## Saturday, November 02, 2013

### Countering the China syndrome

A couple of years ago, in answer to fears expressed by friends and relatives, I posted Uranium 101, to explain what we should fear and what we should not fear, about Uranium and the possible release of radiation in Japan after the earthquake and tsunami.

OK, back from digression. Americans living at sea level are exposed to 3-4 mSv (millisieverts) of radiation yearly. Higher elevations take us above some of the protective atmosphere, so nationwide, the average is about 6 mSv. When you fly in a jet plane at 36,000 ft, you are above 3/4 of the atmosphere, so more space radiation reaches you. However, now you are shielded from most of the radiation coming upward from the ground. Still, you receive a lot more radiation during each hour of flight than you get from the X-ray backscatter scanner at the airport. Better news: many airports are replacing the X-ray scanners with T-ray scanners, which cannot cause harm.

Many people are afraid of all kinds of radiant energy. The electromagnetic spectrum is very, very wide, and only about half of it (in logarithmic terms) is harmful. Too see how wide, we need to talk units. Two sets of units are used, wavelength and energy per photon. Wavelength is used for the longer, less energetic photons, and energy is used to discuss the higher energy, very short-wave photons. The "center" of the spectrum is visible light, and in this region, both units are used depending on the reason for discussing them. So let's start with visible light, and the near-visible regions of near infrared and near ultraviolet.

The limits of normal vision are considered to be at wavelengths of 400 nm at the blue end, and 700 nm at the red end. Actual visual response at these limits is about 0.4% of the response to yellow light near 580 nm. The unit nm is the nanometer, or a billionth of a meter. To convert to energy, the proportionality constant is 1,293.7 eV-nm, and we divide this number by the wavelength to get energy. So blue-limit light's energy per photon is 1,239.7/400 = 3.1 eV, and at the red limit, it is 1,239.7/700 = 1.77 eV. The eV is the electron-volt, the energy an electron has when accelerated by a 1-volt potential. Old CRT type TV sets used an electron gun with about 30,000 volts, so the electrons were hitting the front plate with an energy of 30,000 eV, usually shown as 30 KeV, for Kilo-eV. We'll get back to this.

Near-infrared (NIR) is typically considered to range from 700 to 5,000 nm, AKA 0.7-5 µ (microns; the "consistent" term micrometer hasn't really caught on). Near-ultraviolet (NUV) ranges from 400 to 280 nm, the range of UV that can easily pass through the atmosphere. It has two components, UVA and UVB, with a cutover at 315 nm. UVC that you may have read about is the germicidal UV used in hospitals, ranging down to about 240 nm, where the atmosphere blocks it even over short distances, such as across a room. The UVA-UVB cutoff has an energy of 3.94 eV. Organic chemical bonds have energies in this range, which makes UVA and UVB risky for our skin. The thinner ozone layer is letting through a little UVC from the Sun, also, which is why sunblock is needed more now than in the past. The energy of UVC is at least 4.43 eV per photon, and it can damage exposed skin quickly.

Energetic as these wavelengths may be, they are not ionizing radiation. That takes a lot more energy per photon. Although the C-C bonds in organic materials can be broken by UVC, that produces free radicals, not ions. True ionization needs at least 10 eV/photon, or a wavelength shorter than 124 nm. This is the boundary between Far UV and "soft" X-rays. The X-rays used by your dentist are generated by an electron beam hitting a tungsten anode at 70,000 volts. They have a range of energies peaking at about 40 KeV. These are called medium X-rays, while hard X-rays are in the range above 100 KeV. Such X-rays are used by industrial inspection X-ray machines.

Remember the CRT TV? It produces small amounts of rather soft X-rays at about 20-25 KeV. That's why parents used to tell their children to stay farther from the TV set. Today's flat-screen TV's, whether Plasma, LCD or LED, do not produce any X-rays.

Now, how about your cell phone? Can it cause cancer? While you are talking (not listening), the phone is signaling to the tower using about 1 watt of microwave radio. While "microwave" may sound scary, that just means it is at a wavelength shorter than the UHF band used for analog TV signals (channels 13-65), in the pre-cable days. Microwaves have wavelengths over a wide range, from 1 m to 1 mm. Let's convert the shortest wavelength (most energetic) to nm and check the eV formula: 1mm = 1 million nm, so 1,239.7/1,000,000 = 0.0012 eV per photon. This is much less energetic than visible light. You'd suffer more damage by shining a flashlight into the side of your head! By the way, T-ray scanners use a wavelength near 1 mm.

Other kinds of radio use even longer wavelengths, and their tiny photon energies are why this unit is not used in this range. The longest common frequency to which we are exposed is the 60Hz signal from AC power transmission, which has a wavelength of 5,000 km. Thus the range of non-ionizing radiation is between 5,000 km and about 500 nm, a range of 1 quadrillion to 1. Now let's look at higher energies than X-ray.

There is a big gap in the spectrum of natural radiation to which we are exposed, because of blocking by the atmosphere, and because common radioactive elements produce energetic particles starting at a rather high point, though typically at a low level. Three elements form the foundation of natural radiation in Earth materials, mostly rocks: Uranium, Thorium and Potassium.

