MadSci Network: Physics

Re: What're the criteria for radioactive sources for use in level measurement ?

Date: Sat Aug 14 18:00:33 2004
Posted By: Michael Kay, Haz. Mat Mgmt, Health Physics, Nuclear Science, President and Consultant AMBRY, Inc
Area of science: Physics
ID: 1091268894.Ph

Your question needs to be divided into two parts at the beginning. 
Monitoring levels of liquids in containers requires different techniques 
and radionuclide gauges than determining thickness of paper, plastic, or 
metal while being formed or manufactured.

Liquid level gauges usually use a gamma-ray emitting radionuclide such as 
Cs-137 (half life approximately 30 years) or Co-60 (half life 
approximately 5 years). Co-60 emits a higher energy, more penetrating 
gamma ray than Cs-137. Generally, the higher the energy emitted, the 
shorter the half life of the emitting radionuclide. The main 
consideration in the choice of Cs-137 or Co-60 is the density of the 
material (water, other liquid, solids) that the beam of ganna rays 
penetrates before reaching the gamma-ray detector on the opposite side 
from the radiation source. Cs-137 is commonly used for liquid level 
determination is small containers such as packaged drinks, drug vials, 
liquids is aluminum cans (360 ml or 12 oz) and many other consumer 
packages. Co-60 is used in industrial applications such a determining 
coal level in a storage facility, density of materials flowing in steel 
pipe, and level determination in industrial sized distillation columns.

Beta emitting radionuclides are commonly used in gauges to measure the 
thickness of films or webs that are continually produced (paper, plastic 
film, glass-fiber impregnated plastic film used to make circuit boards, 
or metal foils such as aluminum foil). The film runs at high speed 
between a radioactive source and a detector. The detector signal strength 
is used to control the plastic film thickness. In paper manufacturing, 
beta gauges are used to monitor the thickness of the paper at speeds of 
up to 400 m/s. 

Beta particles (fast moving electrons) are more easily absorbed than 
gamma rays, so that is why they are used for this purpose. The most 
common beta gauges for films having a low mass density (paper, plastic, 
fiberglass) use Kr-85, a radioactive noble gas with a half life about 11 
years. For materials with a higher mass density, such as metals, a higher 
energy beta particle may be needed, so Sr-90 with a half life about 30 
years is used in these gauges. 

For a film (foil, metal, plastic) travelling horizontally, called 
a "web", the source is housed in a shield with a small, shutter 
controlled, aperture that forms a beam directed through the web. The 
source holder is one one end of device that looks like a large tuning 
fork. The other leg of the device holds the detector exactly opposite the 
source aperture. A web gauge moves back and forth at right angles to the 
web, so it traces a repeating diagonal path across the web. Using known 
thicknesses of the material being gauged, the gauge is calibrated so that 
it can read out in a form equivalent to thickness of the web. 

For information on beta and gamma decay, interaction of radiation with 
matter, and a couple of pages on industrial process control, I 
recommend "Radiochemistry and Nuclear Methods of Analysis" by William 
Ehmann and Diane Vance,  Wiley-Interscience, New York, New York, 1991
ISBN: 0-471-30628-2 

One internet reference I found that may be useful is:
This is a table of uses of radionuclides, and includes level and 
thickness gauges.

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