C25 Radioactivity

Question 1

All 3 types of radiation are described as…

Answer 1

ionising radiations (ability ionise atoms by removing some their electrons, leaving +ve ions).

Question 2

What are cloud chambers used for?
(How does it work)

Answer 2

Cloud chamber used detect presence these types radiation. Contains air saturated with vapour at very low temperature. When air mols are ionised, liquid condenses onto the ions to leave tracks of droplets marking the path of the radiation.

Question 3

Nature of Alpha radiation:
(What’s it composed off, charge, range, what’s it stopped by?)

Answer 3

Alpha radiation: consists of positively charged particles. Each alpha particle, 2 protons and 2 neutrons (a He nuclei), has charge +2e (e elementary charge). Shortest range (few centimetres in air), stopped by couple mm of paper.

Question 4

Nature of Beta radiation:
(What’s it composed off, charge, range, what’s it stopped by?)

Answer 4

Beta radiation: consists fast-moving electrons (beta minus), or fast-moving positrons (beta plus). Beta minus, charge -e. Beta plus, charge +e. Medium range (up to 1 metre in air), stopped couple mm of aluminium.

Question 5

Nature of Gammar radiation:
(What’s it composed off, charge, range, what’s it stopped by?)

Answer 5

Gamma radiation: (or rays) consists high-energy photons with wavelengths less than 10^-13m. Travel speed light, has no charge. All emitted from nuclei of atoms as result of changes within unstable nuclei. Greatest range (couple km in air), stopped by couple cm lead or metres of concrete.

Question 6

The Effect of Electric and Magnetic Fields on emitted radiation

Answer 6

-Shows how uniform electric field can distinguish between different types.
-Alpha particles deflected less than beta particles, alpha greater mass.
-Beta (minus) and beta (plus) form mirror images.
-Gamma rays not deflected, uncharged.

Question 7

Dangers of Radioactivity:

Answer 7

-All types radiation cause ionisation, can damage living cells (alpha most dangerous, most ionising).

Question 8

What are precautious methods that can be implemented, when handling radioactive substances?

Answer 8

-Radioactive sources stored in lead-lined storage container (can stop gamma, and hence all of them).

-When transfering use pair tongs, long handles (keep source as far as possible).

Question 9

Transmution

Answer 9

-Radioisotopes like to be stable isotopes so constantly changing to try stabilise. In process, they will release energy and matter from their nucleus, and often transform into new element (process called transmutation).

-The radioactive decay and transmutation continues until new element formed, that has stable nucleus and is not radioactive.

Question 10

The nucleus before decay is known as ….., and new nucleus after decay called ……

Answer 10

Parent nucleus
Daughter nucleus

Question 11

In all nuclear reactions what is always conserved?

Answer 11

In all nuclear reactions, nucleon no. and porton no. conserved.

Question 12

Conservation of mass and energy are …
(Meaning)

Answer 12

Conservation of mass and energy are interchangeable (the energy released in nuclear reactions is produced from mass).

Question 13

Alpha decay (features):

Answer 13

-Loss of alpha particle removes 2 protons and 2 neutrons from parent nucleus.

-Daughter has different proton no. so different element.

-Energy also released in the decay.

Question 14

Gamma decay (features):

Answer 14

-Only emitted if nucleus has surplus energy following an alpha or beta emission.

-Composition of nucleus remains the same.

Question 15

Decay chains (what can not be shown on the graph)?

Answer 15

Gamma decay cannot be shown on graph.

Question 16

Why is radioactive decay complicated?

Answer 16

The radioactive decay of nuclei is complex, because daughter nuclei can still be radioactive.

Question 17

Radioactive Decay is random because…

Answer 17

-we cannot predict when particular nuclei will decay or which one will decay next.
-each nucleus within a sample has same chance of decaying per unit time.

Question 18

Radioactive decay are all…

Answer 18

spontaneous

Question 19

Radioactive decay all spontaneous because, decay of nuclei not affected by:

Answer 19

-presence of other nuclei in the sample.
-external factors such as pressure.

Question 20

Half-life (t1/2)

Answer 20

Half-life of an isotope is the average time it takes for half the number of active nuclei in the sample to decay.

