nuclear physics 1 Flashcards
what is the definition of radioactive decay?
an atomic nucleus undergoes the spontaneous transformation from a higher energy state to a lower energy state
what is radioactive decay?
-parent nucleus must have been previous excited by natural means (core of a star, big bang, cosmic rays) or artificial means (reactor core, particle accelerator)
-emission of electromagnetic or particulate radiation
-radiation is emitted in one of several decay modes
-all radioactive nuclei eventually decay to a stable nuclei
who discovered natural radioactivity?
Henri Becquerel in 1896
-when he noticed that uranium spontaneously produced an invisible, penetrating radiation that affected photographic plates
the nucleons within the nucleus (protons and neutrons) are ____________
IN CONTINUAL MOTION
-as a result of the motion, collisions occur and energy is transferred between particles
what are the differences between a stable nucleus and a radioactive nucleus in radioactivity?
-in a stable nucleus, no particle ever acquires enough energy to escape the nucleus
-in a radioactive nucleus, it is possible for a particle, by a series of chance encounters, to gain enough energy to escape the nucleus, and a new nuclear species is formed
what is radioactivity governed by?
quantum mechanisms, and is thus inherently probabilistic
-it is impossible to know which radioactive atom in a sample will decay, but the average rate of nuclear transformation or decay for a large group of radioactive atoms can be predicted
what is the chart of the nuclides?
-all nuclides are uniquely characterized by their number of protons (Z) and their number of neutrons (N) (A=mass number Z+N)
-plot of stable nuclides (black) forms a curve of stability
-the neutron-rich nuclides fall below the curve of stability, and proton-rich radionuclides fall above the curve of stability
-curve stability is roughly Z-N for low Z nuclides (Z<20) and slow transforms into N-1.5Z with increasing Z due to competing forces
-all radionuclides with Z>83 (Bi) are radioactive
what is stability?
4 fundamental forces of nature
-two, the strong nuclear force and electromagnetic force, contribute to a nucleus’ stability
-the strong nuclear force binds the nucleons
-the electromagnetic force acts between charged particles (like charges repel)
-the strong nuclear force is 2 orders of magnitude stronger than the electromagnetic force, but acts under a much smaller distance
-to bind the nucleons into a stable nucleus a delicate equilibrium between the number of protons and neutrons must exist
-above A (mass number) 40, more neutrons that protons must constitute the nucleus to form a stable configuration in order to overcome the Coulomb repulsion of the charged protons
-if an equilibrium does not exist, the nucleus is unstable (radioactive)
what is some terminology?
where is exponential behavior observed?
-cell growth in culture
-growth of money due to interest
-attenuation of x-rays when passing through matter
-decay of radioactive isotopes
when does exponential behavior occur?
what is the refresher of exponents and logarithms?
what is euler’s number and natural logarithms refresher?
what are the mathematics of radioactive decay?
-the number of radioactive nuclei (N) decaying per unit time is proportional to the number of unstable nuclei present
-the decay constant λ (time -1), is the constant of proportionality
-the fraction of the number of radioactive atoms in a sample that decay per unit time
-depends on the radionuclide
look ar slide 14
what are radioactivity equations?
what is the half-life?
-the time required for the number of radioactive atoms in a sample to decrease by one-half
what is the mean or average life?
-the average lifetime of an individual nucleus
-the time that would be needed for all the nuclei to disintegrate if the initial activity of the group (Ao=λNo) was constant (instead of decreasing exponentially)
what is secular equilibrium?
what is transient equilibrium?
what does the graph look like with no equilibrium?
what are some naturally occurring radionuclides?
-all elements with Z greater than 82 (lead) are radioactive and disintegrate until a stable isotope is formed
-naturally occurring radioactive elements are predominantly members if one of four radioactive series that all begin with very long-lived parents that have half-lives of the order of the age of the earth
what are radionuclides in the environment?
what are nuclear reactors (radionuclide production)?
produce electricity through nuclear fission
-nuclear fuel (most often uranium) is bombarded with neutrons, causing it to split into nuclear fragments, releasing energy
-the released energy heats water to produce steam and turn a turbine
-a high flux of neutrons are present in the reactor core
what are fission products (radionuclide production)?
-when bombarded by neutrons, a fissile material splits into smaller nuclei called fission fragments
-most common target material is uranium-235
-the desired radionuclide can be separated from other fissile fragments using chemical separation techniques
-ex: molybdenum-99 (nuclear medicine imaging)
what is neutron activation (radionuclide production)?
-a neutron is captured by a stable nuclide, creating radioactive product
-often the product has the same number of protons as the target and is difficult to separate chemically (lowers the specific activity)
-Ex; iodine 125 (used in radiation treatment of the prostate) and cobalt-60 (source for the first widely used high-energy radiotherapy unit, developed in SK)
how do cyclotrons work?
-two electromagnets above and below the beam path create a vertical magnetic field
-two evacuated semi-circular cavities (dees) sit between the magnets
-an alternating voltage is applied between the two cavities to increase the particle energy
what are the steps of cyclotrons?
- charged particles injected at center of cyclotron (often positrons)
- the charged particle beam is deflected in the magnetic field and travels in a circular path
- each time the particles reach the gap they are accelerated across (gain energy and velocity)
- as they gain energy, they travel on a circle with a larger radius
- the particles reach their target energy and exit the accelerator and hit the stable target to create the desired daughter product
what is a radionuclide generator?
-the most commonly used radioisotope in nuclear medicine imaging, Technecium-99, is produced in a radionuclide generator
-because of its relatively short half-life (6 hours), it is not feasible to maintain an appropriate supply due to times to produce off-site (nuclear reactor), ship and store
-instead its parent radionuclide (Molybdenum-99, T1/2=66h) is stored and transported in a shielded radionuclide generator and supplied to hospitals
what is the graph of radionuclide generators (radionuclide production)?
-based on the principle of transient equilibrium (longer lived parent and short-lived daughter)
-the daughter activity increases with time and is chemically separated from the parent radionuclide once it approaches the equilibrium activity
-used as a local generator of short-lived radioisotopes (that would not be able to be transported long distances)
-Tc-99 (T1/2=6h) is extracted once daily for one week (2.5 half-lives of Mo-99 (T1/2=66h)) then the generator is replaced
