Gamma rays

It seems that several readers have been searching the site for information about gamma rays. So, here’s a potted summary adapted from Wikipedia.

Gamma rays, or gamma radiation is electromagnetic energy of high frequency (very short wavelength) and so high energy. For electromagnetic radiation wavelength is inversely proportional to frequency, the proportionality constant being the speed of light. Similarly, energy and frequency are proportional in this case the relationship is governed by Planck’s constant). Gamma ray frequencies are usually above 10 exahertz (or more than 10 to the 19 Hz), and so their energies are above 100 kilo electronvolts (keV). Their wavelengths are less than 10 picometres, less than the diameter of an atom.

Gamma rays are produced naturally on Earth and elsewhere in the universe by radioactive decay of high-energy states of atomic nuclei. This form of ionising radiation is also generated when certain high-energy subatomic particles interact or through interactions of cosmic rays.

High-energy nuclear reactions are used to product gamma rays artificially for medical and research applications.  Electron-positron annihilation, neutral pion decay, fusion, and induced fission can be used to generate gamma rays.

Gamma rays are also generated by certain astronomical events in which very high-energy electrons are produced. Such electrons produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation.

The distinction between X-rays and gamma rays is arbitrary. Originally, the electromagnetic radiation emitted by X-ray tubes had a longer wavelength than the radiation emitted by radioactive nuclei (gamma rays). The older scientific literature distinguished between X-rays and gamma rays on the basis of wavelength. Today, gamma rays are now usually distinguished on the basis of their origin rather than an arbitary wavelength cut off: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus.

More on gamma rays here.

 

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Energy, all at sea

Floating wind turbines could capture the energy of higher wind speeds further out to sea and address some of the noise and unsightliness complained about by those with turbines closer to home.

Wind turbines represent one of the most reliable renewable energy solutions, along with solar power and tidal and hydroelectric power. As wind turbine designs increase their size they also get noisier and become more of an eyesore. The solution is either to site them remotely on dry land or to build them at sea with the tower embedded in the seabed of shallow waters, but this restricts them to near-shore waters with depths no greater than 50 metres, which means they cannot utilise the strong winds further out to sea.

Now, naval architect Dominique Roddier of Berkeley, California-based Marine Innovation & Technology has, together with his team, published a feasibility study of a novel platform design – WindFloat – that, as the name suggests uses floating wind turbines. The study is published in the Journal of Renewable and Sustainable Energy this month.

Floating wind turbines could use stronger offshore winds
Floating wind turbines could use stronger offshore winds (Credit: Roddier et al/JRSE/American Institute of Physics)

Roddier and colleagues, Christian Cermelli, Alexia Aubault, and Alla Weinstein, have tested a 1:65 scale model in a wave tank, which shows that a three-legged floating platform, based on existing gas and oil offshore platform designs. The team explains the main issue: “A floater supporting a large payload (wind turbine and nacelle) with large aerodynamic loads high above the water surface challenges basic naval architecture principles due to the raised centre of gravity and large overturning moment,” they say. In other words at first glance such a rig would capsize very easily. However, after several years work, their results show that the current design is stable enough to support a 5-megawatt wind turbine, the largest turbine that currently exists. These mammoth turbines are 70 metres tall and have rotors the size of a football field. Just one, Roddier says, produces enough energy “to support a small town.”

The next step is to continue construction of a prototype with electricity operator Energias de Portugal that will help the developers understand the life-cycle cost of such projects and to refine the economic model. The prototype will be tested in open water by the end of summer 2012, Roddier says. “The WindFloat [design] is envisioned to be located 15-20 km offshore so as to minimize risks/nuisance to the general public, and to mitigate the view impact from the coastline,” the team adds.

Links

J Renewable Sustainable Energy, 2010, 2, 3, 033104
Marine Innovation & Technology

How low can you go?

We’re repeatedly advised to switch off electrical devices, like TVs and DVD players at the mains outlet rather than leaving them in standby mode, to turn to compact fluorescent bulbs and to turn them off when illumination is no longer necessary, to do our laundry at lower temperatures, to run the dishwasher only when it’s full, and to avoid using energy-hungry power showers. All those kilowatts add up to a lot of power wasted if we don’t.

According to a new study into energy use in the UK, by following this advice we might be reducing our carbon footprint a lot more than we thought. Conversely, those who don’t follow the advice might be wasting far more energy than the government thinks and so contributing more to carbon dioxide emissions and so anthropogenic global warming and climate change. Writing in the journal Energy Policy this month, Adam Hawkes, of the Grantham Institute for Climate Change at Imperial College London, has calculated that the figures used by government advisors to estimate the possible carbon dioxide reduction possible might be 60% too low.

Hawkes points out that power stations that supply electricity vary in their carbon dioxide emission rates, depending on the fuel they use: those that burn fossil fuels (coal, gas and oil) have higher emissions than those driven by nuclear power and wind. In general only the fossil fuel power stations are able to respond instantly to changes in electricity demand. He says that the government should keep track of changing carbon emission rates from power stations to ensure that policy decisions for reducing emissions are based on robust scientific evidence.

Hawkes used 60 million data points for electricity production each half-hour period by each power station in Great Britain from 2002 to 2009 and calculated the emissions for each different type of generator by examining government data showing their average annual fuel use. He then calculated emissions rates attributed to a small change in electricity demand from these two data sets.

SPT86-montalto-power-station (Credit: David Bradley)
Montalto power station (Credit: David Bradley)

His new study suggests that excluding power stations with low carbon emission rates, such as wind and nuclear power stations, and focusing on those that deal with fluctuating demand would give a more accurate emission figure. Hawkes’ calculations show that, 0.43 kilograms of carbon dioxide per kilowatt hour of electricity consumed is 60 percent lower than the actual rates observed between 2002 and 2009 (0.69 kilograms of carbon dioxide per kilowatt hour), meaning that policy studies are underestimating the impact of people reducing their electricity use.

“One way governments are trying to mitigate the effects of climate change is to encourage people to reduce their energy consumption and change the types of technologies they use in their homes,” Hawkes says. “However, the UK government currently informs its policy decisions based on an estimate that, according to my research, is lower than it should be.”

Links

Energy Policy, 2010, online