Understand radioactivity
Atoms, subatomic particles and isotopes
An atom is the smallest amount of an element that still has its physical and chemical properties. If we try to split an atom of iron, the products will not be iron, but maybe smaller atoms with different properties. Atoms are rarely alone in ordinary matter, they are most frequently chemically bound to other atoms to form molecules. A substance made with molecules is a compound, like water, which is made with hydrogen (H) and oxygen (O). The smallest amount of a compound that still has its properties is a molecule of that compound.
A standard atom has a diameter around 1 Å (angstrom) which means 10–10 m. Ten million atoms in a straight row would be a 1 mm long queue. An atom has a nucleus and a shell and are made up with three basic particles listed in the table below. Atomic nuclei sizes are in the order of 1 fm (10–15 m), so about a hundred thousand times smaller than the whole atom.
| Particle | Electrical charge | Mass (in rest) | Resides in | |
| Proton | p+ | + 1.602E–19 C | 1.67E–27 kg | nucleus |
| Neutron | n0 | 0 (no charge) | 1.67E–27 kg | nucleus |
| Electron | e– | – 1.602E–19 C | 9.11E–31 kg | shell |
It is important to
notice that:
– Electrical charge of proton and electron is the exactly the same but the sign is
opposite.
– Mass of an electron is about 1800 times less than that of a proton or
neutron.
– "massive" particles of the atom reside in the nucleus, so the
nucleus has almost all the atomic mass in a very small volume. Atoms and
matter are then essentially emptiness.
Atoms are electrically neutral in their basic state. This means they have as many electrons as protons. If electrons are taken out or given to a neutral atom, an ion with positive or negative charge is formed, respectively. The neutrons in the nucleus play an important role in keeping together all the protons that electrically repel each other. The number of neutrons in an atom nucleus is variable between stability limits.
Chemists define elements by the proton
number, called atomic number (Z). All atoms with 6 protons are carbon, and
all atoms of carbon have 6 protons in their nuclei.
If the electron number
mismatch the atomic number, we have an ion of this element. An atom
of carbon with 6 protons and 7 electrons is a carbon negative ion, written
as C–.
All atoms with the same proton number, therefore the same element,
but different number of neutrons are called isotopes of that
element, and are indicated by the mass number (A) which is the sum of
protons and neutrons. Isotopes of an element have very slightly different
physical properties. Carbon has two main isotopes: carbon–12
(12C) with 6 neutrons, and carbon–14
(14C) with 8 neutrons.
It is used extensively
by archaeologists.
Chemical reactions, nuclear reactions and radioactivity.
When atoms undergo chemical reactions, for instance when fuel is burnt in a heating system, only the outermost electrons take part and chemical bonds can be broken. The inner electrons and the nucleus remain the same.
A nuclear reaction is a chemical reaction where the nucleus change. The deeper we go into an atom, the stronger forces and bigger energies we find. So, the nucleus, even if it is minuscule in a minuscule atom, keeps comparative huge forces and energies. Nuclear reactions are dramatically energetic.
When a nuclear reaction happens, some high energy particles or electromagnetic wave are released outside. This is the phenomenon called radioactivity, discovered by H. Becquerel in 1896. Very remarkable is the work by Ms. Curie that discovered radioactivity in thorium and radium.
Atoms don't last forever. They are more or less stable, which means that they have an average lifetime and sooner or later they will undergo a spontaneous nuclear reaction. Atoms regarded as "stable" will last for thousands or millions of years before they change, and unstable atoms can have lifetimes averaging not even a second. These spontaneous nuclear reactions take place in all places in the Universe, and lead to natural radioactivity. Many minerals of Earth, like granite, are naturally slightly radioactive. Natural radioactivity of Earth minerals is one of the sources of energy that keeps Earth core melt at temperatures close to 3000 ºC. A second kind of natural radioactivity are cosmic rays (or cosmic radiation), which are gamma photons produced by nuclear reactions in our Sun and other stars. Cosmic radiation is responsible of producing isotopes like carbon–14, chlorine–39, and sulphur–35.
