[ RadSafe ] News: Thundercloud "accelerator" fires gamma-ray beam

[John Jacobus]

>From 
http://physicsworld.com/cws/article/news/31092

Thundercloud "accelerator" fires gamma-ray beam

Physicists in Japan claim to have the best evidence
yet that thunderclouds can act as high-energy particle
accelerators for seconds or even minutes at a time.
Using an array of radiation detectors installed at a
nuclear reactor, the team recorded a 40-second burst
of gamma radiation during a severe thunderstorm.
According to the researchers, the energy distribution
of the pulse suggests that the radiation is produced
by electrons that have been accelerated by the high
voltages present in a thundercloud (arXiv:0708.2947).

Physicists have known for over a decade that 10 - 20
MeV gamma rays are produced in millisecond bursts
during electrical storms. These bursts are believed to
occur when high voltages in a thundercloud accelerate
electrons to energies up to about 35 MeV. These
electrons are slowed down by colliding with atoms in
the air and as a result give off bremsstrahlung --
gamma rays that are created when an electron is
deflected off its course by an atom. 

Much longer bursts lasting up to several minutes have
also been seen, but these events seem to be much rarer
than their shorter counterparts. Physicists have yet
to work out where in the sky the longer pulses are
coming from – if they are indeed coming from the sky.
The energy distribution of the pulses and whether the
pulses contain any other types of radiation such as
charged particles was also unclear. 

Now Harafumi Tsuchiya of the Cosmic Ray Laboratory of
Japan’s RIKEN research institute and colleagues have
used a new bank of direction-sensitive detectors they
installed at a nuclear power plant to detect a 40-s
gamma ray burst during a very intense thunderstorm on
6 January, 2007. Their system at the
Kashiwazaki-Kariwa plant on the coast of the Sea of
Japan was designed to measure the energy distribution,
composition and source of thundercloud pulses. 

By analysing the energy distribution of the pulse, the
team was able to say that the pulse was made of
bremsstrahlung gamma rays. The directionality of their
detectors allowed the team to confirm that the pulse
came from the storm and because such gamma rays can
only travel short distances in the atmosphere, the
team could also conclude that the pulse was created a
kilometre or less from the detectors. 

Since the gamma rays arrived about a minute before the
first lightning strike, Tsuchiya believes that the
pulse was probably created while electrical energy was
building up in the thundercloud, rather than when
energy is being discharged as lightning. He adds that
the process is likely to begin with a cosmic ray
passing through the cloud and ionizing the air to
produce electrons, which are accelerated to towards
the bottom of the cloud, which has a positive charge.
These electrons ionize other atoms on the way,
creating a stream of high-energy electrons. 

Tsuchiya says that bremsstrahlung at MeV energies
would be focused into a beam that only illuminates a
small area on the ground, which could explain why so
few long-duration pulses have been seen. The team plan
to verify this by placing many radiation detectors
over a wider area. 

David Smith, a physicist at the University of
California at Santa Clara and an expert on atmospheric
gamma ray pulses, agrees that the pulse was made in a
thundercloud accelerator. “The spectrum looks just
right for bremsstrahlung”, he says. According to
Smith, the millions of volts required to produce MeV
electrons could be sustained in a cloud for seconds or
even minutes as long as the stream of electrons does
not become so intense that it causes an avalanche-like
electric breakdown of the type that could lead to
lightning. Smith is now designing an airborne
experiment to test whether lightning flashes are
preceded by gamma-ray pulses.

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"If you guard your toothbrushes and diamonds with equal zeal, you'll probably lose fewer toothbrushes and more diamonds."
- Former national security advised McGeorge Bundy
-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com


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