DETECTING RADIATION

Our eyes react to different types of electromagnetic waves that we call visible light in order for us to see something. We can change our extent or field of vision with the aid of glasses, telescopes, or microscopes. We use our senses in order to understand the things around us. However, when it comes to radiation, we cannot see, hear, touch, smell or taste it. We have to rely on scientific instruments in order to detect and measure radiation for us. When radioactive substances were first discovered, the harmful effects of radiation were not known so detection was not necessary. Many scientists who studied radioactivity were exposed to harmful radiation. When x-rays began to be used by doctors, many reported that patients who were exposed to x-rays suffered bums. In 1896, the physicist Ehhu Thompson deliberately exposed his finger to x-rays so that he could accurately report on the phenomenon of x-ray burns. Thomas Edison was experimenting with x-rays in 1896 when one of his assistants became fatally ill from over exposure to radiation. In 1906, Henri Becquerel, the discoverer of radioactivity, was accidentally burned by a radioactive substance he was carrying in his pocket. When Pierre Curie heard of his injury, he taped a radioactive substance to his own arm to observe the injuries it could cause. Ionizing radiation has high energy capable of knocking off electrons from atoms. Because electrons are negatively charged particles, the atoms that lose these electrons take on a positive charge because the number of positively charged protons left in their centers (nuclei) is greater than the number of remaining electrons. Because the ionizing radiation produces electrically charged particles, we can make instruments that can see these particles, thereby In using this concept, Geiger counters were designed to sense extremely small electrical impulses caused by ionizing radiation. In a Geiger counter, an electric current is passed along the walls of a tube. A thin wire passes through the center of the tube which is filled with a gas argon) that easily loses electrons if it is hit with ionizing radiation. When this happens, an electric current can jump through the gas to the wire. This completes an electrical circuit and the resulting electricity causes a loud clicking noise or moves a needle on a dial. Another instrument often used to detect radiation is a scintillation counter, which takes advantage of the fact that certain substances such as zinc sulfide, give off light when they are struck by high energy radiation. A photocell senses the flashes of light that occur as the radiation strikes and thus measures the number of decay events per unit of time. Henri Becquerel discovered a method of detecting radiation as far back as 1896. He found that invisible rays would affect silver emulsions on photographic plates just like light rays would. This is the principle behind the film badge. Radiation that strikes photographic film affects it much the way light does. The difference is that radiation can penetrate through materials that can stop light. A result, photographic film can be used to test for radioactivity. People who might be exposed to radiation often wear a film badge that contains a small bit of photographic film. This film badge records exposure to ionizing radiation. The photographic film badge is referred to as a dosimeter. The film is covered by a layer of materials such a paper or plastic that prevents light from reaching the film but allows the radiation to pass through. After use, the film is slipped out of the dosimeter and developed. The extent of darkening on the developed film can be translated into a measure of the total amounts of radiation received by the person wearing the dosimeter. Our skin also can act like photographic film. When we are exposed to even small amounts of radiation from the sun, our skin gets darker. This is called a suntan, or a sunburn if exposure is too great. To avoid overexposure to the sun's radiation, we use clothing, umbrellas, or sun screen lotions. We cannot detect radiation with our senses and exposure to too much ionizing radiation is harmful; therefore, a symbol has been developed to warn us when radioactive materials are present. The symbol is used on packages of radioactive materials, such as isotopes, and on doors to rooms or areas where radioactive materials are used or stored.

NATURAL RADIATION SOURCES BACKGROUND RADIATION

Everything in the world is radioactive and always has been. The ocean, the mountains, the air, and our food all expose us to small amounts of natural background radiation. This is because unstable isotopes that give off or emit ionizing radiation are found everywhere. Much of the Earth's natural background radiation is in the form of gamma radiation, which comes from outer space. It also comes, however, from such elements as potassium, thorium, uranium, and radium, which constantly decay and emit radiation. No matter where we go or what we do, we are surrounded by small amounts of radiation. Radiation amounts differ according to where we are on Earth. Different places on Earth have different amounts and types of rocks and minerals. Deposits of substances such as uranium vary in concentration and location as do other materials like coal, copper or lead. In the U.S., some of the best known deposits or uranium are found in New Mexico, Utah, Wyoming and Colorado. In some parts of India and Brazil there are also high amounts of background radiation from their rocks and minerals. The background radiation in parts of India and Brazil exceeds the safety limit of 5 millirems per year that the U. S. Government has set as a maximum limit just outside of nuclear powerplants. A person living in Kerala, India receives about 3,000 millirems of natural background radiation each year. In the U. S., Colorado has the highest average at 170 millirems per year. Location is the important factor; living near a granite rock formation can increase the background radiation by as much as 100 millirems per year to an individual. Many different building materials, such as bricks, wood, and stone also emit natural background radiation. Our homes, schools, businesses and even churches are all sources of natural background radiation. Cosmic rays from outer space are another large contributor of natural background radiation. Many of the cosmic rays are filtered out by the clouds and atmosphere so there are natural controls for the amount of radiation that people receive. Generally, exposure increases by about 1 millirem per year for every 100 foot increase in altitude a person lives above sea level. People who live at high altitudes get more background radiation than people who live at lower altitudes because of the thinner atmosphere. A ski instructor at a mountain resort will receive more background radiation than a fisherman at sea level. An airplane trip across America will expose a person to about 4 millirems of radiation because of the high altitude flying. Natural background radiation is also found in plants, animals and people. Living things are made of radioactive elements such as carbon and potassium; therefore, they are made of naturally radiation-emitting materials. Americans get about 25 millirems of radiation from the food and water they eat and drink each year. This can vary depending upon what is eaten, where it is grown and the amount eaten. However, all foods contain some radioactive elements, and certain foods such as bananas and Brazil nuts contain higher proportions than most other foods. During our lifetimes, our bodies harbor more than 200 billion billion radioactive atoms. About half of the radioactivity in our bodies comes from potassium-40, a natural radioactive form of potassium. Most of the rest of our body's radioactivity is from carbon-14 and tritium, a radioactive form of hydrogen. Still more radiation is gotten from manmade sources. In the United States, most manmade radiation comes from medical and dental sources, mainly x-rays. We also receive radiation from building materials such as bricks, the nuclear industry, coal-fired powerplants, and aboveground testing of nuclear weapons done in the 1950's. The average exposure/dose each American receives each year is about 360 millirems. This is from all natural sources of background radiation. It is a small amount considering that radiation levels much greater (50,000 millirems) have been demonstrated to not show any evident ill effects. Current standards allow people to work with radiation to receive 5,000 millirems per year, although an x-ray technician or a worker in a nuclear power plant control room generally receives only 50 extra millirems in a year. The average person's medical radiation exposure is 70 millirems per year. Wearing a luminous-dial watch gives you 3 millirems per year. An hour of TV each day gives you 0.15 millirems each day. Each time you run through the airport metal detector you get 0.001 millirem. A smoke detector in your home exposes you to 0.02 millirems. The use of Fiesta Ware adds 2.4 millirems per year. A normally operating nuclear power plant adds less than one millirem per year to your average yearly radiation exposure. Radon, the hidden hazard, is one of the pollutants that can build up in our energy efficient homes. Radon-222, an isotope of the noble gas family is extremely radioactive, with a half-life of minutes, is found in one of the decay steps of uranium-238. The granite bedrock under much of the United States contains small amounts of this uranium-238. It is a naturally occurring radioactive isotope with a half-life of more than a billion years. It slowly decays in a series that leads to lead-206. The danger of exposure to radon is considerable to anyone living in a community that is built on granite bedrock. Radon gas can seep into the basement of a house through holes or cracks in the foundation. The half-life of the radon-222 is so short that the danger is beyond the radon. When it is taken into the lungs, some of it will decay before it is exhaled. As radon decays, the products combine with tiny dust particles in the air and stick to the lung tissue. Radiation from the decay products of radon-222 is thereby suspected of playing a significant role in causing lung cancer. The EPA recommends that levels of radon should not exceed 4 picocuries per liter of air. A normal house contains approximately L5pCi-L of air, (IpCi/L = 200 mrem/y EDE or 1.6 rem/yr to the lungs).


 

RADIATION II

NON-IONIZING

Solar (visible light)

Radio broadcast

TV broadcast

Microwave

Heat (infrared)

Low frequency EMF

 

 

IONIZING

(Can cause cancer)

Alpha

Beta

Gamma

X-rays

Ultraviolet light

Fast neutrons

Muons

 

 

 

 

 

 

 

 

 

 

 


 

Basic Types of Ionizing Radiation

Type of radiation Alpha a Beta b Gamma g
What is it? Very fast He nucleus Very fast electron Very high frequency light
Electric charge +2 -1 0
Mass (amu) 4 about 0 0
Speed (% of light speed) 3-8 20-99 100
1 layer of paper, plastic wrap, Al foil, cloth Stops    
1 mm metal. 1cm wood Stops Stops  
5 cm metal Stops Stops Stops
       
Penetration in water or flesh few .01's mm 0.1mm -10 mm few m's
       
Penetration in air few cm 10cm-10m roughly 100m

 

Some Basic Terms

Radiation (ionizing) - fast, high energy sub-atomic particles .which, when they hit matter, knock electrons off atoms and break apart molecules.

Radioactive - containing atoms with unstable nuclei which emit radiation.

Irradiated - matter which has been hit with a significant amount of radiation.

Contaminated - contaminated with radioactive atoms - radioactive atoms where you do not want them to be.

Roentgen (r) - SI and conventional unit used to measure exposure. It can only be used to describe an amount of gamma and x-rays, and only in air. Measures the ability of photons to ionize air.

Rad (radiation absorbed dose) - A unit used to measure the quantity called absorbed dose. This relates to the amount of energy actually absorbed per unit mass in some materials. It is used for any type of radiation and any material. It is the amount of x-ray energy (ergs) absorbed per gram of tissue. Mrad =- 10'3 rad, urad- lO^rad.prd- lO-^rad.

Rem (roentgen equivalent man) - A unit used to derive a quantity called equivalent dose. It relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose.

Curie (Ci) - One curie is that quantity of radioactive material that will have 37,000,000,000 transformations or disintegrations per second. The relationship between Becquerels and curies is: 3.7 x 10' Bq = 1 Ci.

Gray (Gy) - SI unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One gray is equal to one joule of energy deposited in one kg of a material. One gray is equal to 100 rads.

Sievert (Sv) - SI unit used to derive a quantity called absorbed dose. This relates to the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is often expressed in terms of millionths of a sievert or microsievert. One Sv is equivalent to 100 rem.

Becquerel (Bq) - SI unit that is that quantity of radioactive material that will have a transformation or disintegration in one second. One Becquerel is equal to one transformation per second. 37 Gbq equals 1 curie or 37 billion Bq equals one curie.