ÐÓ°ÉÔ­´´

Our car was driving through a subterranean rainstorm. All that we could
see ahead of us were our faces reflected in the windscreen and the red tail
lights of the vehicle we were chasing deep into the mountain. The roar of
the engine, echoing off the bare rock of the tunnel, was ear-splitting.
The driver, Yoichiro Suzuki, professor of physics at the Institute for Cosmic
Ray Research at the University of Tokyo, was yelling: ‘There are over a
thousand kilometres of tunnel down here. Even the miners get lost.’

We were deep in the Kamioka metal mine, in the Japan Alps 300 kilometres
northwest of Tokyo. Zinc, lead and silver have been mined here since the
8th century. Finally, the car came to a halt. ‘We’re now 1.7 kilometres
inside the mountain,’ said Suzuki. We collected our hard hats and torches
and stepped out into the downpour, which came from the wooded mountainside
above, trickling down through fissures in the rock.

We were joined by mining engineer Tetsuo Nakagawa, the driver of the
other vehicle. As we shook hands, thunder filled the air and the ground
began to shake. A giant earth-moving machine lumbered by, belching exhaust
fumes. In its scoop it was carrying 12 tonnes of crushed rock. We headed
down the tunnel the machine had come from, trying not to slip on the wet
floor; in the torchlight, the zinc ore in the rock glistened. Eventually,
we emerged at the bottom of an enormous flood-lit chamber, dome-shaped like
a subterranean St Paul’s Cathedral. Rubble from its excavation was still
piled about. ‘Welcome to Super-Kamiokande,’ said Suzuki.

‘When the chamber is finished,’ he continued, ‘we will fill it with
a cylindrical steel tank containing 50 000 tonnes of ultra-pure water. The
inside surface will be covered with thousands of photomultiplier tubes to
catch tiny flashes of light created by passing neutrinos.’ When this vast
apparatus is completed in April 1996, it will be the first to take neutrino
astronomy into the big time. Super-Kamiokande works on the same principles
as the neutrino detectors which American, Soviet and European scientists
are lowering to the bottom of the sea or burying deep in Antarctic ice.
All are intended to pick up the telltale signatures from particles created
by neutrinos as they hurtle through water or ice. But Super-Kamiokande is
going for a certain bet – neutrinos with energies in the megaelectronvolt
range, which have already been detected coming from a supernova and from
the core of the Sun. The ocean and Antarctic experiments are searching for
a more elusive quarry – high-energy neutrinos in the teraelectron-volt to
gigaelectronvolt range, which no one is sure can be detected at all.

Gigantic explosions

Although Super-Kamiokande is buried deep underground it will be able
to function just like a ‘telescope’. The neutrinos it picks up from supernovae
will enable it to see events totally inaccessible to astronomers using conventional
telescopes – the light coming from a supernova is created in the shell surrounding
it, but the prodigious quantities of neutrinos it creates comes straight
from its heart and will give new insights into what causes these gigantic
explosions.

The neutrinos Super-Kamiokande picks up from the Sun will also enable
it to tackle a problem that goes beyond astronomy, giving scien-tists a
chance to determine whether the neutrino has a mass. If it does the so-called
standard model of particle physics will have to be modified. And lastly,
in a separate series of observations, Super-Kamiokande will be able to peer
inside the water it contains, looking for a signal from a proton in the
act of decaying. If it cannot be found, then Super-Kamiokande will call
into question one of the key predictions of the Grand Unified Theories of
physics, which seek to unite nature’s strong, weak and electromagnetic forces.

But before this wealth of new scientific data can be harvested, there
is much work to be done. Nakagawa, his breath freezing, explained that his
company, Mitsui Mining and Smelting, was excavating a cylindrical cavity
as high as a 10-storey building to house Super-Kamiokande. We were standing
in the top third of this space. The bottom third is currently being blasted,
a procedure that will leave an enormous plug of rock in the middle to be
removed last of all. In all, Mitsui Mining and Smelting plans to excavate
78 000 cubic metres of rock. The job will cost £33 million out of
the total £67 million being spent on the project. Funds are coming
from the Japanese government, with a small contribution from a team at
the Irvine-Michigan-Brookhaven (IMB) observatory in the US.

The job is lengthy and expensive, explains Nakagawa, because the cavern
being excavated is huge and must be strengthened. Above it, a kilometre
of rock is pressing down. The walls of the cavity have to be supported by
steel cables cemented into the rock. Concrete, shot through with steel fibre,
is then sprayed over the steel cables to form a protective layer 16 centimetres
deep.

After this work is finished, the tank will be installed. Its vast volume
is essential for trapping neutrinos because they can fly through matter
almost as though it were empty space. The water in the tank contains about
1.5 times 1034 electrons, enough to ensure that every hour or so, a passing
neutrino will run into one of them, knocking it out of orbit. As the electron
ricochets through the water – in the same direction as the neutrino was
travelling – it will radiate a cone of bluish light that can be picked up
by the photomultiplier tubes lining the tank walls. By measuring where and
when this Cherenkov radiation hits the detector wall, and how bright it
is, the neutrino’s path and energy can be calculated.

