1. Nuclear Reactions Chapter 19

2. Facts About the Nucleus
Very small volume compared to volume of atom
Essentially entire mass of atom
Very dense
Composed of protons and neutrons that are tightly held together
Nucleons
Every atom of an element has the same number of protons
Atomic Number
Isotopes are atoms of the same elements that have different masses
Different numbers of neutrons
Mass Number = number of protons + neutrons 2

3. Facts About the Nucleus
The number of neutrons is calculated by subtracting the atomic number from the mass number
The nucleus of an isotope is called a nuclide
less than 10% of the known nuclides are nonradioactive, most are radionuclides
Each nuclide is identified by a symbol
Element -Mass Number = X-Z 3

4. Radioactivity
Radioactive nuclei spontaneously decompose into smaller nuclei
Radioactive decay
We say that radioactive nuclei are unstable
Decomposing involves the nuclide emitting a particle and/or energy
During radioactive decay, atoms of one element are changed into atoms of a different element
In order for one element to change into another, the number of protons in the nucleus must change
All nuclides with 84 or more protons are radioactive
We describe nuclear changes with using nuclear equations
atomic numbers and mass numbers are conserved 4

5. alpha decay beta decay an  particle contains 2 protons and 2 neutrons
helium nucleus
loss of an alpha particle means
atomic number decreases by 2
mass number decreases by 4
beta decay
a  particle is like an electron
moving much faster
found in the nucleus
when an atom loses a  particle its
atomic number increases by 1
mass number remains the same
in beta decay a neutron changes into a proton 5

6. gamma emission Gamma () rays are high energy photons
Gamma emission occurs when the nucleus rearranges
No loss of particles from the nucleus
No change in the composition of the nucleus
Same atomic number and mass number
Generally occurs whenever the nucleus undergoes some other type of decay 6

7. positron emission electron capture
positron has a charge of +1 c.u. and negligible mass
anti-electron
when an atom loses a positron from the nucleus, its
mass number remains the same
atomic number decreases by 1
positrons appear to result from a proton changing into a neutron
electron capture
occurs when an inner orbital electron is pulled into the nucleus
no particle emission, but atom changes
same result as positron emission
proton combines with the electron to make a neutron
mass number stays the same
atomic number decreases by one 7

8. Artificial Nuclear Transformation
Nuclear transformation involves changing one element into another by bombarding it with small nuclei, protons or neutrons
reaction done in a particle accelerator
linear
cyclotron
made-made transuranium elements 8

9. Detecting Radioactivity
To detect something, you need to identify something it does
radioactive rays cause air to become ionized
Geiger-Müller Counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays
radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical
a scintillation counter is able to count the number of flashes per minute 9

10. Half-Life Not all radionuclides in a sample decay at once
The length of time it takes one-half the radionuclides to decay is called the half-life
Even though the number of radionuclides changes, the length of time it takes for half of them to decay does not
the half-life of a radionuclide is constant
Each radionuclide has its own, unique half-life
The radionuclide with the shortest half-life will have the greater number of decays per minute
For samples of equal numbers of radioactive atoms 10

11. Half-Life
half of the radioactive atoms decay each half-life 11

12. Half-Life First order rate reaction as we’ve done before
k = 0.693/t1/2 or
ln Xo/X = kt
Activity measured in Bequerels (Bq), number atoms decaying per second
Also measured in curies (Ci) x1010 atoms per second

13. Object Dating mineral (geological)
compare the amount of U-238 to Pb-206
compare amount of K-40 to Ar-40
archeological (once living materials)
compare the amount of C-14 to C-12
C-14 radioactive with half-life = 5730 yrs.
while living, C-14/C-12 fairly constant
CO2 in air ultimate source of all C in body
atmospheric chemistry keeps producing C-14 at the same rate it decays
once dies, C-14/C-12 ratio decreases
limit up to 50,000 years 12

14. Medical Uses of Radioisotopes, Diagnosis
radiotracers
certain organs absorb most or all of a particular element
can measure the amount absorbed by using tagged isotopes of the element and a Geiger counter
use radioisotope with short half-life
use radioisotope low ionizing
beta or gamma 13

15. Mass energy relations
The energy change in a nuclear reaction can be determined by finding the change in mass and multiplying by the speed of light squared
DE = Dmc2
To find the change in mass use products – reactants. Use the masses in amu from Table 19.3 (can’t use periodic table because these are average atomic masses)
Nuclear binding energy
A nucleus weighs less than the protons and neutrons it is composed of. The difference is called mass defect and the energy change is called binding energy. Use E=mc2 to find

16. Other Nuclear Changes
a few nuclei are so unstable, that if their nucleus is hit just right by a neutron, the large nucleus splits into two smaller nuclei - this is called fission
small nuclei can be accelerated to such a degree that they overcome their charge repulsion and are smashed together to make a larger nucleus - this is called fusion
both fission and fusion release enormous amounts of energy 14

17. Fissionable Material
fissionable isotopes include U-235, Pu-239, and Pu-240
natural uranium is less than 1% U-235
rest mostly U-238
not enough U-235 to sustain chain reaction
fission produces about 2.1 x 1013 J/mol of U-235
26 million times the energy of burning 1 mole CH4
to produce fissionable uranium the natural uranium must be enriched in U-235 15

18. Fission Chain Reaction
a chain reaction occurs when a reactant is also a product
in the fission process it is the neutrons
only need a small amount of neutrons to keep the chain going
many of the neutrons produced in the fission are either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238
minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass 16

19. Nuclear Power Plants use fission of U-235 or Pu-240 to make heat
heat picked up by coolant and transferred to the boiler
in the boiler the heat boils water, changes it to steam, which turns a turbine, which generates electricity
the fission reaction takes place in the reactor core 17

20. Nuclear Power Plants - Core
the fissionable material is stored in long tubes arranged in a matrix called fuel rods
subcritical
between the fuel rods are control rods made of neutron absorbing material
B and/or Cd
neutrons needed to sustain the chain reaction
the rods are placed in a material used to slow down the ejected neutrons called a moderator
allows chain reaction to occur below critical mass 18

21. Breeder Reactor Design common in Europe
Makes its own fuel by converting U-238 to Pu-239
Use liquid sodium as a moderator
Use water filled radiator to transfer heat to boiler
Plutonium highly toxic and spontaneously combusts in air 19

22. Nuclear Fusion
Fusion is the process of combining two light nuclei to form a heavier nucleus
The sun’s energy comes from fusion of hydrogen to produce helium
Releases more energy per gram than fission
Requires high temperatures and large amounts of energy to initiate, but should continue if you can get it started 20

23. Factors that Determine Biological Effects of Radiation
The more energy the radiation has the larger its effect can be
The better the ionizing radiation penetrates human tissue, the deeper effect it can have
Gamma >> Beta > Alpha
The more ionizing the radiation, the more effect the radiation has
Alpha > Beta > Gamma
The radioactive half-life of the radionuclide
The biological half-life of the element
The physical state of the radioactive material
The amount of danger to humans of radiation is measured in the unit rems 21

24. Somatic Damage
Somatic Damage is damage which has an impact on the organism
Sickness or Death
May be seen immediately or in the future
Depends on the amount of exposure
Future effects include cancer 22

25. Genetic Damage
Genetic Damage occurs when the radiation causes damage to reproductive cells or organs resulting in damage to future offspring 23