Question 21

Second Half-life

Answer 21

Second Half-life, found by getting the half-life of the original half-life value.

Question 22

what is activity measured in?

Answer 22

Decays per second
1 becquerel (Bq) = an activity of one decay per second.

Question 23

Activity definition (2):

Answer 23

The activity A of a source,

the rate , which the nuclei decay or disintegrate

OR

No. of alpha, beta, or gamma photons emitted from source per unit time.

Question 24

Consider a source with a very large no. of nuclei, with N undecayed nuclei at time t=0 that decay into stable daughter nuclei.

The decay, both random and spontaneous. In a small interval of time, it would be reasonable to assume that the no. of nuclei disintegrating would be directly proportional to…

Answer 24

both N and change in t.

change in N, directly proportional to the N x change in time.

therefore Change in N / Change in time, is directly proportional to -N

-Negative sign means that the no. of undecayed nuclei is decreasing.

-Change in N / Change in t = to the rate of decay (A)

Question 25

Decay Constant

Answer 25

Decay constant (lambda), defined as, probability of decay of an individual nucleus per unit time.

Question 26

During experiments, the radiation of a source will also be higher then its true value, why?

Answer 26

due to background radiation (measure its value first then subtract).

Question 27

What is the equation for a known isotope, that links the decay constant, activity and no. of undecayed nuclei?

Answer 27

A = Lambda x N

Question 28

The half-life of radioactive sample can be approximated by an exponential decay:

Answer 28

N=Noe^-lambda x t

-Where No, is the no. of undecayed nuclei at time t=0, N no. undecayed nuclei in sample at time t, and e is the base of natural logarithms.

Question 29

The half-life, 𝑡1/2, of a decay is the average time it takes for half of the sample to decay, so substituting 𝑁 =𝑁0 / 2 into N=Noe^-lambda x t we find that…

Answer 29

𝜆𝑡1/2 = ln(2). Thus, if the decay constant
is known the half-life can be found.

Question 30

Carbon Dating

Answer 30

The determination of the age of old material (such as an archaeological or paleontological specimen) by means of the content of carbon 14.

All loving things contain carbon atoms.

Question 31

Where is Carbon-14 produced and how?

Answer 31

Carbon-14 is produced in the stratosphere by nuclear reactions of atmospheric nitrogen with thermal neutrons produced naturally by cosmic rays.

Question 32

Naturally occurring carbon in atmosphere contains 3 main isotopes:

Answer 32

C-12, C-13 and C-14. The ratio between these isotopes is a known constant, approximately 99% is C-12, 1% C-13 and C-14 exists in trace amounts (around 1 atom per gram of carbon).

Question 33

How can you actually calculate an estimation for the age of an organism?

Answer 33

-Living organisms absorbs C-14 during lifetimes either through photosynthesis of CO2 or through consumption other organisms containing carbon. Therefore, the ratio of C-14 to C-12 in the organism will match the atmospheric ratio, until death were C-14 no longer replenished into organism.

-C-14 is a radioactive isotope that decays via beta emission with a half-life of ~5700 years. Hence, by measuring ratio C-14 to C-12 in dead tissue and comparing this to the atmospheric composition an estimation for the time since organism’s death is calculated.

-The activity of C-14 are extremely small, about 15 counts per min for 1g of carbon

Question 34

Limitations of Carbon Dating (3):

Answer 34

-Radiocarbon dating, not perfect technique however, assumes the ratio of C12 to C-14 has remained constant throughout history.

The ratio could’ve altered due to:
-An increase emission of CO2 (burning fossil fuels may have reduced ratio),
-Natural events (volcanic eruption and solar flares),
-Nuclear weapons testing.

-Also, for small samples the amount of C-14 in the sample can be unnoticeable in comparison to the background radiation.

-Samples much older than 5700 years, the amount of C-14 becomes immeasurably small (technique cannot be used). Instead, rubidium is often used due to its longer half-life.

Question 35

Dating Rocks? (how is it done).

Answer 35

-Cannot use C-14 to date rocks and meteors formed during creation solar system (half life not long enough).

-Use decay of rubidium-87, the nuclei emit beta-minus particles and transform into unstable nuclei of strontium-87. The half-life of rubidium-87 about 49 billion years.