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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.

  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.

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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Specifications subject to change without notice.
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