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becquerel (Bq) is the SI derived unit of radiation activity. The Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The becquerel is therefore equivalent to an inverse second, s⁻¹.


Radiation signs
Radiation signs

Radioactive decay is a process of discharging radioactive particles. When talking about radiation, this article refers to ionizing radiation. Different types of radioactive decay include alpha, beta, and gamma decay. They are named after the particles emitted during this process. During the decay the radioactive particles take the energy away from the nucleus. Some radioactive decay changes the original nucleus of the atom into either a different nucleus or a nucleus in a changed state.

Types of Radioactive Decay

Alpha Decay

Alpha particles, emitted during alpha decay, are made of two neutrons and two protons. Their structure is similar to a helium nucleus. Most alpha particles created by alpha decay do not have high penetration, compared to other particles. Even a sheet of paper can stop them. Alpha particles pose little threat externally because even air can stop them if the wall of air between the radioactive source and the object is wide enough. Skin also stops alpha particles from entering the body. They are very dangerous to living organisms when taken internally, however — much more so than beta or gamma ones. Alpha particles emitted from Polonium-210 are notorious for having been used in murdering a former officer of the Russian secret service, Alexander Litvinenko, in 2006. He was tricked to ingest Polonium-210 in his food during a lunch meeting. It was a widely publicized case, especially because Litvinenko was poisoned in the United Kingdom, where he received political asylum.

Beta Decay

Beta particles, created during beta decay, are positrons or electrons. They have higher penetration than alpha particles, but they cannot penetrate aluminum, as well as a range of other materials. Beta radiation can enter the body during direct exposure. It is used in radiotherapy.

An interesting aspect of beta decay is that sometimes the particles that travel at high speed emit beautiful blue light, called Cherenkov radiation. An example of this was the glow of caesium-137 that attracted people during the Goiânia accident described below. It was because of this glow that at the time of the accident people thought that caesium-137 was a magical substance and displayed it in their houses.

Personal protective equipment. Naval museum complex Balaklava, Crimea, Russia.
Personal protective equipment. Naval museum complex Balaklava, Crimea, Russia.

Gamma Decay

Gamma rays created during gamma decay have very high level of penetration, much higher than the penetration capacity of alpha and beta particles. To protect against gamma radiation one needs to use a shield made from lead or another heavy material. The definition of gamma rays changed a number of times but now they are defined as rays that the nucleus emits, except for gamma rays emitted during astronomic events. They are distinguished from X-rays, which are created by the emission from electrons that are not inside the nucleus.


Each radioactive particle has a half-life, defined as a duration of time in which the total amount of the radioactive substance decreases by half. It represents time and is measured in seconds, minutes, hours, days, years, depending on the duration of the half-life. For example the radioactive particles of iodine-131 and Caesium-137, that were some of the major substances that contaminated the surrounding area after the Chernobyl accident, have half-lives of 8 days and 30 years, respectively. The total amount of time it takes for the radioactive material to decay will depend both on its half-life and on the amount of the material.

Chernobyl Disaster

The 1986 accident on the Chernobyl power plant in Ukraine is notorious for the release of radioactive substances into the atmosphere, and for the contamination of the surrounding area. At the time of the accident a large number of radioactive isotopes were released, including iodine-131, caesium-137, strontium-90, plutonium-241, among others. All of these elements undergo beta decay and as such they can penetrate cells easily if one does not wear protective clothing. They then damage the cells and cause various types of cancer.



Half-life for iodine-131 is the shortest, only 8 days, so it presented a short-term danger at the beginning of the disaster. The original release was extensive, about 1760 petabecquerels (pBqs), where 1 pBq is 10 to the power of 15 becquerels (Bq). Because of its rapid decay, there is very little iodine-131 left in Chernobyl at present.

Iodine-131 is easily absorbed by the body, especially by the thyroid glands. It increases the risk of thyroid cancer. It can be taken in by the body easily through contaminated milk and leafy green vegetables, and children are especially vulnerable. The Soviet government did not inform the public in a timely manner of the harm that radiation presents and how to avoid it. Seven days passed before the news about the accident in Chernobyl appeared in the mass media and in that week many children as well as adults consumed contaminated milk and other foods without knowing. As a result the number of thyroid cancers, especially in children, spiked in the areas affected by the radioactive fallout.


Other Radioactive Materials

Caesium-137, strontium-90, and plutonium-241 still contaminate the area around the reactor because of their significantly longer half-lives of 30, 29, and 14 years respectively. They were released in quantities of 85, 10, and 6 pBqs each. It is important to note that iodine radioisotopes made up only about 10-15% of all radioactive particles released. Amounts of caesium-137 and strontium-90 were much greater, together comprising more than 2/3 of the total radioactive material. These two radioactive elements will remain in the area for another 300 years before they decay.


Currently Caesium-137 is the main element that is potentially dangerous to people working in and visiting the 30 km exclusion zone in Chernobyl. It is also one of the main polluters in Fukushima, that remained in the area after the Fukushima Daiichi Power Plant disaster. Caesium-137 is taken up by the body because it is structurally similar to potassium, which the body needs. It stays in the muscle tissue and damages it. Since heart is a muscle, Caesium-137 affects the heart. Heart problems, especially in children, have increased in recent years in the areas affected by the Chernobyl contamination. Cancer is another risk increased by caesium-137 exposure.


The total amount of the radiation released was estimated by the Soviet government to be between 50 to 100 million curies (2 to 4 million terabecquerels). Scientists across the globe speculate based on the increase in cancers and other illnesses that the amount of radioactive material that escaped is 10 times greater than reported numbers above.

Cleanup Work

To do cleanup work the Soviet government recruited about 600,000 rescue workers, both trained and untrained, from the military and the civilian population, according to the World Health Organization. These workers were called liquidators. Some of the first liquidators were firefighters, many of whom died within days, months, or years later, due to their exposure to high levels of radiation. Many of them were enlisted to do extremely dangerous jobs, like cleaning up the debris on the roofs, because the electronics in the robots, originally used, were damaged due to high levels of radiation. Roof cleanups consisted of removing, among other things, the highly radioactive graphite, which was originally used to cool down the reactors.

It was important to prevent radioactive particles from becoming airborne. A lot of the cleanup, therefore, consisted of removing and burying radioactive materials, such as the debris from the explosions and the contaminated soil. Some initial work also included removing and disposing of food in the evacuated areas and killing domesticated animals left behind. Cleanup work is still continuing in the area.


L-746 Personal Dosimeter. 1980s. Diefenbunker — Canada’s Cold War Museum
L-746 Personal Dosimeter. 1980s. Diefenbunker — Canada’s Cold War Museum

The liquidators, recruited from the military reserves, had no say in the matter. Military service was mandatory for all healthy men in the Soviet Union and everyone who completed their service became a member of the reserves. Therefore, anybody could be recruited for this work, regardless of their current job. Usually people 30 years old or older were called on to join the cleanup efforts. Some managed to escape this by showing real or fabricated medical certificates that rendered them unfit for this work. People who could not provide such documentation had only one alternative to joining the cleanup: a prison sentence. Some volunteered for this work, understanding the risks but realizing that somebody needs to do this and often hoping that nothing will happen to them.

Some of the liquidators later wrote about the conditions they worked in and the violations of health regulations that happened during the cleanup efforts. As evident from the film “Chernobyl: A Chronicle of Difficult Weeks” by Vladimir Shevchenko, liquidators were assigned to work in areas with extremely high levels of radiation. Some liquidators did not wear adequate protection, such as respirators, choosing to ignore the safety regulations. One worker wrote in his memoirs that despite the regulations to wear dosimeters to measure the total exposure to radiation of each person, the officers responsible for keeping track of personnel exposure did not record the data shown on the indicators of the dosimeters. Instead they recorded for each worker an estimate for a given area where he was assigned to work for the day. These estimates were sometimes cut down to prolong people’s stay among the liquidation workers. Personal accounts of liquidators also suggest that even the supposedly safer living quarters were contaminated with radiation because some workers did not change their clothes when going out for cleanup missions, and wore contaminated uniforms back in the living area. Materials used to build the infrastructure in this area and even things like TVs were also sometimes radioactive, being harvested from the contaminated places.


The reactor was covered by a concrete construction, meant to contain radioactive material and to prevent it from spreading across the area. It is referred to as a sarcophagus. This name has a morbid connotation of burying something dead, or something that caused death and disease.

Radiometer-Dosimeter. 1980s. Diefenbunker — Canada’s Cold War Museum
Radiometer-Dosimeter. 1980s. Diefenbunker — Canada’s Cold War Museum

The sarcophagus is now deteriorating and parts of it collapsed in winter 2013. The possibility of collapse was known for a long time and recently construction of a new dome started. It was halted for safety reasons during the most recent collapse of the original sarcophagus, but after a week construction resumed. The estimated completion is scheduled for 2015. If the sarcophagus were to be left without this new incasing, its eminent complete collapse would have caused the release of additional radioactive particles into the air.

Chernobyl Tourism

Once the radiation in much of the exclusion zone was brought down in the mid-nineties thanks to the cleanup, some curious people started visiting the area as extreme tourists. Until recently unofficial “tour guides”, usually local residents, called “stalkers” were the only ones who offered to show people around. They knew safer paths, explained the safety rules to people, and lead them through the area using their own routes. Some did it for the money, some did it for free, to spread the word about the effects of the environmental disaster of such scale. Some introduced the tourists and journalists to local residents, who returned to their homes despite the radiation still present in the area.

Since 1995 an agency responsible for informing the public of the Chernobyl disaster situation also organized official tours to the area. Until 2010 the entry to the exclusion zone was limited but since then the government allowed access to the area for anybody interested to visit. The area was temporarily closed for about half a year in 2011, but is opened again, although the access is more restricted now. The prices as of 2013 start from $150 USD per person and go up depending on the duration of the tour and the number of people in the group.

Radioactive Accidents and Problems Related to Radiation

From the time radiation was discovered over one hundred years ago there have been many civilian accidents. In addition to disasters at power plants, the majority of these accidents are due to improper handling of materials that are still undergoing radioactive decay, often without the knowledge of people that they are radioactive. Some incidents involve melting down Caesium-137 and other radioactive isotopes with scrap metals. These radioactive materials are often mixed into the scrap metal because they were parts of the medical radiotherapy equipment that was not recycled properly.

Some examples include accidents that happened at a recycling facility in Spain and on a steel mill in China. Others include spills and problems with handling radioactive materials by people who are unaware of the dangers. In some cases the source of the contamination is unknown, like in the case with finding radioactive paper money in Russia between 1994 and 1996.

AN/FJW-1 Radiation Detection and Alarm System. 1960s, Diefenbunker — Canada’s Cold War Museum
AN/FJW-1 Radiation Detection and Alarm System. 1960s, Diefenbunker — Canada’s Cold War Museum

Below are just a few of the most notorious cases of accidents related to radiation. Many more accidents and problems happened in the past hundred years. Many of them are due to poor regulations and their enforcement, both in the developing and the developed world.

Radium Girls

Between the 1917 and 1926 radium was used as a glow-in-the-dark agent in paints. Women, who painted watches with this paint breathed in and took unsafe amounts of radium internally. While painting they licked their paintbrushes to be able to draw finer lines. They did not realize that radium was dangerous. Some women also painted their skin and nails with the beautiful paint.

Many later became sick with cancer, having their jaw bone deteriorate, or showing other symptoms of radium poisoning. The factories disputed the causes of the sicknesses for a long time, but several women sued the company they worked for, eventually receiving $10,000 each plus an annual amount of $600 for the rest of their lives. This was a public case, followed closely by the media. It created a precedent for labour disputes, and the government started regulating labour conditions more strictly. Occupational health legislation was developed as a result.

