section 7: Radioactivity and particles

Question 1

important units:

Answer 1

-bequerel (Bq)
-centimetre (cm)
-hour (h)
-minute (min)
-second (s)

Question 2

an atom:

Answer 2

an atom consists of:
-a positive charged nucleus made of:
-positive protons
-neutral neutrons
-surrounded by negatively charged electrons which orbit the nucleus
-the radius of the nucleus is a lot smaller than the radius of the entire atoms, almost all the mass of the atoms lies in the nucleus
proton: relative mass 1, relative charge +1
neutron: relative mass 1, relative charge 0
electron: relative mass 0.0005, relative charge -1

Question 3

isotopes:

Answer 3

-atoms of the same element have the same number of protons and electrons
-isotopes are forms of an element’s atom with the same number of protons and electrons but a different number of neutrons
-for a given nuclide (distinct nucleus):
-X is the symbol of the element
-A is the mass (nucleon) number -> number of neutrons and protons
-Z is the atomic (proton) number -> number of protons

Question 4

radioactive decay:

Answer 4

-radioactive decay is the spontaneous transformation of an unstable nucleus into a more stable one by the release of radiation
-it is a random process which means one cannot know what nucleus will decay or when it will decay because it is down to chance

Question 5

decay processes: alpha

Answer 5

-(helium nuclei) 42,He
-aplha (α) particles are made up of 2 protons and 2 neutrons- they are big, heavy and slow-moving
-they therefore don’t penetrate far into materials but are stopped quickly
-because of their size they’re strongly ionising, which means they bash into a lot of atoms and knock electrons off them before they slow down, which creates a lot of ions
-because they’re electrically charged (positive charge), aplha particles are deflected (direction changes) by electric and magnetic fields
-emitting an alpha particle decreases the atomic number of the nucleus by 2 and the mass number by 4

Question 6

decay processes: beta

Answer 6

-(electrons) 0,-1 e-(on top)
-a beta (β) particle is an electron which has been emitted from the nucleus of an atom when a neutron turns into a proton and an electron
-when a beta particle is emitted, the number of protons in the nucleus increases y 1, so the atomic number increases by 1 but the mass number stays the same
-they move quite fast and they are quite small
-they penetrate moderately before colliding and are moderately ionising too
-because they’re charged (negative), beta particles are deflected by an electric and magnetic fields

Question 7

decay processes: gamma

Answer 7

-gamma (γ) rays are the opposite of alpha particles, they have no mass-they’re just energy
-they can penetrate a long way into materials without veing stopped
-this means they are weakly ionising because they tend to pass through rather than collide, but eventually they hit something and do damage
-gamma rays have no charge, so they’re not deflected by electric or magnetic fields
-gamma emission always happend after beta or alpha decay, you never get just gamma rays emitted
-gamma ray emission has no effect on he atomic or mass numbers of he isotope, if a nucleus has excess energy, it loses this energy by emitting a gamma ray

Question 8

uses of radioactivity: alpha

Answer 8

industry:
-smoke detectors:
-long half-life alpha emitters are used in smoke detectors
-alpha particles cause a current in the alarm, if smoke enters the detector, some of the alpha particles are absorbed and the current drops, triggering the alarm

Question 9

uses of radioactivity: beta

Answer 9

industry:
-thickness monitoring:
-long half-life beta emitters can be used for thickness monitoring of metal sheets
-a source and receiver are placed on either side of the sheet during its production
-if there is a drop or rise in the number of beta particles detected, then the thickness of the sheet has changed and needs to be adjusted

Question 10

uses of radioactivity: gamma

Answer 10

medicine:
-sterilisation of equipement:
-gamma emitters are used to kill bacteria or parasites on equipment so it is safe for operations (this means they can be sterilised through their protective packaging to eliminate the risk of contamination)
-diagnosis and treatment:
-short half-life gamma emitters such as technetium-99m are used as tracers in medicine as they concentrate in certain parts of the body, the half-life must be long enough for diagnostic procedures to be performed, but short enough to not remain radioactive for too long
-other gamma emitters such as cobalt-60 can be used to destroy tumours with a high dose of radiation

Question 11

decay processes: how they are blocked

Answer 11

-alpha particles: are blocked by paper, skin or a few cm of air
-beta particles: are blocked by thin metal
-gamma rays: are blocked by thick lead or very thick concrete

Question 12

practical: investigate the penetration powers of different types of radiation using either radioactive source or simulations

Answer 12

1) set up a Geiger counter without any of the radioactive sources nearby and records the background activity over a period of about 15 minutes and calculate the count rate in counts per minute (divide the total counts by the number of mins)
2) set up a clamp stand directly infront of where the source will be, pointing towards the clamp stand
3) place the geiger counter around 5 cm from where the source will be, pointing towards the clamp stand
4) move the first radioactive source into position and with no absorbers in place, record the number counts over a 5 minute period and calculate the count rate
5) attach different absorberd to the clamp stand, one at a time, and repeat
6) correct all count-rate readings for background radiations by subtracting the background radiation measure in step 1
7) repeat for the other two source and then compare the count rates for each source with each different absorber
8) a higher count rate for a given material means that more radiartion has passed through the absorbed and so the radiation type is more ionising

