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HalfLife and Activity

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HalfLife and Activity
CHAPTER 31 | RADIOACTIVITY AND NUCLEAR PHYSICS
daughter nearer the region of stability. Similarly, those nuclides having relatively more protons than those in the region of stability will
β − decay or
undergo electron capture to produce a daughter with fewer protons, nearer the region of stability.
Gamma Decay
Gamma decay is the simplest form of nuclear decay—it is the emission of energetic photons by nuclei left in an excited state by some earlier
process. Protons and neutrons in an excited nucleus are in higher orbitals, and they fall to lower levels by photon emission (analogous to electrons in
excited atoms). Nuclear excited states have lifetimes typically of only about 10 −14 s, an indication of the great strength of the forces pulling the
nucleons to lower states. The
γ decay equation is simply
A *
Z XN
where the asterisk indicates the nucleus is in an excited state. There may be one or more
radioactive decay,
γ s emitted, depending on how the nuclide de-excites. In
γ emission is common and is preceded by γ or β decay. For example, when
daughter nucleus in an excited state, written 60 Ni* . Then the nickel nucleus quickly
60
These are called cobalt
(31.34)
→ X N + γ 1 + γ 2 + ⋯ (γ decay)
Ni* →
60
60
Co β − decays, it most often leaves the
γ decays by the emission of two penetrating γ s:
(31.35)
Ni + γ 1 + γ 2.
γ rays, although they come from nickel—they are used for cancer therapy, for example. It is again constructive to verify the
γ decay does not change the nuclide to another species, it is not prominently featured in charts of
conservation laws for gamma decay. Finally, since
decay series, such as that in Figure 31.16.
There are other types of nuclear decay, but they occur less commonly than
α , β , and γ decay. Spontaneous fission is the most important of the
other forms of nuclear decay because of its applications in nuclear power and weapons. It is covered in the next chapter.
31.5 Half-Life and Activity
Unstable nuclei decay. However, some nuclides decay faster than others. For example, radium and polonium, discovered by the Curies, decay faster
than uranium. This means they have shorter lifetimes, producing a greater rate of decay. In this section we explore half-life and activity, the
quantitative terms for lifetime and rate of decay.
Half-Life
Why use a term like half-life rather than lifetime? The answer can be found by examining Figure 31.21, which shows how the number of radioactive
nuclei in a sample decreases with time. The time in which half of the original number of nuclei decay is defined as the half-life, t 1 / 2 . Half of the
remaining nuclei decay in the next half-life. Further, half of that amount decays in the following half-life. Therefore, the number of radioactive nuclei
decreases from N to N / 2 in one half-life, then to N / 4 in the next, and to N / 8 in the next, and so on. If N is a large number, then many halflives (not just two) pass before all of the nuclei decay. Nuclear decay is an example of a purely statistical process. A more precise definition of half-life
is that each nucleus has a 50% chance of living for a time equal to one half-life t 1 / 2 . Thus, if N is reasonably large, half of the original nuclei decay
in a time of one half-life. If an individual nucleus makes it through that time, it still has a 50% chance of surviving through another half-life. Even if it
happens to make it through hundreds of half-lives, it still has a 50% chance of surviving through one more. The probability of decay is the same no
matter when you start counting. This is like random coin flipping. The chance of heads is 50%, no matter what has happened before.
Figure 31.21 Radioactive decay reduces the number of radioactive nuclei over time. In one half-life
t 1 / 2 , the number decreases to half of its original value. Half of what
remains decay in the next half-life, and half of those in the next, and so on. This is an exponential decay, as seen in the graph of the number of nuclei present as a function of
time.
−23
16
There is a tremendous range in the half-lives of various nuclides, from as short as 10
s for the most unstable, to more than 10
y for the least
unstable, or about 46 orders of magnitude. Nuclides with the shortest half-lives are those for which the nuclear forces are least attractive, an
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CHAPTER 31 | RADIOACTIVITY AND NUCLEAR PHYSICS
indication of the extent to which the nuclear force can depend on the particular combination of neutrons and protons. The concept of half-life is
applicable to other subatomic particles, as will be discussed in Particle Physics. It is also applicable to the decay of excited states in atoms and
nuclei. The following equation gives the quantitative relationship between the original number of nuclei present at time zero ( N 0 ) and the number (
N ) at a later time t :
N = N 0e −λt,
where
(31.36)
e = 2.71828... is the base of the natural logarithm, and λ is the decay constant for the nuclide. The shorter the half-life, the larger is the
value of
λ , and the faster the exponential e −λt decreases with time. The relationship between the decay constant λ and the half-life t 1 / 2 is
ln(2) 0.693
λ= t
≈ t
.
