Notes of Chemistry BSC-MSC Chapter# 3 RADIOACTIVE TRACERS

chapter 3  Radioactive tracers

      How Can You Detect Radiation?

Radiation cannot be detected by human senses. A variety of handheld and laboratory instruments is available for detecting and measuring radiation. The most common handheld or portable instruments are:

1.1 Geiger Counter, with Geiger-Mueller (GM) Tube or Probe

Principle

A GM tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when radiation interacts with the wall or gas in the tube. These pulses are converted to a reading on the instrument meter. If the instrument has a speaker, the pulses also give an audible click. Common readout units are roentgens per hour (R/hr), milliroentgens per hour (mR/hr), rem per hour (rem/hr), millirem per hour (mrem/hr), and counts per minute (cpm). GM probes (e.g., "pancake" type) are most often used with handheld radiation survey instruments for contamination measurements. However, energy-compensated GM tubes may be employed for exposure measurements. Further, often the meters used with a GM probe will also accommodate other radiation-detection probes. For example, a zinc sulfide (ZnS) scintillator probe, which is sensitive to just alpha radiation, is often used for field measurements where alpha-emitting radioactive materials need to be measured.

Purpose:

On this page, I discuss some of the experiments I've performed with the GM-10 and GM-45 geiger counter radiation detectors Click on the link for more information about these nuclear radiation detectors, and some of the interesting results I've found.

Types of Nuclear Radiation:

There are three types of nuclear radiation which may be detected with a geiger counter:

• Alpha Particles: Helium nuclei, generally emitted from heavy elements such as uranium and thorium. Alpha particles only travel a few inches in the air, and can be stopped by a piece of paper. Special geiger tubes with a mica window are necessary to detect them, as other windows will stop alpha particles.
• Beta Rays: Electrons moving at extremely high (often relativistic) speeds. They are more penatrating than alpha particles. They can pass through light elements, such as paper and aluminum (but only small thicknesses).
• Gamma Rays: Electromagnetic waves, similar to light, but at a much higher energy. Much more penetrating than alpha or beta radiations. High energy gamma rays can pass through several inches of metal. Note that X-Rays and Gamma Rays are really the same thing, the term X-Ray is used when the radiation is produced by electrons striking a material, such as in an X-Ray tube. 

When a particle enters the tube, the gas is ionized, and can conduct current from the 500V DC supply. The 10M resistor limits the current to a safe level. A quenching gas (typically a halogen) stops the flow of current a few microseconds later. Thus, a pulse of current flows. This pulse is passed by the capacitor, which blocks the 500V DC. The output pulse can then go to an amplifier if necessary, and then some sort of counter.

Typically, the counts are summed over a one minute period, and therefore the common unit is Counts Per Minute (CPM). The number of CPM depends on the size and efficiency of the GM Tube. A larger tube naturally will detect more particles.

I've built a circuit which powers the GM tube strictly from the serial port handshake lines - no other power supply is necessary. The circuit then conditions the detected pulse and sends it back to the computer, where software can tally the number of pulses detected over a time period, typically CPM - Counts Per Minute.

This detector is now available as the GM-10, check out the details!

Sources of Radiation:

There are many Sources of Natural Radiation, radiation is all around us, naturally. Radon gas exists in most parts of the US, in varying levels depending upon where you live. Radon is produced from naturally occuring Uranium-238 in the soil. Radon is a problem in some areas today because homes are much more air-tight than they used to be. The radon gas enters the house through the basement. Thorium-232 also exists in the soil. Uranium and Thorium decay into numerous other radioactive isotopes before finally decaying into a stable element such as lead. And all this occurs naturally. In fact, the decay of uranium and thorium is the principle source of energy the heats the center of the Earth. Radiation existed long before Man, even though some would have you believe otherwise.

The US Geological Survey has a Radon . It contains a great deal of useful information, maps of radon levels in the US, and links to other sites.

Note that even though most of the heavy elements are alpha emitters, they can still be dangerous, if they get inside your body. Your skin stops alpha particles if the source is outside your body, but your internal organs and tissues have no such protection. Radioactive dust can be hazardous. Radioactive dust? Read on...

High Altitude Radiation from Airplane Flights

When you fly in an airplane, you're up above much of the Earth's atmosphere. It's the air that protects us from a lot of radiation due to cosmic rays coming from outer space.

Recently I took a transcontinental flight, and of course a GM-10 Radiation Detector came along for the ride!

The graph below shows the background radiation levels on the ground, as well as when we were at our crusing altitude (around 35,000 feet). Note the huge difference in radiation levels! The CPM (Counts Per Minute) went from about 12 on the ground to 360 in flight, or 30 times the level!

Radioactive Dust

Did you know that the dust that's in the air and settling all over your house (and computer monitor) is radioactive? It's true, it contains radioactive decay products from naturally occuring Uranium and Thorium.

As an experiment, I wiped some dust from the TV screen onto a tissue, and placed it in front of the radiation detector. The reading went from a background reading around 10 CPM to around 1300 CPM, or 130 times the reading!

This graph shows the radiation (in Counts Per Minute, CPM) over time, as the daughter products decay. In addition to plotting the raw data, I've also estimated the initial amounts of Radium B (Pb₂₁₄), Radium C (Bi₂₁₄) and Pb₂₁₂:

Radium B and Radium C are decay products of Radon. The Radon is produced by Uranium that naturally occurs in the soil, after a rather long decay process. Radon has a half life of just under 4 days. In addition to being produced by local sources, Radon (and it's daughter products) can be blown in by the winds from distant locations. So just because you don't have a radon problem in your basement doesn't mean that you won't find radioactive dust in your house!

The Radon decays into Polonium, which them decays into Radium B, an isotope of Lead. This decays with a half life of about 27 minutes in Radium C, an isotope Bismuth. This decays with a half life of about 20 minutes into another Polonium isotope, which quickly (164 microseconds) decays into an isotope of Lead. The Lead decays with a very long half life of 22 years, so we don't get much radiation from it, or any further products.

The two Radium isotopes both undergo beta decay, and it is their radiation we detect in this experiment. Notice that we initially start with much more Radium B than Radium C. The Radium B decays, producing more Radium C, initially at a rate faster than the Radium C is decaying. So The amount of Radium C starts to increase. Eventually, there is not enough Radium B decaying into Radium C, and the amount of Radium C starts to drop.

