Re: what's least likey to cause gen. mutation: x rays or sunlight?

You have asked a very good question. However, I’m not sure exactly what you mean by "gen. mutation". I’ll assume you mean general genetic mutations. With this in mind, the answer is sunlight. The reason is very straight forward; sunlight is comprised of a broad range of electromagnetic radiation (which includes x-rays) and high energy particles, which do more damage to the cellular and genetic makeup of an individual than x-rays alone.

The energy spectrum of x-ray emitting equipment or other sources is often a small range of electromagnetic energies. X-rays are defined as radiation emitted from atoms or molecules as excited electrons fall into a stable energy level (non-excited) state. The range of x-ray wavelengths are typically 0.01 to 10 nanometers and are 100 eV to 100 keV in photon energy.

Sunlight's broad spectrum of electromagnetic radiation covers visible, ultraviolet, and infrared light. The wavelengths span from a few picometers up to a kilometer and energy ranges of a few eV to thousands of MeV. For a detailed discussion on the electromagnetic radiation go to:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html

In addition, the sun and other cosmic bodies produce high energy particles. These can range in size from electrons, protons, and neutrons to much large particles bordering on the size of small molecules. Most of these particles are screened out by our atmosphere. However, some still penetrate and can create genetic damage. When the combination of all the different components in sunlight are compared to only one of it’s parts, the sum will create more damage than the single part.

I’ve included several different articles to help further explain x-rays, electromagnetic radiation, sunlight, cellular and genetic mutation, and sunlight related cancers. There is one fact to always keep in mind about genetic mutations; while most are considered dangerous, some mutations are medical miracles and even a break through. I’ve included a few articles with this idea in mind under the Cellular/Genetic Mutation section.

As a side note, more and more environmental causes are being identified as creating genetic and cellular mutations. This may be a topic you may also want to research.

Best of luck with your research and this will answer some of your questions.

X-ray and Electromagnetic Radiation Articles
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html
http://ls7pm3.gsfc.nasa.gov/whatsRM/em.html
http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/
http://csep10.phys.utk.edu/astr162/lect/light/spectrum.html
http://www.pbs.org/wgbh/nova/gamma/spectrum.html

Sunlight Articles
http://science.nasa.gov/headlines/y2000/ast14jul_2m.htm
http://helios.gsfc.nasa.gov/solaract.html
http://helios.gsfc.nasa.gov/cosmic.html

Cellular/Genetic Mutation Articles
http://www-personal.ksu.edu/~bethmont/mutdes.html
http://www.sciam.com/article.cfm?chanID=sa003&articleID=0003CF8E-7A59-1452- BA5983414B7F0000
http://www.thetech.org/genetics/art04_bad.php
http://www.sciencedaily.com/releases/2003/11/031120074728.htm
http://www.msnbc.msn.com/id/5278028

Cancer Articles (sunlight related)
http://homepage.mac.com/herinst/sbeder/HoleStory/intro/intro1.html
http://www.emedicine.com/oph/byname/basal-cell-carcinoma-eyelid.htm