After discovery and understanding the laws of radioactivity, it was possible to control it and to make new nuclear reactions that led to new isotopes and even elements. By having a look at a chemist's Periodic Table, we will find that technetium (Tc) is a synthetic element (on Earth) and elements from neptunium (Np) onwards are all synthetic.
Kinds of nuclear reactions and radioactivity
To better understand nuclear reactions, four new particles have to be introduced (see table below). It must also be said that electromagnetic waves (that include radio and TV waves, infrared, visible light, UV light, X rays and gamma rays) can behave either as a wave or as a flow of particles called photons (in physics books this is called the dual nature of light).
| Particle | Characteristics | |
| positron | e+ | Particle with the mass and charge of an electron, but with positive sign. |
| neutrino | v | Particles with virtually no mass and no charge. They interact very seldom with matter and therefore are very difficult to detect. The Sun is said to produce a huge flux of neutrinos that cross the whole Earth and all the other bodies. |
| antineutrino | v' | |
| photon | γ | Particle with almost no mass and no charge associated to a quantum of an electromagnetic wave (Einstein was given the Nobel prize for its discovery) |
Nuclear reactions can be classified as in the following table.
| Nuclear reaction | Process | Examples |
| disintegration | some nuclear particle(s) split into two or more particles. | Natural disintegration of elements like carbon, sodium, chlorine, etc. |
| (nuclear) fission | a whole nucleus (and atom) splits into two smaller atoms. | Uranium fission in nuclear plants. |
| (nuclear) fusion | two nuclei (atoms) form a bigger atom. | Hidrogen fusion into helium that takes places in the stars' core (and in our Sun) |
But more important is the classification of nuclear reactions by the kind of particle that is emitted. Table below lists the nuclear reaction types, the particle emitted and their basic characteristics. Ionizing power is the capability of electrically interact with matter, creating ions, and induce (bio)chemical alteration. Penetration is the capability of crossing bodies.
| Radiation | Emitted particle | Characteristics | |
| Alpha | α | alpha particle (2 protons and 2 neutrons together) |
Very highly ionizing Low penetration |
| Beta – | β– | electron |
High ionizing power Medium penetration |
| Beta + | β+ |
positron or positive electron (electron with positive charge) |
|
| Gamma | γ | photon (electromagnetic wave) |
Low ionizing power Very high penetration power |
| Neutron | n0 | neutron |
Low ionizing power Low penetration |
And the following table presents the full nuclear reactions for each type and a description of what happens in the nucleus. X stands for the original nucleus, and Xº for the transformed nucleus after the reaction.
| Nuclear process | Description | |
| alpha | X → Xº + α2+ | An alpha particle is emitted from the nucleus, that losses two protons and two neutrons. |
| beta – | n0 → p+ + e– + v' | A neutron disintegrates and gives a proton that remains in the nucleus and an electron and an antineutrino are radiated. |
| beta + | p+ → n0 + e+ + v | A proton disintegrates and yields a neutron, that remains in the nucleus, and a positron and a neutrino are emitted. |
| gamma | X → Xº + γ | A photon (electromagnetic wave quantum) is emitted. The nucleus losses energy with no change in its particles. |
| neutron | X → Y + Z + n0 | It only happens in nuclear fission or fusion. When uranium in power nuclear plants splits into Ba and Kr gives normally 3 neutrons. |
Effects of radiation on live
Living organisms have very specialized tissues and organs formed by cells with very complex biological molecules. Live is a wonderful mechanism with many small tailored pieces and everyone plays a key role. The breakdown of a single piece will cause a serious damage, if not the ruin, of the whole mechanism.
Radiation affects biological
molecules in two ways:
–
ionizing atoms and breaking chemical bonds. Molecules
split or react with different molecules or develop different bonds. Even a
change in the physical shape of a protein can irreversibly stop its
biological
function.
–
provoking nuclear reactions. If an atom changes to another
element by a nuclear reaction induced by radiation, its chemical behaviour
will be different and the molecule will not work and possibly will react
with other molecules or split.
