Unit 3.5 - Nuclear Decay Flashcards
What may radioactive substances do?
Emit one or more of 3 kinds of radiations
3 kinds of radiations
Alpha particles
Beta particles
Gamma rays
Alpha particles:
Symbol
Nature
Consists of
Charge
Mass
Penetrating power
Ionising power
a or He
Particle
2p+ and 2n
+2
4u
Least penetrating : stopped by a thin sheet of paper or 5cm of air at atmospheric pressure
Most ionising
Beta particles:
Symbol
Nature
Consists of
Charge
Mass
Penetrating power
Ionising power
B or e-
Particle
1e-
-1
0.0005u
More penetrating than alpha particles: can travel through a 3mm aluminium sheet or 1m of atmospheric air
Less ionising than alpha particles
Gamma rays:
Symbol
Nature
Consists of
Charge
Mass
Penetrating power
Ionising power
(Photon symbol)
Em radiation
Magnetic field + electric field
No charge
No mass
Most penetrating: passes through several cm of lead or more than 1m of concrete
Least ionising
Describe the wavelengths of gamma rays
Much shorter than that of light
Describe the speeds or alpha and beta radiation and explain
Similar speeds
About the same as the speed of light
Which type of radiation is safest outside of the body?
Alpha
Which type of radiation is safest inside the body?
Gamma
Which type of radiation is the most dangerous inside and outside of the body?
Beta
What type of process is radioactive decay?
Nuclear (not chemical!)
What does the fact that radioactive decay is a nuclear process, not a chemical process, mean?
It’s independent of temperature, pressure, whether it’s ionised e.t.c.
Force that regulates radioactive decay
Weak force
How can we differentiate between the different types of radiations and why does this work?
Using a magnetic field
Since all radiations have a different charge (positive, negative and no charge)
What does moving charge in a magnetic field experience?
A force
Example of moving charge in a magnetic field experiencing a force
Electrons in the coil of a motor
Why will all of the types of radiation bet effected differently by a magnetic field?
Due to charge differences
What happens to gamma radiation In a magnetic field and why?
Undeflected
Has no charge
Which type of radiation has a less severe deflection in a magnetic field and why?
Alpha
Heavier
Which type of radiation has the most severe deflection in a magnetic field and why?
Beta
Negative charge and lightest
What are the relative penetrating powers of radiations due to?
The way they interact with matter
What happens when alpha, beta or gamma radiation passes through matter?
It tends to knock electrons out of the atoms, ionising them
What’s another type of radiation?
Positron emission
What’s another type of radiation that causes ionisation?
X-rays
Ionising radiation
When radiation passes through matter, it knocks electrons out of the atoms, ionising them
Why are alpha particles strongly ionising?
Large
Relatively slow moving
Carry 2 positive charges
Why are gamma rays weakly ionising?
Electromagnetic waves
Carry no charge
Ionising power of beta particles
Falls between alpha particles and gamma radiation in ionising power
Which energy transfer occurs during ionisation?
Form the radiation passing through the matter to the matter itself
Why are alpha particles the least penetrating of the 3 radiations?
The transfer of energy from the radiation to the matter itself happens most avidly in the case of alpha particles since they are the most strongly ionising, and this is why alpha particles are the least penetrating of the three radiations.
Why are gamma rays the most penetrating type of radiation?
They transfer energy from the radiation to the matter less rapidly as they’re weakly ionising
Methods used to detect ionising particles
Spark counter
Cloud chamber
What does a spark counter do?
Counts ionisation of the air caused by alpha radiation (or a match)
Describe how a spark counter works
Alpha particle ionises air particles between the gap, which is accelerated upwards and causes a cascade of ions hitting each other = a spark
Each ionisation particle coming in causes its own spark
What happens within a cloud chamber?
Radioactive sources leave trails as they streak through the saturated vapour
Describe what happens in a cloud chamber
Ionising particles ionise the air as they travel though, ripping electrons from the particles’ outer shells = ionise them
When the particle is ionised, the alcohol is drawn towards it and forms the track in the cloud chamber
Why are alpha particles stopped much more easily than beta particles with the same energy, yet alpha particles are much more massive than beta particles?
