As the packaging speeds of rigid opaque containers increased during the 1950’s, the need for a reliable and accurate means to verify contents and monitor the filling process exceeded the capability of on-line container weighing systems. X-ray systems were initially used e.g., the Hyt-A-Fill manufactured by Industrial Nucleonics, and provided a non-contact means to measure product density at a pre-determined reference height in containers. In 1959, Industrial Dynamics Corporation (IDC) introduced the first gamma-ray systems, which used the gamma rays emitted from the nucleus of americium-241, an isotope of the element americium. Since then, over 100,000 systems from IDC and other manufacturers have been placed into service to meet the continuing and growing demands for accurate, reliable, and economic fill level inspection for vast numbers of opaque packages. These systems are typically referred to as density or fill-height gauges.
Improvement in X-ray technology enabled this alternative density gauging approach to re-emerge as a fill level inspection means in the 1990’s. Both X-ray and gamma-ray systems approach fill level measurement from the same principle: the attenuation of X-rays or gamma rays by the product being measured. The relative attenuation of the radiation beam can be used to determine the density of the package and product at a specific height.
This post addresses the use of low-energy ionizing radiation (less than 100 keV) gamma rays and X-rays in common density gauges. Our discussion also clarifies the two approaches and addresses the regulatory and health and safety concerns relevant to both.
Density Gauges: The System
Peco InspX’ density gauge systems consist of a source housing and a receiver. The source housing contains either americium-241 or an X-ray tube, depending on the system. The systems using americium-241 emit gamma rays in a controlled, direct beam that products then pass through. The systems using an X-ray tube emit X-rays, also in a controlled beam that passes through the products. The only difference between the gauges using gamma rays and the gauges using X-rays is the radiation source. Gamma rays or X-rays pass through the product and then enter the receiver where they are measured with a scintillation material that is coupled to a photomultiplier tube (PMT).
The most commonly used scintillation material is a thin disc of solid sodium iodide containing a trace amount of the element thallium: NaI(Tl). The NaI(Tl) Scintillator converts the gamma rays or X-rays (depending on the system) into lower energy visible photons (blue light) that are then measured by the PMT. The PMT converts the visible photons into electrons (photo electrons) and amplifies the electron current into easily measureable levels. The electrical signal from the PMT is proportional to the density of any material placed between the density gauge source and receiver. Both the gamma-ray and X-ray systems are efficient and safe to use. X-rays and gamma rays are both a form of ionizing radiation.
What is Ionizing Radiation?
Elements, Atoms, and Nuclear Particles
All the materials in the environment are composed of elements. Some of these materials are made of a single element, such as the carbon in a pencil, and other materials are made of combinations of various elements, such as the hydrogen and oxygen in water. An atom is the smallest piece into which an element can be broken and still maintain its original properties. If we cut up the carbon in a pencil until we had the smallest piece that we could still call carbon, we would have a carbon atom.
Each atom consists of a nucleus surrounded by one or more negatively charged particles called electrons, and within the nucleus there are two particles: the proton and the neutron. Both of these particles are about 2000 times more massive than the electron. The number of protons in the nucleus of an atom is an element’s atomic number. Because protons have a positive charge and electrons have negative charge, an atom is neutral when the number of protons equals the number of electrons. For example, a neutral atom of hydrogen always has one proton and one electron. Because it has one proton, the atomic number is 1. A neutral atom of carbon always has six protons and six electrons, and its atomic number is 6. A neutral atom of americium always has 95 protons and 95 electrons, and its atomic number is 95.
The second particle in the nucleus is the neutron. Neutrons have no electrical charge and help hold the nucleus together. Different atoms of the same element can have different numbers of neutrons in the nucleus. These different atoms of the same element are called isotopes. Figure 3 shows the three isotopes of the element hydrogen. All three isotopes of hydrogen have one proton but different numbers of neutrons. The total number of protons and neutrons in the nucleus is referred to as the mass number. The different isotopes of a single element have the same atomic number but different mass numbers. For example, most atoms of uranium have 92 protons and 146 neutrons. Such atoms have an atomic number of 92 (the number of protons) and a mass number of 238 (the number of protons plus neutrons) and are referred to as uranium-238. The isotope uranium-235 also has an atomic number of 92, but because it has 143 neutrons, it has a mass number of 235. There are over 20 isotopes of uranium, but uranium-238 is the most common; 99.3 % of naturally occurring uranium is uranium-238.
