Cerecitas de la Ciencia

Gamma Radiation

Gamma radiation is one of the three types of natural radioactivity. Gamma rays are electromagnetic radiation, like X-rays. The other two types of natural radioactivity are alpha and beta radiation, which are in the form of particles. Gamma rays are the most energetic form of electromagnetic radiation, with a very short wavelength of less than one-tenth of a nanometer.

Gamma radiation is the product of radioactive atoms. Depending upon the ratio of neutrons to protons within its nucleus, an isotope of a particular element may be stable or unstable. When the binding energy is not strong enough to hold the nucleus of an atom together, the atom is said to be unstable. Atoms with unstable nuclei are constantly changing as a result of the imbalance of energy within the nucleus. Over time, the nuclei of unstable isotopes spontaneously disintegrate, or transform, in a process known as radioactive decay. Various types of penetrating radiation may be emitted from the nucleus and/or its surrounding electrons. Nuclides which undergo radioactive decay are called radionuclides. Any material which contains measurable amounts of one or more radionuclides is a radioactive material.

Types Radiation Produced by Radioactive Decay 

When an atom undergoes radioactive decay, it emits one or more forms of radiation with sufficient energy to ionize the atoms with which it interacts. Ionizing radiation can consist of high speed subatomic particles ejected from the nucleus or electromagnetic radiation (gamma-rays) emitted by either the nucleus or orbital electrons.

Alpha Particles

Certain radionuclides of high atomic mass (Ra226, U238, Pu239) decay by the emission of alpha particles. These alpha particles are tightly bound units of two neutrons and two protons each (He4 nucleus) and have a positive charge. Emission of an alpha particle from the nucleus results in a decrease of two units of atomic number (Z) and four units of mass number (A). Alpha particles are emitted with discrete energies characteristic of the particular transformation from which they originate. All alpha particles from a particular radionuclide transformation will have identical energies.

Beta Particles

A nucleus with an unstable ratio of neutrons to protons may decay through the emission of a high speed electron called a beta particle. This results in a net change of one unit of atomic number (Z). Beta particles have a negative charge and the beta particles emitted by a specific radionuclide will range in energy from near zero up to a maximum value, which is characteristic of the particular transformation.

Gamma-rays

A nucleus which is in an excited state may emit one or more photons (packets of electromagnetic radiation) of discrete energies. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead has the effect of moving the nucleus from a higher to a lower energy state (unstable to stable). Gamma ray emission frequently follows beta decay, alpha decay, and other nuclear decay processes.

Gamma-rays were first observed in 1900 by French chemist Paul Villard when he was investigating radiation from radium, according to NASA. A few years later, New Zealand-born chemist and physicist Ernest Rutherford proposed the name "gamma-rays," following the order of alpha-rays and beta-rays — names given to other particles observed from nuclear radiation — and the name stuck.

Gamma-ray sources and effects

Gamma-rays are produced primarily by four different nuclear reactions: fusion, fission, alpha decay and gamma decay. Nuclear fusion is the reaction that powers the sun and stars. It occurs in a multistep process in which four protons, or hydrogen nuclei, are forced under extreme temperature and pressure to fuse into a helium nucleus, which comprises two protons and two neutrons. The resulting helium nucleus is about 0.7 percent less massive than the four protons that went into the reaction. That mass difference is converted into energy according to Einstein's famous equation E = mc2, with about two-thirds of that energy emitted as gamma-rays. (The rest is in the form of neutrinos, which are extremely weakly interacting particles with nearly zero mass.) In the later stages of a star's lifetime, when it runs out of hydrogen fuel, it can form increasingly more massive elements through fusion up to and including iron, but these reactions produce a decreasing amount of energy at each stage. 

Another familiar source of gamma-rays is nuclear fission. Lawrence Berkeley National Laboratory defines nuclear fission as "the splitting of a heavy nucleus into two roughly equal parts (which are nuclei of lighter elements), accompanied by the release of a relatively large amount of energy in the form of kinetic energy of the two parts and in the form of emission of neutrons and gamma-rays." In this process, heavy nuclei, such as uranium and plutonium, are broken into smaller elements, such as xenon and strontium, in collisions with other particles. The resulting particles from these collisions can then impact other heavy nuclei, setting up a nuclear chain reaction. Energy is released because the combined mass of the resulting particles is less than the mass of the original heavy nucleus. That mass difference is converted to energy according to E = mc2 in the form of kinetic energy of the smaller nuclei, neutrinos and gamma-rays. 

Other sources of gamma-rays are alpha decay and gamma decay. Alpha decay occurs when a heavy nucleus gives off a helium-4 nucleus, reducing its atomic number by 2 and its atomic weight by 4. This process can leave the nucleus with excess energy, which is emitted in the form of a gamma-ray. Gamma decay occurs when there is too much energy in the nucleus of an atom, causing it to emit a gamma-ray without changing its charge or mass composition. 

