Radiometric Measurement with
Thurning Instruments Ltd.
What is "radiometric measurement"?
How do you find thickness or density from the measurement?
So, what can we do with this technique?
Often known as "nucleonic" or "gamma" gauging, the technology is the use of X-rays or gamma rays to measure thickness or density.
If you shine a beam of radiation at a material, such as a strip of steel, or water in a pipe, some of the radiation will be absorbed and some will pass right through.
If you know how much radiation is shining on the material and you know how much gets through, you can deduce how much of the material there was in the way of the radiation beam.
This means that you can measure materials that you don't wish to to touch. You can measure through pipes and the measurement will not disturb a flow regime. The instrument will not create a contamination trap in hygienic systems or scratch the surface of a web product.
These advantages are some of the reasons why you may need a radiometric measurement system.
Isn't
radiation dangerous?Yes, of course it is,
if you are exposed to too much. However, we are exposed to naturally occurring radiation from our environment all of the time, the so-called "natural background", so we can tolerate low levels of radiation with no ill effects.People worry about the radiation used in the instrument: this is natural, because we have all heard the consequences of accidents with radioactive materials.
X-rays and gamma rays are
X-rays and gamma rays are
not dirt. They don't stick to the material you are measuring, they don't make it radioactive and you can't breath them in. Like light, they travel in straight lines until they hit something.Thurning Instruments' measurement systems use both X-ray and gamma ray sources.
X-ray sets have technical advantages for making fast measurements: they are
very intense radiation sources. They also have the advantage that they can be switched off when not required.Gamma ray sources are simply radioactive materials such as "Caesium-137" or "Americium-241" in very robust containers. They are much less intense sources of radiation than X-ray sets, but offer better stability and, frequently, lower costs. There are cost and technical advantages in using gamma sources for slow measurements and for measuring very thick materials. Gamma sources cannot be switched off so they have to be installed in a
shielded container with a "shutter" that can be opened to let a beam of radiation out when required.At Thurning Instruments Ltd. we aim to achieve the highest standards of safety with our measurement systems. People who work with them do not normally need to wear a film badge or to be "classified radiation workers", because the radiation dose that they receive from the instrument will not be detectably more than they get from "natural background".
How do you find
thickness or density from the measurement?It is time for a diagram:
This shows a radiation source, emitting a beam of radiation. A collimator (frequently a piece of lead, brass or tungsten with a hole in it) defines a suitably shaped beam. The beam "shines" on the material that you wish to measure and part of it is absorbed. The detector then measures the intensity of the beam that comes out of the other side.
The intensity of the partially absorbed beam is compared with intensity of the unobstructed beam so that we know how much of the radiation was absorbed. The equation is:
I=I0e-
m r xWhere:
I = intensity
I0 = Intensity of the unobstructed beam
e = 2.71828...
m
= absorption coefficient, a function of the material and the energy of the radiation beamr
= bulk densityx = thickness
So: if you know the material and its dimensions, you can deduce the bulk density in the radiation beam. And if you know the density you can deduce the thickness. For real measurements the absorption coefficient is best found by calibration, with a standard sample of known dimensions and density.
Thurning's detectors are normally arranged to measure the unobstructed beam and the partially absorbed beam without a change to the instrument's calibration. This means that you can get an extremely accurate zero measurement. This is only possible due to the exceptional dynamic range of the
RMS1000 and RMS2000.The output from the instrument is routed to a computer: for NDT and experimental applications the instrument readings are normally logged and processed on a PC using "Microsoft Windows" based software. For real time control applications MS Windows is unsuitable and a real time operating system must be used. In practice, that frequently means DOS, or Windows NT with a "real time kernel", and carefully written software in 'C'.
The RMS series instruments can measure at 1ms intervals: this is fast enough for the majority of applications, and is also fast enough to keep a 200MHz PC busy as it receives the output and processes it.
The main uses are thickness or density measurement, as we have seen above. However, thickness and density are frequently equivalent to some other characteristic of the material you are measuring.
Fluid and slurry flows:
You know the dimensions of a flow in a pipe, so you can easily measure the density.
In some applications, the method is used to measure density so that the mass flow in the pipe can be calculated. This is an important measurement if you have a mixture of phases, such as steam and water or oil and gas. The oil industry is interested in mixtures of oil, water, gas and sand.
People in the chemical industry also measure multi-phase mixtures to obtain mass flow rates; the technique is also used to measure the concentration of solutions and solids loading in slurries.
Nuclear and chemical industry researchers have long used the technique for high accuracy measurement of very fast multi-phase flows, the photograph below shows a complex installation with two multi-beam densitometers. The densitometers measure the density of the fluid in the pipe and cross correlation between the two systems is used to measure the flow velocity. This system was designed to study explosive decompression of a chemical reactor vessel.
Chemical industry safety experiment at JRC Ispra, Italy.
Geological samples:
Geologists frequently need to drill and take drill core samples in their investigations. Again, the dimensions of the drill cores are either well known or easily measured by mechanical or optical means. An interesting example of this is the study of the atmosphere in prehistoric times.
Ice Core densitometer used by The Alfred Wegener Institut.
The densitometer uses a narrow beam of radiation to locate air bubbles in the core sample. The air from the bubbles is analysed to study atmospheric change: this has been very important in recent years in supporting the research into global warming. The same technique is also suitable for detecting micrometeorites.
Geologists also use gamma density measurements when they take core samples for mineral exploration. The technique can be adapted to preferentially detect heavy elements (typically tin and heavier) or to give a straight density reading with minimum sensitivity to the chemical composition of the sample.
Web products:
Web products are one of the most economically important tasks for radiometric measurement. The main web products that are measured with radiation based gauges are sheet and plate steel and other metals, plastic sheet and film, and paper.
Metal sheet and plate is manufactured from large billets of material. The billets are shaped by rolling first in hot rolling mills and then in cold rolling mills. The speed and value of "strip rolling" operations comes as a surprise to many who are not familiar with the industry: cold rolled steel frequently travels through the rolling mill at 60 to 100 kilometres per hour. And the speed and productivity of mills is increasing all the time.
In cold rolling mills radiometric gauges have two main advantages: they do not touch the strip, so they cannot scratch it, and they are capable of precision measurements at higher strip speeds than can be achieved with mechanical gauges.
In hot mills, the steel may be at a temperature of 850
The paper industry is a large user of "beta gauges", using beta rays instead of X- or gamma rays. It is possible that as the speed of paper mills increases (modern installations are already faster than strip rolling mills) there will be a move to using low energy X-rays. As the speed of the mills increases the measurements must be made faster, so the radiation source must be more intense. Very large isotope sources can be a nuisance on what is basically a "non-nuclear" plant, and X-ray systems are more acceptable when a very intense source is required. The plastics industry is already using low energy X-rays for gauging.
Pressed products and castings:
This is another area where you know the thickness of your product. Radiometric measurement is used as an inspection tool for quality assurance purposes.
Systems are in service for inspection of pressed refractories, ferrites and for detecting flaws in castings.
Radiometric densitometers are best used for detecting "volume defects" rather than cracks or laminations. When is a "void" so small that it become a crack or lamination? It depends how closely you look: if you reduce the volume that you inspect with the densitometer, you will reduce the size of the smallest detectable defect. Of course there are practical limits to what you can achieve, depending mainly on how much time you have to make the measurement.
There are more applications for radiometric measurement than we could possibly think of, what do you need to measure?
Last updated 06-02-2004
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Richard Fortescue