backgroung radiation

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Background Radiation
· This is the radiation that can be detected all of the time without any identifiable source of Radioactive Emissions present.
· The largest source of background radiation is Uranium deposits in the ground.
This decays to produce other isotopes which are also radioactive.
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A common radioactive isotope is Radon, a gas.
· Cosmic rays from space also contribute to background radiation.
Most are absorbed by the atmosphere.

Taking account of Background Radiation
· When carrying out experiments it is sometimes necessary to allow for background radiation.
· The background count-rate can be measured and then subtracted from the measured count-rates in the experiment.
or
The experiment can be shielded from background radiation by constructing a lead castle around the experiment. The lead will absorb practically all of the background radiation.
The Discovery of Alpha and Beta Radiation
Rutherford used an electrometer to measure an electric current created by the radiation rather than the more crude technique of photograhic plates. Here is the experimental set up reproduced from his 18 paper

A uniform layer of powered uranium compound was spread on plate A and the rays allowed to ionize the gas between plates A and B. The amount of ionization is measured by the saturation current received at B when the potential difference between A and B is great enough to pull all the ions to the plates before they are able to recombine.
Rutherford then proceeded to cover the uranium with aluminum sheets of various thicknesses and measure the current, using the electrometer. Becquerel had already concluded (by March 0, 186; more than two years before Rutherford began working in this area) that the uranium radiation consisted of rays which were unequally absorbed, that is to say that the uranium radiation was made of of two or more distinct parts. With his crude photographic plate technique, he could go no farther.
Rutherford was in the process of checking to see if this was true or not, because Becquerel had also made other claims about uranium radiation which turned out to be incorrect. Now, what Rutherford did find was that there were at least two different rays being emitted by the uranium and he called them a and b. What led him to this conclusion?
He used this equation
r = exp (-ld)
r is the ratio of the intensity of the radiation after passing through a distance d of the substance to the intensity when the substance is removed.
A more modern way of expresing this equation is
I = Io exp (-lt)
Io being the current with nothing covering the uranium on plate A. I is the current at each thickness, which is called t.
The key value is l, the coefficient of absorption. If the uranium rays are homogeneous (i.e., made up of only one type of ray), then l will remain constant as t is increased and I goes down.
This did not happen!! He found that l stayed constant for a few hundredths of a millimeter of aluminum (or a few centimeters of air) covering the uranium on plate A and then suddently dropped to another value, where it remained as the thickness of the aluminum layer was increased.
So he concluded that there were two (at least) part to the uranium rays. The easily absorbed ones, he named alpha (a) and the more penetrating ones he named beta (b).
Well, Ill bet he was glad to get that done. Now, he could relax and be a stuffy professor the rest of his life. Oh no, not our boy!! Discoveries seems to follow Rutherford around, begging him to be discovered. Rutherford was one of the fortunate ones. Oh, he worked hard, but it almost seems like all he had to do was open the door to his lab and a new discovery would bonk him on the head.
NUCLEAR CHANGES
Radioactivity occurs because some atoms have unstable nuclei. Nuclear decay occurs when a nucleus rearranges itself internally to become more stable, at the same time giving out ‘radiation’. Isotopes of elements which are radioactive are called ‘radioisotopes’ (= radioactive isotopes).
An alpha particle is a helium nucleus consisting of protons and neutrons, so when a nucleus undergoes alpha decay its nucleon number decreases by 4 and its proton number decreases by .
So, A è A - 4 and Z è Z -
In the following, radium decays by alpha emission to radon

Notice that the top and bottom balance independently. The radon is also unstable, and a sequence of decays occur until a stable isotope of lead is reached.

ACTIVITY AND THE RADIOACTIVE DECAY LAW
· The activity of a radioactive material is the rate at which disintegrations occur
The SI unit of activity is the becquerel (Bq) - it equals an activity of one disintegration per second.
Activity, A, can be expressed as


In a given set of unstable nuclei we could not predict which particular ones were about to decay. In this sense, the process is said to be ‘random’.
However, the rate of decay, i.e. the activity, is proportional to the number of undecayed nuclei present (E.g. doubling the number present, doubles the rate of decay)



The second of these indicates that the decay is ‘exponential’, as represented in the graphs which follow.

HALF-LIFE, T1/
As nuclei in a radioactive sample decay, the activity gets less, till eventually there are no more nuclei left to decay.
· The half-life of a radioactive source is the time for the activity to fall by half
Or, since the activity of a source is directly proportional to the number of undecayed nuclei, then
· The half-life of a radioactive source is the time for half the undecayed nuclei to decay

Relationship between the decay constant and half-life
Note The following derivation is included for completeness - but your syllabus may not require you to know it

Example
A radioactive source contains 5.11015 atoms and 5000 nuclei decay per second. What is its half-life?


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