Alpha particles…
-move much slower
-have double the charge
= cause much more ionisation than beta particles
Most ionising, leat penetrating
Under which conditions does ionising ration travel further? Why?
If the pressure is reduced
Fewer molecules to interact with
What happens to ionising radiation if the pressure is reduced and why?
Travels further
Fewer molecules to interact with
Working out the range of a particle
Initial energy
———————
KE lost per collision x collisions per cm
=answer in cm
What has the highest range, alpha or beta particles? Why?
Beta particles
Alpha particles can’t travel through more than 5cm of air
When does radioactive decay occur?
When an unstable nucleus emits alpha, beta or gamma radiation in order to become more stable
Why does radioactive decay result in a reduction of the energy of a nucleus?
During the process, the radiation carries away energy as it is emitted from the nucleus, resulting in a reduction of the energy of the nucleus
What do alpha particles consist of?
2 protons, 2 neutrons
= helium Nuclei
When does the emission of an alpha particle occur?
When a nucleus ejects 2 protons and 2 neutrons
What happens to the original nucleus when it emits an alpha particle?
Reduces its nucleon number by 4
Reduces its proton (or atomic) number by 2
Another word for proton number
Atomic number
Name for when a new chemical element is formed
The original element has transmuted
What is a beta particle?
A high energy electron
When is beta decay emitted?
When a neutron in the nucleus decays to form a proton and an electron
How fast do beta particles move?
As much a 99% of the speed of light
What happens to the electron when a nucleus decoys to form a proton and an electron?
Ejected from the nucleus at high speed
What happens to the original nucleus when it decays by beta radiation?
Proton number of the nucleus increases by one
Nucleon number remains unchanged
What would also be present during the decay of carbon-13 that isn’t shown in radioactive decay equations?
Anti neutrino (for lepton number conservation)
How is a nucleus left when it emits an alpha or beta particle?
In an excited state - a state in which it has excess energy
When would a nucleus emit a gamma ray?
When a nucleus emits an alpha or beta particle, it is often left in an excited state, a state in which it has excess energy
In order to lose this energy and so become stable, the nucleus may emit a gamma ray
What type of radioactive emission is often a 2 part process, with alpha or beta emission preceding it?
Gamma emission
What happens to the original nucleus after gamma emission?
Composition of nucleus is unchanged in the process = element remains unchanged
What is often used in producing gamma radiation for industrial use?
Cobalt-60
Explain gamma radiation in the decay of cobalt-60
The product of the first disintegration ( after beta emission), the daughter nuclide, was itself unstable and underwent further changes
Radioactive decay series
When a nucleus undergoes a whole series of decays
Nucleon number
Top number
Proton number (or atomic number)
Bottom number
Why is there a continuous background count of radioactivity?
It is a naturally occurring phenomenon
Examples of naturally occurring radioactivity which contributes to the continuous background count
Rocks (e.g - granite)
Cosmic rays
What must be taken into account when conducting experiment using radioactive materials?
The background count
Why do we work out the background count of radioactivity when conducting experiments using radioactive materials ?
There’s a “zero error” to be considered prior to making any measurements
It helps us work out how much radioactivity is from the source v.s how much is already in the environment
How do we complete a background count of radioactivity?
Measure a background count per minute in the absence of any radioactive materials
What do we do with our background count?
Subtract it from the measured count during the experiment
Unit of radioactivity
Bequerel (Bq)
1 Bq
1 counts-1
Can radioactivity be predicted? Why?
No - the mission of radioactivity by an atom occurs spontaneously and quite unpredictably = its impossible to predict
What is radioactive decay unaffected by?
External conditions like chemical reactions, pressure and temperature
What’s the only things that governs that overall rate of decay of nuclei?
The number of nuclei left un-decayed
What can we write if we have N (number of unstable atoms) at time t?
Can write the rate of change of the number of unstable atoms with time as dN/dt
N
Number of unstable atoms
Activity A of a source unit
Bq
Activity A of a course
The rate of change of the number of unstable atoms with time
dN/dt
What is the rate of change of the number of unstable atoms with time proportional to and what can we therefore write?
