Basic Concepts of Radioactivity

During the course of using this manual you will encounter many rules that you will be required to follow. Such rules are in place to ensure that state and federal laws are not violated. These laws have their basis in scientific principles. It is therefore very useful to understand the basic scientific concepts of radiation for the purpose of understanding the applicable laws and rules. It is also useful to understand these principles so you as a UT researcher can be informed of the measures you must take to ensure your safety. This section includes such items as the definition of radioactivity, the units commonly used to describe radioactivity, the definition of half-life, an explanation of the different kinds of radiation, a short description of radiation producing machines, the definition of dose, radiation levels in the natural environment, the philosophy of radiation protection known as ALARA, radiation detection techniques, and some of the characteristics of radioactive material commonly used at UT.


Contents:

Radioactivity
Radiation
Radiation Producing Machines
Dose
Background Radiation
ALARA
Detecting Radiation
Characteristics of Radioisotopes Commonly Used at UT


Radioactivity

Atoms that are radioactive a known as radioisotopes. The nuclei of radioisotopes are said to undergo disintegrations. These disintegrations result in the emission of charged particles and or photons with various energies. We ordinarily describe radioactivity or activity in units of the number of disintegrations per unit time. Some of the more common units of activity that you may encounter during your work are the following:

                1 becquerel (Bq)        = 1 disintegration per second (dps)
                1 curie (Ci)            = 3.7 x 1010 dps   =   3.7 x 1010 Bq
                1 millicurie (mCi)      = 10-3 Ci
                1 microcurie ((Ci)      = 10-6 Ci

Typical activities used in UT research are on the order of 250-1000 µCi or 0.25-1 mCi.

This activity is not constant through time, however. The activity will decrease at a specific rate depending on the radioisotope present. For some radioisotopes such as 3H (tritium) this rate is such that samples containing it remain radioactive for many years. It is said to be long-lived and have a long half-life. One half-life is the time it takes one half of the radioactive material to decay. For example, if the activity of a sample at a certain time is 1 mCi , the activity will be .5mCi one half-life after the initial time. Other radioisotopes such as 125I are radioactive for shorter periods, thus, they are called short lived and have short half-lives. This change of activity over time can be described with the mathematical equation:

where, A= the activity after time t

                                Ao= 	the initial activity
                                theta = the decay constant
                                 t = the elapsed time since the initial activity

The decay constant theta is given by

whereT1/2 is the half-life.

To demonstrate this, we will imagine that today we have a 100 mCi sample of 12.3 year half-life (long lived) 3H. The second equation above shows that 12.3 years from today there will be only 50 mCi.

Also, about 24 years from now (after two half-lives) there will be only 25 mCi left.

If today we had a 100 mCi sample of the short-lived isotope 125I which has a half-life of about 60 days, in only 120 days (after two half-lives) we would have 25 mCi left.

After 12 years this 100 mCi125I sample would have almost completely decayed.

This clearly demonstrates the difference between long and short-lived isotopes.

 

When the atom of a radioisotope decays the nucleus of that atom changes and thus the identity of that atom changes. Sometimes an atom that has undergone a decay becomes another radioisotope and sometimes it becomes a nonradioactive atom. When 3H decays it always turns into a stable or nonradioactive form of helium. When an 125I nucleus decays sometimes it will decay to a stable form of tellurium and other times it will decay to a radioactive form of tellurium which in turn decays to a stable form of tellurium. The ways in which other radioisotopes decay can be very complex and the radioactive decay equation in this form does not always apply. However, in most cases the above equation can be used to calculate the decay of radioactive samples used at UT.


Radiation

It is not adequate to just know the activity of a sample. One must also know something of the type of radiation being emitted. Of the hundreds of radioisotopes, each emits radiation uniquely. That is, radioisotopes will emit an alpha, ß, neutron, y-ray or x-ray radiation or a combination thereof with various energies. We can distinguish each of the common types of radiation by its nature and origin.

These different types of radiation can have different energies. The unit commonly used is the electron volt (eV). Typical energy ranges are thousands of eV (keV) and millions of eV (MeV). 3H emits a ß- that has an energy of no more than 18.6 keV. This is a very low energy and a ß- of this energy does not travel very far in air. 125I emits, among other things, a 35 keV y-ray. This is also considered a low energy but y-rays can travel much further. As it can be seen, not only is the activity of a radioactive sample important, but so is the type and energy of the radiation.


