RADIATION MEASUREMENT FUNDAMENTALS

-By Shriprasad Kakade

Radiation is a spontaneous emission of energy waves. It’s a random process. There are several natural and man-made sources of radiation viz. Cosmic Radiation (UV rays), Radioactive materials in the earth, manmade sources (e.g. X-rays, Cs-137 and Co-60 etc.). Radioactive nuclei are not stable in nature. To become stable, it keeps emitting energy rays (Alpha, Beta, Gamma and n). This process is called as radioactive disintegration and goes on until 5th HalfLife of isotope (when isotope becomes
stable). Half-Life is Time required for radioactive isotope to reach at its half of initial activity.

TYPES OF RADIATIONS

  1. Alpha : This is a particulate form of energy cannot travel far and can be stopped by Paper. Uranium-238, Am- 241 etc. emits alpha particle during alpha decay. It can be used to measure thickness of plastic film, Paper etc.
  2. Beta : This is also a particulate form of energy. Beta has more ionizing power than gamma but less ionizing power that Alpha particles. This can be stopped by wood or thin aluminum plate. Phosphorus-32 is a Beta emitter widely used in medicine
  3. Gamma : This is an electromagnetic radiation. This has more penetrating power compared to Alpha and Beta particles. This can be obtained while gamma decay of Cs137 or Co-60 isotope. High density material e.g. Lead is required to stop Gamma rays.
  4. Neutron (n) : Neutron has more penetrating power then Gamma and can be stopped with elements of high Hydrogen concentration such as Water.

ISOTOPE REVIEW

Cs-137 and Co-60 are the isotopes more commonly used for Process level and density measurement. The reason to use above isotopes are based on Energy, Penetrating ability required to cover majority of application. Cesium-137 is a gamma emitter with a half-life of 30.2 years. Cesium-137 gamma rays have energy of 662 keV, about half the energy of cobalt-60. Cobalt-60 is also a gamma emitter with a half-life of only 5.2 years. To become stable, cobalt-60 also undergoes beta decay and emits gamma rays with energies of ~1250 KeV.

Is it Safe to Work with Radiometric Products

Let’s look at the Fig. 2, we see that Dose Rate we receive in our daily life, travelling through Plane, X-ray or mountaineering. Dose rate is nothing but Gamma Energy Passing through unit Volume per unit time in Air.

“Field strength,” “field intensity” and “dose rate” are synonymous terms expressing radiation intensity in air. Technically, we measure ionizing energy passing through 1 cubic centimeter of air in 1 hour. Gamma energy passing through unit volume per unit time.
Standard unit is milliroentgen per hour (mR/h) 10 uSv/hr = 1 mR/ hr.

Equivalent dose rate

The equivalent dose defines the energy dose taken in the body through ionizing radiation. Measurement unit for the dose rate is the microsievert/hour (μSv/h).

Scintillation Detector requires very low Radiation to perform in the Range of 0.1 μSv/h.

The sievert (Sv) unit measure the biological effects of radiation. The largest exposure results from a medical xray examination ~100 μSv/h. The dose rate from a source depends on the activity and the distance.

 

Introduction to Radiometric System

Radiometric System consists of four parts:

  1. Radiation source,
  2. Detector (Transmitter)
  3. Process vessel / tank 
  4. Process material.

    Principle of Operation
    A Radiometric gauge is designed to measure the density, level, or weight of a process material by directing a beam of gamma radiation through the process material to a detector assembly (sensor and amplifier) on the other side. Some of the radiation is absorbed passing through the material. This absorption is proportional to the mass of the material through which it passes. The amount of radiation that reaches the detector is measured and converted into electrical impulses that are then amplified. This amplified signal is sent to electronics unit. The electronics has the function of converting the raw data into a useful format that can be displayed to the operator or applied directly to the manufacturing process.

    A detector that can measure the amount of radiation then be calibrated to measure a density or level of the process material. In a density gauge, the gauge measures the density of a process material
    flowing in a pipe by passing a beam of gamma radiation through the material. The radiation is partially absorbed by the process material. The radiation that is not absorbed is measured by the detector. In a
    level gauge, the gauge measures the position of the process inside a vessel by correlating the amount of counts absorbed due to the process level .

Detectors measure radiation field intensity
• Higher counts, field is higher
• Lower counts, field is lower
Based on calibration, it “infers” process change
• Counts go higher, lower density or lower process level
• Counts go lower, higher density or higher process level
Gamma radiation absorption is fundamentally a function of density

Source Holders

The source holder is the device that contains the radioactive source capsule. The job of the source holder is to shield and protect people from radiation exposure, and to direct the radiation energy thru the pipe or vessel and to the detector with various collimation angle. Typical source holders are constructed of carbon or stainless steel and can either use steel or lead for the shielding material.

Source Capsule

Industrial radiation sources Ceramic matrix infused with radioactive liquid. Ceramic “pill” is sealed inside two stainless steel capsules (Double Encapsulated). Each capsule is welded shut to seal in material .For process automation, majorly two different isotopes are used : Caesium Cs-137 and Cobalt Co-60.

 

Detectors

The detectors used in conjunction with source holders can be ionization chambers, GeigerMueller detectors, or scintillation detectors. 

Scintillation Detector

Working principle:-

Scintillation is a flash of light produced in a transparent material by the passage of a particle (an electron, an alpha particle, an ion, or a high-energy photon). Scintillator converts radiation energy into light energy, Photo Multiplier Tube converts light energy into electrical pulses, Electronics counts pulses, Software filters pulse rate and calculates PV.

Radiometric Measurement – Advantage

We have seen Radiometric Technology is a Universal Solution, can be used across all Industries, Processes without affecting process material. It also solves difficult & extreme applications like High Temp, Pressure, Corrosive & build-up media or even with Vessel Internal Obstructions. Installation, Maintenance can be carried out without Shut down of process or with no Intrusions to vessel. Being No moving Parts, this Technology is proven in the market and serves many critical applications likes Level Measurement for Coke Drum, PP Reactor Density measurement, Density Array for Slug-Catchers, De-salters etc.

Proud to say that “Radiometric Technology for Industrial Gauging is One of the top 20 Greatest Engineering achievements of the 20th Century.

Conclusion with Safety aspects is “A person or object is only contaminated or made radioactive if the isotope itself leaks from the capsule and is either ingested by the person or comes in contact with the object. It is extremely unlikely that the material will leak from these capsules. The use of radioactive materials in industry is fairly commonplace and has been for over 60 years. These materials should be respected, but not feared.”

VEGA is the inventor of the modern radiation-based measurement system for level and density, which solve the most difficult measurements to improve our customers’ safety, efficiency, and reliability. Founded as The Ohmart Corporation and in business since 1950. We have over 65 years of application experience in Global Market catering to various Industries, we will be a long-term partner for your radiation-based needs.

This Paper is written by Mr. Shriprasad S Kakade working as Sr. Applications Engineer at VEGA, India and can be reached at s.kakade@vega.com.
You can Reach Vega on :- info@vegaindia.com
References:
Radioactive Isotopes in Process Measurement Mr. Doug Branch VEGA Americas, Inc., Nuclear Product Manager

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