Lecture 19: Atomic and Nuclear Radiation

applications of atomic and nuclear radiation

• nuclear medicine

• MRI

• smoke detector

• radioactive dating

• agriculture and engineering

what is the nucleus

densely packed centre of an atom composed of protons and neutrons

protons are + and their mass is

1.67262 × 10^-27 kg

neutrons and neutral and their mass is

1.67493 × 10^-27 kg

neutrons and protons are collectively called

nucleons

number of protons is recognised as the

atomic number of an atom

total number of nucleons is the

atomic mass number (protons + neutrons)

where to find atomic number and atomic mass number on periodic table

Radius of a nucleus

R = (1.2 × 10^-15)A^(1/3)

• where A is the number of nucleons

what is nuclear mass

measured using mass spectrometer and expressed with reference to the mass of a carbon12 atom which is assigned the mass of 12u, where u is unified atomic mass unit defined as:

1 uamu = 1/12(mass of 1 carbon 12 atom)

1 u = 1/12(12g/6.02×10^(23))=1.661×10²² g 1.66 × 10^(27) kg

masses can also be expressed in terms of their energy equivalent. What is the energy equivalent of 1 amu

E = mc²

• E = energy

• m = mass

• c = speed of light

NOTE: Joules to Ev

1 eV = 1.6 × 10^-19 J

What is binding energy

• The total mass of a nucleus is always less than the mass of protons and neutrons in it.

• This mass difference is a measure of the BINDING ENERGY that holds the nucleus together.

BE = ∆m x 931.5 MeV/u

calculate the binding energy of helium

• helium has 2 protons and 2 neutrons, find the mass of protons and neutrons in u

• 2(mass of proton) + 2(mass of neutron) - atomic mass number of helium = delta m = mass defect

• then use BE = delta m x 931.5 MeV/u.

what holds nucleons together

1. nuclear force (binds nucleons together)

   • attractive between protons, protons and neutrons, and neutrons

   • short range (attracts only nearest neighbours 10^-15m)

2. column force (break protons apart)

   • repulsive for protons

   • long range - acts across the nucleus

for a nucleus to be stable

attractive force between nucleons must overcome the repulsive force between protons

Towards the end of the 19th century, minerals were found that would darken a photographic plate even in the absence of light. Why?

• they emit radiation!

• this phenomenon is now called radioactivity

• marie and pierre curie isolated two very radioactive elements = radium and polonium

what is radioactivity the result of

integration or decay of an unstable nucleus

certain isotopes are unstable, they decay with…

emission of some types of radiation or rays

Rutherford found that

the rays could be classified into three distinct types according to their penetrating power.

3 types of radiowaves

alpha, beta, and gamma rays.

alpha rays

• barely penetrates a piece of paper

beta rays

could pass through as much as 3 mm of aluminium

gamma rays

It could pass through several centimetres of lead and still be detected on the other side

charges of radiation

alpha - positive

beta - negative

gamma - neutral

what is radioactivity

Spontaneous emission of radiation from unstable nuclei.

α, β and γ emission.

types of particles for alpha, beta and gamma rays

All three consisted of familiar kind of particles.

• Gamma rays are very high energy photons.

• Beta rays are electrons, identical to those that orbits the nucleus, but they are created within the nucleus itself.

• Alpha rays are simply the nuclei of helium atoms. That alpha ray consists of two protons and two neutrons bound together.

energy released in nuclear decay

Q = [Mp - (MD + Malpha)] x c²

• Q = disintegration energy

• Mp = mass of parent

• MD = mass of daughter

• Malpha = mass of particle

• c = speed of light

how does a smoke detector work

• contains 0.2 mg of the radioactive americium isotope Am-241 in the form of AmO2. (1.0 µCi)

• α particles from Am-241 continually ionizes the air molecules in the air space between the two oppositely

  charged plates.

• The positive and negative ions are attracted by the charged plates which results in a current flow through the circuit.

• When smoke enters between the plates, the ions becomes attached to the smoke particles, thus resulting in a drop of current.

• The current drop is detected by the electronic circuitry and sets off the

  alarm.

summary of alpha decay and beta decay

gamma decay

• photon is emitted from an excited nucleus

• it is like the emission of photons from an excited atom