When evaluating biological effects of radiation, we have to keep in mind that cells grow and die. Cells have genetic bio molecules (DNA and RNA) that exactly replicate when new cells are born. Sometimes, by hazard, it happens an error in replication that provokes a mutation that gives a slightly different cell. Most mutations don't succeed and die. Some mutations are harmful and attack the organism. Very few mutations are helpful and produce species evolution.
But radiation is not the only factor that produce mutations and damage. Mutations happen at random and they are also induced by some chemicals, even from Nature. Biological damage is also produced by sunlight and toxic metals like lead, cadmium, chrome or mercury that are naturally present in the soil, and therefore naturally present in plants and animals.
When trying to measure biological damage or risk due to radiactivity, the first problem is to answer the question: how to measure damage?. How much is it a hand, an eye or a kidney?. And also we must take into account that radioactivity is not a single phenomenon but a family of them. This means:
– Every type of radiation
(alpha, beta, gamma, neutrons) has a different damaging potential.
– For a given type of radiation, particles can have different energies,
and their damaging potential varies.
– Organs in the body have different sensitivities to radiation.
Measuring radioactivity.
There is a number of methods and variations for the detection and measurement of radioactivity. They all are based on the ionizing power of radiation. Table below lists some common equipments and their performance. Dead time is the time the equipment cannot detect a second particle after a first detection.
| Method | resolution (mm) | dead time |
| Wilson chamber | 0,5 | - |
| Ionization chamber | 50 | 10 s |
| Geiger tube | 50 | 100 μs |
| Semiconductor detector | 5 | 1 μs |
| Scintillation detector | 50 | 1 μs |
| Cerenkov detector | 50 | 10 ns |
This article is centered on Geiger tube. It was invented in the 20's and it hasn't changed very much since then. A Geiger tube is a glass tube surrounded by a steel cylinder with an electrical connection. The tube is filled with argon gas. Finally, a wire inside the tube is the second electrode.
A high DC voltage (from 500 V to 900 V) is set between the central electrode and the shell, that is connected to ground by a resistor. The tube works as a gas cylindrical capacitor. When radioactivity ionizes a single gas atom, a free negative electron and a positive ion are formed and they are accelerated by the electric field. As these particles move, they hit and ionize other atoms, resulting in an avalanche effect and a small current (μA) that lasts for some microseconds and can easily be detected as a voltage pulse (1 to 3 V) in the resistor.
Geiger tubes are manufactured in many sizes, being the smallest around 3 cm long and 1 cm diam. Large Geiger tubes also have higher sensitivities but need higher DC voltages to keep the electrical field.
A Geiger tube detects photons (gamma and X ray) as well as beta particles, with different sensitivity. Some tube models have a thin mica "window" to be able to detect alpha particles as well. They cannot detect neutrons, as neutrons are not ionizing.
A Geiger tube gives an electrical pulse every time it is ionized by radiation. First Geiger counters had simply the tube, high voltage generator, an amplifier and a loudspeaker that sounds a "beep" for each detection. The operator counts the beeps in a given time and gives the result in counts/minute (cpm) or counts/second (cps). This count rate has to be converted to a radioactivity magnitude, as it is explained below. Modern Geiger counters are handheld and light, and the reading can be shown in many units.
Units of radioactivity.
There are several magnitudes and units related to radioactivity. The following table shows the most important magnitudes, their definition and units, as well as equivalences among these units. SI units are in bold letter.
