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aa Radiochemistry > Lecture 4 > Flashcards

Lecture 4 Flashcards

1
Q

What is a use of radioactive isotopes in particular 14C

A
  1. Radiochemical dating
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2
Q

What C isotopes does all organic matter contain

A
  1. All organic matter contains 12C and 13C as well as 14C which is continuously being created by cosmic radiation in the thermosphere
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3
Q

How is 14C formed and what does it decay to

A
  1. 14N + 1n –> 14C + 1H
  2. 14C –> 14N + beta-
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4
Q

How does radiochemical dating work

A
  1. Whilst plant or animal is alive it will have the same proportion of 14,6C as the surroundings
  2. When the plant or animal dies the intake of radiocarbon stops and that already present gradually decays
  3. Thus the smaller A is, the longer the period between death and the present time
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5
Q

What are some assumptions made in carbon dating

A
  1. 14C production is constant and equally spread
  2. Exchange has stopped- microorganisms etc
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6
Q

What do radio-dating methods require

A
  1. Measurement of amount of radio-isotope now
  2. Knowledge or good assumption of the amount of radio-isotope present at the time to be date
  3. Time elapsed can be calculated based on decay laws- since t1/2 is known, need to measure activity
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7
Q

How can you carry out radiochemical dating of geological samples

A
  1. Requires isotope decays with very long half-lives
  2. 3 options
  3. Use of decay series of 238U to 206Pb
  4. Use of 87Rb–> 87Sr + Beta- :date lunar samples
  5. Decay of 40K to 40Ar by electron capture
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8
Q

How does the use of decay series of 238U to 206Pb work for dating

A
  1. Analyse U minerals for their U/Pb ratio
  2. e.g. if find 1:1 ratio and assume no Pb present at start then 1 half life has passed- 4.5 billion years old (approx age of earth)
  3. Useful as half life is close to age of earth- but not very abundant
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9
Q

How does the use of Decay of 40K to 40Ar by electron capture work for dating

A
  1. 40K/40Ar ratio determined by mass spectrometer
  2. Method assumes that all of the argon is trapped in the rock after it has cooled
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10
Q

What needs to be thought about when carrying out radiodating

A
  1. Events such as nuclear testing
  2. Released neutrons into atmosphere
  3. Made 14C
  4. Not reliable carbon dating for this time
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11
Q

How is radiation detected

A
  1. Electrical collection of ions
  2. Scintillation
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12
Q

Describe how radiation is detected by electrical collection of ions

A
  1. Air ionisation- no gas amplification uses air
  2. Gas ionisation- (Geiger-Muller counter)
  3. GM is most common type of detector
  4. Gas amplification
  5. Multiplication factor 10^8-10^10
  6. Long dead time after excitation
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13
Q

Describe how radiation is detected by scinitillation

A
  1. Light production
  2. Liquid Scintillation Counter (Lsc):
  3. Low energy beta and alpha
  4. Solvent mixed with scintillating material
  5. Very low background
  6. Much more sensitive - both tubes must be activated at the same time
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14
Q

Describe nuclear applications in analysis

A
  1. Neutron activation analysis
  2. Mossbauer spectroscopy
  3. Nuclear spectroscopy
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15
Q

Describe neutron activation analysis

A
  1. Bombard sample with neutrons leads to unstable nucleus- produces gamma rays which can be measured
  2. The wavelength of the gamma-rays are characteristic of the element so provides a method of qualitative analysis
  3. Also intensity of rays relate to the concentration of the element so gives quantitative information
  4. Some 50 elements can be analysed using samples as small as 10^-12 g.
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16
Q

Describe Mossbauer spectroscopy

A
  1. a solid sample is exposed to a beam of gamma radiation, and a detector measures the intensity of the beam transmitted through the sample.
  2. The atoms in the source emitting the gamma rays must be of the same isotope as the atoms in the sample absorbing them.
  3. Different chemical environments causes nuclear energy levels to shift
  4. Doppler shift- scans different frequencies by moving source back and forth
  5. Specific for suitable nuclei- especially 57Fe and 119Sn
  6. OS and symmetry around the element
17
Q

Describe nuclear spectroscopy

A
  1. Nuclear transitions are ultranarrow and can be used for very precise frequency measurements
  2. Nuclear clock: 229Th has a very low energy excited state at 160nm that can be reached by UV lasers
  3. Pound-Rebka experiment to confirm that photon gain energy when falling down to Earth
18
Q

What is the band of stability

A
  1. The range of stable nuclei on a graph that plots the number of neutrons in a nuclide against the number of protons
19
Q

Describe stability of 12C and 14C

A
  1. 12C with 6 protons and 6 neutrons is a stable nucleus- doesn’t spontaneously emit radioactivity
  2. 14C with 6 protons and 8 neutrons is unstable and naturally radioactive
20
Q

What is ideal ratio of n/p with atoms with lower atomic number and what does it change to with heavier

A
  1. Ideal ratio of neutrons is approx 1:1
  2. As the atomic number increases the stable neutron-proton ratio gradually increases to about 1.5:1 for the heaviest known elements A>200
21
Q

Give example of a stable nucleus with approx ratio of 1.5:1

A
  1. Lead-206, is a stable nucleus that contains 124 neutrons and 82 protons
22
Q

How else other than passive decay can elements be changed and give example of 1st successful converstion

A
  1. Active bombardment with other particles
  2. 14,7N + 4,2He –> 17,8O + 1,1H
  3. Exposed air to alpha radiation, very unlikely to happen
23
Q

How was the existence of the neutron proved

A
  1. 9,4Be + 4,2He –>12,6C + 1,0n
  2. Easier than alpha particle to convert as neutral so more likely to combine
24
Q

What is a cyclotron

A
  1. Charged particles (electrons, protons and deuterons) are accelerated in an electromagnetic field to high velocities to generate high energy particle beam
  2. The particle beam can be allowed to hit a target
  3. At sufficiently high energy the collisions lead to nuclear reactions - overcome repulsion between positively charged nucleus and bombarding particle
  4. The atom-by-atom reactions are not suitable for bulk production of new isotopes
25
Q

What is a more effective way of carrying out nuclear transformations other than cyclotron

A
  1. By neutron bombardment- no electrostatic repulsion
  2. Used to prepare trans-uranium elements
  3. Generate new elements- irradiate U with neutrons
26
Q

When is neutron bombardment used and when is cyclotron used

A
  1. Neutron bombardment can give trans-uranium elements up to z=100
  2. Beyond, heavy excited particles from cyclotron are used
27
Q

What is the island of stability

A
  1. superheavy spherical nuclei
  2. Not found yet
  3. Need many more neutrons per proton

aa Radiochemistry (11 decks)

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