Syllabus Edition

First teaching 2023

First exams 2025

Radioactive Decay (HL IB Physics)

Topic Questions

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1 mark

Which of the following is the unit for the decay constant λ?

Bq s⁻¹
s
s⁻¹
The decay constant has no units

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1 mark

Which of the following graphs of the number of undecayed nuclei N over time has the smallest decay constant?

CSFNQLD~_12-2-ib-hl-mcqs-easy-q7-answers

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1 mark

Which of the following provides evidence for the existence of neutrinos?

Some particles were indistinguishable from their antiparticles
Nuclear energy levels have a continuous spectrum
The energy of emitted beta particles have a continuous spectrum
The emission spectrum only contains specific wavelengths

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1 mark

A radioactive element has a decay constant λ. At time, t = 0 the number of nuclei of this element is N₀. These are linked by the equation $N_{t} = N_{0} e^{- λ t}$ where N_t is the number of nuclei expected at time t.

What number of nuclei would you expect to remain after 1 second?

$0$
$N_{0} (1 - e^{- λ})$
$N_{0} e^{- λ}$
1

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1 mark

An isotope of ${}_{95}^{241}{Am}$ undergoes alpha-decay.

Which letter on the N-Z graph below represents the correct proton-neutron product of the decay?

7-1-discrete-energy--radioactivity-sl-mcqs-easy-q6-question-diagram

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1 mark

Uranium−238 undergoes radioactive decay to become Thorium−234. A second decay occurs and the product is protactinium−234.

The decay sequence can be represented in equation form, where i, j and k are particles.:

${}_{92}^{238}U \to {}_{90}^{234}{Th} + i {}_{90}^{234}{Th} \to {}_{91}^{234}{Pa} + j + k$

What are the correct names of these particles?

	i	j	k
A	alpha	beta−minus	electron anti−neutrino
B	beta−positive	beta−minus	alpha
C	beta−minus	neutron	photon
D	alpha	beta−positive	neutron

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1 mark

What are the correct numbers of protons, neutrons and electrons in a neutral atom of ${}_{92}^{234}U$ ?

	protons	neutrons	electrons
A	92	92	92
B	92	142	92
C	142	92	142
D	234	142	92

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1 mark

What is the charge on, and mass of, an electron neutrino and during what process is an electron neutrino produced?

	charge /e	mass / u	production of neutrino
A	+1	zero	during β⁺ emission
B	−1	+1	during β⁻ emission
C	zero	zero	during β⁺ emission
D	zero	0.0005	during β⁻ emission

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1 mark

A sample of californium−239 has an activity of 4000 Bq. The half−life of californium−239 is 1 minute.

What will the activity be after 4 minutes?

4000 Bq
1000 Bq
500 Bq
250 Bq

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1 mark

Energy-mass equivalence is given by ΔE = Δmc².

Using the given equation, determine which of the following is a valid unit of mass.

$M e V$
$\frac{M e V}{c}$
$\frac{M e V}{c^{2}}$
$e V$

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1 mark

The average binding energy per nucleon of Neon−20 $({}_{10}^{20}{Ne})$ nucleus is 8.0 MeV.

What is the total energy required to separate the nucleons of one nucleus of ${}_{10}^{20}{Ne}$ ?

0 MeV
8 MeV
160 MeV
800 MeV

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1 mark

The graph shows the binding energy per nucleon against nucleon number.

7-2-nuclear-reactions-sl-mcqs-easy-q4-question

Which row in the table gives possible elements found on the graph at positions X, Y and Z?

	X	Y	Z
A.	Uranium	Calcium	Xenon
B.	Hydrogen	Uranium	Iron
C.	Calcium	Hydrogen	Iron
D.	Hydrogen	Iron	Uranium

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1 mark

Which statement is a definition of binding energy per nucleon?

The difference between an atom's mass and the sum of the masses of its nucleons
The binding energy of a nucleus divided by the number of nucleons in the nucleus
The energy required to break a nucleus into its constituent protons and neutrons
The amount of kinetic energy required for fusion to occur

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1 mark

Following the development of the atomic bomb, scientists discovered that they could create elements heavier than uranium by bombarding nuclei with neutrons.