1/2
1/2
To see how the number of nuclei declines to half its original value in one half-life, let
gives
(31.37)
t = t 1 / 2 in the exponential in the equation N = N 0e −λt . This
N = N 0 e −λt = N 0 e −0.693 = 0.500N 0 . For integral numbers of half-lives, you can just divide the original number by 2 over and over again,
rather than using the exponential relationship. For example, if ten half-lives have passed, we divide
For an arbitrary time, not just a multiple of the half-life, the exponential relationship must be used.
N by 2 ten times. This reduces it to N / 1024 .
Radioactive dating is a clever use of naturally occurring radioactivity. Its most famous application is carbon-14 dating. Carbon-14 has a half-life of
5730 years and is produced in a nuclear reaction induced when solar neutrinos strike 14 N in the atmosphere. Radioactive carbon has the same
chemistry as stable carbon, and so it mixes into the ecosphere, where it is consumed and becomes part of every living organism. Carbon-14 has an
abundance of 1.3 parts per trillion of normal carbon. Thus, if you know the number of carbon nuclei in an object (perhaps determined by mass and
Avogadro’s number), you multiply that number by 1.3×10 −12 to find the number of 14 C nuclei in the object. When an organism dies, carbon
exchange with the environment ceases, and 14 C is not replenished as it decays. By comparing the abundance of 14 C in an artifact, such as
mummy wrappings, with the normal abundance in living tissue, it is possible to determine the artifact’s age (or time since death). Carbon-14 dating
can be used for biological tissues as old as 50 or 60 thousand years, but is most accurate for younger samples, since the abundance of 14 C nuclei
in them is greater. Very old biological materials contain no 14 C at all. There are instances in which the date of an artifact can be determined by
other means, such as historical knowledge or tree-ring counting. These cross-references have confirmed the validity of carbon-14 dating and
permitted us to calibrate the technique as well. Carbon-14 dating revolutionized parts of archaeology and is of such importance that it earned the
1960 Nobel Prize in chemistry for its developer, the American chemist Willard Libby (1908–1980).
One of the most famous cases of carbon-14 dating involves the Shroud of Turin, a long piece of fabric purported to be the burial shroud of Jesus (see
Figure 31.22). This relic was first displayed in Turin in 1354 and was denounced as a fraud at that time by a French bishop. Its remarkable negative
imprint of an apparently crucified body resembles the then-accepted image of Jesus, and so the shroud was never disregarded completely and
remained controversial over the centuries. Carbon-14 dating was not performed on the shroud until 1988, when the process had been refined to the
point where only a small amount of material needed to be destroyed. Samples were tested at three independent laboratories, each being given four
pieces of cloth, with only one unidentified piece from the shroud, to avoid prejudice. All three laboratories found samples of the shroud contain 92% of
the 14 C found in living tissues, allowing the shroud to be dated (see Example 31.4).
Figure 31.22 Part of the Shroud of Turin, which shows a remarkable negative imprint likeness of Jesus complete with evidence of crucifixion wounds. The shroud first surfaced
in the 14th century and was only recently carbon-14 dated. It has not been determined how the image was placed on the material. (credit: Butko, Wikimedia Commons)
Example 31.4 How Old Is the Shroud of Turin?
Calculate the age of the Shroud of Turin given that the amount of 14 C found in it is 92% of that in living tissue.
Strategy
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CHAPTER 31 | RADIOACTIVITY AND NUCLEAR PHYSICS
Knowing that 92% of the 14 C remains means that
N / N 0 = 0.92 . Therefore, the equation N = N 0e −λt can be used to find λt . We also
λt is known, we can use the equation λ = 0.693
t 1 / 2 to find λ and then find t as
know that the half-life of 14 C is 5730 y, and so once
requested. Here, we postulate that the decrease in 14 C is solely due to nuclear decay.
Solution
Solving the equation
N = N 0e −λt for N / N 0 gives
N = e −λt.