Proportional to the number of unstable atoms, N, so we may write dN/dt proportional to N
Or dN/dt = -lambdaN = A
What is the lambda symbol in this unit?
The decay constant
Why is there a negative sign included in dN/dt = -(lambda)N = A?
Because N is a decreasing quantity and therefore dN/dt is negative, while N itself is positive
Equation for radioactive decay
N = Noe ^-(lambda)t
How do we get to the radioactive decay equation?
dN/dt = -(lambda)N = A
If this is integrated with respect to time…
N = Noe ^-(lambda)t
Integrated with respect to time
The area under the graph will be equal to the integral
N in radioactive decay equation
Number of undeclared nuclei
No in radioactive decay equation
Number of undecayed (unstable) nuclei at t=0
What does a larger value of lambda in the radioactive decay equation lead to?
A more rapid radioactive decay
What does a graph of N v.s t show?
Exponential decay
What’s half life labelled as?
t1/2
Ratio of decays in one half life
Always a half
What does a shorter half life lead to on an N against t graph?
More steep curve
What is the activity A of a sample directly proportional to?
The number of undecayed atoms
Radioactive decay equation in terms of A
A = Aoe^-(lambda)t
How do we get the radioactive decay equation in terms of A?
Activity A of a sample is directly proportional to the number of undecayed atoms in it
A
Activity of a source at time t
Ao
Activity of a source at time t=0
What does a higher decay constant lead to?
Faster decay
What happens if something has a shorter half life?
Decays faster
How do we find the value of t with A = Aoe^lambdat?
Take logs
Symbol for half life
T1/2 or t1/2
Half life
The time taken for half the radioactive atoms to decay
What will have happen when t= t1/2?
Half the radioactive atoms will have decayed
Derive the relationship between the decay constant (λ) and the half life (T1/2)
N = Noe^-λt
At one half life, N=No/2
Inputting this…
No/2 = Noe^-λt
The No cancels on both sides so…
1/2 = e^-λt1/2
If we take the natural log on both sides to remove the exponential function…
loge (1/2) = -λt1/2
Using the log rule log (a/b) = log (a) - log (b)…
Log (1/2) = log (1) - log (2) = -λt1/2
Since log (1) = 0
Log (1/2) = -log(2) = -λt1/2
That is…
T1/2 = log(2)/λ
Which is about…
0.693/λ
When does the expression for half life give us the half life in seconds?
If the decay constant has units s-1
Describe the half life for something with high activity
Short half life
Describe the activity of something with a short half life
High activity
What does the equation for half life give us the value of in if the decay constant has units s-1?
Seconds
“Activity”
Decays per second (Bq)
What does decay do in any sample of radioactiv material?
Follows a simple pattern
How many of the atoms remain undecayed in a sample after 2 half lives?
1/2^2
How do we work out how many atoms have remained undecayed after a certain number of half lives?
After n half lives, 1/2^n of the atom remain undecayed
How many half lives will have occurred in a year if the half life is 60 days?
365/60
6.083
What is the activity of a sample directly proportional to?
The number of un-decayed atoms
How do we work out the activity of a sample knowing the initial activity and the number of undecayed atoms? Why does this work?
Initial activity x undecayed atoms
Both of these are directly proportional
How do we work out N from mass and molar mass?
N = mass/molar mass x number of atoms (avogadro constant)
Another word for nucleon number
Mass number
Describe the activity of a sample if the half life is short
High activity
What do we do with percentage exam equations?
Remember that initial N will be 100%
What has to be true in order to prove that a specific type of radiation is present?
There has to be a significant drop in count rate when these are introduced
Why does alpha radiation have less severe reflection than beta particles?
heavier
slower (momentum of beta effects it)
describe the binding energy per nucleon of the products of both fusion and fission - what does this mean?
product nuclei have higher BE per nucleon, so they’re more stable
% decrease
Remember to do 100- % remaining which was worked out from the equation
Equation for the number of half lives using half life
x = t/T1/2
