half life and rate of decay Flashcards
1
Q
radioactivity
A
a random event, we do not know which atom will decay at what time, but can use probability and statistics to tell us how many of the atoms will decay in a certain time period.
2
Q
see pp for
A
equation to determine how much decay in time period
3
Q
radioactive decay law
A
- The number of decays per second is called the activity of the sample
- To signify how fast an isotope decays, the term “half-life” is used. The half-life of an isotope is the time it takes half of the original sample to decay
4
Q
see pp for
A
equation to find the number of atoms left after a specified amount of time to decay
5
Q
half life
A
- The half-lives of known radioactive isotopes vary from about 10-22 seconds 1028 seconds
- Most tables and charts show half life as T1/2
- The half-life and decay constant have an inverse relationship to one another; the longer the half-life, the lower the decay constant.
6
Q
see pp for
A
half life equation
7
Q
see pp for
A
half life sample problem
8
Q
Technetium Tc 99m
A
- Create images of the brain following a stroke, bone scans and also helps to locate stomach and bowel infections e.g sepsis.
- Stroke - assess how much blood is being absorbed into the blood vessels in the brain to determine if parts of the brain are working less efficiently than is normally expected.
- Tc-99m is attached to the chelating agent HMPAO to create technetium (99mTc) exametazime
9
Q
nuclear fission
A
- A heavy nucleus (mass number >200) divides to form smaller nuclei of intermediate mass and one of more neutrons
- Release a large amount of energy
- Fission: process in which the nucleus of a large, radioactive atom splits into 2 or more smaller nuclei
- Caused by a collision with an energetic neutron
10
Q
what is fission
A
• A large nucleus splitting into smaller ones.
11
Q
see pp for
A
fission chain reaction
12
Q
what is fusion
A
• Combination to light nuclei into a heavy one, a good example is
2H + 2H 4He
• It is not quite that simple. Because the nucleus is very small, and protons repel.
• A tremendous amount of energy is needed to get this reaction to go
13
Q
nuclear fission
A
- Fusion: process in which 2 nuclei of small elements are united to form one heavier nucleus
- Requires temperatures on the order of tens of millions of degrees for initiation
- The mass different between the small atoms and the heavier product atom is liberated in the form of energy
- Responsible for the tremendous energy output of stars.
14
Q
artificial transmutation
A
- Rutherford 1919
- Transmutation of lead into gold was achieved by glenn seaborg, who succeeded in transmuting a small quantity of lead in 1980. He also first isolated plutonium for the atomic bomb and discovered ‘created’ many elements
- There is an earlier report (1972) in which soviet physicists at a nuclear research facility in Siberia accidentally discovered a reaction for turning lead into gold when they found the lead shielding of an experimental reactor had changed to gold
15
Q
what is ionising radiation
A
- Ionizing radiation is radiation that has enough energy to remove electrons from atoms or molecules
- The loss of an electron with its negative charge causes the atom (or molecule) to become positively charged (cation).
- Ionisation can also result in gain of an electron by an atom or molecule to form an anion
- Note: Microwave, infrared (IR) and ultra-violet (UV) radiation are examples of non-ionizing radiation. Non-ionizing radiation does not have enough energy to remove electrons.
16
Q
what properties are considered when ionising radiation is measured?
A
- Ionizing radiation is measured in terms of:
- the strength or radioactivity of the radiation source,
- the energy of the radiation,
- the level of radiation in the environment, and
- the radiation dose or the amount of radiation energy absorbed by the human body.
- Occupational exposure limits like the Threshold Limit Values (TLV) are given in terms of the permitted maximum dose. The risk of radiation-induced diseases depends on the total radiation dose that a person receives over time.
17
Q
what units are used for measuring radioactivity
A
- Radioactivity or the strength of radioactive source is Measured in units of becquerel (Bq).
- 1 Bq = 1 disintegration per second.
- One becquerel is an extremely small amount of radioactivity.
- Commonly used multiples of the Bq unit are kBq (kilobecquerel), MBq (megabecquerel), and GBq (gigabecquerel) (109).
- An old and still popular unit of measuring radioactivity is the curie (Ci).
- 1 Ci = 37 GBq = 37000 MBq.
- One curie is a large amount of radioactivity.
- Commonly used subunits are mCi (millicurie), µCi (microcurie), nCi (nanocurie), and pCi (picocurie).
- Another useful conversion formula is: 1 Bq = 27 pCi.
- Becquerel (Bq) or Curie (Ci) is a measure of the rate (not energy) of radiation emission from a source.
18
Q
what units are used for measuring radiation energy
A
- The energy of ionizing radiation is measured in electronvolts (eV).
- One electronvolt is an extremely small amount of energy. Commonly used multiple units are kiloelectron (keV) and megaelectronvolt (MeV).
- 6,200 billion MeV = 1 joule
- 1 joule per second = 1 watt
- Watt is a unit of power, which is the equivalent of energy (or work) per unit time (e.g., minute, hour).
19
Q
what are the units for measuring radiation exposure?
A
• X-ray and gamma-ray exposure is often expressed in units of roentgen (R). The roentgen (R) unit refers to the amount of ionization present in the air.
X-rays
• X-rays are photons (i.e. electromagnetic radiation) with energies typically above 1 keV. They were discovered byWilhelm Conrad Roentgen in 1895.
20
Q
see pp for
A
diagrams
21
Q
used in CT scanners
A
- electrons collide with the target anode, lose kinetic energy; 0.05% is converted into X-radiation, the rest produces heat.To prevent damage to the anode it has to be cooled
- used in CT scanners, airport luggage scanners, X-ray crystallography, material and structure analysis, and for industrial inspection.
22
Q
spectrum of x rays
A
- Spectrum of the X-rays emitted by an X-ray tube with a rhodium target, operated at 60 kV. The smooth, continuous curve is due to bremsstrahlung, and the spikes are characteristic K lines for rhodium atoms
- Bremsstrahlung produced by a high-energy electron deflected and slowed down in the electric field of an atomic nucleus
23
Q
The absorbed dose is the amount of energy absorbed per unit weight of the organ or tissue and is expressed in units of gray (Gy).
A
- One roentgen of gamma- or x-ray exposure produces ~1 rad (0.01 gray) tissue dose
- Equal doses of all types of ionizing radiation are not equally harmful to human tissue.
- Alpha particles produce greater harm than do beta particles, gamma rays and X-rays for a given absorbed dose, so 1 Gy of alpha radiation is more harmful than 1 Gy of beta radiation
- To account for the way in which different types of radiation cause harm in tissue or an organ, radiation dose is expressed as equivalent dose in units of sievert (Sv).
- The dose in Sv is equal to the total “absorbed doses” multiplied by a “radiation weighting factor” (WR ) and is important when measuring occupational exposures.
- WR is 20 for α particles, 5-20 for neutrons, 1 for electrons and photons
24
Q
units of radioactivity and radiation dose
A
- Dose in Sv = Absorbed Dose in Gy x radiation weighting factor (WR
- 1 Gy air dose equivalent to 0.7 Sv tissue dose (UNSEAR 1988 Report p.57)
- 1 R (roentgen) exposure is approximately equivalent to 10 mSv tissue dose
25
Q
what effects do difference doses of radiation have on people
A
- One sievert is a large dose. The recommended TLV is average annual dose of 0.05 Sv (50 mSv).
- The effects of being exposed to large doses of radiation at one time (acute exposure) vary with the dose. Here are some examples:
- 10 Sv - Risk of death within days or weeks
- 1 Sv - Risk of cancer later in life (5 in 100)
- 100 mSv - Risk of cancer later in life (5 in 1000)
- 50 mSv - TLV for annual dose for radiation workers in any one year
- 20 mSv - TLV for annual average dose, averaged over five years
26
Q
radiation damage mechanisms
A
- Direct action: ionization of the DNA molecule, which may result in genetic damage
- Indirect action: radiation ionizes water, which causes free radicals to form. Free radicals attack targets such as DNA. Much more common
- High radiation doses kill cells,
- low doses tend to damage or alter the genetic code (DNA) of irradiated cells.
- High doses can kill so many cells that tissues and organs are damaged immediately causing a rapid body response “Acute Radiation Syndrome”
- low doses – less than 10,000 mrem (100 mSv) – spread out over long periods of time (years) don’t cause an immediate problem to any body organ. The effects of low doses of radiation occurs after many years.
- Genetic effects and cancer are the main outcomes from exposure. Cancer is five times more likely than a genetic effect.
- Genetic effects include chromosome changes, stillbirths, congenital abnormalities, and infant and childhood mortality.
27
Q
see pp for
A
biological effects of radiation in time perspective
28
Q
see pp for
A
radioactivity - labelling the source
29
Q
measuring radiation
A
- Dosimeter - The film badge dosimeter, or film badge, is used for monitoring cumulative exposure to ionizing radiation.
- The film is removed from holder and developed, the darker the film the more the exposure.
- Film darkens on exposure to gamma rays, X-rays and beta particles (useless for measuring neutron radiation)
- Worn by nurses, radiographers
30
Q
ionisation chamber
A
- Geiger-Muller tube
- +ve and –ve ions formed by ionising radiation are collected by an electric field which leads to an electric pulse which is recorded
Detection of higher energy gamma in a thick-walled tube. Secondary electrons generated in the wall can reach the fill gas to produce avalanches. Multiple avalanches omitted for clarity
31
Q
disadvantages of GM tube wall
A
- Not 100% efficient
- The pulse is only detected in direction that the detector is pointing
- Okay for medical work
- Otherwise geometry at source must be taken into account for full 3D counting
- Not all radiation in detector direction is recorded
- Not all radiation striking the detector is recorded
- Particles arriving in close succession – electronics can not cope “dead time”
- Automatic correction in instrument or consult paralysis tables
- Providing paralysis time is constant then
32
Q
scintillation counter - measures low energy beta and v
A
- Schematic of an incident particles hitting a scintillating crystal, triggering the release of photons which are then converted into photoelectrons and multiplied in the photomultiplier
- NaI crystal contains low % Tl, Other crystals could be anthracene or stilbene
- Can have liquid scintillation counters e.g p-terphenyl dissolved in toluene or dioxan
