Radioactivity

Biophysics 8

Radioactivity

Radioactivity refers to the particles which are emitted from nuclei as a result of nuclear instability. Because the nucleus experiences the intense conflict between the two strongest forces in nature, it should not be surprising that there are many nuclear isotopes which are unstable and emit some kind of radiation. The most common types of radiation are called alpha, beta, and gammaradiation, but there are several other varieties of radioactive decay.

Radioactive decay rates are normally stated in terms of their half-lives, and the

half-life of a given nuclear species is related to its radiation risk. The different

types of radioactivity lead to different decay paths which transmute the nuclei

into other chemical elements. Examining the amounts of the decay products

makes possible radioactive dating.

Radiation from nuclear sources is distributed equally in all directions, obeying

the inverse square law

Beta RadioactivityBeta particles are just electrons from the nucleus, the term "beta particle"

being an historical term used in the early description of radioactivity. The

high energy electrons have greater range of penetration than alpha

particles, but still much less than gamma rays. The radiation hazard from

betas is greatest if they are ingested.

Beta emission is accompanied by the emission of an electron antineutrino

which shares the momentum and energy of the decay.

The emission of the electron's antiparticle, the positron, is also called beta decay. Beta decay can be seen as the decay of one of the neutrons to a proton via the weak interaction. The use of a weak interaction Feynman diagram can clarify the process.

Beta Radioactivity

Nuclear ForcesWithin the incredibly small nuclear size, the two strongest forces in nature are pitted against each other. When the balance is broken, the resultant radioactivity yields particles of enormous energy.

The electron in a hydrogen atom is attracted to the proton nucleus with a force so strong that gravity and all other forces are negligible by comparison. But two protons touching each other would feel a repulsive force over 100 million times stronger!! So how can such protons stay in such close proximity? This may give you some feeling for the enormity of the nuclear strong force which holds the nuclei together.

Alpha RadioactivityComposed of two protons and two neutrons, the alpha particle is a nucleus of the element helium. Because of its very large mass (more than 7000 times the mass of the beta particle) and its charge, it has a very short range. It is not suitable for radiation therapy since its range is less than a tenth of a millimeter inside the body. Its main radiation hazard comes when it is ingested into the body; it has great destructive power within its short range. In contact with fast-growing membranes and living cells, it is positioned for maximum damage.

Alpha particle emission is modeled as a barrier penetration process. The

alpha particle is the nucleus of the helium atom and is the nucleus of highest

stability.

Gamma Radioactivity

Gamma radioactivity is composed of electromagnetic rays. It is distinguished from x-raysonly by the fact that it comes from the nucleus. Most gamma rays are somewhat higher in energy than x-rays and therefore are very penetrating. It is the most useful type of radiation for medical purposes, but at the same time it is the most dangerous because of its ability to penetrate large thicknesses of material.

Penetration of Matter

Though the most massive and most energetic of radioactive emissions, the

alpha particle is the shortest in range because of its strong interaction with

matter. The electromagnetic gamma ray is extremely penetrating, even

penetrating considerable thicknesses of concrete. The electron of beta

radioactivity strongly interacts with matter and has a short range.

UltravioletThe region just below the visible in wavelength is called the near ultraviolet. It is absorbed very strongly by most solid substances, and even absorbed appreciably by air. The shorter wavelengths reach the ionization energy for many molecules, so the far ultraviolet has some of the dangers attendent to other ionizing radiation. The tissue effects of ultraviolet include sunburn, but can have some therapeutic effects as well. The sun is a strong source of ultraviolet radiation, but atmospheric absorption eliminates most of the shorter wavelengths. The eyes are quite susceptible to damage from ultraviolet radiation. Welders must wear protective eye shields because of the uv content of welding arcs can inflame the eyes. Snow-blindness is another example of uv inflamation; the snow reflects uv while most other substances absorb it strongly.

Frequencies: 7.5 x 1014 - 3 x 1016 Hz

Wavelengths: 400 nm - 10 nm

Quantum energies: 3.1 - 124 eV

Ionizing Radiation Ionization is the ejection of one or more electrons from an atom or molecule to produce a fragment with a net positive charge (positive ion). The classification of radiation as "ionizing" is essentially a statement that it has enough quantum energy to eject an electron. This is a crucial distinction, since "ionizing radiation" can produce a number of physiological effects, such as those associated with risk of mutation or cancer, which non-ionizing radiation cannot directly produce at any intensity

Non-Ionizing Radiation

Ionization is the ejection of one or more electrons from an atom or molecule to

produce a fragment with a net positive charge (positive ion). In the

electromagnetic spectrum, radiation in the visible or longer wavelength range

does not have sufficient quantum energy to ionize an atom, so we classify it as

non-ionizing radiation. The threshold for ionization occurs somewhere in the

ultraviolet range, with the specific threshold depending upon the type of atom or

molecule. It typically takes a photon with energy in the range of a few electron

volts to ionize an atom.

If an atom absorbs a photon of electromagnetic radiation and remains intact,

there is a strong tendency for it to return to its ground state. Just as water runs

downhill, all physical systems will tend to move to lower energy levels. If the

quantum energy of the radiation absorbed is higher than the average thermal

energy of the molecules (that is, infrared or visible radiation), then the

downward transitions may emit radiation that leaves the material, or it may be

gradually transformed into general thermal energy in the material. Radiation in

the microwave or longer wavelengths generally just contributes to the random

molecular motion which we have described as thermal energy.

The net result of the absorption of non-ionizing radiation is generally just to

heat the sample. Of course if it heats it enough, then chemical changes are

likely to occur, but those chemical changes would be expected to be the same

changes that would occur as a result of any other source of heating.

X-Rays X-ray was the name given to the highly penetrating rays which emanated when high energy electrons struck a metal target. Within a short time of their discovery, they were being used in medical facilities to image broken bones. We now know that they are high frequency electromagnetic rays which are produced when the electrons are suddenly decelerated - these rays are called bremsstrahlung radiation, or "braking radiation". X-rays are also produced when electrons make transitions between lower atomic energy levels in heavy elements. X-rays produced in this way have have definite energies just like other line spectra from atomic electrons. They are called characteristic x-rays since they have energies determined by the atomic energy levels.

In interactions with matter, x-rays are ionizing radiation and produce physiological effectswhich are not observed with any exposure of non-ionizing radiation, such as the risk of mutations or cancer in tissue.

Frequencies: 3 x 1016 Hz upward

Wavelengths: 10 nm - > downward

Quantum energies: 124 eV -> upward

Bremsstrahlung X-Rays

"Bremsstrahlung" means "braking radiation" and is retained from the original German to describe the radiation which is emitted when electrons are decelerated or "braked" when they are fired at a metal target. Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high enough, that radiation is in the x-rayregion of the electromagnetic spectrum. It is characterized by a continuous distribution of radiation which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons is increased. The curves above are from the 1918 data of Ulrey, who bombarded tungsten targets with electrons of four different energies

Characteristic X-Rays Characteristic x-rays are emitted from heavy

elements when their electrons make

transitions between the lower atomic energy

levels. The characteristic x-rays emission

which shown as two sharp peaks in the

illustration at left occur when vacancies are

produced in the n=1 or K-shell of the atom

and electrons drop down from above to fill

the gap. The x-rays produced by transitions

from the n=2 to n=1 levels are called K-alpha

x-rays, and those for the n=3->1 transiton are

called K-beta x-rays.

Transitions to the n=2 or L-shell are

designated as L x-rays (n=3->2 is L-alpha,

n=4->2 is L-beta, etc. ). The continuous

distribution of x-rays which forms the base for

the two sharp peaks at left is called

"bremsstrahlung" radiation.

X-ray Tube

X-rays for medical diagnostic procedures or for research purposes are produced in a standard way: by accelerating electrons with a high voltage and allowing them to collide with a metal target. X-rays are produced when the electrons are suddenly decelerated upon collision with the metal target; these x-rays are commonly called brehmsstrahlungor "braking radiation". If the bombarding electrons have sufficient energy, they can knock an electron out of an inner shell of the target metal atoms. Then electrons from higher states drop down to fill the vacancy, emitting x-ray photons with precise energies determined by the electron energy levels. These x-rays are called characteristic x-rays.