Two incomplete examples are given below:

Reaction 1: ${}_{92}^{238}U + {}_{0}^{1}n \to {}_{. . .}^{. . .}U$

Reaction 2: ${}_{94}^{239}P u + 2 {}_{0}^{1}n \to {}_{. . .}^{. . .}A m + {}_{- 1}^{0}e$

What are the missing products of these reactions?

	Reaction 1	Reaction 2
A.	${}_{92}^{239}U$	${}_{95}^{240}{Am}$
B.	${}_{92}^{239}U$	${}_{95}^{241}{Am}$
C.	${}_{92}^{237}U$	${}_{95}^{237}{Am}$
D.	${}_{92}^{237}U$	${}_{95}^{241}{Am}$

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1 mark

A radioactive source X consists of 10.4 × 10¹¹ atoms of a nuclide of half-life 5 days. A second source Y consists of 5.2 × 10¹⁰ atoms of another nuclide of half-life 6 days.

After how many days will the number of radioactive atoms in X be equal to Y?

$\frac{30 \ln (2)}{\ln (20)}$
$\frac{\ln (20)}{30 \ln (2)}$
$\frac{30 \ln (20)}{\ln (2)}$
$\frac{\ln (2)}{30 \ln (20)}$

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1 mark

Two radioactive elements X and Y have half–lives T_X and T_Y respectively. Initially samples of S, N_X contains three times as many atoms of Y, N_Y .

After a certain time t, which of the expressions for $\frac{n u m b e r o f d e c a y e d a t o m s o f X}{n u m b e r o f d e c a y e d a t o m s o f Y}$ is correct?

$\frac{3 (N_{Y} - N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{X}}})}{N_{Y} - N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}}}$
$\frac{N_{X} {(\frac{1}{2})}^{\frac{t}{T_{X}}} - N_{X}}{N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}} - N_{Y}}$
$\frac{N_{X} {(\frac{1}{2})}^{\frac{t}{T_{X}}}}{N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}}}$
$\frac{{3 N}_{Y} {(\frac{1}{2})}^{\frac{t}{T_{X}}}}{N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}}}$

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1 mark

The initial activity of a radioactive source is 160 counts per second. After a time T, its activity becomes 5 counts per second.

If the half–life of the source is 18 hours, what is T?

$\frac{\ln (32)}{18 \ln (2)}$ hours
$\frac{18 \ln (32)}{\ln (2)}$ hours
$\frac{\ln (2)}{18 \ln (32)}$ hours
$\frac{18 \ln (2)}{\ln (32)}$ hours

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1 mark

A pure sample of a radioactive nuclide has mass m, half-life T_1/2 and initial activity A₀.

Identify the half-life and initial activity of another sample which is otherwise identical but has mass 3m.

	Half-life	Initial activity
A.	$T_{1 / 2}$	$A_{0}$
B.	$3 T_{1 / 2}$	$\frac{1}{3} A_{0}$
C.	$T_{1 / 2}$	$3 A_{0}$
D.	$3 T_{1 / 2}$	$3 A_{0}$

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1 mark

Two radioactive nuclides, P and Q, have half-lives of 70 s and 175 s respectively. At time t = 0, samples of P and Q contain the same number of nuclei.

What is $\frac{n u m b e r o f n u c l e i o f P d e c a y e d}{n u m b e r o f n u c l e i o f Q d e c a y e d}$ when t = 350 s?

8
$\frac{24}{31}$
$\frac{31}{24}$
$\frac{1}{8}$

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1 mark

Two unstable isotopes are initially present in equal numbers. Isotope Y has a half life of 6 minutes and isotope Z has a half life of 3 minutes. Which expression correctly describes the ratio of the activity of Y to Z after 12 minutes?

$\frac{e^{- \frac{l n 2}{2} \times 12}}{e^{- \frac{\ln 2}{2} \times 12}}$
$\frac{3}{6} \times \frac{e^{- \ln 2 \times 12}}{e^{- l n 2 \times 12}}$
$\frac{1}{2} \times \frac{e^{- 4 \ln 2}}{e^{- 3 \ln 2}}$
$\frac{1}{2} \times \frac{e^{- 2 \ln 2}}{e^{- 4 \ln 2}}$

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1 mark

A pure sample of a known element has a very short half-life. What measurement(s), together with the initial activity of the sample, must be made in order to measure the half-life of the element?

