Does the background radiation change during an eclipse?
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Does the background radiation change during an eclipse? |
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Here is a proposal for a co-operative experiment amongst working teachers and senior students. The effect we are looking for may be very small so we need lots of help! As the eclipse occurs on 11th August 1999, your school may not be open but you could always take some equipment home. This experiment can be done even if it is raining on the day!!
When the Moon cuts out the Sunlight it might also affect the Solar Wind particles. These fast particles give rise to a small portion of the background radiation at the Earth's surface.
On the day of the eclipse, set up your data logging equipment to record the background radiation every ten minutes. Start at 9 am and go on to 1 pm BST. Note the time of eclipse maximum and the latitude and longitude of your location. You can do this experiment even if you are not on the path of totality and it does not matter if it is cloudy or raining! If you don't have a data logger, you can record the results with pencil and paper. Send your data to my address below. I will arrange to average the data and will report back to you.
Data logging notes
There are two types of radioactivity data logger;
The background reading using a school GM tube is about 30 counts per minute. With such a low count rate the statistical fluctuations are a problem. Follow the guidelines below to minimise the effect.
Solar physics notes
The Sun continuously ejects fast moving protons, electrons and helium nuclei. For the quite Sun, the particles are ejected at about 500 km s-1. It takes them about 83 hours to reach the Earth. The Moon is 389 times nearer than the Sun, so the time of flight from the Moon to the Earth is about 13 minutes. So if the Moon makes a measurable effect on the background radiation, it might be expected to peak about 13 minutes after the maximum of the eclipse.
Now some words of caution.
Is the Moon charged?
It seems reasonable to assume that the Solar Wind contains a balance of protons, helium nuclei and electrons. The Moon is bathed in this net-neutral stream, so one possibility is that it is also neutral. In which case it will just act as an absorber.
However, it is very difficult to think of an experiment to test for neutrality. If it is charged, it will act as an electrostatic lens. If you have ever used a Maltese Cross tube without earthing the cross, you will have seen the blurred shadow that results - try it!!
Why might the Moon be charged? Well it is bathed in UV and soft X-rays from the Sun and so the surface will be subjected to photo-ionisation. Some of the incoming photons will have energies of hundreds of electron volts. When the surface has charged up to a few hundreds of volts, even the most energetic electrons will not be able to escape. To illustrate this point, put a piece of oxide-free zinc on a gold leaf electroscope. Charge the electroscope positively and then shine UV light onto the zinc. Can you predict what happens to the leaf? The answer is nothing!! Electrons are ejected from the surface but are immediately pulled back as a typical electroscope is charged to many thousands of volts. Charge the electroscope negatively and the UV immediately discharges it; so does sunlight on a summer's day (but not in the winter!).
If we detect a change in the background radiation it might give a clue as to the charge state of the Moon.
Do Solar Wind particles reach the Earth's surface?
The particles are charged and so they interact with the Earth's magnetic field. They spiral along the field lines and are directed towards the magnetic poles. If they reach the top of the atmosphere they excite the atoms there and an auroral display occurs. Otherwise they are trapped in the radiation belts that encircle the Earth out to about ten Earth radii.
Some particles do reach the top of the atmosphere where they strike atomic nuclei. The fragments move downwards, creating yet more fragments until they reach the ground. This is called an Air Shower.
About 13% of the background radiation at the surface of the Earth is due to Cosmic Radiation in general. This is the result of particles from the Sun but also of very energetic protons and heavier nuclei (TeV per atomic mass unit) from outside our Solar System. The Sun's contribution is probably quite small at the surface of the Earth but is important for astronauts and satellites not protected by the Earth's magnetic field.
A strong solar flare can produce a massive barrage of protons. These Solar Proton Events can produce dose rates in space of 0.1 to 1 Sv/hr. As they last many hours these events would be fatal to an unprotected astronaut. It was just good fortune that the missions to the Moon were not affected. Astronauts in low Earth orbit are largely protected by the Earth's magnetic field. Even so, a typical 10 day Space Shuttle mission exposes the astronauts to a dose of about 1 mSv. This is about one third of the annual dose at ground level in the UK.
In her recent Mir space mission, the US astronaut Shannon Lucid received the equivalent of 8 chest x-rays per day. At 0.02 mSv per x-ray this amounts to a dose of 28.8 mSv; the equivalent of 11 year's ground-level background exposure!
Acknowledgements:
I am indebted to Mr Anthony Hopwood, who's articles about the Sun's effect on the ionosphere (Electronics and Wireless World, December 1989 and March 1990) set me thinking about this project.
I am expecting help from the UK Radiation Incident Monitoring Network (RIMNET). This is an array of 92 gamma-ray sensitive GM counters, located at weather stations and airports throughout the country. Every hour a computer system in London calls the outstations and takes their count. The system was put in place after the Chernobyl accident and is set to sound alarms if a cloud of radioactive material drifts over the country. The Department of the Environment has kindly agreed to supply a data set for the day of the eclipse.
Where to send your data
Mr Alan C Pickwick
19 Edale Grove,
Sale,
Cheshire,
M33 4RG,
UK.
(I teach at Manchester Grammar School and am deeply involved in astronomy education as a way to attract young people into physics.)
You can phone me at home: +44 (0)161 973 6796 You can email me at home.
All participants will receive a copy of the final report but make sure you tell me:
Your full name, address with postcode and phone number. Your email address and fax number. The latitude, longitude, postcode and place name of your observation site. The time of eclipse maximum (it does not need to be total at your site).
Please do this experiment even if it is raining!! Good luck and clear skies.