Introduction

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The 2 Atomic Bombs: Little Boy and Fat Man were dropped on Hiroshima and Nagasaki respectively in 1942 in what effectively ended WWII. While you are likely familiar with the controversy surrounding nuclear weapons today, the science behind the atomic bomb is critical for you to fully understand its various impacts.

This blog covers 4 important aspects of the atomic bomb: Chemistry, Physics, Biology and the Aftermath. The first 3 aspects are structured according to levels to present information concisely and clearly in bite-sized chunks. 

Navigation of the website can be done through the labels pane on the left side. It is recommended that you read level 1 of chemistry and physics first before proceeding to read level 1 of biology. Do the same for the other levels. After you have understood the science, watch the video on the aftermath of the atomic bomb blasts in 1942 which provides a visual element on the horror that victims faced. The glossary section will provide you with quick reference for terms that you may not fully grasp at first. The Q&A section at the end of some blog posts help to clarify burning questions and correct common misconceptions. If you have other questions, put it as a comment. One way to effectively absorb the content would be to use free mind-mapping software such as imindmap or free mind.

Glossary

Here is the link to the glossary document for easy reading of the various sections:

http://tinyurl.com/atomicbombglossary

The Physics of the Atomic Bomb (Level 4)

Level 4: Nuclear Fusion

The binding energy of a nucleus is the energy needed to separate a nucleus into its respective individual nucleons and is proportional to the stability of the nucleus. For atoms lighter than Nickel-62, binding energy and also the stability of the nucleus increases as atomic mass increases. Energy given off by hydrogen bombs arise from nuclear fusion in which lighter isotopes such as deuterium and tritium ‘combine’ into a much stable element such as helium. The energy released during nuclear fusion is the difference between the binding energies of the isotope used in the reaction and the fission products. The energy released can be calculated using Einstein’s mass-energy equivalence E = Δmc², where Δm is the difference in mass between the start and end nucleus and c is the speed of light (3 x 10⁸ m/s).

However, nuclei are positively charged due to the presence of positively charged protons. Extremely high temperatures are required for the positively charged nuclei to overcome their mutual electrostatic repulsion and gain enough kinetic energy to fuse.

This diagram compares examples of nuclear fission and nuclear fusion.

The Physics of the Atomic Bomb (Level 3)

Level 3: Nuclear Fission

The binding energy of a nucleus is the energy needed to separate a nucleus into its respective individual nucleons and is proportional to the stability of the nucleus. For atoms heavier than Nickel-62, binding energy and also the stability of the nucleus decreases as atomic mass increases. Energy given off by atomic bombs arise from nuclear fission in which heavier isotopes such as uranium or plutonium split into more tightly bound stable elements. In the example of Atomic bombs using Uranium-235 as the fissile material, free neutrons may hit the uranium, resulting in the formation of the highly unstable Uranium-236. It then splits into the highly radioactive but more stable fission products of Barium-144, Krypton-89 and 3 neutrons which collide with more uranium to cause a chain reaction. The energy released during nuclear fission is the difference between the binding energies of the isotope used in the reaction and the fission products.

Chain reaction refers to self-sustaining nuclear fission. As seen from the example of atomic bombs using Uranium-235, neutrons produce in a fission reaction go on to trigger more fission reactions. The critical mass is the amount of fissile material needed to sustain an exponentially growing chain reaction. What this means is illustrated in the diagram below in which the number of nuclei undergoing fission reactions keeps increasing. However, do note that not all neutrons produced in fission reactions will trigger additional fission reactions.

© BBC Bitesize

The Physics of the Atomic Bomb (Level 2)

Level 2: Nuclear Emissions and their Penetrating Powers

As mentioned in Level 1 of Physics, radioactive decay is the process in which unstable nuclei try to become more stable by ejecting particles or energy. There are 3 ways in which radioactive decay can occur. 

Radiation can be absorbed by objects in its path and becomes less intense the further they are from the radioactive material. This is because the nuclear emissions become more spread out. The thicker the substance, the greater the amount of radiation absorbed.

1. Alpha Particles

When an alpha particle is ejected from an unstable nucleus, the nucleus loses 2 protons and 2 neutrons. Alpha particles have the largest mass among the 3 types of nuclear emissions. They collide with O₂ and N₂ molecules in the air, losing some of their energy in ionizing the air molecules until eventually they give up all of their energy and are absorbed. [Ionization is the process in which an atom or molecule loses or gains electrons. Since electrons have a relative charge of -1, the atom or molecule can become Ions which are positively or negatively charged particles.] Hence, alpha particles have poor penetrating power, and can be absorbed by a thin sheet of paper or a few centimeters of air. They are the slowest type of nuclear emission and have a speed 1/10 of the speed of light (3x10⁸ m/s)

2. Beta Particles

In beta decay, the unstable nucleus converts a neutron into a proton and an electron. The beta particle is an electron that is ejected from the nucleus at a speed 9/10 of the speed of light. Beta particles carry less charge (-1) compared to alpha particles (+2), and thus react less with the atoms and molecules of objects in its path. Beta particles can be stopped by a thin sheet of aluminium.

3. Gamma Rays

After emitting an alpha or beta particle, the remaining nuclei may be at a higher energy level. When it returns to its 'normal' state, gamma radiation is emitted in the form of gamma rays. Gamma rays travel at the speed of light, are the most penetrating and can only be stopped by many centimetres of lead, or many metres of concrete.

Content adapted from: 

http://www.bbc.co.uk/schools/gcsebitesize/science/21c_pre_2011/energy/nuclearradiationrev2.shtml

Q: Suppose you are given 3 radioactive cookies: The 1st is an alpha emitter, the 2nd is a beta emitter and the 3rd is a gamma emitter. You must eat one, hold one in your hand, and put the third in your pocket. What can you do to minimize your exposure to radiation?

Ans: Eat the gamma emitter, hold the alpha emitter in your hand, and put the beta emitter in your pocket. 

Q: What is the significance of knowing about the types of nuclear emissions?

Ans: As you will learn in the biology section, different types of nuclear emissions have different penetrating powers and thus have different health effects.

The Physics of the Atomic Bomb (Level 1)

Level 1: Half-life

Radioactive decay is the process in which unstable nuclei try to become more stable by ejecting particles or energy. Radioactivity is directly proportional to the number of undecayed radioactive nuclei present. Since the number of undecayed nuclei of a sample is proportional to the mass of the sample, the radioactive half-life of a given radioisotope (radioactive isotopes of an element) is the time needed for half of the radioactive nucleus in any sample to undergo radioactive decay. After two half-lives, there will be one-fourth the original sample, so on and so forth. 

Q: Do all radioisotopes have the SAME half-life?

Ans: NO. Polonium-215 has a half-life of 0.0018 seconds while Uranium-235 has a half-life of 4.5 billion years.

Q: So why is knowing the half-life of a radioisotope IMPORTANT?

Ans:  In the case of the atomic bomb blast, it will help to determine whether the location has a level of radioactivity within safe limits. 

Aftermath of the Atomic Bomb Blast

As you would have learnt from the biology section, atomic bomb blasts have serious short-term and long-term health effects ranging from hair loss to the development of cancer in victims.

The following video: http://tinyurl.com/bombblastaftermath provides a detailed visual edge to help you visualise the various impacts of the atomic bomb blast in Hiroshima and Nagasaki. 

Biological Effects of the Atomic Bomb (Level 4)

Level 4: Development of Cancer in Victims of Atomic Bomb Blast

Cancer in victims of the atomic bomb blast is mainly caused by beta, gamma and x-rays. Beta radiation can contaminate food and water which are consumed frequently by victims. Caesium and strontium, which are elements that emit beta particles, possess compounds which are soluble. This means that they can be taken up into plants via the roots if these elements get into the soil, and are eventually consumed by humans and animals. Victims that eat contaminated animals will thus ingest these beta particles.

A nuclear fallout (refer to Level 2 of Biology for information on nuclear fallout) after the explosion also leads to victims ingesting the irradiated materials via breathing the contaminated air. When gamma and x-rays get into the human body via penetration or beta particles via mostly ingestion, they emit ionizing radiation which can produce molecular-bond-breaking energy. It alters the molecular structure of the molecules exposed to radiation, damaging chromosomes and impacting the epigenetic factors which regulate the gene expression. Gene expression is the process in which information in a gene is used in synthesis of a new gene. This results in abnormalities in the tissue, or mutations that result in losses in the cell’s function.

One of the most damaging alterations are the double-strand breaks to the DNA. Double-strand breaks removes a portion of the epigenetic markers of the DNA, and although there are cellular mechanisms that attempt to repair the damage, some of these repairs will be incorrect and the chromosome abnormalities will become irreversible. Cells suffering from major damages die and lose their ability to reproduce, while those suffering from lesser damages remain partly functional and stable. These functional cells can proliferate and create many copies of the abnormalities, and the mutations eventually lead to the development of cancer.

This mutation process is sped up if tumor suppressor genes are damaged. Tumor suppressor genes play a role in stopping a cell from becoming cancerous. They sieve out cells which contain damaged DNA, which in this case is caused by the ionizing radiation, to ensure that the cells do not divide and result into increased chances of mutations in future generations. The tumor suppressor genes then attempt to repair the DNA and if the damage can be repaired, the cells’ reproduction cycle can continue. However, if the damage cannot be repaired, they will program the cell to die in a process called apoptosis. However, if these tumor suppressor genes are mutated or deactivated due to the radiation emitted from the atomic explosion, it results in a loss of function and the genes lose its ability to inhibit cell growth. This causes cells which contain damaged DNA to sometimes undergo uncontrollable cell division, accumulating the DNA damage over time.

The cells then undergo neoplastic transformation, which is the conversion of a tissue with a normal growth pattern into malignant tumors where cells invade nearby tissues. This causes the spread of cancer to different parts of the body.

Free radicals, which are atoms with unpaired electrons, are also created in large amounts by these types of ionizing radiation. They are created when the radiation strikes an atom or molecules, or when it ionizes the water, forming free radicals of hydrogen atoms. A large proportion of the human body contains water, and a lot of damage is caused by free radicals produced from water, also known as the reactive oxygen species. The free radicals are highly reactive and damage the biomolecules which form the various structures of the cells.

This results in oxidative stress, where our body cannot detoxify the huge number of reactive oxygen species or repair the subsequent damage caused by them. This either causes cell deaths or sometimes damages to the DNA, which will then play a role in the development of cancer as stated earlier. 

Biological Effects of the Atomic Bomb (Level 3)

Level 3: Hair Loss in Victims of Atomic Bomb Blast

Hair loss is mainly caused by gamma and x-rays, which are released upon the explosion of the atomic bomb. Upon penetrating a person, these rays can penetrate more than 10,000 cells and cause radiation damage, resulting in cell deaths and the inhibition of cell division. 

The radiation from gamma and x-rays cause the most damage to cells which are actively proliferating. Hair is made up of layers of dead cells stacked on top of each other. Hair follicle cells in the anagen phase constantly divide to produce new cells, which eventually die and stack up on top of each other for hair growth. Hence, impacts on the inhibition of cell division are the most serious to these actively-proliferating hair follicle cells as it significantly decreases hair growth. 

Furthermore, as more cells start to die from radiation, more hair follicles are embedded in the dead cells. This makes it difficult for hair roots to properly anchor into the skin, so hair will not be attached firmly to the skin and can be pulled out easily, increasing the rate of hair loss. 

Q: Are the above reasons why Radiation Therapy for Cancer causes Hair Loss in patients?

Ans: Not necessarily. For example, if radiation treatment is to your head, it can cause alopecia: an autoimmune diseases in which the patient's immune system mistakenly attacks hair follicle cells
(Sources: cancer.org and WebMD)

The Chemistry of the Atomic Bomb (Level 4)

Level 4: Comparison between Atomic and Hydrogen Bombs

For atoms lighter than Nickel-62, the stability of the nucleus increases as atomic mass (nucleon number) increases. Hydrogen bombs derive their energy from nuclear fusion when two light atomic nuclei fuse to form a heavier, more stable nucleus. (Refer to Level 4 of Physics for more information on nuclear fusion) 

As you have learnt in level of chemistry, the nucleus of an atom contains the positively charged protons and electrically neutral neutrons. This thus makes nuclei positively charged. Extremely high temperatures are required for the positively charged nuclei to overcome their mutual repulsion and gain enough kinetic energy to fuse. Deuterium (hydrogen-2) and tritium (hydrogen-3) are isotopes of hydrogen, and carry weak positive charges due to them only having 1 positively charged proton, making it easier to overcome their mutual repulsion and fuse into helium. 