N0
(31.38)
0.92 = e −λt.
(31.39)
Thus,
Taking the natural logarithm of both sides of the equation yields
ln 0.92 = –λt
(31.40)
−0.0834 = −λt.
(31.41)
t = 0.0834 .
λ
(31.42)
so that
Rearranging to isolate
Now, the equation
t gives
λ = 0.693
t 1 / 2 can be used to find λ for
We enter this value into the previous equation to find
14
C . Solving for λ and substituting the known half-life gives
0.693
λ = 0.693
t 1 / 2 = 5730 y .
(31.43)
t = 0.0834
= 690 y.
0.693
(31.44)
t:
5730 y
Discussion
This dates the material in the shroud to 1988–690 = a.d. 1300. Our calculation is only accurate to two digits, so that the year is rounded to 1300.
The values obtained at the three independent laboratories gave a weighted average date of a.d. 1320 ± 60 . The uncertainty is typical of
carbon-14 dating and is due to the small amount of 14 C in living tissues, the amount of material available, and experimental uncertainties
(reduced by having three independent measurements). It is meaningful that the date of the shroud is consistent with the first record of its
existence and inconsistent with the period in which Jesus lived.
There are other forms of radioactive dating. Rocks, for example, can sometimes be dated based on the decay of 238 U . The decay series for 238 U
ends with 206 Pb , so that the ratio of these nuclides in a rock is an indication of how long it has been since the rock solidified. The original
composition of the rock, such as the absence of lead, must be known with some confidence. However, as with carbon-14 dating, the technique can
9
be verified by a consistent body of knowledge. Since 238 U has a half-life of 4.5×10 y, it is useful for dating only very old materials, showing, for
example, that the oldest rocks on Earth solidified about
3.5×10 9 years ago.
Activity, the Rate of Decay
What do we mean when we say a source is highly radioactive? Generally, this means the number of decays per unit time is very high. We define
activity R to be the rate of decay expressed in decays per unit time. In equation form, this is
R = ΔN
Δt
where ΔN is the number of decays that occur in time
honor of the discoverer of radioactivity. That is,
Δt . The SI unit for activity is one decay per second and is given the name becquerel (Bq) in
1 Bq = 1 decay/s.
Activity
(31.45)
(31.46)
R is often expressed in other units, such as decays per minute or decays per year. One of the most common units for activity is the curie
(Ci), defined to be the activity of 1 g of 226 Ra , in honor of Marie Curie’s work with radium. The definition of curie is
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1 Ci = 3.70×10 10 Bq,
or
(31.47)
3.70×10 10 decays per second. A curie is a large unit of activity, while a becquerel is a relatively small unit. 1 MBq = 100 microcuries (µCi) .
In countries like Australia and New Zealand that adhere more to SI units, most radioactive sources, such as those used in medical diagnostics or in
physics laboratories, are labeled in Bq or megabecquerel (MBq).
Intuitively, you would expect the activity of a source to depend on two things: the amount of the radioactive substance present, and its half-life. The
greater the number of radioactive nuclei present in the sample, the more will decay per unit of time. The shorter the half-life, the more decays per unit
time, for a given number of nuclei. So activity R should be proportional to the number of radioactive nuclei, N , and inversely proportional to their
half-life, t 1 / 2 . In fact, your intuition is correct. It can be shown that the activity of a source is
R = 0.693N
t1 / 2
where
(31.48)
N is the number of radioactive nuclei present, having half-life t 1 / 2 . This relationship is useful in a variety of calculations, as the next two
examples illustrate.
Example 31.5 How Great Is the
14
C Activity in Living Tissue?
Calculate the activity due to 14 C in 1.00 kg of carbon found in a living organism. Express the activity in units of Bq and Ci.
Strategy
To find the activity
R using the equation R = 0.693N
t 1 / 2 , we must know N and t 1 / 2 . The half-life of
was stated above as 5730 y. To find
we then multiply by
1.3×10
−12
N , we first find the number of
(the abundance of
14
12
14
C can be found in Appendix B, and
C nuclei in 1.00 kg of carbon using the concept of a mole. As indicated,
C in a carbon sample from a living organism) to get the number of
14
C nuclei in a
living organism.
