Category Archives: radiation

Radiation Measuring Devices

One click on a Geiger counter signifies a single nuclear disintegration, but not the type of radiation released by it. This could be alpha, beta or gamma. The clicks per second can be easily translated into becquerel, and will give the rate at which the living tissue is receiving radioactive particles. The intensity of the radiation source is being measured here.

The biological effect of this, expressed in sieverts, depends on several other factors. A conversion between these units is not easy. Modern devices provide a ‘best guess’ of the sievert equivalent. Some may not detect alpha or beta radiation. Incorporating a mica window allows these particles to be measured, though calibration to sieverts becomes more challenging then.

Microsieverts (µSv) are the most common unit. American equipment may be calibrated in millirems (mrems). One millirem equals ten microsieverts.  Millisieverts (mSv) may also be used; one millisievert (1000 µSv) is a dangerous dose.  [100 mrem; the recommended maximum yearly exposure for the general public]  As radiation is accumulative, you should leave the area as quickly as possible.

Some Geiger counters will give data on dose per hour. The average safety limit for workers in the nuclear industry is 20 mSv/year. Firefighters at the Chernobyl nuclear power plant received an average 12 Sv over their period of exposure, from which all were ill and 30 died quickly.

Radiation on food or in water is harder to measure. Dust from a nuclear incident lands on these and contaminates them. Careful calibration against background radiation and long measuring periods, up to 12 hours, are required. Although the intensity of these sources may be low, the biological effect is compounded by ingesting them. Covering food, bringing farm animals indoors and filtering water can help.

A Geiger counter will not tell you what kind of radioactive sustance is present on food. Safety limits range from only 10 becquerels per kilogram when dealing with plutonium, to 10,000 Bq/Kg for tritium or carbon-14.

The best use of a Geiger counter in a serious emergency is to find a safe place, with a tolerable level of radioactivity. You should remain under cover until the majority of the fallout has dispersed. Four days is a recommended minimum, so a reading of 10 mSv would be the upper limit.

Remember you are keeping dust out, so you are better off in a building. Make sure there is enough water. The longer you can stay there the better, as fallout will now be covering the ground. The danger comes from inhaling or ingesting these fine particles.

Good luck with survival. You might wish you’d been less hostile to wind power.

 

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Measuring Radiation – Sieverts and Greys

The International System of Units (SI) is the most popular system of measurement globally. Radiation units have been brought into this. Becquerels measure quantity and coulombs per kilogram are used instead of roentgens to denote exposure.

Grays (Gy) measure the absorbed dose. A gray is defined as the absorption of one joule of radiation energy by one kilogram of any matter, not just air as with the roentgen unit. One gray is equal to 100 rads. Five grays at once is a lethal dose. Diagnostic medical treatments are usually measured in milligrays (mGy).

An abdominal X-ray gives a dose of 0.7 mGy, while a computerised tomography (CT) scan is higher, at about 6 mGy. Cancer treatments exceed the lethal dose, but in small increments. Up to 80 Gy can be given, in doses of 2 Gy at a time.

Any given amount of radiation may not have the same biological effect. This is influenced by differences in the type of radiation and the conditions of exposure. Where X-rays and gamma rays are concerned, the absorbed dose is the same as the equivalent dose. If alpha particles are involved, the biological damage is more severe and weighting factors are applied. The sievert is the resultant unit.

A sievert (SV) is the standard international measurement of equivalent dose, replacing the rem. Sieverts express the potential for damage to human tissue, and are related to grays. One sievert is equivalent to one hundred rems, which would be a lethal dose. A microsievert (µSv) is one millionth of a sievert. One tenth of a microsievert is the natural radiation found in an average banana.

part four ‘Measuring Devices’ to follow

Measuring Radiation – Roentgens, Rads and Rems

The roentgen is defined as 2.58 x 10-4 coulombs of charge produced by X-rays or gamma rays per kilogram of air.

A roentgen is a lot of radiation. A dose of 500 roentgens within five hours will kill you. So a place with a reading of 100 roentgens per hour or more is very dangerous. Shortly after the Chernobyl nuclear disaster, readings of up to 30,000 R/hr were recorded in some areas.

Devices are usually calibrated in tiny fractions of one roentgen. There are a thousand milliroentgens to one roentgen. The reading will often be given in mR/hr. Flying at high altitudes exposes passengers to around 25 mR/hr, due to cosmic radiation.

Rad stands for Radiation Absorbed Dose. The units used here relate to the amount of radiation absorbed by the irradiated material. This may be you. One rad indicates exposure equivalent to an energy of 100 ergs per gram. It is about the same as 1.07 Roentgen, or 1,070 mR.

Rads are useful when assessing whether acute radiation sickness is a risk. A dose of 10 rads (10,000 mrads) in less than an hour is dangerous. Treatment for ARS will be needed.

The absorbed dose, measured in rads, is adjusted to give the Roentgen Equivalent in Man. The type of radioactive material is taken into account, among other factors. This unit is used to assess the chances of getting cancer from exposure. It is used to calculate safe levels in industry and medicine.

A rem is a large amount, so readings are generally given in millirem (mrem). The general public should not be exposed to over 100 mrem per year. This is just over the natural radiation levels inside a building made of granite.

Roentgens, rads and rems are very roughly equivalent. As the adoption of international standards became important, they were replaced by other units. Some countries, particularly the USA, continue to use the old ones.

part three ‘Sieverts and Grays’ to follow

Measuring Radiation – Curies and Becquerel

Devices are available to measure radiation, from expensive Geiger Counters to smartphone apps. There are several different units used. Some areas or items have naturally high radiation levels which confuse readings. It’s difficult for an amateur to work out how dangerous a source might be.

Curies and Becquerel

A curie (C) is the original unit used to describe the intensity of radioactivity. It is based on the physical properties of radioactive material, as the dangers of exposure were not well understood at the time. One curie is the radioactivity of a single gram of radium.

A microcurie is a millionth of a curie and a picocurie is a trillionth. Thirty seven billion becquerel (Bq) equal one curie. A becquerel is thus about 27 picocuries (pCi). Modern convention has replaced the curie as a unit with becquerels. As these are tiny, measurements are often given in kilobecquerels (1000 Bq) or megabecquerels (1,000,000 Bq).

The unit is named after the French scientist who discovered radioactivity, Antoine-Henri Becquerel.  It describes one atomic disintegration per second. These disintegrations release energetic particles, which are the basis of radioactivity. The process is called decay.

Alpha particles are the largest, the same size as a helium atom. Beta particles are smaller, but can be stopped by a shield only a few millimeters thick. Gamma rays, a sort of proton, can penetrate up to two feet of concrete.

Skin and clothes protect against the larger particles. If radioactive atoms enter the body, however, these particles will hit cells directly during decay. A nuclear incident distributes these atoms in the air, water and food.

Radioactive material presents two types of hazard. There is the danger of being too close to an emitting source, and the risk of ingesting small fragments of it. This risk increases dramatically if the source explodes. Nuclear fallout is the resultant dust.

Although the luminous properties of radium and other elements were exploited for various gadgets, the dangers of emitting sources are now recognised. The public are unlikely to come into contact with them, except during medical treatments.

X-rays and radiotherapy both use radiation. A means of measuring the risks to patients and staff was needed. New systems were explored, designed to express the biohazard aspect, the actual damage done to living tissues.

Part Two ‘Roentgens, Rads and Rems’ to follow

© E J Walker 2014