Radioactivity Flashcards
(26 cards)
What’s radioactivity?
👉🏾 It is the spontaneous emission of (ionizing) radiation (alpha-particles, beta particles and/or gama-ray);
👉🏾 Causes:
* It might be as a result of a nuclear reaction
* Can also be caused by the breakdown of an unstable nucleus
* Also caused by energy transitions the may occur in an atom.
What’s an unstable nucleus?
- An unstable nucleus is one that’s not in a balanced state
- And that’s usually when it’s contents (protons and neutrons) are not present in their right proportion
- If the proton-to-neutron ration is getting more than 1, the nucleus tends to become unstable
What are the types of radiation that may be emitted during radioactivity
👉🏾There are three types of radiation that can be emitted:
* Alpha-particle, λ-particles,
* Beta-particle, β-particles,
* Gama-ray,. γ-ray.
👉🏾NOTE; During the nuclear reaction, all, some or just one of the radiations could be emitted.
👉🏾It should also be noted that radiations ionize the gases that they pass through
Who are the guys that contributed to the discovery of radioactivity till date
👉🏾 Henri Becquerel
*In 1896,
*He accidentally discovered radioactivity while working with phosphorescent and uranium.
*He noted that these materials still emit radiation even when cut off from external source of light.
*His discovery paved way for future research by other scientists who later researched radioactivity.
👉🏾Marie Curie (wife) and Pierre Curie (husband)
*They were the one that discovered polonium and radium from the uranium ore
*The Curie couples coined the term “radioactivity”
*They developed methods to measure amounts of radiation
*Marie Curie’s work with radium paved the way for the application of radioactivity in medicine
*All these contribution led them to receive the physics Nobel prize in 1903,
*Marie Curie, being the first woman to won a Nobel prize also won it again in the year 1911 in chemistry for her discovery or polonium and radium
The differences between chemical reaction and nuclear reaction
*Nuclear reactions produce new elements, while chemical reactions don’t
*Nuclear reactions deals in all sub-atomic particles while chemical reactions involves the exchange of electrons between atoms
*Rate of chemical reactions are affected by external factors while the rate of nuclear reactions are not
*Nuclear reactions involve the exchange of large amount of heat while chemical reactions involves the little exchange of heat
Alpha-particle (λ-particle)
*It has the mass of 4 amu
*And a charge of +2e, hence easily deflected in electric and magnetic fields ;
*This led to it being denoted as helium nucleus (⁴₂He) and defined as stream of extremely fast moving helium nuclei
*It has the least penetrating power among the three types of radiation due to its largest size
*It also has the shortest range amongst them
*When the nuclide of radium-226 loses/emits an alpha particle the nuclide of radon-222 is firmed, it could be written as: ²²⁶₈₈Ra⇒²²²₈₆Rn + ⁴₂He
Beta-particle (β-prticle)
*Has a mass of zero (mass of an electron)
*Has a charge of -1 (the charge of an electron),
*These led to it being referred to as electrons, ⁰₋₁e, and defined as stream of fast moving electrons
*It’s penetrative ability is more than that of alpha-particle but still lower than gama-rays’
*It’s range is also longer than that of alpha-particle, but not as long as gama-rays
* It is also easily deflected in electric and magnetic fields
**It is of two types;
*β⁺radiation, and
*β⁻radiation
The β⁻ radiation
*It is when a particle release an electron (β-particle)
*It results in no change in the atomic mass of the particle,
*but an increase of +1 in it’s charge, since the beta particle releases is charged negatively.
*It can also be explained as the neutron splitting to give electron and proton;
¹₀n⇒¹₁p+⁰₋₁e.
*Example is the formation of protactinium (pa) from thorium-234 (Th) as illustrated below:
²³⁴₉₀Th⇒²³⁴₉₁Pa+⁰₋₁e
Beta-positive (β⁺) radiation
*It is the absorption of an electron by a particle,
*This usually led to no change in the mass… *but a decrease in the atomic charge since the electron is negatively charged
*It can also be explained as a proton absorbing an electron to form a neutron (i.e the conversion of proton to neutron)
¹₁p⇒¹₀n-⁰₋₁e or ¹₁p⇒¹₀n+⁰₁e
*An example is the formation or Argon (Ar) nuclei from potassium-38 (K) nuclei
³⁸₁₉K⇒³⁸₁₈Ar + ⁰₁e
Gama (γ) rays
*Has no charge
*Has no mass,
*Which led to it being referred to as rays, and defined as high-energy electromagnetic radiation.
*Its features are quite similar to x-rays just that it has shorter wavelengths,
*Thereby having more penetrative power
*It is the most penetrative among the nuclear radiations,
*It also has the longest range amongst them
*It is not deflected in electric and magnetic fields because it doesn’t have mass or charge
*They are emitted as a result of internal transitions that occurs in an atom
*They are also emitted, accompanying alpha-particles and beta-particles radiations
What’s K-Capture Transformation
*It is another radioactive process by which the nucleus captures electron from outside the nucleus
*It results in no change in the atomic mass but a decrease in the atomic number, since the captured nucleus neutralize the charge on a proton to form neutron
*An electron from a higher energy level would replace the captured electron (and this leads to the emission of x-rays)
*An example is when Radium-133 (Ra)undergo K-Capture transformation to for Caesium-133(Cs)
¹³³₅₆Ra⇒⇒⇒¹³³₅₅Cs+ γ-ray and/or x-ray
Comparisons between K-Capture transformation and beta-positive reaction
*Though they both involve the loss of electrons…
*the beta-positive reactions release the electrons/beta-particles to the environs, while the K-Capture transformations transfer the electron to the nucleus of the particle.