The number of moles of the sample.
The activity and the number of moles of the sample after a given period of time.
The number of moles after a given period of time.
The activity after a given period of time.

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1 mark

A particular nucleus, ${}_{Z}^{A}X$ decays to form ${}_{z}^{a}Y$ , which then has two possible decay paths, resulting in either an isotope of Thallium (Tl) or Polonium (Po), as shown.

7-1-sl-mcq-hard-q2-phy

Which isotope could not be ${}_{Z}^{A}X$ ?

${}_{82}^{215}{Pb}$
${}_{84}^{214}{Po}$
${}_{85}^{218}{At}$
${}_{82}^{214}{Pb}$

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1 mark

The decay series for Thorium–232 can be plotted as shown.

7-1-ib-sl-hard-mcqs-q3-question

According to the diagram, how many possible routes are available for Thorium–232 to decay to a stable nucleus of lead−208?

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1 mark

The diagram shows the decay chain for a particular isotope.

7-1-sl-mcq-hard-q4-phy

What are the isotopes labelled 1, 2 and 3?

	1	2	3
A	${}_{90}^{230}T h$	${}_{85}^{218}A t$	${}_{82}^{210}P b$
B	${}_{92}^{234}U$	${}_{85}^{218}A t$	${}_{82}^{210}P b$
C	${}_{92}^{234}U$	${}_{84}^{218}P o$	${}_{81}^{210}T l$
D	${}_{90}^{230}T h$	${}_{84}^{218}P o$	${}_{82}^{210}P b$

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1 mark

Polythene film is made by extruding polythene and drawing it into a long sheet which is wound onto a roll, as shown in the diagram below.

7-1-sl-mcq-hard-q5-phy

The manufacturer has to carefully choose a radioactive isotope which will effectively remove any electrostatic charge that has built up on the film. The manufacturer also has to consider the point X, Y or Z at which the radioactive isotope should be placed along the assembly line.

At which point should the radioactive isotope be placed, and which type of radiation should the manufacturer choose to remove excess electrostatic charge?

	Point	Type of radiation
A	X	alpha
B	X	gamma
C	Y	beta
D	Z	alpha

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1 mark

Radioactive isotopes are used in medical imaging.

For example, two isotopes of iodine, 123-I and 131-I are both used by radiographers, who give them to patients in the form of iodide before taking scans.

The possible decay equations for these isotopes are:

${}_{53}^{123}I \to {}_{52}^{123}T e$ + _____

2. ${}_{53}^{131}I \to {}_{54}^{131}X e$ + _____

Which is a correct difference between these two decays?

In decay 1, an electron neutrino is emitted, but in decay 2 an anti-electron neutrino is emitted
In decay 1, an electron has been absorbed but in decay 2 a neutron has been absorbed
In decay 1, an anti-electron neutrino is emitted, but in decay 2 an electron neutrino is emitted
In decay 1, a neutron became a proton but in decay 2 a proton became a neutron

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1 mark

The graph shows the how the neutron-proton ratio of stable nuclei changes as proton number increases.

7-1-sl-mcq-hard-q8-phy

What other changes with increasing proton number are suggested by the graph?

The charge-mass ratio increases
The charge-mass ratio decreases
The likelihood of α-decay increases
The likelihood of β-positive decay increases
Protons outnumber neutrons in large nuclei, with a rapidly increasing ratio

1 and 3
2 and 3
2, 4 and 5
2, 3 and 5

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1 mark

The charge to mass ratio, $\frac{Q}{m}$ , is also known as the specific charge.

What is the ratio of $\frac{specific charge of {}_{3}^{7}{Li} nucleus}{specific charge of {}_{8}^{16}{O^{2 +} ion}}$ ?

$\frac{6}{7}$
$\frac{7}{6}$
$\frac{24}{7}$
$\frac{7}{24}$

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1 mark

The number of undecayed atoms, N, of a radioactive substance after time, t, can be calculated by the equation:

$N = N_{0} {(\frac{1}{2})}^{\frac{t}{T_{\frac{1}{2}}}}$

Where:

$T_{\frac{1}{2}}$ = the half-life
$N_{0}$ = the original number of atoms

Two radioactive elements X and Y have half–lives T_X and T_Y respectively. An initial sample contains three times as many atoms of X, N_X than atoms of Y, N_Y.

