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Natural Radiation

Our environment is permeated by radiation, present around us at all time.  We are constantly exposed to radioactivity from natural sources for the most part naturally occurring radioactive nuclei in rocks and cosmic rays – the ‘background’.  Without ado, this is my lowdown on radioactivity.

Radioactive material is present throughout Nature.  Detectable amounts occur in soil, rocks, water, air and vegetation, from which it is inhaled and ingested into the body.  In addition to this, human beings and all life on Earth, also receive daily external exposure from radioactive materials that remain outside the body.

 

Radioactivity in your Background

Background radioactivity is that ionizing radiation which is naturally and inevitably present in our environment.

Cosmic Radiation

Cosmic rays are extremely high-energy charged particles, mostly hydrogen and helium nuclei, of extra-galactic origin, which reach our home planet from every direction.

The Earth’s atmosphere protects us from direct exposure to these cosmic rays to some extent.  But when they strike the upper atmosphere, they precipitate a cascade of nuclear reactions whose decay products reach the ground.

However, ambient radioactivity comes from a variety of natural and artificial sources.  At an altitude of about 1,500 metres into the atmosphere (roughly one mile up), the natural radioactivity from cosmic rays is about twice as great as at sea level.

 

Mineral samples of Halite and Anhydrite

Terrestrial Radiation

Long-lived radioactive elements have been present in the Earth since its creation.  These massive nuclei are unstable and spontaneously decay by fission into lighter elements, some of which are also radioactive.

Indeed, helium is retrieved from natural gas, having resulted from the  natural decay of Uranium (U) and Thorium (Th) atoms, present in the granitoid rocks of the continental crust.

The richest helium accumulations are found in regions where three geological conditions co-exist:

  1. Granitoid basement rocks are rich in uranium and thorium.
  2. The basement rocks are fractured and faulted to provide escape paths for the helium gas.
  3. Porous sedimentary rocks above the basement faults are capped by an impermeable seal of halite or anhydrite

 

This kind of geological terrain is common in many areas of the United Kingdom and elsewhere in the World.

 

Potential Risk of Radon due to Local Geology

Airborne Radiation: Granite Cities and Radon Gas

Ionising radiation is a fact of life for all of us.

But for some cities, it’s a daily source of worry – and not merely the ones located near Chernobyl or Fukushima.

Radon gas is a natural by-product of uranium decay.  And the second-biggest contributor to lung cancer, after smoking.

A lot of our natural exposure to radioactivity is due to radon, a gas which seeps from the Earth’s crust into the atmosphere.

Not far from home, in Scotland, Aberdeen, the “Granite City”, was founded on radium-rich bedrock.  Without  cracks and fissures in that rock for radon to escape, however, the dangerous gas remains trapped, contained and harmless to the population.

On the other hand, just a few kilometres further North, cracks in the bedrock are abundant.  If the city had been located over that zone, it would have been a different story, and substantial ground-floor ventilation work would have been required.

Near the Caspian Sea, the city of Ramsar (Iran) has such high natural background radiation levels due to nearby hot springs and building materials originating from them, that scientists recommended that the 32,000 residents relocate.  Its neighbourhood of Talesh Mahalleh is the most naturally radioactive inhabited area in the World, and under long-term study.

Record levels of local radiation were found to be over 80 times higher than the World’s average background radiation.

 

The Larger Picture

For the sake of accuracy, we also include fallout from nuclear weapons testing and nuclear accidents, although those are (thankfully) rare incidences making up for less than 0.1% of the total picture.

Overall, we have cosmic radiation… and let’s not dismiss our exposure to the radiation of the Sun.  For better or for worse.  Terrestrial radiation from rocks and soil, and airborne radiation.

Although it’s not always easy to know how radioactive your city is.

Or at the very least, how dangerous the radiation makes it since…

 

Radioactivity = Danger… Right?

Actually, not always…

It really depends on how much of it is around.

 

The analogue display of a Geiger counter, with units CPM and microSievert.

Units of Radioactivity

As a physical process, the effect radioactivity can have on you is time-dependent.

Background radiation is given as a rate of counts per unit time as registered by a Geiger counter, with quantities like counts per minute (cpm) and counts per second (cps).

Count rate measurements are associated with the detection of particles, such as α-particles and β-particles.

For gamma ray and X-ray dose measurements, a unit called the Sievert (Sv) and its subdivisions (mSv or μSv) are normally used.

At high levels, radiation is hazardous.  It can cause damage to matter, particularly living tissue, so exposure to radiation must be monitored and controlled.

At low levels, such as those we all experience naturally, radiation is NOT dangerous.

Anything less than about 100 mSv is harmless.

