chapter 25 - radioactivity

Question 1

ionising radiation

Answer 1

can ionise atoms by removing some of their electrons, leaving positive ions eg alpha, beta, gamma

Question 2

cloud chamber

Answer 2

• used to detect the presence of ionising radiation
• contains air saturated with vapour at a low temp
• when air molecules are ionised liquid condenses onto the ions to leave tracks of droplets marking the path of radiation

Question 3

alpha radiation

Answer 3

consists of 2 protons and 2 neutrons - helium nucleus
2+ charge
strong ionising power
absorbed by few cm of air + paper
constant energy

Question 4

beta radiation

Answer 4

fast moving electron or positron
beta minus has -1 charge beta plus has +1
less ionising
absorbed by thin aluminium
energy varies for each source

Question 5

gamma radiation

Answer 5

consist of high energy photons
travel at speed of light
carry no charge
least ionising
absorbed by thick lead
constant energy

Question 6

electric and magnetic fields on radiation

Answer 6

alpha - deflected towards neg plate
beta - deflected towards pos plate - less due to greater mass
gamma - not deflected due to no charge

Question 7

absorption experiments

Answer 7

use a GM tube and counter
- keep at fixed distance
- count background radiation
- then but an absorber between and count
- corrected count rate = count - background rate

Question 8

dangers of radioactivity

Answer 8

all radiation causes ionisation
- can damage living cells
must
- store radiation in lead
- use tongs to transfer with long handles
- never handle with bare hands

Question 9

transmutation

Answer 9

when the nuclei of one element emit alpha they change (transmutate) into to nuclei of another element
nucleus before is parent and after is daughter

Question 10

when does nuclear decay stop

Answer 10

when stable nuclei are formed

Question 11

alpha decay

Answer 11

the mass no. loses 4
the atomic no. loses 2

Question 12

beta minus decay

Answer 12

caused by weak nuclear force
one neutron decays into a proton and an electron is released with an electron anitneutrino
mass no. same
atomic no. +1

Question 13

beta plus decay

Answer 13

cause by weak nuclear force
one proton decays unto a neutron - a positron is released with an electron neutrino
mass no. same
atomic no. -1

Question 14

gamma decay

Answer 14

gamma photons are emitted if a nucleus has too much energy after alpha or beta emission
doesnt change mass or atomic no

Question 15

random

Answer 15

• cant predict when a nucleus will decay or which will decay next
• each nucleus has the same change of decay /time

Question 16

radioactive decay is described as

Answer 16

random and spontaneous

Question 17

spontaneous

Answer 17

decay is not affected by
- presence of other nuclei
- external factors eg pressure
- cant be sped up or slowed down

Question 18

half life

Answer 18

average time it take for half the number of active nuclei in a substance to decay

Question 19

activity

Answer 19

the rate at which nuclei decay
= number of nuclei decaying /time
1 decay / sec = 1Bq

Question 20

decay constant

Answer 20

λ
the probability of the decay of an individual nucleus per unit time
A = λN
dN/dt = -λN

Question 21

exponential decay

Answer 21

ΔN/Δt = -λN
N = N0 e^-λt
m = m0 e^-λt
A = A0 e^-λt

Question 22

decay constant and half life

Answer 22

when t = t1/2, N = N0/2
so using the decay formula you can solve for half life
t1/2 = ln2/λ

Question 23

carbon dating

Answer 23

• all living things contain carbon
• once they die it stops taking in carbon - carbon decays and isnt replenished
• so if you know the half life of carbon12 and the ratio of carbon14:carbon12 of the dead thing you can find its age

Question 24

carbon-14

Answer 24

• high speed protons in cosmic rays collide with atoms in the upper atmosphere to make neutrons
• neutrons collide with nitrogen14 to make carbon 14
• C14 eventually emits beta- and becomes N14 again
  neutron + N13-> C14 + proton

Question 25

limitation of carbon dating

Answer 25

• activity is very small and close to background
• assume ratio of C14:C12 is constant
  can be changed by
• increased emission of CO2 due to fossil fuels
• volcanic eruption
• solar flares
• testing nuclear bombs

Question 26

dating rocks

Answer 26

• cant use C14 bc half life not long enough
• use rubidium 87
• emit beta minus and transform into stable strontium 87

Question 27

count rate

Answer 27

number of decaying nuclei detected