RADIOACTIVITYUnit IV: Nuclear Physics

What is Radioactivity?

Is the spontaneous breakdown of an unstable nucleus.

Results in the emission of particles or electromagnetic radiation.

It is found naturally and in artificially produced sources.

Radioactivity cannot be detected by human senses.

Pierre and Marie Curie investigated uranium ores using chemical separation. They discovered that pitchblende and chalcocite, naturally occurring ores, were highly radioactive due to the presence of plutonium and radium.

All substance are made of atoms. These have electrons (e) around the outside, and a nucleus in the middle.

The nucleus consists of protons (p) and neutrons (n), and is extremely small.

In some types of atom, the nucleus is unstable, and will decay into a more stable atom. This radioactive decay is completely spontaneous.

You can heat the substance up, subject it to high pressure or strong magnetic fields - in fact, do pretty much whatever you like to it - and you won't affect the rate of decay in the slightest.

All naturally occurring elements with atomic numbers greater than 83 are radioactive, as well as some isotopes of lighter elements.

Isotopes are atoms of the same element that have the same atomic number but different atomic masses due to a change in its number of neutrons.

An element can be written in many forms.

where:   E = element symbol   A = atomic mass   Z = atomic number

For example : Bismuth has an atomic mass of 209 and an atomic number of 83.

where:   E = element symbol   A = atomic mass

Notice: No atomic number is given since each element always has the same atomic number. This number can be found on the Periodic table.

For example :Helium has an atomic mass of 3 and from the periodic table you could determine its atomic number to be 2.

The number of protons in an atom is equivalent to its atomic number.

The number of neutrons plus the number of protons in an atom is equivalent to its atomic mass.

Number of neutrons = atomic mass - atomic number

Carbon consistently has an atomic number of 6 (6 protons are present) but it has two different atomic masses, 12 and 14.

Therefore in this example carbon-12 has 6 neutrons and carbon-14 has 8 neutrons.

Three Types of Radiation

Alpha particles ᾳare positively charged particles emitted from

alpha decay these particles are helium nuclei

are slightly deflected in an electric or magnetic field

are emitted at high speeds

Alpha Particles Cont’dhave the lowest penetration power - up to 5

cm in air can be stopped by a thin layer of paper or

aluminum

Alpha particles are made of 2 protons with 2 neutrons.

This means that when a nucleus emits an alpha particle, it loses 2 protons and so its atomic number decreases by 2.

Also, when a nucleus emits an alpha particle, its atomic mass decreases by 4 (that's 2 protons plus 2 neutrons)

An Example So Americium-241 (an -source used in

smoke detectors), which has an atomic number of 95 and an atomic mass of 241, will decay to Neptunium-237 (which has an atomic number of 93 and an atomic mass of 237).

The equation would look like this:

Note that an alpha particle is the same as the nucleus of a Helium atom (2 protons and 2 neutrons).

Thus we can write  or in the equation.

Alpha-decay occurs in very heavy elements, for example, Uranium and Radium.

These heavy elements have too many protons to be stable. They can become more stable by emitting an alpha particle.

Beta particles ᵦare electrons that are emitted from beta

decay are deflected greatly in an electric or

magnetic field its direction of reflection is opposite to that of

particles they travel at various speeds, sometimes

approaching the speed of light

medium penetration power - 10 m in air can penetrate several centimeters of

aluminum A Beta particle is the same as an

electron.It has a charge of -1, and a mass of

around 1/2000th of a proton.

If a nucleus contains protons and neutrons, what's an electron doing coming out of a nucleus?

To answer this, we need to know more about protons and neutrons:

Protons & neutrons are made of combinations of even smaller particles, called "quarks". Under certain conditions, a neutron can decay, to produce a proton plus an electron. The proton stays in the nucleus, whilst the electron flies off at high speed.

 

Results in the original nucleus changing - atomic mass remains the same and atomic number increases by one

This is because a neutron has changed into a proton (almost the same mass - we can ignore the tiny mass of the electron) and thus the number of protons has gone up.

Example: Strontium-90 undergoes β decay and forms Yttrium-90.

Gamma Rays ᵞare high energy electromagnetic radiation highest penetration power - 2 km in air can penetrate a minimum of 30 cm of lead the composition of the original nucleus does

not change when these rays are emitted

Gamma rays are waves, not particles. This means that they have no mass and no charge. So we sometimes write

We don't find pure gamma sources - gamma rays are emitted alongside alpha or beta particles.

Strictly speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus, it just carries away some energy.

Radioactive nuclides have different properties. All radioactive nuclides have the following properties:They undergo radioactive decay. Their radiation affects the emulsion of

photographic film, it ionizes surrounding air molecules, it makes some compounds fluoresce, and it has certain special biological effects.

Measurement of Radiation Dosimetry is the measurement of

radiation and the study of its effects on living organisms.

Several units are used to measure radiation.

Becquerel (Bq) - Measures the activity of a source. It is not an appropriate unit to use when studying the effect of radiation on a living organism.

One becquerel is equal to one emission per second.1 Bq = 1 emission per second = 1 Curie (Ci)

Absorbed dose - Describes the amount of energy deposited by a source per kilogram of exposed tissue.

When a source gives 1 J of energy to 1 kg of tissue the absorbed dose is 1Gy. It depends on: the type of absorbing material exposed (bone,

organs, flesh, etc.) number of particles per second hitting the

organism the energy per particle

Its SI unit is the gray (Gy) and a common non-SI unit is Rads.1 Gy = 1 J/kg = 100 Rads

The radiation for the treatment of cancer generally involves an absorbed dose of 40 Gy.

Quality factor - A number assigned to each type of radiation to describe its biological effects.

It was defined by comparing its effects with those of a standard radiation of 200 keV X-rays.

1 kiloelectron volt = 1.60217646 × 10-16 joules

Quality factors are approximate since they depend on the energy of the radiation and the type of tissue being irradiated. An absorbed dose of 1 Gy of alpha particles can do 10 times the biological damage as the same dose of beta particles.

Dose equivalent - Measures the biological damage produced on an organism. Its SI unit is the sievert (Sv). Dose equivalent (Sv) = absorbed dose (Gy) quality factor (Q).

Better Safe than Sorry!

Radiation has different effects on different kinds of tissue and no exposure to radioactive emissions, for any period of time, should be considered "safe" to humans or other living organisms.