The structure of nuclei

1
The structure of nuclei

• Nuclei are composed of just two types of
  particles protons and neutrons. These particles
  are referred to collectively as nucleons.
• The atomic number, Z, is equal to the number of
  protons in a nucleus. The neutron number, N, is
  the number of neutrons in a nucleus. The mass
  number of a nucleus, A, is the total number of
  nucleons it contains. Thus A N Z.
• A nucleus with atomic number Z and mass number A
  is designated as follows

2
The structure of nuclei

• Isotopes are nuclei with the same atomic number
  but different neutron number.
• The atomic mass unit, u, which is defined so that
  the mass of is exactly 12u. The value of u is
  as follows
• Neutrons and protons have masses slightly greater
  than 1u.
• The atomic mass unit can be expressed in terms of
  energy (MeV) as follows 1u 931.5
  MeV/c2.

3
The structure of nuclei

• The approximate radius of a nucleus of mass
  number A is given by
• All nuclei have roughly the same density,
  regardless of their mass number.
• Nuclei are held together by the strong nuclear
  force. This force is attractive between all
  nucleons and has a range of only a few fermis.

4
Radioactivity

• Radioactivity refers to the emissions observed
  when unstable nucleus change its composition or
  when an exited nucleus decays to a lower-energy
  state.
• Alpha decay an a particle ( the nucleus of
  helium atom) consist of two protons and two
  neutrons. A nucleus that emits an a particle
  decreases its mass number by 4 and its atomic
  number by 2
• Where X is the parent nucleus and Y is the
  daughter nucleus.
• Beta decay refer to the emission of an electron,
  as when a neutron decays into a proton, an
  electron, and an antineutrino
• This type of decay, which increases the atomic
  number by 1 but leaves the mass number unchanged,
  is referred to as b- decay. If a positron and
  neutrino are given off instead, we refer to the
  process as b decay.
• 3. Gama decay occurs when an excited nucleus
  drops to a lower-energy state and emits a photon.
  In this case, neither the mass number nor atomic
  number is changed.

5
Radioactivity

• The activity of radioactive sample is equal to
  the number of decay per second. The units of
  activity are the curie (Ci) and the Becquerel (Bq)

6
Half-life and radioactive dating

• Radioactive nuclei decay with time in a
  well-defined way. As a result, many radioactive
  nuclei can be used as a type of nuclear clock
• If the number of radioactive nuclei in a sample
  at time t 0 is N0, the number, N, at a later
  time is
• The constant l, in this expression is referred to
  as the decay constant.

7
Half-life and radioactive dating

• The half-life of a radioactive material is the
  time required for half of its nuclei to decay. In
  terms of the decay constant, the half-life is
• The rate at which radioactive nuclei decay is
  proportional to the number of nuclei present at
  any given time, and the decay constant

8
Half-life and radioactive dating

• Carbon-14 can be used to date organic materials
  with ages up to about 15,000y. The age can be
  found using
• Where Ro 0.231 Bq is the initial activity, R is
  the present activity, and l 1.21 x 10-4y-1

9
Practical applications of nuclear physics

• Nuclear radiation can have both harmful and
  useful effects. An important way to characterize
  exposure to radiation is in terms of dosage,
  which can be defined in a number of ways.

10
Units of radiation

• Roentgen, R is related to the amount of
  ionization charge produced by 200-keV x-ray in 1
  kg of dry air at STP
• Rad (radiation absorbed dose) is a measure of the
  amount of energy absorbed by an irradiated
  material, regardless of the type of radiation

11
Units of radiation

• 3. RBE (relative biological effectiveness)
  takes into account that different types of
  radiation produce different amounts of biological
  damage
• 4. Combining the rad and the RBE yields the
  rem
• A dose of 1 rem of any type of radiation causes
  the same amount of biological damage.