6.1 Errors Specific to Mott Polarimetry

Gay & Dunning [10] identify two sources of systematic error in Mott polarimeters:

• The incoming electron beam may be misaligned (figure 6.1.)

  Figure 6.1: A Misaligned Incident Electron Beam. Based on a diagram by Gay & Dunning [10]

  

• The two detectors may have efficiencies $\eta _1$ and $\eta _2$, which are less than unity. More crucially for asymmetry measurements (equation 2.6,) it may be that $\eta _1\neq\eta _2$.

Gay & Dunning [10] show that each of these effects introduces a multiplicative relative systematic error in the electron arrival rate at one channeltron compared with that at the other, which invalidates the use of the raw asymmetry as a polarization indicator (equation 2.6.) A technique has been developed [10,14] which allows the elimination of a systematic error of this nature. In this scenario, the rates of electron arrival $R_{ij}$ on channeltrons $i=1$ and $i=2$ for beams of polarizations of equal magnitude $\vert P\vert$ in opposite directions $j=\uparrow$ and $j=\downarrow$. The polarization is then given by [10,14]

$$SP=\frac{\sqrt{R_{1\uparrow }R_{2\downarrow }}-\sqrt{R_{1\do... ...rrow }R_{2\downarrow }}+\sqrt{R_{1\downarrow }R_{2\uparrow }}}$$ (6.1)