nuclear physics Flashcards
what’s the radius of an atom?
about 0.05nm
what’s the radius of the smallest nucleus?
about 1fm
what happens to the size of the nucleus as more nucleons are added?
the size increases
what does a graph look like of ‘radius of nucleus’ against ‘nucleon number’?
as nucleon number increases so does the radius of the nucleus
gradient becomes less steep as nucleon number increases
what do you need to plot on a graph to get a linear relationship between radius of nucleus and nucleon number through the orgin?
y axis- radius of nucleus
x axis- nucleon number to the power of 1/3
what is R zero equal to in the nucleus radius equation?
about 1.4fm
what does the nucleus radius equation show about the volume and density each nucleon takes up?
shows that it’s about the same R cubed is proportional to A so volume is proportional to A.
also the mass of each nucleon is about the same so the density of nuclear matter is constant.
is nuclear density greater then atomic density and what does this suggest about the structure of atoms?
yes, significantly
-most of an atoms is in its nucleus
-the nucleus is small compared to atom
-atoms contain a lot of empty space
what’s nuclear fission?
when large nuclei are unstable and some can randomly split into two smaller nuclei
is energy released in nuclear fission and why?
yes energy is released
because the newer smaller nuclei have a higher binding energy per nucleon
what’s nuclear fusion?
two light nuclei combine to create one large nuclei
is energy released during nuclear fusion and why?
yes a very large amount of energy
because the new large nuclei has higher binding energy per nucleon
what force do nuclei need to overcome for nuclear fusion to occur?
the electrostatic force of repulsion as all nuclei are positively charged
what force causes the nuclei to get close enough to stay together in nuclear fusion?
the attraction force of the strong interaction
what graph tells you whether a nuclei will undergo nuclear fission or fusion and how?
average binging energy per nucleon-nucleon number
only elements to the right of Fe-56 can undergo fission and only elements to the left of Fe-56 can undergo fusion
how can you work out the energy released during nuclear fission or fusion?
the change In binding energy per nucleon or use E=mc^2
what’s the activity in a sample?
the number of nuclei that decay each second
proportional to the number of unstable nuclei in the sample (N)
what is the decay constant?
the constant of proportionality
it’s the probability of a specific nucleus decaying per unit time
a measure of how quick a isotope will decay- bigger value=faster rate of decay
what’s an iterative process?
radioactive decay is a iterative process- the number of nuclei that decay in one time period controls the number that is available to decay in the next
how does the number of unstable nuclei change with time?
decreases exponentially
what experiment could we do to demonstrate radioactive decay?
rolling over 100 fair 6-sided dice
if they land on a chosen number then they have decayed and are removed
keep rolling until all are decayed
each roll counts as 1 unit time
what is molar mass?
the mass that one mole of the substance would have
its equal to its relative atomic or relative molar mass
how can you calculate number of atoms in a sample from the number of moles?
N=n x the Avogadro constant
where
N=number of atoms
n= number of moles
what is radioactive decay?
if an atomic nucleus is unstable it will break down to become more stable
the nucleus decays by releasing energy until reaching a stable form
what is the most penetrating type of radiation?
gamma radiation
what is the least penetrating type of radiation?
alpha
what is alpha radiation stopped by?
absorbed by paper, skin or a few cm of air
what is beta radiation stopped by?
absorbed by 3mm of aluminium
what is gamma radiation stopped by?
absorbed by many cm of lead or several metres of concrete
what experiment could you do to find out the type of radiation?
you need a source, an absorber, Geiger-Müller tube, Geiger counter
- record the background radiation with no source present
- place the unknown source next to the Geiger-Müller tube and record the count rate
- place a sheet of paper between the source and tube and record count rate
- replace paper with 3mm thick aluminium and record count rate
- take away background radiation and see when count rate significantly decreased
how can you identify type of radiation from magnetic fields?
charged particles move perpendicular to a magnetic field are deflected in a circular path
the direction it curves depends on charge and radius of curvature also depends on mass
what are some uses of alpha radiation?
smoke alarms
how do smoke alarms work?
allow current to flow but not travel very far
when smoke is present, the alpha particles can’t reach the detector which sets the alarm off
what are some uses for beta radiation?
controlling thickness of materials being made
how does beta radiation help control thickness of materials?
