Radioactivity –
types of decays
A presentation for Wed. in the
last week of Physics 125
Atomic and nuclear structure
• Atomic model – nucleus and electrons.
• Nucleus – contains protons and neutrons.
• Protons have charge Qp = +e, where e is defined
as the magnitude of the electron charge.
• e = 1.6 x 10-19 Coulombs = 1.6 x 10-19 C
• Electrons have charge Qe = -e
• Neutrons have charge Qn = 0 (zero, exactly)
• The nucleons (protons and neutrons) are bound
together by the strong nuclear force in a small
nucleus which has a size of about 10-15 m.
Nuclear notation
• Z = atomic number or proton number, is the
number of protons in the nucleus.
• N = neutron number, is the number of neutrons
in the nucleus.
• A = Z + N = mass number, is the number of
nucleons in the nucleus.
A
• In general, the notation is Z X N
• For example,
12
6 C6
has atomic mass 12.000
Nuclear isotopes
• Each element has a distinct proton number Z.
• The neutron number may vary for different
isotopes of the element.
• The mass number A = Z + N will also vary for
different isotopes.
12
6 C6
14
6 C8
• For example,
and
are two isotopes of
carbon. These are also written C-12 and C-14.
• The neutron number is usually omitted, but can
easily be calculated, since N = A – Z .
• Also, the proton number is often omitted: 14C
Radioactivity – unstable nuclides
• Each element may have several different
isotopes, and some of these may be unstable.
• Radioactive decay occurs when a nucleus of an
unstable isotope decays into product particles.
• Three common types of radiation are observed:
• Alpha particles are doubly-charged He nuclei.
• Beta particles are electrons (or positrons).
• Gamma rays are high-energy photons.
Alpha particles
• Alpha particles are doubly-charged He nuclei.
• The particle is a bare nucleus with 2 protons and
2 neutrons, and a charge of +2e
• They are identical to the nucleus of helium atom,
and once the alpha slows down, it will capture
two electrons from surrounding material and
become a neutral He atom.
4
• The nuclear notation for the alpha is a or 2He
• Most of the helium in rocks is due to alpha decay
of heavy elements in the Earth.
Alpha decay
• Alpha particles are emitted in alpha decay.
• The parent nucleus is usually a heavy element.
• For example, polonium-214 decays by alpha
decay to lead-210 and an alpha particle:
•
214
84Po

210
82Pb
+
4
2He
• Notice that this nuclear equation is balanced in
both the proton number (84 = 82 + 2) and the
nucleon number (214 = 210 + 4).
Conservation laws in nuclear decay
• The balance of the proton and mass numbers is
due to conservation laws for nuclear decay:
• Conservation of nucleons (the mass numbers).
• Conservation of charge (the proton numbers).
• Conservation of mass-energy is also observed.
• The energy of the alpha particle from the decay
of 214Po is about 7.7 MeV. This energy is due to
a small difference between the mass of 214Po
and the sum of the masses of 210Pb and 4He.
• The difference in mass between parent and
products is converted to energy by E = mc2.
Beta decay
• Beta particles are emitted in beta decay.
• The parent nucleus is usually an isotope with an
excessive number of neutrons.
• For example, carbon-14 decays by beta decay
to nitrogen-14 and a beta particle:
•
14
6C

14
7N
+
0
-1e
(beta with Q = -e)
• This equation is balanced in the charge number
(6 = 7 + (-1)) and nucleon number (14 = 14 + 0).
• Z increases by one, and N decreases by one.
Beta decay of an isolated neutron.
• Beta decay can be thought of as the decay of a
neutron in the nuclide into a proton and electron.
• An isolated neutron will also decay by beta
decay to a proton and a beta particle:
•
1
0n

1
1p
+
0
-1e
(neutron decay)
• Notice that this nuclear equation is balanced in
both the charge number (0 = 1 + (-1)) and the
nucleon number (1 = 1 + 0).
Two types of beta decay: b+ and b• Beta decay can also produce a positron, the
anti-particle of the electron.
• An example of positive beta decay is oxygen-15:
•
15
8O

15
7N
+
0
+1e
(like a proton decay!)
• This process occurs for some nuclides which
have more protons than neutrons. However, the
proton by itself is stable (we believe).
Electron capture (EC)
• Another type of decay is electron capture, where
an electron of the atom is captured by the
nucleus and a proton is converted into a neutron.
• An example of electron capture is beryllium-7:
•
7
4Be
+
0
-1e

7
3Li
(and an X-ray)
• This process has the same resulting daughter
nucleus, except that no beta particle is emitted.
Gamma decay
• Gamma decay occurs when a nucleus emits a
gamma particle, a high-energy photon.
• This only occurs when a nucleus has extra
energy, perhaps because it was just created in a
previous nuclear decay.
• An example of gamma emission is barium-137m,
which is a nucleus of Ba which has just been
created in a beta decay of Cs-137:
137
137
0
•
 56Ba* + -1e (a beta decay)
55Cs
•
137
56Ba*

137
56Ba
+ g
(a gamma decay)
Energy levels in gamma decay
• Barium-137m is an example of an isomer, which
is a nucleus with excess energy.
• In some ways this is similar to a neutral atom in
an excited state, which can make a transition to a
lower energy level by emitting a photon.
(Fluorescence is one example of this.)
• The nuclei have much larger energies than
atoms, MeV instead of eV.
137
137
•
56Ba*  56Ba + g (661.66 keV)
• This isomer lives long enough to be separated
from it’s parent (Cs-137) and studied in the lab.
Co-60: beta-gamma-gamma decay
• Cobalt-60 is an example of a nuclide that decays
by beta decay to an intermediate excited state.
• The intermediate state then decays by emitting a
gamma rays. Hence both beta and gamma rays
are observed from this nuclide.
• The energy level scheme for this is not too
complicated, and is shown in an on-line database
of nuclide data. (link to Korean database)
• Co-60 decays with emission of a beta of energy
less than 318 keV and two gamma photons of
energies 1173 keV and 1332 keV (link to Lund)