Thursday, April 4, 2013

The Bhangmeter

Its not everyday that you wake up to learn that a small nation of perhaps 3 million is ready for nuclear war with anyone it can find close by.  Perhaps life is that boring in North Korea. Or did the Korean newspaper sales plummet in that last two months? I'm not sure.

However, I hope this does not inconvenience the rest of us, especially if those pesky shocks waves from the Korean Peninsula disrupts my normal running schedule. Man I'll be pissed.

Speaking of shock waves, I was obviously curious about one aspect related to all this nuclear doom and gloom. Suppose you were tasked with the job of measuring the energy of a nuclear bomb just from the explosion, how would you do it? Sounds like a handful.

Actually, yes and no. But one such technique interestingly takes advantage of a peculiar behavior of nuclear explosions. I did some reading on the subject.

It turns out that atmospheric nuclear explosions below 30km in altitude all have a light signal that is quite strongly related to the yield of the bomb. Thermal power of an explosion can be derived from a two peak light pulse that is very characteristic of nuclear explosions. This pulse contains two maximas and one minima, illustrated in diagram below. I stole it from an interesting write up on bomb physics (sources are listed below)

The two peaks have similar instantaneous brightness characteristics but the second maximum lasts 100 times longer and this is where most of the energy of the bomb comes from. At first glance at the plot on the left, you won't get that perspective but its on a logarithmic scale!

The numbers are stupefying obviously. Data from the same paper points out that the peak power generated during the first half second of a 1 kt nuclear bomb is around 4 x 1012 watts! I mean, this is more than the actual installed electric generation capacity of the US for 2011!

Because no other power source on earth can match this pulse and power levels within such fractions of a second, it is distinct to nuclear explosions making it convenient to measure.

Scientists started making cheap prototypes of a pulse measurement instrument using photocells and oscilloscopes in the 1930's. The time to the minimum was measured and averaged followed by a derivation of the capacity of the bomb from established calibration curves. One such curve is displayed below, which I stole from another report.

What the firm EG&G constructed to measure yield in such a fashion was literally called a BHANGMETER! Simple as it was, it was ripe with measurement variation issues but apparently they tuned it well enough to obtain a +/- 15% tolerance band on the yield rating from reflected waves at fairly large distances.

Suffice to say, as dangerous as a nuclear bomb explosion is, its a fascinating thing to study. Perhaps the more dangerous, the more fascinating?

Man's ability to kill those of his own kind is evidently not unique to the human population. Primates do it well and do it efficiently too. But the stupidity of humans in avoiding the exercise of some amount of common sense regarding the use of nuclear bombs given past war data is spectacularly unmatched I think.

Sources :

Bomb Physics and Light Produced In a Nuclear Explosion
Bhangmeter Construction Report

No comments: