Radioactive decay


Radioactive decay generally involves the emission of a charged particle or the capture of an electron by the nucleus to form stable nuclides. The amount of decay = the radioactivity = the number of nuclear transformations per second.

Nomenclature

Nuclide nuclear species with specific number of neutrons and protons that exists in a defined nuclear energy state (e.g. 99mTc is a different nuclide to 99Tc)
Radionuclide radioactive nuclide
Metastable radionuclide a radionuclide that exists for a long time in a higher energy state before falling to ground state (e.g. 99mTc)
Isomer the metastable version of a nuclide (isomer) of a nuclide e.g. 99mTc is an isomer of 99Tc
Isotone nuclides with the same number of neutrons (isotone) but with a different number of protons
Isotope nuclides with the same number of protons (isotope) but with a different number of neutrons

N.B. the number of protons determines the element of an atom. You can change the number of neutrons (and, therefore, the mass number) and the atom will still be the same element.

 

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Nuclear stability

 

The line of stability - Segré chart

The line of stability

 

In the chapter on "Atomic structure" we covered nuclear stability and referred to the Segré chart. What the line of stability shows is that as the number of protons increases, the proportion of neutrons needed to keep the nucleus stable increases. When the nuclide doesn't lie on the line of stability it becomes unstable and radioactive.

 

Decay model of nuclides

Decay Model of Nuclides

 

The decay model of nuclides above includes all nuclides; stable and radioactive. Nuclides in area A have too few neutrons, in area B have too few protons, and in area C are very heavy with excess protons and neutrons. The area the nuclide lies in determines the type of radioactivity the nuclide goes through to become stable and is discussed below.


Radioactive decay

The decay of a nuclide is exponential i.e. it theoretically never reaches zero. 

The S.I. unit of radioactivity is the Becquerel (Bq):

1 Bq = 1 transformation per second

Types of radiation

When a nuclide undergoes radioactive decay it breaks down to fall into a lower energy state expending the excess energy as radiation. The radioactivity released can be:

  1. Alpha particles
  2. Beta particles
  3. Gamma particles (or photons)
  4. Others

1. Alpha particles

  • Symbol: α
  • Formed of 2 protons and 2 neutrons (i.e. a helium atom)
  • Positively charged
  • Relatively heavy
  • Short range of travel

2. Beta particles

  • Symbol: β
  • Electrons emitted from radioactive nuclei
  • Carry negative charge
  • Split into β- (negatron) and an antimatter equivalent β+ (positron)
  • Lighter and smaller than α

3. Gamma particles

  • Symbol: γ
  • Identical to x-rays except for the origin (x-rays originate from electron bombardment, gamma particles from radioactive atoms)
  • Result of transition between nuclear energy levels
  • Very high energy and range of travel

4. Others

  • X-rays
  • Internal conversion: γ ray energy transferred to inner shell electron which is then emitted from the nucleus
  • Auger electron: ejected from electron shells as a result of same radioactive decay processes that create electron shell vacancies. Competes with emission of x-rays.
  • Neutrinos and anti-neutrinos: electrically neutral particles with very little mass emitted from atomic nuclei during β+ and β- decay respectively.
  • Spontaneous fission: very heavy nuclides are so unstable they split into two smaller nuclides emitting neutrons in the process.

 

Decay models

There are several ways in which a nuclide can decay to its more stable form. These are:

  1. Alpha decay
  2. β- decay
  3. β+ decay (aka positron emission)
  4. Electron capture
  5. Isomeric transition
  6. Gamma decay

1. Alpha (α) decay

This occurs in heavier nuclides with too many nucleons. The parent nuclide emits a helium atom (α particle). This type of decay occurs in the nuclides in area C (yellow) of the decay model graph that are very heavy.

Alpha decay radioactivity

 

2. Beta minus (β-) decay

This occurs in nuclides in area B (green) area of the decay model graph that have too many neutrons. The neutral neutron decays into a positive proton (which is retained in the nucleus), a negative electron and an electron antineutrino (i.e. the charge on both sides of the equation remains the same).

np + e- + v-e

Beta decay radioactivity

 

3. Beta plus (β+) decay aka positron emission

This occurs in the nuclides in area A (red) of the decay model graph that have too few neutrons. The extra proton decays into a neutron (which is retained in the nucleus), a positron (β+ or e) and an electron neutrino (ve). This form of radioactivity with the production of a positron is important in PET imaging.

pn + β+/e + ve

Positron emission

 

4. Electron capture

This competes with β+ decay as it also occurs in proton rich nuclei. If the energy difference between the parent and daughter nuclides is too low for positron emission an inner shell electron is captured by the nucleus converting a proton into a neutron (i.e. positive + negative = neutral).

p + e-n

5. Isomeric transition

A radionuclide in a metastable excited state decays to its ground state by isomeric transition and the number of protons and neutrons remain the same. The energy difference (energy released) is emitted as γ radiation. The Z and A remain unchanged.

e.g. Tc-99m → Tc-99 + 140 keV γ rays

6. Gamma (γ) decay

Released by a hyperexcited nucleus to move to lower energy state after β or α decay.

 

  Points to help understanding

1. The charge on both sides of the equation must remain the same

2. Simplistically speaking, a neutron is made of a proton and an electron

n = p + e

n = +ve + -ve

  • This means:
    • A neutron will decay into a proton and an electron (β- decay)
    • A proton and an electron will join to form a neutron (electron capture)

3. Simplistically speaking (again) a proton is made of a neutron and a positron (β+) (β+ decay)

p = n + β+

+ve = n + +ve

4. The mass (A) always remains the same except for in alpha decay


Σ  Summary
  • The number of protons in an atom determines its element
  • Radionuclides transform to a more stable nuclide by releasing energy in the form of radiation
  • Radioactivity is measured in Becquerels (Bq). 1 Bq = 1 transformation / second
  • Radiation can be alpha, beta or gamma particles
  • What is released and the method of decay depends on the characteristics of the radionuclide
Type of decay Occurs in ... Produces Daughter nuclide
Alpha decay Heavy nuclei Helium atom (2p and 2n) A minus 4
Z minus 2
β- decay Too many neutrons np + e- + v-e
neutron becomes proton and electron
A equal
Z plus 1
β+ decay Too few neutrons
Too many protons
pn + β+ + v-e
proton becomes neutron and positron (β+)
A equal
Z minus 1
Electron capture Too few neutrons but not enough energy for β+ decay p + e-n
electron captured and combines with proton to form neutron
A equal
Z minus 1
Isomeric transition Metastable excited nuclides γ radiation A equal
Z equal

Next chapter: X-ray imaging


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