Radioactivity and radioactive decay
Just as a grocery bag can only hold so many items before it will break, the nucleus of a particular atom can only tolerate a certain number of neutrons or protons before it “breaks.” The grocery bag achieves stability by spilling the contents onto the floor. When a nucleus is unstable due to an excess of either protons or neutrons, the nucleus seeks stability by emitting particles, electromagnetic energy, or splitting into two lighter nuclei (fission, which is the energy source in nuclear reactors). Electromagnetic waves span a tremendous range of wavelengths: from low-energy radio waves to extremely high-energy cosmic rays. The individual packets of electromagnetic wave energy are called photons.
Atoms with nuclei that are destined to rid themselves of any excess energy are called radionuclides or simply radioactive material. All elements have at least one isotope that is radioactive, and all elements with 83 or more protons are radioactive. The process of reducing the energy level of the nucleus and the emission of excess energy is called radioactive decay. There are thousands of man-made radionuclides and only 65 naturally occurring radionuclides.
The unit used for the expressing the quantity activity in a radioactive source is the curie (Ci). A curie is defined as that quantity of radioactive material that decays at a rate of 37 billion times per second (37 billion nuclear transformations per second). The americium-241 activity used in gamma fill-height systems is typically 300 mCi or less (1 Ci = 1,000 mCi).
All of these radionuclides decay at different rates, and the time required for 50% of a particular radionuclide to decay is called the radioactive halflife. Radioactive halflives vary from less than millionths of a second to billions of years, depending on the specific radionuclide. The third isotope of hydrogen that was shown in Figure 3 is called tritium (1 proton and 2 neutrons; mass number 3), and it is radioactive with a halflife of 12.3 years. The halflife of americium-241 is 432 years, and a 100 mCi source will be reduced to a 50 mCi source in 432 years.
The energy emitted when the nucleus decays consists of particles and electromagnetic radiation (high energy photons called gamma rays). The two most common particles emitted from a decaying nucleus are alpha and beta particles. The alpha particle is actually a helium nucleus made up of 2 protons and 2 neutrons. Unlike atomic electrons orbiting in proximity of the nucleus, beta particles are electrons emitted from within the nucleus.
When an electron is ejected from a neutral atom, the atom is left with a net positive charge. This positively charged atom is called an ion, and radiation with enough energy to remove electrons from other atoms is called ionizing radiation. The common unit for the measurement of radiation energy is the electron volt or eV, defined as the kinetic energy gained by an electron by its acceleration through an electric potential difference of 1 volt.
Alpha and beta particles are able to directly ionize other atoms; however, the ability of electromagnetic waves to ionize depends on the photon energy. The light by which you are reading this document is a type of electromagnetic radiation, but its photons lack sufficient energy (less than a few eV) to ionize atoms. Non-ionizing radiation includes visible light, infrared, radiation from microwave ovens, and radio waves. An approximate cutoff between non-ionizing and ionizing electromagnetic radiation is ultraviolet light, the same electromagnetic radiation that causes sunburn by damaging skin tissue.
Electromagnetic energy below that of ultraviolet light is called non-ionizing radiation, and electromagnetic radiation with energy greater than that of ultraviolet light (3.3 eV), is called ionizing radiation. There are several types of ionizing radiation, including X-rays, gamma rays, and alpha and beta particles. In this document, we are only concerned with the forms of ionizing radiation used in common density gauges: radiation from americium-241 and X-ray tubes.