Here's the process:

1. Radon (Rn₂₂₂) does an alpha decay into Polonium (Po₂₁₈) with a half life of 3.824 days.
2. Polonium (Po₂₁₈) does an alpha decay into Lead (Pb₂₁₄) with a half life of 3.05 minutes.
3. Lead (Pb₂₁₄) does a beta decay into Bismuth (Bi₂₁₄) with a half life of 26.8 minutes.
4. Bismuth (Bi₂₁₄) does a beta decay into Polonium (Po₂₁₄) with a half life of 19.8 minutes
5. Polonium (Po₂₁₄) does an alpha decay into Lead (Pb₂₁₀) with a half life of 164 microseconds.
6. Lead (Pb₂₁₀) does a beta decay into Bismuth (Bi₂₁₀) with a half life of 22.3 years.
7. Bismuth (Bi₂₁₀) does a beta decay into Polonium (Po₂₁₀) with a half life of 5.01 days.
8. Polonium (Po₂₁₀) does an alpha decay into Lead (Pb₂₀₆) with a half life of 138.38 days.
9. Lead (Pb₂₀₆) is stable.

The black line at the bottom of the graph shows the amount of Lead-212 present. This is a daughter product of Thoron, an isotope of Radon which is produced by Thorium, rather than Uranium. Thoron has a very short half life, about 1 minute. Lead-212 has a much longer half life, almost 11 hours.

A curie is a measure of the activity of radiation. It is equal to 37 billion decays per second. The metric equivilent is the becquerel, which is one decay per second. So a picocurie is 0.037 decays per second. Currently the EPA considers 4 picocuries to be the safe limit. This would be 0.148 radon decays per second. Radon has a half life of about 3.83 days, this equates to around 100 thousand radon atoms per liter of air. Again, radon itself is not very dangerous, the risk comes from the radioactive daughter products (dust) that can get inside your body.

The difference between the GM-10 and GM-45 (and our other models) is the size of the radiation sensor. The sensor in the GM-45 geiger counter has 24 times the surface area of that in the GM-10 making it more sensitive, especially for alpha and beta radiation sources. That means that it can detect weaker levels of radiation.

The typical background levels detected with the GM-10 are around 10 CPM. This can be substantially higher in the basements of homes with high radon levels, for example. A GM-10 on an airplane flight recorded a level of over 400 CPM, due to the large amount of cosmic radiation always present at high altitudes.

The GM-10 and GM-45 geiger counters connect to almost any personal computer (either a PC running Windows or the Macintosh) through a simple serial interface. The powerful software included with them allows you to measure, record, and display radiation readings over any time period.

The GM-10 and GM-45 geiger counters are self powered off the computer's serial port, no batteries or external power supply is required. This makes them ideal for use in the field, or other locations. Just connect to a laptop, and you're ready to go!

A USB version of the GM-10 is available as well. Now you can use your GM-10 with a laptop or other computer with only a USB port available. A USB version of the GM-45, with the larger detector window, is also available.

The GM-10 and GM-45 geiger counter may also be used as radiation sensors and connected to other equipment, we'll be happy to provide connection and operation information to those interested, as well as OEMs. Users have sucessfully used the GM-10 with various microcontrollers. Interfacing details are available. Please feel free to contact us with any questions you may have.

The GM-10 and GM-45 detect alpha, beta, and gamma and x-ray radiation. Full specifications are available.

The GM-10 and GM-45 geiger counter are perfect for educational and industrial uses, the hobbyist or scientific experimenter, as well as anyone interested in determining the radiation levels around them!

3.3   MicroR Meter, with Sodium Iodide Detector — A solid crystal of sodium iodide creates a pulse of light when radiation interacts with it. This pulse of light is converted to an electrical signal by a photomultiplier tube (PMT), which gives a reading on the instrument meter. The pulse of light is proportional to the amount of light and the energy deposited in the crystal. These instruments most often have upper and lower energy discriminator circuits and, when used correctly as single-channel analyzers, can provide information on the gamma energy and identify the radioactive material. If the instrument has a speaker, the pulses also give an audible click, a useful feature when looking for a lost source. Common readout units are microroentgens per hour (μR/hr) and/or counts per minute (cpm). Sodium iodide detectors can be used with handheld instruments or large stationary radiation monitors. Special plastic or other inert crystal "scintillator" materials are also used in place of sodium iodide.

3.4 Portable Multichannel Analyzer — A sodium iodide crystal and PMT described above, coupled with a small multichannel analyzer (MCA) electronics package, are becoming much more affordable and common. When gamma-ray data libraries and automatic gamma-ray energy identification procedures are employed, these handheld instruments can automatically identify and display the type of radioactive materials present. When dealing with unknown sources of radiation, this is a very useful feature.

3.5Ionization (Ion) Chamber

This is an air-filled chamber with an electrically conductive inner wall and central anode and a relatively low applied voltage. When primary ion pairs are formed in the air volume, from x-ray or gamma radiation interactions in the chamber wall, the central anode collects the electrons and a small current is generated. This in turn is measured by an electrometer circuit and displayed digitally or on an analog meter. These instruments must be calibrated properly to a traceable radiation source and are designed to provide an accurate measure of absorbed dose to air which, through appropriate conversion factors, can be related to dose to tissue. In that most ion chambers are "open air," they must be corrected for change in temperature and pressure. Common readout units are milliroentgens and roentgen per hour (mR/hr or R/hr).

3.6 Neutron REM Meter, with Proportional Counter

A boron trifluoride or helium-3 proportional counter tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when a neutron radiation interacts with the gas in the tube. The absorption of a neutron in the nucleus of boron-10 or helium-3 causes the prompt emission of a helium-4 nucleus or proton respectively. These charged particles can then cause ionization in the gas, which is collected as an electrical pulse, similar to the GM tube. These neutron-measuring proportional counters require large amounts of hydrogenous material around them to slow the neutron to thermal energies. Other surrounding filters allow an appropriate number of neutrons to be detected and thus provide a flat-energy response with respect to dose equivalent. The design and characteristics of these devices are such that the amount of secondary charge collected is proportional to the degree of primary ions produced by the radiation. Thus, through the use of electronic discriminator circuits, the different types of radiation can be measured separately. For example, gamma radiation up to rather high levels is easily rejected in neutron counters.

