21.3 Radioactive Decay

Many prominent scientists began to investigate this new phenomenon after the discovery of radioactivity by Becquerel.

Among them were Marie Curie, the first woman to win a Nobel Prize, and the only person to win two Nobel Prizes in different sciences--chemistry and physics, and Ernest Rutherford, who investigated and named gold foil experiment fame.
Many radioactive substances were discovered, the properties of radiation were investigated and a solid understanding of radiation and nuclear decay was developed.

The daughter nuclide can be stable or it can decay.

The parent nuclide undergoes a decay to form the daughter nuclide.

There are two protons and two neutrons in the nucleus.

Although the radioactive decay of a nucleus is too small to see with the naked eye, we can indirectly view it in a cloud chamber.

You can learn about cloud chambers and view a demonstration from the Jefferson Lab by clicking here.

One type of radiation consisted of positively charged and relatively massive particles, a second type was made up of negatively charged and less massive particles, and a third type was un charged.

We now know that particles are high-energy, b particles are highenergy, and g radiation is high-energy.
There are different types of radioactive decay.

Alpha particles that are attracted to the negative plate must be large and positive.

The negatively charged and light particles that are attracted to the positive plate must be negatively charged.
The electric field can't charge the Gamma rays.

Alpha decay occurs in heavy nuclei.

The daughter nuclide has a larger n:p ratio than the parent nuclide because of the loss of an a particle.

The conversion of a neutron into a protons and a b particle is observed in nuclides with a large n:p ratio.

The alpha particle is not part of the electrons surrounding the nucleus.
Emission of an electron does not change the mass number of the nuclide, but it does increase the number of its protons and decrease the number of its neutrons.

The daughter nuclide is closer to the band of stability than the parent nuclide.

There is a nucleus in an excited state.

Nuclear chemistry is one of the modes of decay.

The n:p ratio is low for nuclides that have a +2 b + 7 N Positron emission.

The nuclides are below the band of stability.

The emission of a positron is the result of Positron decay.

The daughter nuclide is closer to the band of stability than the parent nuclide.

An inner shell electron combines with a protons and is converted into a neutron.

One of the outer electrons will fill the void left by the loss of an inner shell electron.
The outer electron will emit energy.
In most cases, the X-ray will be the source of the energy.
Like positron emission, electron capture occurs for "proton-rich" nuclei that lie below the band of stability.
Positron emission and electron capture have the same effect on the nucleus, with the atomic number decreasing by one and the mass number not changing.
The daughter nuclide is closer to the band of stability than the parent nuclide.
Predicting whether electron capture or positron emission occurs is difficult.
The choice is due to the fact that the smaller activation energy is more likely to occur.

Figure 21.7 shows the types of decay, along with their equations and changes in atomic and mass numbers.

The table summarizes the type, nuclear equation, representation, and any changes in the mass or atomic numbers for various types of decay.

Positron emission tomography (PET) scans use radiation to diagnose and track health conditions and monitor medical treatments by revealing how parts of a patient's body function.

A radioisotope is produced in a cyclotron and attached to a substance that is used to investigate a part of the body.
This "tagged" compound, or radiotracer, is put into the patient and how it is used by the tissue reveals how that organ or other area of the body functions.

A patient's body is being imaged with the help of radiation.

The scans it produces can be used to image a healthy brain or to diagnose a condition such as Alzheimer's disease.

9 F + 0 n is incorporated into a FDG.

Critical diagnostic information can be provided by how FDG is used by the body.

The 18F emits positrons that interact with nearby electrons.

A detailed, three-dimensional, color image of that part of the patient's body is created when this energy is detected by the scanner.
Different levels of radiation produce different amounts of brightness and colors in the image, which can be seen by a doctor.
Alzheimer's disease can be diagnosed with the help of a PET Scan, as well as heart damage, cancer, and how much it has spread, and whether or not treatments are effective.
The advantage of PET scans is that they show how something works, unlike magnetic resonance and X-rays, which only show how something looks.
Positron Emission Tomography scans are usually performed in conjunction with a computed tomography Scan.

Most of the naturally radioactive elements of the periodic table are included in three of these series.

They are the thorium, actinide, and uranium series.
The neptunium series is no longer significant on the earth because of the short half-lives of the species involved.
A nuclide on the band of stability is the result of a long half-life and a series of daughter nuclides.
The end-product is a stable isotope of lead.
The neptunium series ends with thallium- 205.

A radioactive decay series consisting of 14 separate steps leads to stable lead-206.

There are eight decays and six b decays in this series.

Radioactive decay is followed by first-order kinetics.

Since first-order reactions have already been covered in detail, we will apply those concepts to nuclear decay reactions.
Half of the atoms in a sample need to decay.
An isotope's halflife allows us to determine how long a sample of a useful isotope will be available, and how long a sample of an undesirable or dangerous isotope must be stored before it decays to a low-enough radiation level that is no longer a problem.

