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Spectral Info
Total Dose Measurements
Ordering Info



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.

  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.

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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Specifications subject to change without notice.
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