Church Rock Uranium Mill Spill

In 1979 a pool for radioactive waste at Church Rock, a uranium mill in New Mexico, overflowed. This was due to negligence by the workers, who did not follow the safety regulations and filled the pool beyond its capacity. The radioactive waste contaminated Puerco River and was carried to the reservation area of the Navajo Nation. People who lived in the contaminated area did not know of the threat for several days and used radioactive water in their daily life and for agricultural needs. The total radioactive decay of the isotopes in the contaminated water was 128,000 picocuries in each liter. In total 4 curies of radioactive particles escaped into the water.

The initial announcements of the danger were in English, the language not understood well enough by some residents. Others did not understand fully the dangers associated with radioactive contamination, despite understanding the announcements. The government provided insufficient aid to the residents in the affected areas and many families suffered the effects of the radioactive contamination and poisoning for many years.

As a result of this disaster a number of livestock died and some residents who bathed in the river, including children had skin damage. Sometimes it was severe enough that people’s limbs had to be amputated. Cases of cancer also increased. Some of the affected areas had no access to clean water after the spill, because the water reserves were contaminated.

The mill was suspended for a short period of time, but continued its operations soon after, further contaminating the area. The case was settled out of court a year later and the Navajo Nation received $525,000 in compensation. The initial cleanup left a significant amount of the toxic waste untreated. It took more than 20 years to restart basic cleanup efforts in 2004 and 2007. From 2008 to 2012 more thorough cleanup efforts were undertaken. Cleanup is not complete yet, and a new initiative is being developed at the time of writing (summer 2013).

Taiwanese Radioactive Apartments

A piece of steel scrap metal from a nuclear plant was used to create construction materials in Taiwan. It was contaminated with radioactive Cobalt-60. The reinforcing bars containing this metal were then used to construct up to 2000 apartments and commercial buildings as well as 30 schools around Taipei, Changhua, Taoyuan, and Keelung, from 1982 to 1984.

In 1992 one of the residents used a Geiger counter in his apartment and discovered the contamination. He made his discovery public and investigation started. The Atomic Energy Council (AEC) of Taiwan allegedly knew of this problem from 1985, while investigating contamination that they thought was caused by a dentist, who operated an x-ray machine in his apartment. They covered up this problem at the time, blaming the radiation on the dentist.

Eventually the authorities checked a range of buildings and found contamination in residential, office, and public buildings, including schools and kindergartens. People who lived, worked, or studied in these buildings have increased cases of cancer because they were exposed to low radiation doses during an extended period of time. 39 deaths were recorded in a nation-wide study related to this case, although it is unclear how many more deaths are caused by this problem. The researchers found a higher rate of cataracts in children who lived in the affected apartment buildings.

Many of these apartments are occupied by new residents and it is unclear if they know of the contamination, but the agencies who currently rent out the units are aware of the problem, but continue renting out the apartments nonetheless. Also, some of the residents refuse to move from the apartments they own, even though they know about the contamination. This is because they cannot sell their apartments at a high enough price, and the government does not provide enough subsidies for them to afford to move.

Goiânia Accident

Another infamous event, the Goiânia accident, happened in Brazil in 1987. There a hospital was abandoned and a radiotherapy unit with radioactive material still inside was left on the premises. The radiotherapy laboratory, IGR, and the owner of the abandoned building had to settle their disagreement about the future of the abandoned site and the ownership of the equipment left inside. Employees of IGR were not allowed to enter the facilities by a court order, so they could not remove the medical equipment left behind, despite their warnings of the dangers of this decision. When the guard stationed at the abandoned building did not come to work, the teletherapy equipment was stolen by scrap metal harvesters, who were not educated well about the dangers of radiation.

The thieves took apart the unit that they stole, and revealed the capsule that contained Caesium-137. One of them punctured a hole in it and found glowing material inside. While working on the unit both of the harvesters received a considerable amount of radiation poisoning, and one of them had some parts of his fingers amputated later, while the other one underwent a partial amputation of his arm, but they did not know the cause of their sickness until later. Several days after stealing the unit they sold the metal, including the capsule to a scrap yard, where the capsule was found by the owner. He took it home and displayed it to several people because of its beautiful blue glow, caused by Cherenkov radiation, described above. He then commissioned a friend to take some of the powdered material out of the capsule, and later gifted it to friends and neighbours.

