nuclear physics - section 8 - radioactivity

Question 1

The gold foil in an alpha scattering experiment is replaced with another foil of the same size made from a mixture
of isotopes of gold. Nothing else in the experiment is changed.
Explain whether or not the scattering distribution of the monoenergetic alpha
particles remains the same. [1]

Answer 1

the scattering distribution remains the same
because the alpha particles
interact with a nucleus with the same atomic number (charge)
so repulsive force remains the same

Question 2

describe the alpha scattering experiment [2]

Answer 2

beam of alpha particles fired at a thin sheet of gold foil
in a vacuum so alpha particles don’t collide with air particles

Question 3

volume of a sphere

Answer 3

4/3 π r^3

Question 4

In an experiment to investigate the structure of the atom, α particles are directed
normally at a thin metal foil which causes them to be scattered.
In which direction will the number of α particles per second be a maximum? [1]

what does this suggest about the structure of the atom? [1]

Answer 4

straight on
deflection of zero degrees

mostly open space

Question 5

In an experiment to investigate the structure of the atom, α particles are directed
normally at a thin metal foil which causes them to be scattered.
A small number of α particles are scattered through 180°.
Explain what this suggests about the structure of the atoms in the metal. [2]

Answer 5

most of the mass of an atom is contained in its nucleus (1)
the nucleus contains a positive charge (1)

Question 6

Why is it necessary to remove the air from the chamber in a Rutherford
scattering experiment? [1]

Answer 6

to prevent interaction
of the α particle with the air (1)

Question 7

2 conclusions about the nucleus from the Rutherford scattering experiment [2]

Answer 7

nucleus has a positive charge
nucleus contains most mass of the atom

Question 8

what were the expectations of the alpha scattering experiment?

Answer 8

plum pudding model suggested that a beam of positively charged alpha particles would be slightly scattered by the atoms of the foil if the positive charge was evenly distributed

Question 9

what were rutherford’s observations in the alpha scattering experiment

Answer 9

some of the particles were bounced back at a much greater force than expected ( small percentage (1 in 10000) were deflected through angles greater than 90 degrees)
most of the alpha particles passed straight through the foil ( about 1 in 2000 were deflected )

Question 10

most alpha particles passed straight through the foil. some bounced back
what inferences can be made from this

Answer 10

most of the atom’s mass is concentrated in a small region in the atom ( the nucleus ) at the centre
the nucleus is positively charged because it repels alpha particles that come close

Question 11

nuclear diameter

Answer 11

10^-15 m

Question 12

rutherford and radiation

Answer 12

he found that radiation ionised the air making it conduct electricity
alpha and beta

Question 13

order the three types of radiation in order of ionisation power ( lowest to highest )

Answer 13

gamma
beta
alpha

Question 14

in the alpha scattering experiment give two reasons why the gold foil should be very thin [2]

Answer 14

the α particles must not be absorbed by the foil (1)
the α particles must only be scattered once(1)

Question 15

order the three types of radiation in order of penetration power ( lowest to highest )

Answer 15

alpha
beta
gamma

Question 16

what are the three types of radiation made up of?
how was this deduced?

Answer 16

when placed in a magnetic field, both alpha and beta radiation were deflected in opposite directions
gamma wasn’t deflected
this meant alpha was composed of positively charged particles and bet of negatively charged particles
gamma was later discovered to consist of high energy photons

Question 17

why is ionising radiation seen as dangerous

Answer 17

can kill or mutate cells
this can lead to mutations and lead to things such as cancer

Question 18

which type of radiation is most dangerous inside the body and why

Answer 18

alpha
Alpha radiation is highly ionising and poorly penetrating
This would mean it would cause localised damage and not be able to leave the body

Question 19

give an example of real life use of beta decay and explain why beta decay is used for this

Answer 19

used to measure the thickness of aluminium foil or paper
Beta is used because it is moderately penetrating and would be able to pass through to reach the detector on the other side
Gamma radiation is too penetrative and would pass through anything.

Question 20

which type of radiation follows the inverse square law

Answer 20

gamma radiation

Question 21

what is the inverse square law

Answer 21

The intensity of radiation is inversely proportional to the square of the distance from the source

Question 22

what is intensity measured in

Answer 22

watts per square metre
W/m^2

Question 23

describe an experiment to show the inverse square law

Answer 23

1. measure the background radiation without the gamma source present in the room using a geiger muller tube
2. Place the source at a set distance from the tube and measure the count rate over a set period of time
3. Take an average count rate
4. repeat for distances at increasing intervals and subtract the background radiation from each recording
5. plot a graph of count rate per minute against 1/distance squared
6. should be a straight line through origin to confirm they are directly proportional

Question 24

what is background radiation

Answer 24

radiation that is constantly present in the surroundings from sources such as rocks and cosmic radiation

Question 25

what is the decay constant

Answer 25

λ
the probability of a nucleus decaying per second

Question 26

what are the units for the decay constant

Answer 26

s^-1

Question 27

what is half life

Answer 27

Time taken for half the unstable nuclei in a substance to decay

Question 28

half life formula

Answer 28

t½ = ln2 / λ

Question 29

activity formula

Answer 29

A = λN

activity = decay constant * Number of unstable nuclei in sample

Question 30

what is activity measured in

Answer 30

Bq
decays per second

Question 31

unstable nuclei remaining formula

Answer 31

N = N0e^-λt

number of unstable nuclei remaining = original number of unstable nuclei * e^-decay constant * time

Question 32

what does the unstable nuclei formula show

Answer 32

that radioactive isotopes decay exponentially

Question 33

which isotope is commonly used in medicine and why

Answer 33

Technetium 99m
- releases gamma radiation
- has a short half-life and doesn’t remain highly radioactive for long
- half life of approximately 6 hours, long enough to be detected
- can be made near to the hospital

Question 34

what does the NZ graph show

Answer 34

shows relationship between proton number and neutron number
shows a stability curve beginning at N=Z until N=20 and then curves up

Question 35

where on the NZ curve does beta minus decay occur

Answer 35

above the stability line
where there are excess neutrons
beta minus decay occurs to turn the neutron to proton and become more stable

Question 36

what type of decay occurs below the stability line and why

Answer 36

beta plus decay
proton rich
beta plus decay occurs to turn protons into neutrons

Question 37

how does the heavier nuclei often decay

Answer 37

through alpha decay
alpha decay emits a helium nucleus (4,2) making the nuclei less heavy and more stable

Question 38

sin 45

Answer 38

root2 / 2