Absence of Interference Effects in theβDecay of PolarizedCo56and

Abstract

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...

An experimental study of beta decay using the radiations from oriented nuclei

“…In this respect Ce is quite good compared to other materials which have been used for low temperature beta spectroscopy [38,39]j for in the present case internal conversion electrons and electrons from…”

Fermi-Gamow-Teller Mixing in the Allowed Isospin-Hindered Positron Decay ofCo56

“…4 Previous published measurements include values of 5 (4.6± 1.9)xl0" 13 and 6 (-0.25 ±3.5)xl0" 13 The allowed isospin-hindered (J ff^J,r , T-*T±1) /3 transitions in nuclei 1 are of particular interest for studies of isospin conservation of nuclear forces 2 and for time-reversal invariance (TRI) tests of the weak interaction. 3 Fermi transitions require AT =0, and consequently, a nonzero value of the Fermi to Gamow-Teller mixing ratio, 1 y = C V M F /C A M GT , violates isospin conservation.…”

“…The mixing ratio, y , has previously been measured for 56 Co(4 + -4 + , 1-2) by several groups using both the asymmetry of the angular distribution of /3 particles from oriented nuclei (NO), 4 and the asymmetry of the /3-y circular polarization correlation methods (CP). 5 " 11 The reported results are in very poor agreement.…”

“…If we assume time-reversal invariance (TRl) and the V -A form of the weak interaction, A$ for 56 Co can We are in quite reasonable agreement (see Table II) with all the previous results except for those of Ambler etal. 4 and Pingot. 10 The results reported by Ambler etal.…”

“…10 The results reported by Ambler etal. 4 were from a very early measurement of A a for 56 Co that used adiabatic demagnetization for cooling. Problems with fringing fields, temperature variations, and positron detection add considerable uncertainty to the value of A d reported.…”