Brief History of Nuclear Physics

1896 - Henri Becquerel (1852-1908) discovered radioactivity 1911 - Ernest Rutherford (1871-1937), Hanz Geiger (1882-1945) and Ernest Marsden (1888-1970) conducted scattering of alpha particles on nuclei 1930 - John D. Cocroft (1897-1967) and Ernest T.S. Walton (1903 1995) conducted the first artificial nuclear reaction 1932 - James Chadwick (1891-1974) discovered the neutron 1933 - Frederick Joliot (1900-1958) and Irene Joliot-Curie (1897 1956) synthesized artificial elements 1938 - discovery of nuclear fission by Otto Hahn (1879-1968) and Fritz Strassman (1902-1980) 1942 - Enrico Fermi (1901-1954) builds a fission reactor

Properties of nuclei

4 He 2

Mass Number A

- number of nuclei

Atomic Number Z

- number of protons (charge, element)

Neutron Number N

- number of neutrons

A nucleus is represented by symbol:

A Z

X

Elements with different numbers of neutrons are called

isotopes

.

relativistic energy & relativistic momentum

relativistic mass: relativistic energy: m  m 0 1  v 2 c 2   m 0 E = mc 2  m 0 c 2  m 0 v 2 2  ...

relativistic momentum: p = mv energy – momentum relation: E 2  p 2 c 2  m 2 0 c 4

attributes of selected particles

proton neutron electron positron photon neutrino kg 1.67262

 10 -27 1.67493

 10 -27 9.1094  10 -31 9.1094  10 -31 0 0 mass a.u.

1.007276

1.008665

0.0005486

0.0005486

0 0 MeV/c 2 938.28

939.57

0.510999

0.510999

0 0 charge spin e 0 -e e 0 0 ½ ½ ½ ½ 0 ½

the spin

Spin – angular momentum like quantity responsible for the magnetic moment of particles.

z

quantum numbers: • spin quantum number I - the magnitude of the spin is S  I  I  1   • magnetic quantum number m I - the z component of the spin is = -I, …. I S z  m I  3 2  1 2   1 2   3 2 

Nucleus size and shape

Rutherford’s experiment m Ze 2e d d  4 kZe 2 mv 2 Conclusion: r  r 0 A 1 3 where r 0 = 1.2 fm

Nuclear Stability

Coulomb interaction - repulsive Nuclear interaction - attractive

line of stability

magic numbers (very stable nuclei):

Z, N = 2, 8, 20, …

Binding Energy

The total (relativistic) energy of a nucleus is always less than the combined energy of the separated nucleons.

The difference E b (MeV) = ( Zm p + Zm n - M A ) · 931.491 MeV/a.u.

is called the binding energy of the nucleus.

Example (alpha particle): E b = (2 · 1.0073au + 2 · 1.0087au – 4.0026au) · 931.491 MeV/au



27.4 MeV

Fission and Fusion

9 8 7 6 5 4 3 2 1 0 region of greatest stability 50 100 150 mass number 200 250

Fission

– heavy nuclei (A>60) split releasing energy

Fusion

– light nuclei (A<60) combine releasing energy

Radioactivity

- spontaneous emission of radiation resulting from disintegration (decay) of unstable nuclei.

Types of radioactive decay: lead shield radioactive source

 +   - positrons - alpha particles - high energy photons  -

photographic plate

- electrons

Activity – the decay rate

The number of disintegrated nuclei in a unit time is proportional to the number of radioactive nuclei in the source.

dN    N dt  – the decay constant Hence N    N 0 e   t N 0 – initial number of radioactive nuclei Activity: R  dN dt  R 0 e   t R 0 – initial activity units: 1 Bq  1 1 s (becquerel), 1Ci = 3.7 · 10 10 Bq (curie)

Half – life time

The decay constant can be expressed in terms of time T ½ , in which activity (the number of radioactive nuclei) decreases by a factor of two.

N 1 2 N 0  N 0 e   T 1 2 N 0 T 1 2  ln 2  1 2 N 0 1 4 N 0 t 0 T 1 2 2 T 1 2