As seen from the diagram above, hydrogen bombs rely on a fission reaction to compress the fusion fuel lithium-6 deuteride of chemical formula ⁶Li²H. Neutrons produced in the fission reaction bombard lithium-6 deuteride to produce tritium. The deuterium and tritium then fuse to produce helium-4 which has a more stable nucleus.

	Atomic Bombs	Hydrogen Bombs
Mechanism	Nuclear Fission: splitting of heavy isotopes into smaller atoms	Nuclear fusion (fusion of nuclei of lighter atoms into larger, more stable ones) caused by fission
Cost	Expensive: rare isotopes used require enrichment to obtain a supercritical mass.	Expensive: needs both fission and fusion components.
Energy needed	Less	More as a high density & high temperature environment is required for nuclear fusion
Energy produced	Less: about 20 kilotons of TNT	More: about 10 megatons of TNT

The Chemistry of the Atomic Bomb (Level 2 and 3)

Level 2: Isotopes in Atomic Bombs

For atoms heavier than Nickel-62, the stability of the nucleus decreases as atomic mass (nucleon number) increases. Energy given off by atomic bombs arise from nuclear fission in which heavier isotopes such as uranium or plutonium split into more tightly bound stable elements.

The atomic bombs ‘Little Boy’ and ‘Fat Man’ were used by the USA in WWII. Little Boy used Uranium-235 and Fat Man used Plutonium-239. 

Level 3: Comparison between Little Boy and Fat Man

Little Boy

Little Boy used the gun-type assembly method. In the diagram on the left, the red rings represent 80% Uranium-235. When the explosive (orange section) is detonated, the uranium ‘bullet’ in front of the explosive is accelerated towards the other uranium section. Each section of uranium is sub-critical, that is of an insufficient mass for an explosion to occur. (For more information of critical mass, refer to Level 3 of Physics) When the uranium sections are 25cm from each other, free neutrons may hit the uranium, resulting in the formation of the highly unstable Uranium-236 which is deformed elastically. It then splits into the highly radioactive fission products of Barium-144, Krypton-89 and 3 neutrons which collide with more uranium to cause a chain reaction. This could cause a pre-donation.

To prevent pre-detonation, the speed of the ‘bullet’ would have to be very high which requires a long and heavy barrel. The gun-type method is unsuitable for Plutonium-239 because it contains about 20% Plutonium-240 which makes pre-donation inevitable. 

In contrast, Fat Man used the more complicated implosion assembly method. As seen from the figure of the left, the center contains a sub-critical mass of plutonium-239. When the explosion lenses are detonated, the density of the plutonium increases until it becomes super-critical (sufficient to start and sustain a chain reaction). This method is much safer as it prevents accidental pre-detonations. However, the complexity of the design leads to higher costs and allows for the creation of a smaller bomb. 

The table below summarises the comparisons

	Little Boy	Fat Man
Method	Gun-type	Implosion: higher complexity, higher costs, safer
Explosiveness	Less explosive: energy generated equivalent to 15 kilotons of TNT (a common explosive)	More explosive: energy generated equivalent to 20 kilotons of TNT
Fissile material	60kg of 80% Uranium-235	8kg of 80% Plutonium-239
Size	Larger and heavier	Smaller and lighter

Biological Effects of the Atomic Bomb (Level 1 and 2)

Level 1: Thermal and Ionizing Radiation

Thermal radiation consists of infra-red and ultraviolet light rays and it is released when an atomic bomb explodes, producing huge amounts of heat energy.