the material is flattened and fed through rollers
the radioactive source is placed on one side of the material and a radioactive detector on the other
the thicker the material, the more radiation will be absorbed so doesn’t reach detector
if too much radiation, rollers will move closer together and other way as well
uses of gamma radiation
radioactive tracers
treatment of cancerous tumours
what are radioactive tracers?
used to help diagnose patients without the need for surgery
a radioactive source with a short half-life to prevent prolonged radiation exposure
a detector eg. PET scanner then detects emitted gamma rays
how can some risks from using gamma radiation in medial treatments be reduced?
shielding- staff leaving room and rotating beams of gamma rays- lessens damage to surrounding tissues
which is the one type of radiation not affected by a magnetic field?
gamma
how can you estimate the radius of an atomic nucleus?
using Rutherfords scattering experiment and find the distance of closest approach
when does an alpha particle scatter through 180 degrees?
at the point where its electric potential energy equals its initial kinetic energy
how can you work out initial kinetic energy for an alpha particle being scattered 180 degrees?
charge of nucleus x charge on alpha particle / 4 x pi x permittivity of free space x distance of closest approach
how is electron diffraction an accurate method for measuring nuclear radius?
electrons are leptons so don’t interact with the strong nuclear force
when investigating nuclear radius, what does the wavelength and energy levels need to be for the electrons?
tiny wavelength and very high energy electrons
where will the first minimum appear in electron diffraction?
sin(angle) approx = 1.22(landa)/2R
with electron diffraction, how does the angle of diffraction effect selective intensity?
the intensity of the maxima decreases as the angle of diffraction increases
how to record background radiation accurately?
-take 3 readings of the count rate using a Geiger counter without a radioactive source present
-average the three readings and subtract the average from each measurement you take of the radioactive sources count rate
what are some examples of background radiation?
-the air, radon gas is realised from rocks and emits alpha radiation
-the ground and buildings
-cosmic radiation, cosmic rays collide with particles in the upper atmosphere and produce nuclear radiation
-living things, all plants and animals contain carbon some is radioactive
-man made radiation, medical devices small proportion of the background radiation
how does the intensity of gamma radiation change with distance from the source?
-doubling the distance, quadruples the area its speed out
-intensity decreases by the square of the distance from the source
how would you calculate intensity of radiation?
constant of proportionality/ distance from source^2
investigating the inverse square law RP12
-taking measurements of intensity at different distances from a gamma source using a Geiger counter
- set up a Geiger counter connected to a geiger-muller tube with one end at the end of a ruler
- turn on the Geiger counter and take 3 readings then an average for the background radiation
- carefully place the radioactive source at a distance d from the tube
- record the count rate at that distance, take three readings
- move the source so the distance is doubled between it and the tube again take 3 readings
- repeat step 5 for 3d, 4d, 5d …
- once finished, put away radioactive source immediately
- average the count rate for each distance and eliminate background radiation
- plot a graph of corrected count rate-distance of the tube from the source, you should see that as the distance doubles, the corrected count rate will drop to a quarter of its value
how to have safe handling of radioactive sources RP12
-always hold a source away from your body when transporting it through the lab
-long handled tongs should be used to minimise the radiation absorbed by the body
-sources of gamma radiation should always be stored in a lead box
-only keep gamma radiation out of the box for the shortest time possible
what is half-life?
the average time it takes for the number of unstable nuclei in a sample to half
applications of radioactive isotopes
date organic material, diagnose medical problems, sterilise food, smoke alarms
what is radioactive dating?
-uses the isotope carbon-14
-living plants take in co2 from the atmosphere including carbon-14
-when they die, the activity of carbon-14 will start to fall so from a once-living material we can test the amount of carbon-14 they have and date them
why can it be difficult to get a reliable age for carbon dating?
-for man made objects crafted from natural materials like wood, you can only find the age of the material used not the object
-the object may have been contaminated by other radioactive sources
-there may be a high background count that obscures the object’s count
-there may be uncertainty in the amount of carbon-14 that existed a thousand years ago
-the sample size or count rate may be small so might be statistically unreliable
what radioactive substance is used in medical tracers and why?
technetium-99m because it emits gamma radiation with a half life of 6 hours so is long enough to record data but short enough to limit the radiation to an acceptable level and decays to a much more stable isotope
what is a medical tracer?
its injected into or swallowed by the patient then moves through the body to the region of interest. the radiation emitted is recorded and an image of inside the patient produced
-shows tissue of organ function
what radioactive substance do nuclear fission reactors use to generate electricity?