Chandra X-ray ObservatorySince its launch on July 23, 1999, the Chandra X-ray Observatory has been NASA's flagship mission for X-ray astronomy,

taking its place in the fleet of "Great Observatories." It is the most sophisticated X-ray observatory built to date.

One of Chandra's investigations was the X-ray image of the Crab Nebula shown at left. It was included in a composited image created by NASA using X-ray, visible, and infrared images.

This set of Chandra images shows evidence for a light echo generated by the Milky Way's supermassive black hole, a.k.a.

Sagittarius A* (pronounced "A-star"). Astronomers believe a mass equivalent to the planet Mercury was devoured by the black

hole about 50 years earlier, causing an X-ray outburst which then reflected off gas clouds near Sagittarius A*. The large image

shows a Chandra view of the middle of the Milky Way, with Sagittarius A* labeled. The smaller images show close-ups of the

region marked with ellipses. Clear changes in the shapes and brightness of the gas clouds are seen between the 3 different

observations in 2002, 2004 and 2005. This behavior agrees with theoretical predictions for a light echo produced by

Sagittarius A* and helps rule out other interpretations.

While the primary X-rays from the outburst would have reached Earth about 50 years ago, before X-ray observatories were in

place to see it, the reflected X-rays took a longer path and arrived in time to be recorded by Chandra.

UltravioletThe region just below the visible in wavelength is called the near ultraviolet. It is absorbed

very strongly by most solid substances, and even absorbed appreciably by air. The shorter

wavelengths reach the ionization energy for many molecules, so the far ultraviolet has some

of the dangers attendent to other ionizing radiation. The tissue effects of ultraviolet include

sunburn, but can have some therapeutic effects as well. The sun is a strong source of

ultraviolet radiation, but atmospheric absorption eliminates most of the shorter wavelengths.

The eyes are quite susceptible to damage from ultraviolet radiation. Welders must wear

protective eye shields because of the uv content of welding arcs can inflame the eyes. Snow-

blindness is another example of uv inflamation; the snow reflects uv while most other

substances absorb it strongly.

Frequencies: 7.5 x 1014 - 3 x 1016 Hz

Wavelengths: 400 nm - 10 nm

Quantum energies: 3.1 - 124 eV

Ultraviolet Interactions

The near ultraviolet is absorbed very strongly in the surface layer of the skin by electron transitions. As you

go to higher energies, the ionization energies for many molecules are reached and the more dangerous

photoionization processes take place. Sunburn is primarily an effect of uv, and ionization produces the risk of

skin cancer.

The ozone layer in the upper atmosphere is important for human health because it absorbs most of the

harmful ultraviolet radiation from the sun before it reaches the surface. The higher frequencies in the

ultraviolet are ionizing radiation and can produce harmful physiological effects ranging from sunburn to skin

cancer.

Health concerns for UV exposure are mostly for the range 290-330 nm in wavelength, the range called UVB.

According to Scotto, et al, the most effective biological wavelength for producing skin burns is 297 nm. Their

research indicates that the biological effects increase logarithmically within the UVB range, with 330 nm

being only 0.1% as effective as 297 nm for biological effects. So it is clearly important to control exposure to

UVB.

Infrared The term "infrared" refers to a broad range of frequencies, beginning at the top

end of those frequencies used for communication and extending up the the low

frequency (red) end of the visible spectrum.

The wavelength range is from about 1 millimeter down to 750 nm. The range

adjacent to the visible spectrum is called the "near infrared" and the longer

wavelength part is called "far infrared".

In interactions with matter, infrared primarily acts to set molecules into vibration.

Infrared spectrometers are widely used to study the vibrational spectra of

molecules.

Infrared does not penetrate the atmosphere well, but astronomy in the infrared is

carried out with the Spitzer Space Telescope.

