radioactive decay

Question 1

half life and rate of decay

Answer 1

• Radioactivity is a random event; we do not know which atom will decay at what time
• We can use probability and statistics to tell us how many of the atoms will decay ina certain period of time
• The equation used to determine how much will decay in that time period is:

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Question 2

Decay constant

Answer 2

• The decay constant is different for different isotopes the greater the decay constant the radioactive the isotope

Question 3

Radioactive decay law

Answer 3

• The previous equation can be arranged to find the number of atoms left after specified amount of time to decay
• look at ppt
• 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

Half life

Answer 4

• Varies from 10^22 seconds to 10^28 seconds (about 10^28 years)
• Most tables and charts show half life as T1/2
• The half lie and decay constant have an inverse relationship to one another; the longer the half life the lower the decay constant (the more slowly it decays)
• The precise relationship is:

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Question 5

Sample problem

Answer 5

• The decay constant of a given nucleus is 5.4 x10-3 sec
• What is its half life
• How much remains of an intial 100- g sample after 6 hours

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Question 6

Technetium Tc 99m

Answer 6

• 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

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Question 7

Nuclear fission

Answer 7

• Process in which the nucleus of a large, radioactive atom splits into 2 or more small nuclei- caused by collision with a energetic neutron
• A heavy nucleus (mass number >200) divides to form smaller nuclei of intermediate mass and one or more neutrons
• Release a large amount of energy
• Urnanium-235

Question 8

What is fission-

Answer 8

• A large nucleus splitting into smaller ones
• The class one is:
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Question 9

Fission chain reaction

Answer 9

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Question 10

What is fusion

Answer 10

• Combination to light nuclei into a heavy one a good example is:
• look at ppt
• 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 (about 40,000,000 K)

Question 11

Nuclear fusion

Answer 11

• Process in which 2 nuclei of the small elements are united to form one heavier nucleus
• Requires temperatures on the order of tens of millions of degrees for initation
• The mass difference between the small atoms and the heavier product atom is liberated in the form of energy
• Responsible for tremendous energy output of stars like our sun

Question 12

Artificial transmutation

Answer 12

• Rutherford 1919
• Transmutation of lead into gold was achieved by glenn seaborg, who suceedeed 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 13

What is ionizing radiation?

Answer 13

• 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 14

What properties are considered when ionizing radiation is measured?

Answer 14

• 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 15

What units are used for measuring radioactivity

Answer 15

• 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 16

What units are used for measuring radiation energy?

Answer 16

• 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 17

What are the units for measuring radiation exposure?

Answer 17

• 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 by Wilhelm Conrad Roentgen in 1895.
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• • 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.

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• Bremsstrahlung produced by a high-energy electron deflected and slowed down in the electric field of an atomic nucleus

Question 18

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 18

• 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 19

Units of radioactivity and radiation dose

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Answer 19

• 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

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Question 20

What effects do different doses of radiation have on people?

Answer 20

• 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 21

Radiation damage mechanisms

Answer 21

• 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

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• 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 22

Biological effects of radiation in time perspective

Answer 22

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Question 23

Radioactivity- labelling the source

Answer 23

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Question 24

Measuring radiation

Answer 24

• 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 25

ionization chamber

Answer 25

- 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 look at ppt

Question 26

Disadvantages of GM tube wall

Answer 26

- 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 look at ppt

Question 27

Scintillation counter- measures low energy beta and v

Answer 27

look at ppt - 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