Radioactive Decay Overview
Read this document and try the practice problems.
1. What is RADIOACTIVITY?
o Radioactivity is when an unstable nucleus emits particles or electromagnetic rays in order to gain
stability
o This emission actually changes the element
o The emission is called RADIATION
2. What is RADIATION?
o Radiation is ENERGY in the form of PARTICLES or ELECTROMAGNETIC RAYS released from
radioactive atoms
o The Particles released are Alpha, Beta, and Neutrons.
o The Energy released are Gamma Rays.
3. Alpha Particles ( α ):
o The largest of the particles
o Helium nucleus (2 protons and 2 neutrons)
o Symbol: α or 42𝐻𝑒
o Shielding: can be stopped by a piece of paper, a layer of skin, a few centimeters of air
234
4
o 238
92𝑈  90𝑇ℎ + 2𝐻𝑒
4. Beta Particle (β )
o An extra neutron from the nucleus that converts from a neutron into a proton and electron (over
charge is still neutral)
o The converted proton stays in the nucleus but the electron is ejected from the nucleus as a β
particle
o Usually accompanied by the emission of a gamma ray as well if the emission of the β doesn’t rid
the nucleus of enough of the extra energy
o Symbol: β or −10𝑒 − or −10𝛽 −
o Shielding: few millimeters of aluminum or plastic, or a block of wood
90
0
o 38
𝑆𝑟  90
39𝑌 + −1𝑒 −
5. Gamma Ray ( γ )
o High-energy WAVES which can travel long distances and have a great ability to penetrate other
materials
o Often used in medical applications for treatment of cancer or equipment sterilization
o They do NOT have the ability to make other materials radioactive
o Occur after alpha or beta emissions as a way for the element to release energy (not to change the
nucleus)
o Symbol: γ
o Shielding: several feet of concrete or a few inches of dense material (such as lead)
137
o 137
56𝐵𝑎  56𝐵𝑎 + γ
1
6. Neutrons ( 0𝑛)
o Elements with atomic masses over 90 can undergo SPONTANEOUS FISSION in which the
unstable nucleus will spontaneously break apart into smaller nuclei releasing with it one or more
NEUTRONS
o These neutrons are high-speed nuclear particles with exceptional penetration ability
o The only form of radiation that can actually make another substance RADIOACTIVE
1
o Symbol: 0𝑛
o Shielding: VERY thick hydrogen-containing materials (such as concrete and water)
140
108
1
o 252
98𝐶𝑓  54𝑋𝑒 + 44𝑅𝑢 + 4 0𝑛
Practice Questions:
1. Complete the blanks with the correct type of particle or ray. Remember that matter and energy are
conserved this means that the total mass on the left side and the right side of the equation should be
equal.
o
232
92𝑈
→ 228
90𝑇ℎ + ______
o
144
58𝐶𝑒
o
______ →
o
222
86𝑅𝑛
→ ______ + −10𝑒
99
43𝑇𝑐
+𝛾
→ ______ +
o
129
53𝐼
→ 129
54𝑋𝑒 + ______
o
239
94𝑃𝑢
o
237
92𝑈
→ 237
93𝑁𝑜 + ______
o
236
92𝑈
92
→ 36
𝐾𝑟 +
→ ______ + 42𝐻𝑒
4
2𝐻𝑒
2. Write the balanced decay reaction for the following:
o 45
20𝐶𝑎 (beta decay)
o
210
84𝑃𝑜
(alpha decay)
o
234
94𝑃𝑢
(alpha decay)
o
24
11𝑁𝑎
(beta decay)
o
235
92𝑈
(neutron decay making cesium-140, rubidium-92, and 3 neutrons)
141
56𝐵𝑎
+ ________