V_||-Alpha: Setup

Measurement procedure

The test chamber is located in the center of the cyclotron magnet with wires horizontal (z coordinate) in the vertical (y) B field.

The beam of the UV laser is deflected via a movable mirror into the chamber volume. The beam of the N₂ laser (337 nm) is sent from the former experimental cyclotron hall into the cyclotron containing the chamber. The beam is refocused by a f=1 m quartz lens to provide ~200 e^-/cm of path of ionization in the chamber.

Schematic of the test chamber, x-y section. The mirror moves along x and parallel to the electric field E.

z-x section, including three beam positions (shown in red) for differential measurements.*

The chamber and the gas lines are first evacuated and then filled with the desired gas mixture thereafter continuing with a flow of one volume exchange per hour. The gas is premixed in a container tank by adding partial pressures. This method is accurate to <0.3% for binary mixtures. The high voltage on the wires is adjusted, for ~200 mV peak response to laser shots after amplification (24 mV/uA). The gas gain is ~4*10⁴, which is far from saturation effects.

100 events (laser shots) are taken in each of the three mirror positions x_l, x₂, and x₃, with x₂-x₁ = x₃-x₂ = 10.00+-0.01 mm setting error. Since x is parallel to E, the recorded times measure the parallel drift velocity component of interest by

v_||(E,B) = (x₃-x₁)/(t₃-t₁),

which is consistent with (x₂-x₁)/(t₂-t₁) to within 0.6%. The distribution of arrival times from laser shots at the same x position has a 4 ns rms time jitter. This variation causes <0.15% error in the measured drift velocity.

The pad charges Q_i, (i = 1,..., 20) are recorded from outputs of amplifiers calibrated to 0.5%. The y coordinate along the wire is derived from the weighted average

y = (·y_iQ_i)/(·Q_i)

The nonlinearity has been checked with micrometer controlled mirror movements and was found to be <200 um. The statistical fluctuation of each y measurement is 66 um in rms. The drift angle due to the Lorentz force depends on the gas as well as on the magnitude of E and B. and is determined by

alpha(E,B) = tan^-1[(y₃-y₁)/(x₃-x₁)].

Note that both drift velocity and deflection angle are differential measurements, which eliminates the effect of different behavior of the amplification gap.

Because the differential measurements of positions 1-2 and 2-3 agreed well, the data sets past 1995 only use positions 1-2.

Data Adjustment

Because sea level is 760 torr and most US labs are at 24 degrees C or higher, the E fields have been corrected for this.

Corrected E Field = (E field)*(760/pressure)*((273+temperature)/(273+24))

References