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On the polarization of electrons in β-decay
Nuclear Physics 4 (1957) 271--272; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher
/} ON T H E P O L A R I Z A T I O N OF E L E C T R O N S IN [3-DECAY M. E. V I S H N E V S K Y , V. K. G R I G O R I E V , V. A. E R G A K O V , S. J. N I K I T I N , E. V. P U S H K I N A N D YU. V. T R E B U K H O V S K Y
U.S.S.R. Academy o/ Sciences, Moscow Received 9 April 1957
Extending the concept of non-conservation of parity in weak interactions, introduced by Lee and Yang 2), Landau has recently suggested 1) that a so-called two-component or longitudinal neutrino m a y exist. According to this idea the neutrino is polarized along its momentum. One of the possibilities of proving this hypothesis would be to s t u d y the polarization of r-electrons since the electrons emitted in allowed transitions should be longitudinaUy polarized, the polarization being equal to v/c. We accordingly performed experiments with electrons emitted by Cu e~ (AJ = 1, no). In order to observe longitudinal polarization, the electrons emitted by Cu 84 were deflected in an electric field of a cylindrical condenser and the azimuthal a s y m m e t r y in the number of electrons scattered by a thin foil was studied. The angle between the spin and momentum of the electrons deflected within the condenser was 90 °. A deflected beam of 145 keV electrons (v/c = 0.63) was scattered by a thin gold foil, the direction of the beam making an angle of 45 ° with the plane of the scatterer, The scattered electrons were counted by means of two tel~copes, each containing two G-M counters. The axis of the telescopes formed an angle equal to 180 °. In order to determine the azimuthal asymmetry the telescopes and scattering target were rotated about the beam axis and the number of scattered electrons at various azimuthal angles were counted; one of the telescopes thus counted electrons reflected by the foil and the other electrons transmitted by the foil. The source consisted of a thin layer of CuO (=< 2 mg/cm ~) deposited on a 0.5 mg/cm 2 A1 foil. The electrons were scattered by a thin gold layer, evaporated on a 0.04 mg/cm 2 cellophane film. Gold layers of 0.120 mg/cm ~ and 0.240 mg/cm 2 were used. An azimuthal asymmetry in the number of scattered electrons, corresponding to an electron polarization opposite to the direction of momentum (right-left asymmetry) was observed. 271
272
M. E. VISHNEVSKY 8~ al.
The value of the observed asymmetry A -~ ( 2 1 - I ) / ( I +I.._) averaged over several runs for electrons penetrating the scatterer, measured with two slightly different arrangements, was found equal to 0.35 4- 0.05 and 0.27 4- 0.05 respectively. The asymmetry obtained with electrons reflected b y the scatterer was considerably less than the results given above owing to a larger multiple-scattering effect within the scatterer. The errors given correspond to the statistical errors only and do not take into account depolarization effects within the source, multiple scattering within the target or possible small apparatus errors. No up-down asymmetry exceeding the statistical errors was observed. The theoretical value for the asymmetry in the case of the two-component neutrino is equal to 0.34 3); our result is in qualitative agreement with this value. At present a more refined experimental study of the polarization of relectrons is being carried out with the aim of quantitatively checking Landau's theory. A similar investigation has recently been performed b y A. I. Alikhanov, G. P. Yeliseyev, V. A. Liubimov and B. V. Ershler *) with Sr 9° and ygo. Some time before completion of our experiments we were informed about the experiments performed b y the Illinois University group 5). The data obtained b y both of the aforementioned groups of investigators closely agree with our results. The authors wish to thank V. V. Vladimirsky for numerous helpful discussions. The apparatus used was constructed b y N. I. Afanasiev, E. G. Savinov, V. M. Berezin and I. I. Yermakov. The high specific activity Cue* sources were prepared b y the cyclotron crew headed b y B. M. Stasevich. The autl~rs wish to express their thanks to all of-them. References 1) 2) 3) 4)
L. D. Landau, Nuclear Physics 3 (1957) 127 T. D. Lee and C. N. Yang, Phys. Rev. 104 (1956) 254 N. Sherman, Phys. Rev. 103 (1956) 1601 A. I. Alikhanov, G. P. Yeliseyev, V. A. Liubimov and B. V. Ershler, Nuclear Physics (in press) 5) H. Frauenfelder, R. Bobone, E. von Goeler, N. Levine, H. R. Lewis, R. N. Peacock, A. Rossi and G. de Pasquali, Phys. Rev. 106 (1957) 386