uranium-235
reasons why a nucleus may become unstable
too many neutrons
too few neutrons
too many nucleons altogether
too much energy
when does alpha emission occur?
in very heavy atoms like uranium and radium
the nuclei of these atoms are too massive to be stable
when does beta-minus decay occur?
in isotopes that are neutron rich (many more neutrons then protons in the nucleus)
what is beta-minus decay?
the emission of an electron from the nucleus along with an antineutrino. one of the neutrons in the nucleus is changed to a proton
when does beta-plus decay occur?
in isotopes that are proton rich (have high proton to neutron ratio). a proton gets changed to a neutron in the nucleus. a neutrino is also emitted
what happens during gamma emission?
there is no change to the nuclear constituents, the nucleus just loses energy
what is electron capture?
when a nucleus captures and absorbs one of its own orbiting electrons, which causes a proton to change into a neutron. a neutrino is also released
produces gamma radiation
has the same effect on nucleon and proton number as beta decay
what qualities must be conserved in all nuclear reactions?
momentum, charge, nucleon number and lepton number
what is mass defect?
the difference between the mass of a nucleus and the mass of its constituent nucleons
what is binding energy?
the energy needed to separate all the nucleons in a nucleus
equivalent to the mass defect
how would you calculate binding energy from mass defect?
- convert mass defect into kg
- use E=mc^2
what is 1u in MeV?
931.5
how can you work out the average binding energy per nucleon?
binding energy / nucleon number
what are the key features in a thermal nuclear reactor?
control rods
fuel rods
moderator
pump
coolant
concrete casing
what are the fuel rods in nuclear reactors made off?
uranium-235 or sometimes plutonium-239
what is a chained reaction in the nuclear reactor?
the fission reactions produce more neutrons which then induce other nuclei to fission
what condition do the neutrons need to be for a chained reaction to occur?
they need to be slowed down - thermal neutrons
what are thermal neutrons?
slowed down neutrons
what do the fuel rods need to be placed in in a nuclear reactor?
a moderator eg water
why is a moderator important in nuclear reactors?
the slow down the neutrons
how does a moderator slow down neutrons?
through elastic collisions with nuclei of the moderator material
when neutrons collide with particles that are of a similar mass, they are slowed down more efficiently
why is water often used as a moderator?
it contains hydrogen which is of similar mass to a neutron
what can we assume about a collision between a moderator particle and a neutron?
perfectly elastic so KE and momentum are conserved
you can assume the moderator particle is stationary before the collision
if both masses are roughly equal, final velocity of the neutron =0
all KE and momentum would be transferred to the moderator particle
what is critical mass?
the amount of ‘fuel’ you need for the chain reaction to continue on its own at a steady rate
what do control rods do?
control the rate of fission
they control the chain reaction by limiting the number of neutrons in the reactor
they absorb neutrons so the rate of fission is controlled
what does coolant do in a nuclear reactor?
its sent around the reactor to remove heat produced by fission
the material should be liquid or gas at room temperature and be effective at transferring heat
its often the same water being used in the reactor as a moderator
safety precautions in place for nuclear reactors
-surrounded by a thick concrete case which acts as shielding, prevents radiation from escaping
-in an emergency, the reactor can be shut down automatically by the release of control rods into the reactor
-unused fuel rods emit only alpha radiation so is easily contained
what happens to spent fuel rods after?
they are initially very hot and are placed in cooling ponds until temperature falls to a safe level
then stored in sealed containers until its activity has fallen sufficiently
benefits of using nuclear power for generating electricity
there is enough fuel for us to keep generating electricity for centuries to come
fission doesn’t release greenhouse gases
very efficient
State what happens to a neutron that is incident on the moderator.
the neutron undergoes an elastic collision/bounces off with less speed/kinetic energy
State what happens to a neutron that is incident on a control rod
the neutron is absorbed
A slow-moving neutron is in collision with a nucleus of an atom of the fuel which causes fission.
Describe what happens in the process.
the neutron is absorbed/U-236 is formed
(causing) the nucleus (of fuel/uranium) to split into (two smaller) daughter nuclei / nuclei / fragments
releasing (several fast-moving) neutrons
the main source of the most dangerous waste
The (highly radioactive/ most dangerous) waste are the fission fragments from the fission of uranium-235 or from (spent) fuel rods.
how to calculate binding energy
(proton number x mass of proton) + (Neutron number x mass of neutron) - nuclear mass
