The branching ratio of the two modes of decay of Fm'", i.e. , E.C. /n, was found to be about 8.5which gives 89.5% decay by electron capture and 10.5% by alpha emission. It was not possible to measure the cross section for the Cf'"(n, 3n)Fm'" reaction because Fm'" could also be produced from other californium isotopes in the target. A previous publication4 on a possible identification of the Fm'" gave the values of 6.85&0.04 Mev for the alpha-particle energy, and a half-life &10 days. It is a pleasure to thank the crew of the 60-inch cyclotron for their extremely careful and skillful operation of the machine during the bombardment. We wish to thank Professor Glenn T. Seaborg for his continued interest.

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in case screening is important. If there is no screening, then Q= (m 2 k/2EoE) for bremsstrahlung and Q= (m 2 k/ 2E + EJ) for pair production.Now consider the problem of integrating dcrs given by Eq. (1) over the angular variables u, v, s, and t. For purely dimensional reasons the result of the integration can be a function only of (Q/m) multiplied by Eo 2 , k 2 , or JE 2 according as M is given by Eqs.(3)-(5). But from the Bethe-Heitler formula, 3 the cross section summed over final spin-states iswhere R is a function of (Q/m) which is given the name of "radiation length." From this it necessarily follows that the angular integrals of Eq.(1) must have the values (f-jR" 1 , ^Rr 1 , f^" 1 ) for the three separate final spin combinations. We have also checked these values by a direct integration. We therefore conclude that the integrated cross sections for bremsstrahlung with assigned polarizations are given bywhere the suffixes refer respectively to the incident electron (energy Eo), the photon (energy k), and the outgoing electron (energy E).A precisely similar argument applied to the pairproduction process gives the integrated cross sectionswhere the suffixes refer to the polarization of photon, positron, and electron, respectively. These cross sections are of interest for two reasons. First, they show more clearly than the unpolarized cross sections the symmetry between bremsstrahlung and pair production, and they explain the origin of the unsymmetrical factors (2E 0 2 +2E 2 +k 2 ) and (k 2 +2E+ 2 -\-2EJ) which appear in the unpolarized cross sections. Second, they clearly indicate the possibility of a large-scale persistence of longitudinal polarization in an electromagnetic cascade originated by a single polarized electron of high energy. 4 The latter effect will be the subject of a separate communication.I N a previous communication 1 we reported that we observed a large asymmetry in the angular distribution of electrons from polarized Co 60 nuclei. It was concluded that unequivocal proof was thereby established of the nonconservation of parity as well as of noninvariance under charge conjugation in beta decay. It was also pointed out that according to Lee, Oehme, and Yang, 2 invariance under time reversal could also be investigated by studying the momentum dependence of the asymmetry parameter 0. Since then we have made further measurements and checks. In particular we have carried out similar experiments 3 with Co 58 and observed an asymmetry in the positron emission with a coefficient opposite in sign and roughly one third of that from Co 60 . Through more detailed measurements on Co 60 we have obtained the general behavior of the momentum dependence of /3. The linear dependence of P on v/c in the range from 0.4 to 0.75 is good.In order to put upper limits on possible spurious effects in our experimental method, we have performed a similar experiment with Bi 210 incorporated in the crystal. Since the bismuth ion in cerium magnesium nitrate is diamagnetic, there can be no significant nuclear polarization s...

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LETTERS TO THE EDITOR 503For the reactions listed above, forward and backward cross sections satisfied the reciprocity condition to within 20%.A second conclusion of our study, which we expect to amplify in a forthcoming detailed paper, is that many experiments which initially seem to test TR invariance, actually may not be sensitive to this symmetry. Thus, the lack of TR invariance does not rule out detailed balance in many reactions. This is assured by the Hermitian property of the Hamiltonian, for example, when first-order perturbation theory applies and spins are not measured. A less familiar restriction is imposed by the unitarity property of the S matrix, which implies that {a\S\b)=exp(idab)(b\S\a) on the energy shell, when, for example, only two channels are open. An academic illustration is the 5-wave interaction 7 r +4-^=± 7r°-|-^. In most nuclear reactions a model is necessary before the sensitivity of detailed balance with respect to TR invariance can be predicted. The usual models 9 * 10 predict a lack of sensitivity to TR invariance in p-{-p-^±Tr + -\-d, and in the forward angular distributions of direct processes such as (d,p) and (p,d) reactions.There are effects in elastic scattering which can in principle reveal a breakdown of TR invariance, but those we have examined are only of second order in the force terms that change sign under time reversal. For example, in a double scattering in which the second process takes place at the same energy and angle as the first, o-(left-left) -o-(left-right) can be negative only if TR invariance is violated. It is unfortunate that experimental evidence exists 11 only for a system of total spin J, in which special case the positiveness of the above quantity follows from parity conservation and rotational invariance alone.We hope that the above discussion will encourage physicists to perform high-precision experiments to test TR invariance in nuclear physics. In detailed balance experiments it is important to have many competing channels open. 12 For correlation experiments of successive radiations, the most sensitive measurement of the interference phase of two competing radiations occurs when these are about equal in strength and are followed or preceded by a pure radiation. In correlation experiments, a null-type test of TR invariance has been suggested by Lee and Yang. 13 The detection of a term, of the form (p-kXk')k-k', where p is the momentum of the electron preceding gamma-ray emission and k and k' specify the directions of two successive gammas, would prove that TR invariance cannot hold in strong interactions. A further test of TR invariance in nuclear interactions occurs in beta decay; for example, experiments suggested by Jackson, Treiman, and Wyld 14 determine not only TR invariance in beta decay, but also in strong interactions. If TR invariance is found not to hold in such experiments, it becomes all the more important to determine whether the breakdown occurs because of weak or strong interactions. I N an earlier communication 1 we reported that...

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