3.8  Radon Detectors

A number of different techniques are used for radon measurements in home or occupational settings (e.g., uranium mines). These range from collection of radon decay products on an air filter and counting, exposing a charcoal canister for several days and performing gamma spectroscopy for absorbed decay products, exposure of an electret ion chamber and read-out, and long-term exposure of CR-39 plastic with subsequent chemical etching and alpha track counting. All these approaches have different advantages and disadvantages which should be evaluated prior to use.

The most common laboratory instruments are:

Liquid Scintillation Counters

A liquid scintillation counter (LSC) is a traditional laboratory instrument with two opposing PMTs that view a vial that contains a sample and liquid scintillator fluid, or cocktail. When the sample emits a radiation (often a low-energy beta) the cocktail itself, being the detector, causes a pulse of light. If both PMTs detect the light in coincidence, the count is tallied. With the use of shielding, cooling of PMTs, energy discrimination, and this coincidence counting approach, very low background counts can be achieved, and thus low minimum detectable activities (MDA). Most modern LSC units have multiple sample capability and automatic data acquisition, reduction, and storage.

Proportional Counter

A common laboratory instrument is the standard proportional counter with sample counting tray and chamber and argon/methane flow through counting gas. Most units employ a very thin (microgram/cm²) window, while some are windowless. Shielding and identical guard chambers are used to reduce background and, in conjunction with electronic discrimination, these instruments can distinguish between alpha and beta radiation and achieve low MDAs. Similar to the LSC units noted above, these proportional counters have multiple sample capability and automatic data acquisition, reduction, and storage. Such counters are often used to count smear/wipe or air filter samples. Additionally, large-area gas flow proportional counters with thin (milligram/cm²) mylar windows are used for counting the whole body and extremities of workers for external contamination when exiting a radiological control area.

Multichannel Analyzer System

A laboratory MCA with a sodium iodide crystal and PMT (described above), a solid-state germanium detector, or a silicon-type detector can provide a powerful and useful capability for counting liquid or solid matrix samples or other prepared extracted radioactive samples. Most systems are used for gamma counting, while some silicon detectors are used for alpha radiation. These MCA systems can also be utilized with well-shielded detectors to count internally deposited radioactive material in organs or tissue for bioassay measurements. In all cases, the MCA provides the capability to bin and tally counts by energy and thus identify the emitter. Again, most systems have automatic data acquisition, reduction, and storage capability.

3.9 Semiconductor Radiation Detectors

Building upon Centronic’s extensive experience in the design and manufacture of electro optic devices, our expertise has been extended to detection of other types of radiation. This has focused initially on detection of x-rays. X-ray detectors utilise silicon photodiode technology but with the addition of a crystal or membrane of scintillator material that is able to convert the higher energy x-rays into the visible spectrum, which can be detected by a photodiode.

There are a variety of scintillator materials available today such as Caesium Iodide (CsI), Cadmium Tungstate (CdWO4) crystals and Gadolinium Oxide membranes. Any of the available photodiodes (single element or linear / matrix arrays) in the 5T range (refer to electro optics product range) can be used as a basis for x-ray detectors and if required, custom diodes can be designed to suit a specific application.

Scintillators can be provided in any of the available materials with the thickness tailored to suit a particular energy range.

 3.9 Radioactive tracers

Radioactive tracers are substances that contain a radioactive atom to allow easier detection and measurement. (Radioactivity is the property possessed by some elements of spontaneously emitting energy in the form of particles or waves by disintegration of their atomic nuclei.) For example, it is possible to make a molecule of water in which one of the two hydrogen atoms is a radioactive tritium (hydrogen-3) atom. This molecule behaves in almost the same way as a normal molecule of water. The main difference between the tracer molecule containing tritium and the normal molecule is that the tracer molecule continually gives off radiation that can be detected with a Geiger counter or some other type of radiation detection instrument.

One application for the tracer molecule described above would be to monitor plant growth by watering plants with it. The plants would take up the water and use it in leaves, roots, stems, flowers, and other parts in the same way it does with normal water. In this case, however, it would be possible to find out how fast the water moves into any one part of the plant. One would simply pass a Geiger counter over the plant at regular intervals and see where the water has gone.

The use of radioactive tracers in cell research is an effective and safe means of monitoring molecular interactions. There is simply no other technique which allows the precision and specificity of radioactive tracers.

Radiation is to be taken seriously. At a minimum, its misuse can lead to increased environmental pollution, and at worst can lead to serious long term injury. It can be handled safely, however.

Radioactivity is caused by the spontaneous release of either particulate and/or electromagnetic energy from the nucleus of an atom. Atoms are composed of a positively charge nucleus, surrounded by the negatively charged electrons. In an uncharged atom, the number of orbital electrons equals the number of positively charged protons in the nucleus. In addition, the nucleus contains uncharged neutrons. A proton has a mass of 1.0076 amu (Atomic Mass Units), while a neutron has a mass of 1.0089 amu.

If the mass of a helium nucleus is examined, there is a difference between the expected mass based on its proton and neutron composition, and the actual measured mass. Helium contains two protons and two neutrons in its nucleus, and should have a corresponding mass of 4.0330 amu. It has an actual mass, however, of 5.0028 amu. The difference (0.0302 amu) is the equivalent energy of 28.2 Mev and is known as the binding energy . It would require 28.2 Mev to fuse two protons and two neutrons into a helium nucleus, and the fission of the helium nucleus would yield the same energy.

In addition, the electrons orbit the nucleus with precise energy levels. When the electrons are in their stable orbits, they are said to be in their ground state. If the electrons absorb energy (e.g. from photons), they jump to different, yet characteristic orbits and enter the excited state. The energy difference between a ground state and an excited state can take the form of an electromagnetic radiation.

The number of protons in the nucleus of an atom is called the atomic number, while the number of protons plus neutrons is the mass number. The mass number is approximately equal to the atomic weight. In the representation of an atom used in the periodic table of elements, the atomic number is a subscript written to the left of the letter(s) designating the element, while the mass number is written as a superscript to the left.

