Beta Particles Essay

This essay has a total of 2209 words and 16 pages.

Beta Particles

I will investigate how the field strength varies the deflection of Beta Particles.

Preliminary Work
I started my preliminary work because, when I started my measurements using 2 coils used
in experiments to deflect electrons from and electron gun. While testing for the
deflection of beta particles, I found that beta radiation was scattered in a very large
cone, I can not get any readings with amount of beta radiation scattering.

So I would have to construct some type of shielding for this investigation, this is so I
can measure the deflection more easily. The angle at which the beta particles are being
scattered is 48o.

Deciding on the Type of Shielding
I will test for the best shielding. The best properties of the shield will be; it can be
malleable to form different shapes and can be punctured, can stop radiation at a small

Strontium 90 beta source
GM tube counter
Different thickness of different metals
Clamps, bosses and clamp stand to hold the source and the material being tested.

1. Set-up equipment as in the diagram
2. Record the thickness and the material being used.
3. Record 5 readings of the radiation count, and record them in a table
4. Replace material being tested with different material or a different sized material.
5. Repeat steps 2 to 4 as required.

The background radiation reading is 2, 4, 6, 4, 5, 2. The average count is 3.8 (1dp).

This shows that aluminium stops radiation at 3.5 mm, this would be difficult to use
because, this thickness of Aluminium is not malleable and the aluminium is not soft enough
to puncture. Lead can stop radiation at very thin thickness', also lead is very malleable
and is soft enough to puncture. I will use Lead shield at 0.6mm thick, since it is the
most abundant thickness' available and it is the easiest to form to any shape I want.

Deciding how the shielding can be used.
I want to have a tight beam of beta particles in this investigation, so I will use my
knowledge on what would be the best way to shield the source.

An unshielded source
The source is unshielded and has beta particles spreading out. Angle Theta is the angle
which the beta particles are scattered through. The path of the beta particles is not a
straight line, but a curve because the beta particle are deflected by the moles in the
air. The points A B are the furthest points where beta ration is detected.

Using a plate shielding
The beta particles have a smaller angle, that they are being scattered through. Also if
the shield isn't wide enough the beta particles that travelled either side of the
shielding could cause problems. For example, when measuring the deflection of the beta
particles in the electric field, the beta particles that passes outside of the shield will
interfere with detecting the deflection of the Beta.

Using a cylindrical shielding
The cylindrical shielding produces a smaller angle which the beta particles are being
scattered through than the source. But this is too wide to for I want to use. So I will
need a compromise of the plate shielding and the cylindrical shielding.

Using Compressive Shielding
This shielding produces the smallest angle for scattering because the cylinder stops the
beta particles that spread out to the side, the plate stops all the beta particles that
don't pass through the hole.

The smaller the hole in the plate shield will affect the counts, because the smaller the
hole then greater the number of beta particles are absorbed by the shielding. The size of
the cylinder does not matter as long it has a diameter greater than sources width, because
the beta particles will do the same thing as in fig 3.

Making the shielding
To make the shielding, I must be accurate in making the hole in the plate, I will do this
by using a tool designed to make regular indentations. This tool can make holes from 5mm
wide to 9mm with intervals of 1mm. In making the cylindrical shielding, it doesn't have to
be a cylinder to produce the same desired affect. I will use a round metal bar with which
to roll the lead shielding. The both parts of the shield will be stuck together with tape,
because if I piece become damaged or deformed, it can replaced easily.

Measuring the Angle at Which Beta Particles Are Scattered Through With different Diameters of punctures

Strontium 90 beta source
GM Tube
Lead Shielding (43.6mm long)

Note: To make sure that the GM tube, the puncture and the source is in alignment, draw a
straight-line with a rule on the table. Then use a set square to get the perpendicular
vertical line. Mark a set distance, this mark will be placed in the centre of the source,
GM and the punter along the table. (Careful with the source, approximate the centre, see
safety notes).

1. Set-up the equipment as the diagram, measure the distance (20cm) with an accurate rule,
stating with plate shield have in a puncture of 5mm to start with.

2. Take 10 readings per 100s, record these results in a table
3. Change the time base for the counter to 10s and move the GM tube along parallel with
the shielding. Using the method of trial and improvement, find the places where the is a
sufficient drop in counts, mark this with a sharp piece of chalk or any other marker.

4. Measure the distance between each point and record the results in the table
5. Repeat step 1 to 4 replacing the plate shielding.

The background count had an average of 28.9 per 100 seconds.

I worked out the angle by using this equation

Tan-1 (AB/D) = Angle beta particles scatter though.

The graphs show a linear relationship between the puncture size and distance form AB. I
would of expected an squared relation ships because as the diameter increases with a
factor, the area would increase with 4 times that factor, so less beta particles would be
stopped by the shielding. If I had more time to investigate this I would take more
readings and examine the relationship.

The best puncture size is 5mm, it has the smallest angle which the beta particles scatter
through and the count is sufficient to use in my final experiment.

Measuring the Deflection of Beta Particles.

Strontium 90 beta source
2 electric plates
GM tube and counter

1. Set up equipment as in the diagram
2. Make sure that all the equipment is in line
3. Place the plates at 5cm apart, using the set square making sure that the plates are 2.5 cm form the centre line.
4. Turn on the EHT
5. Raise the voltage to 1kV
6. Use the 10 second count, move the GM tube until there is a drop in the background
count. Change to 100s and move the GM slightly until you just get a back group count.

7. Record this result in the table.
8. Repeat step 5 to 7 increasing the voltage by 1kV

This experiment is similar to an electron gun experiment I have done in the past. This
experiment could be compared with a electron gun, it has the same principles. For example;
Electrons are being released from the source and being deflected by the electric field.
The differences are that this experiment was not carried out in a low vacuum, the beta
particles are being decelerated, due to impacts with air molecules.

V = speed of an electron,
E = Electric field strength,
e = charge of an electron,
Vp = p.d. between the 2 plates,
d = plate spacing,
m = mass of electron,
x = horizontal distance from point of entry,
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