/} ON T H E P O L A R I Z A T I O N OF E L E C T R O N S IN [3-DECAY M. E. V I S H N E V S K Y , V. K. G R I G O R I E V , V. A. E R G A K O V , S. J. N I K I T I N , E. V. P U S H K I N A N D YU. V. T R E B U K H O V S K Y
U.S.S.R. Academy o/ Sciences, Moscow Received 9 April 1957
Extending the concept of non-conservation of parity in weak interactions, introduced by Lee and Yang 2), Landau has recently suggested 1) that a so-called two-component or longitudinal neutrino m a y exist. According to this idea the neutrino is polarized along its momentum. One of the possibilities of proving this hypothesis would be to s t u d y the polarization of r-electrons since the electrons emitted in allowed transitions should be longitudinaUy polarized, the polarization being equal to v/c. We accordingly performed experiments with electrons emitted by Cu e~ (AJ = 1, no). In order to observe longitudinal polarization, the electrons emitted by Cu 84 were deflected in an electric field of a cylindrical condenser and the azimuthal a s y m m e t r y in the number of electrons scattered by a thin foil was studied. The angle between the spin and momentum of the electrons deflected within the condenser was 90 °. A deflected beam of 145 keV electrons (v/c = 0.63) was scattered by a thin gold foil, the direction of the beam making an angle of 45 ° with the plane of the scatterer, The scattered electrons were counted by means of two tel~copes, each containing two G-M counters. The axis of the telescopes formed an angle equal to 180 °. In order to determine the azimuthal asymmetry the telescopes and scattering target were rotated about the beam axis and the number of scattered electrons at various azimuthal angles were counted; one of the telescopes thus counted electrons reflected by the foil and the other electrons transmitted by the foil. The source consisted of a thin layer of CuO (=< 2 mg/cm ~) deposited on a 0.5 mg/cm 2 A1 foil. The electrons were scattered by a thin gold layer, evaporated on a 0.04 mg/cm 2 cellophane film. Gold layers of 0.120 mg/cm ~ and 0.240 mg/cm 2 were used. An azimuthal asymmetry in the number of scattered electrons, corresponding to an electron polarization opposite to the direction of momentum (right-left asymmetry) was observed. 271
272
M. E. VISHNEVSKY 8~ al.
The value of the observed asymmetry A -~ ( 2 1 - I ) / ( I +I.._) averaged over several runs for electrons penetrating the scatterer, measured with two slightly different arrangements, was found equal to 0.35 4- 0.05 and 0.27 4- 0.05 respectively. The asymmetry obtained with electrons reflected b y the scatterer was considerably less than the results given above owing to a larger multiple-scattering effect within the scatterer. The errors given correspond to the statistical errors only and do not take into account depolarization effects within the source, multiple scattering within the target or possible small apparatus errors. No up-down asymmetry exceeding the statistical errors was observed. The theoretical value for the asymmetry in the case of the two-component neutrino is equal to 0.34 3); our result is in qualitative agreement with this value. At present a more refined experimental study of the polarization of relectrons is being carried out with the aim of quantitatively checking Landau's theory. A similar investigation has recently been performed b y A. I. Alikhanov, G. P. Yeliseyev, V. A. Liubimov and B. V. Ershler *) with Sr 9° and ygo. Some time before completion of our experiments we were informed about the experiments performed b y the Illinois University group 5). The data obtained b y both of the aforementioned groups of investigators closely agree with our results. The authors wish to thank V. V. Vladimirsky for numerous helpful discussions. The apparatus used was constructed b y N. I. Afanasiev, E. G. Savinov, V. M. Berezin and I. I. Yermakov. The high specific activity Cue* sources were prepared b y the cyclotron crew headed b y B. M. Stasevich. The autl~rs wish to express their thanks to all of-them. References 1) 2) 3) 4)
L. D. Landau, Nuclear Physics 3 (1957) 127 T. D. Lee and C. N. Yang, Phys. Rev. 104 (1956) 254 N. Sherman, Phys. Rev. 103 (1956) 1601 A. I. Alikhanov, G. P. Yeliseyev, V. A. Liubimov and B. V. Ershler, Nuclear Physics (in press) 5) H. Frauenfelder, R. Bobone, E. von Goeler, N. Levine, H. R. Lewis, R. N. Peacock, A. Rossi and G. de Pasquali, Phys. Rev. 106 (1957) 386