Frequencies: .003 - 4 x 1014 Hz

Wavelengths: 1 mm - 750 nm

Quantum energies: 0.0012 - 1.65 eV

Infrared Ear Thermometer Body temperature is routinely monitored in clinical settings with

infrared ear thermometers which measure the infrared energy emitted

from the patient's eardrum in a calibrated length of time. A short tube

with a protective sleeve is inserted into the ear, and a shutter is

opened to allow radiation from the tympanic membrane to fall on an

infrared detector for a period which is typically from 0.1 to 0.3

seconds in the varieties surveyed. The device beeps when data

collection is completed and a readout of temperature is produced on

a liquid crystal display.

This kind of temperature from the eardrum has been found to be a clinically reliable

indicator of body core temperature. The eardrum is located close to the

hypothalmus, which is the body's temperature regulator. The membrane itself is thin

and almost transparent in the visible, so you would presume that it reliably tracks

the temperature inside the membrane so that the infrared energy it emits gives a

good indication of the inside temperature.

The infrared energy falls on a thin pyroelectric crystal which develops a charge

proportional to that collected energy. Discharging the crystal sends a current pulse

through filters and conversion circuits which compare the signal to tabulated data on

temperature and calculate a body temperature for the display.

TV and FM Radio Band The carrier frequencies for VHF television Channels 2-4 cover the frequency range 54 to

72 MHz. There is a band from 72-76 MHz which is reserved for government and non-

government services, including a standard aeronautical beacon at 75 MHz. VHF TV

channels 5 and 6 are between 76 and 88 MHz. The FM radio band is from 88 to 108 MHz

between VHF television Channels 6 and 7.Above the FM is a range 108-122 MHz for

aeronautical navigation including localizers, radio ranging and airport control. From 122 to

174 MHz is another general service band for both government and non-government

signals. It includes fixed and mobile units and amateur broadcast. Channels 7 through 13

span the frequency range 174-216 MHz. 216-470 MHz includes a number of fixed and

mobile communication modes, including some aeronautical navigation and citizens radio.

470-890 MHz includes UHF television channels 14 to 83. Frequencies 890-3000 MHz

include a variety of aeronautical and amateur uses, studio-transmitter relays, etc. There

are radar bands 1,300-1,600 MHz.

The FM stations are assigned center frequencies at 200 kHz separation starting at 88.1

MHz, for a maximum of 100 stations. These FM stations have a 75 kHz maximum

deviation from the center frequency, which leaves 25 kHz upper and lower "gaurd bands"

to minimize interaction with the adjacent frequency band. Television channels have 5 MHz

separation.

The frequency range for mobile cellular telephones is listed as 824.040 - 848.970 MHz.

The Electromagnetic Spectrum

The Electromagnetic Spectrum

Speed of light

Radioactive Half-Life

• The radioactive half-life for a given radioisotope is the time for half the radioactive nuclei in any sample to undergo radioactive decay. After two half-lives, there will be one fourth the original sample, after three half-lives one eight the original sample, and

so forth.

Radiation Risk

• Because the energies of the particles emitted during radioactive processes are extremely high, nearly all such particles fall in the class of ionizing radiation.

Activity of source Absorbed doseBiologically

effective doseIntensity

Old standard unit Curie Rad Rem Roentgen

SI unit Becquerel Gray Sievert ...

Meet the Millirem

• The millirem is a unit of absorbed radiation dose.

• A person would get this amount of radiation from

• Three days of living in Atlanta

• Two days of living in Denver

• About seven hours in some spots in the Espirito Santo State of Brazil.

• You increase your dose by a millirem by:

• an average year of TV watching

• a year of wearing a luminous dial watch

• a coast-to-coast airline flight

• a year living next door to a normally operating nuclear power plant

Biologically Effective Dose

• The biologically effective dose in rems is the radiation dose in rads multiplied by a "quality factor" which is an assessment of the effectiveness of that particular type and energy of radiation. For alphaparticles the relative biological effectiveness (rbe) may be as high as 20, so that one rad is equivalent to 20 rems. However, for x-rays and gamma rays, the rbe is taken as one so that the rad and rem are equivalent for those radiation sources. The sievert is equal to 100 rems.

