Radioactive Decay Processes, Chemistry tutorial

Introduction:

Radioactive decay is the procedure through which the atomic nucleus of an unstable atom loses energy via emitting ionizing particles or electromagnetic radiation. The emission is spontaneous, in that the atom decays devoid of any physical interaction by the other particle from outside the atom. Generally, radioactive decay occurs due to a procedure confined to the nucleus of the unstable atom. Most of the nuclei are radioactive. This signifies that they are unstable, and will ultimately decay via emitting a particle, transforming the nucleus into other nucleus, or to a lower energy state. A chain of decays occurs till a stable nucleus is reached.

Radioactive Decay:

Radioactive decay can only be stated as a spontaneous nuclear transmutation or transformation of unstable nuclei which exist outside in the formation of stable isotope. The decay method is unaffected via temperature, pressure, chemical forms of the elements. The decay or loss of energy, outcomes whenever an atom with one kind of nucleus, termed as the parent radionuclide, converts to an atom having a nucleus in a different state or an entirely dissimilar nucleus, either of which is termed the daughter nuclide. Often the parent and daughter are of various chemical elements. An illustration of this, a carbon-14 atom (that is, the parent) emits radiation (that is, a beta particle and a gamma ray) and converts to a nitrogen-14 atom (the daughter).The daughter nuclide of a decay event might as well be unstable (radioactive). In this condition, it will as well decay, producing radiation. The resultant second daughter nuclide might as well be radioactive. This can lead to the sequence of some decay events, phenomenon termed as decay chain.  Throughout the radioactive decay, principles of conservation apply. A few of these laws are as follows:

  • Conservation of energy 
  • Conservation of momentum (linear and angular) 
  • Conservation of charge 
  • Conservation of nucleon number. 

The kind of decay which takes place based on the position of the nuclei undergoing the decay and as a result the kind of radiation which accompanies the process. Therefore, the decay method is characterized via the decay period, mode and the energy without regards to either chemical or physical conditions.

Kinetics of Radioactive Decay:

The particles emitted are of various kinetics or kinetic energies.  All radioactive decays follow first-order kinetics thus rate of decay at time (t) = λN

Here, 'λ' is the first order rate constant and 'N' is the number of nuclei

Note:  'N' at time zero is (No) and at time t is 'Nt'

Rate of decay = K (N) as 

Ln(No/N) = aλt

It is as well noted that each atom decay independently of the other, thus stoichiometrically a = 1

Thus, Ln(No/N) = λt

In nuclear chemistry, decay rate is generally deduced in terms of half life of the process. That is, the amount of time needed for half of the original sample to react.

For first order process:

t1/2k = ln2/k = 0.693/k

Decay Mode and Energy:

The Radioactive decay comprises a transition from a definite quantum state of original nuclide to a definite quantum state of product nuclide. The energy difference between the two quanta levels comprised in the transition corresponds to what is termed as decay energy. As for kinds of radioactive radiation emitted, it is found that an electric or magnetic field can split such emissions into three kinds of beams or sub-atomic particles. They are alpha, beta and gamma. While alpha decay is seen only in heavier elements (that is, atomic number 52, tellurium and above), the other two kinds of decay are seen in all of the elements. In analyzing the nature of the decay products, it is apparent from the direction of electromagnetic forces produced on the radiations via external magnetic and electric fields which alpha rays carried a positive charge, beta rays taken a negative charge, while gamma rays were neutral. From the magnitude of deflection, it is illustrated that alpha particles are much huger than beta particles. Passing alpha particles via a very thin glass window and trapping them in a discharge tube allowed researchers to study the emission spectrum of the resultant gas and eventually confirm that alpha particles are helium nuclei. Other experiments exhibited the similarity between the classical beta radiation and cathode rays: They are both streams of electrons. Similarly, gamma radiation and X-rays are found to be identical high-energy electromagnetic radiation. However alpha, beta and gamma were found most generally, there are other kinds of decay which are eventually discovered. Shortly after the discovery of the positron in cosmic ray products, it was realized that the similar process which operates in classical beta decay can as well produce positrons (that is, positron emission). In an analogous procedure, rather than emitting positrons and neutrino, some of the proton-rich nuclides were found to capture their own atomic electrons (that is, electron capture), and emit merely a neutrino (and generally as well as gamma ray). Each of such kinds of decay comprises the capture or emission of nuclear electrons or positrons, and acts to move a nucleus in the direction of the ratio of neutrons to protons which have the least energy for a given net number of nucleons (that is, neutrons plus protons).

The mode of decay is based on the specific kind of nuclear comprised in the reaction (that is, position of unstable nuclei). It is significant to keep in mind that in radioactive decay, there are numbers of conservation laws which should be valid for a true decay to take place.

Consider the reaction:

X1 + X2    X3 + X4 ---------- y

Here, 'X' symbolizes nucleus or elementary particles. X1 and X2 might be unstable nucleus and bombarding particles whereas X3 and X4 are products formed. Therefore for this general reaction (y) the following number of conservation law holds.

a) The net energy of the system should be constant 

E1 + E2 = E3 + E4

Here, 'E' comprises all forms of energy (that is, kinetic and electrostatic energy)

b)  The linear momentum should be constant 

P = MV

P1 + P2 = P3 + P4

It will be noted that Ekin = P2/zm

Here, Ekin is the kinetic energy

c) The net charge (proton + electron) of the system should be constant

Z1 + Z2 = Z3 + Z4

d) The mass number of the system should be constant

A1 + A2 = A3 + A4

MA = zmMH + NMn = 2.016 u

e)  The angular momentum PI of the system should be conserved 

(PI)1 + (PI)2 = (PI)3 + (PI)4

Alpha Decay (α):

An unstable nucleus experiences alpha decay via emitting alpha particles.

