RP 12 Inverse Square Law for Gamma Radiation Flashcards

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1
Q

What safety precautions must be taken when working with a radioactive source?

A

● Limit the time of exposure
● Warning signs should be displayed so people are aware that a radioactive source is in use
● Keep sources an arm’s length away from your body at all times, and only ever handle the source using long-handled tongs

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2
Q

How should a radioactive source be handled safely?

A

Long-handled tongs should be used to handle a radioactive source, and you should remain at least an arm’s length away from it at all times.

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3
Q

How should a radioactive source be stored safely?

A

Radioactive sources should be locked away in a sealed lead-lined container. A hazard symbol should be visible on the container as well as at the location where it is stored.

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4
Q

What is the inverse square law of radiation?

A

The intensity of radiation is inversely proportional to the square of the distance from the source. As the distance doubles, the intensity quarters.

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5
Q

What device can be used to measure a radioactive count rate?

A

A Geiger Counter or Geiger-Muller Tube connected to a scaler.

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6
Q

Why is Cobalt-60 a suitable source for this experiment?

A

Cobalt-60 is safe for use in schools and has a half-life of around 5 years, meaning it can be reused for a number of years. The activity is low enough to be safe, but high enough for measurements to be taken easily.

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7
Q

When measuring the count-rate, what advantage comes with measuring over a longer period of time?

A

The longer the period of time over which it is recorded, the lower the measurement uncertainty will be.

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8
Q

What preliminary recording should be taken before bringing the radioactive source into the lab?

A

Before bringing the radioactive source into the lab, the background radiation count should be taken.

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9
Q

How should the background radiation count be accounted for in the experimental data?

A

The background count should be subtracted from the counts for each distance, to produce corrected counts (C’).

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10
Q

How do you convert from a count to a count rate for a given distance?

A

The count should be divided by the length of time over which it was taken to acquire the count rate (mean number of counts per unit time).

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11
Q

What preliminary experiment could you carry out with the source before commencing this experiment?

A

Recordings could be taken to find the maximum distance from the source at which you can still get a measurable count. You could also take recordings closer to the source to determine a suitable time period over which to take readings.

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12
Q

What graph should be plotted to confirm that the inverse square law has been obeyed?

A

A graph of 1/√C’ against x should be plotted. If the inverse square law has been obeyed, this should form a straight line graph with a positive gradient.

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13
Q

Why should a graph of 1/√C’ against x be plotted instead of a graph of C’ against 1/x²?

A

Plotting a graph of 1/√C’ against x removes any systematic error in the distance measurements.

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14
Q

Why is there likely to be a systematic error in your distance measurements in this experiment?

A

The location of the radioactive source within the sealed capsule, and the precise location of ionisation in the GM tube are not known to a high degree of precision.

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15
Q

How can you obtain the systematic distance error from a graph of 1/√C’ against x?

A

The straight line plotted will not cross the x-axis at the origin. The difference between x=0m and the point at which the straight line crosses the x-axis will be the systematic error.

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16
Q

Why may an old gamma source be unsuitable for this experiment?

A

Depending on the source’s half-life and its age, the activity of the source may have fallen to a level that is too low to obtain recordable counts over a large enough range of distances to conduct a meaningful experiment.