Supplementary Course Topic 1:

Nuclear and Radiation Chemistry

• Nucleons, nuclides and isotopes • Nuclear fusion and stellar nucleogenesis • Natural Radioactivity • Which nuclides are stable and why • Where decay mechanisms come from • How fast does an unstable nucleus decay? Half-lives • Radiocarbon Dating • How does radiation interact with (biological) matter? • How is radiation exposure measured?

• What are common sources of radioactivity?

• Medical Imaging

Nucleons - The Sub-Atomic Particles

Particle

proton neutron electron positron

Symbol

p n e - e +

Charge

+1 0 -1 +1

Mass (a.m.u.)

1.007276 1.008665 0.000549 0.000549 Not present in stable atoms .

The unit of mass is atomic mass units (a.m.u.), defined by 12 setting the mass of the isotope to exactly 12. 6

C

1 a.m.u. = 1.66 x 10 -27 kg.

Nuclides and Isotopes

The composition of any nucleus is defined by two numbers.

• The • The

atomic number mass number

is the number of protons in the nucleus.

• This defines the chemical nature of the atom.

• It is equal to the total charge on the nucleus.

is the total number of nucleons (protons and neutrons) in the nucleus.

E.g. 6

C

has an atomic number of 6 and a mass number of 12.

• A nuclide is an atom with a particular mass number and atomic number.

• Nuclei with the same atomic number but different mass numbers are called isotopes .

Nuclides and Isotopes

Nuclei with the same atomic number but different mass numbers are called

isotopes

.

E.g. Carbon may exist as a number of isotopes 11

C

6 12 6

C

13 6

C

14 6

C

15

C

6 Unstable nucleus; prepared by nuclear reaction in a cyclotron.

Stable nucleus; accounts for 98.89% of natural carbon.

Stable nucleus; accounts for 1.11% of natural carbon.

Unstable nucleus; trace amounts present in living matter.

Unstable nucleus.

How Mass Spectrometry Works

In a mass spectrometer, the atoms or molecules to be studied are vapourised and then ionised, usually by an electrical discharge.

In the conventional design of a mass spectrometer, ions follow a curved path and their

deflection m/z

depends on the mass-to-charge ratio, (sometimes denoted

m/e

). This deflection was originally recorded as impact on a strip of photographic film, but now use digital current or luminescence detectors.

Mass Spectrometry

Aston’s results established the existence of isotopes. (They were already known for radioactive elements, but never shown for stable elements.) 1920 - Aston measured two isotopes of Ne (20 and 22), three of S (32, 33, 34), three of Si (28, 29, 30), six of Kr (78, 80, 82, 83, 84, 86), and many others

Nuclides and Isotopes

The

atomic mass of an element

is the average of the atomic masses and abundances of each of the

naturally-occurring isotopes

.

E.g. The atomic mass of carbon is 12.01.

That is (12.0000x98.89 + 13.00335x1.11)/100 12

C

6 Mass of nuclide is the reference for a.m.u scale.

13

C

6 Mass of nuclide taken from a reference table

Nucleogenesis

Where do the elements come from?

How are atoms (nuclei) formed?

All atoms are generated from the simplest element, hydrogen 1 1

H

, by

nuclear reactions

. Clouds of atomic hydrogen are pulled together by gravity and begin to heat as they are compressed. Eventually high enough temperatures for nuclear fusion are achieved and the cloud ignites as a star.

Nucleogenesis

The fundamental nuclear reaction is 1 1

H

 1 1

H

1 2

H

 0 1

e

This denotes a positron of mass 0 and charge 1 In nuclear reactions, where the nuclide is changed, we must balance both the charge as well as the mass numbers In a nuclide, the charge is the same as the atomic number – the number of protons.

See - http://www.nobel.se/physics/articles/fusion/index.html

Nucleogenesis

The fundamental nuclear reaction is 1 1

H

 1 1

H

1 2

H

 0 1

e

This is rapidly followed by two other nuclear reactions and 1 2

H

 1 1

H

2 3

He

 2 3

He

 2 4 2 3

He

 

He

 2 1 1

p

This denotes high energy, short wavelength gamma radiation, which has no mass or charge.

Again note that both mass numbers and charges (atomic numbers) must balance.

See - http://www.nobel.se/physics/articles/fusion/index.html

Nucleogenesis

The overall “

hydrogen burning reaction”

4 1 1

H

2 4

He

 2 0 1

e

  releases energy into the surroundings as heat (

exothermic

) and radiation (also releases neutrinos n ).

As the star exhausts its hydrogen, it begins

helium burning

to fuse heavier nuclei to form increasingly larger atoms.

E.g.

2 3

He

 2 4

He

 4 7

Be

  4 7

Be

 1 1

p

 5 8

B

  Heavier nuclei like 13 C, 13 N, 14 N, 15 N, 15 O... are produced by red giant stars, still heavier nuclei in supergiants, and true heavy elements form in supernovae.

