Week 11: Nuclear forensics and dating

Question 1

What three pillars is the non-proliferation treaty (NPT) built on?

Answer 1

Prevention of the spread (proliferation) of nuclear weapons.

Promotion of the peaceful uses of nuclear energy.

Goal of achieving nuclear disarmament (mutual rather than unilateral).

Question 2

What are the three recognised nuclear weapon states?

Answer 2

USA
Russia
UK
France
China

Question 3

How does the IAEA department of safeguards deter the proliferation of nuclear weapons?

Answer 3

1) Providing credible assurance that states are honouring their international obligation, thus helping to build international confidence.

2) Being able to detect early any misuse of nuclear material or technology, thereby alerting the world to potential proliferation.

Question 4

How does the IAEA check states’ compliance to the NPT?

Answer 4

Through inspectors.

Question 5

How is isotopic composition measured?

Answer 5

Isotopic ration or radioisotopes can be measured by gamma ray spectroscopy provided each isotope has a suitable gamma radiation signature.

Question 6

Why is a high purity germanium detector usually used when measuring isotopic composition?

Answer 6

In order to get the most detailed information in the gamma ray energy.

Question 7

How is the setup for composition measurement calibrated?

Answer 7

For absolute efficient against Egamma.

Calibration is performed with standard sources such as Eu-125.

A background spectrum is measured and the sample spectrum is collected.

Question 8

How can the activity of an isotope be determined from a gamma ray spectrum?

Answer 8

From the number of counts recorder under each peak in the (background-corrected) spectrum n, te absolute efficiency, epsilon, the collection time, T, and the intensity, I, of the gamma ray yield for the transition per disintegration.

(46)

Question 9

If the nuclide of interest does not emit a gamma ray that is easily detected, how may the nuclide be identified?

Answer 9

A suitable gamma radiation signature may be a transition from a nuclide further down the decay chain.

For this to give an accurate idea of the amount of the parent nuclide, we need to know the relationship between the two.

Question 10

What is secular equilibrium?

Answer 10

(47)

Question 11

For a sample with activity per unit volume A, what is the count rate for a simple model proportional to?

Answer 11

(48)

Question 12

For a sample with activity per unit volume A, what is the count rate for a model accounted for self-absorption, proportional to?

Answer 12

(49)

Question 13

What is the absorption length?

Answer 13

1 / linear attenuation coefficient

Question 14

When is a sample described as having infinite thickness?

Answer 14

If d > 7 / linear attenuation coefficient.

i.e. it is seven absorption lengths or more in thickness.

Question 15

What happens if a sample has infinite thickness?

Answer 15

The quantity of the sample in the visible volume is directly proportional to the count rate of the gamma ray detected.

Question 16

What is the intrinsic calibration method?

Answer 16

Using ratios of the yields of the different gamma emission and detected intensities, this method allows the efficiency relative to a reference energy to be determined.

Once this is known, the relative amounts of different isotopes in the sample can be found.

Question 17

How can isotopes be identified in samples of unknown origin?

Answer 17

From the emitted gamma radiation.

The energies of gamma ray transitions observed from the sample are accurately determined by using e.g. a high purity Ge detector.

The measured energies can be found in a look-up table.

Question 19

What is the half life of 14C and how is it useful for dating?

Answer 19

Half-life: 5730 years

It is continually being produced in the atmosphere by neutrons produced by cosmic rays causing (n,p) reaction in 14N.

Question 20

How is carbon-14 used in dating?

Answer 20

Living matter is continuously exchanging carbon with its surroundings, so will have the same ratio of 14C to 12C as the atmosphere.

When an organism dies, the carbon exchange processes stop, so the ratio will decrease as the 14C decays away.

Question 21

How and why does carbon-14 dating need to be calibrated?

Answer 21

Is calibrated by comparison with dead organisms of a known age, such as tree rings.

Needs to be calibrated as the ration of 14C to 12C is not entirely constant over time.

Question 22

Why is a high sensitivity, low background detection system needed for carbon-14 dating activity measurement?

Answer 22

In 1g of living matter, 14C has an activity of less than 0.25 Bq and old material is even less active.

Question 23

What two methods are used for measuring activity in carbon-14 dating?

Answer 23

Beta counting
Accelerator mass spectrometry

Question 24

What is the average energy of the beta particle produced by the decay of 14C?

Answer 24

48.233 keV.

Question 25

Why do detectors have to be sensitive to low beta energies in beta counting?

Answer 25

The average energy of the beta particle produced by the decay of 14C is 48.233 keV.

Question 26

As modern facilities use liquid scintillators for beta counting, what must happen to the sample beforehand?

Answer 26

The sample must be processed to convert the carbon into benzene.

Question 27

In beta counting, how is the detector shielded against background radiation and provided with anti-coincidence detection?

Answer 27

Having additional detectors outside the counting chamber, and any signal that is detected inside and outside the chamber at the same time can be attributed to cosmic rays and discounted.

Question 28

What measurements are made in beta counting?

Answer 28

The activity of the sample The activity of the standard The background

Question 29

What results are gained from beta counting?

Answer 29

Absolute activity - The specific activity Bq (kq carbon) ^-1 Activity radio - the ratio of the absolute activity to that of the standard measured at the same time. Relative activity - the ratio of the difference between the absolute activity of the sample and that of the standard, to the absolute activity of the standard.

Question 30

What is the difference between beta counting and accelerator mass spectrometry?

Answer 30

Rather than trying to detect the radiation from the 14C, instead it directly counts the number of atoms of the two isotopes.

Question 31

List the main components of an accelerator mass spectrometer in order.

Answer 31

Negative ion source Low energy inflector magnet tandem accelerator High-energy analyser magnet Foil Second analyser magnet Particle detector

Question 32

In AMS how is the deflection of ions by the magnets related to the mass and the charge?

Answer 32

(50)

Question 33

How are the ions detected in AMS?

Answer 33

In two stages: The first detector measures the amount of energy that the particles lose as the move through it. The second stops the particles and measures the energy deposited. The total energy of the particles, E, is then measured by summing the signals from the detectors.

Question 34

How are the ions identified in AMS?

Answer 34

The measurement of the change of E gives a measure of the stopping power which is proportional to mz^2/E. Plotting delta E against E gives a hyperbola for each mz^2, which allows the ions to be identified.

Question 35

Why does the ratio of 14C to 12C vary?

Answer 35

Due to changes in the cosmic ray flux due to interaction with the varying solar wind and magnetic field, as well as the Earth's magnetic field.

Question 36

What is the Suess effect?

Answer 36

The increased use of fossil fuels with industrialisation lead to a dilution of the 14C content of the atmosphere. The carbon stored in fossil fuels is sufficiently old that it does not contain any 14C, and when it is burnt it releases carbon dioxide, resulting in more 12C being in the atmosphere relative to the amount of 14C.

Question 38

How can the amount of 87Sr in a sample be written as a function of time?

Answer 38

(51)

Question 39

State the value at which uranium is considered to be highly enriched.

Answer 39

20%

Question 40

Define the absolute efficiency of a detector setup.

Answer 40

the fraction of the radiation emitted from the source that is detected.

Question 41

State the condition required for secular equilibrium to occur between a nuclide N1 and its immediate decay product N2 in terms of their decay constants.

Answer 41

lambda1 << lambda2

Question 42

Give an example of another dating technique that can be used to calibrate the 14C dating results.

Answer 42

Comparisons to values from dead organisms of known age such as tree rings (dendrochronology).

Question 43

Explain why scintillation detectors are used for beta counting of 14C rather than semiconductor detectors.

Answer 43

Scintillation detectors have higher efficient than semiconductor detectors, but lower energy resolution. Beta counting of 14C needs to detect as many particles as possible, but accurate energy values are not necessary so the higher efficiency of scintillation detectors is proffered.

Question 44

Explain why the accelerator used in AMS is referred to an "tandem".

Answer 44

The Van de Graaff generator is positively charged so it attracts the negative ions when they enter the chamber. When they reach the stripper, which is aligned with the generator, they are converted to positive ions, so they are now repelled from the positively-charged generator, giving a second acceleration.

Question 45

List three effects that have caused variation in the amount of atmospheric 14C.

Answer 45

Variations in cosmic ray flux Emissions of old carbon from fossil fuels Production of 14C from atmospheric nuclear weapons testing.

Question 46

List the isotopes that start the decay chains used in uranium-lead dating along with their half lives.

Answer 46

238U (4.468 x 10^9 yrs) 235U (7.038 x 10^8 yrs)

Question 47

List the isotopes that start the decay chains used in rubidium-strontium dating along with their half lives.

Answer 47

87Rb (4.810 x 10^10 yrs)

Question 48

List the isotopes that start the decay chains used in samarium-neodymium dating along with their half lives.

Answer 48

147Sm (1.06 x 10^11 yrs)