Built deep underground, at the bottom of the Kamioka mine, the tank
is shielded from other high-speed particles, such as muons, that can also
produce Cherenkov light. Showers of muons are created when cosmic rays collide
with nuclei in the atmosphere, but very few can penetrate a kilometre of
rock. ‘We chose the Kamioka mine because it is the deepest in Japan,’ says
Suzuki. ‘Down here, we get only one cosmic ray every three seconds, making
them easy to eliminate.’ Super-Kamiokande should be able to pick up about
30 neutrinos a day from the Sun. If a supernova goes off at the centre of
the Galaxy, 28 000 light years away, Super-Kamiokande will detect about
4300 neutrinos from the event.

Will Super-Kamiokande work as planned? Suzuki is confident it will –
and for a very good reason. It can be found not far away, down a narrow
tunnel shared by a narrow-gauge railway, a gurgling stream and some heavy-duty
wiring. ‘Don’t touch the overhead cable,’ Suzuki warns. ‘You’ll get a 400-volt
²õ³ó´Ç³¦°ì.’

Perfect timing

Here, behind a steel door, is another cavern, similar to the one under
construction to house Super-Kamiokande. Inside it is Kamiokande II, a neutrino
detector converted from an experiment to look for proton decay in 1985.
Although much smaller than Super-Kamiokande – only five storeys high and
filled with 3000 tonnes of water – it has already made history. On 23 February
1987, it picked up a burst of neutrinos from a supernova 160 000 light years
away in another galaxy. Overnight, the science of neutrino astronomy was
born. The same event was also registered by the IMB observatory in a salt
mine near Cleveland, Ohio, which explains the American involvement in
Super-Kamiokande.

The neutrinos had come from the first supernova to be visible to the
naked eye since the German astronomer Johannes Kepler spied his nova in
1604. The explosion subjected the Earth to a 10-second blast of neutrinos.
‘Sixty billion neutrinos flashed through every square centimetre of our
detector,’ says Suzuki. ‘Eleven were stopped.’

Supernova 1987A detonated in the Large Magellanic Cloud, a satellite
of our Galaxy visible only from the southern hemisphere. ‘The neutrinos
passed right through the Earth,’ says Suzuki. ‘They came up from beneath
our feet.’ It was like pointing a conventional telescope at the ground and
seeing a star on the far side of the world.

‘We were extremely lucky to detect the neutrinos,’ admits Suzuki. ‘If
the supernova had gone off one minute earlier, we would have missed it.’
Every two hours, the detector was turned off to calibrate it. By enormous
good fortune, the supernova occurred just moments after the detector was
turned on again.

Super-Kamiokande will be much more sensitive. When it starts hunting
for neutrinos in April 1996, its interior will be lined with 11 200 photomultipliers,
compared with the mere 948 of Kamiokande II. They will cover 40 per cent
of the surface, twice as much as Kamiokande II. Super-Kamiokande should
be able to see any supernova within 650 000 light years, four times the
distance of Supernova 1987A.

Energetic spread

Although a supernova can outshine a galaxy of 100 billion stars, it
emits a hundred times as much energy in neutrinos as it does in light. Measuring
how those neutrinos are spread in time, and their energies, should help
theorists understand what triggers these catastrophes.

Supernovae occur in massive stars which have burnt all the nuclear fuel
in their cores. Once heat energy is no longer being generated, the core
begins a catastrophic collapse under its own gravity. This is stopped only
when protons and electrons in the centre of the core are crushed together
to form a ‘neutron star’, a ball of neutrons as dense as an atomic nucleus
and virtually incompressible. Material raining down on the neutron star
from the outer regions of the core is brought to an abrupt and dramatic
halt, creating a tremendous shock wave, which rebounds outwards through
the star, blowing it apart. Thus nature turns an implosion into an explosion.

However, there is a problem with this picture. ‘Simple theoretical models
just cannot make a supernova explode,’ says Suzuki. The outward shock wave
stalls long before it gets to the star’s surface. Suzuki believes that neutrinos,
which are created in fantastic numbers in the formation of a neutron star,
play a key role in keeping the shock wave going.

Studies of spiral galaxies like our Milky Way show that supernovae occur
about once every 25 years. We see so few because most are obscured by inter
– stellar dust in our Galaxy. However, dust is no barrier to neutrinos.
‘The next supernova in our Galaxy could go off tomorrow,’ says Suzuki.
‘I’m betting on 1999.’ The most likely place to see one is at the centre
of the Galaxy, 28 000 light years away, where the density of stars is greatest.