The brother of the scrapyard owner used the radioactive material to decorate his house and placed some on the table. His daughter, who touched the radioactive powder while eating, was exposed to a lethal dose of radiation both though ingestion and by proximity to the source. She was only six years old. After she died she was buried in a coffin lined with lead, and some people from the neighbourhood tried to stop the burial, fearing that her remains will contaminate the cemetery.

The wife of the owner became sick soon after the exposure and her mother came to take care of her at the hospital. The mother then returned to her home, spreading contamination in her village. Later, two employees of the scrap yard worked on the parts of the teletherapy unit, trying to extract valuable metals such as lead from it. They exposed themselves to high levels of radiation during that time and became sick.

The wife started to suspect that the scrap metal might be to blame for numerous sicknesses among her relatives. She recovered the radioactive scrap materials from another junkyard, where they were sold to by then, and took them to a hospital for examination. Initially the symptoms that she and other people were experiencing were thought to be caused by a tropical illness, but the package she brought was examined carefully and the doctors understood that radiation was to blame.

The physicist, who studied the package at the request of the hospital staff, concluded that it was radioactive. The hospital notified the government and cleanup started shortly after. It took more than two weeks for the cleanup to start from the time the unit was stolen; by that time contamination spread through the area. The wife of the scrap yard owner is credited with saving people and preventing further spread of the radioactive contamination because she brought the radioactive material for examination.

This accident resulted in the deaths of the niece and the wife of the scrap yard owner, as well as of the two of his employees who were working on extracting lead from the unit. The owner received more radiation than the other four people, but did not die from the exposure, likely because his exposure was spread out across a longer period of time. Many more people were treated because of their radiation exposure, and cleanup operations to decontaminate the area that the government undertook included demolition of several houses among other efforts.

Kramatorsk Accident

A sealed vial with radioactive caesium-137, originally used as part of a measuring device, was lost in a quarry near Kramatorsk, Ukraine, in the end of 1970s. It emitted 200 roentgens per hour. After some efforts to locate it, the workers gave up. The vial was accidentally buried in the materials used to construct panels for apartment buildings. In 1980 an apartment building was built using the panel that contained the vial inside. Three of the residents in one of the apartments in that building passed away, and later, when the new family occupied this apartment, a child in that family also died. The child’s father managed to have the area investigated and the authorities discovered high radiation levels. Two adults and four children died from radiation exposure as a result of this accident.

Zaragoza Accident

In some cases deaths were caused by negligence of technicians or medical staff that serviced or operated the radiotherapy equipment. This was the case in 1990 in Zaragoza, Spain. The technician servicing a radiotherapy unit, used in the hospital to treat cancer patients, set the power more than five times higher than the norm. As a result eleven out of the twenty five cancer patients died from overexposure to radiation.

Samut Prakan Accident

In 2000 in Samut Prakan province in Thailand scrap metal collectors found and opened a container that held cobalt-60 that was emitting 15.7 terabecquerels of radiation. This was originally part of a radiotherapy unit in a Bangkok hospital. After upgrading to a new machine, the hospital sold the old one to the electric company that sold them the replacement. They did not complete the necessary transfer documentation, and the unit was not registered with the agency that monitors the location of all radioactive objects in Thailand. The electric company stored the unit together with two other unlicensed units at a property with limited security.

It is unclear how the unit was stolen, but the scrap metal collectors that had it originally claimed that they bought it. They cut it open with the help of scrapyard workers, and soon became ill, because they were exposed to high levels of ionizing radiation. They also contaminated the surrounding area and exposed people in the vicinity to radiation. At the hospital, several days after the first patients were admitted the doctors suspected radiation poisoning. This was 17 days after the first exposure. The hospital contacted the national agency responsible for radiation monitoring.