Question 13

balancing nuclear equations:

Answer 13

-alpha emission: mass number decreases by 4, atomic number decreases by 2
-beta emission: mass number stays the same, atomic number increases by 1
-gamma emission: mass number stays the same, atomic number stays the same
-neutron emission: mass number decreases by 1, atomic number stays the same

Question 14

dangers of ionising radiation:

Answer 14

-radiation can cause mutations in living organisms
-radiation can damage cells and tissue
-the problems arising from the disposal of radioactive waste and how the associated risk can be reduced

Question 15

detecting ionising radiation:

Answer 15

photographic film:
-the more radiation absorbed by the film, the darker it gets (the film is initially white)
-they are worn as badges by people who work with radiation, to check how much exposure they have had
Geiger-Müller tube:
-a gm tube is a tube which can detect radiation
-each time it absorbs radiation, it transmits an electrical pulse to the machine, which produces a clicking sound
-the greater the frequency of clicks, the more radiation present

Question 16

safety measure of ionising radiation:

Answer 16

-minimising the time of exposure to radiation, keeping as big a distance from the radioactive source as possible, and using shielding against radiation (such as protective clothing made from dense materials such as lead)
-radioactive waster from nuclear reactors must be disposed of carefully, usually by burying it in sealed drums deep underground and remotely handling it after it has been thoroughly cooled

Question 17

background radiation:

Answer 17

-weak radiation can be detected from external sources called background radiation
sources of background radiation include:
-from space:
-cosmic rays include high-energy charged particles penetrating the atmosphere
-from earth:
-radioactive rocks which give off radioactive radon gas
-food and drink which contains radioactive isotopes (such as Carbon 14)
-fallout from nuclear weapons testing
-medical source such as x-rays from MRI scanners
-nuclear power plants which produce radioactive waste

Question 18

activity of radioactive sources:

Answer 18

-the activity of radioactive source is the number of decays which occur per unit time and is measured in becquerels (Bq where 1Bq = 1 decay per second)
-the activity of a radioactive source decreases over a period of time

Question 19

half-life:

Answer 19

-the half-life of an isotope is the time taken for half the nucleis to decay, or the time taken for the activity to halve
-a short half-life means the activity falls quickly, because of lots of the nuclei decay quickly
-a long half-life means the activity falls more slowly because most of the nuclei don’t decay for a long time-they just sit there, basically unstable, but kind of biding their time

Question 20

contamination and irradiation:

Answer 20

-contamination: occurs when a radioactive source has been introduced into or onto an object, the contimanated object will be radioactive for as long as the source is in or on it
-irradiation: occurs when an object is exposed to a radioactive source which is outside the object, the irradiated object does not become radioactive

Question 21

nuclear fission:

Answer 21

-the process of splitting a nucleus is called nuclear fission
-when uranium-235 nucleus absorbs a thermal (slow-moving) neutron, it splits into two daughter nuclei and 2 or 3 neutrons, releasing energy in the process
-the neutrons then can induce furhter fission events in a chain reaction by striking other uranium-235 nuclei
-when uranium-235 splits into two it will form two daughter nuclei, both lighter elements than uranium
-these nuclei are usually radioacive, this is a big problem with nuclear power-radioactive waste
-each nucleus splitting gives out a lot of energy-this energy is in he kinetic energy stores of the fission products
-in a reactor, this energy is transferred to thermal energy stores to produce steam to drive a turbine

Question 22

nuclear reactor:

Answer 22

-control rods: (usually made of boron) are used to absorb excess neutrons and keep the number of neutrons such that only one fission neutron per event goes on to induce further fission
-the moderator: (usually water) slows down neutrons by collisions so that they are moving slow enough to be absorbed by another uranium-235 nucleus
-a coolant: (also water) is used to prevent the system from overheating
-the reactor core: is a thick steel vessel which withstands the high pressures and temperature and absorbed some of the radiation
-the whole core is kept in a building with thick reinforced concrete walls that act as radiation shields to absorb all the radiation that escaped the reactor core

Question 23

nuclear fusion:

Answer 23

-the process of fusing two nuclei to form a larger nucleus is called nuclear fusion
-there is a very small loss of mass in the process, accompanied by a release of energy
-nuclear fusion is how the sun and other stars release energy
-nuclear fusion dows not happen at low temperatures and pressure because the electrostatic repulsion of the protons is too great
-which is because the positively charged nuclei have to get very close to fuse, so they need to be moving very fast to overcome the strong force due to electrostatic repulsion