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EXPLOSIONS
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Arson or Explosions
• Often an explosion starts a fire. However, the bomb specialist will investigate the scene to determine if the explosion was intentionally set. The fire is considered a by-product of the explosion.
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What is an Explosion?
• An explosion is the product of combustion accompanied by the creation of gases and heat at a very rapid rate.
• As these gases are released from confinement, they produce a blast effect, or an outward rush of gas—at a rate of up to 7,000 mph.
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Explosive Components
An explosive device is comprised of three components:
1. A type of fuel2. An oxidizer 3. An igniter
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Categories of Explosives
• Explosives are categorized by the speed at which they decompose.
• The two categories of explosives are low and high.
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Low Explosives
•A low explosive decomposes at the speed of deflagration. •Deflagration (burning) is a rapid oxidation reaction accompanied by a sub-sonic pressure wave.•Low explosives decompose at a velocity of less than 1000 meters per second.•These explosives burn only at the surface.
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Low Explosives (cont)
Low explosives produce a propelling or throwing action. This makes them suitable as propellants for ammunition or sky rockets.
The danger of low explosives should not be underestimated. If confined in a small container, the force of deflagration is very lethal.
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Examples of Low Explosives• This category includes:
black powderchlorate powderfireworksflash powdernatural gas-air
smokeless powder
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Examples of Low Explosives (cont)
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Low Explosives (cont)
• The most widely used low explosive is black powder. This powder is readily available in gun stores or may be homemade.
• The most powerful low explosive is smokeless powder.
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Low Explosives (cont)
• Chlorate powder is more sensitive and may ignite from the heat generated in a chemical reaction with sugar.
• A mixture of air and gaseous fuel will explode only when combined in certain ratios.
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Low Explosives (cont)
• Fireworks or pyrotechnics are low explosives that are used as a form of entertainment.
• Due to the potential for danger, fireworks are legally restricted.
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High Explosives
• A high explosive decomposes at the speed of detonation.
• Detonation is an extremely rapid oxidation reaction accompanied by an intense, high-speed shock wave.
• High explosives decompose instantaneously at a velocity of 1000-8500 meters per second.
• These explosives produce a smashing or shattering effect on the target.
• High explosives generally require a primer or blasting cap.
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Two Groups of High Explosives
• High explosives are divided into two groups: primary and secondary.
• These groups are based on the sensitivity to heat, shock or friction.
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Primary Explosives
• Primary explosives are the group of high explosives that are ultrasensitive to heat, shock or friction.
• Primary explosives detonate violently. Therefore, they are used as primers to detonate other explosives in a chain reaction.
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Examples of Primary Explosives
• Primary Explosives, the main ingredient in blasting caps, include:
Lead AzideLead Styphnate
Diazodinitrophenol (DDNP)
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Examples of Blasting Caps
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Secondary Explosives
• Secondary explosives are relatively insensitive to heat, shock and friction.
• These explosives are used for commercial and military blasting.
• Secondary explosives generally require a detonator.
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Example of Secondary Explosives
• Secondary explosives include:
Dynamite
TNT (trinitrotoluene)PETN (pentaerythritoltetranitrate)RDX (cyclotrimethylenetrinitramine)ANFO (ammonium nitrate/fuel oil)
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Examples of Secondary Explosives (cont)
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Dynamite
• In 1866, Alfred Nobel experimented with methods that would desensitize nitroglycerin. His experiments led to the invention of dynamite.
• In 1867, Nobel patented his invention.• When he died, in 1896, Nobel left a 9
million dollar endowment for the establishment of the Nobel Prize.
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Dynamite (cont)
• Dynamite is sold in the form of a stick that is approximately 20 cm in length and 2.5 cm in diameter.
• Dynamite has 60% greater energy density than TNT.
• The main uses of dynamite are mining and construction.
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TNT
• TNT is the most commonly used explosive by the military.
• TNT is poisonous and any contact with skin turns the skin a bright yellow.
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PETN• PETN is an ultrasensitive high explosive.• It is used in detonating cords or primacords.
These cords are used to connect a series of explosive charges so all will detonate simultaneously.
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RDX• RDX was one of the
first plastic explosives.
• It was widely used in WWII.
• This explosive is used in controlled demolition. The demolition of the Jamestown Bridge used RDX.
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ANFO
• ANFO accounts for 80% of all explosive use in North America.
• Ammonium nitrate is widely used as a fertilizer, but when mixed with the correct concentration of fuel oil it becomes an explosive.
• ANFO is relatively inexpensive to manufacture.
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Homemade Explosives
• TATP (triacetone triperoxide) is one type of homemade explosive. It is a mixture of acetone, hydrogen peroxide and hydrochloric acid.
• It is an impact-sensitive explosive that is very powerful when packed into small containers.
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A Flow Chart of Explosives
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The Damage
• The main damage from explosives occur due to:• flying shrapnel
• rapidly expanding gases create extreme pressure on the walls of the container. The container burst and sends shrapnel in all directions.
• self-propagating wave• caused by rapidly expanding gases
compressing the surrounding air.• can reach speeds of 7000 mph.
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Detection and Collection of Evidence• First, investigators will locate the
crater that is at the origin of the blast.
• All loose dirt and debris is removed from the crater and sent to the lab.
• Any object penetrated by the shrapnel is collected.
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Detection and Collection of Evidence (cont)• Residues from the
surfaces of nearby objects is also collected.
• All pieces of shrapnel are packaged and sent to the lab.
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Detection and Collection of Evidence (cont)• The Ion Mobility Spectrometer (IMS) is a
portable detection machine that uses a vacuum to collect explosive residues from surfaces.
• The vaporized residue is converted to electrically charged ions that move through an electric field.
• Different ions move at different rates depending on their size and structure, which allows for identification.
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Ion Mobility Spectrometer
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Bomb ThreatTrained dogs are able to detect the scent of
explosives, even a few yards away. They are useful anywhere--airplanes, schools, hotels, parking lots, etc.
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Bomb Threats
Once a bomb threat or a suspicious package has been detected, the bomb squad or a robot can be dispatched to disarm it.
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Analysis of the Evidence
• Any recovered debris is rinsed with organic solvent and analyzed using color tests, thin-layer chromatography and gas chromatography/mass spectrometry.
• Unexploded devices are identified through X-ray diffraction or infrared spectrophotometry.