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VL 12 - Nuclear Decay & Half-Life Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles (ionizing

radiation). There are many different types of radioactive decay. A decay, or loss of energy, results when an atom with one type of

nucleus, called the parent radionuclide, transforms to an atom with a nucleus in a different state, or to a different nucleus containing

different numbers of protons and neutrons. Either of these products is named the daughter nuclide. In some decays the parent and

daughter are different chemical elements, and thus the decay process results in nuclear transmutation (creation of an atom of a new

element). As for types of radioactive radiation, it was found that an electric or magnetic field could split such emissions into three types

of beams. The rays were given the alphabetic names alpha, beta, and gamma, in order of their ability to penetrate matter. While alpha

decay was seen only in heavier elements (atomic number 52, tellurium, and greater), the other two types of decay were seen in all of the

elements. Spontaneous decay is evident in elements of atomic number ninety or greater. In analyzing the nature of the decay products,

it was obvious from the direction of electromagnetic forces induced upon the radiations by external magnetic and electric fields that alpha particles carried a positive charge, beta particles carried a negative charge, and gamma rays were neutral. From the magnitude of

deflection, it was clear that alpha particles were much more massive than beta particles. Passing alpha particles through a very thin glass

window and trapping them in a discharge tube allowed researchers to study the emission spectrum of the resulting gas, and ultimately

prove that alpha particles are helium nuclei. Other experiments showed the similarity between classical beta radiation and cathode rays:

They are both streams of electrons. Likewise gamma radiation and X-rays were found to be similar high-energy electromagnetic

radiation. Radioactivity is one very frequent example of exponential decay. The law describes the statistical behavior of a large number

of nuclides, rather than individual ones. Given a sample of a particular radionuclide, the half-life is the time taken for half the of the

parent nuclei to decay to daughter particles.

Before the famous Rutherford hypothesis and subsequent experiments from 1909-1911 the “plum pudding

model’ of the atom was widely assumed. In this experiment Alpha particles from a radioactive source were

allowed to strike a thin gold foil. Alpha particles produce a tiny, but visible flash of light when they strike a

fluorescent screen. Surprisingly, alpha particles were found at large deflection angles and some were even

found to be back-scattered.

This experiment showed that the positive matter in atoms was concentrated in an incredibly small volume and

gave birth to the idea of the nuclear atom. In so doing, it represented one of the great turning points in our

understanding of nature. If the gold foil were 1 micrometer thick, then using the diameter of the gold atom from

the periodic table suggests that the foil is about 2800 atoms thick.

Please navigate to the PHET: https://phet.colorado.edu/sims/html/rutherford-scattering/latest/rutherford-

scattering_en.html You can consider the Rutherford atom to understand the description above better.

Now go to the “Plum Pudding Model” to consider the following question: If you are given three cookies: One

that is an alpha source, a second that is a beta source, and finally the third that is a gamma source and you are

forced to decide under threat of your life which cookie to eat, which cookie to put in a lead box, and which cookie

to put in your back pocket… explain your choices! Hint: consider the ability of each of the sources to emit particles

that can penetrate various tissues and materials as well as to harm your tissues if they interact with them.