Radiation Producing Machines

Machines specifically designed for the purpose of producing radiation are another source of radiation. Mostly these are the familiar x-ray machines. These machines produce useful radiation in two ways by accelerating electrons toward a heavy metal target. The first way that x-rays are produced is when these accelerated electrons are sharply deflected in the vicinity of the atomic nuclei of the target atoms. In the process the electrons lose energy by emitting x-ray photons. These type of x-rays are called bremsstrahlung which is German for "braking radiation." The second way x-rays are produced by these machines is when the electrons interact with the target causing excitation and ionization of the target atoms. X-rays of this type are called characteristic x-rays.

At UT there are 40 x-ray units used by various departments for various purposes. All units are inspected at least annually by the Radiation Safety Officer. The main difference between these machines and radioactive materials is that machines can be turned off so that no radiation is being produced. Radioactive materials cannot be turned off. Furthermore, the energy and amount of x-rays emitted from the machine can be controlled. The energy and amount cannot be "adjusted" for radiation originating from radioactive materials. The energies of machine-produced x-rays usually range from 15 to 150 keV.


Dose

One of the main goals of radiation protection is to measure the amount of radiation to which a person's body is subjected. When radiation interacts with tissue some or all of the radiations energy is imparted to that tissue. In this energy absorbing process electrons are liberated from atoms leaving positively charged atoms and energetic electrons. Chemical changes are produced by these ion pairs and these in turn can result in damage to sensitive tissues and or molecules. Ionization and energy absorption measurements are therefore the basis of quantifying radiation in a meaningful way. In fact measurements like this are the only way in which to communicate the characteristics of radiation fields. It is not adequate to describe a radiation field in terms of the activity of the source since the type of radiation is not always known. In the following paragraphs we present the definitions of these various measurements so that you can understand their meaning in a physical sense.

Exposure is the measurement used to describe the amount of ionization caused by x-rays and y-rays in air. The unit of exposure is the Roentgen and typical values for this measurement are in milliRoentgen (mR). This measurement only describes the amount of charge produced in a given mass of air by photons. Instruments used to measure radiation fields typically read out in exposure rate in units of mR/hr. If you spent 8 hours in a 10 mR/hr radiation field you would have been subjected to an exposure of 80 mR.

Absorbed dose is defined as the amount of energy absorbed per unit mass of any material. The absorbed dose in tissue is the measurement of interest for radiation protection purposes. The unit used for absorbed dose is the rad.

The dose equivalent is defined as the amount of energy deposited per unit mass of material (the absorbed dose) times what is known as the quality factor(QF). The quality factor takes into consideration the varying effectiveness of different radiations in causing biological damage for the same amount of energy deposited.

                Type                    QF
        x-rays and y -rays              1
                ß-                      1
                alpha                20
                n                       10

Alpha particles and neutrons are more efficient in causing damage than photons and ß- particles. The unit for dose equivalent that we use is the rem. Typical values encountered are millirem (mrem).

A dose can result from radiation originating external to the body or internal to the body. For the former, a deep dose equivalent (DDE) is specified. For the latter, an internal dose is specified. The sum of the DDE and the internal dose is called the total effective dose equivalent (TEDE). For those whose job it is to work with radioactive materials an occupational dose is specified. Occupational doses are usually reported as the sum of the occupational internal dose and the occupational external dose.

Biological Half-Life

Associated with the ingestion of a radionuclide is a biological half-life (Tb). The biological half-life is a measure of the rate at which substances are eliminated from the body. For example, assume an individual takes in 1 liter of water in a short period of time. The time it takes for his or her body to eliminate or excrete one half of a liter of that same water can be thought of as the biological half-life for water. Water has a Tb of 12 days. The biological half-life depends on the particular element and its chemical form and is independent of the activity contained in the substance of interest.

Effective Half-Life

For radiation protection purposes (such as determining an internal dose equivalent) we must consider two types of half-lives. The first is the rate at which the substance containing radioisotopes is eliminated from the body, the biological half-life. The second is the rate at which the isotope of interest decays, the physical half-life. These two half-lives can be combined into an effective half-life Teff.

For example, the rate at which some compounds of phosphorus are eliminated from the body is described by a biological half-life of 19 days. The radioactive decay of 32P is described by the physical half-life of 14 days. Thus the rate at which radioactivity is "removed" from the body from decay and elimination is described by the effective half life of Teff = 8 days. Values for Teff can be obtained from your PI.


