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Geiger Counter Experiment #7

Magnetic Deflection of Beta Particles

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magnetic-deflection of beta particles

Beta particles and electrons are the same particle. The reason we classify them differently is that the beta particle originates from the nucleus of the atom being generated by radioactive decay. So we could say that a beta particle is a freshly minted electron. If we place a strong magnetic field across the path of beta particles being emitted from a radioactive source we can make the path of the beta particles bend. The energy of the beta particles and the strength of the magnetic field will determine the degree of deflection.

The following graph provides an illustration of the deflection of charged particles in a magnetic field. Notice the gamma ray photon is undeflected by a magnetic field.

deflection of charged particles in a magnetic field

The objective of this experiment is to deflect beta particles using a magnetic field and to detect the deflection using a geiger counter.

magnetic deflection of beta particles

Performing the Experiment:

Step 1) Begin by using the Geiger counter to take a set of background radiation readings. Set GCA-07 or GCA-07W to CPM mode using the front panel switch. Log a minimum of three one-minute readings. If you are using the radiation graphing program included with the CGA-07(W) start the program. Set the time increment to one minute.

Step 2) Next, place the SR-90 source in position by the detector, with no magnetic feild. Take a minimum of three one minute recordings. The position and height of the SR-90 as it relates to the GM tube detector is critical. It should be at the same distance and height as it will be when placing the source between the magnetic field.

Failure to maintain this exact distance will render the background numbers inaccurate for the experiment.

Step 3) Place the SR-90 source in position by the detector, with N-S magnetic field. Take a minimum of three one minute recordings.

Step 4) Place the SR-90 source in position by the detector, with S-N magnetic field. Take a minimum of three one minute recordings.

In steps 2, 3 and 4 the distance and height between the SR-90 source and detector should remain the same, or as close as possible to record proper data.


Images Radiation Graphing program allows you to save data and to reload it into its own data viewer program.

I do not have a horseshoe magnet strong enough for this experiment. So I fabricated an equivalent out horseshoe magnet out of wood blocks and two very strong neodymium magnets .

fabricated horseshoe magnet

The wood blocks are 1.5" x 1.5" x 1.5" and are attached to the wood base (1.5" wide x 4.75" long x 0.5" thick) using two wood screws. The steel cups that will be securing the magnets are attached to the inner facing surfaces of the wood blocks are from KJ Magnetics. The poles of the magnets (KJ Magnetics DX04B-N52) are checked with a compass to determine the North and South poles of each disc magnet. The Surface Field of the magnets are: 3309 Gauss.

neodymium-disc-magnet            Steel-cup-holder-for-magnet

The magnets are then place inside the steel cups (KJ Magnetics NSCX02 or equiv.) making sure that one disc magnet has it North pole facing inward while the opposing magnet has its South pole facing inward.

The wand holder used in this experiment is Images wand holder


A close up of the experiment is shown in the following picture.

deflection of beta particles experiment
The results of the experiment show that beta particles are deflected by the magnetic field.

If we outline the magnetic field as it is applied in the experiment when it is directing beta particles toward the detector would look like the following illustration.


Going Further

If we changed our Sr-90 source to a Na-22 source that emits positive beta particles (Positrons) what would you expect the deflection of the positrons to be compared to the normal beta particles?

Components for Sale

Sr-90 Source - $79.95

Na-22 source - $79.95

Wand Holder - $12.95

Lead Shield - $14.95
2.5" x 2" x 0.5"


Nickel-plated Steel Cup