February 8, 11 1 Radioactivity

February 8, 11 2 Radioactivity

Radionuclides - can spontaneously emit particles and radiation which can be expressed by a nuclear equation.

Spontaneous Emission: Mass and charge are conserved.

Alpha emission

Beta emission

Gamma emission

Positron emission

Electron capture

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92238U → 90

234 Th + 24 He

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53131 I → 54

131Xe + -10e

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01n → 1

1p + -10e

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00γ

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611C → 5

11B + 10e

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11p → 0

1n + 10e

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11p + -1

0e → 01n

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+10e + -1

0e → 00γ

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-10β

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24α

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10e

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-10e

February 8, 11 3 Radioactivity

Particle Charge Mass (g) Nomenclature

alpha 2+ 6.64e-24 4 He 4 α 2 2

beta 1- 9.11e-28 0 e 0 β -1 -1

gamma 0 0 0 γ

proton 1+ 1.673 e-24 1 H 1 p 1 1

neutron 0 1.675 e-24 1 n 0

electron 1- 9.11e-28 0 e -1

position 1+ 9.11e-28 0 e +1

February 8, 11 4 Radioactivity

A plot of neutrons vs. protons for the stable nuclides. A plot of N vs. Z for all stable nuclides gives rise to a narrow band that veers above N/Z=1 shortly beyond Z=10. The N/Z values for several stable nuclides are given. The most common modes of decay for unstable nuclides in a particular region are shown: nuclides with a high N/Z ratio often undergo β decay; those with a low ratio undergo e- capture or position emission; heavy nuclei beyond the stable band (and a few lighter ones) undergo α decay.

The blue box in the larger diagram is expanded to show the stable and many of the unstable nuclides in that area. Note the modes of decay; α decay decreases both N and Z by two; b decay decreases N and increases Z by one; position emission and e- capture increase N and decrease Z by one.

Neu

tron

s (N

)

February 8, 11 5 Radioactivity

Predicting Nuclear Stability -

Radionuclides sometimes go through a series of emission (Radioactive series) before becoming a stable nuclei.

Nuclear disintegration series for U-238 under goes α-emission (blue arrows) and β-emission (red arrows) until it forms stable Pb-206 (which is an isotope within the belt of stability.

February 8, 11 6 Radioactivity

Transmutation - Change of nuclear identity by artificially striking nucleus with a particle.

• Many medical chemotherapy isotopes are formed by transmutation. • Many new elements are discovered by transmutation

Nomenclature: Target (bombard, ejected) product 17O + 1H 14N + 4He In this example, The target is 17O, the product is 14N, the bombarding particle is a proton 1H (or p) and the ejected particle is the alpha particle 4He (or α). The nomenclature is therefore

17O (p,α) 14N

Example: solve the following. The answer is in the next slide.

i) 238U + 1n 239Np + 0β ( see next slide for answer) ii) 238U (n,γ) 239U ( see next slide for answer) iii) 18O (n,β) 19F ( see next slide for answer)

February 8, 11 7 Radioactivity

€

t12

= ln2k

t12

= 5730 yr

k = ln2t

12

=ln2

5730 yr=

0.6935730 yr

= 1.21•10-4 yr−1

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12 C14 C

=1012

1

14 C[ ]= 50.000 ppt based on 12C

14 C[ ]= 46.114 ppt today

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Ao = 50.000A = 46.114

⎫ ⎬ ⎭

= lnAoA

= kt

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ln50.00046.114

1.21• 10−4 yr−1 = t

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t = 668.6 yr

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1990 -669 = 1321 ±50 AD

The Shroud of Turin is a linen cloth over 4 m long. It bears a faint, straw-colored image of an adult male of average build who had apparently been crucified. Reliable records of the shroud date to about 1350, but for these past 600 years it has been alleged to be the burial shroud of Jesus Christ. Numerous chemical and other tests have been done on tiny fragments of the shroud in recent years. The general conclusion has been that the image was not painted on the cloth by any traditional method, but no one could say exactly how the image had been created. Re-cent advances in radiochemical dating methods, however, led to a new effort in 1987–1988 to estimate the age of the cloth. Using radioactive 14 C, the flax from which the linen was made was shown to have been grown between 1260 and 1390 A.D. There is no chance that the cloth was made at the time of Christ.

February 8, 11 8 Radioactivity

• Radioactive substance possess special rates of decay and half lives, t1/2 .

• Radioactive decay is a first-order kinetic process. ln {N/No} = -k t t1/2 = 0.693/ k

Half-life - The time in which

a substance will decay to one-

half its original mass.

February 8, 11 10 Radioactivity

Each isotope has its characteristic half-life unaffected by external conditions, i.e., temp, pressure, chemical nature.

February 8, 11 11 Radioactivity

Radiation can be used as a diagnostic tool to non-invasively probe the body

February 8, 11 12 Radioactivity

• Penetrating power of radiation is a function of mass.

• γ -radiation penetrates further than β-radiation which penetrates further than α-radiation.

• Radiation absorbed by tissue causes excitation (non-ionizing radiation) or ionization (ionizing radiation).

• Ionizing radiation is much more harmful than non-ionizing radiation

• Most radiation result in radical formation.

• Free radicals undergoes a chain reaction to form other radicals.

February 8, 11 13 Radioactivity

A graph of the sources of the average annual exposure of the US population to high-energy radiation. On average ~300mrem/yr sea level, but 400 mrem/yr Denver.

Radioactivity measurement:

February 8, 11 14 Radioactivity