atomic nucleus and radioactivity

Question 1

what is radioactivity

Answer 1

• Spontaneous emission of small particles and/or radiation by unstable atomic nuclei to attain more stable nuclear state

Question 2

who discovered radioactivity

Answer 2

• Marie and Pierre Curie isolated two previously unknown radioactive materials, polonium and radium
• Radioactivity was found to be unaffected by chemical and physical testing, showing that the radiation came from the atom itself – specifically from the disintegration or decay of an unstable nucleus

Question 3

Ernest Rutherford 1893

Answer 3

began studying the nature of the rays that were emitted and they were classified into three distinct types according to their penetrating power

Question 4

what are the three distinct types of rays emitted

Answer 4

alpha decay
beta decay
gamma decay

Question 5

Alpha decay

Answer 5

positively charged, can barely penetrate a piece of paper. alpha rays are nuclei of helium atoms

Question 6

Beta decay

Answer 6

negatively charged, pass through as much as 3mm of aluminium. Beta rays are electrons

Question 7

Gamma decay

Answer 7

neutral, extremely penetrating. Gamma rays are high energy protons

Question 8

what is alpha decay

Answer 8

the loss of 2 neutrons and 2 protons = changed atom. see pp for equation

Question 9

alpha decay : smoke detector

Answer 9

• Americium 241 has a half-life of 432.6 years
• Emitted alpha particle ionises the air molecules which conduct current between two terminals
• Smoke clings to ionised air molecules and slows them down
• Current decreases and a transistor switch activate the alarm
• Contains 0.3ug of the isotopes or 37Bq or 1 Ci of radioactivity
• Radiation risk much smaller than background radiation
• Sensitive to flaming stage of fire
• Optical smoke detectors sensitive to smouldering stage of fire

Question 10

what is beta decay

Answer 10

• also called negatron decay
• The electron emitted in beta decay is not an orbital electron; the electron is created in the nucleus itself
• One of the neutrons changes to a proton and in the process throws off an electron
• These particles are referred to as beta particles so as not to confuse them with orbital electrons

Question 11

beta decay and the neutrino

Answer 11

• 1930 – Wolfgang Pauli proposed that there was a new particle that was very difficult to detect was emitted in beta decay, as well as the electron
• This new particle was names neutrino by Enrico Fermi 1934
• In 1956, complex experiments produced further evidenced for the neutrino, but by then physicists had already accepted existence
• The symbol for the neutrino is the Greek letter “nu”
see pp for equation

Question 12

positron decay

Answer 12

• Occurs when there are too few neutrons compared to the number of protons. These isotopes lie below the stable isotype line
• In this type of beta decay, the particle that is emitted is called a positron, as it has the same mass as an electron of +1
• The positron is called the antiparticle to the electron
see pp for equation

Question 13

positron-emission tomography

Answer 13

• Nuclear medicine functional imaging technique that is used to observe metabolic processes in the body
• The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide, which is introduced into the body on a biologically active molecule. 3D images of tracer concentration within the body are then constructed by computer analysis
• Biological molecule e.g. fludeoxyglucose (FDG), an analogue of glucose, shows tissue metabolic activity corresponding to glucose uptake. Indicates cancer metastasis
• 20 min scan
• Red areas show more accumulates tracer substance and blue areas are regions where low

Question 14

electron capture

Answer 14

• Electron capture occurs when a nucleus absorbs one of the orbiting electrons. Usually one of the innermost electrons
• Electron disappears and a proton becomes a neutron in the process
• A neutrino is emitted in electron capture decay
• This process has been detected by the emission of x-rays due to other orbital electrons jumping down to fill in the lower energy levels and releasing energy of that energy
see pp for equation

Question 15

give 4 sources of instabilities

Answer 15

• Too big
• Too many neutrons for the protons
• Not enough neutrons for the protons
• Too much excess energy

Question 16

nuclear stability

Answer 16

• Determined by the number of neutrons to protons
• Atomic number(Z) less than 20:
- Ration close to 1 fall in zone of stability
• Atomic number going from 21-83:
- Number of neutrons always greater than the number of protons
- Neutrons help to stabilise the nucleus and overcome repulsive electric forces of the protons
- Ration increases from 1 to 1.5 as Z increases from 21 to 83
• Atomic number greater than 83
- Ration greater than 1.5 (proportionally more neutrons needed to stabilise larger number of protons)
- All unstable and decay and are radioactive
- Outside zone of stability

Question 17

see pp for

Answer 17

neutron to proton ratio graph

Question 18

left of zone

Answer 18

• Neutron rich
• Changes neutrons to protons by β emission
• n –> p + e

Question 19

right of zone

Answer 19

• Proton rich
• Tries to lose protons and gain neutrons
• P + e –> n
• Positron emission or electron capture

Question 20

just in zone but Z>83

Answer 20

• Decay by emitting α particle to try to achieve a more stable nucleus

Question 21

odd-even rule

Answer 21

• When neutrons and protons both have even numbers, isotopes tend to be far more stable than when they are both odd
• Of all 264 stable isotopes, number of protons/neutrons
- 168 even/even
- 57 even/odd
- 50 odd/even
- 4 odd/odd

Question 22

radioactive decay series

Answer 22

• Some nuclei cannot obtain stability in just one transformation
• Uranium goes through 8 alpha decays and 6 beta decays before it reaches stability at lead

Question 23

gamma decay

Answer 23

• Photons having very high energies. The decay of a nucleus by the emission of a gamma ray is much like the emission of photons by excited electrons
• The gamma rays come from an excited nucleus that is trying to get back to its ground state
• The nucleus could be in an excited state either from a collision with another particle or because it is a nucleus left over from a previous radioactive decay
• A nucleus may remain in an excited state for some time before it emits a gamma ray. The nucleus is then said to be in a metastable state and is called an isomer
• The nucleus can also return to its ground state by a process called internal conversion, where no gamma rays are emitted
• The nucleus interacts with an electron and loses its energy in that way
• The electron then loses energy as an x-ray
• X-rays come from atom-electron interactions, while gamma rays come from nuclear processes

Question 24

conservation of nucleon number

Answer 24

• The following laws all hold true:
- Law of Conservation of Energy
- Law of Conservation of Linear Momentum
- Law of Conservation of Angular Momentum
- Conservation of Electric charge
- Law of Conservation of Nucleon Number
• The total number of nucleons stays the same in any decay, but the different types can change