First and foremost, we cannot avoid Potassium (symbol K). The human body contains 0.25% K. Thus, I weigh 200 lbs (91 kg), so my body contains half a pound of potassium, or about 0.23 kg. The radioactive isotope of potassium is K-40, and makes up 0.0118% of the total, or 0.027 g; just over 1/40th of a gram. That isn't much, and K-40 is weakly radioactive, with a half life of 1.28 billion years. But that 40th of a gram is about 4x1020 atoms, of which nearly 7,000 decay each second. Now we get to energy. K-40 decays by the beta process, ejecting an electron or positron (it can do either, to become either Ca-40 or Ar-40, both of which are stable). The ejected particle has an energy of 1.3 or 1.5 MeV (million eV), some 1,000 times as energetic as a hard X-ray. It also produces energetic photons with an energy of 0.5 or 1.5 MeV. The beta particle stays in the body, while the gamma photon can exit, meaning that during a hug (or sleeping together) we receive some gamma radiation from our partner!

K-40 gamma radiation is near the low end of the range of natural radioactivity, but is not the lowest. Uranium and Thorium in the soil, particularly in areas with a lot of granite, produce energetic alpha particles, but these are absorbed by almost anything, such as a sheet of paper. A typical room with gypsum sheetrock contains a tenth of a gram of U and half as much Th, but their alpha radiation is stopped by the paper and the paint on the wall. Not so their gamma photons, which are actually in the hard X-ray region, at 48 KeV and 59 KeV respectively. Also, they have long half lives, 1.41 billion years for Th-232 and 4.51 billion years for U-238.

What about Radon? When U-238 emits an alpha particle, it becomes Th-234. That emits a beta particle (24 day half life) to become Pa-234 (Protactinium), and the chain continues. After a few more decays, Radium (Ra-226) is produced, which has a half life of 1,600 years. After an alpha emission, the next daughter element is Radon, specifically Rn-222, with a half life of 3.8 days. This is a gas, and is a concern everywhere there are soils derived from granitic rock (most of the U.S.). Radon is the primary cause of lung cancer in nonsmokers. Although it produces gamma radiation, with an energy of 500 KeV, it is the 5 MeV alpha particle, with nothing to stop it, that damages the lung. In sum, the ionizing range of radiations goes from about 10 eV to about 10 MeV, and there are cosmic rays with much higher energies. This is about a million-to-one range, a much smaller part of the entire spectrum than the non-ionizing range.

CT scans are another situation entirely. A chest-abdomen spiral scan totals 50-60 mSv, equal to 5-10 years of background radiation for most of us. This is the greatest radiation exposure most of us will ever have. When your doctor orders a CT scan, make sure it is for a good reason!

The authors of Radiation dwell much on what was learned from the casualties and survivors of the Hiroshima and Nagasaki nuclear explosions. This sets another baseline, the high end of survivable exposure. The LD50 (lethal dose for 50% of victims) for whole-body radiation dosage is 5 Sv or 5,000 mSv. That's only about 100 CT scans! However, that is a single-event dose; little is yet known about doses spread over years or decades. It seems the body can repair radiation damage up to a point.

The authors stress several times, when a doctor prescribes any kind of radiation beyond a simple X-ray, you need to ask what the exposure is, as compared to background (stated in mSv or in milli-Grays, which is equivalent). If the doctor can't state that, or won't, you need a different doctor! The doctor also should be able to explain the expected benefit and how it outweighs the risk of the radiation dose, whether from a CT scan, radiation applied to a cancer, or an ingested or injected radioisotope for some therapeutic or test purpose. This is a general rule, but is particularly important regarding such therapies and tests: if your doctor can't or won't explain, get a new doctor!

Finally, I have to tout nuclear power generation. The authors make it clear that we are much more likely to get radiation-induced cancer from coal burning power plants than from nuclear power plants. There are radioactive elements in coal, and they go right into the air when coal is burned. Also, the slag remaining from burnt coal contains heavy metals and other toxins, and these don't have a half-life like U or Ra, so they are toxic forever and ever. A nuclear power plant produces a few tons of high-level radioactive waste per year. A coal fired power plant produces hundreds or thousands of tons of toxic waste per year. The waste dump for a single coal plant could be used to store all the output from all U.S. nuclear plants for decades or centuries, and not run out of room. Just put the canisters on pallets on the ground, fence it off, and guard the stuff.

Really, we ought to be recycling spent uranium. 95% of its energy is still in there, just "poisoned" by the fission products. The problem isn't scientific; the science and technology are well known and safe. The problem is political. Even better, we ought to be using breeder reactors, to turn U-238, which won't "fizz", into Pu-239, which will. There's 140 times as much U-238 as there is U-235, the usual fuel. I suggest having the U.S. Navy oversee the design, construction and operation of nuclear power plants. They've been running aircraft carriers and submarines with nuclear power for more than half a century, and they seem to be able to do it without meltdowns or other accidents.

OK, I really like this book, and got quite inspired as you can see above. Without minimizing or distorting the risks, the authors make it clear that current fears about radiation are unfounded. Knowledge is the enemy of unwarranted fear. This book belongs on everybody's reading list.