The chemical identity of an element is determined by the number of protons in the nucleus of the atom. The number of neutrons may vary, however. Elements sharing the same number of protons, but having different numbers of neutrons are known as isotopes. Hydrogen, for example, has one proton. All nuclei containing one proton are hydrogen nuclei. It may have one, two, or three neutrons. The isotopes of hydrogen would be written as H H H (in all further references, the atomic number subscript 1 is left off for clarity). H is the most stable form of hydrogen and is therefore the most abundant (99.985% of all forms). H is also a stable form of hydrogen, but less stable than H, and constitutes about 0.015% of the total hydrogen found. It is known as deuterium.

H is unstable and constitues a very small fraction of the amount of hydrogen available. Termed tritium , this element readily reorganizes its nucleus, and is said to decay. The emission of its sub-atomic particles and energy is therefore known as radioactive decay, or simply radioactivity. Deuterium is a stable, but heavy isotope of hydrogen, tritium is a radioactive isotope of hydrogen.

Note that each of the three will chemically react as hydrogen. This is important for tracer work in cell biology. The substitution of either deuterium or tritium for hydrogen in a molecule will not effect any chemical or physiological changes in the activity of the molecule. Tritium will, however, tag the molecule by making it radioactive.

Radiation emissions have several forms. When an atom reorganizes its sub-atomic structure to a more stable form, it may emit neutrons, protons, electrons, and/or electromagnetic waves (energy). An alpha particle is 2 protons plus 2 neutrons (essentially a helium nucleus). A beta particle is an electron. Gamma rays are electromagnetic energy waves similar to x-rays. The release of sub-atomic particles and energy, resulting in the change of one element to another is known as radioactivity.

Radioactive elements thus, by their very nature, self destruct. The loss of their sub-atomic particles is a spontaneous process, and once it has occurred, the element is no longer radioactive. With time a percentage of all radioactive elements in a solution will decay. Statistically, it is nearly impossible to predict which individual element will radioactively decay, but we can make a prediction about large numbers of the elements. That is, we can say that if we wait 14,000 years, half of the radioactivity in a sample of C (a radioactive isotope of C) will be lost (1/2 remains). We then say that C has a half- life of 14,000 years. After a second 14,000 years, half of the remaining half would have been lost, or 3/4 of the original amount. Based on this information, could you predict how long it would take for all radioactivity to have disappeared from the sample?

With a half-life of 14,000 years, radioactive carbon will be around for a very long time. This is why it is used for dating rocks and fossils. If one makes some assumptions about the activity of the carbon when the fossil was formed, and measures the current level, the age of the fossil may be determined.

The amount of radioactive material is measured by how many nuclei decay each second, and this value is known as the activity. It is measured in curies Each radioisotope has three important properties; the type of particles emitted, the particle energy, and the half-life. The energy and kind of decay particle will determine the penetration of the radiation, and therefore determine the degree of shielding necessary to protect the user. The half-life determines both the remaining activity after storage or use, and the time that the isotope must be stored before disposal.

In cell biology, only a few of the many radioactive elements are used routinely. The primary elements used are H (Tritium), C (Carbon-14), P 20(Phosphorus-32), I 20(Iodine-125) and I 20(Iodine-137).

 3.10 Applications

Industry and research. Radioactive tracers have applications in medicine, industry, agriculture, research, and many other fields of science and technology. For example, a number of different oil companies may take turns using the same pipeline to ship their products from the oil fields to their refineries. How do companies A, B, and C all know when their oil is passing through the pipeline? One way to solve that problem is to add a radioactive tracer to the oil. Each company would be assigned a different tracer. A technician at the receiving end of the pipeline can use a Geiger counter to make note of changes in radiation observed in the incoming oil. Such a change would indicate that oil for a different company was being received.

Another application of tracers might be in scientific research on plant nutrition. Suppose that a scientist wants to find out how plants use some nutrient such as phosphorus. The scientist could feed a group of plants fertilizer that contains radioactive phosphorus. As the plant grows, the location of the phosphorus could be detected by use of a Geiger counter. Another way to trace the movement of the phosphorus would be to place a piece of photographic film against the plant. Radiation from the phosphorus tracer would expose the film, in effect taking its own picture of its role in plant growth.

Medical applications. Some of the most interesting and valuable applications of radioactive tracers have been in the field of medicine. For example, when a person ingests (takes into the body) the element iodine, that element goes largely to the thyroid gland located at the base of the throat. There the iodine is used in the production of various hormones (chemical messengers) that control essential body functions such as the rate of metabolism (energy production and use).

Suppose that a physician suspects that a person's thyroid gland is not functioning properly. To investigate that possibility, the patient can be given a glass of water containing sodium iodide (similar to sodium chloride, or table salt). The iodine in the sodium iodide is radioactive. As the patient's body takes up the sodium iodide, the path of the compound through the body can be traced by means of a Geiger counter or some other detection device. The physician can determine whether the rate and location of uptake is normal or abnormal and, from that information, can diagnose any problems with the patient's thyroid gland.

3.11 Mossbauer Spectroscopy

Introduction

The Mossbauer effect is the recoil-free emission of gamma radiation from a solid radioactive material. Since the gamma emission is recoil-free, it can be resonantly absorbed by stationary atoms, i.e., also in a solid. The nuclear transitions are very sensitive to the local environment of the atom and Mossbauer spectroscopy is a sensitive probe of the different environments an atom occupies in a solid material.

Production of gamma rays for ⁵⁷Fe Mossbauer spectroscopy

Approximately 90% of the ⁵⁷Fe nuclear excited state decays through the intermediate level to produce 14.4 keV gamma radiation. These gamma photons can then be absorbed by ⁵⁷Fe in a sample.

Instrumentation

The gamma ray source is a radioactive element that is mechanically vibrated back and forth to Doppler shift the energy of the emitted gamma radiation. The schematic below shows a transmission Mossbauer experiment. As the energy of the gamma radiation is scanned by Doppler shifting, the detector records the frequencies of gamma radiation that are absorbed by the sample.

Schematic of an experimental set-up for transmission Mossbauer spectroscopy

  

Dosimetry in cases of radiation accidents

IONIZING radiation is an extremely potent agent in inducing chromosomal aberrations in all types of cells both in vivo and in vitro. The types of aberrations induced depend on the stage of the cell cycle irradiated. Irradiation of cells in G0 and G1 stages will yield chromosome type of aberrations (i.e. involving both the chromatids of a chromosome) whereas in G2 cells chromatid type of aberrations are induced. The frequencies of aberrations increase with the dose. The yield of chromosomal aberrations, especially exchange type of aberrations, such as dicentrics for a given radiation dose is similar both in vivo and in vitro. This characteristic feature can be used to estimate absorbed radiation dose in victims of radiation accidents. This applied aspect of radiation cytogenetics will be reviewed in this part.

