratories efforts are being made to study nuclear matrix elements in first-forbidden *The survey of the literature was closed on October 1, 1960. t This work was performed under the auspices of the U. S.
Atomic Energy Commission.1 For these references, see Bibliography at the end of the article.(:J decay it is felt that a consistent presentation and discussion of the theoretical description of forbidden (:J decay may be desirable. The purpose of this paper is threefold: It attempts to give a concise introduction to the theoretical treatment of forbidden (:J decay, it presents a review of the pres:nt ~tatus o! the theory of first-forbidden (:J decay, and 1t...
