Relative Biological Effectiveness - What Kind of Radiation is the Most Risky?

As you read this right now, you are being bombarded by radiation – cosmic rays flying in from the dark reaches of space, photons streaming out of the hot core of the earth, and miniscule particles issuing from your computer and other objects around you. But not all radiation is created equal. A new field of study has unearthed the fact that different kinds of radiation affect us differently.  

Although quite a mouthful, this study is called Relative Biological Effectiveness (abbreviated RBE). It seeks to put all radiation on an equal plane and find out what kind poses the highest risk to our organism. Higher values of RBE mean that certain types of radiation are more harmful. Ionizing radiation, which is made up of alpha, beta, and gamma rays, constitute electrically charged particles that interact with matter. These interactions can cause ionization, which refers to changes within the structure of an atom that can cause it to destabilize or behave differently.

Alpha particles are the largest kind of ionizing radiation, each consisting of two protons and two neutrons. Because they are highly charged and quite large, they are quickly stopped by as little as 4 cm (1.4 inches) of open air or a sheet of paper¹. Beta particles are much smaller, meaning they can penetrate further: through 9 meters (19 ft) of open air or 11 mm (.4 inches) of body tissue. Gamma rays are high-energy photons, meaning that they penetrate much further and interact differently with matter than alpha or beta particles. Thick, dense materials such as lead are necessary to block gamma rays. Another related form of radiation is neutron radiation, which is commonly referred to as indirectly ionizing radiation. Free neutrons, which are emitted from nuclear materials such as uranium and plutonium, have about a quarter of the mass of an alpha particle². Neutrons are not charged but readily cause ionization by knocking away electrons or slamming into atomic nuclei. The neutral charge of these fast-traveling neutrons also allows them to penetrate much further into most materials than other types of ionizing radiation, even through many feet of concrete.

Source: American Nuclear Society

 To test how damaging different types of radiation is on the human body, scientists expose living tissue to equal amounts of energy from each type. Surprisingly enough, scientists have found that beta and gamma radiation are nearly equally damaging, so the RBE value of beta and gamma radiation is 1. It gets more complicated from here, though. Alpha and neutron radiation have different RBE values depending on what kind of cells are exposed to them. The RBE for bacteria is 2-3, but can be 6-8 for more complex cells like those found in the human body. This means that a certain amount of alpha radiation is 6-8 times more damaging then the same amount of beta radiation. Neutrons are even more damaging with a RBE of 4-6 for bacteria and 12-16 for complex cells³.  

The high RBE values of alpha and neutron radiation should make us think twice about how we deal with these types. Because incoming alpha particles are stopped by a single layer of skin, they can’t do much damage unless they get in our bodies. That’s why breathing in alpha radiation (from radon or radioactive dust) or ingesting it (in contaminated food or water) is so dangerous. When alpha particles get to really important cells in our organs, the RBE can shoot up: scientists have measured RBE values of 1,000 for alpha radiation inside hamsters⁴. Neutron RBE values are more constant because neutrons penetrate just about everything, but they are also much harder to contain. That’s why materials that emit neutrons are highly controlled, very hard to transport, and large neutron sources are only found in research facilities and power plants.

The Rad-ID device by D-tect Systems has a special way of finding neutron radiation. A container filled with Helium-3, a rare and stable gas, is included with other radiation detectors inside the Rad-ID. As neutrons shoot through the detector, they collide with some of the He-3 atoms, causing them to change into charged particles. These particles are quickly identified and counted by a detector and a measurement of this radiation is sent to the user. Since neutron sources give off varying levels of gamma radiation, the Rad-ID can also identify these materials and let the user know what they are dealing with.

The Rad-ID can detect neutron radiation sources.

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D-tect Systems is a supplier of advanced radiation and chemical detection equipment sold around the world. www.dtectsystems.com.

  

Isotope Identification: How Does it Work?

Although we can’t feel, see, or hear it, we live in a sea of radiation. Cosmic rays from outer space continually bombard our planet, natural radioactive materials produce a steady stream of radiation, and many man-made materials constantly emit radiation in our homes and vehicles. Although much of this radiation is weak and harmless, there are some sources, as well as elevated radiation levels, that are best avoided. This is the reason that isotope identification is so important.

The term isotope is often misunderstood: not all isotopes emit radiation. Rather,  the term isotope has to do with the number of neutrons in the nucleus of an atom. Each element is defined by a set number of protons (or atomic number), by which it is listed on the Periodic Table. Iodine, for example, has 53 protons. But there are different versions or isotopes of iodine with varying numbers of neutrons, which are denoted in the isotope name. I-127 is the most common isotope of Iodine and is stable, meaning that its atoms don’t give off radiation or change to other isotopes.  But I-129 and I-131, which are produced in nuclear processes, are unstable and give off radiation that can be dangerous.

Isotope identification consists of finding out which radioactive isotopes are responsible for radiation. It is possible to figure this out by closely measuring the energy levels of radiation. Each radioactive particle or photon has a certain energy level, and each radioactive isotope emits a different set of energy levels. For example, here is a radiation measurement taken by a Rad-ID device:

Isotope Identification from the Rad-ID

As you can see, there are two energy peaks (at 25 keV and 88 keV) shown on the graph.  These two peaks appear because the isotope gives off a higher percentage of 25 keV and 88 keV radiation than radiation at other levels. By matching these measured energy peaks to pre-programmed energy peaks for known radioactive isotopes, isotope identifiers can narrow down and found out which isotopes are emitting the radiation.

