Jörg R. Hörandel Homepage Joerg R. Hoerandel
Measurement of drift velocities of electrons in gases
An apparatus has been developed to measure drift velocities of electrons in
gas mixtures. Free electrons are liberated in the gas by laser ionization and
the drift time is measured with a two-beam differential method.
The experimental set-up allows the determination of electron drift velocities
as a function of the reduced electric field E/p. Absolute pressures up to
2500 hPa can be realized.
The drift chamber contains 40 electropolished stainless
steel ring electrodes, separated by 10+/-0.1 mm using Teflon spacers. The
electrodes are interconnected by a precision resistor chain to form a
homogeneous drift field. The stack is stabilized by four rods made of
polyoximethylen (POM), a plastic material with high dielectric strength which
combines hardness and form stability.
The top electrode is connected to a high voltage supply which provides an
output voltage up to -50 kV, the bottom electrode is on ground potential. At
a total stack length of 520 mm, drift fields up to approximately 960 V/cm are
possible. Each electrode has a central hole of 25 mm diameter. The electrons
drift through these holes in a homogeneous electric field to a sense wire
below the bottom electrode. The wire is partly surrounded by an u-shaped
cathode on ground potential. In order to protect the signal wire from
induced-charge effects, the central hole of the bottom electrode is covered
with a nickel Frisch grid with suitable spacing to allow high transmission
coefficients for electrons ( 20 micron wire diameter, 234 micron wire
The drift range is mounted inside a cylindrical pressure vessel
made of stainless steel (612 mm height, 257 mm diameter).
The pressure vessel is equipped with four pairs of opposite quartz windows
which serve as beam entrance and exit. Thus, three different drift lengths
of dx=117+/-1 mm, dx=234+/-1 mm and dx=351+/-1 mm are available. The gas
pressure inside the vessel is measured with an accuracy of 0.5%.
Measuring dT, the arrival time difference between two individual
electron signals belonging to one single laser pulse, provides v=dx/dT,
the drift velocity component parallel to the electric Field E.
The counting gas in the chamber is ionized with a 337 nm N2 laser
(MSG 803 SD, LTB Lasertechnik, Berlin, Germany) with a pulse duration of 500
ps and an average laser pulse energy of 500 mm J. It operates at a maximum
repetition rate of 30 Hz. The laser beam is split up into two separate beams
which are focused on the axis of the drift chamber. At the focus, the UV
light produces electron clusters essentially by ionizing impurities in the gas
via multi-photon absorption processes.
The anode wire signals are capacitively decoupled and fed into a spectroscopy
amplifier. From the signals of the first and second electron cluster arriving
at the wire a start and stop pulse for a TAC (Ortec 566) are derived in a
logic unit consisting of standard NIM modules. An UV-sensitive photodiode
placed behind the lower exit window delivers a gate to the logic unit for each
laser pulse in order to avoid cosmic-ray generated background. The TAC output
signal is digitized by a voltage-sensitive ADC (National Instruments
PCI-MIO-16E DAQ card) in a PC. PC software allows to automate series of
measurements stepping through the range of high-voltage settings.