Measuring Radiation

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Figure 1 Radiation units.
Figure 1 Radiation units.

Depending on whether we refer to radiation emitted by a source (activity), absorbed or deposited in an object (exposure), or the biological damage it causes (dose), different units of measurement are most commonly used (Figure 1).

Becquerel is the unit most commonly used for ionizing radiation; it is simply the number of nuclei disintegrations per second. A larger unit is the curie, which represents the amount of radiation emitted by one gram of radium, or 3.7x107 nuclei per second. Both curie (Ci) and becquerel (Bq) are measured by an appropriate radiation detection device such as Geiger counters and are used when the amount of radiation emitted from a source is of interest. They tell nothing about the radiation dose that is actually absorbed by objects. Roentgen (R) is the measure of the radiation intensity of x-rays and gamma rays and refers to the number of ionizations produced in air.

Rad (radiation absorbed dose) gives the amount of radiation absorbed and deposited in a matter (exposure) — be it bone, fat, muscle, or concrete, irrespective of the effects this deposit has on the material. For water and soft tissues, rad and roentgen are approximately equal. Another commonly used unit of absorbed dose is a gray, which is equal to 100 rads. Equal doses of different types of radiation cause different degrees of biological damage.

Rem (roentgen equivalent man) is a measure of the damage caused by one rad of radiation in the human body (dose). It takes into account the effectiveness of both the source and the living tissue. Since rem values are normalized to different tissue types, one rem of any type of radiation does the same biological damage. For x-rays and gamma rays, the rem and rad are taken to be equivalent. Alpha particles have 20 times the risk of x-ray and gamma ray radiation, so one rad of alpha particle radiation is equivalent to 20 rem (both singular and plural). Since one rem of radiation is a large quantity, the unit most often used is the millirem (mrem), which is one thousandth of a rem. Another commonly used unit is the sievert, which is a measure of the biological effect of one gray of gamma ray. One sievert is equal to 100 rem.


Radiation Dosimetry

Individuals receive radiation from natural sources and from various activities over their lifetimes. The cumulative effect is not equivalent to the effect of an equal amount of radiation received at once or over a short time. Chronic exposure refers to relatively low doses of radiation for periods of months and years. This is typically experienced in normal daily activities such as handling nuclear materials and living next to a nuclear waste dump, or is accumulated as a result of years of radiation therapy and routine medical checkups. Acute exposure refers to a large dose of radiation received over a short time, such as that resulting from a nuclear accident or a nuclear war.

Although people cannot sense radiation directly, sensitive instruments called dosimeters can measure the amount of radiation they receive. Dosimeters are small sensors that a person can carry to measure the total dose of radiation received over a period of time. Geiger counters are normally used to detect the presence of radiation. In the United States, each healthy person receives an average of 300 mrem of radiation per year. The amount can be significantly higher for those who receive radiation therapy or who work in nuclear power plants, radiology units, and some research laboratories. The US Environmental Protection Agency (EPA) is responsible for setting limits on the amount of radiation humans can receive. According to these guidelines, the maximum annual permissible dosage is 500 mrem for the public. Furthermore, it has set a limit of 5,000 mrem per year, not to exceed 1,250 mrem in any given three month period, as the safety limit for adults working with radioactive materials from man-made radiation sources. The EPA guideline for the maximum one-time radiation dose for emergency workers volunteering for lifesaving work is 75 rem. You can estimate the amount of radiation you receive every year by filling in the dosimeter chart given below.

Effects of Radiation on Health

There are over 300 different products derived from fission of uranium and plutonium isotopes. Some have very short half-lives and practically disappear after a few seconds. Others, such as strontium-90 and cesium-137 with half-lives of about 30 years, enter the body by inhalation and the ingestion of contaminated foods and slowly decay by the continuous emission of beta and gamma radiation. Iodine-131 has half-life of only 8 days, so it accumulates in victim’s thyroid gland very fast. This is why doctors prescribe the normal (nonradioactive) iodine pills to saturate the body before radioactive isotope has a chance to be absorbed. This of course has no protective effect on other types of radioactive materials.

When molecules of living organisms are exposed to ionizing radiation, they become unstable; chromosomes and strands of DNA (deoxyribonucleic acid) break apart to form new molecules. DNA is a double helix of thousands of atoms strung along carbon-linked struts that contains all the information needed to produce and control living tissues. When mutations occur in reproductive cells (eggs or sperm), the changes can be passed on to subsequent generations (1).

Effects of ionizing radiation on human health are hard to assess and are highly dependent on the dose, the duration of exposure, and the type of cells involved (2). For example, if a body receives a massive amount of radiation in a short time, such as what might occur following a nuclear accident or an atomic blast, the effects are immediate – usually resulting in radiation sickness and death within hours or days. On the other hand, chronic exposure to low-level radiation may not be observed for many years or until future generations. Rapidly-growing cells like bone marrow and soft tissues such as ovaries, testes, and lenses of the eyes are most susceptible to destruction by radiation. Furthermore, depending on the general state of health of the individual, the same amount of radiation may result in different symptoms.

It is difficult to quote a number as the threshold at which death by radiation exposure is a certainty. Most nuclear scientists, however, agree that a single radiation dose of 500 rads or higher will almost certainly result in death within hours or days. If exposed to radiation doses of 100-250 rads, symptoms will include nausea, vomiting, and diarrhea, and many may eventually die as a result. No death is expected with a radiation dose of 100 rads or less, although in future years the risk of development of cancer, leukemia, cataracts, or sterility is significantly higher.

Question: Does a single 10 rem dose cause the same amount of damage as 10 doses of 1 rem spread over many years?

Answer: Many believe that genetic damage is probably independent of the dose rate since all doses cause non-reversible mutations. The somatic effects of several smaller doses are less, however, because time allows for some repair between administration of small doses.

Most data on high-level radiation doses are available from the two nuclear bombs dropped on Japan and the nuclear accident in Chernobyl. Over 200,000 people perished in a few days following the Hiroshima and Nagasaki bombings, and numerous others suffered from cancer, miscarriages, birth defects, and stillbirths in the years after. Even third-generation offspring of bombing survivors show higher rates of leukemia, thyroid, and colon cancers. Data from the Chernobyl accident point to similar catastrophes. More than 28,000 square kilometers of prime farmland were contaminated and another 200,000 people were forced to leave their homes as a result. According to the World Health Organization (WHO) (3), rates of thyroid cancer have climbed five to 30-fold depending on the indivisual’s distance from the power plant. There has also been a considerably higher rate of spontaneous miscarriages.

The effect of low-level radiation is much harder to assess, as factors that can affect one’s health increase with time. However, it has been established that low-level radiation is responsible for increased rates of several types of cancer. Iodine-131, a byproduct of nuclear reactions, concentrates in the thyroid gland and ovaries. Strontium-90 attacks bone marrow, increasing the risk of leukemia and other blood diseases. Plutonium-239 is one of the most toxic substances known. If inhaled, one thousandth of a gram can cause massive fibrosis of the lungs. Another radioactive waste, cesium-137, is mostly absorbed by the liver, kidneys, and sexual organs.

Useful Radiation

Radiation is not always associated with ills and disaster. Radiation has many uses in diagnosis and treatment of diseases, as well as industrial applications. Radiation can be used in numerous research and diagnostic tools, such as x-rays, MRI, and CAT scanners. Radioactive solutions can be ingested by a patient as a tracer to allow mapping of the blood stream for detection of tumors and restricted blood vessels, as well as photography of a particular internal organ. Radiation therapy can also be used in the treatment of disease, primarily to kill cancerous cells. Other applications include crop improvement and protection, manufacturing of consumer products such as watches, and ionization smoke detectors, and in the sterilization of such products as cosmetics and medical supplies. Foods are also irradiated to kill germs and other microorganisms, to aid in preservation, and to increase their shelf life. Radiation is a convenient tool for dating art and antiques, even dating such things as the age of the earth or a meteorite. Industrial uses of radiation include process monitoring, desalination, welding, detection of cracks and seams, and numerous others.

Question: Radiation therapy involves irradiating and killing the cancerous cells without damage to the healthy cells. How does radiation distinguish normal cells from cancerous cells?

Answer: Patients will be subjected to crossed beams of radiation that pass through the cancerous region. Alternatively, the patient can be rotated as he is radiated by a single beam of radiation. This way, the normal, non-cancerous, cells are exposed to much less radiation than the damaged cells that receive continuous exposure.


(1) The Los Alamos national laboratory maintains a website that provides information on radiation health physics and a radiation exposure calculator (

(2) US Environmental Protection Agency (

(3) World Health Organization (

(4) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

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