Transcript The buoyant force on an object totally submerged in a

Discussion question: If a nucleus undergoes
radioactive decay it must be older than the
other nuclei in the sample that haven’t
decayed.
A.
B.
True
False
Types of radioactive decay
Gamma
 Beta (+,-, e.c.)
 Alpha

Neutron emission
 Fission
 Fusion

Gamma Decay
Nucleus in excited state gives off energy as a photon
14
6𝐶
∗
→
14
6𝐶
+𝛾
Beta Decay – electron and positron decay
Changes a proton to a neutron or a neutron to a proton
β-
14
β+
141 Sm
62
e.c.
6C
18 F
9
+e
Alpha Decay--42He given off
Alpha particle is a very low
energy system / very tightly bound
226
88Ra
238
A.
B.
C.
D.
E.
U
92
alpha
beta +
beta –
e.c.
gamma
234
Th
+
?
90
T1/2 = 4.5 Gy
234
A.
B.
C.
D.
E.
90Th
alpha
beta +
beta –
e.c.
gamma
234
91Pa
+ ?
T1/2 = 24 days
Why do some things decay, other don’t?
Magic
Numbers
Image from http://pms.iitk.ernet.in/ICT/physics_courses/akj/AKJain_IITR_Ch_2.htm
Why do some things decay, other don’t?
Magic
Numbers
Image from http://pms.iitk.ernet.in/ICT/physics_courses/akj/AKJain_IITR_Ch_2.htm
Small nuclei – N ~ Z
Large nuclei – N > Z
Radiometric Dating

Carbon Dating
– Cosmic rays release neutrons
– 14N + n → 14C + 1H
 Half life of ~5760 years
– Compare carbon-12 to carbon-14 ratio
– Limitations
 Must be organic
 Has the ratio of C-12 to C-14 always been the
same?
– Calibration
 50,000 years limit (not much C-14 left)
2 Nephi 2:26
And the Messiah cometh in the fulness of time,
that he may redeem the children of men from the
fall. And because that they are redeemed from the
fall they have become free forever, knowing good
from evil; to act for themselves and not to be
acted upon, save it be by the punishment of the
law at the great and last day, according to the
commandments which God hath given.
More Proof
226 Ra
88
eventually becomes 20682Pb.
What is released in this process?
A.
B.
C.
D.
E.
only alphas
only betas
both but more alphas than betas
both but the same number of alphas and
betas
both but more betas than alphas
232
90
Th
237
93
238
92
Np
235
92
U
U
N
238
92
234
90
U
Th
145
234
91
234
92
230
90
Th
140
226
90
80
85
Pa
U
Ra
90
Z
http://en.wikipedia.org/wiki/Decay_chain
Penetration Depth
Alpha– only a few cm through air
 Beta—only through air—blocked by paper
or sheet of metal
 Gamma rays—short wavelength light—
penetrate several cm lead or a meter of
concrete. Easily penetrates the skin and
interacts with human cells—these are the
dangerous ones.

What has to be true for a
particular type of nuclear decay
to happen?

Conservation of . . .
– Energy, momentum, angular momentum
 Compare rest energy
 OK if we go down in energy – turn into kinetic
– Lepton number, Baryon number, …

Force to make it happen
For radioactive decay where No
is the number of nuclei you start with and
N is the number of nuclei after a time t
N  N oe
 t
How do I solve for λ?
Discussion question: Over the course of 3
hours, 15% of a radioactive material decays.
What is its half-life?
A.
B.
C.
D.
E.
4.1 hrs
12.8 hrs
24.0 hrs
68.6 hrs
84.2 hrs
#decays/sec=λN

1 Ci = 3.7x1010 decays/sec
Fission:
Heavy Elements can reduce
energy (i.e. increase binding
energy per nucleon) by
splitting roughly in half.