Ionizing radiation is caused by beta particles and gamma rays. Ionization is the process in which an atom or molecule loses or gains electrons. Since electrons have a relative charge of -1, the atom or molecule can become Ions which are positively or negatively charged particles. Beta particles can penetrate the skin partially. Gamma rays are highly penetrating and can penetrate the skin and even pass through thick layers of concrete. For more information on beta particles and gamma rays, refer to Level 2 of Physics.

Level 2: Skin Burns

Thermal Burns

Thermal Burns are caused by thermal radiation and are differentiated into primary and secondary thermal burns. Primary thermal burns are flash burns, experienced when one is 2-3 kilometers from the center from the explosion. Thin scars that form will thicken to become keloids after 3-4 months. Keloids is an overgrowth of scar tissue on the surface of a wound, and is an irregularly shaped protrusion.

Secondary thermal burns are flame, scorch and contact burns, which are experienced when one is 1-2 kilometers from ground zero. The victim's skin will be heavily scarred or even melt if he is very near the center of the nuclear explosion, due to the internal tissues being destroyed by extreme temperatures and radiation. 

Beta Burns

Beta particles from Ionizing radiation can penetrate the skin partially, causing 'beta burns'. Beta burns in victims of the atomic bomb blast are usually a burning sensation or itching in the first 2 days.Some victims may experience hyperemia: an increased amount of blood flow to areas suffering from beta burns. 

The burnt areas start to redden as ionizing radiation damages the cells and cause redness around burnt areas (erythema). Other symptoms include increased skin pigmentation around the burnt region.

Beta burns are classified from 1st to 4th degree burns.

Victims of first degree beta burns suffer damages mostly at the epidermis (surface of the skin) and the skin will also start to peel off in a process called desquamation. Furthermore, scabs will gradually form at the burnt region. 

Victims of second degree beta burns suffer from blisters in the burnt region. 

Victims of third and fourth degree beta burns suffer from various damages from deep and wet ulcerated lesions to ulcerated necrotic dermatitis, which is the inflammation of the skin when the victim suffers from heavy tissue damages in the region. 

Beta burns are also caused by exposure to the radioactive fallout after the atomic bomb blast. A nuclear fallout forms when residual radioactive material released by the atomic blast rises into the upper atmosphere. It can either fall back to the ground after the shockwave of the blast has passed, or mix with precipitation and fall as black rain. When the victims are exposed to the radiation or they come into contact with the precipitation, they will also suffer from beta burns. 

The eye lens are the most vulnerable to beta burns due to their exposed nature and they can only be protected with the usage of instruments like safety goggles. 

Gamma Burns

Gamma rays pose a huge radioactive threat to the human body due to its high penetrative powers, resulting to internal “gamma burns” on the skin in some cases. Gamma rays also damage the cells of the body, causing erythema in the affected region 

The Chemistry of the Atomic Bomb (Level 1)

Level 1: Elements and Isotopes

In Year 1, you would have learnt that an element is a substance that cannot be broken down into simpler substances by chemical methods and that an atom is the smallest unit of an element.

Atoms consist of subatomic particles: protons, neutrons and electrons. Protons and neutrons are contained in a nucleus and electrons surround the nucleus. In normal atoms, there are the same number of protons and electrons. The term 'nucleons' refer to both protons and neutrons. The table below compares these 3 subatomic particles.

©cronodon.com

These elements are arranged in a periodic table according to their proton number. Isotopes are atoms of the same element that have the same number of protons but different number of neutrons and are commonly expressed as [element]-[nucleon number]. For example, Carbon-12 means that carbon has 12 nucleons, that means a total of 12 protons and neutrons. As all isotopes of carbon have the same number of 6 protons, Carbon-12 has 6 neutrons. 

Q: Do ALL elements have naturally-occurring Isotopes?

Ans: A few elements do not have isotopes. Examples include Beryllium, Fluorine and Sodium.

Q: Do all elements have the SAME number of Isotopes?

Ans: NO. Hydrogen has 3 naturally-occurring isotopes: protium (hydrogen-1), deuterium (hydrogen-2), and tritium (hydrogen-3). In contrast, Tin has 10 naturally-occurring isotopes.