25 - Radioactivity Flashcards
ALPHA RADIATION - nature and charge
particle (helium nucleus), +2e
ALPHA RADIATION - ionising and penetrating effect
most ionising, least penetrating (stopped by a few mm of paper)
BETA RADIATION - nature and charge
particle (fast electron), -e
BETA RADIATION - ionising and penetrating effect
(medium) ionising effect, stopped by a few mm of aluminium
GAMMA RADIATION - nature and charge
EM wave, 0
GAMMA RADIATION - ionising and penetrating effect
least ionising, most penetrating (stopped by many cm of lead)
range of each type of radiation in air
alpha - a few cm
beta - a few m
gamma - a few km
radioactive decay is the…
emission of a radioactive particle from an unstable nucleus
why is alpha particle deflected less towards the negative plate when passing perpendicular to a uniform electric field?
alpha particle is heavier than beta+ particle so affected less
how to find the direction of deflection of particles due to a magnetic field?
Fleming’s left hand rule
dangers of radioactivity and precautions
ionising effect of radiation can destroy cells and mutate them (could lead to cancer)
so lead-lined storage, pair of tongs with long handles, not handling sources with hands
procedure to investigate the absorption of different radioactive materials
using GM tube and Geiger Counter, measure background count rate without source. Do three times and calculate average
number of particles = count rate / time interval
place source and calculate count rate for this source when a material is placed at fixed distance in front of it. vary thickness and make sure to minus background count rate to get count rate for source.
+ mention safety precautions
why is it not possible to carry out absorption experiments with positrons in the school lab?
positrons and electrons would annihilate each other
glow in the dark hands on clock explained
painted with zinc sulfide and radium. zinc sulfide glows in the dark due to being near the decay of radium emitting alpha particles
what is transmutation?
process of radioisotopes stabilising to create stable isotopes, releasing energy and matter from nucleus to a new element (parent to daughter nuclei)
what is conserved in all nuclear reactions?
nucleon number, proton number
conservation of energy and mass (energy released produced from mass)
nuclear transformation equation for alpha decay
A A-4 4
X —> Y + He
Z Z-2 2
nuclear transformation equation for beta minus decay
A A 0 -
X —> Y + e + Ve
Z Z+1 -1
nuclear transformation equation for beta plus decay
A A 0
X —> Y + e + Ve
Z Z-1 +1
why does gamma decay occur?
gamma only emitted if a nucleus has surplus energy following alpha or beta emission (composition of nucleus remains same)
nuclear transformation equation for gamma decay
A A
X —> X + 𝜸
Z Z
why might a decay series/chain occur
some daughter nuclei are still unstable so will continue to decay until stable
patterns for stability graph
all stable nuclei on narrow stability band surrounded by possible unstable nuclei, prediction of decay based on relative position to band
Z>82, decays by alpha emission
right of band, proton rich, beta + decay
left of band, neutron rich, beta - decay
why may count rate be lower than expected?
not all radiation travels in direction of detector (random + spontaneous)
RANDOM nature of decay
cannot predict when a nucleus will decay or which one will decay next (each nucleus has same chance of decaying per unit time)
SPONTANEOUS nature of decay
decay of nuclei unaffected by presence of other nuclei in sample and external factors such as pressure
simulation of radioactive decay using dice
each die has one side coloured black
roll all dice and count ‘decayed’ nuclei (black dice)
repeat until all dice have decayed
plot undecayed nuclei against roll number
(decay rate high at start and as number of undecayed nuclei decreases, so does decay rate)
define half life of an isotope
average time it takes for half the number of active nuclei in a sample to decay
define activity A of a source
rate at which nuclei decay or disintegrates
(number of alpha, beta, gamma photons emitted per unit time)
units of activity
Bq
(decays per second)
power equals
A X E
activity x energy
decay constant
defined as the probability of decay of an individual nucleus per unit time
ΔN∝NΔt
ΔN/Δt∝-N (-ve sign to show nuclei in sample is decrasing)
A=λN
radioactive decay is…
an exponential decrease
N=Noe^-λt
A=Aoe^-λt
N - number of undecayed nuclei
A - activity
also…
C=Coe^-λt (count rate)
P=Poe^-λt (power)
decay constant and half life
at t=t1/2, N=No/2
λt1/2 = ln2
therefore decay constant and half life are inversely proportional to each other
calculating N when number of half lives is known
N=(0.5)^n x No
N is number of nuclei left
n is number of half lives elapsed
No is nuclei at t=0
n is equal to t/t1/2
carbon dating
(method for determining age of of organic material)
1) the ratio of carbon-12 to carbon-14 is constant in all living things
2) once a living organism dies, it stops absorbing carbon and the carbon-14 continues to decay
3) the ratio of carbon-12 to carbon-14 is measured in a dead and a living sample
4) the activity equation is used to estimate the age of the sample
limitations to carbon dating
assumes the ratio of carbon-12 to carbon-14 is constant over time - may not be true due to increased emission of CO2 due to burning fossil fuels, volcanic eruptions)
carbon-14 cant date very old objects as its half life is not long enough (e.g. rocks)