Illustration:

23892U → 3490Thi + 42He

The Alpha particle cause extensive ionization of matter. Alpha particles interact with matter that might as well cause molecular excitation thus resultant in fluorescence.

Alpha decay is noticed for elements heavier than lead (Pb) and for some nuclear that are as light as Lanthanide (Ln). The decay energy can be computed from known atomic mass since binding energy corresponds to a:

E = 931.5 ΔM

ΔM = mass defect

Here ΔM = (Mz-2 + MHe - Mz)

That is,

Change in mass = mass reactant - mass product

Beta Decay β:

β decay is the spontaneous disintegration throughout which beta particles are emitted or electrons captured. The Radioactive beta decay is depicted as β decay, it could be in any of the three forms; as electron emission β- or 0-1e, as position β+ or 0+1e or and as election capture (EC).

a) Electron emission β- or 0-1e:

Illustration: 

146Co → 147N + 0-1e

The Energetic electrons cause ionization and molecular excitation in matter, however the effect is weaker and harder to detect than alpha particle. Therefore, there is a requirement to amplify the effect for counting of individual beta particles. 

Illustration:    

137Cs → 137Ba + β-

b) Electron capture (EC):

The EC decay method is represented as AZX → (EC) → AZ+X + v

The captured electron comes out from one of the inner orbital of the atom. Based on the electron shell from which the electron originates, the procedure is at times termed to as K-capture or L-capture electron. The probability of capture of electron in higher shell reduces by quantum number of shell. Thus, the probable capture of e-from K-shell is far greater than capture of e- from L shell

The computation of decay energy in electron capture is represented as:

QEC = -931.5 (MZ-1 - MZ)

c) Positron decay β+ or 0+1e: In positron emission, a proton in an unstable nucleus is transformed to a neutron and a positron.  The neutron remain in the nucleus consists positron is emitted.

Illustration:

2413Al → 2412Mg + 0+1e

Gamma Ray Emission (00γ):

The emission of gamma rays is for all time in company of emission of other particles. It is the emission that takes place where transition between energy levels of similar nucleus occurs.  Gamma rays are high energy radiation, emitted whenever an unstable nucleus experiences a rearrangement of as constituent particles give more stable, lower energy nucleus gamma rays are frequently emitted all along with other kind of particles

Illustrations:

99m43Tc → 9943Tc + 00γ

99m43Tc + 9943Tc → 9943Tc + 00γ

It will be noted that, as Tc is in unstable form; it quickly decays to emit γ ray and becomes stable. mTc is the metastable form of Tc. Note as well that pure gamma emitters are rare, however instead, the radiation accompanies either on alpha or beta radiation.

Chain Reaction:

A chemical reaction in which most of the molecules undergo chemical reaction after one molecule becomes activated. This is a continuous procedure in which either splitting of bigger molecules takes place to produce daughter nuclei and neutron or joining of smaller molecules take place to form big or a new parent molecule, the chemical methods termed as fission and fusion correspondingly.

Nuclear Fission:

The Isotopes of unstable nuclei having atomic number greater than 80 are capable of undergoing a nuclear reaction termed as nuclear fission, in which they split to nuclei of intermediate masses and emit one or more neutrons. The energy produced is termed as atomic energy. Some of the fission reactions are spontaneous whereas some are not spontaneous; therefore, the non-spontaneous need activation energy from bombardment. A given nucleus is split in many ways discharging enormous energy a typical illustration is illustrated below.

1540_Example of Nuclear Fission.jpg

Fig: Example of Nuclear Fission

The 236U is an intermediate nucleon and is short lived producing fragment as illustrated above. Particles which can supply the requisite activation energy comprise neutrons, protons, alpha particles and fast electrons.

Experiment exhibits that whenever comparing the mass of original or starting materials with that of product, there is a small reduction. This missing mass has been transformed to energy and is derived via Einstein equation:

E = mc2

Here 'E' is the energy liberated, 'm' is the loss mass and 'c' is the speed of light.

Nuclear Fusion:

This is coming altogether of small nuclei to form heavy nucleus. Though, the fusion reaction needs a temperature of around 1,000,000,000oC to overcome the repulsion of Hydrogen nucleus, subsequent to which they are forced to experience fusion. Spectroscopic evidence exhibits that sun is a tremendous fusing reactor comprising of 73% H, 26% He and 1% other element. This is a main reactor which comprises fusing of deuterium, 21H and tritium 31H at high temperature.

21H 31H → 42He + 10n + energy

Nuclear Fusion Reactor:

In a fusion reactor, fusion reaction is controlled via injecting materials which absorb some of the neutrons so as to prevent the explosion. Therefore, the energy produced can be productively transformed to heat source in a power plant.

There are different kind of nuclear reactors, these comprise: 

  • Light water reactor
  • Breeder reactor

Nature of Radiation:

However different electromagnetic rays and sub-atomic particles comprised in radioactivity is still essential to reveal more regarding the nature of such radiations and their properties. Such radiations comprise; alpha particles, beta particles, gamma rays, proton, neutron and positrons.

The penetrating capacities of rays and particles are proportional to their energies. Particles such as positions are around 100 times more penetrating than the heavier ones, such as alpha particles. Beta particles can be stopped by a (1/3) inch trek (0.3cm) aluminum sheet. Beta particles can pierce a skin however can't touch internal organ.

Alpha particles encompass low penetrating ability, therefore, can't damage or penetrate the skin although; they can damage internal tissue if inhaled. The high energy gamine rays encompass great penetrating power as severely damage both skin as internal organ. They travel at a tie speed of high and can just be stopped by timer layers of concrete or lead.

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