Nuclear physics

Question 1

how was the alpha scattering experiment setup?

Answer 1

• alpha particles used as projectiles to study the structure of gold atoms
• a very thin gold foil was used as the target for the alpha particles
  -a fluorescent screen was used as the detector, when alpha particles struck the screen a flash of light was produced allowing the team to see the trajectories

Question 2

what was the alpha scattering experiment measuring?

Answer 2

• the number of alpha particles deflected by an angle theta

Question 3

why did the apparatus have to be evacuated?

Answer 3

• to stop the alpha particles being stopped by air

Question 4

what was the first Rutherford conclusion?

Answer 4

• atom has a very small positively charged nucleus
• responsible for the repulsion force on the positively charged alpha particle, causing it to change direction

Question 5

what was the second Rutherford conclusion?

Answer 5

• nucleus contains nearly all the mass of the atom
• conservation of momentum explains this

MaVa + Mn x 0 = MaVa

nucleus barely moves after repulsion of a particles so Mn>Ma

Question 6

what was the closest distance of approach?

Answer 6

• minimum distance an alpha particle can get to the nucleus before being repelled due to Coulombs Force

Question 7

what was the equation for closest distance of approach?

Answer 7

kinetic energy at start = potential energy at repulsion

Question 8

what was electron scattering

Answer 8

• experiment used to study the structure of matter
• high energy electron beams fired at a target
• deflection was analysed giving conclusions

Question 9

what are the three equations for electron scattering?

Answer 9

lambda = h / p

p = E / c

sin theta = 1.22 lambda / d

Question 10

what is meant by an empirical equation?

Answer 10

• an equation based entirely on experimental results

Question 11

what was the empirical equation for radius of nucleus?

Answer 11

R = Ro x A^1/3

Question 12

derive how i can plot this graph

Answer 12

R = Ro x A^n

ln R = lnRo + lnA^n

ln R = n lnA + lnRo

Question 13

whats the equation for nuclear density

Answer 13

p = 3/(4piR^3) x m

Question 14

what is radiation?

Answer 14

• when an unstable nuclei emits energy

Question 15

give the nature of each three types of radiation

Answer 15

alpha particle - helium nucleus

beta particle - fast electron

gamma ray - electromagnetic photon

Question 16

give the charge of each of the three types of radiation

Answer 16

alpha particle is +2e

beta particle is -e

gamma ray is 0

Question 17

give the masses of each of the three types of radiation

Answer 17

alpha particle - 6.6x10^-27kg

beta particle - 9.1x10^-31kg

gamma ray - 0

Question 18

give the speed of each of the three types of radiation

Answer 18

alpha particle - 5% of c

beta particle - 98-99% of c

gamma ray - c

Question 19

give the ions per mm of air for a particle of 3MeV

Answer 19

alpha particle - 10000

beta particle - 100

gamma ray - 1

Question 20

give the detection for each of the three types of radiation

Answer 20

alpha particle - slight deflection in electric and magnetic fields

beta particle - significant deflection in electric and magnetic fields

gamma ray - no deflection in electric and magnetic fields

all affects photographic film

Question 21

give the ionisation of each of the three types of radiation

Answer 21

alpha particle - strongly ionising

beta particle - much less ionising than a particles

gamma ray - very weakly ionising

Question 22

how does the charged gold leaf electroscope illustrate the strong ionising power of alpha radiation?

Answer 22

• alpha particles create a large number of positive and negative ions in the air
• the charge of the electroscope becomes neutralised depending on the inital charge
  ( free electrons attracted if positively charged )
  ( positive ions attracted if negatively charged )
• as the charge on the electroscope is reduced, the gold leaf gradually collapses

Question 23

what is background radiation?

Answer 23

there is always radiation present in the environment, such as rocks that contain radioactive uranium

the sun also emits protons which can create ions in the atmosphere

Question 24

how does a smoke alarm work?

Answer 24

• alpha particles ionise the air between plates so a current flows
• smoke particles absorb the alpha particles so the air is no longer ionised and current decreases
• this sets off the alarm

Question 25

describe the emission of radiation from a nucleus

Answer 25

- the emission is both random and spontaneous

Question 26

what is the decay constant?

Answer 26

- probability of a nucleus decaying per unit time

Question 27

what is the significance of the minus sign in the equation

Answer 27

- the number of radioactive nuclei in a sample of material decreases with time

Question 28

what is the activity of a radioactive source

Answer 28

- the number of disintegrations per second

Question 29

give the equation to predict the number of nuclei at any time

Answer 29

N = No x e^-kt where k = decay constant

Question 30

what is half life

Answer 30

- one half life is the time taken for half of a sample of radioactive nuclei to decay

Question 31

how is half life connected to the decay constant?

Answer 31

T1/2 = ln2 / decay constant

Question 32

what can the half life of a radioactive nucleus be used for?

Answer 32

- dating of objects - medical diagnosis

Question 33

how does dating of objects work?

Answer 33

- nuclei with long half lifes such as carbon 14 can be used to date organic objects - done by measuring the current amount of carbon-14 and comparing it to the initial amount

Question 34

how does medical diagnosis work

Answer 34

- nuclei with relatively short half-lives are used as radioactive tracers in medical diagnosis - eg pure gamma emitters, half life of 6 hours, short enough to limit exposure but long enough for tests to be carried out

Question 35

why do radioactive nuclei have to be stored in a certain way

Answer 35

- to prevent nuclei from damaging the environment and the people that may be living around them hundreds of years into the future

Question 36

what is meant by electron capture?

Answer 36

when an unstable nucleus absorbs one of its own orbiting electrons, to stabilise itself

Question 37

why can electromagnetic radiation be given off sometimes when decaying

Answer 37

the daughter nucleus may be excited

Question 38

what are the two ways in which i can take safety precautions when dealing with radioactive sources?

Answer 38

- minimise time of exposure - keep as big a distance between you and the source

Question 39

what is nuclear stability

Answer 39

- nucleus held together by strong force - protons experience a force of repulsion - if forces are out if balance, nuclei will become unstable

Question 40

what decay occurs if a nuclei has too many neutrons?

Answer 40

- beta minus

Question 41

what decay occurs if a nucleus has too many protons

Answer 41

beta plus electron capture

Question 42

what decay occurs if a nucleus has too many nucleons?

Answer 42

- alpha decay

Question 43

what happens if a nucleus has too much energy

Answer 43

it will decay and emit a gamma ray

Question 44

what is the equation linking count and area

Answer 44

( area / surface area ) x count rate

Question 45

why us technetium used in medicine

Answer 45

- metastable isotope, so it emits gamma rays to become more stable - physical half life is short

Question 46

what are the dangers of nuclear radiation , alpha radiation

Answer 46

- outside body does little harm - inside body very dangeous - never used for medicine as danger of inhaling / digesting

Question 47

what are the dangers of beta radiation

Answer 47

- more penetrating than alpha radiation - able to penetrate inside and outside the body

Question 48

what are the dangers of gamma rays?

Answer 48

- in high doses, dangerous in and outside the body

Question 49

how does a Geiger tube work

Answer 49

- radiation interacts with gas knocking off electrons - electrons are accelerated towards the anode via e field - as the electron accelerates it collides into more gas molecules, causing more ionisation - avalanche creates a sudden surge of current, detected as a pulse of electricity - pulse corresspondes to a single ionising event - external circuit count pucks up number of times