Nuclear physics

What is Radioactive Decay?

Nuclei are made up of protons and neutrons, which are held together by the so-called strong force. Some nuclei have a combination of protons and neutrons that do not lead to a stable configuration. These nuclei are unstable or radioactive. Unstable nuclei tend to approach the stable configuration by emitting certain particles. The types of radioactive decay are classified according to the class of particles emitted.
Alpha decay: The radioactive element of atomic number Z, emits a nucleus of Helium (two protons and two neutrons), the atomic number decreases by two units and the mass number by four units, producing a new element located in place Z-2 of the Periodic Table.
Beta-decay : The nucleus of the radioactive element emits an electron, consequently its atomic number increases by one unit, but the mass number is not altered. The new element produced is located at Z + 1 on the Periodic Table.
Gamma decay: The nucleus of the radioactive element emits a high-energy photon, the mass and the atomic number do not change, only a readjustment of the energy levels occupied by the nucleons occurs.
The interactive program model describes a radioactive substance A disintegrating a stable substance B . N radioactive nuclei of unstable substance A are arranged. The decay constant l is entered. As time passes, the number of nuclei that remain undisintegrated is noted. Subsequently, the decreasing exponential law will be verified from the data taken.
From the observation of the disintegration process we can extract the following qualitative relationships:

  • The decay rate decreases as the radioactive nuclei decay.
  • We cannot predict at what moment a specific nucleus will disintegrate, nor what nucleus will disintegrate at a certain moment.

See Also: Properties of Types of radiations
Explanation:

“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 it is 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

                                          radioactive decay

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:

1/2R0=R0e-λt/2

Which leads readily to

t1/2=ln2/λ   ————(4)

a relationship between the half-life and the disintegration constant.

Types of Radioactive Decay

What is Alpha decay?

The radionuclide 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, the 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:

32 → S32 + e +v      ——–(6)       (t 1/2=13.3d)

and

Cu64   →Ni64 +e++v ————-(7)          (t1/2=12.7h)

The symbols v and v  represent the neutrino and its antiparticle, the antineutrino, 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)

or

p → n +e++ v                         (β+decay)

These are the basic beta decay process.

Radioactive decay (video)

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2 Comments

  1. Radioactive decay rates are normally stated in terms of their half-lives , and the half-life of a given nuclear species is related to its radiation risk . The different types of radioactivity lead to different decay paths which transmute the nuclei into other chemical elements. Examining the amounts of the decay products makes possible radioactive dating .

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