Life Cycle of Stars

Hydrogen burning T ~ 10 7 K Helium burning T < 2 x 10 8 K Carbon core Hydrogen burning T ~ 10 7 K Helium burning T < 2 x 10 8 K 40 Ca… 58 Ni formed (C and O burning) T < 3 x 10 9 K Heavy elements

Second-generation stars

Supernova explosions inject carbon, oxygen, silicon and other heavy elements up to iron into interstellar space. They are also the site where most of the elements heavier than iron are produced. This heavy element enriched gas will be incorporated into

future generations of stars and planets

.

We know from the presence of heavy elements in our sun that it is (at least) a second-generation star, currently undergoing hydrogen burning.

Without supernovae, the fiery death of massive stars, there would be no carbon, oxygen or other elements that make life possible.

Nucleogenesis

…and the periodic table He burning. Star expands to red giant C burning. Core of red giant Red supergiant core.

Supernova (everything heavier) H burning

Natural Radioactivity

Nucleogenesis produces nuclides that can be stable or unstable. Unstable nuclei decay through a range of mechanisms involving the release of particles with high kinetic energy or of  -radiation. These high-energy products are collectively known as

radioactivity

.

Henri Becquerel

"in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity"

Pierre Curie

"in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel"

Marie Curie

Natural Radioactivity

The four most important radioactive decay mechanisms are

1.

a

decay

e.g.

212 83

Bi

2.

b

decay

208 81

Tl

 4 2 a The a particle is simply a helium nucleus with mass 4 and charge 2+.

As with all nuclear reactions, both mass and charge are balanced.

e.g.

12

B

5 12

C

6   0 1

e

 b (or b ) is an electron ejected from the nucleus. One neutron is changed into a proton in this nuclear reaction to balance the charge.

Natural Radioactivity 3. Positron (

b +

) emission

e.g.

12

N

7 12

C

6  0 1

e

 When a positron ( b + ) is ejected from the nucleus is usually collides with its antiparticle (the electron) in the surrounding environment very soon:

e

 

e

 

4. Electron capture

e.g.

55

Fe

26   0 1

e

 55

Mn

25 Electron capture is followed by emission of x-rays as electrons fall into lower energy states to fill the vacancy left by the captured electron.

(x-rays are not generally classified as radioactivity, although they can cause radiation damage.)

Natural Radioactivity - worked example

Balance the following nuclear decay reactions and identify the emitted particle where appropriate.

1.

234 92

U

230 90

Th

 2 4

He

or 4 2 a 2.

63

Ni

28 63 29

Cu

  0 1

e

 3.

36

Cl

17  1 36

S

16

Nuclear Reactions - worked example

Nuclear reactions are balanced in the same way, but may involve more than one reactant. Balance the following nuclear reactions and identify the missing nuclide or particle.

1.

14

N

7  2 4

He

17 8

O

 1 1

H

or 1 1

p

2.

239

Pu

94  2 4

He

242 96

Cm

 0 1

n

3.

28

Si

14  1 2

H

29

P

15  0 1

n

Natural Radioactivity -



and x-rays

Both x-rays and  radiation are

high energy (= high frequency or short wavelength) forms of light

.

x-rays have shorter wavelengths than visible or ultraviolet light between 0.01nm and 10nm .

 rays have very short wavelengths - less than 0.01nm

or 0.1Å

Natural Radioactivity

Unstable heavy nuclei decay spontaneously by a series of steps through unstable intermediates. Over time, unstable nuclei give rise to a family of decay products in a

decay series

.

E.g. 238 U decays into… 238 92

U

234 90

Th

234

Pa

91 234 92

U

230 90

Th

234 90

Th

 234

Pa

91  4 2 a 0  1 b 234 92

U

 0  1 b 230 90

Th

 4 2 a 226

Ra

88  4 2 a …etc, etc,...

Natural Radioactivity

A radioactive decay sequence (e.g. of 238 U) can be represented more concisely as a graph of atomic number versus neutron number.

a decay is shown as a decrease of two protons (

Z

) and two neutrons (

N

).

b decay is shown as a decrease of one neutron and an proton.

increase

of one Isotopes (same

Z

, different

N

) lie along vertical lines in this graph.

Natural Radioactivity

A radioactive isotope like 238 U thus generates a family of

daughter isotopes

in a decay series.

Naturally-occurring uranium contains 238 U, and so

will also contain components of the decay series

.

Radioactive Decay Series

…and the periodic table 206 Pb 238 U

Marie Curie

•Born Maria Sklodowska in Warsaw, Poland on November 7, 1867.

•In 1891 at 24, Sklodowska went to Paris to study mathematics, physics and chemistry at the Sorbonne. •July 25, 1895 married Pierre Curie •1903 Nobel Prize for Physics •1911 Nobel Prize for Chemistry •Discovered Radium and Polonium, coined term “Radio-activity”.