Half of the 60 27 Co nucleus decays every 5.27 years in a given cobalt-60 source, both the amount of material and the intensity of the decay.

This is what it would be for a process after first-order kinetics.
The source that is used for cancer treatment needs to be replaced regularly.

50% remains after 5.27 years, 25% remains after 10.54 years, and 25% remains after 15.81 years for cobalt-60, which has a half-life of five years.

The mathematical relationships used for first-order chemical reactions can be adjusted.

The activity of the radioactive sample is referred to as the rate in nuclear decays per second.

27 Co decays with a half-life of 5.27 years to produce 28 Ni.

The co that will remain after 15 years is 0.138.

After 15 years, 13.8% of the 27 Co originally present will remain.

86 Rn has a half-life of 3.823 days.

Half-lives of radioactive nuclide vary widely because they have a specific number of nucleons, balance of repulsion and attraction, and degree of stability.

94 Ra is 24,000 years and 86 Rn is 3.82 days.

The half-lives of a number of radioactive isotopes important to medicine are shown in Table 21.2 and Appendix M.

Several radioisotopes have half-lives and other properties that make them useful for purposes of "dating" the origin of objects such as archaeological artifacts, formerly living organisms, or geological formations.

This is an unstable, high-energy state of Tc-99, and the "m" in Tc-99m indicates this.
Excess energy can be rid of by emitting g radiation.

The history of the earth, the evolution of life, and the history of human civilization can be found in this OpenStax book.

Some of the most common types of radioactive dating will be explored.

The method for dating objects that were part of a living organisms is provided by the radioactivity of carbon-14.

The natural abundance of 6 CO in the atmosphere is 1 part per trillion; until recently, this has been constant over time, as seen by gas samples found trapped in ice.

The 6 C: 6 C ratio found in a living plant is the same as the 6 C: 6 C ratio in the atmosphere.

When the plant dies, it no longer traps carbon.
The concentration of 126C in the plant does not change because it is a stable isotope.

After the plant dies, the 6 C ratio decreases.

The time that has elapsed since the death of the plant is provided by the decrease in the ratio.

Plants and animals take in carbon-12 and carbon-14 at a constant level, and remain radioactive even after death.

We can determine how long ago the organisms lived by comparing this ratio to the C-14:C-12 ratio in living organisms.

If the 6 C: 6 C ratio in a wooden object found in an archaeological dig is half what it is in a living tree, this indicates that the wooden object is 5730 years old.

6 C: 6 C ratios can be obtained from very small samples.

There are simulations of dating.

A piece of paper taken from the Dead Sea Scrolls has an activity of 10.8 disintegrations per minute per gram of carbon.

The age of the Dead Sea Scrolls can be estimated if the initial C-14 activity was 13.6 disintegrations/min/g of C.

The Dead Sea Scrolls were written on paper made from plants that died between 100 BC and AD 50.

Plants that were preserved in the tombs of ancient Egyptian pharaohs have been used to determine more accurate dates of their reigns.

It is not always valid to say that a living plant's 6 C ratio is the same as it was in the past.

The atmosphere may be changing.

The ratio in organisms currently living on the earth is affected by the 6 C ratio decreasing.

We can use other data, such as tree dating, to calculate correction factors.

Accurate dates can be determined with these correction factors.
The limit for carbon-14 dating is about 57,000 years, since radioactive dating only works for 10 half-lives.

Other radioactive nuclides can also be used to date older events.

The age of rocks can be established using the decays in a series of steps into lead-206.
It takes 4.5 billion years for half of the original U-238 to decay into Pb-206.
We can assume that lead was not present in the sample when it was formed.
The age of the rock can be determined by measuring and analyzing the ratio of U-238:Pb-206.

The lead-206 present is assumed to have come from the decay of uranium-238.

It is necessary to make an adjustment if there is additional lead-206 present.

A similar method is used for dating.

Ar-40 has a half-life of 1.25 billion years.
The age of the rock is determined by the amount of Ar-40 gas that escapes and the Ar-40:K-40 ratio.
rubidium-strontium dating (Rb-87 decays into Sr-87 with a half-life of 48.8 billion years), is one of the methods that operate on the same principle.
The age of various rocks and minerals is used by scientists to estimate the age of the earth.
The Jack Hills zircons from Australia are the oldest known rocks on the planet and were found to be almost 4.5 billion years old.

The rock has U-238 and Pb-206 in it and less than 10 g of Pb-206.

Determine when the rock formed.

We can assume that all the Pb-206 in the rock was produced by the radioactive decay of U-238, because the sample of rock contains very little Pb-208, the most common isotope of lead.

When the rock was formed, it contained all of U-238 and some that have since been radioactive.