After a certain time, t, which of the expressions for $\frac{n u m b e r o f d e c a y e d a t o m s o f X}{n u m b e r o f d e c a y e d a t o m s o f Y}$ is correct?

$\frac{3 (N_{Y} - N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{X}}})}{N_{Y} - N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}}}$
$\frac{N_{X} {(\frac{1}{2})}^{\frac{t}{T_{X}}} - N_{X}}{N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}} - N_{Y}}$
$\frac{N_{X} {(\frac{1}{2})}^{\frac{t}{T_{X}}}}{N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}}}$
$\frac{3 N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}}}{N_{Y} {(\frac{1}{2})}^{\frac{t}{T_{Y}}}}$

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1 mark

A nucleus ${}_{Z}^{A}X$ has a mass M.

Which of the following expressions correctly represents the binding energy per nucleon of this nucleus in terms of the speed of light c, the mass of a proton m_p and the mass of a neutron m_n?

$(\frac{Z m_{p} + A m_{n} - M}{A}) c^{2}$
$(\frac{Z m_{p} + (A - Z) m_{n} - M}{Z}) c^{2}$
$(\frac{Z m_{p} + (Z - A) m_{n} - M}{A}) c^{2}$
$(\frac{Z m_{p} + (A - Z) m_{n} - M}{A}) c^{2}$

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1 mark

A nuclear reaction can be written in the form:

$W + X = Y + Z$

Energy is released during this reaction.

Which of the following is correct regarding the masses m and the binding energies b of the nuclides?

	Binding energy	Mass
A.	$b_{W} + b_{X} < b_{Y} + b_{Z}$	$m_{W} + m_{X} < m_{Y} + m_{Z}$
B.	$b_{W} + b_{X} > b_{Y} + b_{Z}$	$m_{W} + m_{X} < m_{Y} + m_{Z}$
C.	$b_{W} + b_{X} < b_{Y} + b_{Z}$	$m_{W} + m_{X} > m_{Y} + m_{Z}$
D.	$b_{W} + b_{X} > b_{Y} + b_{Z}$	$m_{W} + m_{X} > m_{Y} + m_{Z}$

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1 mark

One possible fission reaction for uranium−235 is:

${}_{92}^{235}U \to {}_{X}^{141}{Ba} + {}_{Z}^{Y}{Kr} + 3 {}_{0}^{1}n$

The proton number of barium is 20 more than the proton number of krypton. The measured mass of this nucleus is M.

Which expression gives the mass defect associated with the krypton nucleus?

$56 m_{p} + 35 m_{n} - M$
$36 m_{p} + 55 m_{n} - M$
$\frac{36 m_{p} + 55 m_{n}}{M}$
$56 m_{p} + 55 m_{n} + M$

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1 mark

The mass of a nucleus of rutherfordium−254 is 254.1001u.

Which of the following is not equal to the energy of this nucleus?

^{$254.1001 \times 931.5 M {eV c}^{- 2}$}
$254.1001 \times 931.5 \times (1.6 \times 10^{- 19}) J$
$254.1001 \times (1.661 \times 10^{- 27}) \times {(3.00 \times 10^{8})}^{2} J$
$254.1001 \times 931.5 \times (1.6 \times 10^{- 13}) J$

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1 mark

A stationary uranium-238 nucleus decays by alpha-emission, as shown in the equation below:

${}_{92}^{238}U \to {}_{90}^{234}{Th} + {}_{2}^{4}α$

This decay generates a total energy E.

Which of the following statements is correct regarding the kinetic energy of the alpha particle?

It is slightly less than 0.5E
It is equal to E
It is slightly greater than 0.5E
It is slightly less than E

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1 mark

Burning 1000 kg of coal can yield 3 × 10¹⁰ J of energy. When a thermal neutron is absorbed by a uranium-235 nucleus it produces fission products as well as three neutrons.