•In 1906, Pierre Curie, was hit by a horse-drawn carriage and killed.

•Marie Curie died at the age of 67 in 1934 of leukemia. Her cremated remains are kept in the Pantheon in Paris •In 1935, the Curie's daughter, Irene Joliot-Curie won a Nobel Prize for Chemistry in 1935, making them the first mother and daughter to share this honor.

Nuclear Stability

What factors determine whether a nucleus is stable or unstable?

If we look at the range of stable nuclides that exist in nature, then there are two main observations 1. The size of the nucleus.

2. The composition of the nucleus (proton:neutron)

Nuclear stability

1. Nuclear size There are no stable nuclei heavier than 209

Bi

83

Nuclear Stability

2. neutron:proton (N:Z) All known stable nuclides fall inside the

zone of stability

.

This zone has a N:Z ratio near to 1, but “bends” towards more neutrons per proton as the nucleus gets larger.

These two observations are enough to give us a “rule” for nuclear stability that goes something like “Unstable isotopes must decay towards the zone of stability, finally falling below 209 Bi.”

Nuclear Stability and Decay Mechanisms

Consider some of the known isotopes of carbon from the last lecture.

11

C

6 12

C

6 13

C

6 14

C

6 15

C

6 Unstable nucleus N/Z = 0.83 too low Stable nucleus; N/Z = 1 Stable nucleus; N/Z = 1.17

Unstable nucleus; N/Z = 1.33

too high Unstable nucleus; N/Z = 1.5

too high

Nuclear Stability and Decay Mechanisms

N/Z too low gives b + decay.

11 6 Each nuclide decays

towards the zone of stability

changing its N/Z ratio at constant mass number.

by

C

11

B

5  1 0

e

 N/Z too high gives b decay.

14 6

C

14

N

7   0 1

e

 N/Z = 0.83 N/Z = 1.2

Or equivalently by electron capture.

55

Fe

26  0  1

e

 55 25

Mn

N/Z = 1.33

N/Z = 1.0

15 6

C

15

N

7  0  1

e

 N/Z = 1.11

N/Z = 1.2

N/Z = 1.5 N/Z = 1.14

Nuclear Stability

The “rule” for nuclear stability: “Unstable isotopes must decay towards the zone of stability, finally falling below 209 Bi.” Heavier nuclides than 209 Bi decay by a combination of mechanisms, using a decay to reduce mass (with N/Z = 1) and the other mechanisms to change N/Z.

N/Z too high b decay.

Mass too high a decay.

N/Z too low b + decay or electron capture.

Nuclear Stability

E.g. Here is how the 238 U decay sequence looks on our

zone of stability

graph.

Nuclear Stability - Origin of Decay Mechanisms

The nuclear stability “rule” is

empirical

, based on the simple experimental observation of which nuclides are stable and which are not.

We can apply it like an

algorithm

to solve some nuclear decay problems, without understanding the reasons for nuclear stability.

To understand the reasons, the rule, and the observations, we need to consider the forces between nucleons within the nucleus.

Nuclear Stability - Origin of Decay Mechanisms

The stability of a nucleus involves the competition between two forces.

1. Coulomb or electrostatic repulsion between protons acts to push these nucleons apart over a long range.

2. The strong nuclear force is a short range attraction between all nucleons.

This is the main function of neutrons in the nucleus. They contribute to the binding of the nucleus without also contributing to the electrostatic destabilisation.

Nuclear Stability - Origin of Decay Mechanisms

How does this explain our observations?

1. In nuclides with too few neutrons, the electrostatic repulsions overwhelm the strong nuclear attractions.

2. As the nucleus gets larger, the long-range electrostatic repulsion between protons accumulates and eventually overwhelms the strong nuclear attraction, even if N/Z is optimised.

This microscopic model does not explain how nuclides with too many neutrons can be unstable. To do so will involve

quantum mechanics

.

Rate of Nuclear Decay

Unstable nuclides are present in nature for two reasons.

• Some unstable nuclides have long half-lives, so they simply haven’t decayed yet.

• Some unstable nuclides continue to be formed by nuclear reactions.

For each parent nuclide and a particle or



that decays, one daughter nuclide is emitted. e.g.

is produced

212

Bi

83 12

B

5 208 81

Tl

 4 2 a 12

C

6   0 1

e



Emitted Particle Parent nuclide Daughter nuclide

The decay of an unstable nuclide is characterised by a

half-life

. This is the time required for

half of the nuclei

present to undergo a decay event.

Rate of Nuclear Decay - Half-Life

E.g. 32 P decays into 32 S with a half-life of 14.7 days.

32 15

P

32

S

16   0 1

e

 The number of 32 P nuclei halves in 14.7 days, and halves again after a further 14.7 days...

So, after 14.7 days, half of an initial 10g of 32 P will have decayed, leaving 5g. At the same time 5g of 32 S will have formed.

