Radioactivity Flashcards
Atomic number Z
number of protons
Atomic mass # (A)
number of nucleons
neutron to proton ratio
- At close distances, a nuclear force between the nucleons holds the nucleus together
- Greater need for neutrons to interact with more protons
- most of the first 20 elements have a 1:1 ratio
how to calculate # of neutrons?
number of nucleons - number of protons
neutron to proton ratio past the first 20 elements
the number of neutrons exceeds the number of protons
why are neutrons present?
Positively charged protons would repel each other without the right number of neutrons present
Radioactivity
- unstable nuclei disintegrate to achieve the ideal balance of neutrons to protons
- emits radiation and/or fast-moving particles
- nuclear process
- not possible to control the rate of radioactive breakdown of a nuclide
- parent radionuclide disintegrates -> daughter product (stable or unstable)
isotope
atom with the same number of protons but a different number of neutrons
- naturally occurring or man made
radioisotope
an unstable isotope
- aka radionuclide, radioactive nuclides, radioactive isotopes
nuclear belt of stability
- non-radioactive isotopes
- belt of stability ends at element 83 (bismuth)
- all elements with 84 or more protons are radioactive
understand slide 11 digram
blue dots and red line
what is the law of conservation of matter?
matter is never created or destroyed
- in a nuclear reaction all the protons and neutrons in a nucleus must be accounted for
Format of a reaction
X -> R+Y
X - reactants
R and Y - products
-> chemical change occurred
radioactive decay
- unstable nucleus loses energy through radiation to achieve stability
- type depends largely on how its n:p compares with those of nearby nuclei that lie within the belt of stability
- Alpha decay, Beta decay and/or Gamma decay
alpha particle decay/emission
- heavy nuclei
- unstable nuclide
- nucleus is too large (high atomic mass) to balance forces
what is an alpha particle (mass and make up)?
2 protons and 2 neutrons, atomic mass 4
alpha particle emission solves instability by…..
- release alpha particles to lose atomic mass
- moves nucleus diagonally towards the belt of stability
properties of an alpha particle
- heaviest of particle emissions
- positive electric charge
- highly ionizing
- very short range
how far can an alpha particle travel in air?
1-10cm
how far can an alpha particle travel in soft tissue?
0.1mm
what is the decay scheme?
- energy diagram depicting decay
- graphical representation of all the transitions occurring in the decay of a radioactive substance
- height of the horizontal lines represents the different nuclear energy states
- changes in proton numbers are represented by a horizontal displacement
beta particle emission
- neutron rich nuclei
- atomic number 0
- decrease neutrons by 1 increase protons by 1
solves high n:p ratios by decreases the ratio
Beta particle properties
- identical to an electron (same mass and charge)
- negative electric charge
- ionizing
- short range (3mm in soft tissue, stopped by a layer of Al a few mm thick)
positive beta particle/positron emission
- proton rich nuclei
solves the problem of: too many protons and a n:p that is too low
what solutions does positron emission provide?
need to give off radiation that reduces number of protons or repulsion force or increases the number of neutrons
positron particle properties
- positive electric charge
- ionizing
- combines with negative electrons and disappears in an annihalation radiation
if n:z ratio is too high
neutrons are converted to protons via beta decay
if n:z ratio is too low
protons are converted to neutrons via positron emission
if amu is too high
alpha decay - change to both neutrons and protons
isomeric reaction
- nucleus may be excited following the emission of a beta or alpha particle
- excited nuclei release energy without changing number of protons or neutrons (typically gamma rays)
- ranges from instantaneous to years
gamma radiation emission
0,0 y
- does not effect the n:z ratio
- releases excess energy from nucleus
properties of gamma radiation
- no charge and no mass
- ionizing
- highly penetrating
- radioactive cobalt used in radiotherapy
- requires several mm of Pb to produce significant attenuation
what is a metastable state?
too much energy
isomeric transition
metastable state element -> gamma photon + stable state element
Deposition of radiant energy
- the amount of radiation deposited in matter is expressed in terms of the absorbed dose, measured in gray (gy)
- absorbed dose = energy absorbed per unit mass of tissue
- consider spatial distribution of the ionizing event
equivalent dose
absorbed dose * radiation weighting factor
effective dose
equivalent dose * tissue weighting factor
penetrating ability or linear energy transfer
- average energy deposited per unit path length along the track of an ionizing particle
- keV/um
what does LET depend on?
- nature of radiation
- material traversed
high LET radiation
3-200keV/um
- alpha particles
- protons
- neutrons
greater density of interactions at cellular level more likely to produce biological damage in tissue
Lower LET radiation
0.2-3 keV/um
- electrons
- positrons
- gamma rays
- x-rays
less likely than high LET to produce tissue damage in the same volume of tissue
what is radioactive half life?
- time required for one half if the sample to decay
- decay is exponential
if you start off with 50g of Cs which has a HL of 14 years, how much Cs will be present 42 years later?
6.25 g
what is the number of radioactive decays per second?
Activity (A)
SI unit for radioactive activity
Becquerel (Bq) - one transformation/decay per second
activity depends on?
- amount of substance
- half life
how does the amount of substance affect activity?
- higher mass = greater number of nuclei = more activity
- mass and activity are directly proportional
how does half life affect activity?
- longer the half life, lower the activity
- half life and activity are inversely proporitonal
decay equation
decay constant (⋋) = # of atoms breaking down per second/total number of atoms
- the rate at which a radionuclide decays
what is the relationship between decay constant and each nucleotide
- fixed characteristic value for each radionuclide
half lives in medical imaging
Isotopes with short half lives are useful for several reasons:
- Cheaper to manufacture
- The patients and staff receive less radiation dose because the activity reduces more quickly
- Post-treatment/investigation activity is low
therapeutic uses of radioactivity in medicine
- Sources of ionizing radiation applied to malignant tissue to prevent or reduce cell division
- Administered internally (injection or ingestion) or externally (external beam therapy)
diagnostic uses of radioactivity in physiological studies
a chemical labelled with a radionuclide is injected/ingested and its uptake by an organ or system is monitored to determine function
diagnostic uses of radioactivity in blood volume studies
total volume of blood can be estimated by measuring its diluting effect on a known amount of radionuclide
diagnostic uses of radioactivity in imaging studies
a radionuclide is introduced in the body and a measurement of the spatial distribution in an organ or system creates an image