Gamma-ray therapy

Gamma-rays are sometimes used to treat cancerous tumors in the body by damaging the DNA of the tumor cells. However, great care must be taken because gamma-rays can also damage the DNA of surrounding healthy tissue cells. One way to maximize the dosage to cancer cells while minimizing the exposure to healthy tissues is to direct multiple gamma-ray beams from a linear accelerator, or linac, onto the target region from many different directions. This is the operating principle of the CyberKnife and the Gamma Knife. According to the Mayo Clinic website, "In Gamma Knife radiosurgery, specialized equipment focuses close to 200 tiny beams of radiation on a tumor or other target. Although each beam has very little effect on the brain tissue it passes through, a strong dose of radiation is delivered to the site where all the beams meet."

Gamma-ray astronomy

One of the more interesting sources of gamma-rays is gamma-ray bursts (GRBs). These are extremely high-energy events that last only a few milliseconds to several minutes. They were first observed in the 1960s, and they are now observed somewhere in the sky about once a day. 

"For a long time, it was believed that GRBs must come from within our own galaxy," the University of California, Berkeley website states. "It seemed impossible that they could be much more distant — for a gamma-ray burst to have come from a distant galaxy, it would have to be incredibly powerful to explain its observed brightness." We now know that most GRBs actually do come from galaxies that are more than 100 million to billions of light-years away. 

According to Robert Patterson, a professor of astronomy at Missouri State University, GRBs were once thought to come from the last stages of evaporating mini black holes. They are now believed to originate in collisions of compact objects such as neutron stars. Other theories attribute these events to the collapse of supermassive stars to form black holes. In either case, GRBs can produce enough energy that, for a few seconds, they can outshine an entire galaxy. Because the Earth's atmosphere blocks most gamma-rays, observations are typically conducted using high-altitude balloons and orbiting telescopes.

The Atomic Bombings of Hiroshima and Nagasaki

Radiation Injuries

The radiations from the nuclear explosions which caused injuries to persons were primarily those experienced within the first second after the explosion; a few may have occurred later, but all occurred in the first minute. The other two general types of radiation, viz., radiation from scattered fission products and induced radioactivity from objects near the center of explosion, were definitely proved not to have caused any casualties.

The proper designation of radiation injuries is somewhat difficult. Probably the two most direct designations are radiation injury and gamma ray injury. The former term is not entirely suitable in that it does not define the type of radiation as ionizing and allows possible confusion with other types of radiation (e.g., infra-red). The objection to the latter term is that it limits the ionizing radiation to gamma rays, which were undoubtedly the most important; but the possible contribution of neutron and even beta rays to the biological effects cannot be entirely ignored. Radiation injury has the advantage of custom, since it is generally understood in medicine to refer to X-ray effect as distinguished from the effects of actinic radiation. Accordingly, radiation injury is used in this report to mean injury due only to ionizing radiation.

According to Japanese observations, the early symptons in patients suffering from radiation injury closely resembled the symptons observed in patients receiving intensive roentgen therapy, as well as those observed in experimental animals receiving large doses of X-rays. The important symptoms reported by the Japanese and observed by American authorities were epilation (lose of hair), petechiae (bleeding into the skin), and other hemorrhagic manifestations, oropharyngeal lesions (inflammation of the mouth and throat), vomiting, diarrhea, and fever.

Epilation was one of the most spectacular and obvious findings. The appearance of the epilated patient was typical. The crown was involved more than the sides, and in many instances the resemblance to a monk's tonsure was striking. In extreme cases the hair was totally lost. In some cases, re-growth of hair had begun by the time patients were seen 50 days after the bombing. Curiously, epilation of hair other than that of the scalp was extremely unusual.

Petechiae and other hemorrhagic manifestations were striking findings. Bleeding began usually from the gums and in the more seriously affected was soon evident from every possible source. Petechiae appeared on the limbs and on pressure points. Large ecchymoses (hemorrhages under the skin) developed about needle punctures, and wounds partially healed broke down and bled freely. Retinal hemorrhages occurred in many of the patients. The bleeding time and the coagulation time were prolonged. The platelets (coagulation of the blood) were characteristically reduced in numbers.

Nausea and vomiting appearing within a few hours after the explosion was reported frequently by the Japanese. This usually had subsided by the following morning, although occasionally it continued for two or three days. Vomiting was not infrequently reported and observed during the course of the later symptoms, although at these times it generally appeared to be related to other manifestation of systemic reactions associated with infection.

Diarrhea of varying degrees of severity was reported and observed. In the more severe cases, it was frequently bloody. For reasons which are not yet clear, the diarrhea in some cases was very persistent.

Lesions of the gums, and the oral mucous membrane, and the throat were observed. The affected areas became deep red, then violacious in color; and in many instances ulcerations and necrosis (breakdown of tissue) followed. Blood counts done and recorded by the Japanese, as well as counts done by the Manhattan Engineer District Group, on such patients regularly showed leucopenia (low-white blood cell count). In extreme cases the white blood cell count was below 1,000 (normal count is around 7,000). In association with the leucopenia and the oropharyngeal lesions, a variety of other infective processes were seen. Wounds and burns which were healing adequately suppurated and serious necrosis occurred. At the same time, similar ulcerations were observed in the larynx, bowels, and in females, the gentalia. Fever usually accompanied these lesions.

