Can the decay half-life of a radioactive material be changed?
Category: Physics Published: April 27, 2015
Yes, the decay half-life of a radioactive material can be changed. Radioactive decay happens when an unstable atomic nucleus spontaneously changes to a lower-energy state and spits out a bit of radiation. This process changes the atom to a different element or a different isotope. Since radioactive decay is a spontaneous event, you may think that the half-life of the decay process is completely fixed and cannot be altered by outside influences. However, this statement is not completely true.
First of all, it is worth pointing out that the time when an individual radioactive atom decays is completely random. It is impossible to predict when an individual radioactive atom will decay. The half-life of a certain type of atom does not describe the exact amount of time that every single atom experiences before decaying. Rather, the half-life describes the average amount of time it takes for a large group of amounts to reach the point where half of the atoms have decayed.
The half-life of a radioactive material can be changed using time dilation effects. According to relativity, time itself can be slowed down. Everything that experiences time can therefore be given a longer effective lifetime if time is dilated. This can be done in two ways. Traveling at a speed close to the speed of light causes time to slow down significantly, relative to the stationary observer. For instance, a number of radioactive atoms shot through a tube at high speed in the lab will have their half-life lengthened relative to the lab because of time dilation. This effect has been verified many times using particle accelerators. Time can also be dilated by applying a very strong gravitational field. For instance, placing a bunch of radioactive atoms near a black hole will also extend their half-life relative to the distant observer because of time dilation.
The half-life of radioactive decay can also be altered by changing the state of the electrons surrounding the nucleus. In a type of radioactive decay called "electron capture", the nucleus absorbs one of the atom's electrons and combines it with a proton to make a neutron and a neutrino. The more the wavefunctions of the atom's electrons overlap with the nucleus, the more able the nucleus is to capture an electron. Therefore, the half-life of an electron-capture radioactive decay mode depends slightly on what state the atom's electrons are in. By exciting or deforming the atom's electrons into states that overlap less with the nucleus, the half-life can be reduced. Since the chemical bonding between atoms involves the deformation of atomic electron wavefunctions, the radioactive half-life of an atom can depend on how it is bonded to other atoms. Simply by changing the neighboring atoms that are bonded to a radioactive isotope, we can change its half-life. However, the change in half-life accomplished in this way is typically small. For instance, a study performed by B. Wang et al and published in the European Physical Journal A was able to measure that the electron capture half-life of beryllium-7 was made 0.9% longer by surrounding the beryllium atoms with palladium atoms.
In addition to altering the chemical bonds, the half-life can be altered by simply removing electrons from the atom. In the extreme limit of this approach, all of the electrons can be ripped off of a radioactive atom. For such an ion, there are no longer any electrons available to capture, and therefore the half-life of the electron capture radioactive decay mode becomes infinite. Certain radioactive isotopes that can only decay via the electron capture mode (such as rubidium-83) can be made to never decay by ripping off all the electrons. Other types of radioactive decay besides electron capture have also been found to have the decay half-life depend on the state of the surrounding electrons, but the effects are smaller. The change in half-life due to changing the electron environment is generally very small, typically much less than 1%.
Lastly, the half-life of a radioactive material can be changed by bombarding it with high-energy radiation. This should not come as a surprise since radioactive decay is a nuclear reaction, and inducing other nuclear reactions at the same time as the decay can interfere with it. However, at this point, you don't really have stand-alone radioactive decay. Rather, you have nuclear reaction soup, so this approach may not really count as "changing the half-life".
When reference books list values for the half-life of various materials, they are really listing the half-life for the material when its atoms are at rest, in the ground state, and in a particular chemical bonding configuration. Note that most changes to the half-life of radioactive materials are very small. Furthermore, large changes to a half-life require elaborate, expensive, high-energy equipment (e.g. particle accelerators, nuclear reactors, ion traps). Therefore, outside of specialized labs, we can say that as a good approximation radioactive decay half-lives don't change. For instance, carbon dating and geological radiometric dating are so accurate because decay half-lives in nature are so close to constant.