To Next Page

To Previous Page

2–2

The Model 3440 gauge utilizes two modes of operation: direct

transmission mode (with the source rod extended into the

material) and backscatter mode. Figure 2 and Figure 3 illustrate

these two modes of operation.

WARNING!

The source rod should always be locked in the

SAFE position when the gauge is not in use.

Source rod positions are described on page 3–5.

In direct transmission mode, the rod containing the Cesium-137

(8 mCi/0.3 GBq) source is lowered to the desired depth. The

detectors in the gauge base measure the radiation emitted by

the source rod. Gamma photons reaching the detectors must

first pass through the material, colliding with electrons present

in the material. Generally, the lower the number of photons that

reach the detectors, the more dense the material is.

In backscatter mode, the gamma photons that enter the material

must be scattered (or reflected) to reach the detectors. With the

rod locked in the first notch, the source and detectors are in the

same plane, referred to as the backscatter position. Photons

emitted from the source penetrate the material and the

detectors measure the scattered photons.

While the direct transmission geometry measures the average

density of the material from the source to the surface, the

backscatter geometry yields an average heavily weighted by the

density close to the surface.

Figure 4 shows two normalized top layer effect curves,

illustrating the percentages of photons at the detectors for

various depths. The two curves can be used to compute the

gauge response to layered material of different densities. For

example, the density of the top inch of a surface layer accounts

for about 52% of the backscatter density measurement.

Troxler RoadReader 3430 User Manual

Questions and Answers: