These substances are absorbed by our bodies, into our tissues, organs, and bones, and are constantly replenished by ingestion and inhalation. From the radionuclides that are present in our bodies, the average man in the United States receives an effective dose of about 0. This is about one-tenth or 10 percent of the 3.
For women and children, the dose is less, in rough proportion to their smaller bodies. A pie chart in this report shows dose contributions from various natural background radiation sources, and the contribution from our own bodies can be found by adding the dose from potassium and from thorium and uranium and their decay products discussed in more detail below.
All of us have a number of naturally occurring radionuclides within our bodies. The major one that produces penetrating gamma radiation that can escape from the body is a radioactive isotope of potassium, called potassium This radionuclide has been around since the birth of the earth and is present as a tiny fraction of all the potassium in nature.
Potassium 40 K is the primary source of radiation from the human body for two reasons. First, the 40 K concentration in the body is fairly high. Potassium is ingested in many foods that we eat and is a critically important element for proper functioning of the human body; it is present in pretty much all the tissues of the body.
The amount of the radioactive isotope 40 K in a kg person is about 5, Bq, which represents 5, atoms undergoing radioactive decay each second. Second, 40 K emits gamma rays in a little over 10 percent of its decays and most of these gamma rays escape the body. A gamma ray is emitted in about one out of every 10 disintegrations of 40 K, implying that about gamma rays are produced each second.
These will be moving in all directions, some will be attenuated in the body, and the dose rate from these gamma rays outside of the individual's body will represent a very small fraction of the normal background dose rate from all natural sources outside the body.
If a person is above average in weight, the dose rate outside of this person's body will expectedly be higher than the dose outside the body of a lower-weight individual. In both cases, however, the dose rate will be extremely small compared to the normal background dose rate. The heavier person will receive a greater internal dose because the decay of the 40 K produces other low-penetrating radiation beta radiation that deposits its energy within the body.
However, the dose to the heavier individual will not be significantly different from that to the lower-weight individual because the energy deposited per unit body mass is the dose-determining factor, and this will be about the same for both individuals. There are many other radionuclides in the human body, but these either are present at lower levels than 40 K for example, U, Th, and their decay products or they do not emit gamma rays that can escape the body for example, 14 C and 87 Rb.
Radon and its decay products is not a significant source of radiation from humans because it is present at very low levels in the body. There is one other very minor mechanism by which the human body acts as a source of radiation: some of the gamma rays emitted by the radionuclides in the environment interact with the atoms in our bodies by what is known as the photoelectric effect.
The result is the emission of x rays by these atoms. Potassium content of the body can be obtained from its natural abundance of 0. The potassium content of the body is 0. Carbon content of the body is based on the fact that one 14 C atom exists in nature for every 1,,,, 12 C atoms in living material.
Using a half-life of 5, y, one obtains a specific activity of 0. Though these isotopes make up most of our body's radiation, we take in only about 0. Some foods have higher concentrations of radioactive isotopes — like bananas , which contain a small amount of potassium 40, and Brazil nuts , which contain radium. Of course, the amounts of these foods an average person consumes does not significantly increase radiation-related health risks, according to the U.
Environmental Protection Agency. Other environmental factors can lead the human body to become far more radioactive. Radon is a radioactive, odorless gas that occurs naturally in the environment. In , Stanley Watras , a radiation worker in Pennsylvania, unexpectedly set off an alarm that detected people's exposure to radiation.
Safety personnel were puzzled to find that Watras was not physically carrying any sources of radiation, but it was later discovered that his body had absorbed huge amounts of radon gas from his basement — which he was told significantly increased his risk of lung cancer. Related: Why do nuclear bombs form mushroom clouds?
Short said that the radioactive isotopes humans take in are created through different processes. Potassium 40, for instance, is a " primordial nuclide ," meaning it has existed in its current form since before Earth's genesis. Primordial nuclides take so long to break down, or decay, that they are essentially the same today as they were at their creation in stars or in the Big Bang. For example, of the four stable isotopes for iron, iron naturally accounts for about 92 percent, and the rarest is iron at 0.
A scientist gives a test subject doses of iron and monitors the amounts of different iron isotopes in blood and other biological samples. Because iron is heavier than iron, a mass spectrometer distinguishes them easily.
Positron Emission Tomography produces three-dimensional images of organs and tissues through the use of radioactive isotopes. The isotopes, such as fluorine, give off gamma radiation -- a form of energy that passes through the body and into a detector. When combined with sugar and given to a patient, the fluorine migrates to those tissues that are actively metabolizing sugar, such as areas of the brain in a person working on math problems.
PET scans show these body parts in clear detail. By observing the different levels of metabolism, a doctor can identify tell-tale signs of abnormalities such as tumors and dementia.
A Myocardial Perfusion Imaging scan uses radioactive isotopes to produce images in a method similar to a PET scan, but for monitoring the heart in real time.
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