half life and rate of decay

Question 1

radioactivity

Answer 1

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.

Question 2

see pp for

Answer 2

equation to determine how much decay in time period

Question 3

radioactive decay law

Answer 3

• 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

Question 4

see pp for

Answer 4

equation to find the number of atoms left after a specified amount of time to decay

Question 5

half life

Answer 5

• 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.

Question 6

see pp for

Answer 6

half life equation

Question 7

see pp for

Answer 7

half life sample problem

Question 8

Technetium Tc 99m

Answer 8

• 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

Question 9

nuclear fission

Answer 9

• 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

Question 10

what is fission

Answer 10

• A large nucleus splitting into smaller ones.

Question 11

see pp for

Answer 11

fission chain reaction

Question 12

what is fusion

Answer 12

• 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

Question 13

nuclear fission

Answer 13

• 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.

Question 14

artificial transmutation

Answer 14

• 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

Question 15

what is ionising radiation

Answer 15

• 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.

Question 16

what properties are considered when ionising radiation is measured?

Answer 16

• 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.

Question 17

what units are used for measuring radioactivity

Answer 17

• 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.

Question 18

what units are used for measuring radiation energy

Answer 18

• 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).

Question 19

what are the units for measuring radiation exposure?

Answer 19

• 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.

Question 20

see pp for

Answer 20

diagrams

Question 21

used in CT scanners

Answer 21

• 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.

Question 22

spectrum of x rays

Answer 22

• 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

Question 23

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).

Answer 23

• 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

Question 24

units of radioactivity and radiation dose

Answer 24

• 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

Question 25

what effects do difference doses of radiation have on people

Answer 25

• 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

Question 26

radiation damage mechanisms

Answer 26

• 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.

Question 27

see pp for

Answer 27

biological effects of radiation in time perspective

Question 28

see pp for

Answer 28

radioactivity - labelling the source

Question 29

measuring radiation

Answer 29

• 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

Question 30

ionisation chamber

Answer 30

• 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

Question 31

disadvantages of GM tube wall

Answer 31

• 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

Question 32

scintillation counter - measures low energy beta and v

Answer 32

• 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