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Spectral Info
Ordering Info


The FWT LET-1/2 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 thin aluminum shell which serves as a vacuum tight housing. The 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 has a connector block at one end carrying 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 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 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, 0.050 inch (0.127 cm); inside diameter, 0.500 inch (1.27 cm); Shonka Type A-150 plastic, 1.12 g/cm .
Gas Filling Requirements: TE proportional counting gas; 5.63 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. Output is AC coupled.
Signal Connector: Mates with BNC UG260 B/U or equal.
High Voltage Requirements: Positive polarity, 450 volts at 2.6 cm Hg, 550 volts at 5.25 cm Hg, 650 volts at 10.5 cm Hg.
High Voltage Connector: MHV mates with UG932 A/U or equivalent.
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 length: 10”
Modifications Available: Thicker aluminum can, up to 0.020 inch, (0.051 cm); no alpha source; 0.100 inch thick wall.
Calibration: Includes a graph of the detectors alpha peak and a graph of Cf252 spectra.

Ordering Information
LET-1/2: Tissue equivalent proportional counter with storage box
-Cm244 Optional Curium source in counter

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Specifications subject to change without notice.
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