What are we doing todayDecay

Types of RadiationProperties of nuclear radiation

Decay and ProbabilityProtactinium

RadiationRadiation: The process of emitting energy in the form of waves or particles.

Where does radiation come from?Radiation is generally produced when particles interact or decay.

A large contribution of the radiationon earth is from the sun (solar) or from radioactive isotopes of the elements (terrestrial).

Radiation is going through you atthis very moment!

http://www.atral.com/U238.html

IsotopesWhat’s an isotope?

Two or more varieties of an element having the same number of protons but different number of neutrons. Certain isotopes are “unstable” and decay to lighter isotopes or elements.

Deuterium and tritium are isotopes of hydrogen. In addition to the 1 proton, they have 1 and 2 additional neutrons in the nucleus respectively.

Another prime example is Uranium 238, or just 238U.

Radioactivity

By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:

1) Alpha particles ()

2) Beta particles ()

3) Gamma-rays ()

By the end of the 1800s, it was known that certain isotopes emit penetrating rays. Three types of radiation were known:

1) Alpha particles ()

2) Beta particles ()

3) Gamma-rays ()

Where do these particles come from ?

These particles generally come from the nuclei of atomic isotopes which are not stable.

The decay chain of Uranium 238produces all three of these formsof radiation.

Let’s look at them in more detail…

Alpha Particles ()

Radium

R226

88 protons138 neutrons

Radon

Rn222

Note: This is theatomic mass number,which is the number ofprotons plus neutrons

86 protons136 neutrons

+ nnp

p

He)

2 protons2 neutrons

The alpha-particle is a Helium nucleus.

It’s the same as the element Helium, with the electrons stripped off !

Beta Particles ()

CarbonC14

6 protons8 neutrons

NitrogenN14

7 protons7 neutrons

+ e-

electron(beta-particle)

We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. In symbolic notation, the following process occurred: _

n p + e ( + Yes, the same anti-neutrino we saw previously

Gamma particles ()In much the same way that electrons in atoms can be in an excited state, so can a nucleus.

NeonNe20

10 protons10 neutrons

(in excited state)

10 protons10 neutrons

(lowest energy state)

+

gamma

NeonNe20

A gamma is a high energy light particle.

It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

A gamma is a high energy light particle.

It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

Gamma Rays

NeonNe20 +

The gamma from nuclear decayis in the X-ray/ Gamma ray

part of the EM spectrum(very energetic!)

NeonNe20

How do these particles differ ?

ParticleMass*

(MeV/c2)Charge

Relative to the electronic charge

Gamma () 0 0

Beta () ~0.5 -1

Alpha () ~3752 +2

* m = E / c2* m = E / c2

Rate of DecayBeyond knowing the types of particles which are emittedwhen an isotope decays, we also are interested in how frequentlyone of the atoms emits this radiation.

A very important point here is that we cannot predict when aparticular entity will decay.

We do know though, that if we had a large sample of a radioactive substance, some number will decay after a given amount of time.

Some radioactive substances have a very high “rate of decay”,while others have a very low decay rate.

To differentiate different radioactive substances, we look toquantify this idea of “decay rate”

Half-LifeThe “half-life” (t ½ ) is the time it takes for half the atoms of a radioactive substance to decay.

For example, suppose we had 20,000 atoms of a radioactive substance. If the half-life is 1 hour, how many atoms of that substance would be left after:

10,000 (50%)

5,000 (25%)

2,500 (12.5%)

1 hour (one lifetime) ?

2 hours (two lifetimes) ?

3 hours (three lifetimes) ?

Time No. atomsremaining

% of atomsremaining

Decay constant, The decay constant is the probability a given nucleus will decay in one second

The rate of decay is directly proportional to the number of un-decayed atoms

dN/dt N or dN/dt = - N

This is a differential equation – when it is solved it leads to

N = No e – t

N0 = starting number of particles = number of particles at time t

Lifetime Note by slight rearrangement of this formula:

Fraction of particles which did not decay: N / N0 = e- t

Number of half lives

Time

(min)

Fraction of remainingneutrons

0 0 1.0

1 14.7 0.5

2 29.4 0.25

3 44.1 0.125

4 58.8 0.063

5 73.5 0.026

N = No e – t

Not all particles have the same half-life.

Uranium-238 has a half life of about 4.5 billion (4.5x109) years !

Some subatomic particles have half-lives that are less than 1x10-12 sec !

Given a batch of unstable particles, we cannotsay which one will decay.

The process of decay is statistical. That is, we can only talk about either,

1) the half-life of a radioactive substance, or2) the “probability” that a given particle will decay.

Not all particles have the same half-life.

Uranium-238 has a half life of about 4.5 billion (4.5x109) years !

Some subatomic particles have half-lives that are less than 1x10-12 sec !

Given a batch of unstable particles, we cannotsay which one will decay.

The process of decay is statistical. That is, we can only talk about either,

1) the half-life of a radioactive substance, or2) the “probability” that a given particle will decay.

Summary Certain particles are radioactive and undergo decay.

Radiation in nuclear decay consists of , , and particles

The rate of decay is give by the radioactive decay law:

Some elements have half-lives ~billions of years.

Subatomic particles usually have half-lives which are fractions of a second…

Certain particles are radioactive and undergo decay.

Radiation in nuclear decay consists of , , and particles

The rate of decay is give by the radioactive decay law:

Some elements have half-lives ~billions of years.

Subatomic particles usually have half-lives which are fractions of a second…

N = No e – t