After a further 14.7 days, only 2.5g of 32 P will remain, and 7.5g of 32 S will be present… This also tells us that the

rate of decay

, the number of nuclei that disintegrate each second, also halves every 14.7 days. The

rate of decay halves

after every half-life.

The Rate of Nuclear Decay

The disappearance of a radionuclide by radioactive decay is described by 

N

0 exp(  

t

) The number of nuclei remaining after time

t

.

The number of nuclei present at the beginning.

1 The

decay constant

.

E.g. Decay curve for 3 H, T 1/2 = 12.26 years.

The number if nuclei halves every 12.26 years.

0.75

0.5

0.25

0 0 25 50

time (years)

75 100

The Rate of Nuclear Decay

The half-life is the time required for half of the nuclides to decay (and for half to be left). So if we let

N

=

N

0 /2

N

0

2



N

0

exp(

 

t

1 2

)

and solve, this gives

t

1 2 

ln(2)

 

0.693

 So finally we can write the decay in terms of half-life as 

N

0  1 2

)

Activity is the

Rate

of Nuclear Decay

The

activity, A ,

of a radionuclide is simply the rate of emission, or

minus

the rate of disappearance of the nuclide.

i.e.

A

   

dN N dt

0   exp(

d



dt



t N

) 0  

N

exp(  

t

) The activity (rate of decomposition) of a sample is proportional to the number of nuclei present.

i.e.

when the number of nuclei present is halved, the activity is also halved.

Units of Activity

• Fundamental unit of activity - Disintegrations per second, also known as the becquerel (Bq) • Curie (Ci) – 1 Ci equals the number of nuclei disintegrating each second in 1g of 226 Ra.

= 3.70 x 10 10 counts per second (or Bq).

Activity and Half-Life

• Activity and Half-Life are related.

• Low activity (few disintegrations per second) = long half-life.

• High activity = short half-life

A

 

N

 0.693

N t

1 2 • Molar Activity = Activity/mole • Specific Activity = Activity/gram

A S A M

 

N A

 0.693

N A t

1 2  

N A M

 0.693

N A t

1 2

M

M = atomic mass Molar Activity, Specific Activity and Half-Life are both independent of the amount of radioactive material present in the sample.

Activity and Half-Life - worked example

What is the molar activity of 13 N, which has a half life of 9.96 minutes?

Answer.

A M

 

N A

 0.693

N A t

1 2 9.96 minutes = 598s

A S

= 0.693 x 6.022 x 10 23 /598 = 6.98 x 10 20 disintegrations mol -1 s -1 (or Bq mol -1 ) or 6.98 x 10 20 /3.70 x 10 10 = 1.88 x 10 10 Ci mol -1

Rate of Nuclear Decay - Decay Series

In a decay series, each step in the mechanism has its own half-life.

Notice that the half-life of 238 U is 4.5x10

9 y, so that many of the atoms present when the earth was formed still have not decayed. However the daughter nuclides decay much more quickly.

Notice also that half lives can be as long as billions of years or as short as a ms or less.

Radiocarbon Dating

14 C is an example of an isotope that is continuously produced in our environment.

99.9% of the naturally abundant 14 C is produced in the upper atmosphere by neutrons reacting with 14 N, which then enters the carbon cycle. 14

N

7  0 1

n

14

C

6  1 1

p

The production rate is 2.5 atom cm -2 s -1 with a global inventory of 3 x 10 30 14 C atoms (90% oceans, 8% biosphere and soils, 2% atmosphere). Typical 14 C concentration in sea waters is 1.2 x 10 9 14 C atoms/L (2 x 10 -15 M). www.ansto.gov.au/ansto/environment1/ams/ams_14c.htm

Radiocarbon Dating

Other Sources of 14 C: In-situ (0.1%): 14 C is produced by spallation reactions induced by neutrons and muons incident on the surface of the Earth. The 14 C production rate from quartz (at sea-level) is 20 atoms g -1 yr -1 and varies with elevation and geomagnetic latitude. A wide range of geophysical problems can be studied by this method including erosion histories, uplift rates, glacial histories, eruption ages, rates of movements of sand dunes, accumulation and ablation rates of ice and climatic change.

Radiogenic: 14 C can be produced underground, directly or indirectly, by the decay of uranium and thorium series. An estimate of this 14 C can be useful in the study of hydrological environments where uranium and thorium contents are high.

Anthropogenic: 14 C levels in the atmosphere show a major peak in 1963 (with about 100% increase in 14 C concentration) because of contributions from nuclear weapons testing and a slow drop since then. This

bomb pulse

is useful in the study of environmental problems such as the air enclosure process in ice and the circulation of groundwaters.

http://www.ansto.gov.au/ansto/environment1/ams/ams_14c.htm

How Radiocarbon Dating Works

All organic

living

matter contains a fixed fraction of 14 C amongst all its carbon. This comes mostly (from atmospherically generated) material taken up by biochemical paths.