There are a few difficulties that occur in the isotope identification process. First, no matter how well a radiation reading matches the pre-programmed energy levels, there is always at least a slight chance of an incorrect identification. This probability grows dramatically if the measured radiation levels can’t be matched very well. This is why it’s important to understand how good the match is and if there are lots of isotopes involved. Confidence bars can help out with this. Another problem is shielding. These two pictures show measurements an unshielded source on the left and a shielded source on the right:  

If you look closely at the two pictures you'll notice that a large energy peak on the left side of the first  (unshielded) reading is missing in the second. This  happens because shielding tends to block high-energy photons much better than the low-energy ones, which penetrate better. This can drastically change the shape of radiation measurements, and with enough shielding, completely block the radiation.

Measuring energy levels precisely enough to make an identification takes a very specialized instrument, different than a normal radiation detector. Identifiers usually have a LaBr3 or CZT detector, as does the Rad-ID.  These detectors are much more accurate than other types of radiation detectors. The Rad-ID also uses a large scintillation detector (NaI(Tl)), a Geiger-Mueller detector and an optional He3 tube to find a wide range of gamma and neutron radiation. In fact, the Rad-ID can identify 107 different radioactive isotopes, even if a measure sample is reading radiation from several isotopes.

With a good isotope identifier, you can be sure that you know what isotopes are in the environment and if you need to worry about them.

D-tect Systems is supplier of advanced radiation and chemical detection equipment sold around the world. www.dtectsystems.com.

Japan's Nuclear Crisis

Last week one of the largest earthquakes on record shook Northern Japan and triggered a devastating tsunami.  The damage is extensive: so few roads and runways are open that even humanitarian supplies have been seriously delayed.  But the greatest fear of the country isn’t the washed out roads or flattened villages.  It’s an invisible phenomenon with huge historical significance to the Japanese: the threat of nuclear radiation is rising like a ghost recalled from the past.

Nuclear power doesn’t make many headlines these days.  Until last Friday, nuclear plants have been considered in many parts of the world to be the best economical solution to growing power needs.  Japan has 55 nuclear reactors, providing approximately a quarter of the country’s power.  Advancing nuclear technologies have made power more efficient and seemed to invalidate radiation risks illustrated so horrifically by incidents at Chernobyl and Three-Mile Island.  But it is clear that innate nuclear power risks, however diminished, remain.

Japanese security personnel at the nuclear complex.  Photo credit cnn.com.

 The setting for the nuclear showdown in Japan is the Fukushima Dai-ichi nuclear complex.  Although this reactor, as well as two others, ceased operations as soon as the magnitude 9.0 earthquake hit, consequent damage to the structure has destabilized the normal cooling operation of the plant and lead to an atomic crisis.  Three hydrogen gas explosions have already rocked the plant, providing evidence that the fuel rods are at above normal temperatures.  Japanese authorities have already announced that steam from a nuclear cooling pond (used to cool the fuel rods) has been released into the atmosphere, meaning that some radiation has already leaked from the plant.  At this point, quantities of released radiation are unknown, but could rise dramatically if cooling of the reactor core is unsuccessful or a breach in the reactor wall occurs. 

But what is the real danger of nuclear radiation?  Unlike other forms of radioactive materials, such as those used commonly in hospitals and industry, nuclear materials are very heavily controlled throughout the world, and for good reason.  Nuclear materials, such as plutonium and uranium, give off neutrons at extremely high energy levels as their nuclei decay.  This kind of radiation easily passes through most matter, but can affect body tissues enough to cause serious medical problems. Short-term nuclear exposure can cause infections, hair loss, and fevers, and in extreme cases, organ failure and death.  Long-term exposure can cause cancer, tumors, and genetic damage.  Even shielded nuclear radiation sources can emit gamma radiation, which brings other health risks. 

The nature of this nuclear crisis, as well as many related scenarios, requires the use of a combination of radiation detectors all working together to minimize risks.  Our products are designed for just this.  In case of a radiation release, a perimeter could be set up using small, handheld MiniRad-D devices.  These pager-sized radiation detectors can sense radiation from tens of meters away.  The MiniRad-D could also be used to check personnel leaving the nuclear zone to determine if decontamination is needed.

The MiniRad-D is self-calibrating and uses a high-sensitivity scintillation detection system.

 The Rad-D unit is ideal for placement in unmanned locations to monitor ambient changes to radiation levels.  The system requires no maintenance and sophisticated neutron detectors can be configured into the system as well as gamma detectors. 

At the forefront of the crisis, specialized equipment designed for finding and identifying the type of radiation is needed.  The high-energy nature of nuclear radiation tends to saturate detectors and is hard to differentiate from gamma radiation.  Special neutron detector systems, such as the Helium-3 gas-filled tubes used by D-tect Systems in both the Rad-ID and Rad-D systems, sort out gamma rays and detect and identify neutron radiation.  The Rad-ID also contains a combination of detector types to find radiation over a wide range of energies, and from large amounts of radiation to sources emitting just above background radiation.

The Rad-ID can identify over 110 radioactive isotopes.

 We hope for the best in the Japan’s current nuclear crisis and that future wise decisions will mitigate the risks involved with nuclear power.