 

Background Exposure

The 30-40 millisieverts per year of radiation that people in part of Brazil, Sudan, India and China get is higher than the annual dosage limits for non-radiation workers in the United States.  Source: World Nuclear Association

Levels of background radiation exposure vary hugely according to the local terrain.  For instance, residents staying in granitic areas or on mineralised sands, receive more terrestrial radiation than others.  While people living or working at high altitudes are more affected by cosmic radiation.

The UK’s annual average is 2.2 mSv (millisieverts).  If you live in Chennai in the Indian state of Kerala, it rises above 30 mSv, and it is as high as 40 mSv in parts of Brazil and Sudan, according to the World Nuclear Association.

The energy industry has strict guidelines and devotes considerable effort to ensuring that those working with nuclear power are not exposed to harmful levels of radiation from it.

Standards for the general public are set about 20 times lower still, well below the levels normally experienced by any of us from natural sources.

Annual radiation exposure is 2.2 mSv in the United Kingdom.

 

The amount of radioactive material in the environment is not the only factor: many of the bioactive effects do not arise from direct radiation, but rather from that radioactive material’s access to your body, in the air you breathe or the plants you eat.

 

Physics of Radioactivity

Radioactive decay or ‘radioactivity’ is a physical process, whereby unstable atomic nuclei break up spontaneously and liberate energy.

The reaction proceeds via a sequence of steps – a decay chain.

Uranium (238U) is a silvery-gray metal, occurring in Nature.  It is the most common form of radioactive uranium isotope on Earth.

The radioactive decay process for Uranium (238U) goes like this:

{}^{238}_{92} U \rightarrow {}^{234}_{90} Th \rightarrow {}^{234}_{91} Pa \rightarrow {}^{234}_{92} U ...

The half-life describes how quickly unstable radioactive nuclei undergo radioactive decay.  And conversely, how long stable atoms survive.  It is the time required for exactly half of the entities to decay on average.

For many identical atoms, the law of large numbers assumes that half of the atoms remain after one half-life.

 

Types of Radioactivity

A diagram showing the Alpha Decay event of a Uranium atom (parent nucleus) into a Thorium atom (daughter nucleus) and Helium atom (or alpha particle).

Alpha Decay of a Uranium-238 Nucleus

How dangerous radioactivity is also depends on the radiation type.

The types of radioactive decay include

  • α-decay (alpha decay), in which a nucleus emits an α-particle
  • β-decay (beta minus decay)
  • γ-decay (gamma decay).

 

The uranium nucleus spontaneously decays into a lighter element (thorium), while releasing α-radiation.  With 2 protons and 2 neutrons, the emitted α-particle soon attracts 2 electrons to form a helium atom.

{}^{238}_{92} U \rightarrow {}^{234}_{90} Th + {}^{4}_{2} He + energy

Beta-decay involves the emission of an electron e from the nucleus of an atom.

Gamma-decay involves no change in neutrons and protons.  When a nucleus is in an excited state, a quantum jump down to the ground state is accompanied by the emission of a photon.

All these types of radioactive decay liberate energy.

Where does the energy come from?

The answer lies in Einstein’s famous equation

E=mc^2

 

A Diffusion-type Cloud Chamber.  Source: Wikimedia

Visualising the ‘Background’

Although ionising particles are invisible to the naked eye, a diffusion cloud chamber can be used to visualise background radiation.

Cloud chamber are particle detectors for visualising the passage of ionizing radiation.  They work using the same principle as bubble chambers, but are based on supersaturated vapour, rather than super-heated liquid.

Basically, alcohol (isopropanol C3H8O) is vaporised by a heater in a duct in the upper part of the device.  Solid CO2 (dry ice) cools the bottom of the chamber.

The evaporated alcohol spreads throughout the chamber.  As the cooling vapour descends to the black refrigerated plate, it condenses.

Due to the temperature gradient, a layer of supersaturated vapour is formed above the bottom plate.  At this point, the alcohol condenses below its normal temperature to form narrow clouds around the ions from the radiation.

 

In any nuclear decay, electric charge and mass number are always conserved.

Since the principle of operation is based on the ionization caused by the passage of the particle, it follows that the higher the ionization capacity and the greater the visibility of the trace.

 

Which Particles Can you Detect?

  • Depending on the emission energy, α-particles have a range of a few centimetres.  They produce straight, short and fairly thick tracks.

 

  • Electrons and positrons come from the β-decay of radioactive elements, but also from the interaction of primary cosmic rays.  The traces left by the β-particles (electrons, positrons) are quite thin because the ionization power is rather small.  If the energy is sufficiently high, the traces will be straight.  Otherwise, the traces will be irregular due to scattering.  As a result, the tracks will be curly or curved.  The most irregular traces can be produced by photo-electrons produced by the photoelectric effect.

