UNIT 5 Nuclear Physics

Question 1

Describe the structure of an atom in terms of …

Answer 1

a positively charged nucleus and negatively charged electrons in orbit around the nucleus

Question 2

Rutherford’s alpha scattering experiment

Describe how the scattering of alpha (α) particles by a sheet of thin metal supports the nuclear model of the atom, by providing evidence for:

a) a very small nucleus surrounded by mostly empty space
(b) a nucleus containing most of the mass of the atom
(c) a nucleus that is positively charged

Answer 2

OBSERVATION, DESC, PROOF of atomic model

a) MOST ALPHA particles - passed straight thru gold foil WITHOUT deflection - atom is mostly empty space

b) approximately 1 in 8000 alpha particles [INTO] - deflected back towards the source - nucleus is very small & dense compared to the rest of the atom; charge concentrated at nucleus

c) SOME alpha particles [TOUCHES] - deflected at angle greater than 90° - presence of a dense, positively charged nucleus which repels the alpha particles; almost all mass of atom

Question 3

A beam of α-particles and β-particles passes, in a vacuum, between the poles of a strong magnet.

Compare the deflections of the paths of the two types of particle.

Q: A beam of β-particles passes, in a vacuum, through the electric field between a pair of
oppositely charged metal plates.
Describe the path of the particles.

Answer 3

• α and β deflected in opposite directions
• β deflected more (than α)
• paths (of particles) are curves

Q: curved path
(deflected/attracted) towards positively charged plate

Question 4

i) alpha particle is a ____ nucleus

ii) desc path of gamma rays in magnetic

Answer 4

helium nucleus
X4 2
ii) not deflected

Question 5

Describe how an electric field between two charged plates could be used to determine
whether a beam of particles consists of α- or β-particles.

Answer 5

• α towards negative (plate)
• β towards positive (plate)
• α and β deflected in opposite directions

Question 6

State what is meant by the term isotopes

[note to know: an element may have more than one isotope]

Answer 6

• versions of same element
• (isotopes of same element have) same proton number

(isotopes of same element have) different nucleon numbers

Question 7

half-life of a particular isotope

• define
• steps

Answer 7

the time taken for half the nuclei of that
isotope in any sample to decay

so, 1. check the original activity or count rate (where the line crosses the y-axis). C0

2. halve this value and look for this activity
3. go across from the halved value (on the y-axis) to the best-fit curve, and then straight down to the x-axis
4. point where you reach the x-axis should be the half-life

Question 8

half-life of isotope shld be calc’ed by -

Answer 8

• removing the background radiation from data or decay curves

BY: 1. measuring the background radiation (with no sources present) => background count
2. carry out the experiment
3. subtract the background count from each reading, to provide a corrected count 4. the corrected count = best estimate of the radiation emitted from the source and should be used to measure its half-life

Question 9

eqn - beta decay

Answer 9

14 C => 14 N + 0 e
6 7 -1

Question 10

deduce the type of emissions coming from the radioactive source. Explain your reasoning.

Answer 10

GAMMA = HIGHEST BC range in air HIGHEST, beta 2nd highest

γ rays
(γ rays) detected at B
(γ rays) not deflected by field / not charged
charged particles / β particles (accept α for charged particles)
β particles detected at C
reference to direction of deflection / LH rule
no α-particles OR only background detected at A

Question 11

The technician tries to locate an area above the pipe where the radioactive count rate is higher than in the surrounding area.

(i) State and explain the type of radiation that must be emitted by the isotope for the leak to be detected.

ii) half-life of the isotope used is 6.0 hours. Explain why an isotope with this half-life is suitable.

Answer 11

i) gamma emitter used
can penetrate ground to surface

ii) long enough to find leak
short enough to disappear quickly

Question 12

which type of radiation, α, β or γ, is the most strongly ionising.

Answer 12

α

[& most to least anything: y, B, a)

Question 13

background radiation

• define
  -types?

Answer 13

the radiation that exists around us all the time

1. natural sources from radioactive elements that always existed on Earth & in outer space
2. man-made sources from human activity that adds to the amount of radiation humans are exposed to on Earth

Question 14

RMBR - sources that make a significant
contribution to background radiation

Answer 14

-radon gas (in the air)
-rocks and buildings
-food and drink
-cosmic rays

Question 15

1. why are isotopes more likely to decay?
2. instability is due to…

& to become more stable…

Answer 15

1. tend to be more unstable due to their imbalance of protons and neutrons
2. • nucleus having too many protons or neutrons. - nucleus very large

• emit radiation to become more stable; radiation moves away from nucleus, takes some energy w/ it => reduces overall energy of nucleus => makes the nucleus more stable

Question 16

desc processes of nuclear fission and
nuclear fusion

Answer 16

nuclear reactions that change the nucleus of an atom to produce high amounts of energy from the energy stored in the nucleus of an atom

Question 17

📌 nuclear fission - decay by fission

Answer 17

the splitting of a large, unstable nucleus into two smaller nuclei /

[to produce smaller nuclei and neutrons with a lot of kinetic energy]

Question 18

during fission

Answer 18

• neutron collides with unstable nucleus
• neutron and nucleus are reactants
• nucleus splits into 2 smaller nuclei (daughter nuclei) + two or three neutrons
• the daughter nuclei & neutrons are products of reaction. gamma rays also emitted

Question 19

nuclear fission nuclide equations - e.g. fission of uranium-235 [or plutonium]

what is conserved? describe the energy transfer?

Answer 19

235 92 U + ¹₀ n => 92 36 Kr + 141 56 Ba + 3 ¹₀ n + energy

• Energy is conserved. energy is transferred from nuclear energy store of the parent nucleus to the kinetic energy store of the reactants

Question 20

FISSION: why is the mass of the products less than the mass of the original nucleus?

FUSION: why is the mass of the product (fused nucleus) less than the mass of the two original nuclei (reactants)?

Answer 20

1. bc the remaining mass has been converted into energy, which is released during the fission process
2. remaining mass has been converted into the energy released when the nuclei fuse

Question 21

📌 nuclear fusion

produce? requires?

Answer 21

when two light nuclei join to form a heavier nucleus

[releases HUGE amount of energy; requires extremely high temperature and pressure]

Question 22

nuclide equation for fusion - hydrogen to helium

Answer 22

²₁H + ¹₁H => ³₂He + energy

[energy produced during nuclear fusion comes from very small amount of a particle’s mass converted into energy]

Question 23

nuclear fusion mass and energy values

Answer 23

E = mc²

E = energy released from fusion, (J)
m = mass converted into energy, (kg)
c = the speed of light, (m/s)

Question 24

nuclear power station
the sun

fusion/fission?

Answer 24

fission = nuclear power station
sun = fusion

Question 25

with what can ionising nuclear radiation be measured using?

what does it use? what does this mean?

when does count rate decrease?

Answer 25

a detector connected to a counter,
- detector uses count rate (number of decays per second)

• count rate decreases the further the detector is from the source; this is bc the radiation becomes more spread out the further away it is from the source

Question 26

most common device used to measure and detect the count rate of radiation

does so by…

Answer 26

Geiger-Müller tube/ GM tube/ Geiger counter - absorbs radiation, it transmits an electrical pulse to a counting machine

Question 27

desc the emission of radiation from a nucleus as _____ and ____ in ____

Answer 27

spontaneous and random in direction

Question 28

radioactive decay

Answer 28

a change in an unstable nucleus that can result in the emission of α-particles or β-particles and/or γ-radiation and know that these changes are spontaneous and random

Question 29

relation btwn charge on ionising power

Answer 29

the greater the charge of the radiation, the more ionising it is

SO alpha radiation is the most ionising (+1 charge)
beta mod, gamma least

Question 30

relation btwn kinetic energy on ionising power

Answer 30

the higher the kinetic energy of the radiation, the more ionising it is

alpha particle is still most ionising bc it has the greatest mass
beta mod. (electron travels at high speeds), gamma weakly ionising,

Question 31

conditions for deflection in
1. electric &
2. magnetic fields

WHY is path curved?

Answer 31

particle is deflected in…

1. an electric field if it has charge
2. in magnetic field if it has charge and is moving

• WHY: force because particle is charged

Question 32

electric fields

Answer 32

• alpha particles are deflected towards the negative plate
• beta particles are deflected towards the positive plate
• gamma radiation is not deflected and travels straight through between the plates

Question 33

magnetic fields

WHEN DRAWING BETA and ALPHA paths:

Answer 33

alpha and beta deflected in opposite directions due to their opposite charges ;

α-particle curve up the page in at least half of width of field B1
β-particle curve opposite to α-particle curve OR down page if α line has no B1
curvature anywhere
smaller radius of β path clear

Question 34

during α-decay or β-decay, …

Answer 34

the nucleus changes to that of a different element

Question 35

isotopes of an element may be
radioactive due to…

Answer 35

an excess of neutrons in the
nucleus and/or the nucleus being too heavy

Question 36

effect of alpha & beta decays and gamma emissions on nucleus

Answer 36

• an increase in stability
• a reduction in the number of excess neutrons

Question 37

during beta emission specifically… neutron -> ____ + ____

Answer 37

neutron → proton + electron

Question 38

alpha decay

MINUS - mass FOUR, proton TWO

A X => A-4 Y + ⁴₂a
Z Z - 2

Answer 38

• alpha particle is emitted from an unstable nucleus
• mass number decreases by 4; atomic number decreases by 2
• charge on the nucleus also decreases by 2
• completely new element formed

