2.1 Nature of Ionizing Radiation

Many persons are aware that ionizing radiation in normal intensities cannot be seen, felt, tasted or smelled; hence, it is considered very mysterious. Such mystery is not warranted. Radiation, per se, is simply a means of propagating energy through space. Light radiation and heat radiation are not considered mysterious. Radio waves are not well understood by most persons, although their existence is well accepted. Ionizing radiation is very similar to these other radiation types, and really no more mysterious.

Ionizing is also an unnecessarily frightening term. All matter is composed of molecules. Each molecule is composed of atoms. Each atom is composed of a group of electrons orbiting around a nucleus containing protons and neutrons. Electrons have one unit negative electric charge. Protons have one unit positive charge. Neutrons have no charge. Normal atoms and molecules have the same number of orbiting electrons as protons in the nuclei. Hence, they are electrically neutral. Ionization is the process whereby molecules or atoms are divided into parts having unequal numbers of electrons and protons and thus have net electric charges. The charged fragments are called ions. This process can occur by several means. For instance, rubbing cat fur with a plastic rod will ionize the plastic; putting table salt into water will ionize the salt. Ionizing goes on all the time in nature for beneficial purposes.

Ionizing radiation is simply any radiation which can impart enough energy to cause ionization of the material being struck. There are many types of such radiation. However, only four are of particular importance at OSU.

a.Alpha particles are composed of two protons plus two neutrons; thus, they have a charge of +s. Although possessing high kinetic energy (3-6 MeV) they are relatively non-penetrating due to their relatively large mass and large charge. A sheet or two of paper will stop them. They can travel only a few mm in air.

b.Beta particles are high speed electrons; thus, they have a charge of -1. Kinetic energies ranging from 10-3 MeV to several MeV. Their mass and charge result in their being moderately low-penetrating; they can be stopped by a few cm of water or plastic if of higher energies, less if of lower energies. They can travel up to several feet in air, depending on energy.

c.X-rays and gamma rays are photons, not particles; they have neither mass nor electric charge. Energies range from several eV to several MeV, thus shielding needed varies widely. Note that x-rays and gamma rays are identical except for origin; x-rays originate outside the nucleus, while gamma rays originate from within the nucleus. They can travel from a few cm to very long distances in air, depending on energy.

d.Neutrons have mass, but no electric charge. Their kinetic energies vary from about 1/40 eV to many MeV. Here also, the amount of shielding required varies greatly. Due to the physics of neutron shielding, it is a very complicated subject.

2.2 Units

Energies of ionizing radiation are usually measured in eV (electron volts). An eV is the kinetic energy possessed by an electron after acceleration by a potential of one volt. 1 eV = 1.6 x 10-19 J (joule), 1.6 x 10-12 erg, or 6.1 x 10-20 gm-cal. Obviously, an eV is a very small unit of energy. One usually uses units of keV (thousands of eV) and MeV (millions of eV) for real life situations.

Radioactive material is usually measured in terms of activity. The "old" unit is the curie (Ci), which is the amount of material which undergoes 3.7 x 1010 transforms per second. The new SI unit is the becquerel (Bq) which is the amount of material which undergoes 1 transform per second. Since most transforms are disintegrations, a curie is often thought of (improperly) as 3.7 x 1010 dis/sec. Note that neither unit is a direct measurement of particles or photons emitted nor of energy liberated; they measure only the number of atoms undergoing change per unit time.

Common measurement of radiation dose involves three different concepts including five dose units.

a. Exposure or exposure dose is the amount of energy passing through a given location at a given time. The unit is the roentgen (R), which is the amount of x or gamma radiation which will cause one esu of charge of either sign per 0.001293 g of air at the location of interest. Note that the R is defined only for photon radiation, not for alphas, betas, or neutrons.

b. Absorbed dose is the amount of energy absorbed by the material of interest. The "old" unit is the rad, which is the absorption of 100 ergs per gram of material. The new SI unit is the gray (Gy), which is absorption of 1 J/kg. If one assumes that humans are similar to water, then 1 rad to the whole body will result in a temperature rise of about 2 x 10-6 0C; a rad is a very small amount of energy. However, it is large compared to many doses actually received. The dose unit commonly used is the millirad (mrad), which is 1/1000 rad. Note that 1 Gy = 100 rad.

c. Dose equivalent is a measure of result of irradiation. The old unit is the rem, which is the effect caused by one rad of medium energy photons to the material of interest. The new SI unit is the sievert (Sv), which is the effect caused by 1 Gy of photons. The rem, small though it is, is also, like the rad, too large for many uses; the unit often used is the mrem. As with Gy vs. rad, 1 Sv = 100 rem.

The difference between dose and dose rate is often forgotten. For most real life situations, the radiation effect is independent of rate and proportional to total dose. Many measuring instruments measure only dose rate; to determine dose one must multiply the rate times the time of exposure. Since rate is often not constant during exposure, one must integrate a series of products of rate times time to determine the dose incurred.

One question often asked about radiation units is "How do I convert from mCi to mrem?" Such conversion is not trivial to calculate. It requires knowledge of the radiation emanation scheme (how many particles and photons of what energies per transform), absorption coefficients for each radioemission and each target material, absorption coefficients for all intervening material, secondary radiation production coefficients, scatter coeffecients, relative effects of each radioemission for each target material, etc. It is usually much easier to simply look up gamma radiation dose factors in a table, or use simple rules of thumb for beta radiation.