After death, 14 C no longer accumulates, and decays into 14 6

C

14 7

N

 0  1

e

 with a half-life of 5730 y.

Thus comparing the concentration of 14 C in dead and comparable living matter can tell us how long since the sample died.

Radiocarbon Dating

How is the amount of 14 C determined?

As noted before, 14 C undergoes b decay.

14

C

6 14

N

7   0 1

e

 Radiocarbon dating can be achieved either • by measuring the concentration of 14 C present in a sample, or • by measuring the

activity

due to b emission. (Recall that activity is

A

 

N

 0.693

t

1 2

N

http://www.c14dating.com/

Radiocarbon Dating

Measuring Activity - scintillation counter.

Scintillation is the emission of light when exposed to ionizing radiation. Scintillation counters simply measure the intensity of light emitted as a result of exposure to a radiation source. By calibration against a standard, this can read activity directly.

Measuring 14 C - accelerator mass spectrometry (AMS) AMS is a high precision mass spectrometry technique that can measure small amounts of sample and resolve isotopic composition. In 14 C dating, AMS is used to measure the ratio of 14 C/ 13 C and 14 C/ 12 C. As 13 C and 12 C are stable, these ratios can be used directly to obtain radiocarbon age. As carbon from different sources can have slightly different isotopic ratios due to various chemical processes, the ratio of 13 C/ 12 C is measured directly in AMS as an additional calibration correction.

Radiocarbon Dating - Activity Ratio

The

activity

is proportional to the number of nuclei present.

A

 

N Thus the ratio of the activity after death to activity while alive is equal to the ratio of the number of 14 C nuclides.

A A

0

dN



dt



dN

0

dt

 

N



N

0 

N N

0  exp(  

t

) To determine the age of a sample we compare the activity

A

with the activity of a still-living (or recently dead) sample,

A

0 , and use the half-life or decay constant.

1 1 0.75

0.75

0.5

0.5

0.25

0.25

0 0 25 50

time (years)

75 100 0 0 25 50

time (years)

75 100

Radiocarbon Dating

This method

assumes

that the concentration of 14 C in living matter has been constant over the dating period.

This assumption is known not to be exactly true, so a number of qualifications and corrections are applied to 14 C dates, and a standard method is always used to report radiocarbon age.

1. The age of a sample is reported as its radiocarbon age. This may be reported as “years BP” (before present, where present = AD1950 when radiocarbon dating was invented).

2. An uncertainty or “error range” is often reported based on known changes in 14 C levels as well as on experimental uncertainty.

3. The radiocarbon age may be corrected using a calibration graph obtained from independent data.

4. Variations in natural isotopic ratios between sources are also corrected.

Calibration of Radiocarbon Dating

Willard Libby, who invented 14 C dating in 1946 (Nobel Prize, 1960), prepared a primary calibration graph, shown below, using samples with independently determined ages.

The curve shows the “Libby half-life” of 5568y, which is used to determine the radiocarbon age of materials and effectively assumes a constant rate of 14 C production. Note that all the independent data is <5000 years old.

Radiocarbon Dating - worked example

E.g. A one gram sample of carbon from peat moss has an activity of 0.350mCi. A reference or modern standard sample yields 0.446mCi. What is the radiocarbon age of the sample?

This pre-factor is obtained from the “Libby half life” and is equal to 5568/ln(2)

t

 8033ln

A A

0 This is a ratio. The units of activity must be the same in the numerator & denominator.

 8033ln 0.446

0.350

 1950

Years BP

(rounded up from 1947) The units of time are determined by the units of the pre-factor or half-life.

Radiocarbon Dating by AMS

AMS has two major advantages over activity measurements.

1. Sensitivity. AMS can measure samples as small as a few mg of carbon, or much older samples in which the fraction of the original 14 C remaining is very small. AMS has an effective limit of around 26,000 years, whereas activity ages are limited to about 10,000y.

2. Internal Calibration. AMS can measure the ratios between all carbon isotopes directly. This means that local variations in isotopic composition ( fractionation ) for stable ( 12 C and 13 C) and unstable 14 C isotopes are determined

in situ

.

In AMS, ratios are often expressed as deviations from a standard. For the stable isotopes the National Institute of Standards and Technology (NIST) gives the value R std = 13 C/ 12 C = 0.011237. Deviation from this value for different materials is expressed in parts per thousand (per mille, or ‰)  

R



R std R std

 1000

Radiocarbon Dating by AMS - The Shroud of Turin

Read the original scientific article at http://www.shroud.com/nature.htm

AMS was used to determine the age of the Shroud of Turin by radiocarbon dating in 1989. Each sample investigated consisted of 50mg of cloth, which was analysed independently by three different laboratories.

 13 C was measured directly, and gave results around 25‰, consistent with calibration standards for such fibres (independent of age).