Background to the formation of chromosomal aberrations by ionizing radiation

The target for induction of aberrations is DNA. Ionizing radiation induces several types of damage in DNA, such as, single strand breaks, double strand breaks (DSB), base damage and cross links. Among these, DSB have been shown tobe the critical lesion leading to radiation-induced chromosome aberrations1. Sparsely ionizing radiations, such as X-rays and gamma rays have low LET (linear energy transfer) values and induce ionizations as well as DNA damage and chromosomal aberrations randomly distributed among the cells. This has been shown to be the case following X- or gamma irradiation and induced aberrations fit a Poisson distribution. With densely ionizing high LET radiations, such as neutrons, alpha particles, the ionization tracks will be non-randomly distributed between cells. This characteristic results also in non-random distribution of chromosome aberrations among the cells and at any observed mean aberration frequency, there will be more cells with multiple aberrations and with zero aberrations than expected from a Poisson distribution. Two major types of aberrations are recognized, namely exchange aberrations (interaction between two chromosomes) and deletions. Chromosome exchanges include dicentrics and centric rings (asymmetrical exchanges), as well as translocations (symmetrical exchanges). The dose response curve for induction of exchange aberrations induced by low LET radiations is linear-quadratic, exemplifying contributions of both one and two track events and generally fits the equation:

          where Y is the yield of dicentrics, D is the dose, A is the background frequency, a is the linear coefficient and b is the dose-squared coefficient. With chronic exposure (low dose rate) to low LET radiation, the yield of dicentrics is linear. Following high LET radiation, the dose response for induction of dicentrics is predominantly linear.

Human lymphocytes

Human peripheral blood lymphocytes are predominantly in dormant G0 (pre-DNA synthetic) stage. The majority of the circulating lymphocytes are T cells (thymus derived), which can be stimulated to proliferate in vitro by a mitogen such as phytohaemagglutinin (PHA). This makes lymphocytes ideal target cells for looking for induced aberrations. There are several types of T lymphocytes with different average life spans.

Radiation accidents

In radiation accidents, it is essential to estimate the absorbed dose in the victims to help plan their therapy. In most accidents, no physical dosimetry is available. Even in situations in

which physical measurement is feasible, an independent estimation by biological methods can be very useful. As blood can be drawn easily in a non-invasive way, the circulating lymphocytes have been employed as target cells for estimating absorbed radiation dose in case of accidents. Since in vitro and in vivo irradiation of lymphocytes induces similar yields of chromosome damage per unit dose, the absorbed dose can be estimated by comparing the observed frequency of aberrations in the lymphocytes of accident victims to dose response curves generated from in vitro experiments. To assess the extent of damage in the lymphocytes, several end points such as dicentrics, translocations, micronuclei and fragments in prematurely condensed chromosomes (PCC) can be used. The choice of the end point depends on the urgency and accuracy needed in the radiation dose estimate. However, the most commonly employed end point is the frequency of dicentrics. In most of the radiation accidents, such as the ones which occurred in Mexico, Chernobyl, Goiania (Brazil), San Salvador, Istanbul (Turkey), etc. dose estimates were made

mainly using the data on the frequencies of dicentrics.

One of the authors (A.T.N) had the responsibility to set up the biological dosimetry laboratory for the Brazilian National Commission for Nuclear Energy in Rio de Janeiro in 1986 under the auspices of the International Atomic Energy Agency and therefore, was involved in estimating radiation doses of the victims of Goiania accident. In this accident, which occurred in 1987, a 1375 Ci cesium-137 teletherapy unit was broken by ignorant individuals looking for scrap metals. The radiactive cesium which was in the powder form was distributed to several individuals and the whole neighbourhood was contaminated. Of 11,200 individuals monitored, 249 were contaminated either internally or externally. Immediately after the detection of the accident, more than 110 samples of affected persons were analysed for the frequencies of dicentrics and rings in the lymphocytes4. A prerequisite for conducting radiation dosimetric studies is the availability of a reliable dose response curve generated following in vitro irradiation of lymphocytes with the same type of radiation involved in the accident, in the participating laboratory. Since no dose response curve for low dose rate 137Cs was available at that time, an existing calibration curve generated for 60Co g rays at a dose rate of 0.12 Gy/ minute was used for estimates. Of the 110 individuals analysed, 29 had an estimated dose of 0.5 Gy and above (0.5–7.0 Gy). When new calibration curves used similar exposure rates as in the accident, the estimated doses were reduced by about 20% (ref. 5). Though most of the individuals received an inhomogeneous dose, suggested by the presence of localized lesions in the skin, all cases except six showed a Poisson distribution, suggesting whole body irradiation.

In this accident, the exposure pattern was complicated (fractionated, protracted and internal) resulting in most cases in a distribution indistinguishable from Poisson. The dose estimates made by this method were very similar to those generated by physical and chemical dosimetry. The individual doses estimated fitted very well with the chronology of events in this accident.

Based on the persistence of lymphocytes carrying dicentric chromosomes following in vivo exposure, the mean life time of lymphocytes can been estimated. These estimates made by earlier studies vary from 530 to 1600 days6. Some classes of lymphocytes persist for more than 50 years as exemplified by the atom bomb victims from Hiroshima and Nagasaki who still carry circulating lymphocytes with dicentrics. The Goiania accident provided a good opportunity to estimate the average life span of lymphocytes in vivo. In the follow up studies of 10 exposed individuals, the disappearance of lymphocytes carrying dicentrics was monitored and a mean half-life of about 130 days with a range from 95 to 220 days was estimated7. The lifetime of lymphocytes may vary according to the health status of the exposed individuals. Most of the victims under study had a mild to severe leukopoenia, which is expected to accelerate the repopulation of lymphocyte pool. In the case of two victims, who had high137 Cs body burden, the aberration yield increased initially with time (up to about 110 days) in reasonable agreement with estimated doses due to internal contamination and began to fall following decorporation of radioactive Cs.

