3.3 UNITS OF MEASUREMENT

3.3.1 Exposure and Dose

When people are exposed to radiation, the energy of the radiation is deposited in the body. This does not make the person radioactive or cause them to become contaminates.

An analogy would be to shine a bright light upon your body. The body absorbs the light (energy), and in some cases the absorption of the light energy may cause noticeable heating in the body tissue. Your body does not become a light source or emit light now that it has absorbed the light.

In a similar way, when exposed to radiation, your body absorbs the radiation energy. As this absorption takes place, the tissue of your body may be damaged by the penetration and conversion of the radiation energy. Again, your body does not become radioactive or emit radiation due to this energy absorption.

Since absorption of radiation can damage tissue, a way to measure that damage and ensure that it is kept to a minimum is necessary. The amount of radiation Energy absorbed in a body is known as dose. The special unit for measuring dose in a person (called dose-equivalent) is the rem.

The rem is the unit used for equating radiation absorption with biological damage.

Since the rem is a fairly large unit, radiation exposure is usually recorded in thousandths of a rem - or millirem. 1000 millirem = 1 rem. Millirem is usually abbreviated as mrem.

For example, if you receive a chest x-ray, the amount of exposure - or dose - would be approximately 10 mrem (0.010 rem). This same amount of dose or biological harm, could be received from making two or three coast to coast airline flights - each round trip involves about 5 mrem (From elevated cosmic radiation levels in the upper atmosphere). The important point is, the source of the exposure is relatively unimportant, once the dose has been measured in a standard unit, it can be compared to other doses, added to other doses, or used in risk comparisons regarding non-radiation risks. We will make some of these comparisons later.

Other related units are used to make radiation measurements. You sill hear several terms such as "exposure", "dose" or "absorbed dose" associated with some of these units. Check the glossary for more information on these terms. Since these units and terms are used frequently, and have similar meanings; they are mentioned here for comparison. The following units are among the most common English units used. The limitations of each of the units are described. These units (the 'R', the 'rad', and the 'rem') are often interchanges, but each has a specific definition.

Roentgen(abbreviated 'R')

(Pronounced: "renken")

The roentgen is a unit for measuring exposure. It is defined only for effect on air. The roentgen is essentially a measure of how many ion pairs are formed in a given volume of air when it is exposed to radiation. Therefore it is not a measure of energy absorbed, or dose. It applies only to gamma and x-rays. It does not relate the amount of exposure to biological effects of radiation in the human body.

1 R (Roentgen) = 1000 mR (milliRoentgen)

Rad (Radiation Absorbed Dose)

The rad is a unit for measuring absorbed dose in any material. Absorbed dose results from energy being deposited by the radiation. It is defined for any material. It applies to all types of radiation. It does not take into account the potential effect that different types of radiation have on the body. Therefore, it can be used as a measure of energy absorbed by the body, but not as a measure of the relative biological effect (harm or risk) to the body.

1 rad = 1000 millirad (mrad)

Rem (Roentgen equivalent man)

As stated above, the rem is the unit for measuring the special quantity called dose equivalent. The rem takes into account the energy absorbed (dose) and the relative biological effect on the body due to the different types if radiation (expressed as the "quality factor" of the radiation). It is therefore a measure of the relative harm or risk caused by a given dose of radiation when compared to any other doses of radiation of any type. Occupational radiation exposure is recorded in rems.

The rem can be thought of as the unit of biological hazard.

Note: For purposes of this training, the term dose will be used to mean dose-equivalent.

Dose is the amount of radiation you receive. Dose Rate indicates how fast you receive the dose. Dose rate is the intensity of the radiation.

For Example
a) Dose - usually measured in mrem
b) Dose rate - usually measured in mrem/hr

Note: The units of R, rad, and rem can sometimes be acceptably interchanged. For instance, for gamma radiation, and exposure of 1 R causes an absorbed dose in a person of about 1 rad, which results in a dose equivalent of 1 rem. This is due to the basis for the definitions of the units and the relative biological effectiveness of gamma radiation. An absorbed dose of 1 rad from fast neutrons, however would result in a dose equivalent of about 10 rem.

