Asymmetry In Mott Research Articles

Polarized electron Mott scattering model for the Geant4 Monte Carlo toolkit

A new method for simulation of polarized electron interactions with matter, based on the Geant4 Monte Carlo toolkit, is presented. The extension consists of a Mott scattering model taking into account the polarization dependence of the cross section, as well as the change of electron polarization in the scattering. The results regarding azimuthal asymmetry in Mott scattering of polarized electron beams off gold and lead targets are compared to available experimental data for energies up to 14 MeV.

A new measurement of electron transverse polarization in polarized nuclear β-decay

The Mott polarimetry for T-violation (MTV) experiment tests time-reversal symmetry in polarized nuclear [Formula: see text]-decay by measuring an electron’s transverse polarization as a form of angular asymmetry in Mott scattering using a thin metal foil. A Mott scattering analyzer system developed using a tracking detector to measure scattering angles offers better event selectivity than conventional counter experiments. In this paper, we describe a pilot experiment conducted at KEK-TRIAC using a prototype system with a polarized 8Li beam. The experiment confirmed the sound performance of our Mott analyzer system to measure T-violating triple correlation (R correlation), and therefore recommends its use in higher-precision experiments at the TRIUMF-ISAC.

Measurements of the transverse polarisation of K-shell photoelectrons produced by unpolarised 334 and 439 keV photons in thorium

An experimental investigation of the transverse polarisation normal to the production plane of K-shell photoelectrons from a thin thorium (Z=90) converter produced by unpolarised 334 and 439 keV gamma rays has been carried out. Neutron capture gamma rays from an in-pile target enriched in 149Sm provided an intense source of photons. The photoelectrons were analysed in momentum with a high resolution, iron beta spectrometer which viewed the combined target plus converter situated near the core of the high flux reactor at the Institut Laue-Langevin. The electrons were detected after being scattered by a thin Bi foil in four silicon surface barrier detectors. The polarisation was deduced from the left-right asymmetry in Mott scattering from the Bi foil at backward angles. The left-right asymmetry in the two forward detectors allowed correction for spurious and instrumental asymmetries. The measurements were carried out at a mean angle of 45 degrees between the electron and photon momenta with the results: Egamma =334 keV, C02=0.074+or-0.018; Egamma =439 keV, C02=0.070+or-0.046.

Elastic electron scattering by mercury atoms

The cross section and Mott asymmetry function of elastic electron scattering by mercury atoms have been calculated for energies of 46, 79, 100, and 204 keV and the angular range 45‡–180‡. Some progress in the computational method is reported which remedies the difficulties at backward angles.

Intensity shift in small-angle multiple scattering of transversely polarized electrons

Mott asymmetry in repeated small-angle scatterings of transversely polarized electrons by heavy atoms produces a shift in the distribution of the particles emerging from the scattering foil. An experiment for the measurement of this effect is proposed and possible applications as a means of gaining information on the scattering law at small angles and as a detector for the transverse polarization of free electrons are discussed. Numerical data are given for electron energies between 100 keV and 300 keV.

Double scattering of electrons

The Mott asymmetry function for electrons with energies from 26 to 261 keV has been investigated with an improved arrangement for double scattering of accelerated electrons. First, a beam of singly scattered electrons with a reproducible and well defined degree of transverse polarization was obtained by performing a double scattering experiment with identical scattering conditions for polarizer and analyser. In this experiment scattering angles of 105° and gold foils of a standard thickness were used. The beam of singly scattered electrons was then used for calibration of a rotatable Mott analyser, consisting of a second scattering foil and four scintillation counters. Two counters were set at large scattering angles for detection of the asymmetry due to polarization and two were set at 45° for simultaneous observation and elimination of instrumental asymmetries. It is shown that the 45°-zero measurement is very effective in reducing the influence of several instrumental effects. The calibrated analyser can be ussed for polarization detection of other electron beams with a relatively large efficiency. The ratio of the numbers of incident and scattered electrons is about 0.6×10 −3. The instrument yields the absolute degree of transverse polarization of the incoming electrons with an accuracy of better than 1% in favourable cases. Measurements are reported of the energy dependence of the Mott function S at 45° and 105° and of its angular dependence at 121 keV. The results agree with those of Mikaélyan et al. 9) but have higher accuracy. Qualitatively the present results show the energy and angular dependence of S predicted by theory. Measurements at small angles show a change of sign, as do the calculations. However, quantitative agreement is often poor. Especially at low energies and small angles the calculated S values are significantly larger than the observed values.