One can monitor radiation workers in nuclear industry who receive cumulative exposures. as well as workers accidentally exposed to radiation, using the frequencies of dicentrics in lymphocytes8. From the distribution of dicentrics among the lymphocytes one can make dose estimates both for whole body or partial body irradiation, as well as for exposure to high LET or low LET radiation or a mixture of these two types of radiation2

Retrospective dosimetry

In cases where biological dosimetry could not be performed immediately following a radiation accident, it can be done later. As pointed above, the lymphocytes have limited life span and dicentric carrying lymphocytes will be eliminated with time. Reciprocal translocations are relatively stable and in theory can be used successfully for retrospective dosimetry. Fluorescence in situ hybridization (FISH) technique using chromosome-specific DNA libraries allows one to detect translocations with relative ease9. Though claims have been made that from the frequencies of translocations detected by FISH, one can estimate radiation doses decades after exposure, based on the study of the atom bomb victims from Japan10, further studies have shown that there are several limitations in this approach. It is assumed that translocations and dicentrics  following irradiation are formed in 1 : 1 ratio and hence the frequencies of translocations retrospectively detected should reflect the initially induced dicentrics. In vitro studies employing FISH have shown that many more translocations than dicentrics are induced for a given dose. In a detailed 8-year follow up study on the victims of Goania radiation accident for which data on initial frequencies of dicentrics are available, the frequencies of translocations have been determined11. This study demonstrated that it isfeasible to use translocation frequencies to estimate radiation dose retrospectively only in cases where the exposure dose is about 1 Gy and below. At higher doses the frequencies of translocations decrease, similar to dicentrics, though at a slower rate. However, translocation frequencies can be helpful to estimate the cumulative doses in individuals working in the nuclear industry8. Populations living in the monazite area in Kerala who are chronically exposed to high LET radiation at a low dose rate will form an ideal cohort for such a study, using FISH technique to detect accumulated translocations. It is a pity that such a study has not been taken up, though the FISH technique has been in use for the past 20 years. Such a study could demonstrate if this population has developed a natural defence mechanism against radiation damage, as there appears to be no increase in the incidence of cancer or childhood mortality in this population.

Fingerprint of past radiation exposure

Several types of translocations are induced by ionizing radiation. Interchanges such as reciprocal, terminal, interstitial and complex translocations are formed between two or more chromosomes, whereas intrachanges formed between the arms of one chromosome (pericentric inversions) or between segments within one arm of a chromosome (paracentric inversions) are formed. It has been observed high LET radiations such as neutrons induce many more complexes and interstitial translocations in Chinese hamster splenocytes12, which has been confirmed in other studies using human cells. Intrachanges are relatively stable aberrations and remain in circulating lymphocytes over decades after exposure in plutonium workers from former USSR

and their high frequencies can be used as a signature for past exposure to high LET radiation13.

In conclusion, radiation cytogenetics has several practical applications, one of which is biological dosimetry, which has been exploited successfully in many radiation accidents all over the world. This approach is also useful in discriminating between false claims of exposure to radiation and real accidental exposure and is routinely used in national radiation protection agencies.

Assessing the safety of irradiated food material

During the period 1927–1945, the effects of ionizing radiation on living systems were largely interpreted on a biophysical basis. The target or treffer theory of radiation action14–16 was

based on the linear relationship observed between dose and effect. However, the discovery of ‘oxygen effect’ in radiobiology17–19 strongly suggested a chemical (physicochemical as well as biochemical) pathway in the actions of ionizing radiation in cells and organisms. The end of the Second World War in 1945 marked a spurt of intensive research on the biological effects of ionizing radiation both for peaceful applications and for setting the radiological protection standards.

It was then that Stone et al.20 reported the production of mutations in Staphylococcus aureus by irradiation of the substrate and the possible mutagenic activity of the radiolytic degradation products of the culture medium. Further studies brought out the possible role of peroxide in the adverse biological effects of irradiated broth. Wagner et al. showed that both the irradiated medium and peroxide enhanced the mutation rate in Neurospora. While these findings of basic value supported a chemical pathway in the development of radiobiological effects, the potential use of ionizing radiation in destroying the food spoilage microorganisms also came to light. One who realized the implications of these findings much in advance and also envisioned its usefulness as early as 1958 is M. S. Swaminathan. In his article, ‘Atoms and agriculture’ in The Statesman (6 October 1958), he wrote, ‘The use of ionizing radiation in food preservation is based on its ability to destroy the microorganisms and insects which cause food spoilage. The particular attraction that irradiation offers is that it does not lead to any appreciable rise in temperature of the foodstuff during treatment. The use of radiation thus opens up the possibility of a wider distribution of perishable foods in a fresh state’. At the same time, he emphasized the urgent need for rigorous genetic toxicological evaluation of the safety of the irradiated food materials for human consumption. Keeping in view that bulk of the above-mentioned reports on the indirect effects of ionizing radiation (irradiated medium) involved prokaryotic test systems, he organized research to investigate the possible clastogenic activity of irradiated substrate/media on the unirradiated eukaryotic systems such as plant meristematic cells, human peripheral blood lymphocytes, and sex-linked recessive lethal mutations as well as visible mutations in Drosophila melanogaster.

The first of a series of papers elucidating the clastogenic activity of indirect effects of radiation was published by Natarajan and Swaminathan23 who had treated Triticum monococcum, T. dicoccum and T. aestivum and Allium cepa bulbs with X-irradiated water and cultured the embryos of T. monococcum and T. dicoccum in irradiated White’s medium. The authors found increased frequency of chromosomal aberrations which they attributed to free radicals/biochemical products resulting from radiolysis of water and the medium. A little later, Natarajan reported that irradiated thymidine solution (0.7% irradiated with a total dose of

7.5 ´ 10⁶ rad (7.5 ´ 10⁴ Gy) of gamma rays) caused a high frequency of chromosome aberrations in the root meristems of barley.

