Two Chapter 10

ARTIFICIAL TRANSMUTATION

• Rutherford and his colleagues produced artificial transmutation by firing alpha particles into a gas.
• The apparatus used is:

• Rutherford increased the distance of the source from the screen until scintillation stopped (particles did not have enough energy to reach the screen).
• He introduced nitrogen gas into the tube and scintillation began again.
• The source of the scintillation was protons which had been knocked out of the nitrogen nuclei to produce oxygen.

• Rutherford and Chadwick produced other transmutations as well.
• An added benefit from this process was the production of energy which had been stored in the nuclei.
• In 1932, Cockcroft and Walton produced transmutation by accelerating protons and colliding them with nuclei.
• A particle accelerator was used to accelerate the protons. After acceleration, the protons struck a lithium nucleus. They fused with the nucleus to produce an unstable beryllium nucleus which disintegrates into 2 helium nuclei.

• Lawrence, Van de Graaf and others built devices to accelerate particles.
• Accelerators - devices to accelerate particles.
• The basic methods for studying the nucleus are:
  • use accelerators.
  • cause collisions between particles and nuclei.
  • Study the radiation and products produced by using detection systems such as bubble chambers and mass spectrometers.
• Atom Smashers - High energy accelerators.
  • Rated in terms of the energy of the accelerated particles.
  • Measured in eV.
    • 1 eV = 1.6 X 10^-19 J
    • The energy an electron gains by accelerating through a potential of 1 V.
  • Other units used are:
    • 1 keV = 10³ eV
    • 1 MeV = 10⁶ eV
    • 1 GeV = 10⁹ eV
    • 1 TeV = 10¹² eV
• Sometimes accelerators are connected together.
• Accelerators may be 6.3 km in circumference as is the Fermi Accelerator. Beams of particles are often caused to collide.

CHADWICK'S DISCOVERY OF THE NEUTRON

• The neutron is a basic particle in the nucleus of all atoms.
• Rutherford predicted its existence.
• The German physicists, Bothe and Becker, found that beryllium when bombarded with alpha particles produced high energy emissions that penetrated several centimetres of lead.
• This emission had no charge.
• Curie and his wife Irene found that these emissions caused protons to be emitted from compounds containing hydrogen (e.g. paraffin).
• Chadwick was able to identify the emissions with the following apparatus.

• Using a cloud chamber, Chadwick calculated the energy of the individual protons.
• This energy of the protons had to come from the energy of the unknown emission.
• Using conservation of energy and momentum, he calculated the mass of the unknown particles.
• The mass was the same as that of the proton. Therefore, it had to be the neutron that Rutherford predicted (it had the same characteristics as those predicted by Rutherford).

NEUTRON TRANSMUTATION

• The discovery of the neutron gave physicists another tool.
• Since protons and alpha particles are positively charged, the alpha particles are repelled by the nucleus.
• The neutron had no charge and therefore, is not repelled by the nucleus.
• As a result, when neutrons are used to bombard a nucleus, transmutation can occur.
• The symbol we use for the neutron is:

• An example of neutron transmutation occurs with the formation of plutonium.

• In this example, the neptunium and plutonium produced do not occur naturally and are heavier than uranium.
• Other elements heavier than uranium have been similarly produced.
  • All of these elements do not exist naturally.
• Transuranic elements - The manufactured elements that do not occur naturally.

WRITING NUCLEAR REACTIONS

• The most basic rule is that the sums of the mass numbers and atomic numbers on either side of the equation representing the reaction, must be equal.
• e.g.

IONIZING RADIATION AND LIFE

• X-rays and radioactive sources produce ionizing radiation.
• This radiation may encounter an electron in orbit around an atom, and remove it (ionize the atom).
• Ions cause chemical reactions. These chemical reactions are what cause damage to living cells.
• In each cell are molecules of DNA which contain the information for cellular reproduction, tissue repair etc.
• DNA is damaged by:
  • itself
  • viruses
  • industrial chemical
  • chemicals in diet
  • smoking
  • the sun's ultraviolet light
  • ionizing radiation
  • etc.
• If the DNA is permanently damaged, the life of the cell is threatened, or even the life of the organism.
• Most DNA is repaired properly, but there is always the chance it won't be repaired properly.
• To kill a cell, ionizing radiation must be over 10 000 times larger than our average yearly dose of radiation from natural sources.
• The Becquerel is not a good unit to use to measure the effects of radiation on living organisms.
• The SI unit that measures absorbed radiation is the Gray (Gy)

1 Gy = 1 J/kg

Dose = Energy/mass

• When radiation gives 1 J of energy to 1 kg of living material, the dose is 1 Gy.
• A typical cancer treatment is 40 Gy.
• Absorbed dose depends upon:
  • The number of particles arriving per second.
  • The energy per particle.
  • The type of absorbing material.
• To compare doses of radiation on biological matter, the Sievert is used.
• Sievert (Sv) - The dose in grays times the quality factor.
• Quality factor - The comparison of the effect of a dose of radiation with those of a standard radiation of 200 keV x-rays.
• Any dose can cause damage, therefore, precautions must be taken to prevent any exposure to radiation.
• Dosimeter - a badge which people who work with radiation wear to monitor the radiation they receive.
  • must be checked periodically
  • If exposure is too high, the person must take a break until their bodies repair themselves.
• Natural background radiation and medical doses cause little damage.
• High doses from damaged reactors or bombs can produce massive damage.
• There are four ways to protect oneself from radiation exposure.
  • Distance - Ionizing radiation obeys the inverse square law.
  • I a 1/r²
  • - if r is doubled, intensity becomes 1/4 as much.
  • - distance from source reduces radiation risk considerably.
  • Time - Generally the shorter the exposure, the less the effect.
  • - This is why dosimeters are worn.
  • Shielding - Thin layers of paper and plastic will stop alpha and beta
  • particles but not x-rays and gamma rays.
  • - Substantial thickness of dense concrete or lead are
  • required to stop x-rays or gamma rays.
  • - Sources of x-rays and gamma rays are therefore kept in
  • thick walled containers with concrete and lead walls.
  • Containment - for safety to the environment and humans, radioactive
  • materials are confined to specific areas and stored in
  • specifically designed containers.

THE FUNDAMENTAL FORCES OF NATURE

• The fundamental forces of nature are
  • gravity
  • electromagnetic
  • strong nuclear
  • weak nuclear
• The strong nuclear force binds only the particles of the nucleus together because it operates over distances less than 10^-14 m.
• Gravity influences to infinity.
• The weak nuclear force is believed to be involved with beta decay and other elementary particle decay.
• The quest for a theory to unify all the forces of nature began with Maxwell in the 1860's.
• Maxwell united electrical and magnetic forces (electromagnetic forces).
• Einstein enhanced this unification with his special theory of relativity.
• Einstein tried to unify electromagnetic and gravity forces but failed.
• Sheldon Glashow, Steven Weinberg and Abdus Salam pieced together a theory that combined electromagnetism and the weak nuclear force (1960's). Their theory is called electroweak theory.

Strong Nuclear Electromagnetic Weak Nuclear Gravity

Range less than 10^-14 infinity 10^-10 to 10^-11 infinity

Relative Strength 1 10^-3 10^-14 10^-40

Particles Acted On proton charged subatomic everything

and neutrons particles particles with mass

USES OF ARTIFICIAL RADIOISOTOPES

• The nuclear structure of lighter elements can be changed by neutron bombardment. The element is placed in or near a nuclear reactor.
• Medical Therapy
• Consider: