Helium diffusion is one type of nuclear decay dealing with the emission of Helium nuclei known as an alpha emission. Elements like uranium and thorium produce helium in zircons as a biproduct of their radioactivity. This helium seeps out of zircons quickly over a wide range of temperatures. If the zircons really are about 1.5 billion years old (the age which conventional dating gives assuming a constant decay rate), almost all of the helium should have dissipated from the zircons long ago. But there is a significant amount of helium still inside the zircons, showing their ages to be 6000 +/- 2000 years. Accelerated decay must have produced a billion years worth of helium in that short amount of time.
A Helium Diffusion Model Proposed
As early as the 1970s, Robert Gentry pointed out that rocks appeared to be retaining more Helium than they should using standard models. Russell Humphreys, Steven Austin, Andrew Snelling, and John Baumgardner, the first three professors at the Institute for Creation Research and the last, a scientist at Los Alamos National Laboratory, conceived a model using what was known regarding helium diffusion in zircons. Their report  suggested that the amount of helium in the zircon was well over what would be observed in an old universe, citing a 5 order of magnitude error. Following the scientific method, they used their model to predict a collection of diffusion rates at hitherto untested temperatures. Humphreys claims these predictions were validated in a separate experiment reported in his 2004 report .
Kevin Henke, an instructor at the University of Kentucky, spent 25000 words challenging these results.  Humphreys responded in a 2005 report.  It should be noted that Henke's attack was on the 2003 model, not the putatively correct predictions found after the original report.
Helium in the Atmosphere
There is also insufficient mass of helium in earth's atmosphere to account for 4.6 billion years of radioactive decay. Helium is a noble gas which doesn't combine with any other element, but there is not enough of it to account for the radioactive decay which should have occurred in an old earth scenario. Malcolm Davis gives a careful treatment of the mathematics involved in calculating the rate at which helium escapes, and notes that the rate at which Helium is being produced is thirty times higher than its rate of escape. If the earth were very old, much more Helium should have accumulated in the atmosphere. Malcolm mentions some of the processes scientists have offered to explain the difference such as periods of higher than average temperature, but notes these have not been observed.
"Also, the rate of helium buildup would be slower now than in the past, because the radioactive sources have decayed. This would put an even lower upper limit on the age of the earth.
The only way around this problem is to assume that the helium is escaping into space. But for this to happen, the helium atoms must be moving fast enough to escape the earth’s gravity (i.e., above the escape velocity). Collisions between atoms slow them down, but above a critical height (the exobase) of about 500 kilometres (300 miles) above the earth, collisions are very rare. Atoms crossing this height have a chance of escaping if they are moving fast enough—at least 10.75 kilometres per second (24,200 miles per hour).3 Note that although helium in a balloon will float, helium when unenclosed will just mix evenly with all the other gases, as per normal gas behaviour."
Uniformitarians suggest models for which helium can escape due to the solar wind.
- Helium diffusion rates support accelerated nuclear decay by D. Russell Humphreys, Ph.D., Steven A. Austin, Ph.D., John R. Baumgardner, Ph.D., and Andrew A. Snelling, Ph.D. Proceedings of the Fifth International Conference on Creationism, R. L. Ivey (Ed.), pp. 175–195, 2003.
- Helium diffusion age of 6,000 years supports accelerated nuclear decay by Russell Humphreys, Steven Austin, John Baumgardner, and Andrew Snelling. 2004. Creation Research Society Quarterly 41(1): 1-16.
- Helium Escape Problem by Larry Vardiman. ICR IMPACT No. 143 May 1985
- RATE Group Confirms Fast Diffusion of Helium in Rocks by Larry Vardiman, RATE group chairman
- Helium Evidence for A Young World Remains Crystal-Clear by Russell Humphreys, Ph.D.
- Helium Diffusion Experiments Give Nuclear Evidence for a Young World Powerpoint presentation by Russell Humphreys
- Helium in the Earth’s Atmosphere by David Malcolm. TJ. August, 1994.
- Blowing old-earth belief away by Jonathan Sarfati. Creation Magazine, June 1998.
- Helium Diffusion Rates Support Accelerated Nuclear Decay
- Radioactive Crystals Make and Lose Helium