Review:

If you stand in an area where the dose rate is 40 mrem/hr for 1/2 hour, what would your dose be? 10)

Exposure to radiation 11) (does/does not) result in a person becoming contaminated.

The dose rate in a room you are working in is 3 mrem/hr. If you work in this area for a full 8 hour shift, your total dose would be 12) mrem.

What is the unit for biological damage done by radiation? 13)

There are 1000 milliem in one rem. It may be helpful to think of decimal places to convert from one to the other.

For example:

__ __ 2 0 0 . mrem
__ __ . 2 0 0 rem

3.3.2 Measuring Radioactivity/Contamination

The amount of radioactive material in a given object or sample can be visualized by thinking of the unstable atoms in the material. These atoms are continuously decaying, so the more unstable atoms there are, the greater the decay rate. This rate of decay is measured in units of Curies. Curies are related to the decay rate, or disintegration rate, as follows:

One curie = 2,200,000,000,000 disintegrations per minute or 2.2 x 10¹² dpm

Since the curie is a larger number of disintegrations per minute, sub-units of the curie such as the microcurie or millicurie are often used. Also, the dpm is used when measuring surface contamination. For example, when using a frisker (contamination monitoring instrument) to measure contamination, the instrument reading - in counts per minute (cpm) - is coverted to dpm by a simple conversion of 1 cpm =~ 10 dpm. Therefore, if the frisker reads 100 cpm, the contamination level is 1000 dpm. This is a very small amount of radioactivity.

If your work involves working around contamination, you will receive training on the use of a frisker or other contamination monitoring instruments.

Review:

15. A sample of radioactive material is said to have 22,000 dpm. This means that:

a. there are 22,000 atoms in the material
b. every minute, 22,000 atoms escape from the material
c. every minute, 22,000 atoms decay and give off radiation
d. the dose rate on the surface is 22,000 mrem/hr.

3.4 TYPES OF IONIZING RADIATION

The four basic types of ionizing radiation of concern in most radiological work situations are alpha particles, beta particles, gamma rays and neutron particles. These may exist in various amounts, depending on the exact location and nature of the work. We will examine each type of radiation for its characteristics here, and later in the training look more closely at where we might find each type at CEBAF.

Alpha particles

• Physical characteristics
  
  Alpha particles are emitted during the decay of certain types of radioactive materials. Compared to other types, the alpha particle has a relatively large mass. It consists of two protons and two neutrons. (Positive charge of plus two.) It is a highly charged particle that is emitted from the nucleus of an unstable atom. The positive charge causes the alpha particle (+) to strip electrons (-) from nearby atoms as it passes through the material, this ionizing these atoms.
  
  
• Range

  The alpha particle deposits a large amount of energy in a short distance of travel. This large energy deposit limits the penetrating ability of the alpha particle to a very short distance. This range in air is about one to two inches.
  
  

• Shielding

  Most alpha particles are stopped by a few centimeters of air, a sheet of paper, or the dead layer (outer layer) of skin on our bodies.
  
  

• Biological hazard

  Alpha particles are not considered an external radiation hazard. This is because they are easily stopped by the dead layer of skin. If alpha emitting radioactive material is inhaled to ingested, it becomes a source of internal exposure. Internally, the source of the alpha radiation is in close contact with body tissue and can deposit large amounts of energy in a small volume of body tissue.

Beta particles

• Physical characteristics

  The beta particle is an energetic electron emitted during radioactive decay. Compared to an alpha particle, a beta particle is nearly 8000 times less massive and has half the electrical charge. Beta radiation causes ionization by the same forces at work with alpha radiation - mainly electrical interactions with atoms which is encountered as it travels. However, because it is not as highly charged, the beta particle is not as effective at causing ionization. Therefore, it travels further before giving up all its energy and finally coming to rest.
  
  

• Range

  The beta particle has a limited penetrating ability. Its typical range in air is up to about 10 feet. In human tissue, the same beta particle would travel only a few millimeters.
  
