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Further reading:

Alpha particles


Genaral information on this virtuell experiment “Alpha particles” 

This animation allows you to learn more about the properties of alpha particles and their behaviour in air, vacuum or crossed fields. A virtual experiment shows how the natural speed of alpha particles can be determined. You can proceed through the animation step by step in line with the options you have chosen using the options button.

The apparatus for the virtual experiment is as follows:

–  Vacuum pump
–  Chamber with preparation and condenser plates
–  Alpha particle source Am 241
–  Condenser plates with spacing d=0.9mm and a fixed voltage of U=3000V (E field)
–  External B field, continuously controllable, perpendicular to E field
–  Blind with small 3.5 mm aperture
–  Counter tube

 

More information on this virtual experiment

Explanation 1
The options selected are:

Vacuum   – “OFF”
E field   – “OFF”
B field – “OFF”

Particles emerge continuously from the alpha particle source (Am 241) and interact with the air. As their range is shorter than the length of the chamber, the counter tube only indicates residual radiation.

Now switch on the vacuum. The E field and the B field stay OFF.

Explanation 2
The options selected are:

Vacuum   – “ON”
E field   – “OFF”
B field – “OFF”

Alpha particles move in a straight line through the diversion chamber under vacuum without loss to the counter tube. About 975 particles are counted in 100 seconds.

However, our task is to determine the natural speed of the alpha particles. Of course, you can’t use a stopwatch, as you might for a 100 metres race, so you have to measure the interaction between an alpha particle and something else. We can use the experimental setup for this purpose. We have an electric and a magnetic field which can interact with the alpha particles. So let’s try.

Perhaps you are wondering why we selected precisely this experimental setup. If you are still wondering, just read on.

Start by switching on the E field. The B field stays OFF and we know that we will not be able to measure any alpha particles in air, so we need to switch the vacuum ON.

Explanation 3
The options selected are:

Vacuum   – “ON”
E field   – “ON”
B field – “OFF”

The alpha particles emitted by the source pass through the diversion chamber under vacuum through the condenser plate towards the counter tube. However, they do not reach the counter tube and only residual radiation is measured. There is no air which could act as an obstacle and the particle beam is moving through a vacuum. There must be another factor preventing straight-line movement of the particles to the counter tube. The only reason for the diversion of the particles is the effect of the electric field, or more precisely, electric force Fel .

Result:
Alpha particles can be influenced by the force exerted by an electric field

Fel = q * E

The next logical step is to check the effect of the magnetic field on the alpha particles. To do so, switch the vacuum and the B field ON and the E field OFF.

 

Explanation 4
The options selected are:

Vacuum   – “ON”
E field   – “OFF”
B field – “ON”

The alpha particles emitted by the source pass through the diversion chamber under vacuum through the condenser plate towards the counter tube. However, they do not reach the counter tube and only residual radiation is measured, as was the case with the electric field. There is no air which could act as an obstacle and the electric field was switched OFF. The only reason for the diversion is the magnetic force Fmagn exerted by the magnetic field.

Result:
Alpha particles can be influenced by the force exerted by a magnetic field  (F= q * v * B ) This force is also known as the Lorentz force.

Important

At this point, we would like to remind you of the question which we asked above. Why did we select precisely this test setup? We would also like to ask you how the natural speed of alpha particles can be determined.

Have you noticed the physical factors which we can use to influence the alpha particle beam?

the electric force ( Fel = q * E  ) and

the magnetic force or Lorentz force ( F= q * v * B ).

You will see what we need to do with the equations to calculate the speed. We simply set both equations to the same value and the result is

Fel = FL or,

following simplification and transformation,

v= (U/d) / B

We also know that E = U / d (the electric field E can be calculated by dividing the voltage “U” by the condenser plate spacing “d”. (Please refer to the BIGS animation of a plate-type condenser).
Therefore:

v = E / B

q * E = q * v * B

For the purpose of the calculation, we have set the two equations to the same value, but what does that mean in physical terms? Please think about what that means for the alpha particle beam.

If the two equations are set to the same value, the two forces are equal. The beam of alpha particles moves in a straight line through the fields and reaches the counter tube at maximum intensity.

Now, it is easy to calculate the natural speed of alpha particles. The animation shows that the particle beam moves in a straight line at a magnetic field of B=0.23 T. As the “producers” of the virtual experiment, we can also supply the values of the plate spacing “d” and the electric field “E”. These values cannot be determined from the animation itself.

d =  0.9 * 10-3m

U =  3000 V

If you use these values in the formula

v = (3000V/0.9*10-3m) / 0,23 T

v = 1.45 * 107 ms-1

With our virtual experiment, we have succeeded in determining the natural speed of

alpha particles v=1.45*107 ms-1.

Please note that this is about 5% of the speed of light. A relative calculation is therefore justified. But what about determining the speed of other radioactive particles, such as beta particles?


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