Radiocarbon age (corrected for  13 C) was determined from the 14 C/ 13 C ratio to be 690 ±30 years BP. Three similar references samples were also dated: • 11-12th century linen dated at 940y BP • Linen from the mummy of Cleopatra dated at 1960y BP • Threads independently dated to 1300AD, 14 C dated at 724y BP.

Conclusion: “...the linen of the shroud of Turin is mediaeval.”

The “Bomb Pulse”

The ambient 14 C level increased due to atmospheric nuclear testing, peaking in 1962 known as the “Bomb Pulse.” This has been accurately tracked over time as the deviation from the pre bomb isotope ratio,  14 C, and can be used to accurately determine the (recent) age of carbon-containing materials.

E.g. • wine dating & detecting false labels or blends.

• dating drug crops.

U.S. Department of Energy photograph http://www.nv.doe.gov/news&pubs/photos&films/atm.htm

Biological Effects of Radiation

How do various forms of radiation interact with (biological) matter?

The basic characteristic of radiation produced by radioactivity is that it is

high energy

, and causes the

ionization of matter

ejecting an electron from an atom. (It’s generally called ionizing radiation.) by When radiation is stopped my matter, it has interacted with it and therefore caused ionization.

Highly penetrating radiation passes through matter

without ionizing it

.

Radiation protection and biological effects are the concern of

Health Physicists

. Nuclear facilities employ health physicists to train staff, monitor activites, and develop safety protocols.

Biological Effects of Radiation

Ionization produces

free radicals

. These are highly reactive chemical species.

E.g. Gamma irradiation of water ejects an electron creating a radical ion 

+ H 2 O



H 2 O +.

+ e -

Both products lead to the production of more free radicals

H 2 O + + H 2 O



H 3 O + + OH .

e + H 2 O



H .

+ OH -

Free radicals attack biomolecules such as DNA strands or membrane lipids. This can (infrequently) lead to genetic damage, cancers, disruption of cell membranes, or malfunctions in enzymes that regulate biological processes.

Biological Effects of Radiation

Penetrating power of a , b • a and  radiation: are heavy, highly charged particles.

When an a particle strikes matter, it is stopped sheet of paper.

by as little as a single • • • a particles ionize the surfaces of matter effectively. Surface damage.

b are lighter, and more penetrating.

A b particle can be stopped by a few sheets of paper or plastic. They ionize to a greater depth than a particles.

 are highly penetrating.

It takes several cm of water to gradually attenuate and stop  radiation.

 radiation is less efficient, but causes internal damage.

b + are highly ionizing and penetrating.

A b + particle reacts with an electron causing ionization release of two gamma photons in opposite directions.

and the

Radiation Exposure

Biological exposure arises through a variety of mechanisms.

E.g. Direct exposure to a radioactive source (external) Solid or liquid: Localised source. Radiation can be shielded.

Gas or vapour: Diffuse source. May diffuse around shielding.

E.g. Contamination (external and internal) External: Picking up radioactive material on hands, shoes, etc… Internal: Ingestion or inhalation.

The risk of damage from a particles through internal contamination is much higher than external, where they are quite well shielded by your skin.

For more details, you can install and run the U.S. National Centre for Neutron Research safety training programme available on the First Year Chemistry web site. This is the same programme used to train visitors, temporary and permanent workers in the nuclear reactor at the National Institute of Standards and Technology.

Biological Effects of Radiation

Half-Life and Radiation damage Nuclides with longer half-lives disintegrate at lower frequency.

A

 

N

 0.693

N t

1 2 That is, longer half lives equals lower (molar) activity, so lower potential for ionization and radiation damage.

From this point of view 238 U, with a half-life of 4.5 billion years, or 230 Th (T 1/2 = 83,000 y) is less damaging than 3 H (T 1/2 = 12 y) or 234 Th (T 1/2 = 24.5 days).

Units of Radiation Dosage

(Human) Radiation dosage is measured in rems (or millirems) or in Sieverts (Sv).

Dosage

attempts

to include all the factors that can affect a living organism activity, energy, penetration, and the mass of living matter irradiated.

Source Activity a b Bq or Ci Energy of Radiation Joule Energy per particle x activity x exposure time Energy Absorbed per unit mass (

dose

) Gray (J/kg) or Rad Radiation distributed or localised?

Relative Biological Effectiveness Effective Dosage Equivalent 10 1 Q-factor rem = rad x Q Sievert =Gy x Q 1Sv = 100 rem 1Gy = 100 rad Less penetrating radiation does more damage.

Dosages and Their Effects

The total expected dosage for an average person is about 360 mrem/year.

What are the short-term effects of radiation dosage?

25,000 mrem in 24h 50,000 mrem in 24h 100,000 mrem in 24h 200,000 mrem in 24h 500,000 mrem in 24h No detectable effects Slight temporary blood change Nausea & fatigue First death (no medical intervention) LD50 (50% of humans exposed die.) N.B. The probability of longer-term effects increases with dose. Most health physicists use a linear no-threshold model. That is, they assume that there is no level of exposure that is free from effects. However the time-scale and statistical nature of the effects make low-dose response hard to determine.