During the early 1960s, Swaminathan and his co-workers published a series of significant papers25–29, which all clearly established the clastogenic activity of irradiated potato mash, fruit juices and culture media. In particular, the observation that the barley embryos cultured on X-irradiated potato substrate (20 kR/0.20 kGy) and stored at 2–3°C for about seven months showed 8-fold increase in the frequency of occurrence of cells with micronuclei was of particular concern. In a critical review, Kesavan and Swaminathan have discussed in detail all aspects of food irradiation from the point of view of their genetic safety for human consumption. The major points in conclusion are:

1. Potatoes irradiated with 6–10 kR (0.06 to 0.10 kGy) do not exhibit any detectable cytotoxic effect. The earlier experiments used much higher doses 20 kR, (0.2 kGy) and the test systems consisted of plant meristems, which do not have detoxification systems as of animals particularly

the mammals.

2. In the cytological studies, it was demonstrated27 that there occurs a decrease in pH and an increase in the peroxide content of the irradiated fruit juices. Bradley et al. reported that when the pH of the irradiated sucrose solution was between 4.6 and 7.2 no more chromosome aberration was found than by control solutions, but at pH values lower than 4.6, the percentages of abnormal anaphases in root tips of Vicia faba exposed to either control or irradiated sucrose (2%) solutions increased with decreasing pH. However, Kesavan, et al.32 found that the decreased pH of the irradiated sucrose solutions is not the major cause of the observed chromosomal aberrations and growth inhibition. They implicated radiolytic products with considerable life-span. In this regard, the detailed review on the radiation chemistry of carbohydrates is relevant. The radiolytic products of sucrose solutions have further received a great deal of attention. These studies have shown that the deleterious compounds produced in the irradiated sucrose solutions appear to be hydroxyalkyl peroxides (HAP) derived from the interaction of radiolytic hydrogen peroxide (H2O2) with carbonyl compounds produced in the radiolysis of sucrose. It has also been demonstrated that histidine- peroxide adduct strongly inhibits the growth of Salmonella typhimurium in glucose medium at pH 7.0. The catalase reactive hydroperoxides predominate in irradiated oxygen-free sucrose, while the catalaseresistant dialkyl peroxides predominate in irradiated oxygenated sucrose solutions.

What has emerged finally from the application of cytogenetic methods for the genotoxic evaluation of irradiated food materials by Swaminathan and co-workers is: (i) that milk, fruit juices and many liquids rich in sugars are just not suitable for preservation by irradiation. This is because the radiolytic products of water are the major cause of the degradation of the organic molecules (e.g. sucrose, glucose)

as follows:

H2O ® °OH, °H, eaq

–·

(1)

RH2 ® °RH + °H (Direct effect) (2)

(organic

molecule)

RH2 + °OH ® °RH + H2O (Indirect effect) (3)

When O2 is present, °RH is converted into °RHOO. When water is absent, the in situ °RH and °H can undergo harmless recombination:

°RH + °H ® RH2 (Restitution) (4)

The restitution of °RH to RH2 is indeed favoured by storage at high (~ 37°C) temperatures even if oxygen diffuses slowly into the irradiated seeds

(ii) The studies by Swaminathan and his co-workers with irradiated potato mash substrate essentially brought out that both the dose of irradiation, and the temperature as well as the duration of storage are important. It is noted that Chopra and Swaminathan26 had stored irradiated potatoes (20 kR/0.20 kGy) at 2–3°C for about seven months and these potatoes still exhibited clastogenic activity though much reduced. In this regard, the studies by Ehrenberg and his co-workers are of relevance. They showed that the main fraction of the free radicals induced in food by irradiation have a half-life around 48 h at 25°C. After six months of storage at +25°C, the radical content was zero or less than one percent of the value, whereas at –20°C, the decay was slower, leaving 10–20 percent of the initial concentration.

It should, therefore, be appreciated that in radiation preservation of potatoes, onions, food grains, spices, etc. the very purpose is to extend their shelf-life at temperatures between 15 ± 3°C (potatoes) and 30 ± 5°C (for grains, pulses, spices, etc.) for at least three months to one year or more. The permitted doses of irradiation are also relatively much lower than used in experimental studies.

The extensive studies by Kesavan and his students clearly brought out the influence of initial seed moisture content and post-irradiation hydration temperature on the kinetics of reactivity towards oxygen or decay of oxygensensitive sites in barley system. It was also evident that a slow process of ‘thermal annealment’ of the radiationinduced free radicals results in their harmless decay. In the case of irradiated (75 krad or 7.5 kGy) wheat, the storage at the summer room temperatures (35 ± 5°C) would result in harmless decay of the free radicals. While these irradiated grains are safe from stored grain pests, there will be no development of post-irradiation long-lived/stable radiolytic products. In fact, quite extensive cytogenetic investigations by the Bhabha Atomic Research Centre (BARC), Mumbai, have unequivocally established that even the

freshly irradiated (75 krad/7.5 kGy) wheat fed to mice and rats does not result in genotoxic effects and are absolutely safe for human consumption.

A two-member committee (P. C. Kesavan and P. V. Sukhatme) appointed by the Ministry of Health and Family Welfare, Government of India in 1975 carefully examined all the published as well as the unpublished data and concluded in their report that the irradiated (75krad) wheat

was safe for human consumption. The details are published elsewhere. The Food and Drug Administration (FDA), USA cited the report of the Kesavan–Sukhatme Committee and published their final rule in 1986 that the gamma-irradiated (75 krad) wheat is absolutely safe, and could be consumed by the humans with impunity.

Thus, starting with his vision in 1958, that ionizing radiation could be used to destroy the pests and organisms, which cause the spoilage of food, and through a series of cytogenetic studies with his students and co-workers, Swaminathan has played a very significant and responsible role in establishing the food irradiation programme in India on a safe and sound scientific premise. The Bhabha Atomic Research Centre (BARC, Mumbai) with its own in-depth research on genetic toxicological safety of irradiated food materials has come to the same conclusion that liquids rich in sugar (e.g. milk, fruit juices) are not suitable for radiation preservation, while on the other hand, food grains, spices, pulses, fish, poultry, meat and beef irradiated at the prescribed doses using gamma rays or electrons up to 10 MeV are absolutely safe. With these radiation sources, there is no induced radioactivity in the food materials. The sprout inhibition doses for potatoes and onions are really in the low dose (0.06 to 1.00 kGy) regime.

It is indeed very gratifying that the Ministry of Health and Family Welfare, Government of India has given clearance for trade in irradiated food materials ranging from potatoes, onions, turmeric, garlic and ginger for sprout-inhibition, mangoes for delaying the ripening; wheat, rice and pulses for disinfestation during storage and fish, poultry, meat, beef and spices for sterilization against pathogenic microorganisms. Food irradiation is now widely accepted in the

globalized international trade and it is emerging as an energy- effective, post-harvest processing and preservation technology.