Question 39

beta decay (+1 proton, mass same)

A X => A Y + ⁰-₁β
Z Z+1

Answer 39

• a neutron changes into a proton and an electron
• the electron is emitted and the proton remains in the nucleus
• atomic number changes => new element; mass number same, but atomic number increases by ONE

Question 40

gamma decay

A X => A X + ⁰₀γ
Z Z

Answer 40

• a gamma ray is emitted from an unstable nucleus
• makes the nucleus less energetic / reduces energy but doesn’t change its structure bc gamma radiation has no mass or charge
• does not affect the mass number or the atomic number of the radioactive nucleus

Question 41

type of radiation emitted and half-life of an isotope determine which isotope is used for the following applications:

Answer 41

• household fire (smoke) alarms [ALPHA]
• irradiating food to kill bacteria [GAMMA]
• sterilisation of equipment using GAMMA rays
• measuring and controlling thicknesses of materials with the choice of radiations [DIFFERENT] used linked to penetration and absorption
• diagnosis and treatment of cancer using GAMMA rays

Question 42

use of ALPHA radiation: household fire alarms

Answer 42

• alpha particles used in smoke detectors
• alpha radiation ionises the air within the detector, creating a current across circuit gap
• alpha emitter is blocked when smoke enters the detector; no current created
• alarm is triggered by a microchip when the sensor no longer detects the alpha particles
• isotope of alpha radiation with long half-life used for smoke detectors so don’t need replacing often

Question 43

use of GAMMA: sterilisation of equipment

1. why is it most suited?
2. half-life? means what for equipment?

Answer 43

1. most penetrating out of all the types of radiation. penetrating enough to irradiate all sides of the instruments. Instruments can be sterilised without removing the packaging
2. source of gamma rad. used for steri. half-life of around 5 years, ∴ sterilisation equipment does not need to be replaced often

Question 44

use of GAMMA: irradiating food to kill bacteria

Answer 44

• to kill any microorganisms present; makes food last longer & reduces risk of food-borne infections

Question 45

use of DIFFERENT radiation: measuring the thickness of materials

rad. commonly used? uses of other ones?

Answer 45

📌 beta radiation most commonly used bc it will be partially absorbed by most materials

🔗 Alpha particles used for thinner materials bc lower penetrating power & are absorbed by a thin sheet of aluminium

📍 gamma radiation can be used for very thick materials because they have a higher penetrating power and are mostly absorbed by thick pieces of lead.

Question 46

uses of GAMMA: diagnosis and treatment of cancer

what happens? why is gamma used? half-life? why?

Answer 46

• radiation can kill living cells. Beams of gamma rays are directed at the cancerous tumour

📌 gamma rays are used because they can penetrate the body, reaching the tumour. Beams are moved around to minimise harm to healthy tissue whilst still being aimed at the tumour

• half-life of around 5 years; does not need to be replaced often within the machine that uses it

Question 46

PROCESS - measuring thickness of materials

Answer 46

• material moves across a radiation source
  -the particles that penetrate it are monitored using a detector above
  -thickness of the material is monitored

-if material gets thicker, more particles will be absorbed by the material, meaning that less will get through and be detected by the detector

-if the material gets thinner the opposite happens [less particles absorbed, more get through & detected]

-machine makes adjustments to keep the thickness of material constant

-radiation used to measure thickness of materials has a half-life of many years (10-20 years) so that count rate remains relatively constant each day

Question 47

tracer - what is it? what does it do? half-life?

Answer 47

• a radioactive isotope that can be used to track the movement of substances, like blood, around body. a PET scan can detect the emissions from a tracer to diagnose cancer and determine the location of a tumour
• the half-life of a tracer is several hours; provides time for a scan to be conducted and then the radiation to leave the body quickly

Question 48

explain why alpha radiation is used in smoke detectors, and beta or gamma radiation is not.

Answer 48

Alpha is the most weakly penetrating and strongest ioniser

Beta and gamma have stronger penetrating power and weaker ionising power

since alpha is absorbed by smoke, and beta and gamma are not, this makes it most suitable for use in a smoke detector

Question 49

effects of ionising nuclear radiations on
living things

Answer 49

cell death, mutations and cancer

Question 50

safety precautions for all ionising
radiation

Answer 50

📌 reducing exposure time [reduces the amount of radiation dose received, sieverts (Sv)]

📌 increasing distance between source and living tissue [reduces the size of the dose received, use of tongs, remote areas, underground]

📌 using shielding to absorb radiation [limit exposure]

Question 51

safe handling - risks associated with handling radioactive sources can be minimised by:

A student plans to use a gamma source to conduct an experiment. List four things that the student should do in order to minimise the risk to themselves when using the source.

