Nucleonic measurement

Nucleonic gauges can be found everywhere where extreme conditions would mean the end for any other measurement technology. That’s because nucleonic devices measure contactlessly, which makes the measuring systems wear free and practically maintenance free as well. However, since this technology uses radioactive radiation, safety is the top priority.

The use of radioactivity for level, limit level, density or mass flow measurement has many advantages in comparison with other measuring methods, but also two disadvantages: nucleonic instruments are somewhat expensive and their operation requires special permits and safety precautions. That’s why nucleonic technology is only used in measurement engineering when measuring probes that protrude into the vessel are not able to handle the measuring task. Indeed, to this day, some areas of application do not allow any alternative to nucleonic measurement. This is because only measurement technology using gamma radiation is completely unaffected by high vessel pressures, corrosive media, extreme temperatures or problematic physical product characteristics and able to continuously deliver exact and reliable measuring results. Such demanding measurement applications, which require media to be detected without contact and without gauge maintenance, can be found primarily in large processing installations in the chemical and petrochemical industry, but also in the offshore and oil industry and in cement, power generation and sewage treatment plants.

In 1896, the French physicist Antoine Henri Becquerel discovered the radioactive radiation of uranium, and thus radioactivity itself. But the specific use of radioactivity in industry and technology started only after people were able to produce artificial radioisotopes through nuclear fission. Today, more than 100 years after the discovery of radioactivity, it’s hard to imagine life without it. In medicine, it has revolutionised many areas of diagnosis and therapy, particularly cancer therapy. In industry, it is used for examining welded seams or for quantitative and qualitative substance analysis. But also in the area of industrial measurement, control and process automation, it would hardly be possible to do without radioactivity: measurement engineering utilises the steady, uniform radiation properties of beta and gamma rays for level, density and thickness measurement.

Compared with some other applications of radioactivity, nucleonic measurement gets by with radiation sources of relatively low intensity. If protection regulations are adhered to, radiation hazards are practically nonexistent. The radiation intensity of nucleonic devices for measurement of level, limit level, density or mass flow is so low today that a conventional Geiger counter does not respond to it at all. But it is nonetheless radioactivity, and so nucleonic gauges must be one thing above all: safe.

Until now, point level sensors were the only nucleonic gauges that did justice to the high requirements of the SIL certificate. As of late, however, nucleonic devices developed according to SIL specifications for continuous level measurement and level detection, as well as interface and density measurement, are also available on the market. To provide these capabilities with safety, new developments have focused on three points. First: enhancement of the responsiveness of the scintillation detector. Second: improvement of the radiation protection containers. Third: the selection of a suitable radioactive material.

Principle of operation

The nucleonic measuring principle is based on the fact that gamma rays are weakened when they penetrate matter. At first glance, a nucleonic gauge is not really that much different from other gauges which operate according to the same principle, using radar, ultrasound or microwaves: they consist of a transmitter, a receiver and processing electronics which convert the measured signals into correct level, limit level, density or mass data. The transmitter sends signals in the direction of the medium that is to be measured, and these signals are picked up by the receiver installed on the opposite side of the medium. The larger the amount or density of the medium, the more the signals are attenuated on their way to the receiving device. From the strength of the incoming waves, impulses or beams, the software can calculate how full a container is, how great the density of a medium is and other parameters.

The first difference between a nucleonic gauge and other technologies is, of course, the radiation protection container, which encloses the capsule with the barely rice-grain-sized particle of radioactive isotope. This radiation protection container allows the radiation to exit only in the direction of the receiver (detector) and shields off the radiation in all other directions.

On the receiving side, the detector contains a scintillator, a photomultiplier and evaluation electronics. Scintillators are some of the most sensitive nuclear measuring instruments. They react to even the tiniest amounts of radioactivity, which allows comparatively weak radioactive preparations to suffice as a radiation source. Impinging gamma radiation generates flashes of light in the crystal or plastic scintillator. These flashes of light reach a photomultiplier, which amplifies them and converts them into electrical impulses. The impulse rate is a measure of the intensity of the radiation. Depending on how the device was calibrated, the impulse rate is converted by the processing electronics into a level, switching, density or concentration signal.

Improving safety

In order to raise the SIL level for nucleonic gauges, the developers began with the points already mentioned: as a first step they increased the responsiveness of the scintillation detector. This way, radiation sources with low radiation activity suffice for the new devices. To give a sense of the amount of radiation being used, the natural radioactive radiation present at an altitude of about 3000 m is more intense than the radiation the new detector needs to provide high-precision measuring results.

Next, the developers focused on the radiation protection container. This component has two functions - on the one hand, it protects the surroundings from the radiation and on the other, it protects the capsule containing the radioactive material from mechanical or chemical damage. With the new radiation protection container, the capsule is safely ‘wrapped’ in every respect: the radioactive preparation is completely enclosed by multiple, hermetically welded stainless steel capsules, which satisfies the highest classification and strictest safety criteria according to ISO 2919. The capsule is then placed in a cast metal housing, where it finally receives a protective cast lead mantle. As a result of its design, the radiation protection container offers maximum protection with minimal weight. Only a narrow slit remains open through which the radiation can exit, focused in the direction of the detector. The radiation outlet is adapted to the respective application and can be closed off completely when required: the radiation protection container can be pneumatically or electrically closed, for example when maintenance work has to be carried out on the vessel the nucleonic gauge is mounted on. It is also no longer necessary to have people come into direct contact with the detector - thanks to bus systems and DTM/EDD, the detector can be parameterised directly from the process control system.

From a safety-engineering standpoint, the most important element in a nucleonic gauge is, of course, the radioactive preparation itself. That’s why the measurement technology experts have taken a very close look at the different radionuclides that are selected as possible radiation sources. They came to the realisation that each measurement application requires an individual decision about which factors are important for the measurement. “Until now, we always had to do a balancing act with respect to the time constants in the measurement”, says Winfried Rauer, project manager for nucleonic sensors at VEGA. “There were either exceedingly precise or exceedingly fast results. Whoever wanted both had to go for a stronger radioactive radiation source.”

Many uses, greater safety

Nucleonic gauges are being used in more and more areas of process engineering. They are no longer used only for level detection and continuous level measurement under extremely difficult measuring conditions, but also for interface, density and concentration measurement in connection with toxic or abrasive liquids, for mass flow measurement - such as on dredgers, or as belt weighers for throughput measurement in mines. This means more and more people are handling these devices.

As a result of safety-related improvements, the detector is more sensitive; therefore, the dose rate that operating staff are exposed to in the vicinity of, for example, the Vega PROTRAC detector is considerably less than 1 microsievert per hour (µSv/h). To get a perspective on how small this level is, we should mention that every person on earth is exposed to a natural radiation dosage that is far higher. In Germany, the average value of terrestrial radiation exposure is 350 µSv per year. The consumption of 170 litres of mineral water in one year exposes us to, believe it or not, an average of 100 µSv, which is just about as much as a flight from Frankfurt to New York and back. And one single CAT scan in the area of the abdomen exposes us to a dosage of 10,000 to 25,000 µSv.

By: Roland Bonath, VEGA Grieshaber KG