Absorbed Dose of Radiation

• The rad is a unit of absorbed radiation dose in terms of the energy actually deposited in the tissue. The rad is defined as an absorbed dose of 0.01 joules of energy per kilogram of tissue. The more recent SI unit is the gray, which is defined as 1joule of deposited energy per kilogram of tissue. To assess the risk of radiation, the absorbed dose is multiplied by the relative biological effectiveness of the radiation to get the biological dose equivalent in rems or sieverts.

Typical Radiation Doses in Millirem

Note: This does not include radon exposures, which may be very high.

Environmental Radiation Exposure

Source Exposure in mrem/yr

Cosmic rays 45

External radiation from

radioactive ores, etc. 60

Internal exposure from

radioactive material

ingested into the body

25

Diagnostic X-rays 70

Total: 200

U.S. Average Exposure

Note: This does not include radon exposures, which may be very high.

Radon compared to other radiation sources

• The "action level" recommended by the Environmental Protection Agency for radon in the air is 4 picocuries/liter of air. It is difficult to convert air concentrations to actual exposures in rems or sieverts, but estimates are in the range of 4 to 14 rem per year at that concentration. That makes it greater that all the other routine environmental exposures combined.

The percentages for radon exposure and

other sources in the pie chart at left were

attributed to the EPA in an article by

Charles Seabrook in The Atlanta Journal

on July 10,1990. Seabrook says 40000

lung cancer deaths a year are blamed on

radon. 15-20% of homes have

unacceptably high levels (presumably

meaning > 4 pCi/l).

At what level is Radon dangerous?• It is generally assumed that no level of ionizing radiation is completely safe,

but one must try to set a threshold of reasonably tolerable risk. The EPA suggests an "action level" of 4 picocuries per liter in air. Lutgens & Tarbuck suggest that this level corresponds ot about 8 to 9 atoms of radon decaying every minute in every liter of air. According to Cohen, the mean radon level in houses throughout the western world is about 1 pCi/l.

• Cohen has expressed the risks of radiation in terms of "loss of life expectancy". He places the loss of life expectancy of 1 millirem of radiation at 1.2 minutes, and the loss of life expectancy of 20 hours for a years exposure to 1 pCi/l in your home. By this admittedly over-simplified model, you can project the equivalent exposure of 4 pCi/l for a full year as equivalent to 4000 millirem or 4 rem. The maximum permissible occupational exposure for persons working in radiation related occupations is 5 rem per year.

• Other sources estimate the dose equivalent of 4 pCi/l to be as high as 14 rem, but I'm not clear on the rationale for that high figure.

Press Overreaction to Radiation

• When it became evident in the 70's and 80's that radon exposure in one's own home is probably greater than the radiation exposures which had regularly been trumpeted in headlines, we entered another awkward era in the reporting of radiation issues.

Radiation in your own home?

What is radon? And why are they saying all those bad things about it?

• Radon is a colorless, odorless gas, a radioactive byproduct of radium. It is part of the natural radioactive decay series starting with uranium-238. It is radioactive with a half-life of 3.8 days, decaying by the emission of alpha particles to polonium, bismuth, and lead in successive steps.

• The decay of radon-222 with emission of an alpha particle is followed within about an hour by a series of four further decays, two of them accompanied by emission of alpha particles and the other two accompanied by other types of radiation. The short-lived atoms into which a radon atom decays are actually isotopes of polonium, lead, and bismuth, but they are referred to collectively as radon daughters, or, by those sensitive to questions of gender, as radon progeny. The radon daughter atoms float around in the air during their few minutes of existence, often becoming attached to dust particles.

• In summary, a radon atom in the air decays within a few days into its short-half-life radon daughters, which decay within about an hour; with these decays, three alpha particles are emitted, one by radon and two by its daughters.

Where does Radon come from?• Radon is a naturally occurring radioisotope. Radioactivity is, and always has

been a part of the earth. Radon-222 is one of the elements in the long radioactive decay chain from uranium-238, and the less common isotope radon-220 is part of the decay series from thorium-232. The elements above radon in the chain are relatively long-lived and of less concern for radiation exposure, but radon and the elements immediately following it in the chain are short-lived and therefore more hazardous.

• Whereas the predecessors to radon in the chain are solids and will not migrate far from their place in the soil, radon is a gas and can migrate through a few feet of earth. Cohen says that on the average, about six atoms of radon emerge form every square inch of soil every second. Radon in outside air is diluted rapidly, but if it enters through a basement floor and is trapped in a tight house, it can reach high concentrations.

• From Lutgens & Tarbuck there is an estimate that the top 6 feet of soil from an average acre of land contains about 50 lbs of uranium. This corresponds to about 2 to 3 parts per million.

At what level is Radon dangerous?• It is generally assumed that no level of ionizing radiation is completely safe,

but one must try to set a threshold of reasonably tolerable risk. The EPA suggests an "action level" of 4 picocuries per liter in air. Lutgens & Tarbuck suggest that this level corresponds ot about 8 to 9 atoms of radon decaying every minute in every liter of air. According to Cohen, the mean radon level in houses throughout the western world is about 1 pCi/l.

• Cohen has expressed the risks of radiation in terms of "loss of life expectancy". He places the loss of life expectancy of 1 millirem of radiation at 1.2 minutes, and the loss of life expectancy of 20 hours for a years exposure to 1 pCi/l in your home. By this admittedly over-simplified model, you can project the equivalent exposure of 4 pCi/l for a full year as equivalent to 4000 millirem or 4 rem. The maximum permissible occupational exposure for persons working in radiation related occupations is 5 rem per year.

• Other sources estimate the dose equivalent of 4 pCi/l to be as high as 14 rem, but I'm not clear on the rationale for that high figure.

Radon in the Air

• One would think that radon was the least of our radiation problems since it is an inert gas. That would be so except that when we breathe, we are constantly passing air into our lungs and out of them. In this process, the radon gas simply goes in and out, doing little damage, but the radon daughters, being basically solid materials, and sometimes being electrically charged, can stick to the surfaces of our bronchial tubes. This puts them right where they can do the most harm, for the cells lining our bronchial tubes are among the cells of our body most sensitive to radiation-induced cancer. The alpha particles emitted in the decay of radon daughters, in spite of their poor penetrating power, can reach these very sensitive cells because they are deposited so close to them. To make matters very much worse, alpha particles are much more efficient than other types of radiation for inducing cancer. The very fact that they are not penetrating means that they dump a lot of their energy into each of the biological cells they pass through, and this large release of energy into a single cell is just what is needed to initiate a cancer. As a result an alpha particle is a hundred times more likely to cause cancer than other types of radiation, if it can reach the target cells. Our breathing processes allows the alpha particles from radon daughters to reach these cells.

• Radon is believed to be an important cause of lung cancer, killing about 10,000 Americans each year. Only cigarette smoking causes more lung cancer deaths per year. And in perhaps one out of a thousand American homes, radon levels are so high they pose a greater lung cancer risk than smoking a pack of cigarettes per day.

Correlation between smoking and lung cancer

More about the Watras home

• A new nuclear power plant was preparing to start up near Pottstown, Pennsylvania, and its management began requiring workers to pass through a radiation monitoring portal to check whether any radioactive contamination was being carried out as they left the plant. Curiously, one of the workers, Stanley Watras, set off the alarm on entering the plant. The problem was traced to radon in his home. In fact, to this day his house still holds the world record, 2,700 pCi/l!

• The Watras house is on the Reading Prong, a granite formation that extends from near Reading in southeastern Pennsylvania, through a wide band of northern New Jersey State (e.g., Morris County), a narrow band in New York State (e.g., Putnam County), and into Connecticut. This whole formation is known to have high uranium content and hence was expected to have radon problems. However, measurements now make it clear that the radon problems are largely confined to the Pennsylvania section plus extreme western New Jersey.

Acute Radiation Exposure

Effect Dose (rems)

No observable effect 0-25

Slight blood changes 25-100

Significant reduction in blood

platelets and

white blood cells (temporary)

100-200

Severe blood damage, nausea,

hair loss,

hemorrhage, death in many

cases

200-500

Death in less than two months

for over 80% >600

Effects of Large, Whole-Body Radiation Doses