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The
FWT LET-SWI/2 is a spherical tissue-equivalent (TE) proportional
counter suitable for measuring the spectra of absorbed dose in LET
(actually P(Y) spectra) or absorbed dose directly. In the latter
case the counter acts as a multiplying ionization chamber.
The detector is housed in a 0.007-inch-thick aluminum shell. This
detector utilizes a single wire design based on work by P.W.Benjamin.
The aluminum shell and attached stem are completely submersible
and capable of being operated in any of the common TE fluids. The
active volume of the detector is 0.500 inch in diameter and has
a 0.05-inch-thick wall of TE plastic. The stem is approximately
10 inches long and has a connector block at one end which contains
the high-voltage and signal connectors and a vacuum fitting. The
block is suitable for clamping the detector to laboratory fixtures
and is gasketed for moisture resistance.
The counter is designed to be evacuated and filled by the user with
TE proportional-counting gas at a low pressure, typically 2.5 to
40 cm Hg. These low pressures enable the spherical detector volume
to simulate small tissue spheres from less than 1 to more than 8
microns in diameter.
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Spectral
Measurements
Connection of the detector to a preamplifier, such as used with
Ge(Li) detectors, and a linear amplifier will enable a P(Y) spectrum
to be accumulated on a pulse-height analyzer when the detector is
exposed to a radiation source. Proper choice of amplifier gain settings
will enable data to be obtained for Y values from 0.5 to 1000 keV
per micron. These data may be converted to a spectrum of absorbed
dose in LET with the aid of specific software.
The optional internal Cm244 alpha source makes calibration of the
Y axis in keV per micron particularly simple. The source allows
collimated alpha particles to cross the detector on a diameter,
producing a peak in the pulse-height analyzer display. The peak
channel is characteristic of the average keV per micron expended
by the alpha particles in traversing the sphere. The alpha source
has a gravity-operated shutter mechanism that allows operation free
of unwanted alpha counts when desired.
Total Dose
Measurements
Connection of the detector to an electrometer permits simple absorbed
dose measurements. Because of the tissue equivalent design the dose
measured will be rads-tissue. Very simple techniques will yield
sensitivities of 4 x 10-11 A/rad/ hr. Special guard ring construction
reduces detector leakage to 1 to 2 x 10-13 A at this multiplication.
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Specifications
 Tissue-Equivalent
(TE) Plastic Sphere: Wall thickness, 0.050 inch (0.127 Cm); inside
diameter, 0.500 inch (1.27 Cm); Shonka Type A-150 plastic.
Gas Filling
Requirements: TE proportional counting gas; 5.25 cm Hg of TE gas
of density 1.062 g/liter corresponds to a cavity of I micron diameter;
gas must be made up of high purity components and be moisture free.
Gas Filling:
Bellows seal valve, Connector mates with Swagelok O-QC4-D-400
Signal Output:
Suitable for use directly into a low noise preamplifier.
May also
be applied directly to the input of an electrometer for absorbed
dose measurements and yields 4 x 10-" A/rad/hr. at 1200 V and
40 cm Hg TE gas pressure.
Signal Connector:
BNC Mates with BNC UG260 B/U or equal.
High Voltage
Requirements: Negative polarity, -450 volts at 2.6 cm Hg, -500 volts
at 5.63 cm Hg, -800 volts at 10.5 cm Hg.
High Voltage
Connector: MHV Mates with UG932 A/U or equiv.
Optional
Internal Source: CM244,~0.3 microcuries, average alpha energy 5.8
MeV, gravity operated shutter.
Exterior
Can: Vacuum tight aluminum; wall thickness, 0.007 inch (0-0178 cm);
grounded to serve as electrostatic shield; maximum O.D. of detector
end 0.825 inch (2.1 cm).
Overall Size
and Weight: Length including vacuum connector 15,75 inches (40.0
cm); weight, 230 grams; Stem 10 inches (25.4 cm) long
Modifications
Available: Thicker aluminum can - up to 0.020 inch, @0.051 cm):
no alpha source; 0. 1 00 inch thick wall.
Ordering Information
LET-SW1/2:
Tissue equivalent proportional counter with storage box
-Cm244 Curium
source in counter option
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