Far West LET-SW2 Operation Manual - page 4
MODEL LET -2
FAR WEST TECHNOLOGY
3
III.
FILLING THE INSTRUMENT WITH GAS
The standard instrument is provided with a Quick-Connect manufactured by Crawford Fitting Company. This
connector mates with a Swagelok B-QC4-D-400 DESO-type connector. These connectors contain spring loaded
plungers that seat against elastomeric o-rings providing a gas-tight seal. Experience will allow the gas filling tube
to be "snapped off" the instrument with no detectable change in instrument gain. Dirt must not be allowed to get
into the Quick-Connect. It will invariably cause instrument gain shifts with time. When a bellows valve is used,
this is not a problem.
The usual procedure for gas filling is, initially, a pump down to 5 to 10 microns with a good quality rotary pump.
Liquid nitrogen cold traps may be useful, but have not been found necessary. The instrument is then filled to about
50 cm Hg with tissue equivalent gas and pumped down to 5 to 10 microns. This procedure may be repeated if the
instrument has not been in use for some time.
The instrument is next filled to the proper pressure for operation. This is determined by the tissue equivalent gas
density. Our usual gas mixture is 29.9% CO
2
, 2.75% N
2
and 67.54% CH
4
by volume with a density of 1.062 grams
per liter at 20 c and 760 mm Hg. The instrument cavity is 2.24 inch in diameter (5.69 cm) and thus a pressure of
1.26 cm Hg will simulate a cavity of about 1 x 10
-6
meter diameter in density 1.00 tissue. This effective diameter
can be made smaller or larger by variation of the gas pressure. A more recent tissue equivalent gas mixture
5
composed of 39.6% CO
2
, 5.4% N
2
and 55% propane has been used with equivalent results. Addition of 10%
isobutane will provide better operation at very high multiplications.
Gas purity is of considerable importance in counter operation. Poor resolution, gain or a low voltage arc-over point
are evidences of gas problems. Water in the gas will invariably cause arc-over. A dew point of -40°c or lower is
recommended.
IV.
ELECTRICAL CHARACTERISTICS
The end plate carries the signal and high voltage connectors. The high voltage should be negative with respect to
ground, i.e. the center wire of the H.V. cable should be positive with respect to the shield. The voltages needed for
operation is given below:
Counter Diameter
Gas Pressure
Negative Operating
Volts
Arc-Over Voltage
Methane Base TE
1 micron
1.26 cm Hg
600-700
800
2 micron
2.52 cm Hg
700-800
950
Propane Base TE
1 micron
0.74 cm Hg
500-600
650
2 micron
1.48 cm Hg
600-700
750
Arc-overs should be avoided due to the inevitable insulator carbon tracking which occurs. This can seriously
degrade the chamber performance. Large arc-overs can transfer enough charge to destroy the input FET on some
preamplifiers. If a solid-state preamplifier is used, it is usually necessary to limit the rate of rise of the high voltage
so that switching transients do not destroy the first amplifier stage semi-conductor. If high voltage is applied to the
bias connector on your preamplifier, use a positive voltage.
The signal from the detector appears on the connector block marked SIG. This connector mates with BNC cable
connector UG-260/U. Low noise cable such as Microdot (Malco) 250/3834-0000 should be used for best low noise
performance. The cable length should be as short as possible to reduce input capacity. There are no coupling
capacitors in the signal circuit. The pulses produced are negative going.