Ionizing Radiation Measurement
Ionization chamber survey instrument
Ionizing radiation ejects electrons from other atoms, leaving behind a positively charged atom called an ion. If the ionization occurs in a gas, such as air, the ions and ejected electrons can easily be collected using an electrical field. The higher the energy of the interacting X-ray or gamma ray, the greater the number of ions created, and we have a means of measuring the intensity of the ionizing radiation field. A common instrument for measuring the ionization in air is the ionization survey instrument. The instrument collects the ions generated in a specific volume of air and displays the intensity of the radiation field in a special unit of exposure called the roentgen (R). The exposure is often stated in smaller units of milliroentgen or mR or exposure rate (mR/hr). Typical ionization survey instruments are capable of measuring exposure rates from 0.02 mR/hr to 50,000 mR/hr (50 R/hr).
Geiger Mueller detector
Another very common instrument based on this detection principle is the Geiger Mueller detector or GM detector. GM detectors consist of a sealed tube filled with various types of gases. The GM tube is either internal or external to an electronic meter. Every time an X-ray or gamma ray interacts with the gas in the tube, a single large voltage pulse is generated. This signal results in a single “count” and can be displayed as total counts or counts per minute, or the GM detector can be connected to a speaker, with each detected event causing an audible click. GM instruments are very sensitive and excellent for locating sources of ionizing radiation. However, because a single count is produced for each ionization event, regardless of the intensity of X-ray or gamma-ray energy, a GM instrument is not suitable for measuring the exposure rate. It is only instrumental in detecting the presence of ionizing radiation.
Units for Radiation Measurement
Three units are normally used in radiation measurement: the roentgen, the rad, and the rem. The roentgen (R) is a unit of radiation-produced ionization in the air, and it is called a unit of exposure. Ionization chamber survey instruments are used to measure the ionization in the air, and the meter dials are marked in milliroentgen per hour (mR/hour; 1 R =1,000 mR). The rad is a unit of absorbed energy and measures the radiation energy absorbed in any substance, not just air. Finally, the rem is a unit of effective dose. Occupational and environmental radiation levels are expressed in rem or millirem (1 rem = 1,000 mrem), and rates are stated in mrem per time period (such as mrem/hour or mrem/year). The rem takes into account both the energy of the radiation and the fact that some types of radiation cause greater biological damage than others. For the specific case of X-rays and gamma rays, the radiation weighting factors are 1. Therefore, for the types of radiation used in the fill-height gauges discussed in this paper, all three of the above units are essentially the same: 1 roentgen = 1 rad= 1 rem. The international units for dose and end effective dose are the gray (Gy), and Sievert (Sv): 1 Gy = 100 rad, and 1 Sv = 100 rem.
Natural and Manmade Ionizing Radiation
The inhabitants of Earth are continuously exposed to several natural sources of ionizing radiation. These sources of radiation are often referred to as natural background radiation. This background radiation comes from solar and cosmic rays, radioactive substances in the earth, and radiation from within the human body. Exposure to radiation from the sun and outer space increases as one travels higher in elevation. A person standing at sea level is exposed to lower doses of solar and galactic cosmic rays than a person standing in the High Sierras. On average, people are exposed to about 300 mrem of ionizing radiation per year from all of the above natural sources.
There are 65 naturally occurring radionuclides, such as uranium-235, uranium-238, thorium-232, and radium-226. Most of the radioactive materials used today, however, are man-made in nuclear reactors or particle accelerators. Since the discovery of radioactivity in 1896, thousands of radionuclides have been created. Americium-241, the principle radionuclide used in low-energy fill-height gauges, is man-made and decays by the emission of an alpha particle and a series of gamma rays. The 59.54 keV (59,540 eV) gamma ray is the most abundant of the americium-241 gamma rays and is emitted in 36% of the time when the americium-241 nucleus decays. The alpha particle has a large mass, so although the alpha particle is very energetic (5,500 keV), it is stopped in less than 2 inches of air and completely stopped within the stainless steel source capsule surrounding the americium-241 source material. The gamma rays, however, do pass through the thin stainless steel outer layer of the americium source capsule and are emitted in all directions. However, the entire source capsule is shielded with tungsten metal (more effective in shielding low energy gamma rays than lead), and only a small portion of the gamma rays escape the source housing via a small hole (aperture) in the tungsten shield analysis. The resulting narrow gamma-ray beam is directed across the measuring gap and into the receiver housing for detection and fill height analyse
Another type of ionizing radiation is the X-ray, which is used in density gauges as an alternative to the americium-241 source (gamma rays). X-rays are produced by accelerating the electrons emitted from the negatively charged cathode of an X-ray tube into a metal target, or anode, which is held at a high voltage potential (4,000 to 100,000 volts in typical density gauge systems). When the electrons are stopped in the target material, X-rays are produced.
X-rays are produced whenever electrons are accelerated, in a vacuum, across a high voltage potential and stopped in material. For example, a vintage color television picture tube operates at a potential of about 25,000 volts. Consequently, X-rays are produced in color television picture tubes. However, the amount of X-rays produced is low, and the X-rays are well shielded by the thick, glass television tube (which is why a large tube color TV weighs so much). An X-ray tube, on the other hand, is designed to produce the maximum amount of X-rays and to emit those X-rays in a carefully directed beam. Unlike the “exact” 59.54 keV gamma rays from americium-241, the energy of the X-rays is a spectrum from low energy up to the maximum X-ray tube voltage (Fig 13). Different thicknesses of material are used to filter the low-energy (“soft”) X-rays out of the useful beam, as they can interfere with the density and fill height measurements.
The only difference between the X-ray and gamma-ray density gauges is the source. The X-ray gauges use an X-ray tube to generate X-rays, and the radionuclide gauges use americium-241 to emit gamma rays. Both types of gauges use a scintillator coupled to a PMT to receive the radiation and measure products.
Occupational Radiation Guidelines
Table 2 shows the annual occupational ionizing radiation exposure guidelines and the annual levels that require monitoring individual dose using radiation dosimeters (clip on badge dosimeters). As shown in Table 1, the radiation levels near either X-ray or gamma-ray systems are below 0.04 mrem/hr. Conservatively assuming an employee remains in proximity of the fill height system in a constant radiation field of 0.04 mrem/hr for the entire work year (2,000 hr), the annual dose would be 80 mrem. This dose is below all of the annual dose criteria requiring a dosimetry program and is about 30% of the average annual natural background of 300 mrem. Table 3 compares ionizing radiation levels for several medical procedures to natural background.
Manufacturers of X-ray systems must meet the specific requirements established by the Food and Drug Administration (FDA). Customers are typically required to register all
X-ray systems with their state radiation regulatory authority and pay an annual registration fee.
As for the gamma-ray systems, the United States Nuclear Regulatory Commission (NRC) regulates the americium-241 used in fill-height systems. Many states agree to meet or exceed the NRC requirements and register the material themselves; these are referred to as “Agreement States.” Manufacturers of gamma systems are required to have a specific license for the possession and use of the americium-241. Manufactures are also required to have a distribution license for each specific fill-height system. Customers are automatically granted a “general” license and only need to register the system with the applicable authority (NRC /Agreement State).
Although regulations governing radioactive materials vary somewhat in detail from state to state, persons who possess a device pursuant to a general license must comply with the following general requirements:
- Register receipt of the device with the State radiation control authority.
- Not transfer, abandon, or dispose of the gamma system except to a specific license holder (usually the original system manufacturer).
- Ensure that all the labels affixed to the device are maintained and comply with all instructions contained on such labels.
- Have the device tested for proper operation of the on-off indicator and shutter mechanism at installation and every six months thereafter.
- Have the device tested for leakage of radioactive material at installation and every three years thereafter. The tests and all other services involving the radioactive material are to be performed by a person holding an appropriate specific license (usually the original system manufacturer).
- Maintain records of all tests performed on the fill system.
- Maintain records on each receipt, transfer, and disposal of a fill system.