${}_{0}^{1}n + {}_{92}^{235}U \to {}_{40}^{98}{Zr} + {}_{52}^{135}{Te} + 3 {}_{0}^{1}n + energy$

The following data is available:

Mass defect ≈ 0.2 u
Mass of ${}_{92}^{235}U$ ≈ 4 × 10⁻²⁵kg
Atomic mass unit, u ≈ 2.0 × 10⁻²⁷ kg

What is an estimate of the mass of coal needed to give the same amount of energy as 1 kg of Uranium-235?

3 × 10⁴kg
2.6 × 10⁶ kg
4.7 × 10⁸ kg
7.7 × 10¹³ kg

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1 mark

Two samples P and Q of different radioactive isotopes have the same initial activity. Sample P has a third of the number of atoms as sample Q. The half-life of P is T seconds.

What is the half-life of sample Q?

$\frac{T}{3}$
$\frac{T}{3 \sqrt{2}}$
$3 T$
$3 \sqrt{2} T$

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1 mark

The half-life of a radioactive isotope is 10 days. What is the approximate percentage of the sample remaining after 25 days?

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1 mark

A pure sample of a radioactive nuclide undergoes a decay resulting in a stable daughter nuclide. What fraction of the sample is made up of the daughter nuclide after five half-lives?

$\frac{1}{16}$
$\frac{1}{32}$
$\frac{15}{16}$
$\frac{31}{32}$

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1 mark

A series of radioactive decays produce both alpha particles and anti-neutrinos. Which answer option describes the nature of the energy spectrum of these two particles?

	Anti-neutrino	Alpha-particle
A.	discrete	discrete
B.	discrete	continuous
C.	continuous	discrete
D.	continuous	continuous

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1 mark

The decay of several unstable nuclei are observed in a radiation chamber. The following identifications are made:

Alpha-particles have discrete kinetic energy levels
Beta-particles have a continuous range of kinetic energies
Gamma-ray photons have discrete kinetic energy levels

These observations directly relate to two important discoveries; the use of emission spectra to identify elements and the existence of the neutrino.

Which row correctly identifies the observations that lead researchers to the discovery of emission spectra and the existence of the neutrino?

	Emission spectra	Existence of the neutrino
A.	II only	II only
B.	I and III only	II only
C.	II only	I and III only
D.	I and III only	I and II

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1 mark

A radioactive sample of initial activity 10.0 Bq has a half-life of 1 day.

What is the activity of the sample after 2.5 days?

0.8 Bq
1.2 Bq
1.8 Bq
7.6 Bq

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1 mark

${}_{92}^{238}U$ decays to thorium-234 by emitting an alpha particle and two gamma rays. Thorium-234 then decays into protactinium via beta decay.

Which point on the N-Z graph below represents the position of the granddaughter nucleus, protactinium?

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1 mark

The half-life of carbon-14 is 6000 years.

An ancient elephant tusk has been uncovered and its age is unknown. A 20 g sample of the tusk has an activity of 1.25 Bq due to carbon-14.

A 80 g sample of tusk taken from a living elephant has an activity of 20 Bq.

Use this information to determine the age of the ancient tusk.

3000 years
12 000 years
18 000 years
24 000 years

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1 mark

Fluorodeoxyglucose is a compound used as a tracer in medical imaging. The isotope fluorine-18 is used, which is a positron emitter.

The way these positrons interact with electrons in the body allows PET (positron emission tomography) scanners to determine the rate of respiration certain cells are performing.

Fluorine-18 decays into an isotope of oxygen.

Which equation below represents the correct nuclear equation for this decay?

${}_{9}^{18}F \to {}_{8}^{18}O + {}_{+ 1}^{0}β + υ_{e}$
${}_{9}^{18}F \to {}_{8}^{18}O + {}_{- 1}^{0}β + \bar{υ_{e}}$
${}_{9}^{18}F \to {}_{9}^{18}O + {}_{+ 1}^{0}β + υ_{e}$
${}_{9}^{18}F \to {}_{8}^{18}O + {}_{+ 1}^{0}β + \bar{υ_{e}}$

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1 mark

Protactinium-231 ( ${}_{91}^{231}{P a}$ ) is a radioactive element, it decays by alpha radiation and then beta-minus decay as shown below:

${}_{91}^{231}{P a} \to A + α \to B + β^{-} + \bar{υ_{e}}$

What proton number and mass number will element B have?