2.3 Radioactive Transforms

Most radioisotopes of interest at OSU undergo transforms via beta particle emission, often accompanied by gamma ray emission. This creates an atom of a different species from the parent atom. For instance, tritium decays to helium, C-14 decays to nitrogen, P-32 decays to sulfur. Examples also include Cs-137 which decays to barium and Co-60 which decays to nickel, both of which are important for the gamma emission rather than the betas.

Some isotopes transform by gamma emission only, resulting in no change of chemistry. Tc-99m, for instance, transforms to Tc-99.

Some isotopes transform via electron capture, i.e., the nucleus absorbs one of its orbital electrons and thus becomes an atom of another species. Rearrangement of the remaining orbital electrons results in an assortment of photons being emitted from the atom. I-125, an example of e.c. transform, thus becomes Te-125.

Some atoms transform via alpha particle emission, resulting in an atom of a different species. Uranium and radium are prominent examples.

Alpha transforms are used for neutron production in some neutron sources. Am-241 alphas, if allowed to impinge on beryllium, will transform the beryllium atoms into carbon atoms, with accompanying emission of a neutron. Pu-239 is also used for this purpose at OSU.

2.4 Radiation Absorption - Shielding

For any type of ionizing radiation the energy absorbed by shielding material will be mostly via molecular excitation (heat) and only to a small degree via ionization. Alpha particles lose energy via collisions with atoms, causing excitation and some ionization, until they collect two electrons and become helium atoms or stick to nuclei and become new atoms. Neutrons lose energy via collisions with atoms, causing excitation and ionization until they stick to nuclei and create new atoms. This is often accompanied by x-ray or gamma emission. Beta particles lose energy via collisions causing excitation and ionization plus creation of secondary photons by bremsstrahlung until they lose enough energy to act like a free electron and attach to an ion. Note that bremsstrahlung production is increasingly efficient at higher beta energies. Gamma and x-rays lose energy via excitation, ionization and beta particle production (photoelectric, Compton and pair-production) until their energy is all given up; they then disappear. All three beta-producing effects are proportional to some power of Z, and vary with energy of the photon.

The practical result of shielding phenomena is that for gamma and x-ray shielding use high Z material such as lead for greatest absorption per inch or per pound. For beta radiation shielding use low Z material such as plastic to absorb the betas with minimum bremsstrahlung production. (Note that this applies to unshielded beta emitters; for those in, for instance, glass bottles one must shield the secondary photons.) For neutron use low Z material such as water or polyethylene to get maximum absorption per unit amount of absorber. For alpha particles, use just about anything, but in small quantity, since alphas are very easily absorbed.

2.5 Radiation Detection and Measurement

Of the myriad ways to detect or measure ionizing radiation, only a few are of importance for radiation safety purposes.

a. Exposure of photographic film to ionizing radiation renders silver grains developable. Development results in increased optical density compared to unexposed film. This optical density is a function of the dose applied; unfortunately, it is dependent also upon the types and energies of radiation actually getting to the film emulsion. Personnel dosimeter film cannot be evaluated unless the types and energies of radiation are known; this is done by using a multi-filter film holder (badge) during exposure. Note that the film packet will not respond to alphas or weak betas even though the film emulsion can do so, since these radiations cannot get through the packet paper wrappers.

b. Exposure of LiF or other thermoluminescence material causes absorption of energy within the crystal lattice. Controlled heating will release this energy as light photons which can be measured by photomultiplier tubes. Light emitted is proportional to dose and reasonably independent of energy over a wide range of doses and energies. However, energy absorption, and thus light emission, does vary with type of radiation.

c. Exposure of liquid scintillation fluid, NaI, or other scintillators causes absorption of energy by the scintillator followed by immediate release of energy as light photons proportional to absorption. Energy released is proportional to energy absorbed, hence the energy of the incident ionizing radiation can be determined. Efficiency of the process is strongly dependent upon the type of radiation.

d. Solid state detectors such as Ge(Li) are also used to determine energies of incident particles or photons. Such detectors usually are specific for a type of radiation (photons, betas, alphas, neutrons, etc.).

e. Some portable survey instruments use gas ionization in a detector chamber to produce an electric current proportional to energy absorbed, and thus proportional to dose. Chamber wall design will limit the types and energies of radiation accepted and dictate any correction factors.

f. The most common portable survey meter detector is the Geiger-Mueller gas amplification tube, which produces a large pulse whenever a particle or photon produces an interaction within the tube. Response is proportional to particle interactions, hence to particle flux, but not to dose unless specifically calibrated for the spectrum being measured. As with ioniziation chambers, G-M tube design will dictate radiation accepted and correction factors.

Film and TLD dosimeters integrate low doses of radiation over periods of time. They are commonly used for personnel dosimetry. Scintillation is usually very sensitive and is used to detect very small quantities of radioactive material. Ionization chamber survey instruments are used to determine dose rates. G-M survey instruments are used to detect radiation at low levels, but not to measure doses except in special cases.