Tuesday, November 16, 2010

20.1: Radioactivity-Nuclear Bombardment Reactions

Nuclear Stability:

Why don't all the protons in the nucleus repel each other and cause the nucleus to blow apart?

Nuclear Force: a strong force of attraction between nucleons that acts only at very short distances, overwhelms the electrostatic force (the positive charges repelling each other.)

Shell Model of the Nucleus: a nuclear model in which protons and neutrons exist in levels, or shells, analogous to the shell structure that exists for electrons in an atom.



First Test For Stability:

Magic Numbers: the number of nuclear particles in a completed shell of protons or neutrons. Think of the way full electron shells make noble gases stable, this is similar. If a nucleus has two magic numbers, it is very stable.

Magic Numbers for p+ → 2, 8, 20, 28, 50, 82, (114)
Magic Numbers for no → 2, 8, 20, 28, 50, 82, 126

Example:

42He
p+ → 2
no → 2
Both are magic numbers, very stable.

Example:

Which is more stable?
10251Sn or 10151Sn?

10251Sn → p+ = 51, no = 51
10151Sn → p+ = 51, no = 50

10151Sn has a magic number and 10251Sn doesn't.

Second Test for Stability:

Even/Odd Test:



Nuclei with an even number of protons and an even number of neutrons are very likely to be stable.
Nuclei with an odd number of protons and an odd number of neutrons are not at all likely to be stable.
Nuclei with variably even and odd numbers of protons or neutrons may or may not be stable.

Example:

Which is more likely to be stable, 6231Ga or 6432Ge?

6432Ge because it has both an even number of protons and an even number of neutrons.

Third Test for Stability:

Band of Stability: the region in which stable nuclides lie in a plot of number of protons against number of neutrons.




When it turns out to be unstable, what does the nucleus do about it?

The Six Types of Radioactive Decay:

1. Alpha Emission: emission of a 42He nucleus, or alpha particle from an unstable nucleus. Happens for very large nuclei.

Example:

22688Ra → 22286Rn + 42He

2. Beta Emission: emission of a high-speed electron (β) from an unstable nucleus.

Example:

146C → 147N + 0-1β

Equivalent to the conversion of a neutron to a proton.

10n → 11p + 0-1e

3. Positron Emission: emission of a positron (β+, or 01e) from an unstable nucleus.

Example:

9543Tc → 9542Mo + 01e

Equivalent to the conversion of a proton to a neutron.

11p → 10n + 01e

Professor said a positron is an electron traveling backward in time, but I have no idea what that means.

4. Electron Capture: the decay of an unstable nucleus by capturing, or picking up, an electron from an inner orbital of an atom.

Example:

4019K + 0-1e → 4018Ar

In effect, a proton is changed to a neutron, as in positron emission:

11p → 01e- + 10n

5. Gamma Emission: emission from an excited nucleus of a gamma photon (denoted γ). Often, gamma emission occurs very quickly after radioactive decay. In some cases, however, an excited state has a significant lifetime before it emits a gamma photon.

Metastable Nucleus: a nucleus in an excited state with a lifetime of at least one nanosecond (10-9s). In time, the metastable nucleus decays by gamma emission.

Example:

99m43Tc → 9943Tc + 00γ

99m43Tc → the m denotes metastable, excited state for nucleus
9943Tc → ground state

6. Spontaneous Fission: the spontaneous decay of an unstable nucleus in which a heavy nucleus of mass number greater than 89 splits into lighter nuclei and energy is released.

Example:

23692U → 9639Y + 13633I + 410n

When A is greater than 89 → spontaneous fission

Recall the Band of Stability:



*To the left of the band, nuclides have a neutron to proton ratio larger than that needed for stability, so they tend to decay by beta emission.
*To the right of the band, nuclides have a smaller neutron to proton ratio that that needed for stability, so they tend to decay by either positron emission or electron capture.
*As the curve follows Z as it becomes larger than 83, decay is often by alpha emission.


Radioactive Decay Series: a sequence in which one radioactive nucleus decays to a second, which then decays to a third, and so forth. Eventually, a stable nucleus, which is an isotope of lead, is reached.

Each step will involve an alpha or a beta decay.



Example:

What is the nucleus formed from Uranium–238 after six alpha (42He) and two beta (0-1β) emissions?

23892U → □ + 642He + 20-1e

□ = 21482Pb


Nuclear Bombardment Reactions:

Alchemists → wanted to turn lead to gold and live forever

Transmutation: the change of one element to another by bombarding the nucleus of the element with nuclear particles or nuclei.

Notation:

Target Nucleus + Subatomic Particle → Product Nucleus + Subatomic Particle

Example:

147N +42He → 178O + 11H

147N(42He, 11H)178O

Example:

94Be +42He → 126C + 10n

94Be(42He, 10n)126C


Elements with large atomic numbers deflect alpha particles (large positive nucleus deflect positive alpha particles.) To shoot large particles into heavy nuclei, charged particles must be accelerated.

Particle Accelerator: a device used to accelerate electrons, protons, and alpha particles and other ions to very high speeds. The kinetic energies of these particles is units of electron volts.

Electron Volt: Denoted eV, the quantity of energy that would have to be imparted to an electron to accelerate it by one volt potential difference.

1 eV = 1.602 x 10-19 J

Cyclotron: a type of particle accelerator consisting of two hollow, semicircular metal electrodes called dees (because the shape resembles the letter D), in which charged particles are accelerated by stages to higher and higher kinetic energies.



Ions introduced at the center of the cyclotron are accelerated in the space between the two dees. A magnetic field keeps the ions moving in a spiral path.



The dees are connected to a high-frequency electric current that changes their polarity so that each time the ion moves in the space between the dees, it is accelerated. Then it leaves the cyclotron at high speed and hits its target.

This guy really loves cyclotrons:

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