	Proton Number	Mass Number
A.	89	229
B.	90	229
C.	89	227
D.	90	227

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1 mark

A radioactive source is known to emit $β$ radiation. A Geiger-Muller tube is used to measure the count rate at increasing distances from the source.

Wh_ich graph correctly represents the variation in count rate over these distances for $β$ radiation?

q16_discrete-energy-_-radioactivity_ib-sl-physics-mcq

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1 mark

Unstable nuclei make up 10% of a sample’s mass. The count rate of the sample is measured over a time period of 8 hours.

q17_discrete-energy-_-radioactivity_ib-sl-physics-mcq

After some time has passed, the percentage of the sample which is unstable reduces to 2.5%. What is the count rate of the source at this time?

90 cpm
60 cpm
45 cpm
30 cpm

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1 mark

A source is known to be radioactive but the type of radiation being emitted is unknown.

A Geiger-Müller tube is placed close to the source and different materials are placed between the two. A table of the count rates recorded for each material is shown below. The background count rate is 15 counts per minute.

Material	Count rate recorded / counts per minute
Paper	528
Nothing	1064
Thick lead	17
Aluminium	524

What types of radiation are being emitted by the source?

$α, β$ and $γ$
$α$ only
$β$ and $γ$
$α$ and $γ$

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1 mark

Three of the four isotopes below are the same element. Which isotope represents a different element?

	Nucleon number	Neutron number
A.	233	141
B.	235	143
C.	238	146
D.	239	146

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1 mark

The image below shows a simplified version of the binding energy per nucleon E of nuclei versus the nucleon number N. Which of the following positions represents nuclei that are the most stable?

q11_nuclear-reactions_ib-sl-physics-mcq

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1 mark

The rest mass of a nucleus of Boron-11 ( ${}_{5}^{11}B$ ) can be considered as m_B. The rest-masses of a neutron and proton can be considered as m_N and m_P respectively. Which of the following equations is the correct representation for the binding energy of Boron-11?

(5m_P + 6m_N – m_B)c²
(5m_P + 6m_N + m_B)c²
(5m_P + 11m_N – m_B)c²
(6m_P + 5m_N – m_B)c²

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1 mark

The binding energy per nucleon is 7.98 MeV for an atom of $_{8}^{16} O$ . Approximately how much energy would be needed to completely separate the nucleons of this atom?

33.2 MeV
63.9 MeV
88.5 MeV
127.7 MeV

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1 mark

The mass defect for Helium-4 is 5.04 × 10^–29kg. What is the binding energy of Helium-4 closest to?

0.02 MeV
28 MeV
190 MeV
1225 MeV

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1 mark

Two identical nuclei of mass m fuse to form a single heavier nucleus (with no other products) with mass M. Which of the following statements is correct?

m = M
2m = M
2m > M
2m < M

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1 mark

Nitrogen-14 can be transformed into Oxygen-17 by bombardment with high energy alpha particles, as described in the nuclear reaction equation below:

${}_{7}^{14}N + {}_{2}^{4}{H e} \to {}_{8}^{17}O + {}_{1}^{1}H$

The total rest mass of the reactants is 18.006 u and total rest mass of the products is 18.007 u.

Which of the following statements about this reaction is correct?

A mass of 0.001 u has been converted to about 1 MeV of energy
The kinetic energy of the products exceeds the kinetic energy of the reactants by about 1 MeV
The kinetic energy of the reactants exceeds the kinetic energy of the products by about 1 MeV
The mass defect of this reaction is 0.002 u

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1 mark

The graph below shows how the average binding energy per nucleon varies with nucleon number for stable nuclei.

q20_nuclear-reactions_ib-sl-physics-mcq-tif

Approximately how much energy is released when the nucleus ${}_{70}^{181}X$ forms?

7.90 MeV
1430 MeV
533 MeV
888 MeV

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1 mark

Which statement about nuclear binding energy is correct?

It is the energy equivalent of the mass of the neutrons in a nucleus
It is the energy required to separate nucleons in a nucleus
It is the energy required to overcome the electrostatic force between nucleons in the nucleus
It is the energy required to remove a single nucleon from a nucleus

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