Radioactive decay pt. 2
Class 37: (ThT Q)
2
mc
What role does E =
play
in nuclear physics?
Did you complete at least
70% of Chapter 44:5-7?
A.Yes
B.No
More on T½,
Writing Nuclear reactions.
4-5 common ones
Q of reaction.
Applications/ facts about
Radioactivity.
Natural Radioactivity. Facts
Lab 8 & 9 began.
Today: More on T½, Writing Nuclear
reactions.
4-5 common ones
Q of reaction. Applications/ facts about
Radioactivity. Natural Radioactivity. Facts
If you used the physics tutorial lab this
semester, we invite you to rate the tutors you
received help from. The rating form, as well as
the photos of the tutors, is at
http://gardner.byu.edu/tas/tutorrating.html
►
Lab 8 Postponed.
-►
Exam Redo was Due
on Wed
Viewing assignment for Friday Apr. 10 Microwaves,
Icebergs, and Global Warming; David Long; BYU Forum Address: January 27, 2009
http://www.byub.org/talks/Talk.aspx?id=2994
• Decay rate R = dN/dt = -λN;
• N = # nuclei λ = decay constant
• Solve for N: N = Noe–λt
• Half-life N = ½ N0 
• T½ = ln 2/λ
-λt
• Note: R = R0e
Fig 44-9, p.1453
Suppose we are using a Geiger
counter to measure the decay rate of
several radioactive samples. One
sample is more active than the
others. Which of the following is a
possible reason? (pp)
A. more radioactive nuclei
B. shorter half-life
C. either of the above
Video Encyclopedia of Physics Demos #16
What role does E = mc2 play in
nuclear physics?
• Half-lives
• Alpha decay
– Smoke detectors
– Radon in environment
• Beta decay (also positron decay)
– Evidences for the little neutral one
– K 40
– Carbon dating
• Natural radioactivity
– U and Th (Np)
– Formation of solar system and extinct light elements
Fig 44-8, p.1453
When there is an alpha decay, the
charge on the resulting nucleus
A.
B.
C.
D.
E.
Stays the same
Increases by one
Increases by two
Decreases by one
Decreases by two
Fig 44-11, p.1457
Consider the alpha decay of
uranium 238 to thorium 234,
which mass is greater?
A.U 238
B.Th 234 + helium-4
Hint: think about
2
E=Mc
Alpha decay
•
•
•
•
U238 to Th234 + alpha + Q
mUc2 = mThc2 + mHec2 + Q
Thus, Q = 4.3 MeV
The greater the Q the shorter
the t½ . And not just a little bit.
Tunneling
Fig 44-12, p.1459
Yellow is alpha decay
Fig 44-10, p.1457
• Americium 241- half-life 432 years
Fig 44-13a, p.1459
Fig 44-13b, p.1459
Fig 44-17, p.1465
Radon in environment
222
86
Rn 
218
84
Po
+
4
2
He
T½ = 4 days
average home: 25 million
entire earth composed of
222
86
222
86
3
Rn/m
Rn
Mearth
49
= 1.6 x 10 atoms
MRn
 164 half-lives  less than 2 y
There are other alpha series.
Red is beta decay
Green is beta plus or e-capture.
Fig 44-10, p.1457
Beta decay
• Evidences for the little neutral one
• K 40
• Carbon dating
Fig 44-14, p.1460
Fig 44-14a, p.1460
Fig 44-15, p.1461
Fig 44-15a, p.1461
Fig 44-16, p.1464
Table 44-3, p.1464
Carbon-14 dating
14
Generation of
C
6
Cosmic rays  neutrons
n+
N C + H
Decay
–
C  N + β + ˉν
T½ = 5700 y
14
7
14
6
14
6
14
7
1
1
Potassium 40
• This is really weird
– It is a heavier atom with odd number of
protons and neutrons.
– Decaying could even up the score
– Either β+ or β- or e- capture. Why doesn’t it do
it rapidly?
40
19
K  Ca + β + ˉν
40
20
–
It does; it also does this.
40
19
K
40
18
Ar + β+ + ν
• But why does it take so long?
• T ½ = 1.3 billion years.
• Carrying away angular momentum.
Table 44-5, p.1467
Natural radioactivity
Table 44-4, p.1465
Fig 44-17, p.1465
Which element has
long-lived mass 5? pp
A. Hydrogen (quintium)
B. Helium
C. Lithium
D. More than one of the
above
E. None of the above
Nuclear-isotope trivia
•
•
•
Are there any even numbered (even atomic
number, Z) elements which only have one
stable isotope?
What elements with an atomic number less
than 22 have an isotope which is
radioactive with a half life greater 10,000
year? Which isotopes are they? Extinct
isotopes in meteors.
What element has an isotope with an A =8?
•
•
•
Are there any nuclear reactions with a
Q>10 which do not involve absorption
of a neutron? What is the consequence
for nuclear research/devices?
What element has a stable isotope with
an A =5?
Why are there only four alpha-decay
series of the heaviest elements? Why
are only 3 of the 4 series naturally
producing daughters on earth?