Measurement of Mott asymmetry at 100 keV, of longitudinal electron polarization of Co 60 and Pm 147 from 40 to 210 keV and of a large source depolarization unexpected by theory

Measurement of Mott asymmetry at 100 keV, of longitudinal electron polarization of Co 60 and Pm 147 from 40 to 210 keV and of a large source depolarization unexpected by theory

Photon Asymmetry in the Bremsstrahlung of Transversely Polarized Electrons

There is a left-right asymmetry in the bremsstrahlung emitted by transversely polarized electrons analogous to the left-right asymmetry in Mott scattering. The magnitude of this effect has been calculated from the second Born approximation, where it appears in the cross terms between the matrix elements of order $Z{e}^{3}$ and those of order ${Z}^{2}{e}^{5}$. Principal contributions come from the Feynman diagram in which the photon is emitted between the Coulomb interactions with the nucleus, while the largest terms from the diagrams with the photon emitted before or after both Coulomb interactions are of relative order $\frac{v}{c}$ compared to the principal terms. The contributions of all three diagrams to relative order ${(\frac{v}{c})}^{2}$ are included in the final result. As is the case in Mott scattering, the asymmetry in bremsstrahlung depends only on intermediate states which lie on the energy shell. The reason for this has not been investigated here. It does not seem feasible to integrate the cross section over electron momenta analytically. In order to reduce geometric complexity, the calculation is performed in detail for forward electron momentum. The matrix elements for this case are listed explicitly so that other polarization-dependent effects may be conveniently calculated. An outline of the evaluation of the integrals which appear, and a discussion of the divergences associated with the no-cutoff limit of the Coulomb field, as they apply to bremsstrahlung, are included.

Mott-Scattering Analysis of Longitudinal Polarization of Electrons fromCo60

Mott scattering was used to analyze the degree of longitudinal polarization of beta particles emitted from radioactive nuclei. The reliability of this method and the influences of the various systematic errors associated with this methed on the accuracy of the measurement were investigated in detail and are discussed. On the basis of a linear extrapolation of the inverse of the Mott asymmetry to zero scatterer thickness, the polarization of 194 kev electrons from Co/sup 60/ was found to be - (0.994 plus or minus 0.057) v/c with all known corrections applied. The effects of atomic screening and finite nuclear size were not included. Using the quoted value for the polarization measured in the pure Gamow-Teiler transition in Co/sup 60/ yields C/ sub A/ = (0.7 to 1.45)C/sub A/. ( auth)

Determination of Electron Polarization by Means of Mott Scattering

The results of a numerical calculation of the polarization asymmetry factor, S( THETA ) = ( sigma ( THETA )!/sup 1/2/, and the single scattering cross section for the scattering of electrons by the unscreened Coulomb fields of gold and aluminum at energies of 75 and 121 kev are presented. The results for gold at 121 kev are compared with the Mohr and Tassie calculations for the same element and energy which include the effects of screening. This comparison (indicating a Mott asymmetry about 50% greater for the screened field at angles near 165 deg ) suggests the desirability of more detailed investigation of screening effects in order to be confident of Mott scattering as a means of measuring the polarization of electrons is this energy region. (auth)

Measurement of the Mott Asymmetry in Double Scattering of Electrons

The Mott asymmetry was observed at 121 kev for gold targets and scattering angles of ${\ensuremath{\theta}}_{1}=90\ifmmode^\circ\else\textdegree\fi{}$ and ${\ensuremath{\theta}}_{2}=80\ifmmode^\circ\else\textdegree\fi{} \mathrm{to} 140\ifmmode^\circ\else\textdegree\fi{}$. The cosine dependence of the asymmetry on the azimuthal angle was shown in all cases. A weak magnetic lens (maximum field: 12.5 gauss) was used between the two scatterers, and a low-resolution (8%) electrostatic energy analyzer was used after second scattering. The measured asymmetry amplitudes are compared with the calculations of Sherman (pure Coulomb field) and of Mohr and Tassie (screened Coulomb field). The experimental values for ${\ensuremath{\theta}}_{2}=90\ifmmode^\circ\else\textdegree\fi{}, 100\ifmmode^\circ\else\textdegree\fi{}, 110\ifmmode^\circ\else\textdegree\fi{}$ agree well with either theoretical curve but are not accurate enough to distinguish between them. The measured value at ${\ensuremath{\theta}}_{2}=80\ifmmode^\circ\else\textdegree\fi{}$ is enough higher (17%) than the pure Coulomb field curve to indicate a screening effect. The experimental values for ${\ensuremath{\theta}}_{2}=120\ifmmode^\circ\else\textdegree\fi{}, 130\ifmmode^\circ\else\textdegree\fi{}, \mathrm{and} 140\ifmmode^\circ\else\textdegree\fi{}$ fall from 15% to 20% below the theoretical curves. This discrepancy is attributed to plural scattering. Measurements made using the magnetic lens but not the energy analyzer yield asymmetries reduced in amplitude about 33% and shifted in phase by 26\ifmmode^\circ\else\textdegree\fi{} when compared with the measurements discussed above. This is believed due to the presence of an appreciable number of polarized electrons at a considerably lower energy than 121 kev in the first scattered beam.