*The K-Capture transformation is accompanied by the release of gamma-ray or x-ray, while the beta-positive reaction release beta-particle
Detection and measurement of Radioactivity
*The activity of radioactive materials is
the radioactivity per unit mass of the material.
*It is measured in counts per minute (cpm) or counts per seconds (cps)
Methods used for the detection and measurement of radioactivities
The methods used for the detection and measurement of radioactivities are:
*Gas ionisation
*Scintillation methods
*Autoradiography, and
*Cloud chamber
Gas ionisation chamber
*The Geiger Muller counter and tube rely on gas ionisation
*The radiation ionises the argon to form the positive argon ions,
*Which move to a cathode and the electrons to an anode.
*This creates an electric pulse, which is then amplified.
*The pulse registers as clicks, light flashes on meter reading.
*The magnitude of the radioactivity is proportional to the magnitude of pulse registered.
*Different types of gas filled chambers used are Geiger Müller (GM) counters, proportional counters and ionization chambers
The scintillation method of radioactivity detection
*Some (fluorescent) materials absorbs radioactive energies and release light energy.
*When these materials absorbs the radioactive energy, they gets excited;
*then these excited atoms returns back to their ground states,
*which is usually through series of internal transformations,
*that are accompanied by the release of light photons.
*Photomultiplier tubes or photodiodes are used to detect and amplify these photons,
*also converting them into electric signals that are then measured.
*Example of such materials is zinc sulphide.
Autoradiography method of radioactivity detection and measurement
*This method is usually used to measure the type of radioactive elements in the sample, and their distribution.
*The radioactive sample is brought in direct with a photographic emulsion (or film) containing silver halide (AgX)
*The radiation from the sample oxidizes the silver ion to form black silver metals
*The distribution and intensity of the dark spots on the emulsion/film is then studied (in the same way photographic films are studied)
The cloud chambers method of radioactivity detection and measurement
*Was discovered in 1911 by C.T.R Wilson
*It is made of supersaturated vapour (of water or alcohol) sealed off in a glass chamber.
*When particles from a radioactive source passes through this chamber, it ionizes the vapours on its path
*These ionized vapours acts as ‘nucleation center’ for the formation of visible droplets
*These droplets are observed in the apparatus against a dark background…
*for information about the radiation particle that passed through the chamber
👉🏾Nucleation center/site is the location where nucleation occur
*Nucleation is the process by which new phase 0f a substance is formed from supersaturated or unstable states
Stable and Unstable isotopes
*Nuclei whose proton to neutron ratio lie outside the unstable region undergo spontaneous radioactive decay
*Most naturally occurring elements usually contain high amount of a stable isotope and very little amount of one or more stable or unstable isotope.
*Unstable isotopes are radioactive, and are called radioisotope
*Radioisotopes can be either natural or artificial
*Both decays spontaneously emitting alpha and beta particles till a stable nuclei is formed
**Note that all naturally occurring isotopes with atomic number greater than 83 (Bismuth) are unstable
Half life of a radioactive substances
*Half life is the time taken for the particle to disintegrate till half it’s initial amount remains
*Radioactive disintegration is a first-order kinetics
*The rate is directly proportional to the initial amount present
Calculating the half life of a radioactive element
*The half life is directly proportional to the initial amount present
(dN/dt) α Nᵢ
dN/dt = kNᵢ
dN/Nᵢ = kdt (integrate both sides)
ln(Nᵢ) = -kt + C.
*The negative there indicated that the amount is reducing
*when t=0, N=Nᵢ
C =lnN
therefore; ln(Nᵢ) = -kt + lnN
ln(Nᵢ/N) = -kt
ln(N/Nᵢ) = kt……eqn (i)
*At half life,
N/Nᵢ = 2
Therefore,
ln2 =kt₀·₅ ,where t₀·₅ is the half life
*Half life doesn’t depends on the initial amount present, the rate does
*Both the half life and decay constant are characteristics of a particular isotope
Decay series
*An unstable (parent) nuclei undergo nuclear reaction to form a daughter nuclei
*If the daughter nuclei formed is also unstable, it repeats the process, this usually go on till a stable daughter nuclei is formed.
*These series of processes is termed ‘decay series’
**Parent nuclei is the nuclei that undergo nuclear transformation and daughter nuclei is the one formed during nuclear transformation
Artificial nuclear transformation (radioactivity)
*Both stable and unstable isotopes can be producedby bombarding nuclei with high-energy particles
*Elements with atomic number of 93 or more do not occur naturally
*They have to be produced artificially .
*They are termed “trans-uranium elements”
Nuclear fission
*Binding energy is related to mass defects using Einstein equation (E= mc²); where unit of E is I’m eV [1eV = 9.6×10¹⁶ J/mol ]
*Nuclear fission is the breaking down of heavy nuclei into fragments
*Isotopes of some elements with Z > 80 are more susceptible to undergo nuclear fission, and they emit one or more neutrons in the process
*For instance, fissioncan be induced by bombarding heavy nuclei with neutrons. Since two or more neutrons are emitted from every uranium-235 nucleus, a nuclear chain reaction is initiated and this results in an explosive chain reaction with the release of massive amount of energy. The complete chain reaction and loss of energy take place in less than one microsecond (10-⁶ sec). The energy that is producedor lost can be safely used as a heat source in a power plant.