If there is another supernova, the scientists at Super-Kamiokande are
ready for it – and ready to alert the astronomical world as well. Next door
to the cavern housing Kamiokande II is the ‘counting room’, crammed with
racks of electronics and bundles of coloured wires. Video cameras fixed
to brackets on the wall relay pictures of digital read-outs to a laboratory
on the surface. ‘If the software spots a supernova, it dials the laboratory
automatically,’ says Suzuki.

The neutrinos in a supernova are generated before the visible light,
so they can give an advance warning of the coming cataclysm. When the alarm
sounds, Suzuki and his colleagues will embark on a phoning frenzy. With
luck, when the supernova flares in the sky every major telescope in the
world will be pointing towards it within an hour or so.

Super-Kamiokande promises to show us supernovae and who knows what else
in the wider Universe, but there is much more it can do besides. Astronomy
is just a sideline for many of the scientists at Super-Kamiokande. ‘Most
of our people are particle physicists not astrophysicists,’ says Suzuki.
Just as important to him and his colleagues is what Super-Kamiokande can
tell us about the fundamental nature of matter – whether the neutrino has
mass and how quickly protons decay. Super-Kamiokande has a chance to answer
both questions by the end of the century.

The implications for cosmology could be dramatic if the neutrino has
a mass. Much of the Universe’s ‘dark matter’, known only by its gravitational
effect on visible matter, could turn out to be made up of massive neutrinos
left over from the big bang. If so, the combined gravity of these particles
could have had a profound effect on the evolution of galaxies and clusters
of galaxies. In the Universe, neutrinos outnumber protons by about a billion
to one.

Good acoustics

Whether or not the neutrino has a mass may depend on the answer to another
question: why do far fewer neutrinos arrive on Earth than is predicted
by current theories of the nuclear reactions occurring within the Sun?
According to one possible solution of this long-standing puzzle, neutrinos
‘oscillate’ between three different ‘flavours’ associated with the electron,
the muon and the tau particle. Current neutrino detectors are most sensitive
to only one type, neatly explaining why there appears to be a shortage of
neutrinos coming from the Sun.

The importance of neutrino oscillations is that they imply the neutrino
has a small mass. It is simply impossible for a massless particle – for
instance, a photon – to change spontaneously into another particle. Neutrino
oscillations could be forced by the matter of the Sun so that the proportions
of the three types of neutrino are altered, a possibility known as the MSW
effect, after the theorists who proposed it: S. P. Mikhayev, Alexei Smirnov
and Lincoln Wolfenstein.

Kamiokande II picked up its first solar neutrinos back in 1987. ‘Because
Kamiokande II can determine the direction of neutrinos, it was the first
neutrino detector to actually make an image of the Sun,’ says Suzuki. ‘It
also was able to register neutrinos as they arrived.’ Earlier ‘radiochemical’
experiments waited for neutrinos to trigger chemical changes in a liquid
mass. The changes were discovered only after chemical processing, so both
the direction and timing of neutrinos was lost.

Super-Kamiokande will go one better than Kamiokande II. It will be able
to measure the energy spectrum of solar neutrinos and daily variations in
the number of neutrinos reaching the detector with much greater precision.
If the neutrinos oscillate, then its spectrum should vary in a specific
way because of the MSW effect. It should also result in a diurnal variation
in the number of neutrinos, because at night neutrinos must pass right through
the Earth, encountering more mass than during the day.

Neutrino oscillations are not the only fundamental physics Suzuki and
his colleagues hope to explore with Super-Kamiokande. They also hope to
glimpse the spontaneous disintegration of protons, predicted by the Grand
Unified Theories of particle physics. In fact, Kamiokande was originally
built to look for the distinctive Cherenkov signature of proton decay. Only
in 1985 was the KAMIOKA Nucleon Decay Experi-ment upgraded to observe cosmic
neutrinos as well as proton decay.

The Cherenkov signature of proton decay should be easy to distinguish
from that of solar neutrinos because the energy is very different. Super-Kamiokande
will contain about 1034 protons, which means that if the average lifespan
of a proton is 1034 years the experimenters will see one decay a year. If
the experimenters are lucky enough to show that the proton decays, they
can be sure their names will join the great physicists of history.

But deadly serious fundamental physics is not all that goes on deep
in the Kamioka mine. Above the roar of the engine as we headed to the surface,
Suzuki explained that a singer had recently come to perform in the mine
because of the acoustics. There was now talk of building an underground
concert hall. ‘But that’s nothing,’ Suzuki added. ‘There is also a plan
to put an athletics centre down here, with an Olympic-size swimming pool.’
It is a serious idea. The rock is airtight and the plan is to pump out some
of the air so Japanese athletes could gain the benefits of exercising at
high altitude. Imagine athletes training in the bowels of the Earth to prepare
themselves for the Olympics? The mind boggles – it is almost as implausible
as astronomers watching supernova from deep underground.

Marcus Chown’s book, Afterglow of Creation, is published by Arrow Books
( £5.99).

Further reading: Spaceship Neutrino by Christine Sutton (Cambridge UP).