Common Sources of Radioactivity

We are exposed to several common natural sources of radioactivity. These account for about 300mrem/year. The most common is radon, which is part of the decay series of 238 U and other heavy elements, and decays into polonium with a half-life of 3.82 days.

222 86

Rn

218

Po

84  2 4

He

a decay of radon gas causes damage to lungs and is thought to be responsible for up to 10% of lung cancers.

Other ambient isotopes include 40 19

K

40 18

Ar

 0 1

e

as dissolved potassium ions, and 14 6

C

14

N

7   0 1

e

 in CO 2 and organic compounds.

Other Common Sources of Radioactivity

• Other common sources of radiation include cosmic rays in the upper atmosphere. Average annual exposure from this source at ground level is ~26mrem.

• A 4h plane flight increases dosage by a few mrem.

• Medical exposure (x-rays and nuclear medicine) is around 40mrem/year • Consumer products ~10mrem/y • Nuclear fallout <1mrem/y • Nuclear power ~.05mrem/y

Medical Imaging

Basic principles of medical imaging.

• Use a radioisotope to specifically target a chemical agent, organ or process in the body with high selectivity .

• Isotope should emit low-energy, highly-penetrating radiation to minimise effective dosage equivalent to patient. In practice this usually means  .

• Image distribution of radioisotope (by its activity) using scintillation counting • gamma camera (planar image like an x-ray) or • computer-assisted tomography (CAT or CT scan - cross section or three-dimensional reconstruction) • Images may be a simple gray scale density or pseudo-colour signal. Pseudo colour is especially common in computer-reconstructed imaging.

Gamma camera CT scanner

E.g.  -camera image of solution) uptake in a normal and diseased thyroid gland, showing localisation of iodine.

131 I (from NaI E.g. tomographic image of a single anatomical level of the brain using 18 F labelled glucose.

Medical Imaging -

99m

Tc

Technetium-99m ( 99m Tc) is used in about 85% of radionuclear chemistry. It is formed by the decay of 99 Mo by 99 42

Mo

99

m

43

Tc

  0 1

e

 N/Z = 1.36 N/Z = 1.30

Tc is a wholly synthetic element and is unknown in nature. It has no stable isotopes.

The first isotope prepared was 98 Tc obtained in 1937 by the nuclear reaction 98

Mo

42  1 2

H

98 43

Tc

 2 0 1

n

Medical Imaging -

99m

Tc

Technetium-99m a metastable

isomer

that decays into 99 Tc by  emission with a half-life of 6h. 99

m

43

Tc

99 43

Tc

  Highly penetrating  radiation.

Then decays into ruthenium by b emission, but with a half-life of 2.1 x 10 5 years.

99 43

Tc

99

Ru

44   0 1

e

 Long half-life = low activity.

N.B. As a gamma emitter, 99m Tc remains the same element during its residence in the body so it doesn’t change its chemistry when it decays.

Chemical Generation of

99m

Tc from

99

Mo

• • • 99 Mo has a half-life of 67h. It can be used for ~1 week as a continuous source of 99m Tc, which it replenishes after extraction. (Commonly referred to as a “Molly cow” periodically “milked for” Tc.) Extraction is based on the differential solubility of molybdate (MoO 4 2 , insoluble) and pertechnetate ions (TcO 4 , soluble) in NaCl solution. 99 MoO 4 2 is supplied in a cartridge, precipitated or

adsorbed

onto the surface of a support like alumina (Al 2 O 3 ). This is flushed with saline solution to extract the daughter isotope 99m Tc for use.

Pertechnetate ion may be used directly for imaging, e.g. thyroid or blood circulation, or it may be chemically transformed to target other sites.

Several other

generators

are also used, exploiting chemical changes accompanying radioactive decay to form and separate short half-life nuclides for imaging. E.g.

82 38

Sr

82

Rb

37  1 0

e



t 1/2 ( 82 Rb)

= 76 s

Medical Imaging

How is 99m Tc used?

99m Tc is incorporated into a wide variety of compounds that are used to specifically target sites in the body.

E.g.1. Technetium pyrophosphate is used to target bones and identify bone cancer, as in this gamma camera image.

E.g.2. Real-time imaging of technetium penetrate is used to monitor kidney function.

http://www.nuclearonline.org/PIbyGeneric2.htm

Medical Imaging - Positron Emission Tomography

Unlike 99m Tc and other direct gamma emitters, positron emitters undergo a nuclear transformation when they decay.

e.g.

11 6

C

11

B

5  0 1

e

 18 9

F

18

O

8  0 1

e

 This means that chemical reactions may ensue from both the nuclear change and the reaction with an electron that produces the two  ’s for tomographic scanning.

e

 

e

 2  The annihilation of the positron by its antiparticle produces energy in the form of two  ’s. Conservation of momentum ensures that they travel in exactly opposite directions, so the tomographic detector gets two signals from each decay event .

Medical Imaging - Positron Emission Tomography

Positron Emitting Isotopes are generally formed in a cyclotron, which bombards a stable sample with protons or deuterons.

Charged protons or deuterons are generated and accelerated in electric field and magnetic fields along a spiral path until they strike their target (stable) nuclide.

14

N

7  1 1

H

11

C

6  2 4

He t 1/2

= 20.3 min 16

O

8  1 1

H

13

C

6  1 1

H

13 7 13

N

7 14

N

7

N

   0 1

n

2 1 2 4

He H t 1/2

= 9.97 min 15

O

8  0 1

n

20

Ne

10  1 2

H t 1/2

= 2.07 min 18

F

9  2 4

He t 1/2

= 109.7 min The short half-life of these radionuclides means that cyclotrons need to be on-site at hospitals. E.g. The Australian Medical Cyclotron is located next door to RPAH.

Medical Imaging - Positron Emission Tomography

The nuclear reactions that generate PET radioisotopes transform atoms

within stable molecules

. The desired products then need to be chemically isolated from the product mixture.

E.g 14 N in target N 2 is transformed by 1 H into 11 C, producing CN. 14 7

N

 1 1

H

• Target N 2 mixed with H 2 leads to H 11 CN formation, which is an important reagent used to produce many radiolabelled organic molecules.

11

C

6  2 4

He

• Target N 2 mixed with O 2 yields a mixture of products including 11 CO 2 and 11 CO.

Medical Imaging - Positron Emission Tomography

These radiolabelled molecules must be purified, and can then be used directly for imaging purposes, or used to synthesise more complex molecules for research or diagnostic use at specific chemical sites. Fast synthetic procedures are needed to keep the isotope active for use, often using automated procedures or “robots.” E.g. fluorodeoxyglucose (FDG) is synthesised in 35 minutes with 50% radiochemical yield in a two-step process.

20

Ne

10  1 2

H

18

F

9  2 4

He

1. 18 F salt added in CH 3 CN solution and heated to evaporate (10 min) 2. HCl added and heated (10 min)

Precursor glucose derivative reagent FDG

Recognising organic molecules and synthetic techniques will come up later in this course.

Medical Imaging - Positron Emission Tomography

Fast synthesis of radiolabelled molecules allows them to be used to study specific biochemical processes and responses.

E.g.1. FDG is absorbed by the brain like glucose, but the fluorine substitution stops it from being completely metabolised. 18 FDG It can be used to examine the reaction mechanism, and changes in uptake & metabolism accompanying various diseases.

These PET scans of a single anatomical level in the brain show changes in the brain chemistry and glucose metabolism by 18 FDG with the progress of AIDS Dementia Complex.

Colour scale increases: black-blue-yellow-red-white

Medical Imaging - Summary

Nuclear imaging is useful because it allows us to radiolabel molecules that specifically target organs, molecules or chemical processes for diagnosis or biochemical research. The synthetic chemistry to design these target molecules differs widely: • Cyclotron-produced PET isotopes ( 11 C, 18 F…) are often exploited in the synthesis of organic molecules (drugs, peptides, carbohydrates, steroids, vitamins…) • Metals ( 99m Tc, 82 Rb,…) may be used as soluble or insoluble salts, or as

co-ordination compounds

, to mimic biological molecules, toxins, as heavy heavy metal tracers,… Footnote: Magnetic Resonance Imaging (MRI) is not a radiochemical technique.

Although based on the technique of nuclear magnetic resonance (NMR), MRI uses the quantum mechanical properties of 1 H and other stable nuclei (e.g. 13 C) to measure and map the concentration of chemical species at different positions in an organ or whole body.

There are no nuclear reactions or decay processes involved.

Summary I

You should now be able to • Recognise nuclear reactions, including the major spontaneous decay mechanisms.

• Define and distinguish between nucleons, nuclides & isotopes, x-rays & gamma rays, decay series and daughter isotopes.

• Explain stellar nucleogenesis.

• Calculate the average atomic mass from isotope information.

• Balance nuclear reactions.

Summary II

You should now be able to • Recognise stable and unstable nuclides.

• Predict the decay mechanism for an unstable isotope.

• Calculate the activity or half-life of an unstable nuclide from appropriate data.

• Calculate the age of a sample using the carbon-14 method, and know the underlying assumptions and appropriate timescale for its application.

Summary III

You should now be able to • Explain the main factors that contributes to effective radiation dose, including penetrating power, activity, energy.

• Explain the main mechanism of biological damage by ionizing radiation.

• Explain the use of radioactive isotopes in medical imaging, and distinguish the information obtained from x rays.

• Explain how isotope generators produce e.g. 99m Tc for medical imaging, and give some examples of its use.

• Explain PET, the generation of radioisotopes by a cyclotron, and the know the kinds of isotopes produced.