Explosions: How We Shook the World Page #7

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the exact dimensions of that potential was made clear

in a laboratory in Copenhagen.

An experimental physicist, Otto Frisch,

who'd escaped Hitler's regime in Germany, constructed

a piece of apparatus to measure the energy released when an atom splits.

Now, it's not my field of science,

but he knocked his up in a weekend and did the measurements,

so I feel there's a fighting chance for an amateur like me.

All the apparatus really consists of

is a metal box with a metal plate in it.

Now, when an atom splits,

you end up with two high-energy fission products.

Now, as they fly through the gas around them, they can

smash electrons off other atoms, causing ionisation,

producing positive and negative particles.

The atoms that Frisch split were of the element uranium

and he did it by bombarding them with particles called neutrons.

A couple of hundred volts between here and here

should enable us to detect if there's been any ionisation in here

and from that we'll be able to deduce

the energy released when an atom splits.

Obviously, now all I need is a source of uranium

and some neutrons to bombard it with.

The National Physical Laboratory near London have

the sort of thing I need, so I've brought my part of the kit.

That is you ion chamber, is it?

Well, yes. This is my ion chamber.

It's not at the top end of the sophistication that you've got here,

- but can we try it?

- By all means.

I've got a piece of uranium here

- which I borrowed from our radioactivity group.

- That's not a phrase you hear a lot.

- No.

Uranium does have a reputation.

How safe is it? How long can I be near it?

Provided you stay a few centimetres away from it, you're out of the range of the alpha particles.

Right, so I'll put the lid on here.

- The thing that we're missing now is the thing to split the atoms.

- You need a neutron source.

- Yes.

We are the Neutron Standards Authority for the UK and we produce neutrons and use them

to calibrate personal dose-meters, like the ones we gave you to wear.

Yeah, I've got mine.

I can get a neutron source, but you will have to leave while I put it up here.

I'm happy to get out of the way while that's happening.

The neutron source contains an element whose radioactivity

is much more penetrating than uranium's,

so it has to be treated with care.

OK, so we've now got our uranium being blasted with neutrons.

- Yeah.

- How do we tell if we're splitting any atoms?

We'd have to see some pulses from our ion chamber.

- OK.

- So if I turn up the volts we might begin to see something

and the first thing that we might see, if it works,

is the natural radiation from the uranium.

I'm rather astounded, but they look like genuine pulses.

So this'd be what you'd hear on a Geiger counter, going kcrr-kcrr?

- Yes, but you're not seeing any fission yet.

- Are we not?

If it was fission, you would see some very much bigger pulses.

So if I turn the discriminator up...

That is a massive pulse.

- So is that a split atom?!

- Yes.

Wow!

- Considerably bigger than the pulses from the natural decay of the uranium.

- Yeah!

This is a completely different thing.

Yes, yes. Very much more energetic.

Now, from this can we get a measure of how much energy

is being produced every time an atom splits?

Well, the classical figure is 200 MeV, 200 mega-electronvolts.

That makes that, the energy released when one of those atoms gets split,

is about 50 million times more than a molecule of nitroglycerine.

That's...

You can see they were onto something.

They were, indeed.

Frisch finished his weekend's work in the early hours

of January 13th 1939 and was soon woken by a telegram

with news that his Jewish father had been released

from a concentration camp.

He said he remembered it as his lucky day,

but would have liked a few more hours sleep.

As war rumbled across Europe and then the world, physicists

in many countries grasped the potential of Frisch's experiment.

In the early morning of the 16th July 1945, a team of

international researchers became the first to see that potential realised

in the deserts of New Mexico.

Inside a giant sphere of shaped charges, like the ones Sidney showed

me, they placed radioactive material no bigger than an orange.

The whole contraption was hoisted up a tower and then the charges detonated.

The initial flash of light and heat

travelled out at 200,000km a second,

with temperatures reaching over 100 million degrees,

20,000 times hotter than the surface of the sun.

It melted the sand in the desert.

Just like other explosions, this heat causes a massive expansion

in the surrounding air.

There's no production of gas, like in a chemical reaction.

It's simply the staggering quantity of heat released

by a runaway nuclear reaction that causes mankind's biggest explosion.

That expanding air slams into the air around it,

causing an abrupt shock wave which crushes the air

and just like in the fire piston, heats it,

but to such a temperature that the air itself begins to glow.

You can see the white hot bubble-like shock wave

in these astonishing pictures.

Then it cools to a dark, transparent layer

and the fireball inside shows through.

The Trinity explosion, as it's known,

had the equivalent power of 20,000 tons of TNT,

all from just a few kilos of radioactive material.

And all that power, and that enormous shock wave,

is produced just simply by heating the air.

In some ways, it's similar to the heat causing the bamboo to burst

back in ancient China, but with a nuclear explosion,

the heat is almost unimaginably intense and sudden.

In little more than 2,000 years, the journey of understanding

that mankind has so far travelled is immense.

We've gone from crackling bamboo to creating something like a star here on Earth

and man-made explosions terrify us as much now as they always have.

The advent of the nuclear age was as shocking to us

as gunpowder was to medieval Europe.

Throughout history, explosives have been used first

as weapons and then had their power harnessed to more constructive ends.

They have shaped our world, through warfare and engineering.

Even nuclear power has been turned to peaceful uses.

New explosives may always be discovered

and wreak terrifying havoc.

But if history has taught us anything,

it's that by properly understanding these things

we can create instruments of unrivalled power.

If you want to find out more about the science of explosions,

go to the website -

And follow the links to the Open University.

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