Detecting Radiation

Radiation can be detected using a variety of instruments and methods that read out in activity, exposure rate or dose equivalent. The method employed is dependent upon the radiation type and the measurement needed. The three basic methods employed at UT involve the use of survey instruments, liquid scintillation analysis and film badges.

Survey instruments are used to locate contamination or radioactive material where it is not wanted. Ionizations in the sensitive volume of the detector result in electronic pulses that cause meter and audio responses. The meters typically read out in mR/hr and counts per minute (cpm). There is a vast selection of survey instruments available specifically designed for the detection of all types of radiation. These instruments are used for rough measurements when the isotope present is known. Typically Geiger counters or ion chambers are used at UT. A Geiger counter is used for the detection of y -rays and higher energy ß- radiation. Ion chambers are used for the detection of x-rays and y -rays.

Another detection technique is liquid scintillation analysis. Liquid scintillation analysis is the process in which a wipe is taken of a surface and is then counted for radioactive material that has been removed from that surface. At UT this technique is used to detect lower energy ß- radiation. It can also be used to detect alpha radiation. Liquid scintillation is used for surveying surfaces such as table tops, door knobs, phones and lab equipment for contamination that cannot be detected with survey instruments. 3H is an example of a low energy ß- emitter that cannot be detected with survey instruments. Liquid scintillation will yield results as activities, and if done properly can even identify the radioisotope present.

The film badge that you are required to wear during your work measures the deep dose equivalent from external sources of radiation. These film badges consist of film that darkens when exposed to radiation. State and federal regulations require that the degree of darkening be roughly related to the deep dose equivalent (DDE). Film badges are sensitive to y-ray, higher energy ß-, and x-ray radiation. However they do not respond to radiation from 3H and 14C or alpha emitters.


Background Radiation

Everyone on the face of the earth is subjected to background radiation (commonly referred to as simply background) from the sun, the ground, and other natural sources. These background levels are always present but pose little hazard. Typical background doses that an individual would receive from living in the Knoxville area for one year are:

        Source                          Dose equivalent (mrem/yr)*
        40K (in the body)                       20
        cosmic                                  40
        terrestrial                             60
        radon                                   180
        Total                                   300
* 1 millirem (mrem) = 10-3 rem.

It is important to remember that 300 mrem is the approximate dose that everyone receives every year from just living. Later this background dose will be compared to typical occupational doses received by UT researchers.


ALARA

As it can be seen it is impossible to avoid a dose due to the presence of background radiation. However, it is still important to ensure that doses remain as close to background as possible or As Low As Reasonably Achievable.

                                As
                                Low
                                As
                                Reasonably
                                Achievable

The principle of ALARA is the cornerstone of radiation protection and is the basis of the University's radiation protection policy. This concept refers to keeping doses and releases to the environment as low as we can achieve based on state of the art technology and socio-economic factors. This is a balance between the benefits gained from research and the costs associated with using radioactive materials. ALARA is based on the very conservative "linear hypothesis" which states that any dose, no matter how small, may harm the body. Therefore it is not permissible to operate at or just below the legal limits of dose but rather the goal is to stay as far below the limits as possible.

There are three basic laboratory practices used to achieve ALARA. The first is spending as little time as necessary in a radiation field. The second is keeping the distance between oneself and a radiation source as large as possible. A good rule of thumb is the inverse square law. That is, if you double the distance from a source the intensity of the field will decrease by at least a factor of four. Or, if you triple the distance from a source the intensity of the field will decrease by at least a factor of nine. The third method is to use shielding. These methods used for achieving ALARA are commonly referred to as time/distance/shielding.


Characteristics of Radioisotopes Commonly Used at UT

Among the most common radioisotopes used at UT include 3H, 32P, 14C, 35S, 125I and 131I. Below are listed the characteristics of four of these.

As it can be seen, many factors such as activity, whether the sample is long or short-lived, the type of and energy of the radiation, and the behavior of the isotope in the body must be considered. The type of protective measures taken is dependent upon these characteristics. University policy on the proper use of radioisotopes and laboratory safety techniques are based on these factors. Your PI will provide you with all the information necessary for the proper use of radionuclides specific to your laboratory.


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Comments or suggestions to Sandy Lindsey
at the Office of Research
at the University of Tennessee
Revised August 26, 1996