# Radioactive decay for OCR GCSE Physics

3. Count rate

A nucleus is said to be stable if it has a certain amount of neutrons depending on the number of protons it has. If there are too many or too few neutrons, the nucleus is said to be unstable and will decay by emitting radiation. The same number of protons and neutrons are needed for nuclei of elements of fewer protons to be stable. The greater the number of protons, the more neutrons are needed for the nucleus to be stable. Radioactive decay occurs randomly and each decay is independent of the rest, therefore predictions about when an atom will decay cannot be made.

The activity of a radioactive object is the average number of radioactive decays per unit time and it is measured in becquerels (Bq). This can also be described as the rate of radioactive decay. The greater the time that has passed, the smaller the number of nuclei present, therefore the decay of the sample of the object will decrease. The number of decaying nuclei is directly proportional to the original number of them present.

Count rate is the number of decays recorded each second by a radiation detector. An example of a radiation detector is a Geiger-Muller tube. When radiation enters the GM tube, the counter clicks and the count is displayed on the screen. The count rate before the radioactive sample is brought into a room must be measured, since this constitutes for the background radiation. This must be subtracted from any readings of the count rate taken to account for background radiation.

# 1

Sketch a graph of the number of radioactive nuclei present against time.

image

# 2

Define the activity of a radioactive object.

The activity of a radioactive object is the average number of radioactive decays per unit time.

# 3

Draw the apparatus needed to record the count rate of a radioactive sample.

image

# 4

Label the given diagram for recording the count rate of a radioactive sample.

A: counter, B: Geiger-Muller tube.

# 5

Count rate must be corrected after it is recorded. Explain how it must be corrected.

Before the count rate is recorded for the radioactive sample, it msut be measured for the room. This is the count rate of the background radiation. Any following recordings of the count rate must be corrected by subtracting this value to account for the background radiation.

End of page