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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The International Temperature Scale of 1990, ITS-90, came into effect on 1 January 1990, replacing the International Practical Temperature Scale of 1968, IPTS-68, and the "Echelle Provisoire de Température de 1976 entre 0,5 K et 30 K", EPT-76. The present paper reviews the various experimental measurements of thermodynamic temperature T (often expressed as differences (T - T68)) in the range 0,65 K to 1 357,77 K, which were available prior to September 1989, and on the basis of which values of temperature were adopted in the ITS-90. Estimates are given for the uncertainties in the derivation of these values.

The Provisional Low Temperature Scale from 0.9 mK to 1 K, PLTS-2000, was adopted by the Comité International des Poids et Mesures in October 2000. It is defined using an equation for the melting pressure of 3 He over the complete temperature range, and forms an extension of the International Temperature Scale of 1990, ITS-90, below its lower limit of 0.65 K. An internationally-accepted ultra-low temperature scale is needed to provide the basis for reliable thermometry in the temperature range in which commercial dilution refrigerators operate, and at lower temperatures where experiments investigating the thermodynamic properties of 3 He and other condensed matter are carried out in many research centres. This paper is a summary of a fuller publication describing the background and derivation of the scale, published in the Journal of Low Temperature Physics [1], and includes tables of values of melting pressure, p m / MPa, and temperature T 2000 / K, and the derivative, dp m /dT 2000 in MPa/K.

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After a historical introduction the general principles of the magnetic cooling method are outlined. First, the case of the ideal paramagnetic salt is treated, and this is followed by a discussion of the electric and magnetic interaction effects which cause departures from ideal behaviour, and set a lower limit to the temperatures which can be reached upon

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...