This week’s question was asked by a friend and hypochondriac.

QUESTION: When I went to the dentist, why did they put a lead blanket over me before taking an X-ray?

ANSWER: Lead is very dense. With 82 protons and roughly 126 neutrons in its nucleus, lead is the densest non-radioactive element occurring in nature. Lead can block a lot of radiation, and it is also cheap. Lead is used in all kinds of shielding, especially around nuclear reactors for generating electrical power and nuclear-powered ships at sea.

Shielding is talked about in terms of “halving thickness.” How thick of a shield do you need to absorb half of the radiation coming from a given sample. For example, lead has a halving thickness of .4 inches, steel needs one inch, concrete needs 2.5 inches, water needs seven inches and open air requires 48 feet.

Of course, the thicknesses described above only blocks half of the radiation. You need to double up to stop the majority of radioactive particles coming through the shield. The fact remains that lead can be much thinner and more practical than steel or concrete.

Radiation comes in different forms: particles and waves. Particles can be protons, neutrons, and electrons. High-energy waves could be gamma rays or X-rays.

Alpha particles, consisting of two protons and two neutrons, are the most massive and easiest to stop. A sheet of paper will stop alpha radiation. Beta particles, which are electrons, can be halted by a thin sheet of aluminum. Betas can still be dangerous, as high-speed betas hitting metal can produce X-rays. About the only thing that can effectively stop gamma rays is lead.

Let’s explore this idea of rays and radiation. Radio waves, television waves, light waves, X-rays, and gamma waves (rays) are all the same kind of wave, namely an electromagnetic wave. As the length of these waves get shorter and their frequency (number of waves per second) increases, the more penetrating they become. Hence more dangerous to humans.

In everyday life, we’re pretty well shielded from radiation. The earth’s magnetic field deflects most all particles streaming at us from the sun. The earth’s atmosphere blocks nearly all dangerous rays, including ultraviolet rays. We can protect ourselves with a few smears of sunscreen for any rays that do get through.

Nuclear power plants are really big sources of radiation. Shielding consists of super-dense concrete, lead, and even water in a clever multi-layer shield.

Recall how alpha particles can be stopped by a sheet of paper or even an inch of air. You could hold an alpha source in your hand and you would be fairly safe. Alpha sources are used in smoke detectors. But if that alpha source was ingested, massive damage occurs.

That was brought to world-wide attention in November, 2006, when Russian agents put an alpha source (Polonium-210) in the tea of former KGB operative Alexander Litvinenko in a London hotel bar. Litvinenko had fled Russian and received political asylum in Britain. Word has it that there was a plan to kill Litvinenko to “set an example as a punishment for a traitor.” He had publicly criticized the KGB for illegal activities.

An alpha source in the stomach or any internal organ is like a bull in a china shop. Alpha particles kill living cells at short range. Doctors were baffled by Litvinenko’s illness. Tests by Geiger counters were negative. Geiger counters don’t pick up alpha particles, only beta and gamma radiation. The two assassins had chosen well.

Litvinenko lived for three weeks after that really bad cup of tea. Just hours before he died, doctors and scientists tested for alpha-emitters using special equipment. Litvinenko was buried in a lead-lined coffin in London’s Highgate Cemetery.

Lead-lined garments may protect sensitive body parts in the dentist office or hospital, but lead can be a poison. Lead causes serious neurological problems. Lead had been banned from paints and gasoline but is still used in batteries, solder, pipes, pottery, roofing materials and some cosmetics. Manufacturers have been ordered to get the lead out. Signs and symptoms of lead poisoning in children include developmental delay, learning difficulties, irritability, loss of appetite and weight loss.

Lead played a major role in the early history of Wisconsin. Miners, many from England, poured into southwestern Wisconsin in the early 1800s. By 1829, 4,000 miners worked in the Mineral Point, Shullsburg and New Diggings area, producing 13 million pounds per year. Some newly arrived miners pitched tents but some simply burrowed holes into the hillsides for shelters, earning the miners the nickname “badgers.”

Larry Scheckel is a retired Tomah High School physics teacher.


Steve Rundio is editor of the Tomah Journal. Contact him at 608-374-7785.

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