Eye injuries produced by the atomic bombings in both cities were the subject of special investigations. The usual types of mechanical injuries were seen. In addition, lesions consisting of retinal hemorrhage and exudation were observed and 75% of the patients showing them had other signs of radiation injury.

The progress of radiation disease of various degrees of severity is shown in the following table:

Summary of Radiation Injury

Clinical Symptoms and Findings

Day 

after 

Explosion Most Severe Moderately Severe Mild

1. 1. Nausea and vomiting after 1-2 hours. 1. Nausea and vomiting after 1-2 hours. 

------

2. ------ ------ ------

3. NO DEFINITE SYMPTOMS

4. ------ ------ ------

5. 2. Diarrhea ------ ------

6. 3. Vomiting NO DEFINITE SYMPTOMS ------

7. 4. Inflammation of the mouth and throat ------ ------

8. 5. Fever ------ ------

9. 6. Rapid emaciation ------ ------

10. Death (Mortality probably 100%) ------ NO DEFINITE SYMPTOMS

11. ------ 2. Beginning epilation. ------

12. ------ ------ ------

13. ------ ------ ------

14. ------ ------ ------

15. ------ ------ ------

16. ------ ------ ------

17. ------ ------ ------

18. ------ 3. Loss of appetite and general malaise. ------

19. ------ ------ 1. Epilation

20. ------ 4. Fever. 2. Loss of appetite

21. ------ 5. Severe inflammation and malaise of the mouth and throat ------

22. ------ ------ 3. Sore throat.

23. ------ ------ 4. Pallor.

24. 

------ ------ 5. Petechiae

25. ------ ------ 6. Diarrhea

26. ------ ------ 7. Moderate emaciation.

27. ------ 6. Pallor. ------

28. ------ 7. Petechiae, diarrhea and nose bleeds (Recovery unless complicated by previous poor health or super-imposed injuries or infection). ------

29. ------ ------ ------

30. ------ ------ ------

31. ------ 8. Rapid emaciation Death (Mortality probably 50%) ------

It was concluded that persons exposed to the bombs at the time of detonation did show effects from ionizing radiation and that some of these patients, otherwise uninjured, died. Deaths from radiation began about a week after exposure and reached a peak in 3 to 4 weeks. They practically ceased to occur after 7 to 8 weeks.

Treatment of the burns and other physical injuries was carried out by the Japanese by orthodox methods. Treatment of radiation effects by them included general supportative measures such as rest and high vitamin and caloric diets. Liver and calcium preparations were administered by injection and blood transfusions were used to combat hemorrhage. Special vitamin preparations and other special drugs used in the treatment of similar medical conditions were used by American Army Medical Corps officers after their arrival. Although the general measures instituted were of some benefit no definite effect of any of the specific measures on the course of the disease could be demonstrated. The use of sulfonamide drugs by the Japanese and particularly of penicillin by the American physicians after their arrival undoubtedly helped control the infections and they appear to be the single important type of treatment which may have effectively altered the earlier course of these patients.

One of the most important tasks assigned to the mission which investigated the effects of the bombing was that of determining if the radiation effects were all due to the instantaneous discharges at the time of the explosion, or if people were being harmed in addition from persistent radioactivity. This question was investigated from two points of view. Direct measurements of persistent radioactivity were made at the time of the investigation. From these measurements, calculations were made of the graded radiation dosages, i.e., the total amount of radiation which could have been absorbed by any person. These calculations showed that the highest dosage which would have been received from persistent radioactivity at Hiroshima was between 6 and 25 roentgens of gamma radiation; the highest in the Nagasaki Area was between 30 and 110 roentgens of gamma radiation. The latter figure does not refer to the city itself, but to a localized area in the Nishiyama District. In interpreting these findings it must be understood that to get these dosages, one would have had to remain at the point of highest radioactivity for 6 weeks continuously, from the first hour after the bombing. It is apparent therefore that insofar as could be determined at Hiroshima and Nagasaki, the residual radiation alone could not have been detrimental to the health of persons entering and living in the bombed areas after the explosion.

The second approach to this question was to determine if any persons not in the city at the time of the explosion, but coming in immediately afterwards exhibited any symptoms or findings which might have been due to persistence induced radioactivity. By the time of the arrival of the Manhattan Engineer District group, several Japanese studies had been done on such persons. None of the persons examined in any of these studies showed any symptoms which could be attributed to radiation, and their actual blood cell counts were consistently within the normal range. Throughout the period of the Manhattan Engineer District investigation, Japanese doctors and patients were repeatedly requested to bring to them any patients who they thought might be examples of persons harmed from persistent radioactivity. No such subjects were found.

It was concluded therefore as a result of these findings and lack of findings, that although a measurable quantity of induced radioactivity was found, it had not been sufficient to cause any harm to persons living in the two cities after the bombings.