Chapter 30:Nuclear

Physics and Radioactivity

Radioactivity• Radioactivity is the

discentigration of an unstable nuclei.

• when the nuclei decays the nucleus emits alpha rays, beta rays, and gamma rays.

Henri BecquerelBecquerel was a French physicist that

came from a family of scientists. He used potassium uranyl sulfate,

K2UO2(SO4)2, and exposed it to sunlight by placing it on photographic plates and wrapping it in black paper.

This method revealed uranium crystals.

Becquerel’s Results

More Becquerel Results

• He concluded that the phosphorescent substance in question emitted radiation which penetrated the paper.

• He demonstrated that radiation emitted by uranium shared certain characteristics with x-rays, but could be deflected by a magnetic field, thus it must consist of charged particles.

Becquerel’s Accomplishments

• He was awarded the Nobel prize for physics in 1903 for his discovery of spontaneous radioactivity.

• The SI unit of radioactivity was named after him, the Becquerel (Bq), which is one transformation (or decay) per second.

Marie and Pierre Currie

• They investigated radioactivity in uranium after Becquerel made his discovery.

• Pierre died and Marie finished the work they started.

• After Marie finished her chemical extraction, she said the compound she was working with was more radioactive then the uranium.

• This in turn led to the discovery of Po and Ra (polonium and radium).

Marie’s Recognition

• In 1903 she and her husband were awarded the Nobel prize in physics for spontaneous radiation.

• In 1904 she was awarded the Nobel prize in chemistry for the discovery of two elements.

• She was the first person to receive two Nobel prizes.

Observation of

Radioactive Rays•Alpha Rays which barley penetrate a piece of paper.

•Beta Rays which can penetrate 3mm of aluminum.

•Gamma Rays which can penetrate several centimeters of lead.

We now know:

• Alpha rays are helium nuclei

• Beta rays are electrons

• Gamma rays are electromagnetic radiation

Rays reacting to magnetic field

XAZ

X= Chemical symbol for elementA= Atomic massZ= Atomic Number

N = N0e- λt

No = initial amount of Substance λ = decay constant (different for every substance)t= timee= natural exponential who’s value is 2.718…

Radioactive Decay Law

14 6C

∆N∆t

The number of decays per second is called the activity.

The previous formula can also be written as:

= e∆N∆t

∆N∆t( ) - λt

The half-life is the time it takes for half the nuclei in a given sample to decay. This formula is derived from the previous formula we looked at.

Half-life Formula

Example

• A radioactive material is known to produce 3000 decays per minute at one time, and 4.6 hours later it produces 750 decays per minute. What is its half life?

3333 1500 750

2.3 hrs

Example

• What fraction of a sample whose half life is 6 months will remain after 2 years?

• 24 = 4 ½ life: ½ after 6 months

6 ¼ after 12 months

1/8 after 18 months

1/16 after 24 months

Section 30.13Section 30.13

Detection of RadiationDetection of Radiation

Detection of RadiationDetection of Radiation

Individual particles such as electrons, Individual particles such as electrons, protons, protons, αα particles, neutrons, and particles, neutrons, and γγ rays rays are not detected directly by our senses.are not detected directly by our senses.

Several instruments have been developed Several instruments have been developed to make up for this.to make up for this.

Geiger CounterGeiger Counter• A cylindrical metal tube filled with a A cylindrical metal tube filled with a

certain type of gas (usually helium, certain type of gas (usually helium, neon, or argon) with a wire running neon, or argon) with a wire running down the center. The wire is kept at down the center. The wire is kept at a very high positive voltage (slightly a very high positive voltage (slightly less than that required to ionize the less than that required to ionize the gas) with respect to the cylinder.gas) with respect to the cylinder.

• Charged particles passing through Charged particles passing through the window ionize a few gas atoms. the window ionize a few gas atoms. The freed electrons accelerate The freed electrons accelerate toward the wire ionizing more toward the wire ionizing more atoms along the way. When this atoms along the way. When this “avalanche” of electrons hits the “avalanche” of electrons hits the wire a voltage pulse is produced wire a voltage pulse is produced which can be amplified and which can be amplified and displayed in the form of audible displayed in the form of audible clicks or by a needle meter.clicks or by a needle meter.

Scintillation CounterScintillation Counter A scintillator is a material that emits visible A scintillator is a material that emits visible

light when struck by charged particles. light when struck by charged particles. Typically crystals of NaI or certain plastics.Typically crystals of NaI or certain plastics.

The scintillator is attached to a The scintillator is attached to a photomultiplier tube which converts the photomultiplier tube which converts the energy of the scintillator-emitted photons energy of the scintillator-emitted photons into an electrical signal.into an electrical signal.

The photons emitted strike a photoelectric The photons emitted strike a photoelectric surface called a photocathode. This emits surface called a photocathode. This emits electrons which travel through the tube electrons which travel through the tube striking several electrodes of successively striking several electrodes of successively higher voltages along the way. When each higher voltages along the way. When each electrode is struck, more electrons are electrode is struck, more electrons are ejected. By the time they reach the end they ejected. By the time they reach the end they have multiplied into a large number of have multiplied into a large number of electrons, close to 10electrons, close to 106 6 or more. These or more. These electrons produce an electric signal which electrons produce an electric signal which can be sent to a counter just like the Geiger can be sent to a counter just like the Geiger counter.counter.

Bubble ChamberBubble Chamber In addition to detecting the presence of charged particles, some devices can be used to In addition to detecting the presence of charged particles, some devices can be used to

determine the path.determine the path.

The bubble chamber uses a superheated liquid kept close to boiling point (usually liquid The bubble chamber uses a superheated liquid kept close to boiling point (usually liquid hydrogen). The bubbles characteristic of boiling form around ions produced by the hydrogen). The bubbles characteristic of boiling form around ions produced by the passage of charged particles. Photos can determine the path of the particles.passage of charged particles. Photos can determine the path of the particles.

A magnetic field is usually applied across the chamber and the momentum can be A magnetic field is usually applied across the chamber and the momentum can be determined from the radius of curvature of the particle paths.determined from the radius of curvature of the particle paths.

Wire Drift ChamberWire Drift Chamber The wire drift chamber is a more modern The wire drift chamber is a more modern

way of detecting the paths of charged way of detecting the paths of charged particles. It can be likened to a very large particles. It can be likened to a very large and more complex Geiger counter.and more complex Geiger counter.

It consists of many thin wires immersed in It consists of many thin wires immersed in gas. Some are grounded while others are gas. Some are grounded while others are kept at a high voltage. Charged particles kept at a high voltage. Charged particles passing through the chamber ionize the passing through the chamber ionize the gas. Electrons drift toward the high voltage gas. Electrons drift toward the high voltage wires and produce an “avalanche” along the wires and produce an “avalanche” along the way. These produce voltage pulses.way. These produce voltage pulses.

The positions of the particles are The positions of the particles are determined electronically by the position of determined electronically by the position of the wire and by the time it takes for the the wire and by the time it takes for the pulses to reach the sensors at the end of pulses to reach the sensors at the end of the wires. These are used by computers to the wires. These are used by computers to reconstruct the paths.reconstruct the paths.

Homework

Problems 36 & 39

ConceptRadioactivity is unaffected by…

a. Heating

b. Cooling

c. Chemical reagents

d. a & b

e. All of the above

The decay constant is…

a. The same for all isotopes

b. Different for different isotopes