What is radioactive decay?
“Radioactive decay is the process by which the nucleus of an unstable atom loses energy by emitting radiation, including alpha particles, beta particles, gamma rays and conversion electrons.”
Most of the nuclides that have been identified are radioactive.That is,they spontaneously emit a particle,transforming themselves in the process into a different nuclide . In this section we discuss the two most common situations,the emission of an α particle (alpha decay) and the emission of an electron (beta decay).
No matter what the nature of the decay,its main feature is that its statistical.Consider,for example,a 1 -mg sample of uranium metal.It contains 2.5 ×1018 atoms of the very long lived alpha emitter U238.The nuclei of these atoms have existed without decaying since they were created in the explosion of a supernova.During any given second about 12 of the nuclei in our sample will decay,emitting an α particle in the process.We have absolutely no way of predicting ,however,whether any given nucleus in the sample will be among those that do so.Every single U238 nucleus has exactly the same probability as any other to decay during any 1-s observation period ,namely,12 /(2.5 ×1018),or one chance in 2×107.
In general,if a sample contains N radioactive nuclei,we can express the statistical character of the decay process by saying that the ratio of the decay rate R(=-dN/dt) to the number of nuclei in the sample is equal to a constant,or
Here N0 is the number of radioactive nuclei in the sample at t=0.We see that the decrease of N with time follows a simple exponential law.
We often more interested in the activity or decay rate R(=-dN/dt) of the sample than we are in N.Differentiating equation (2) yields:
R =R0e-λt ———(3)
In which R0(=λN0) is the decay rate ,at t=0.Note also that R=λN at any time t.We assumed initially that the ratio of R to N is constant,so we are not surprised to confirm that they both decrease with time according to the same exponential law.
A quantity of interest is the time t1/2 ,called the half life after which both N and R are reduced to one half of their initial values.Putting R=1/2 R0 in equation (3) we get:
Which leads readily to
a relationship between the half life and the disintegration constant.
What is Alpha decay?
The radio nuclide U238 a typical alpha emitter,decays spontaneously according to the scheme.
U238 →Th234+He4 ———-(5)
With a half life of 4.47 ×109y.In every such decay,an energy of 4.27 MeV is emitted appearing as kinetic energy shared between the α particle (He4) and the recoiling residual nucleus (Th234).
We now ask ourselves:”If energy is released in every such decay event,why did the U238 nuclei not decay shortly after they were created?”The creation process is believed to have occurred in the violent explosions of ancestral stars,predating the formation of our solar system.Why did these nuclei wait so very long before getting rid of their excess energy by emitting an α Particle?To answer this question,we must study the detailed mechanism of alpha decay.
What is beta decay?
A nucleus that decays spontaneously by emitting an electron is said to undergo beta decay.We give two examples here:
P 32 → S32 + e– +v ——–(6) (t 1/2=13.3d)
Cu64 →Ni64 +e++v– ————-(7) (t1/2=12.7h)
The symbols v and v– represent the neutrino and its antiparticle ,the anti neutrino,neutral particles that are emitted from the nucleus along with the electron or positron during the decay process.Neutrinos interact only very weakly with matter and for that reason are so extremely difficult to detect that ,for many years ,their presence went unnoticed.
So,it with the electrons and the neutrons emitted from nuclei during beta decay.They are both created during the emission process,a neutron transforming itself into a proton within the nucleus,according to:
n→p +e– +v¯ (β– decay) ——(8)
p → n +e++ v (β+decay)
These are the basic beta decay process.