 

  • Muons, the heavy “cousins” of electrons, are continuously produced by the interaction of primary cosmic rays with the nuclei present in high atmosphere.  These muons are very energetic and penetrating, they have energies of order 100 MeV to 1 GeV.  They can travel hundreds of metres within dense matter, such as rock or water.  Tracks left by muons are straight and clearly visible given the high ionization power of these particles.

 

The higher the ionization, the greater the trace visibility.

The particle tracks are seen twirling, stretching, and interacting.  It’s like the particles are dancing…

 

Hearing the ‘Background’

As well as seeing background radioactivity, you can hear it and quantify it with a different kind of detector, called a Geiger counter.

 

Typical Figures from the Environment

We each receive about 2 mSv per year from the natural background, and maybe more from medical procedures.

The following figures may be of interest: 

  • In the United Kingdom, the typical dose equivalent experienced as a result of natural background radiation ranges from about 6.8 μSv per day up to an average of 21 μSv per day if you live in a location with a high concentration of radioactive rocks, such as Cornwall or Scotland.
  • A 3-hour aeroplane flight would mean you receive a dose equivalent of about 10 μSv from cosmic rays.
  • A typical chest X-ray is about 20 μSv.
  • Some survivors of the Hiroshima and Nagasaki atomic bombs were exposed to an instantaneous dose of γ-radiation of approximately 5 Sv.

 

Environmental Radiation Doses

Clicks Per Minute (CPM)MicroSievert Per HourMilliSievert Per YearAction
120.100.876This is low, it does not get much lower.
250.211.825Pretty normal.
500.423.65Happens once in a while for no real reason. Just be vigilant.
1000.837.3ALERT - No need to panic. But try to figure out what is going on. Stay out of the rain. Avoid unnecessary trips.
1501.2510.95Real cancer risk if exposed for a year.
5004.1736.5Real cancer risk if exposed for 90 Days.
20Limit for normal nuclear plant workers.
100Limit for Fukushima nuclear plant workers.
Based on NO INTERNAL EXPOSURE
- If radiation gets inside your body, it is 20 times more powerful!
(Sievert calculations based on Caesium-137 isotope)

 

Effect of Distance

The effect radioactivity can have on you is also distance-dependent.

If you were to use a Geiger counter to count the radiation 1 cm away from a point source, then repeat the readings at a distance of 10 cm from the same source, you would find that the count rate would be reduced by a factor of approximately 100.

The variation of count rate with distance obeys an inverse square Iaw.

The count rate decreases by a factor of x2 every time the distance

increases by a factor of x.

 

In the lab, if you use tongs to handle an encased uranium-rich mineral sample, your hand will be at a distance of approximately 20 cm from the mineral sample, rather than at a mere distance of about 1 cm.

Therefore, the dose of radiation you would receive will be reduced by a factor of (20)2 = 400.

 

Absorption

The marked variation of count rate with distance explains why it is so important to keep the positions of the Geiger tube and isotope generator fixed when investigating the absorption of γ-rays by different thicknesses of lead.

Lead is a stable element and an effective absorber for all types of radiations.  As a protection, it is used to shield people from radiation sources in ‘lead aprons’ and also to store radioactive sources.

Different things happen when α- or β-particles, or γ-rays pass through different materials.

 

Gamma-rays, α-particles and electrons released in the Earth from radioactive minerals are absorbed by the minerals themselves and their surrounding rocks.

Alpha-particles are the most readily absorbed, being completely stopped by a single sheet of paper.

Electrons are absorbed easily by low atomic number materials, such as the carbon atoms in Perspex.

Neither of these materials is very effective at stopping γ-rays.

Alpha and beta radiation have low penetrating power.  As a result, they are unlikely to affect vital internal organs from outside the body.  Although any type of ionizing radiation can cause burns, α- and β-radiation can only do so if radioactive contamination or nuclear fallout is deposited on the individual’s skin or clothing.

Gamma and neutron radiation can travel much further distances and penetrate the body easily, so whole-body irradiation generally causes acute radiation sickness, before skin lesions are evident.  Local gamma irradiation can cause skin effects without any sickness.

 

The Hourly Perspective

Consider the following…

About 500,000 cosmic rays from outer space penetrate the average human being every hour.

About 30,000 radioactive atoms of radon, polonium, bismuth or lead in the air we breathe disintegrate each hour in our lungs.

About 15 million atoms of Potassium-40 and 7000 atoms of Uranium, from the food we eat, disintegrate each hour inside of us.

Over 200 million γ-rays from the soil and buildings pass through an average person each hour.

Not to mention for the unlucky few, medical radiation for diagnosis or treatment (~ 14%).  What you get from the dentist’s occasional X-rays, etc.  I hate that guy…

 

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