 3.12 Major Radiation Exposure in Real Life Events

Hiroshima and Nagasaki

For more information on what happened at Hiroshima and Nagasaki, consult the nuclear past page and the nuclear warfare page.

Many people at Hiroshima and Nagasaki died not directly from the actual explosion, but from the radiation released as a result of the explosion. For example, a fourteen-year-old boy was admitted to a Hiroshima hospital two days after the explosion, suffering from a high fever and nausea. Nine days later his hair began to fall out. His supply of white blood cells dropped lower and lower. On the seventeenth day he began to bleed from his nose, and on the twenty-first day he died.

At Hiroshima and Nagasaki, the few surviving doctors observed symptoms of radiation sickness for the first time. In his book Nagasaki 1945, Dr. Tatsuichiro Akizuki wrote of the puzzling, unknown disease, of symptoms that "suddenly appeared in certain patients with no apparent injuries." Several days after the bombs exploded, doctors learned that they were treating the effects of radiation exposure. "We were now able to label our unknown adversary 'atomic disease' or 'radioactive contamination' among other names. But they were only labels: we knew nothing about its cause or cure... Within seven to ten days after the A-bomb explosion, people began to die in swift succession. They died of the burns that covered their bodies and of acute atomic disease. Innumerable people who had been burnt turned a mulberry color, like worms, and died... The disease," wrote Dr. Akizuki, "destroyed them little by little. As a doctor, I was forced to face the slow and certain deaths of my patients."

Doctors and nurses had no idea of how their own bodies had been affected by radioactivity. Dr. Akizuki wrote, "All of us suffered from diarrhea and a discharge of blood from the gums, but we kept this to ourselves. Each of us thought: tomorrow it might be me... We became stricken with fear of the future." Dr. Akizuki survived, as did several hundred thousand others in or near Hiroshima and Nagasaki. In fact, at least ten people who had fled from Hiroshima to Nagasaki survived both bombs.

The survivors have suffered physically from cataracts, leukemia and other cancers, malformed offspring, and premature aging, and also emotionally, from social discrimination. Within a few months of the nuclear explosions, leukemia began to appear among the survivors at an abnormally high rate. Some leukemia victims were fetuses within their mothers' wombs when exposed to radiation. One child who was born two days after the Hiroshima explosion eventually died of acute leukemia at the age of eighteen. The number of leukemia cases has declined with time, but the incidence of lung cancer, thyroid cancer, breast cancer, and cancers of other organs has increased among the survivors.

3.13 Three Mile Island

For more information on what happened at Three Mile Island, consult the nuclear past page.

On a Wednesday morning, maintenance workers cleaning sludge from a small pipe blocked the flow of water in the main feedwater system of a reactor at Three Mile Island near Harrisburg, Pennsylvania. The sift foreman heard "loud, thunderous noises, like a couple of freight trains," coming. Since the reactor was still producing heat, it heated the blocked cooling water around its core hot enough to create enough pressure to have popped a relief valve. Some 220 gallons of water per minute began flowing out of the reactor vessel. Within five minutes after the main feedwater system failed, the reactor, deprived of all normal and emergency sources of cooling water, and no longer able to use its enormous energy to generate electricity, gradually started to tear itself apart.

The loss of coolant at the reactor continued for some 16 hours. Abort a third of the core melted down. Radioactive water flowed through the stuck relief valve into an auxiliary building, where it pooled on the floor. Radioactive gas was released into the atmosphere. An estimated 140,000 people were evacuated from the area. It took a month to stabilize the malfunctioning unit and safely shut it down. The reactor was a total loss and the cleanup required years of repair and hundreds of millions of dollars.

No one was reported injured and the little radiation that leaked out was quickly dispersed. Although this accident did cost lots of money and time, no one was hurt.

3.14 Chernobyl

For more information on what happened at Chernobyl, consult the nuclear past page.

A far more serious accident occured at Chernobyl, in what was then still the Soviet Union. At the time of the accident, the Chernobyl nuclear power station consisted of four operating 1,000 megawatt power reactors. Without question, the accident at Chernobyl was the result of a fatal combination of ignorance and complacency. "As members of a select scientific panel convened immediately after the... accident," writes Nobel laureate Hans Bethe, "my colleagues and I established that the Chernobyl disaster tells us about the deficiencies of the Soviet political and administrative system rather than about problems with nuclear power."

Although the problem at Chernobyl was relatively complex, it can basically be summarized as a mismanaged electrical engineering experiment which resulted in the reactor exploding. The explosion was chemical, driven by gases and steam generated by the core runaway, not by nuclear reactions. Flames, sparks, and chunks of burning material were flying into the air above the unit. These were red-hot pieces of nuclear fuel and graphite. About 50 tons of nuclear fuel evaporated and were released by the explosion into the atmosphere. In addition, about 70 tons were ejected sideways from the periphery of the core. Some 50 tons of nuclear fuel and 800 tons of reactor graphite remained in the reactor vault, where it formed a pit reminiscent of a volcanic crater as the graphite still in the reactor had turned up completely in a few days after the explosion.

The resulting radioactive release was equivalent to ten Hiroshimas. In fact, since the Hiroshima bomb was air-burst--no part of the fireball touched the ground--the Chernobyl release polluted the countryside much more than ten Hiroshimas would have done. Many people died from the explosion and even more from the effects of the radiation later. Still today, people are dying from the radiation caused by the Chernobyl accident. The estimated total number of deaths will be 16,000.

3.15 Medical Treatment

For a more in-depth view of current medical technologies available to the treatment of radiation, go to the medical imaging page.

There is currently no effective medical treatment available for potentially fatal radiation doses. The case of the Japanese boy mentioned above illustrates an important fact about radiation sickness. The boy had probably received a dose of 450 rems or more, yet his symptoms were about the same as those of a person who received about 300 rems. Medical science has no way of telling the difference between people who have received fatal doses and will die despite all efforts and others who received less radiation and can be saved. Treatment for the ones that can be saved includes blood transfusions and bone-marrow transplants. Bone-marrow transplants rejuvenate the supply of white blood cells which was affected by the radiation.