  

• Shielding

  Beta particles are easily shielded by relatively thin layers of plastic, glass, aluminum, or wood. Dense materials such as leas should be avoided when shielding beta radiation sue to the increase in production of x-rays in the shield.
  
  

• Biological Hazard

  Externally, beta particles are potentially hazardous to the skin and eyes. They cannot penetrate to deep tissue such as the bone marrow or other internal organs. We call this type of external exposure shallow dose. When taken into the body, materials that emit beta radiation can be a hazard in a similar way to that described from alpha emitters - although comparatively less damage is done in the tissue exposed to the beta emitter.

Gamma rays/x rays

• Physical characteristics

  Gamma/x ray radiation is an electromagnetic wave or photon and has no electrical charge. Gamma rays and x rays can be thought of as physically identical. The only difference is in the place of origin. These photons have no mass or charge but can ionize matter as a result of direct interactions with orbital electrons. Like all electromagnetic radiations, gamma rays travel at the speed of light.
  
  

• Range

  Because gamma/x ray radiation had no charge and no mass, it has a very high penetrating power (said another way, the radiation has a low probability of interacting in matter). Gamma rays have no specific "range" but are characterized by their probability of interacting in a given material. There is no distinct maximum range in matter, but the average range in a given material can be used to compare materials for their shielding ability.
  
  

• Shielding

  Gamma/x ray radiation are best shielded by very dense materials, such as lead, concrete, or steel. Shielding is often expressed by thicknesses that provide a certain shielding factor, such as a "half-value layer" (HVL). An HVL is the thickness of a given material required to reduce the dose rate to one half the unshielded dose rate.
  
  

• Biological hazard

  Due to the high penetrating power, gamma/x ray radiation can result in radiation exposure to the whole body rather than a small area of tissue near the source. Therefore, a photon radiation has the same ability to cause dose to tissue whether the source is inside or outside the body. This is in contrast to alpha radiation for example which must be received internally to be a hazard. Gamma radiation is considered an external hazard. Refer to the definition of "whole body" in the glossary.

Neutron particles

• Physical characteristics

  Neutron radiation consists of neutrons that are ejected from the nuclei of atoms. A neutron has no electrical charge. Due to their charge, neutrons do not interact directly with electrons in matter. A direct interaction occurs as the result of a "collision" between a neutron and the nucleus of an atom. A charges particle or other radiation which can cause ionization may be emitted during these interactions. This is called indirect ionization.
  
  

• Range

  Because neutrons do not experience electrostatic forces, they have a relatively high penetrating ability and are difficult to stop. Like gamma radiation, the range is not absolutely defined. The distance they travel depends on the probability for interaction in a particular material. You can think of neutrons as being "scattered" as they travel through material, with some energy being lost with each scattering event.
  
  

• Shielding

  Moderate to low energy neutron radiation is best shielded by materials with a high hydrogen content, such as water (H₂O) or polyethylene plastic (CH₂-CH₂-X). High energy neutrons are best shielded by more dense materials such as steel or lead. Sometimes a multi-layered shield will be used to first slow down very 'fast' neutrons, and then absorb the 'slow' neutrons.
  
  

• Biological hazard

  Like gamma radiation, neutrons are an external "whole body" hazard due to their high penetrating ability.

Summary of radiation types and characteristics

Review:

The four basic types of ionizing radiation are: 16), 17), 18), and 19).

20. In order of increase penetrating power, rank the following types of radiation:

beta	123
gamma	123
alpha	123

21. Classify the following as ionizing radiation or non-ionizing radiation.

Alpha particles	ionizingnon-ionizing
Ultraviolet radiation	ionizingnon-ionizing
RF (microwave) radiation	ionizingnon-ionizing
Neutron radiation	ionizingnon-ionizing
Laser radiation	ionizingnon-ionizing

A good shield for beta radiation is .

Gamma rays have no or charge, and are therefore moreless likely to interact in a given thickness of material than beta particles.