Answer 51

any 4 from:

• keep source in lead-lined container until needed
• use tongs to move source, rather than handling it directly
• source should be kept at as far a distance from student as possible during experiment
• time that source is being used should be minimised
  -after experiment, student should wash their hands
  -date and the time that the radiation has been used should be recorded

Question 52

two of the social, economic or environmental issues involved in the storage of radioactive
materials with very long half-lives. [2]

Answer 52

• causes cancer
• high cost of storage
  -pollution of atmosphere

Question 53

Many of the charged particles produced by the Sun are emitted from its surface at high
speeds and travel out into space.
(i) Explain why these particles constitute an electric current.

Answer 53

charges are moving (and current is the (rate of) flow of charge)

Question 54

α-particles pass through air, they are more strongly ionising than β-particles.
Suggest two reasons why this is so.

[α-particles are more strongly ionising and have a shorter range in air than γ-rays. explain these differences.]

Answer 54

larger charge
slower moving
more massive
more chance of collision
more energy

[γ is not charged and does not have mass.
* α has mass and α has large size.
α more ionising because it has greater charge.
* γ has no charge so less ionising.]

Question 55

Sample 1 is in a container of thickness 0.5 mm and radiation can be detected coming
through the container.

i) State the type of radiation coming through the container of Sample 1. [1]
(ii) Explain your answer to (b)(i). [2]

Answer 55

beta;
beta stopped by 5 mm/thick Al ;
alpha stopped by 0.5mm/thin Al ;

Question 56

At first, the detector continues to register a low count rate sometimes slightly less than the
count rate registered without the source. The count rate suddenly increases to a very high
value when the source is very close to the detector.
Explain these changes in the count rate. [3]

Answer 56

any 3 of:

• low count rate due to background radiation only
• slightly less reading due to random nature of radioactivity
• very high reading due to α-particles

[sudden increase of count rate at limit of range of α-particles]

Question 57

In a second experiment, α-particles pass between two parallel, horizontal metal plates in a vacuum. They then continue to the detector.

A positive charge is established on the upper plate and a negative charge on the lower plate. [2]

Answer 57

• (count rate) decreases / background only
• deviation starts at start of plates

Question 58

He notices that the count rate registered by the detector every year is slightly smaller than it was the previous year.

Suggest why this is so [2]

Answer 58

-decay of source

• radioactive source half-life decreasing counts per second over time, so activity decreases

Question 59

i) one source of this background radiation [1]

ii) gamma-rays [2]

Answer 59

i) rocks and buildings; cosmic rays

ii) - electromagnetic radiation
(very) high frequency

Question 60

A lead sheet of thickness 10 mm is positioned between the detector and the radioactive source.

State and explain what happens to the count rate on the detector. [2]

Answer 60

• count rate decreases
  only some gamma rays detected

[gamma rays no deflection; bc gamma rays are uncharged]

Question 61

α-particles from the uranium source being directed at a very thin gold foil, in a vacuum. To investigate the scattering of α-particles, a detector is moved to different positions around the very thin gold foil and measurements are recorded.

Describe the results from this scattering experiment and explain what they show about the structure of atoms. [4]

Answer 61

most α-particles travel straight (through the foil)

nucleus small / atom mostly empty space

small number deflected (through large angles)

most of mass in nucleus

Question 62

State the effect of carrying out the experiment in air.

with gamma & alpha

Answer 62

α will be stopped by air/won’t move far
γ will continue OR air ionised by α

Question 63

State and explain the purpose of the lead cylinder.

Answer 63

only particles/rays in line with hole can pass through OR lead absorbs radiation

to produce a (thin) beam of α

Question 64

After 6 days the count rate hardly decreases and, in fact, increases a little at times.

Explain [2]

Answer 64

any mention background / background radiation varies randomly

varies due to random nature of radioactivity, OR rate of decay very small

Question 65

Use the graph to determine the half-life of the sample. Explain your working carefully. [4]

Answer 65

• choose a point alrdy done/plotted, & another.
• find halfway point, then do btwn this value & your lower val.
• e.g., 50 and 14. 50-14=36. 36/2=18. 14+18=32.
  -drawing lines on graph that look like ⁀I but straight, see where they intersect.
• this is half-life.
  i.e. 1.6