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

Atomic Number Z - number of protons (charge, element)

Neutron Number N - number of neutrons

Mass Number A - number of nuclei +

+

+

+

+

+

A nucleus is represented by symbol: XAZ

Elements with different numbers of neutrons are called isotopes.

He42

relativistic energy & relativistic momentum

relativistic mass: 0

2

2

0 m

c

v1

mm

relativistic energy: E = mc2

relativistic momentum: p = mv

...2

vmcm

2

02

0

energy – momentum relation:

42

0

222 cmcpE

attributes of selected particles

proton 1.6726210-

27

1.007276 938.28 e ½

neutron 1.6749310-

27

1.008665 939.57 0 ½

electron 9.1094 10-31 0.0005486 0.510999 -e ½

positron 9.1094 10-31 0.0005486 0.510999 e ½

photon 0 0 0 0 0

neutrino 0 0 0 0 ½

mass

kg a.u. MeV/c2charge spin

the spin

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

2

3

2

1

2

1

2

3

z

quantum numbers:

• spin quantum number I- the magnitude of the spin is

1IIS

• magnetic quantum number mI = -I, …. I- the z component of the spin is

Iz

mS

Nucleus size and shape

Rutherford’s experiment

Ze

d

m

2e2

2

mv

kZe4d

Conclusion:3

1

0Arr where r0 = 1.2 fm

Nuclear Stability

Coulomb interaction - repulsive

Nuclear interaction - attractive

magic numbers (very stable nuclei):

Z, N = 2, 8, 20, …

line of stability

Binding Energy

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

The difference

Eb (MeV) = ( Zmp + Zmn - MA ) · 931.491 MeV/a.u.

is called the binding energy of the nucleus.

Example (alpha particle):

Eb= (2 · 1.0073au + 2 · 1.0087au – 4.0026au) · 931.491 MeV/au 27.4 MeV

Fission and Fusion

0 50 100 150 200 2501

2

3

4

5

6

7

8

9

mass number

bind

ing

ener

gy p

er n

ucle

on (

MeV

)

region of greatest stability

Fission – heavy nuclei (A>60) split releasing energy

Fusion – light nuclei (A<60) combine releasing energy

Radioactivity

lead shield

radioactivesource

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

photographicplate

- high energy photons - alpha particles

- - electrons

+ - positrons

Types of radioactive decay:

Activity – the decay rate

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

Ndt

dN – the decay constant

Hence t0eNtN N0 – initial number of

radioactive nucleiActivity:

t0eR

dt

dNtR R0 – initial activity

units:s1

1Bq1 (becquerel), 1Ci = 3.7 · 1010 Bq (curie)

Half – life time

The decay constant can be expressed in terms of time T½, in which activity (the number of radioactive nuclei) decreases by a factor of two.

21T

00 eNN2

1

2lnT

21

t

NN0

0N2

1

0N4

1

21T

21T20