Fluke Biomedical 943-35 - Manual (Page 8)

Victoreen 942-35,943-36,943-36H,943-37,943-237A
Operators Manual
1-4
Max. Count Rate 2E7 CPM
Operating Temperature 32° to 122°F (0° to 50°C), (160°F for 943-36H)
Storage Temperature 32° to 122°F (0° to 50°C), (160°F for 943-36H)
Relative Humidity 0 to 95% non-condensing
1.5 Functional Description
Refer to Figure 1-1 for aid in understanding the functional description.
Figure 1-1. Typical Model 943-36 Series Gamma Scintillation Detector (Not Drawn To Scale)
Gamma or x-rays that have enough energy to penetrate the crystal or outer housing and interact with the
Nal crystal scintillator, will produce light pulses proportional to the energy deposited in the scintillator. A
photomultiplier tube, optically coupled to the scintillator detects visible light emitted and converts this light
into pulses of electrical energy, whose voltage is proportional to the energy deposited. The electrical
pulses are sent to the preamplifier in the detector housing.
The preamplifier circuit amplifies the pulses received from the photomultiplier tube/scintillator assembly
with a fixed gain of six (6). The current drive output of the preamplifier circuit will drive a 50 ohm
transmission line up to 1500 feet without degradation of the signal.
Should a very high intensity radiation source be detected, the photomultiplier tube may become saturated.
That is, visible light from successive photon collisions within the scintillator may occur so often that they
are essentially continuous. This would have the effect of holding the output voltage of the preamplifier at a
relative constant output voltage. Since the readout is designed to count pulses that represent individual
particle collisions, a high intensity signal could be interpreted as a low or no radiation condition. The
readout device should include an overrange setpoint of 2E7 CPM that will result in the display of an
overrange error message instead of a radiation value. Although the actual radiation value will not be
displayed under this extreme condition, it will prevent misrepresentation of the radiation value during
overrange conditions.
Integral Sodium Iodine (Nal) Crystal/Photomultiplier Tube
The Nal crystal and photomultiplier tube are supplied as an integral assembly within the detector housing.
The Nal crystals used in the detector are single crystal elements. They are hermetically sealed into a
metal can with an optical window on one end of the can to permit the light pulses to escape the crystal.
The interior of the can is lined with a highly reflective media to enhance the collection of light.
Outer Housing