1 Radioacti vit y and Modes of Radioactive Decay Stuart Jackson PhD FCCPM University of Alberta 2007 Radiation Dose and Radioactivity Radiation Dose Units Exposure Surface Exposures Absorbed Dose Kerma Integral Dose Equivalent Dose Effective Dose Relative Biological Effectiveness Quality Factors Radioactivity • Decay Constant • Half life • The Decay Equation • Modes of Decay • Decay Diagrams • Parent-Daughter Decay • Radioactive Equilibra
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8/6/2019 Radioactivity and Modes of Radioactive Decay
of energetic electrons, results whenan N/P ratio is too high for stability;
Positron emission or electron captureoccurs when it is too low for stability.
These two conditions are representedby specific areas of the nuclide chart
shown. Beta emitters are above thestable nuclides, and positron emitters
and electron capture nuclides are
below.
Beta minus Decay In this process a neutron essentially transforms into a
proton and an electron .
Also a neutrino (anti-neutrino) is involved in theenergetics of the process.
n -----> p+ + e- + + energy
YX1Z
A
Z
A
+
−→ β
At the fundamental level this is due to the conversion of a down quark to anup quark by emission of a W- boson; the W- boson subsequently decays intoan electron and an anti-neutrino.
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Internal conversion is another electromagnetic process which can
occur in the nucleus and which competes with gamma emission.
Sometimes the multipole electric fields of the nucleus interact withorbital electrons with enough energy to eject them from the atom.
This process is not the same as emitting a gamma ray which knocks
an electron out of the atom.
It is also not the same as beta decay, since the emitted electron waspreviously one of the orbital electrons, whereas the electron in betadecay is produced by the decay of a neutron.
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Internal Conversion (IC) The internal conversion coefficient may be empirically determined
by:
Important points:
- emissions have a spectrum of energies and originate from the
nucleus.
IC electrons have discrete energies + accompanyingcharacteristic x-rays and originate from the atomic electron orbits.
Metastable Radionuclides
Are important for nuclear imaging sincethey have relatively long half lives whichfacilitates chemical separation and theproduction of “pure gamma emitters”.
However, conversion electrons are alsoemitted and only add to patient dose.
The conversion coefficient needs to besmall in order to minimize the dose.
Metastable transitions are isomeric.
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PHARMACEUTICAL GIANT DEDICATES BUILDING TOBROOKHAVEN LAB RESEARCHER
Upton, NY - Powell "Jim" Richards, who retired from the U.S.Department of Energy's Brookhaven National Laboratory in 1983,was honored earlier today, when Mallinckrodt Medical dedicated anew building to him at the pharmaceutical firm's Europeanheadquarters in Petten, The Netherlands.
While working in BNL's Medical Department in 1960, Richards wrotethe first scientific publication suggesting the medical use oftechnetium-99m.
These words, which are taken from that 1960 article, are engravedon the bronze plaque set in the Mallinckrodt building:
"Technetium-99m should be a useful research tool; it combines a short half-life and unique radiation characteristics. The absence of beta radiation reduces the amount of damage to biological systems usually associated with radioisotopes."
Technetium-99m is involved in nearly 20 million diagnosticprocedures annually worldwide.
Also, technetium-99m accounts for nearly 85 percent of thediagnostic imaging procedures used in nuclear medicine.
"Nuclear medicine to a large extent owes its emergence andexistence to technetium-99m
(partial list of emissions)
K
Mode Yield Energy
8/6/2019 Radioactivity and Modes of Radioactive Decay
Branching Decay Some radioactive elements undergo decay by more
than one method. In this case, the total decayconstant is given by the sum of the individual
constants.
Example: (18F: 97% by + and 3% by EC)
The fraction decaying by a particular mode is calledthe branching ratio, (B.R.)
..........321 +++= λ λ λ λ total
total
i R B
λ
λ =..
Units of Activity
S.I. units of activity are the
Becquerel (Bq) which correspondsto disintegrations per second.
Conventional units are the Curie
(Ci), where 1 Ci = 3.7x1010 dps.
This is roughly the activity of 1 gram of the
radium isotope 226 Ra
(1mCi = 37 MBq)
In 1896, while investigatingphosphorescence in uranium salts,Becquerel accidentally discoveredradioactivity.
Investigating the work of Wilhelm ConradRöntgen, Becquerel wrapped a fluorescent
mineral, potassium uranyl sulfate, inphotographic plates and black material inpreparation for an experiment requiringbright sunlight.
However, prior to actually performing theexperiment, Becquerel found that thephotographic plates were fully exposed. Thisdiscovery led Becquerel to investigate thespontaneous emission of nuclear radiation.
)107.3(
.)(
10×=
N Ci A
λ
8/6/2019 Radioactivity and Modes of Radioactive Decay
The Curie's began a search for the source of the radioactivity anddiscovered two highly radioactive elements, "radium" and "polonium."The Curie's won the 1903 Nobel prize for physics for their discovery.They shared the award with another French physicist, Antoine HenriBecquerel, who had discovered natural radioactivity. In 1906 Pierre,overworked and weakened by his prolonged exposure to radiation,died when he was run over by a horse drawn wagon.
Madame Curie continued her work on radioactive elements and wonthe 1911 Nobel prize for chemistry for isolating radium and studyingits chemical properties. In 1914 she helped found the RadiumInstitute in Paris, and was the Institute's first director. When the firstworld war broke out, Madame Curie thought X-rays would help tolocate bullets and facilitate surgery. It was also important not to movethe wounded, so she invented X-ray vans and trained 150 femaleattendants.
On July 4, 1934, at the age of 67 Madame Curie died of leukemia(aplastic pernicious anemia), thought to have been brought on byexposure to the high levels of radiation involved in her research. Afterher death the Radium Institute was rename the Curie Institute in herhonor.
Exponential Decay
The number of atoms remaining at time (t)in a radioactive sample is given by:
Since activity is related to the number ofatoms:
The exponential curve represent therepeated reduction of the same fraction ofactivity for each period of time.
t t e N N λ −= .)0()(
t t e A A λ −= .)0()(
8/6/2019 Radioactivity and Modes of Radioactive Decay