| Magnitude: | ACTIVITY (A) | ||
| Definition: | disintegrations or nuclear reactions par unit time | ||
| Units: | Becquerel | Bq | 1 Bq = 1 dps |
| disintegrations/second | dps | ||
| Curie | Ci | 1 Ci = 3,7E10 dps | |
| Notes: | Specific activity is given in Bq/g , Ci/kg , etc. | ||
| Magnitude: | EXPOSURE (X) | ||
| Definition: | electrical charge produced by ionization par unit mass or volume of air | ||
| Units: | Coulomb/ kg | C/kg | |
| Roentgen | R | 1 R = 2,58E–4 C/kg | |
| Notes: | only for photons (X ray and gamma) in air | ||
| Magnitude: | EXPOSURE RATE (X/t) | ||
| Definition: | instantaneous or average exposure par unit time | ||
| Units: | ampere/ kg | A/kg | |
| Roentgen/h | R/h | 1 R/h = 7,167E–8 A/kg | |
| Magnitude: | ABSORBED DOSE (D) | ||
| Definition: | energy gained par unit mass of material | ||
| Units: | Joule/ kg | J/kg | |
| Grey | Gy | 1 Gy = 1 J/kg | |
| radiation absorbed dose | rad | 1 rad = 0,01 Gy = 10 mJ/kg | |
| Notes: |
Absorbed
dose depends on radiation type, its energy and material properties.
For photons in air, there is an equivalence between exposure and dose: |
||
| Magnitude: | ABSORBED DOSE RATE (D/t) | ||
| Definition: | instantaneous or average absorbed dose par unit time | ||
| Units: | Watt/kg | W/kg | |
| Grey/ hour | Gy/h | 1 Gy/h = 2,778E–4 W/kg | |
| rad/hour | rad/h | 1 rad/h = 0,01 Gy/h = 2,778E–6 W/kg | |
| Magnitude: | EQUIVALENT DOSE (H) | ||
| Definition: | weighted absorbed dose to evaluate potential damage | ||
| Calculated as: | H = Q·N·D | ||
| Units: | Joule/ kg | J/kg | |
| Sievert | Sv | 1 Sv = 1 J/kg | |
| rem | rem | 1 rem = 0,01 Sv = 10 mJ/kg | |
| Notes: |
Q =
quality factor of radiation, depending on type and energy (from 1 to
20) |
||
| Magnitude: | EQUIVALENT DOSE RATE (H/t) | ||
| Definition: | instantaneous or average equivalent dose par unit time | ||
| Units: | Watt/kg | W/kg | |
| Sievert/ hour | Sv/h | 1 Sv/h = 2,778E–4 W/kg | |
| rem/hour | rem/h | 1 rem/h = 0,01 Sv/h = 2,778E–6 W/kg | |
| rem/year | rem/y | 1 rem/y = 1,14E–6 Sv/h = 3,169E–10 W/kg | |
Now the question is: how can I convert the count rate from a Geiger tube to any of these units? The answer is that the manufacturer of the Geiger tube provides a graph with the relationship between Geiger count rate (cps or cpm) and either dose rate or exposure rate. As an example, graph below is from LND Geiger tube model 713. From the graph, 700 cps equals 100 mR/h or 877 μGy/h. This kind of graphs are obtained with a given radioactive source, in this case 60Co. In scientific work, a sensitivity curve for the radiation under study must be obtained and used, but this standard curve can be used for general purposes as the differences are very small.
We designed and built a Geiger-Muller counter at JC Electronica using an LND 713 tube. It works from four 1V5 battery cells, generates a nominal 525 V with high energy efficiency and has a microcontroller and five pushbuttons. It counts Geiger pulses and time, and shows either counts/min (cpm) or yearly dose (mrem/y). Some values obtained after years of use are shown below.
| Place |
Geiger count (LND 713) (cpm) |
Dose rate D/t (mrad/year) |
Equiv. dose rate H/t (mrem/year) (Q=1) |
Exposure rate X/t (μR/h) |
| Nature (beach, mountain, etc.) | 5 | 91,5 | 91,5 | 12 |
| City, in the street. | 9 | 165 | 165 | 22 |
| City, inside an old, stony building. | 13 | 238 | 238 | 31 |
The yearly dose limit is 500 mrem/year for general population and 5000 mrem/year for professionally exposed people. Marie and Pierre Curie worked all their lifes with radioactive elements and they isolated radium, the most radioactive element known, and worked with no special protection, because the risks were not known. Pierre died at the age of 47 in a traffic accident and Marie died at the age of 67. They had two daughters, Irene and Eve.