Solution
One mole of carbon has a mass of 12.0 g, since it is nearly pure 12 C . (A mole has a mass in grams equal in magnitude to
A found in the
periodic table.) Thus the number of carbon nuclei in a kilogram is
23
–1
N( 12 C) = 6.02×10 mol ×(1000 g) = 5.02×10 25 .
12.0 g/mol
(31.49)
So the number of 14 C nuclei in 1 kg of carbon is
N( 14 C) = (5.02×10 25)(1.3×10 −12) = 6.52×10 13.
Now the activity
(31.50)
R is found using the equation R = 0.693N
t1 / 2 .
Entering known values gives
R=
or
0.693(6.52×10 13)
= 7.89×10 9 y –1,
5730 y
(31.51)
7.89×10 9 decays per year. To convert this to the unit Bq, we simply convert years to seconds. Thus,
R = (7.89×10 9 y –1)
or 250 decays per second. To express
1.00 y
= 250 Bq,
3.16×10 7 s
(31.52)
R in curies, we use the definition of a curie,
R=
250 Bq
= 6.76×10 −9 Ci.
3.7×10 10 Bq/Ci
(31.53)
Thus,
R = 6.76 nCi.
(31.54)
Discussion
Our own bodies contain kilograms of carbon, and it is intriguing to think there are hundreds of 14 C decays per second taking place in us.
Carbon-14 and other naturally occurring radioactive substances in our bodies contribute to the background radiation we receive. The small
number of decays per second found for a kilogram of carbon in this example gives you some idea of how difficult it is to detect 14 C in a small
sample of material. If there are 250 decays per second in a kilogram, then there are 0.25 decays per second in a gram of carbon in living tissue.
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CHAPTER 31 | RADIOACTIVITY AND NUCLEAR PHYSICS
To observe this, you must be able to distinguish decays from other forms of radiation, in order to reduce background noise. This becomes more
difficult with an old tissue sample, since it contains less 14 C , and for samples more than 50 thousand years old, it is impossible.
Human-made (or artificial) radioactivity has been produced for decades and has many uses. Some of these include medical therapy for cancer,
medical imaging and diagnostics, and food preservation by irradiation. Many applications as well as the biological effects of radiation are explored in
Medical Applications of Nuclear Physics, but it is clear that radiation is hazardous. A number of tragic examples of this exist, one of the most
disastrous being the meltdown and fire at the Chernobyl reactor complex in the Ukraine (see Figure 31.23). Several radioactive isotopes were
released in huge quantities, contaminating many thousands of square kilometers and directly affecting hundreds of thousands of people. The most
significant releases were of 131 I , 90 Sr , 137 Cs , 239 Pu , 238 U , and 235 U . Estimates are that the total amount of radiation released was about
100 million curies.
Human and Medical Applications
Figure 31.23 The Chernobyl reactor. More than 100 people died soon after its meltdown, and there will be thousands of deaths from radiation-induced cancer in the future.
While the accident was due to a series of human errors, the cleanup efforts were heroic. Most of the immediate fatalities were firefighters and reactor personnel. (credit: Elena
Filatova)
Example 31.6 What Mass of
137
Cs Escaped Chernobyl?
It is estimated that the Chernobyl disaster released 6.0 MCi of 137 Cs into the environment. Calculate the mass of 137 Cs released.
Strategy
N released.
Since the activity R is given, and the half-life of 137 Cs is found in Appendix B to be 30.2 y, we can use the equation R = 0.693N to find N
t1 / 2
We can calculate the mass released using Avogadro’s number and the concept of a mole if we can first find the number of nuclei
.
Solution
Solving the equation
R = 0.693N
t 1 / 2 for N gives
Rt 1/2
.
0.693
(31.55)
(6.0 MCi)(30.2 y)
.
0.693
(31.56)
N=
Entering the given values yields
N=
Converting curies to becquerels and years to seconds, we get
(6.0×10 6 Ci)(3.7×10 10 Bq/Ci)(30.2 y)(3.16×10 7 s/y)
0.693
26
= 3.1×10 .
N =
One mole of a nuclide A X has a mass of
A grams, so that one mole of
the mass of 137 Cs released was
m =
137
Cs has a mass of 137 g. A mole has 6.02×10 23 nuclei. Thus
⎛ 137 g ⎞
26
3
⎝6.02×10 23 ⎠(3.1×10 ) = 70×10 g
= 70 kg.
(31.57)
(31.58)
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