Editorial Feature

Bhangmeter - Measuring the Power of Electromagnetic Radiation

The bhangmeter is a non-imaging radiometer that was invented in 1948 with the purpose of detecting nuclear detonations and measuring the yield of a nuclear weapon. To achieve this, the bhangmeter is fitted onto reconnaissance and navigation satellites.

The bhangmeter is used for detecting electromagnetic radiation. Bhangmeters have become regular instruments used to examine nuclear tests conducted in the USA.

The term ‘bhangmeter’ was coined as a joke by Nobel Prize-winning American physicist Frederick Reines who worked with a group of scientists to build the instrument. Reines was referring to ‘Bhang’, an Indian word for cannabis, which when smoked or drunk caused intoxication. According to him, the simplicity of the instrument to perform such an immense task as measuring nuclear denotation seemed almost absurd, and one would have to be intoxicated to believe that it would work.

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Working Principle

During a nuclear explosion, a double flash can be observed, which helps to differentiate between traditional explosives and nuclear explosives. After detonation, traditional explosives produce only one primary flash.

Bhangmeters are designed to recognize the double flash. The first flash is a very bright fireball and is caused by the actual detonation of the weapon. It lasts for about 1ms, and is then hidden by the expanding shock wave.

The second flash is caused due to the cooling of the ionized gas shock wave, which allows light from the fireball to escape and illuminate the surrounding area. For example, the second flash occurs at 150ms for a 20 kiloton bomb, and at 900ms for a 1 megaton bomb. The bhangmeter captures both the flashes, and the time taken between the two flashes can be used to determine the weapon’s nuclear yield.

The bhangmeter consists of a simple electronic circuit fitted with a quick-response photodiode or phototransistor and a high speed timer circuit. Additional circuitry such as a high-pass filter is also used.


Bhangmeters are mainly used in the field of defense to detect detonation of a nuclear bomb. This can also help to keep a check on any nuclear denotation taking place illegally in any part of the world. Bhangmeters help in the study of the power of a nuclear weapon.

Sources and Further Reading

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