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The
FWT LET-2 is a spherical tissue equivalent (TE) proportional counter
suitable for measuring the spectra of absorbed dose in LET (actually
P[Y] spectra). It is based upon designs originated by Dr. H. H.
Rossi of Columbia University and uses a spiral grid for uniform
collection characteristics.
The detector is housed in a 0.060 inch-thick aluminum shell which
serves as a vacuum tight housing. The aluminum shell is thus watertight,
but the electrical connectors are not. The aluminum shell is grounded
electrically and is sturdy enough to allow direct clamping.
The counter is designed to be evacuated and filled by the user with
TE proportional counting gas (either propane or methane based) at
a low pressure, typically 1 to 20 cm Hg. These low pressures enable
the spherical detector to simulate small tissue volumes from less
than 1 to more than 8 microns in diameter.
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Spectral
Measurements
Connection of the detector to a preamplifier 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.
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Specifications
 Tissue-Equivalent
(TE) Plastic Sphere: Wall thickness, 0 .1 25 inch (0-318 cm); inside
diameter, 2.24 inch (5.69 cm); Shonka Type A-150 plastic, 1.12 9/CM3.
Gas Filling
Requirements: TE proportional counting gas; 1.17cm Hg of TE gas
of density 1.062 g/liter corresponds to a cavity of 1 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. Output
is AC coupled.
Signal Connector:
Mates with BNC UG260 B/U or equiv.
High Voltage
Requirements: Positive polarity, @700 volts at 1 .17 cm Hg, @750
volts at 2.0 cm Hg.
High Voltage
Connector: MHV, Mates with UG932 A/U or equiv.
Optional
Internal Source: CM244, ~0.3 microcuries; average alpha energy ~5.80
meV; gravity operated shutter.
Exterior
Can: Vacuum tight aluminum; wall thickness, 0.060 inch (0. 152 cm)
, grounded to serve as electrostatic shield, maximum O.D. of detector
3.62 inches.
Overall Size
and Weight: Length including vacuum connector: 6.125 inches (15.5
cm) ; weight, 560 grams
Calibration:
Includes a graph of the detectors alpha peak and a graph of Cf252
spectra.
Modifications
Available: No alpha source.
Ordering Information
LET-2: Tissue
equivalent proportional counter with storage box
-Cm244 Curium
244 source in detector option
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